Factors influencing free intracellular calcium concentration in quiescent ferret ventricular muscle

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required.

Systolic [Ca2+ ]i regulates diastolic levels in rat ventricular myocytes

KEY POINTS

For the heart to function as a pump, intracellular calcium concentration ([Ca2+ ]i ) must increase during systole to activate contraction and then fall, during diastole, to allow the myofilaments to relax and the heart to refill with blood. The present study investigates the control of diastolic [Ca2+ ]i in rat ventricular myocytes. We show that diastolic [Ca2+ ]i is increased by manoeuvres that decrease sarcoplasmic reticulum function. This is accompanied by a decrease of systolic [Ca2+ ]i such that the time-averaged [Ca2+ ]i remains constant. We report that diastolic [Ca2+ ]i is controlled by the balance between Ca2+ entry and Ca2+ efflux during systole. The results of the present study identify a novel mechanism by which changes of the amplitude of the systolic Ca transient control diastolic [Ca2+ ]i .

ABSTRACT

The intracellular Ca concentration ([Ca2+ ]i ) must be sufficently low in diastole so that the ventricle is relaxed and can refill with blood. Interference with this will impair relaxation. The factors responsible for regulation of diastolic [Ca2+ ]i , in particular the relative roles of the sarcoplasmic reticulum (SR) and surface membrane, are unclear. We investigated the effects on diastolic [Ca2+ ]i that result from the changes of Ca cycling known to occur in heart failure. Experiments were performed using Fluo-3 in voltage clamped rat ventricular myocytes. Increasing stimulation frequency increased diastolic [Ca2+ ]i . This increase of [Ca2+ ]i was larger when SR function was impaired either by making the ryanodine receptor leaky (with caffeine or ryanodine) or by decreasing sarco/endoplasmic reticulum Ca-ATPase activity with thapsigargin. The increase of diastolic [Ca2+ ]i produced by interfering with the SR was accompanied by a decrease of the amplitude of the systolic Ca transient, such that there was no change of time-averaged [Ca2+ ]i . Time-averaged [Ca2+ ]i was increased by β-adrenergic stimulation with isoprenaline and increased in a saturating manner with increased stimulation frequency; average [Ca2+ ]i was a linear function of Ca entry per unit time. Diastolic and time-averaged [Ca2+ ]i were decreased by decreasing the L-type Ca current (with 50 μm cadmium chloride). We conclude that diastolic [Ca2+ ]i is controlled by the balance between Ca entry and efflux during systole. Furthermore, manoeuvres that decrease the amplitude of the Ca transient (without decreasing Ca influx) will therefore increase diastolic [Ca2+ ]i . This identifies a novel mechanism by which changes of the amplitude of the systolic Ca transient control diastolic [Ca2+ ]i .

An explant muscle model to examine the refinement of the synaptic landscape

Signals from nerve and muscle regulate the formation of synapses. Transgenic mouse models and muscle cell cultures have elucidated the molecular mechanisms required for aggregation and stabilization of synaptic structures. However, far less is known about the molecular pathways involved in redistribution of muscle synaptic components. Here we established a physiologically viable whole-muscle embryonic explant system, in the presence or absence of the nerve, which demonstrates the synaptic landscape is dynamic and malleable. Manipulations of factors intrinsic to the muscle or extrinsically provided by the nerve illustrate vital functions during formation, redistribution and elimination of acetylcholine receptor (AChR) clusters. In particular, RyR1 activity is an important mediator of these functions. This physiologically relevant and readily accessible explant system provides a new approach to genetically uncouple nerve-derived signals and for manipulation via signaling molecules, drugs, and electrical stimulation to examine early formation of the neuromuscular circuit.

Regulation of ion gradients across myocardial ischemic border zones: a biophysical modelling analysis

The myocardial ischemic border zone is associated with the initiation and sustenance of arrhythmias. The profile of ionic concentrations across the border zone play a significant role in determining cellular electrophysiology and conductivity, yet their spatial-temporal evolution and regulation are not well understood. To investigate the changes in ion concentrations that regulate cellular electrophysiology, a mathematical model of ion movement in the intra and extracellular space in the presence of ionic, potential and material property heterogeneities was developed. The model simulates the spatial and temporal evolution of concentrations of potassium, sodium, chloride, calcium, hydrogen and bicarbonate ions and carbon dioxide across an ischemic border zone. Ischemia was simulated by sodium-potassium pump inhibition, potassium channel activation and respiratory and metabolic acidosis. The model predicted significant disparities in the width of the border zone for each ionic species, with intracellular sodium and extracellular potassium having discordant gradients, facilitating multiple gradients in cellular properties across the border zone. Extracellular potassium was found to have the largest border zone and this was attributed to the voltage dependence of the potassium channels. The model also predicted the efflux of [Formula: see text] from the ischemic region due to electrogenic drift and diffusion within the intra and extracellular space, respectively, which contributed to [Formula: see text] depletion in the ischemic region.

Calcium flux balance in the heart

This article reviews the consequences of the need for the cardiac cell to be in calcium flux balance in the steady state. We first discuss how this steady state condition affects the control of resting [Ca(2+)]i. The next section considers how sarcoplasmic reticulum (SR) Ca content is controlled by a feedback mechanism whereby changes of SR Ca affect the amplitude of the Ca transient and this, in turn, controls sarcolemmal Ca fluxes. Subsequent sections review the effects of altering the activity of individual Ca handling proteins. Increasing the activity of the SR Ca-ATPase (SERCA) increases both the amplitude and rate constant of decay of the systolic Ca transient. The Ca flux balance condition requires that this must be achieved with no change of Ca efflux placing constraints on the magnitude of change of amplitude and decay rate. We analyze the quantitative dependence of Ca transient amplitude and SR content on SERCA activity. Increasing the open probability of the RyR during systole is predicted to have no steady state effect on the amplitude of the systolic Ca transient. We discuss the effects of changing the amplitude of the L-type Ca current in the context of both triggering Ca release from the SR and loading the cell with calcium. These manoeuvres are considered in the context of the effects of β-adrenergic stimulation. Finally, we review calcium flux balance in the presence of Ca waves.

K201 improves aspects of the contractile performance of human failing myocardium via reduction in Ca2+ leak from the sarcoplasmic reticulum

In heart failure, intracellular Ca2+ leak from cardiac ryanodine receptors (RyR2s) leads to a loss of Ca2+ from the sarcoplasmic reticulum (SR) potentially contributing to decreased function. Experimental data suggest that the 1,4-benzothiazepine K201 (JTV-519) may stabilise RyR2s and thereby reduce detrimental intracellular Ca2+ leak. Whether K201 exerts beneficial effects in human failing myocardium is unknown. Therefore, we have studied the effects of K201 on muscle preparations from failing human hearts. K201 (0.3 microM; extracellular [Ca2+]e 1.25 mM) showed no effects on contractile function and micromolar concentrations resulted in negative inotropic effects (K201 1 microM; developed tension -9.8 +/- 2.5% compared to control group; P < 0.05). Interestingly, K201 (0.3 microM) increased the post-rest potentiation (PRP) of failing myocardium after 120 s, indicating an increased SR Ca2+ load. At high [Ca2+]e concentrations (5 mmol/L), K201 increased PRP already at shorter rest intervals (30 s). Strikingly, treatment with K201 (0.3 microM) prevented diastolic dysfunction (diastolic tension at 5 mmol/L [Ca2+]e normalised to 1 mmol/L [Ca2+]e: control 1.26 +/- 0.06, K201 1.01 +/- 0.03, P < 0.01). In addition at high [Ca2+]e) K201 (0.3 microM) treatment significantly improved systolic function [developed tension +27 +/- 8% (K201 vs. control); P < 0.05]. The beneficial effects on diastolic and systolic functions occurred throughout the physiological frequency range of the human heart rate from 1 to 3 Hz. Upon elevated intracellular Ca2+ concentration, systolic and diastolic contractile functions of terminally failing human myocardium are improved by K201.

Ca2+/calmodulin-dependent protein kinase: a key component in the contractile recovery from acidosis

Intracellular acidosis exerts substantial effects on the contractile performance of the heart. Soon after the onset of acidosis, contractility diminishes, largely due to a decrease in myofilament Ca(2+) responsiveness. This decrease in contractility is followed by a progressive recovery that occurs despite the persistent acidosis. This recovery is the result of different mechanisms that converge to increase diastolic Ca(2+) levels and Ca(2+) transient amplitude. Recent experimental evidence indicates that activation of the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is an essential step in the sequence of events that increases the Ca(2+) transient amplitude and produces contractile recovery. CaMKII may act as an amplifier, providing compensatory pathways to offset the inhibitory effects of acidosis on many of the Ca(2+) handling proteins. CaMKII-induced phosphorylation of the SERCA2a regulatory protein phospholamban (PLN) has the potential to promote an increase in sarcoplasmic reticulum (SR) Ca(2+) uptake and SR Ca(2+) load, and is a likely candidate to mediate the mechanical recovery from acidosis. In addition, CaMKII-dependent phosphorylation of proteins other than PLN may also contribute to this recovery.

Cardiovascular effects of the aqueous extract of Gynostemma pentaphyllum Makino

In the present study, the cardiovascular activity of the aqueous extract of Gynostemma pentaphyllum Makino leaves was investigated in the anaestetized guinea-pigs and has been compared with two of its isolated gypenosides (III, VIII) and with verapamil, a well-known Ca-antagonistic drug. The results obtained showed that the intravenous administration of the decoction of G. pentaphyllum (2.5, 5 and 10mg/kg) produced a protective effect against pitressin-induced coronaryspasm, arrhythmias and pressor response. Extract also increased the dose of ouabain required to cause ventricular tachyarrhythmias and lethality. Further extract reversed ouabain-induced persistent ventricular tachycardia and restored sinus rhythm in a dose-dependent manner. The results obtained have also shown that gypenosides III and VIII caused similar protective effects in both experimental models used; however, the duration of the action is lower than that of the extract containing corresponding quantities of gypenosides III and VIII.

