The interaction of sodium and calcium ions at the cell membrane and the control of contractile strength in frog atrial muscle

The Cardiac Pacemaker Story-Fundamental Role of the Na+/Ca2+ Exchanger in Spontaneous Automaticity

The electrophysiological mechanism of the sinus node automaticity was previously considered exclusively regulated by the so-called "funny current". However, parallel investigations increasingly emphasized the importance of the Ca²⁺-homeostasis and Na⁺/Ca²⁺ exchanger (NCX). Recently, increasing experimental evidence, as well as insight through mechanistic in silico modeling demonstrates the crucial role of the exchanger in sinus node pacemaking. NCX had a key role in the exciting story of discovery of sinus node pacemaking mechanisms, which recently settled with a consensus on the coupled-clock mechanism after decades of debate. This review focuses on the role of the Na⁺/Ca²⁺ exchanger from the early results and concepts to recent advances and attempts to give a balanced summary of the characteristics of the local, spontaneous, and rhythmic Ca²⁺ releases, the molecular control of the NCX and its role in the fight-or-flight response. Transgenic animal models and pharmacological manipulation of intracellular Ca²⁺ concentration and/or NCX demonstrate the pivotal function of the exchanger in sinus node automaticity. We also highlight where specific hypotheses regarding NCX function have been derived from computational modeling and require experimental validation. Nonselectivity of NCX inhibitors and the complex interplay of processes involved in Ca²⁺ handling render the design and interpretation of these experiments challenging.

Ups and downs of calcium in the heart

Contraction and relaxation of the heart result from cyclical changes of intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ). The entry of Ca²⁺ into the cell via the L-type Ca²⁺ current leads to the release of more from the sarcoplasmic reticulum (SR). Compared to other regulatory mechanisms such as phosphorylation, Ca²⁺ signalling is very rapid. However, since Ca²⁺ cannot be destroyed, Ca²⁺ signalling can only be controlled by pumping across membranes. In the steady state, on each beat, the amount of Ca²⁺ released from the SR must equal that taken back and influx and efflux across the sarcolemma must be equal. Any imbalance in these fluxes will result in a change of SR Ca²⁺ content and this provides a mechanism for regulation of SR Ca²⁺ content. These flux balance considerations also explain why simply potentiating Ca²⁺ release from the SR has no maintained effect on the amplitude of the Ca²⁺ transient. A low diastolic [Ca²⁺ ]_i is essential for cardiac relaxation, but the factors that control diastolic [Ca²⁺ ]_i are poorly understood. Recent work suggests that flux balance is also important here. In particular, decreasing SR function decreases the amplitude of the systolic Ca²⁺ transient and the resulting decrease of Ca²⁺ efflux results in an increase of diastolic [Ca²⁺ ]_i to maintain total efflux.

Low sodium inotropy is accompanied by diastolic Ca2+ gain and systolic loss in isolated guinea-pig ventricular myocytes

We measured sarcolemmal Ca2+ fluxes responsible for the positive inotropic effects of solutions with reduced Na+ concentration in voltage-clamped guinea-pig ventricular myocytes; intracellular Ca2+ concentration ([Ca2+]i) was measured with Indo-1. Reduction of external Na+ concentration by 50 % (to 67 mM) produced an increase in systolic [Ca2+]i accompanied by a decrease in Ca2+ entry via the L-type Ca2+ current. With reduced Na+ concentration, there was an initial decrease in the Na+-Ca2+ exchange current on repolarization followed by an increase to greater than control. We attribute this initial decrease to a decrease in the Na+ gradient and the subsequent increase to a fall in intracellular Na+ concentration and increase in systolic [Ca2+]i. The decreased L-type Ca2+ current and increased Ca2+ efflux on Na+-Ca2+ exchange resulted in a calculated systolic loss of Ca2+. The calculated systolic loss of Ca2+ was accompanied by a measured increase in sarcoplasmic reticulum (SR) Ca2+ content. Reduction of the external Na+ concentration also produced an outward shift of holding current which was blocked by Ni2+. This is taken to represent Ca2+ influx via Na+-Ca2+ exchange. When diastolic influx is taken into account, the observed gain in SR Ca2+ content can be predicted. The measurements show that, in reduced Na+, much of the entry of Ca2+ into the cell occurs during diastole (via