Ouabain increases myofibrillar Ca 2+ sensitivity but does not influence the Ca 2+ release in human skinned fibres

The present study investigated the impact of the Na(+) pump inhibitor ouabain (g-strophanthin) on Ca(2+) sensitivity and Ca(2+) release in human right auricular trabeculae (coronary bypass) and in skinned muscle fibres from left ventricular myocardium (cardiac transplantation, dilated cardiomyopathy). A time-dependent increase in force of contraction was observed in right auricular trabeculae in response to ouabain (100 nM) before the intracellular Ca(2+) transient (fura-2) increased (n=6). In triton X-skinned fibres (no sarcoplasmic reticulum) of human failing myocardium, ouabain (1-100 nM) concentration-dependently increased tension at a free extracellular Ca(2+) concentration of 1 microM and the Hill coefficient of the Ca(2+)-dependent tension development. Ouabain (1-100 nM) did not directly induce a Ca(2+) release out of the sarcoplasmic reticulum, nor did it alter the caffeine (10 mM) induced sarcoplasmic reticulum Ca(2+) release in saponin-skinned fibre preparations in which the sarcoplasmic reticulum had been Ca(2+)-loaded. In conclusion, ouabain increases myofibrillar Ca(2+) sensitivity possibly due to an increase in the cooperativity of the thick and thin myofilaments. This mechanism may additionally contribute to the positive inotropic effect of ouabain.

The role of sarcolemmal Ca2+-ATPase in the regulation of resting calcium concentration in rat ventricular myocytes

1. The aim of this work was to investigate the role of sarcolemmal Ca2+-ATPase in rat ventricular myocytes. We have measured intracellular Ca2+ concentration ([Ca2+]i) using indo-1. The actions of the ATPase inhibitor carboxyeosin were studied. 2. Carboxyeosin increased resting [Ca2+]i and the magnitude of the systolic Ca2+ transient and slowed the rate of its relaxation by 5 %. 3. Carboxyeosin increased the magnitude of the caffeine-evoked increase in [Ca2+]i and slowed its relaxation by 20 %. Removal of extracellular Na+ slowed the rate constant by 80 %. When Na+ was removed in a carboxyeosin-treated cell, the caffeine-evoked increase in [Ca2+]i did not decay. 4. Carboxyeosin increased the integral of the Na+-Ca2+ exchange current activated by caffeine. This is, in part, due to an increase in sarcoplasmic reticulum Ca2+ content. 5. When extracellular Na+ was removed, there was a transient increase in [Ca2+]i which then decayed. The rate of this decay was slowed by carboxyeosin by a factor of about four. The residual decay could be abolished with caffeine. 6. In the absence of extracellular Na+, increasing extracellular Ca2+ concentration ([Ca2+]o) elevated [Ca2+]i. In carboxyeosin-treated cells, [Ca2+]i was much more sensitive to [Ca2+]o. 7. These results demonstrate the role of a carboxyeosin-sensitive Ca2+-ATPase in the control of resting [Ca2+]i and the reduction in [Ca2+]i following an increase in [Ca2+]i.

Antiarrhythmic effects of magnesium on rat papillary muscle and guinea pig ventricular myocytes

1. Despite widespread use of magnesium ion (Mg2+) for antiarrhythmic purposes, little direct information is available regarding its antiarrhythmic mechanisms. To elucidate the possible cellular mechanism, the effects of Mg2+ on early afterdepolarization (EAD), delayed afterdepolarization (DAD), triggered activity (TA), transient inward current (TI) and aftercontraction (AC) were examined in various cardiac preparations. The effects of Mg2+ on myoplasmic Ca2+ concentration were also studied. 2. The effects of Mg2+ on AC, induced by overdrive stimulation, were studied in isolated rat ventricular papillary muscle superfused with low K+ solution. In enzymatically isolated guinea pig myocytes, EAD, DAD and/or TA were induced after overdrive stimulation under conditions of superfusion with low K+ solution, using the whole-cell current-clamp method, and TI was also induced by the whole-cell voltage-clamp method. 3. Immediately after changing the solutions, containing varying concentrations of Mg2+, the effects of Mg2+ were examined. In addition, effects of Mg2+ on Ca transient were studied, using fura-2. 4. We found that: (1) in the rat papillary muscle, 10 mM Mg2+ effectively inhibited AC, which was produced after stimulation at both 3.3 Hz and 5 Hz, although 5 mM Mg2+ was without effect in the case of AC induced after 5-Hz stimulation; (2) in the myocytes, 5 mM Mg2+ did not inhibit DADs, EADs and TA, but 10 mM Mg2+ inhibited them completely; (3) the amplitude and frequency of TI decreased significantly in the presence of 10 mM Mg2+; and finally (4) 10 mM Mg2+ inhibited the Ca transient underlying DAD and/or TA. 5. The findings suggest, but do not prove unequivocally, that Mg's actions are probably due to a combination of a shift of the threshold of various ion channels to less negative potentials, a decrease in Ca2+ influx via Ca channels, a block of several K channels, and/or a block of Na-Ca exchanger. In conclusion, the present study indicates that extracellular Mg2+, via whatever mechanism, exerts antiarrhythmic activities.

Effect of inotropic interventions on contraction and Ca2+ transients in the human heart

The present study investigated the influences of inotropic intervention on the intracellular Ca2+ transient (intracellular Ca2+ concentration ([Ca2+]i)) and contractile twitch. Isometric twitch and [Ca2+]i (fura 2 ratio method) were measured simultaneously (1 Hz, 37 degrees C) after stimulation with Ca2+ (0.9-3.2 mM), the cardiac glycoside ouabain (Oua; 0.1 microM), the beta1- and beta2-adrenoceptor-agonist isoprenaline (Iso; 1-10 nM), and the Ca2+ sensitizer EMD-57033 (30 microM) by using isolated human nonfailing right auricular trabeculae (n = 19). Inotropic interventions increased force of contraction and peak rate of tension rise (+T) significantly. Only Iso stimulated peak rate of tension decay (-T) higher than +T (P < 0.05), thereby reducing time of contraction (Ttwitch). EMD-57033 increased +T more effectively than -T and prolonged Ttwitch (P < 0.05). Ca2+, Oua, and Iso, but not EMD-57033, increased systolic Ca2+. Diastolic Ca2+ increased after stimulation with Oua or Ca2+, but not in the presence of EMD-57033. Iso shortened the Ca2+ transient and did not influence diastolic Ca2+. In conclusion, positive inotropic agents differently affect force and [Ca2+]i depending on their mode of action. Inotropic interventions influence diastolic Ca2+ and thus may be less advantageous in a situation with altered intracellular Ca2+ homeostasis (e.g., heart failure due to dilated cardiomyopathy).

Comparative inotropic effect of calcium, endothelin-1, and forskolin on ferret papillary muscles during acidosis

Acidosis affects multiple steps in the excitation-contraction coupling pathway of myocardium, producing decreased calcium sensitivity of myofibrils and modification of the function of the sarcoplasmic reticulum. Our aim was to evaluate the effectiveness of three different classes of inotropic agents under acidotic conditions: 1) forskolin, an adenylate cyclase activator that enhances cellular cyclic AMP concentrations, 2) elevated extracellular Ca2+ and 3) endothelin-1, an activator of the inositol triphosphate, diacylglycerol pathway. Ferret papillary muscles were mounted in organ baths containing normal physiological solution (pH = 7.4). After baseline tension was measured, the muscles were bathed in an acidotic solution (pH = 6.98) that decreased tension to 40% of the control; subsequently, the muscles were washed with normal physiological solution until they returned to baseline. Each inotropic agent was added to the bathing solution in a concentration sufficient to increase tension by 40% above the baseline. Then the solution was made acidotic (pH = 6.98) in the continuous presence of that concentration of inotropic agent and the resultant steady-state developed tension measured. The increases in tension induced by each inotropic agent at normal pH were adjusted to be similar: in contrast, the response to each drug in acidosis was significantly different. Under acidotic conditions, endothelin-1 was the most effective inotropic agent in restoring the depressed developed tension. This was possibly due to enhancement of the myofilament sensitivity to Ca2+, which was more effective than increasing [Ca2+]i through elevating extracellular Ca2+ or the addition of forskolin which increased [Ca2+]i but desensitized the myofilaments to Ca2+.

The sarcolemmal mechanisms involved in the control of diastolic intracellular calcium in isolated rat cardiac trabeculae

We performed experiments using the calcium indicator Indo-1 to determine the relative roles of the sarcolemmal mechanisms involved in the regulation of diastolic intracellular calcium concentration ([Ca2+]i) in trabeculae from the rat heart. Ryanodine was used to eliminate sarcoplasmic reticulum (SR) function. In the functional absence of the SR, 76.8 +/- 3.9% of the calcium was extruded by the Na-Ca exchange carrier in the [Ca2+]i range of diastolic concentration +/- 200-400 nM. This was assessed by measuring the recovery of [Ca2+]i from small perturbations in the presence and absence of extracellular sodium. The steady-state relationship between [Ca2+]o and [Ca2+]i was linear over the range of 1-40 mM, a 20-fold increase of [Ca2+]o produced a 1.97-fold +/- 0.13-fold increase in [Ca2+]i (n = 5). In the absence of extracellular sodium raising [Ca2+]o had a variable effect. In some preparations there was little change of [Ca2+]i while in others the response was almost as large as in control conditions. We conclude that the Na-Ca exchanger contributes approximately 77% of sarcolemmal calcium extrusion following small perturbations in [Ca2+]i and that this fraction does not diminish as the [Ca2+]i declines. In addition we have shown a sodium-independent entry of calcium into quiescent cardiac muscle under resting conditions.

Propagating calcium waves initiated by local caffeine application in rat ventricular myocytes

1. Caffeine was applied locally to one region of a resting cell via an extracellular pipette while simultaneously imaging the concentrations of intracellular calcium ([Ca2+]i) and intracellular caffeine ([caffeine]i). 2. Local application of caffeine produced a rise of [caffeine]i which was confined to the region of the cell near the pipette. There was also a local increase of [Ca2+]i which then, in most resting cells, propagated along the cell as a linear Ca2+ wave. The initial magnitude of the rise of [Ca2+]i was greater than that of the electrically stimulated Ca2+ transient. 3. As the wave of increase of [Ca2+]i propagated along the cell it decreased in both amplitude and velocity in cells that had not been treated to elevate the cellular Ca2+ load. 4. In some cells the caffeine response did not propagate significantly. In these cases an increase of the cellular Ca2+ load enabled caffeine-induced Ca2+ wave propagation along the entire cell length without significant decay in amplitude and velocity. 5. Previous work has shown that an electrically evoked local systolic Ca2+ transient does not propagate. The fact that the caffeine-evoked response does propagate and the correlation between decay of amplitude and velocity suggest that the transient has to be a certain size before it can propagate. It is suggested that one of the factors which favour propagation of waves under conditions of elevated sarcoplasmic reticulum Ca2+ content is the increased release of Ca2+.

Spatial non-uniformities in [Ca2+]i during excitation-contraction coupling in cardiac myocytes

The intracellular calcium ([Ca2+]i) transient in adult rat heart cells was examined using the fluorescent calcium indicator fluo-3 and a laser scanning confocal microscope. We find that the electrically evoked [Ca2+]i transient does not rise at a uniform rate at all points within the cell during the [Ca2+]i transient. These spatial non-uniformities in [Ca2+]i are observed immediately upon depolarization and largely disappear by the time the peak of the [Ca2+]i transient occurs. Importantly, some of the spatial non-uniformity in [Ca2+]i varies randomly in location from beat to beat. Analysis of the spatial character of the non-uniformities suggests that they arise from the stochastic nature of the activation of SR calcium-release channels. The non-uniformities in [Ca2+]i are markedly enhanced by low concentrations of Cd2+, suggesting that activation of L-type calcium channels is the primary source of activator calcium for the calcium transient. In addition, the pattern of calcium release in these conditions was very similar to the spontaneous calcium sparks that are observed under resting conditions and which are due to spontaneous calcium release from the SR. The spatial non-uniformity in the evoked [Ca2+]i transient under normal conditions can be explained by the temporal and spatial summation of a large number of calcium sparks whose activation is a stochastic process. The results are discussed with respect to a stochastic local control model for excitation-contraction (E-C) coupling, and it is proposed that the fundamental unit of E-C coupling consists of one dihydropyridine receptor activating a small group of ryanodine receptors (possibly four) in a square packing model.

Anti-arrhythmic profile of a garlic dialysate assayed in dogs and isolated atrial preparations

The effects of garlic (Allium sativum L., Liliaceae) dialysate were studied on arrhythmias induced in anaesthetized dogs and on isolated left rat atria. Garlic dialysate suppressed premature ventricular contractions (PVC) and ventricular tachycardia (VT) in ouabain-intoxicated dogs as well as the ectopic rhythms induced by isoprenaline (10(-6) M) and aconitine (10(-8) M) on electrically driven left rat atria. The effective refractory period (ERP) and the sinus node recovery time (SNRT) of isolated rat atria were prolonged in a dose-dependent manner by the administration of this extract. Garlic dialysate decreased the positive inotropic and chronotropic effects of isoprenaline in a concentration-dependent manner. These last effects were increased by propranolol. The results suggest that garlic dialysate has a significant antiarrhythmic effect in both ventricular and supraventricular arrhythmias.

Digoxin activates sarcoplasmic reticulum Ca(2+)-release channels: a possible role in cardiac inotropy

1. The effect of digoxin on rapid 45Ca2+ efflux from cardiac and skeletal sarcoplasmic reticulum (SR) vesicles was investigated. Additionally the interaction of digoxin with single cardiac and skeletal muscle SR Ca(2+)-release channels incorporated into planar phospholipid bilayers and held under voltage clamp was determined. 2. Digoxin (1 nM) increased the initial rate and amount of Ca(2+)-induced release of 45Ca2+ from cardiac SR vesicles, passively loaded with 45CaCl2, at an extravesicular [Ca2+] of 0.1 microM. The efflux in the presence and absence of digoxin was inhibited at pM extravesicular Ca2+ and blocked by 5 mM Mg2+. 3. To elucidate the mechanism of action of digoxin, single-channel recording was used. Digoxin (1-20 nM) increased single-channel open probability (Po) when added to the cytosolic but not the luminal face of the cardiac channel in the presence of sub-maximally activating Ca2+ (0.1 microM-10 microM) with an EC50 of 0.91 nM at 10 microM Ca2+. The mechanisms underlying the action of digoxin appear to be concentration-dependent. The activation observed at 1 nM digoxin appears to be consistent with the sensitization of the channel to the effects of Ca2+. At higher concentrations the drug appears to interact synergistically with Ca2+ to produce values of Po considerably greater than those seen with Ca2+ as the sole activating ligand. 4. Digoxin had no effect on single-channel conductance or the Ca2+/Tris permeability ratio. In channels activated by digoxin the Po was decreased by Mg2+. Single-channels were characteristically modified to along lasting open, but reduced, conductance state when 100 nM ryanodine was added to the cytosolic side of the channel.5. Activation of the cardiac SR Ca2+-release channel was observed with similar concentrations of digitoxin, however, higher concentrations of ouabain were required to increase PO. In contrast, a steroid which is not positively inotropic, chlormadinone acetate, had no effect on either cardiac or skeletal SR Ca2+-release channel activity.6. At concentrations up to 1 microM, digoxin had no effect on Ca2+-induced 45Ca2+ efflux from skeletal muscle SR vesicles nor did it affect skeletal SR Ca2+-release channel Po, reflecting a difference between the cardiac and skeletal isoforms of the Ca2+-release channel.7. Since activation of the cardiac SR Ca2+-release channel occurs within the range of concentrations of digoxin encountered therapeutically, it is possible that activation of this channel contributes to the positive inotropic effect observed with this drug. Further, activation of the channel by higher concentrations of digoxin may contribute to the toxic effects seen clinically.

Intracellular calcium and myocardial function during ischemia

Cardiac ischemia causes a rapid decline in mechanical performance and, if prolonged, myocardial cell death occurs on reperfusion. The early decline in mechanical performance could, in principle, be caused either by reduced intracellular calcium release or by reduced responsiveness of the myofibrillar proteins to calcium. It is now known that intracellular calcium rises during ischemia and that the early decline in mechanical performance is caused largely by the inhibitory effects of phosphate and protons on the myofibrillar proteins. The rise of intracellular calcium during ischemia is related to the acidosis and is probably caused by calcium influx on the Na/Ca exchanger. This is triggered by a rise in intracellular sodium which enter the cell in exchange for protons on the Na/H exchanger. Intracellular calcium rises still further on reperfusion the elevation of calcium and the degree of muscle damage are closely correlated.

Intracellular calcium transients and arrhythmia in isolated heart cells

Intracellular calcium ([Ca2+]i) elevation may mediate cardiac arrhythmias. However, direct measurement of the rapid alterations of [Ca2+]i on a beat-to-beat basis using fast temporal resolution and without signal averaging in the spontaneously beating in vivo heart is lacking. Furthermore, data from an isolated spontaneously beating myocyte preparation that develops arrhythmia similar to that in the in vivo heart are unavailable. We measured rapid changes of [Ca2+]i with fast temporal resolution in isolated spontaneously beating neonatal rat ventricular myocytes with cell-to-cell communication and characterized the interrelation between [Ca2+]i and arrhythmia. An elevated extracellular calcium ([Ca2+]o) concentration of 10.8 mM induced premature beats, a rapid beating rate (tachyarrhythmia), and chaotic or fibrillatory beating activity in a small group of myocytes. [Ca2+]i levels during systole increased from the nanomolar to micromolar concentration range before arrhythmia development. Spontaneous oscillations of [Ca2+]i during diastole could evoke a spontaneous tachyarrhythmia. In the presence of [Ca2+]i elevation, a spontaneous tachyarrhythmia could induce severe [Ca2+]i overload. Reduction of [Ca2+]i with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid AM (5 microM) in the presence of 10.8 mM [Ca2+]o reversed the arrhythmia. In single ventricular myocytes superfused with 10.8 mM [Ca2+]o, oscillations of membrane potential characteristic of transient inward current occurred that were prevented by ryanodine (0.1 microM), an inhibitor of Ca2+ flux across the sarcoplasmic reticulum. This study characterizes 1) an isolated multicellular myocyte model of arrhythmia similar to that evident in in vivo hearts, 2) elevation of [Ca2+]i with systolic [Ca2+]i levels of 1-3 microM and diastolic [Ca2+]i oscillations before the initiation of arrhythmia, 3) tachyarrhythmia as a cause of severe [Ca2+]i overload, which may be important in the perpetuation and degeneration of arrhythmias, and 4) reversal of arrhythmia with reduction of [Ca2+]i. The results in the isolated myocyte model may have relevance to the generation and perpetuation of certain cardiac arrhythmias associated with calcium overload.

Diastolic, systolic and sarcoplasmic reticulum [Ca2+] during inotropic interventions in isolated rat myocytes

1. The fluorescent indicator Fura-2 has been used to monitor intracellular [Ca2+] (Ca2+i) in myocytes isolated from the ventricles of rat hearts. 2. The relationships between diastolic Ca2+i, systolic Ca2+i and the Ca2+ content of the sarcoplasmic reticulum (SR; assayed using caffeine) have been studied during changes of stimulation rate and bathing [Ca2+] (Ca2+o). 3. When stimulation rate was increased, there were increases in diastolic Ca2+i, systolic Ca2+i and the Ca2+ content of the SR. 4. The SR inhibitor ryanodine (1 mumol l-1) decreased the size of the Ca2+i transient, and abolished the increase of Ca2+i produced by caffeine (10 mmol l-1). In the presence of ryanodine, increasing stimulation rate increased diastolic Ca2+i but not systolic Ca2+i. 5. Increasing Ca2+o led to increases of diastolic Ca2+i, systolic Ca2+i and SR Ca2+ content similar to those observed during changes in stimulation rate. 6. Ryanodine altered the relationship between systolic and diastolic Ca2+i during changes of Ca2+o. 7. These results are consistent with a change of diastolic Ca2+i leading to an increase in the Ca2+ content of the SR, and hence an increase in the size of the Ca2+i transient during changes in stimulation rate and Ca2+o.

Flux of Ca2+ across the sarcoplasmic reticulum of guinea-pig cardiac cells during excitation-contraction coupling

1. A method has been developed for calculating the flux of Ca2+ across the sarcoplasmic reticulum (SR) during excitation-contraction coupling in mammalian heart cells. FSR will symbolize the net rate of movement of Ca2+, per litre of accessible cytoplasm, into or out of the sarcoplasmic reticulum. FSR has the units MS-1. 2. A theory of the cytoplasmic [Ca2+]i transient in mammalian heart cells is presented in which the [Ca2+]i transient results from the various cellular processes that tend to increase or decrease cytoplasmic [Ca2+]i. According to the theory, FSR can be calculated if all cellular processes that contribute to the [Ca2+]i transient (other than Ca2+ fluxes across the SR) are either eliminated or are known quantitatively. 3. To obtain the measurements required to apply this theory, [Ca2+]i transients and membrane currents were recorded in guinea-pig single ventricular myocytes subjected to whole-cell voltage clamp and internal perfusion. [Ca2+]i transients were recorded through the use of the Ca2+ indicator, Fura-2 (pentapotassium salt). 4. Ca2+ fluxes through the sodium-calcium exchanger were eliminated in all experiments, by perfusing the cells, internally and externally with Na(+)-free solutions. Ca2+ flux through the sarcolemmal L-type Ca2+ channel was measured as the verapamil-sensitive current. Influx of Ca2+ through all other voltage-dependent pathways was found to be negligible for the calculation of FSR over the time course of a single [Ca2+]i transient. 5. In the combined absence of Ca2+ current, Na(+)-Ca2+ exchange and fluxes across the SR (10 mM-caffeine), the net rate of removal of Ca2+ from the cytoplasm, which includes presumed contributions from sarcolemmal Ca(2+)-ATPase and mitochondrial Ca2+ transport, was found to be a negligible quantity in the calculation of FSR, over the time course of a single [Ca2+]i transient. 6. Calculation of FSR requires that the Ca(2+)-binding capacity of cytoplasm be known. [Ca2+]i transients recorded during measurable total Ca2+ influx into the cytoplasm (verapamil-sensitive current in the absence of fluxes across the SR) were compared with theoretical Ca2+ transients computed on the assumption that the entering Ca2+ could bind only to intracellular ligands (values for ligands taken from literature) and to Fura-2 (30 microM). The slope of the regression line relating calculated total change in [Ca2+]i to the measured total Ca2+ influx was 0.99, not different from the perfect theoretical slope of 1.0 (correlation coefficient, 0.81; standard deviation of slope, 0.14; n = 7).4+ the SR and FSR had a similar time course to that on depolarization. 10. The unidirectional efflux of Ca2+ from the SR, symbolized FSR, rel was calculated utilizing assumed characteristics of the Ca2+ pump of the SR. The value of FSR, rel was not affected by repolarization from voltage-clamp pulses greater than 150 ms in duration.(ABSTRACT TRUNCATED AT 400 WORDS)

Dependence of intracellular free calcium and tension on membrane potential and intracellular pH in single crayfish muscle fibres

The dependence of intracellular free calcium ([Ca2+]i) and tension on membrane potential and intracellular pH (pHi) was studied in single isolated fibres of the crayfish claw-opener muscle using ion-selective microelectrodes. Tension (T) was quantified as a percentage of the maximum force, or as force per cross-sectional area (N/cm2). In resting fibres, pHi had a mean value of 7.06. Contractions evoked by an increase extracellular potassium [( K+]0) produced a fall in pHi of 0.01-0.05 units. The lowest measured levels of resting [Ca2+]i corresponded to a pCai (= -log [Ca2+]i) of 6.8. Intracellular Ca2+ transients recorded during K(+)-induced contractions did not reveal any distinct threshold for force development. Both the resting [Ca2+]i and resting tension were decreased by an intracellular alkalosis and increased by an acidosis. The sensitivity of resting tension to a change in pHi (quantified as -dT/dpHi) showed a progressive increase during a fall in pHi within the range examined (pHi 6.2-7.5). The pHi/[Ca2+]i and pHi/tension relationships were monotonic throughout the multiphasic pHi change caused by NH4Cl. A fall of 0.5-0.6 units in pHi did not produce a detectable shift in the pCai/tension relationship at low levels of force development. The results indicate that resting [Ca2+]i is slightly higher than the level required for contractile activation. They also show that the dependence of tension on pHi in crayfish muscle fibres is attributable to a direct H+ and Ca2+ interaction at the level of Ca2+ sequestration and/or transport. Finally, the results suggest that in situ, the effect of pH on the Ca2+ sensitivity of the myofibrillar system is not as large as could be expected on the basis of previous work on skinned crustacean muscle fibres.

Effects of changes of pH on the contractile function of cardiac muscle

It has been known for over 100 years that acidosis decreases the contractility of cardiac muscle. However, the mechanisms underlying this decrease are complicated because acidosis affects every step in the excitation-contraction coupling pathway, including both the delivery of Ca2+ to the myofilaments and the response of the myofilaments to Ca2+. Acidosis has diverse effects on Ca2+ delivery. Actions that may diminish Ca2+ delivery include 1) inhibition of the Ca2+ current, 2) reduction of Ca2+ release from the sarcoplasmic reticulum, and 3) shortening of the action potential, when such shortening occurs. Conversely, Ca2+ delivery may be increased by the prolongation of the action potential that is sometimes observed and by the rise of diastolic Ca2+ that occurs during acidosis. This rise, which will increase the uptake and subsequent release of Ca2+ by the sarcoplasmic reticulum, may be due to 1) stimulation of Na+ entry via Na(+)-Ca2+ exchange; 2) direct inhibition of Na(+)-Ca2+ exchange; 3) mitochondrial release of Ca2+; and 4) displacement of Ca2+ from cytoplasmic buffer sites by H+. Acidosis inhibits myofibrillar responsiveness to Ca2+ by decreasing the sensitivity of the contractile proteins to Ca2+, probably by decreasing the binding of Ca2+ to troponin C, and by decreasing maximum force, possibly by a direct action on the cross bridges. Thus the final amount of force developed by heart muscle during acidosis is the complex sum of these changes.

Effects of intracellular acidosis on [Ca2+]i transients, transsarcolemmal Ca2+ fluxes, and contraction in ventricular myocytes

We examined the effects of intracellular acidosis produced by washout of NH4Cl on [Ca2+]i transients (indo-1 fluorescence), cell contraction (video motion detector), and 45Ca and 24Na fluxes in cultured chick embryo ventricular myocytes. Exposure of cells to 10 mM NH4Cl produced intracellular alkalosis (pH 7.6), and subsequent washout resulted in a transient acidosis (pH 6.5). Exposure to 10 mM NH4Cl slightly decreased [Ca2+]i transients but increased the amplitude of cell contraction. Subsequent washout of NH4Cl initially increased diastolic [Ca2+]i and decreased the peak positive and negative d[Ca2+]i/dt, while the amplitude of cell contraction was markedly decreased. Subsequently, peak systolic [Ca2+]i increased with partial recovery of contraction. A similar increase in [Ca2+]i and decrease in contraction after washout of NH4Cl was observed in single paced adult guinea pig ventricular cells. Acidosis decreased 45Ca uptake by sarcoplasmic reticulum vesicles isolated from chick embryo ventricle. However, the [Ca2+]i increase caused by intracellular acidosis was also observed in the presence of 10 mM caffeine, suggesting that altered sarcoplasmic reticulum handling of calcium is not the only mechanism involved. Intracellular acidosis only slightly increased total 24Na uptake under these conditions, an effect resulting from the combination of a stimulation of amiloride-sensitive sodium influx (Na(+)-H+ exchange) and inhibition of sodium influx via Na(+)-Ca2+ exchange, manifested by a significant decrease in 45Ca efflux. Further support for a lack of involvement of an increased [Na+]i in the observed increase in [Ca2+]i during acidosis was low-sodium, nominal 0-calcium extracellular solution, an experimental condition that minimizes the possible effects of Na(+)-H+ exchange and Na(+)-Ca2+ exchange. We conclude that the [Ca2+]i increase caused by intracellular acidosis in cultured ventricular cells is primarily due to changes in [Ca2+]i buffering and [Ca2+]i extrusion, rather than to an increase in transsarcolemmal calcium influx. Intracellular acidosis also markedly decreases the sensitivity of the contractile elements to [Ca2+]i in cultured chick embryonic and adult guinea pig ventricular myocytes.

Effect of intracellular and extracellular pH on contraction in isolated, mammalian cardiac muscle

1. Intracellular pH (pHi) and Na+ activity were recorded (ion-selective microelectrodes) in guinea-pig papillary muscle and the sheep cardiac Purkinje fibre while simultaneously recording twitch tension. The effects of intracellular acidosis and alkalosis upon contraction were investigated. 2. A fall of pHi produced by reducing pHo was associated with a fall of twitch tension. Similarly, a rise of pHi produced by raising pHo produced a rise of twitch tension. The time course of the changes in tension correlated with the time course of changes of pHi rather than pHo. These results are consistent with previous work showing that acidosis inhibits contraction and that the inhibition depends upon a fall of pHi. 3. Changes of pHi were produced while maintaining pHo constant at 7.4. Removal of NH4Cl or addition of sodium acetate (pHo 7.4) reduced pHi but this gave either an increase of tension (papillary muscle) or an initial fall followed by a subsequent recovery of tension (Purkinje fibre). The increase or recovery of tension occurred despite the fact that there was an intracellular acid load. Thus, reducing pHi at constant pHo can increase tension whereas reducing pHi at low pHo (6.4, see paragraph 2) inhibits tension. 4. The increase of recovery of tension during intracellular acidosis produced at a constant pHo (7.4) was associated with a rise of intracellular sodium activity (aiNa). Amiloride (1.5 mmol/l), an inhibitor of Na(+)-H+ exchange, prevented the rise of aiNa during intracellular acidosis and also prevented the recovery of tension. It is concluded that the increase or recovery of tension at low pHi is secondary to a rise of aiNa caused by stimulation of Na(+)-H+ exchange. A rise of aiNa will elevate Ca2+ via sarcolemmal Na(+)-Ca2+ exchange and thus will elevate tension. 5. An intracellular acidosis produced by reducing pHo (6.4) does not elevate aiNa in the Purkinje fibre. In papillary muscle, aiNa rises but this occurs slowly and the rise is 50% smaller than that seen when the same intracellular acidosis is induced at normal pHo (7.4). The net depression of tension under these conditions thus correlates with the lack of a large rise of aiNa. 6. Knowing the quantitative dependence of tension upon both aiNa and pHi in the two tissues it is possible to predict the recovery of twitch tension during intracellular acidosis at constant pHo (7.4), using the changes of pHi and aiNa measured under these conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological abnormalities and enhanced reperfusion arrhythmias in the isolated hearts of hyperthyroid rats

1. The influence of hyperthyroidism on electrophysiological characteristics and on reperfusion arrhythmias was examined in rat hearts. 2. Electrophysiological studies were performed with glass microelectrodes, and the experiments on reperfusion arrhythmias were done in isolated perfused hearts. 3. Ventricular muscle from hyperthyroid rats was more prone than that from euthyroid rats to develop triggered activity under conditions believed to cause myoplasmic Ca2+ overload. 4. The severity of reperfusion arrhythmias was significantly enhanced in hyperthyroid preparations as compared with euthyroid ones. 5. The enhanced reperfusion arrhythmias in hyperthyroid rats were significantly reduced by propranolol (3 x 10(-7) M), lignocaine (1 x 10(-5) M) and verapamil (3 x 10(-8) M), but not by nadolol (3 x 10(-7) M) or prazosin (3 x 10(-7) M). 6. These results suggest that increased heart rate due to hyperthyroidism and responses mediated via either alpha- or beta-adrenoceptors were not dominant causes of enhanced reperfusion arrhythmias in hyperthyroid hearts. 7. The increased tendency to develop triggered activity which was observed in the electrophysiological study, may be one possible explanation of enhanced reperfusion arrhythmias in hyperthyroid hearts.

Cellular origins of the transient inward current in cardiac myocytes. Role of fluctuations and waves of elevated intracellular calcium

Activation of the transient inward current (ITI) by a rise in intracellular calcium concentration ([Ca2+]i) is believed to be responsible for generating triggered cardiac arrhythmias. In this study, the cellular basis of the rise in [Ca2+]i that activates ITI and aftercontractions in single rat ventricular myocytes was examined. [Ca2+]i was measured both indirectly by cell contraction and directly with fura-2. Under conditions that caused steady-state [Ca2+]i to increase (i.e., calcium overload) membrane repolarization after a voltage-clamp depolarization resulted in the appearance of ITI that was similar in many respects to that observed in multicellular preparations. This ITI occurred at the same time that [Ca2+]i spontaneously increased and preceded the aftercontraction by 60-90 msec. However, ITI recorded from a single cell was variable in time course and amplitude (unlike that observed in multicellular preparations). Examination of cell contraction and digital imaging of fura-2 fluorescence showed that ITI was often associated with propagating regions of increased [Ca2+]i, which arose from discrete sites of origin within the cell. Apparently synchronous aftercontractions could also be associated with multiple propagating waves of [Ca2+]i. The variation in the time course and amplitude of ITI in single cells appeared to be due to changes in the location and number of sites of origin for the waves of [Ca2+]i. After the first aftercontraction and ITI, desynchronization of the sites of origin of increased [Ca2+]i occurred, and this resulted in a decrease in the amplitude of ITI and an increase in its duration. We conclude that the variability seen in single cells arises from changes in the pattern of spontaneous Ca2+ release. Such phenomena will seriously complicate interpretation of multicellular data, even when [Ca2+]i is measured directly.

The effects of metabolic inhibition on intracellular calcium and pH in isolated rat ventricular cells

1. Intracellular calcium concentration [( Ca2+]i) and pH (pHi) were measured in single, isolated rat ventricular myocytes using, respectively, the fluorescent indicators Fura-2 and BCECF (2',7'-bis(carboxyethyl-5(6)-carboxyfluorescein). Contraction was measured simultaneously. The intracellular calibration of BCECF is demonstrated. In a HEPES-buffered bathing solution of pH 7.4, pHi had a mean value of 7.16 +/- 0.05 (mean +/- S.E.M.). 2. Addition of NH4Cl (5-20 mM) produced an intracellular alkalosis that was associated with an increase of contraction amplitude. Removal of NH4Cl produced an acidosis and decrease of contraction. 3. The addition of 2 mM-cyanide (CN-) to inhibit oxidative phosphorylation had variable effects on contraction amplitude. Changes of contraction amplitude could largely be accounted for by changes in the systolic Ca2+ transient. 4. CN- addition increased lactic acid production. However, in the majority of experiments, this was not accompanied by an intracellular acidosis. 5. Anaerobic glycolysis was inhibited by either removal of glucose, addition of deoxyglucose, or addition of iodoacetate. Under these conditions the application of CN- decreased systolic [Ca2+]i and contraction amplitude. This was sometimes preceded by a transient increase of systolic [Ca2+]i and contraction amplitude. 6. When glycolysis was inhibited, the subsequent addition of CN- always increased diastolic [Ca2+]i and produced a contracture. The increase of [Ca2+]i occurred before the contracture. However, once the contracture had developed, decreasing [Ca2+]i (by removal of external Ca2+) did not cause relaxation. 7. With glycolysis inhibited, addition of CN- resulted in a large (0.51 +/- 0.05 pH unit) acidosis that was sometimes preceded by an alkalosis. This acidosis was unaffected by removal of external Ca2+ or external alkalinization. Calculations show that some of this acidosis may result from protons released by ATP hydrolysis. 8. If the acidosis produced by metabolic blockade was partly reversed by adding NH4Cl then a contracture immediately developed. This suggests that the acidosis delays the onset of the contracture. 9. We conclude that metabolic inhibition increases diastolic [Ca2+]i. The accompanying acidosis prevents contraction. Once the contracture has developed it is maintained by factors other than increased [Ca2+]i, possibly by a fall of [ATP].

SR Ca loading in cardiac muscle preparations based on rapid-cooling contractures

The influence of rest periods on twitches and rapid-cooling contractures (RCCs) was examined in trabeculae from rabbit, rat, guinea pig, and frog ventricle and rabbit atrium. RCCs were used as a relative index of sarcoplasmic reticulum (SR) Ca content. After increasing rest duration, rabbit and guinea pig ventricles exhibit a decline of both twitch force and RCC force (rest decay). When stimulation is resumed, both twitches and RCCs recover to steady-state levels. The SR (and cells) in these tissues may lose Ca during quiescence and become reloaded with progressive stimulation. Rat ventricle and rabbit atrium exhibited an increase in both twitch and RCC tension as a function of rest duration (rest potentiation). Resumption of stimulation resulted in parallel declines of both twitch and RCC tension approaching steady state. Thus stimulation in rat ventricle and rabbit atrium may lead to a net Ca loss from the SR (and the cell) and quiescence may lead to replenishment of cellular Ca. This major difference in Ca metabolism in mammalian cardiac muscles might be due to a fundamental difference in SR properties or, alternatively, different sarcolemmal transport properties (e.g., action potential configuration, Na-pump). After long rest intervals in rabbit and guinea pig ventricle, RCCs return toward their steady-state value in considerably fewer beats than does twitch tension. This implies that something other than SR refilling is responsible for the slow phase of twitch recovery after rest. In rabbit ventricle increasing frequency or extracellular Ca concentration ([Ca]o) generally increases both twitch and RCC tension. However, decreasing [Ca]o (to 0.2 mM) does not decrease RCCs much despite a dramatic decline in twitch tension (suggesting low twitch tension despite a loaded SR). Rapid rewarming during an RCC usually results in a transient rise in tension (or rewarming "spike"), which is due to a warming-induced increase in myofilament Ca sensitivity. Differences in rewarming spikes among the tissues studied suggest differences in temperature effects on myofilament Ca sensitivity.

Fall in intracellular pH and increase in resting tension induced by a mitochondrial uncoupling agent in crayfish muscle

1. The influence of the mitochondrial uncoupling agent carbonylcyanide-m-chlorophenylhydrazone (CCCP) upon resting tension and intracellular pH (pHi) was studied in the dactyl opener muscle of the crayfish. pHi was measured with liquid sensor H+-selective microelectrodes. 2. CCCP (10(-6)-10(-5) mol l-1) induced a reversible, tonic contracture which was associated with a depolarization of the membrane potential. Both effects were augmented by a fall and inhibited by a rise in extracellular pH. The action of CCCP on tension was not mimicked by cyanide + oligomycin or by cyanide + dicyclohexylcarbodiimide nor was it inhibited by pre-exposure to these agents. 3. CCCP produced an initial alkalosis of less than 0.1 units and thereafter a fall in pHi of 0.4-0.6 units during which the sarcolemmal H+ driving force decreased from 61 to 15 mV. The apparent influx of H+ due to CCCP had a maximum of 2.7 mequiv l-1 min-1. The CCCP-induced acidosis was unaffected by iodacetate (0.5 mmol l-1) but it was inhibited by a depolarization of the membrane potential. 4. The contraction caused by CCCP was not due to the simultaneous fall in pHi since an intracellular acidosis of equal magnitude, produced by propionate (50 mmol l-1), did not lead to force generation. In addition, propionate had an inhibitory effect on the depolarization and contracture caused by CCCP. 5. Both the depolarization and the contracture caused by CCCP were inhibited by gamma-aminobutyric acid (GABA). The contracture was blocked by Cd2+, Mn2+ and by a nominally Ca2+ -free medium but not by a pre-exposure to caffeine (20 mmol l-1). Cd2+ and Mn2+ had no influence on the fall of pHi caused by CCCP. 6. It is concluded that CCCP induces a sarcolemmal H+ conductance which leads to a fall in pHi and to a depolarization of the membrane potential. This depolarization activates sarcolemmal, voltage-dependent calcium channels and thereby induces an increase in tension. The initial alkalosis produced by CCCP may be due to a transient uptake of H+ by mitochondria.

Rapid regulation of the 'second inward current' by intracellular calcium in isolated rat and ferret ventricular myocytes

1. Single cells were isolated from the ventricles of ferret and rat hearts. Cells were voltage clamped using a single conventional microelectrode. Membrane voltage, membrane currents and cell length were monitored. 2. The current elicited by decreasing the membrane potential from a holding potential of -40 or -45 mV to potentials more positive than -20 mV was abolished by D600, by Cd2+ and by removal of Ca2+ from the cell superfusate. This current activated within 20 ms and inactivated over several hundred milliseconds; it had a bell-shaped current-voltage relation, and was maximal at about +10 mV. It is concluded that this is the fast Ca2+ current ICa. 3. Increasing bathing [Ca2+] (Ca2+o) led to the appearance of transient inward currents (Iti). If ICa was triggered during Iti, it was reduced in magnitude, and inactivated more slowly. 4. The sarcoplasmic reticulum inhibitor ryanodine (1 mumol/l) abolished Iti, and reduced twitch contraction, but had no direct effect on the magnitude of ICa, although its rate of inactivation was slowed. 5. Iti produced by depolarization of the holding potential, or by lowering bathing [K+] or [Na+], led to similar changes to those described in paragraph 3. 6. Gradually increasing diastolic cytoplasmic [Ca2+] (Ca2+i) by rapid stimulation in the presence of ryanodine, by lowering bathing [K+], or lowering bathing [Na+], led to a parallel decrease of ICa. 7. The effects of lowering bathing [Na+] could be abolished by using an electrode-filling solution containing EGTA. 8. In some ferret cells a slow component of the second inward current was observed. The size of this current was directly related to the size of the twitch: changes in the size of the twitch produced by changing the pattern of stimulation or application of ryanodine were paralleled by changes in the size of this current, but had no effect on the size of ICa. 9. It is concluded that the magnitude of ICa can be decreased by an increase of either resting Ca2+i, or the spontaneous increase of Ca2+ which underlies Iti, but it is not affected by the size of the stimulated calcium transient (although the time course of inactivation is dependent on the calcium transient). The size of the slow component of the second inward current, however, is directly related to the size of the twitch and may, therefore, be activated by Ca2+.

The effects of changes in muscle length during diastole on the calcium transient in ferret ventricular muscle

1. Ferret papillary muscles were isolated and injected with aequorin to measure intracellular Ca2+ concentration [( Ca2+]i). Developed tension and [Ca2+]i were measured in response to length changes. 2. A maintained reduction in muscle length produced an immediate decrease in developed tension followed by slow decline over 10-20 min. This slow decline in tension was accompanied by a slow decline in the amplitude of the systolic [Ca2+]i rise (the Ca2+ transient). The immediate decrease in tension was accompanied by a prolongation of the Ca2+ transient and an abbreviation of the twitch. 3. Repeated reductions in muscle length timed to occur only during the period of contraction (systolic shortening) produced an immediate decrease of developed tension but the subsequent slow decline was substantially smaller. The slow decline in the amplitude of the Ca2+ transients was also smaller. The prolongation of the Ca2+ transient and abbreviation of the twitch were similar to those observed with a maintained reduction of length. 4. Repeated reductions in muscle length during the period between contractions (diastolic shortening) did not produce the immediate decrease of tension but the slow decline of tension was present. The slow decline in the amplitude of the Ca2+ transients was also present. However no change in the duration of the Ca2+ transient or the twitch was present under these conditions. 5. These results suggest that diastolic muscle length can influence the amplitude of the Ca2+ transients achieved during systole. This conclusion was confirmed by experiments in which the recovery of tension and Ca2+ transients was observed after periods of rest. Both developed tension and Ca2+ transients on recovery from a rest were reduced when the rest occurred at a short length in comparison with a long length. 6. We suggest that muscle length influences resting [Ca2+]i and this in turn affects the Ca2+ transients and developed tension.

The physiological basis of left ventricular diastolic dysfunction

Overall cardiac pump function requires adequate ventricular diastolic filling as well as normal systolic ejection. Abnormalities of the rate or extent of myocardial relaxation (diastolic dysfunction) have been described in a large variety of clinical conditions, including hypertrophy, ischemia, and after cardiac surgery. Diastolic and systolic dysfunction can be readily distinguished by analysis of pressure volume loops and utilization of echocardiography or nuclear cardiology gated blood pool scans. The mechanisms by which diastolic dysfunction can occur may be structural (hypertrophy, fibrosis) or dynamic (hypoxia, ischemia, alteration of diastolic cytosolic calcium levels). Hypertrophied myocardium is particularly susceptible to diastolic dysfunction by virtue of both structural changes (increased LV mass and interstitial fibrosis) and greater susceptibility to develop impaired myocardial relaxation during hypoxia or ischemia than nonhypertrophied myocardium.

Effects of acidosis on ventricular muscle from adult and neonatal rats

We compared the response of ventricular muscle from adult and neonatal rats to hypercapnic acidosis. In adult muscle, acidosis caused an initial rapid fall of developed tension to 30 +/- 5% of control (mean +/- SEM, n = 6). However, tension recovered slowly to a steady state that was 56 +/- 6% of control. In neonatal muscle, acidosis caused a significantly smaller initial fall in tension to 43 +/- 3% (n = 8, p less than 0.05), but the tension then showed a subsequent slower fall to a steady state that was 29 +/- 4% of control, significantly less than in the adult (p less than 0.01). We have attempted to identify the mechanisms underlying these differences in response. In detergent-skinned myofibrils, reducing the pH from 7.0 to 6.5 caused a reduction in the pCa50 of 0.61 units in the adult muscle, but only 0.27 units in the neonatal ventricular muscle. Myofibrillar Ca2+ sensitivity in neonatal ventricular muscle is thus less susceptible to the effects of acidic pH than that of adult muscle. Since intracellular pH decreases rapidly on application of increased external CO2, these results are consistent with the finding that, initially, developed tension in neonatal muscles is less sensitive to the effects of acidosis. Sodium dodecylsulfate gel electrophoresis of myofibrillar preparations from adult and neonatal rats demonstrated differences in thin filament proteins, including troponin I, which may underlie the observed differences in Ca2+ sensitivity. In adult rat ventricular muscles, the slow recovery of tension during acidosis is associated with an increase in the amplitude of the Ca2+ transients to 263 +/- 34% of control (n = 4).(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced sensitivity to hypoxia-induced diastolic dysfunction in pressure-overload left ventricular hypertrophy in the rat: role of high-energy phosphate depletion

Isolated buffer-perfused rat hearts with pressure-overload hypertrophy develop a greater decrease in left ventricular (LV) diastolic distensibility and a greater impairment in extent of LV relaxation in response to hypoxia than do normal hearts. Using 31P-NMR spectroscopy, we tested the hypothesis that the enhanced susceptibility of hypertrophied hearts to develop hypoxia-induced diastolic dysfunction is due to an accelerated rate of ATP and/or creatine phosphate depletion. Twelve minutes of hypoxia were imposed on isolated isovolumic (balloon-in-left-ventricle) buffer-perfused hearts from 14 rats with pressure-overload hypertrophy (LVH; LV/body wt ratio = 3.43 +/- 17) secondary to hypertension induced by uninephrectomy plus deoxycorticosterone and salt treatment and from 17 age-matched controls (LV/body wt ratio = 2.22 +/- 0.12, p less than 0.001). Coronary artery flow per gram left ventricle was matched in the LVH and control groups during baseline oxygenated conditions and held constant thereafter. Balloon volume was held constant throughout the experiment so that an increase in LV end-diastolic pressure during hypoxia represented a decrease in LV diastolic distensibility. LV systolic pressure was 165 +/- 9 mm Hg in the LVH group compared with 120 +/- 5 mm Hg in the controls during baseline aerobic perfusion (p less than 0.001). LV end-diastolic pressure rose significantly more in response to 12 minutes of hypoxia in the LVH group (12 +/- 1 to 44 +/- 10 mm Hg) than in the controls (12 +/- 1 to 20 +/- 3 mm Hg, p = 0.04). During baseline aerobic conditions, ATP content was the same in the LVH (17.1 +/- 0.5 mumol/g dry LV wt, n = 4) and control (18.8 +/- 0.6 mumol/g dry LV wt, n = 4, p = NS) groups. During hypoxia, ATP declined at the same rate in the LVH and control groups (3.2 +/- 0.5 versus 3.0 +/- 0.5%/min, p = NS) despite the greater rise in end-diastolic pressure in the LVH group. Creatine phosphate content during baseline aerobic perfusion was 14% lower in the LVH group compared with controls, but the rate of creatine phosphate depletion during 12 minutes of hypoxia was the same. During hypoxia, intracellular pH declined modestly and to the same degree in both groups. Thus, the greater susceptibility to hypoxia-induced diastolic dysfunction observed in isolated buffer-perfused hypertrophied rat hearts cannot be explained by an initially lower total ATP content or by an accelerated rate of decline of ATP or creatine phosphate.(ABSTRACT TRUNCATED AT 400 WORDS)

Sodium/calcium exchange in ventricular muscle

Ventricular cells possess two Ca extrusion mechanisms, a Na/Ca exchange system and a Ca pump. Reversing the exchanger by extracellular Na removal causes [Na]i to decrease, and the cells take up mmolar quantities of calcium. Since [Ca]i shows only a marginal increase the calcium load must be buffered. The capacity of the SR is limited so the mitochondria probably buffer a large part of this load. However, when Ca uptake into the mitochondria is blocked, the gain in Ca is still mmolar and the increase in [Ca]i still marginal, suggesting an additional buffering site. Measurements of the Na/Ca stoichiometry on sarcolemmal vesicles gave a value of 3, but in ventricle values of around 2.5 or 3 are found. Reasons for this are discussed, as are the differences amongst the different methods of Ca measurement. The interaction of the sarcolemmal Ca pump and the exchanger are considered and it is suggested they could interact via [Na]i. At rest both systems could remove Ca from the cell but on a large perturbation the Na/Ca exchange would be the more important of the two.

Influence of sodium-hydrogen exchange on intracellular pH, sodium and tension in sheep cardiac Purkinje fibres

1. The influence of sarcolemmal Na+-H+ exchange upon intracellular Na+ activity (aiNa), intracellular pH (pHi), extracellular surface pH (pHs) and tonic tension was investigated in sheep cardiac Purkinje fibres. Intracellular ion activities were measured with liquid sensor ion-selective micro-electrodes. A two-micro-electrode voltage-clamp was also used to control membrane potential while simultaneously recording tonic tension. 2. Inhibition of the sarcolemmal Na+-K+ pump by strophanthidin (10 mumol/l) produced a rise in aiNa, an increase in [Ca2+]i as evidenced by a rise in tonic tension, and a fall in pHi of 0.1-0.3 units. The intracellular acidosis has been shown previously to be linked to the rise in [Ca2+]i (Vaughan-Jones, Lederer & Eisner, 1983). 3. Amiloride (1-2 mmol/l), an inhibitor of Na+-H+ exchange, produced a small reversible decrease in pHi and aiNa. Both effects became more pronounced in strophanthidin-exposed fibres. In addition, pHi decreased during application of strophanthidin and this decrease was reversibly inhibited by amiloride. It is concluded that sarcolemmal Na+-H+ exchange is stimulated following inhibition of the Na+-K+ pump. 4. In strophanthidin-exposed fibres, a rise in [Ca2+]i resulted in an intracellular acidosis which could still be observed in the presence of amiloride (1 mmol/l). This suggests that the fall in pHi was not caused by a modulatory effect of [Ca2+]i on sarcolemmal Na+-H+ exchange. 5. Tetrodotoxin (TTX) produced a small fall in aiNa (ca. 0.5 mmol/l) which was not augmented in the presence of strophanthidin. Furthermore, the effects on aiNa of TTX and amiloride were additive. Thus the influence of amiloride on aiNa does not involve blockade of voltage-gated Na+ channels. 6. The stoicheiometry of Na+-H+ exchange, estimated from the rates of change of pHi and aiNa in amiloride, appeared to be electroneutral (1:1). The stoicheiometry was unaffected by changes in pHi. 7. In strophanthidin-exposed fibres (i.e. aiNa is elevated), the recovery of pHi from an intracellular acidosis (brought about by brief exposure to NH4Cl) was slowed greatly by amiloride (1-2 mmol/l). The rise in aiNa that occurred during pHi recovery was also reduced by amiloride. It is concluded that Na+-H+ exchange can be stimulated by a fall in pHi under conditions where aiNa is elevated. However, at a given pHi, its rate of recovery was slower in the presence than in the absence of strophanthidin.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of beta-adrenoceptor stimulation on intracellular Ca transients and tension in rat ventricular muscle

Effects of beta-adrenoceptor stimulation on intracellular Ca2+ transients and tension were explored in rat ventricular muscles injected with aequorin. Adrenaline (0.05-5.0 microM) and isoproterenol (0.05-1.0 microM) increased the peak of twitch tension and accelerated relaxation. The former effect depended on Ca2+ concentration in Tyrode's solution ([Ca2+]o) and the stage of the experiment. Low concentrations of these drugs added to normal Tyrode's solution containing 2 mM [Ca2+]o did not potentiate twitch tension in the early stage of the experiments. These drugs increased the peak of the aequorin light signal and slightly accelerated the falling phase of the light especially the tail. Effects of dibutyryl-cyclic AMP (DB-cAMP)(0.1-5.0 mM) and 3-isobutyl-1-methylxanthine (IBMX) (0.01-0.5 mM) were qualitatively similar to those of adrenaline and isoproterenol. Isoproterenol applied at the peak of Na-deficient contracture decreased tension without significantly changing the light signal; similar results were obtained in the presence of ryanodine (1 microM). The results were interpreted as follows: The increase of intracellular cAMP induced by beta-adrenoceptor stimulation facilitated Ca2+ uptake by sarcoplasmic reticulum (SR) and decreased Ca2+ sensitivity of contractile elements. Faster relaxation induced by cAMP was considered to be due to the decrease of Ca2+ sensitivity of contractile elements and faster Ca2+ uptake by SR. The slightly faster falling phase of light transient might be due to the faster Ca2+ uptake by SR, which predominates over the slower fall of [Ca2+]i induced by the decreased Ca2+ sensitivity of the contractile element.

Cell pairs isolated from adult guinea pig and rat hearts: effects of [Ca2+]i on nexal membrane resistance

Cell pairs isolated from adult rat and guinea pig ventricles were used to study the resistance of the nexal membrane, rn. Each cell of a cell pair was connected to a voltage-clamp circuit to obtain simultaneous whole-cell, tight-seal recordings. With this technique, rn was determined under experimental conditions aimed at primarily modifying [Ca2+]i. Moderate changes in [Ca2+]i (produced by trains of depolarizing voltage-clamp pulses activating the slow inward current, or alterations in [Ca2+]o from 0.5 to 10 mM), resulted in no change in rn for normally coupled cell pairs (rn = 5 M omega), but small and reversible changes in slightly uncoupled preparations (rn greater than or equal to 50 M omega). Large increases in rn developed with substantial elevations in [Ca2+]i (secondary to [Na+]o-withdrawal, exposure to strophanthidin in conjunction with isi, or Ca2+-dialysis). Increases in rn brought about via elevation in [Ca2+]i always were accompanied by cell shortening consistent with a sustained contracture. The current-voltage relationship of the nexal membrane was ohmic regardless of whether rn was low (control) or elevated (after increasing [Ca2+]i).

Control of cytosolic calcium activity during low sodium exposure in cultured chick heart cells

We investigated the roles of sodium-calcium exchange, sarcoplasmic reticulum, and mitochondria in Cai homeostasis in cultured chick ventricular cells. Specifically, the influence of low sodium medium on contractile state, calcium fluxes, and cytosolic free [Ca] [( Ca]i) was examined. [Ca]i was measured using fura-2. Mean [Ca]i in control medium was 126 +/- 14 nM. Exposure of cells to sodium-free or sodium- and calcium-free medium (choline-substituted) resulted in contracture development, which returned toward the baseline level over 2-3 minutes. The Nao-free contracture was associated with a tenfold increase in [Ca]i (1,280 +/- 110 nM) followed by a gradual decrease to a level fourfold above control [Ca]i (460 +/- 58 nM). Nao- and Cao-free contracture was associated with a fivefold increase in [Ca]i (540 +/- 52 nM) followed by a rapid decrease to below 80 nM. Sodium-free medium failed to produce an increase in [Ca]i or contracture in cells preexposed to calcium-free medium, although caffeine, when subsequently added to sodium- and calcium-free medium, was able to elicit a transient increase in [Ca]i and contracture. Brief, 5-second preperfusion of cells with La3+ (1 mM) or EGTA (1 mM) abolished the Nao-free contracture and the increase in [Ca]i. In the presence of 20 mM caffeine, removal of Nao resulted in minimal changes in the resting position of the cell although 45Ca uptake and [Ca]i were increased in response to sodium-free medium; the subsequent decrease in [Ca]i was greatly slowed. Addition of caffeine during the relaxation phase of the sodium-free contracture produced an additional transient contracture and transient increase in [Ca]i. Ryanodine (1 microM) abolished this effect of caffeine. Caffeine or ryanodine abolished Nao- and Ca-free contracture. CCCP (2 microM), a potent oxidative phosphorylation inhibitor, did not significantly affect calcium efflux rate. In the presence of 2 microM CCCP, removal of sodium resulted in an augmented contracture signal and a rise in [Ca]i, followed by a slow decrease. We conclude that removal of extracellular sodium enhances transsarcolemmal entry of calcium via sodium-calcium exchange, but this effect alone does not lead to the development of sodium-free contracture. Calcium displaceable by lanthanum or EGTA appears to contribute to Nao-free or Nao- and Cao-free contracture. Studies using caffeine and ryanodine suggest that removal of Nao leads to release of calcium from the sarcoplasmic reticulum (presumably via calcium-induced calcium release).(ABSTRACT TRUNCATED AT 400 WORDS)

The role of the sarcoplasmic reticulum in the response of ferret and rat heart muscle to acidosis

1. The photoprotein aequorin was micro-injected into papillary muscles from the right ventricle of ferrets and rats. Tension and aequorin light (a function of intracellular [Ca2+]) were monitored. 2. In stimulated ferret papillary muscles, increasing the [CO2] of the bicarbonate-buffered superfusate from 5% (pH 7.35) to 20% (pH 6.8) led to a rapid decrease of developed tension, with no significant change in the size of the intracellular Ca2+ transient which accompanies contraction. There was then a small brief recovery of tension which was accompanied by a large brief increase in the size of the Ca2+ transient. Tension then declined again before recovering more slowly, with no significant change in the size of the Ca2+ transient. 3. The time course of the Ca2+ transient was prolonged on exposure to the acid solution, but shortened on continued exposure to the acid solution. Relaxation of twitch tension became faster on exposure to the acid solution, but slowed again on continued exposure to the acid solution. 4. In the presence of 10 mM-caffeine the size of the Ca2+ transient increased during the initial decline of developed tension, the short-lived recovery of tension was abolished, and the Ca2+ transient became smaller during the slower recovery of developed tension. 2 microM-ryanodine had similar effects on developed tension. 5. Addition of 10 mM-lactic acid to the superfusate produced changes similar to those described in 2 and 3 above. 6. An intracellular acidosis, produced by the addition and subsequent withdrawal of 20 mM-NH4Cl from the superfusate also caused changes similar to those described above. In the presence of caffeine, withdrawal of NH4Cl produced changes similar to those described in 4 above. 7. In unstimulated ferret papillary muscles, increasing superfusate [CO2] produced an increase of aequorin light when the bathing [Ca2+] was increased or in the presence of ouabain (10 microM). This increase was not inhibited by verapamil (5 microM), carbonyl cyanide p-trifluoromethoxyphenylhydrazone (1 microM) and oligomycin (2.5 microM), but was reduced by ryanodine (2 microM). 8. Rat papillary muscles showed responses which were quantitatively different from those observed in ferret papillary muscles: the initial recovery of tension developed more slowly, and sarcoplasmic reticulum (s.r.) inhibitors had a greater inhibitory effect on the recovery of tension. 9. It is concluded that the early decline of developed tension observed during acidosis is due to a decrease in Ca2+ release by the s.r. and a decrease in Ca2+ binding by the myofilaments.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiac dysrhythmias in the acute setting: pathophysiology or anyone can understand cardiac dysrhythmias

Cardiac dysrhythmias are easy. Unlike the lung (which has formidable neuroendocrine, metabolic, and respiratory responsibilities), the heart is simple. It is an innervated muscular pump. A resting Purkinje or ventricular muscle cell membrane maintains a charge of about 90 millivolts. The five phases of a cardiac action potential are similar to the action potential in skeletal muscle, however, the cardiac action potential lasts a hundred times longer. When sodium specific "fast" channels and calcium specific "slow" channels open, positive ions rush into the myocardial cell, thus causing rapid membrane depolarization. In order to produce an action potential, some stimulus must decrease the membrane potential from -90 millivolts to "threshold" or -60 millivolts. Purkinje fibers do not have a stable phase for diastolic potential. These fibers continuously depolarize during diastole. Hypoxemia or hypokalemia may exacerbate this diastolic depolarization, thus promoting "hyperexcitability" or "automatic" ectopy. When myocardium is damaged, characteristically with myocardial ischemia, rapid conduction of cardiac impulses may be slowed dramatically. Very slow impulses may course through muscle such that by the time the activation wave front returns to the initiating site, this origin has had a chance to repolarize. This is the basis for re-entrant dysrhythmias. All cardiac dysrhythmias are automatic, re-entrant or both.

Effects of membrane potential on intracellular calcium concentration in sheep Purkinje fibres in sodium-free solutions

1. The intracellular Ca2+ concentration [( Ca2+]i) was measured in voltage-clamped sheep cardiac Purkinje fibers while recording tension simultaneously. 2. When [Na+]i was elevated (by Na+-K+ pump inhibition) depolarization produced an increase of tonic tension. 3. Replacement of external Na+ by Li+ or choline produced a contracture which then relaxed spontaneously. Following this relaxation, depolarization either had no effect on tonic tension or produced a small decrease. 4. When external Na+ was replaced by Ca2+, depolarization (over the range -120 to -20 mV) produced a decrease of tonic tension and [Ca2+]i. Hyperpolarization increased tonic tension and [Ca2+]i. 5. An after-contraction and accompanying increase of [Ca2+]i were produced by repolarization in both Na+-free and Na+-containing solution. This eliminates the possibility that the stimulus for the after-contraction is the increase of [Ca2+]i during the depolarization and suggests that the stimulus may be the change of membrane potential. 6. The increase of [Ca2+]i on hyperpolarization seen in Na+-free solutions persisted in the presence of ryanodine. 7. These results show, in contrast to previous work, that in Na+-free solutions tonic tension is still sensitive to membrane potential. The results support the hypothesis that, in Na+-containing solutions, the increase of tonic tension on depolarization results from a voltage-dependent Na+-Ca2+ exchange. The reduction of tonic tension on depolarization in Na+-free solutions may be due to the decrease of the electrochemical gradient for Ca2+ to enter the cell.

Acute alterations in diastolic left ventricular chamber distensibility: mechanistic differences between hypoxemia and ischemia in isolated perfused rabbit and rat hearts

Changes in diastolic chamber distensibility (DCD) during hypoxemia and ischemia were studied in isolated-buffer-perfused rabbit hearts. Two minutes of hypoxemia (low PO2 coronary flow) resulted in a shift of the diastolic pressure-volume curve to the left, i.e., distensibility was decreased (hypoxemic contracture). In contrast, 2 minutes of ischemia (zero coronary flow) resulted in an initial shift of the diastolic pressure-volume curve to the right indicating increased distensibility, which was followed by a later (30 minutes) shift to the left (ischemic contracture). Two minutes of ischemia superimposed on hypoxemia caused complete reversal of contracture. A quick stretch and release applied to the myocardium reversed late ischemic contracture but did not effect early hypoxemic contracture. The role of intracellular pH in modulating changes in DCD during hypoxia and ischemia was studied using phosphorus-31 nuclear magnetic resonance spectroscopy of isolated-buffer-perfused rat hearts that demonstrated changes in DCD similar to rabbit hearts during hypoxemia and ischemia. Intracellular pH decreased from 7.03 +/- 0.02 to 6.87 +/- 0.03 (p less than .01) during 2 minutes of ischemia but did not change significantly during 4 minutes of hypoxemia. When 2 minutes of ischemia were superimposed on hypoxemia, pH decreased from 6.99 +/- 0.01 during hypoxemia to 6.88 +/- 0.02 after 2 minutes of ischemia (p less than .01), concomitant with the complete reversal of hypoxemic contracture. These results suggest different mechanisms for late ischemic and early hypoxemic contracture and also suggest an explanation for the opposite initial changes in DCD seen after brief periods of ischemia and hypoxemia. The early development of contracture during hypoxemia and rapid redevelopment of diastolic tension after quick stretching are consistent with the hypothesis that hypoxemic contracture results from persistent Ca++-activated diastolic tension secondary to impaired calcium resequestration by the sarcoplasmic reticulum. In contrast, the late development of contracture during global ischemia and reversal by quick stretching is compatible with rigor bond formation. The initial increase in distensibility during early ischemia and the reversal of hypoxemic contracture by a brief period of superimposed ischemia probably is the result of two factors present during ischemia but not during hypoxemia: the collapse of the coronary vasculature and loss of the "erectile" effect and, the rapid development of intracellular acidosis, which has been shown to affect myofibrillar calcium sensitivity, and this may lead to a decrease in Ca++ activated diastolic tension.

Measurement of intracellular calcium during the development and relaxation of tonic tension in sheep Purkinje fibres

The photoprotein aequorin was micro-injected into several cells in a sheep Purkinje fibre. The intracellular Ca concentration [( Ca2+]i) was measured from the resulting light emission. Inhibition of the Na-K pump with strophanthidin resulted in the development of tonic tension which increased on depolarization. This increase was accompanied by an increase of aequorin light. Increasing external Ca concentration [( Ca2+]o) or the magnitude of the depolarization increased both light and tension. If the depolarizing pulse was maintained for several minutes then both tonic tension and aequorin light slowly decayed. The relationship between tension and light was unaffected during this decay. On repolarization the light decayed to below the level before the depolarization before slowly increasing. During this period a test depolarization produced increases of aequorin light and tension which were smaller than control. The application of ryanodine (1-10 microM) abolished all components of tension other than the tonic component. Under these conditions the time course of the increase of tonic tension and aequorin light on depolarization was sufficiently slow to be measured. In most (five out of six) experiments the relationship between light and tension during this development of tonic tension was found to be similar to that during the subsequent spontaneous decay. However, in one experiment the decay of force was greater than could be accounted for by the fall of [Ca2+]i. It is concluded that most of the spontaneous relaxation of tonic tension can be attributed to a fall of [Ca2+]i rather than to other explanations such as an intracellular acidification or increase of inorganic phosphate concentration.

Na+-Ca2+ exchange contributes to increase of cytosolic Ca2+ concentration during depolarization in heart muscle

The possible role of Na+-Ca2+ exchange in contributing to depolarization-induced increase in cytosolic Ca2+ concentration ([Ca2+]i) of isolated rat ventricular myocytes was investigated. Measured with the Ca2+-sensitive indicator quin 2, [Ca2+]i increased from 177 +/- 12 (mean +/- SE, n = 11) to 468 +/- 41 nM when cells were depolarized with solutions containing 50 mM KCl [high extracellular K+ concentration ([K+]o)]. Approximately 73% of this high-[K+]o-induced increase in [Ca2+]i was abolished by the Ca2+ channel blocker verapamil (5 microM). For cells pretreated with 10 mM caffeine to deplete the Ca2+ stored in sarcoplasmic reticulum, 50 mM KCl still produced an increase in [Ca2+]i, even in the presence of 5 microM verapamil. However, if extracellular Na+ was replaced by Li+ or tris(hydroxymethyl)aminomethane, this increase was completely abolished. The results suggest that, in addition to voltage-sensitive Ca2+ channels, voltage-sensitive Na+-Ca2+ exchange can also contribute to the increase in [Ca2+]i on depolarization. Therefore both Ca2+ transport systems may play important roles in regulating cardiac excitation and contraction.

A study of intracellular calcium oscillations in sheep cardiac Purkinje fibres measured at the single cell level

Previous work has shown that an elevation of intracellular calcium concentration [( Ca2+]i) produces spontaneous oscillations of [Ca2+]i. However the fact that the oscillations are unsynchronized between different cells has made it difficult to study them. We have therefore injected only one cell in a Purkinje fibre with aequorin in order to avoid these problems. The addition of strophanthidin (10 microM) produced an increase of mean aequorin light over the course of several minutes. During this period spontaneous oscillations of light developed and, with time, their frequency and magnitude increased. The oscillations could first be seen at levels of [Ca2+]i of less than 1 microM. The amplitude of the oscillations of [Ca2+]i could be up to 10 microM and was modulated at a slow rate (about 0.3-0.5 Hz). This suggests that, even within one cell, different regions may oscillate at different frequencies. Elevating [Ca+]o, removing extracellular Na+, or depolarization increased the magnitude of the aequorin light oscillations. Converting the records to [Ca2+]i showed that this increase in the magnitude of the aequorin oscillations was accompanied by a real increase of mean [Ca2+]i and of the magnitude of the oscillations [Ca2+]i. The frequency of the oscillations increased up to a point but saturated at a maximum value of 3-4 Hz. Since previous experiments have used the mean aequorin light to estimate mean [Ca2+]i, we have calculated the error produced in this calculation by the presence of [Ca2+]i oscillations. We estimate that the error is greatest at low levels of Ca2+ loading when the frequency of the oscillations is low. However, at higher Ca2+ loads, when the frequency is above 2 Hz, the error is probably less than 10%. If oscillations were produced by removal of external Na+ after the application of strophanthidin, then either ryanodine or caffeine abolished the oscillations. Furthermore, in both cases, the resulting steady level of [Ca2+]i was similar to the mean level before the addition of the drugs. In another series of experiments we examined the effects of these drugs on oscillations produced by the application of strophanthidin. Caffeine produced a transient increase in both the frequency of the oscillations and mean [Ca2+]i before abolishing the oscillations and decreasing [Ca2+]i to below the level in the absence of caffeine. In contrast ryanodine gradually decreased both the mean [Ca2+]i and the frequency until the oscillations were abolished. During this period of slowing of the oscillations their magnitude was often increased.

Potassium changes the relationship between receptor occupancy and the inotropic effect of cardiac glycosides in guinea-pig myocardium

K+ (2.4-15.6 mmol l-1) antagonized the positive inotropic effect of dihydro-ouabain. The concentration-effect curves became steeper with the shift to higher concentrations of the glycoside. At 1.2 mmol l-1 Ca2+, an increase in K+ from 2.4 to 12 mmol l-1 required tenfold higher concentrations of dihydro-ouabain to produce equal inotropic effects. This factor was reduced to four at 3.2 mmol l-1 Ca2+. The same change in K+ concentration, at 1.2 mmol l-1 Ca2+, diminished the inotropic effect of ouabain on rested-state contractions by a factor of six. The positive inotropic effect of Ca2+ was also antagonized by K+ (1.2-12 mmol l-1). Reduction of Na+ from 140 to 70 mmol l-1 abolished the antagonistic action of K+ (1.2-8.0 mmol l-1). Moreover the inotropic effect of Ca2+ was enhanced. Reduction of Na+, from 140 to 70 mmol l-1, antagonized the positive inotropic effect of dihydro-ouabain more at low (2.4 mmol l-1) than at high (8.0 mmol l-1) K+. Accordingly, the extent of the dihydro-ouabain-K+ antagonism was reduced. When the K+ concentration was increased from 2.4 to 12 mmol l-1, [3H]-ouabain binding was reduced by a factor of three. This is less than the reduction in the inotropic effectiveness of ouabain or dihydro-ouabain. Reduction of stimulation frequency from 1 to 0.1215 Hz did not significantly alter the antagonistic effect of K+. Diminution of Vmax of the action potential was observed only at K+ concentrations greater than 5.9 mmol l-1, whereas the resting membrane potential was continuously depolarized over the entire range of K+ concentrations. The results support the view that the reduction in receptor affinity cannot be the sole cause of the antagonism between the glycoside and K+. Impairment of passive Na+ influx during diastole, due to the K+-dependent depolarization of the resting membrane potential, contributed to about one half of the glycoside-K+ antagonism.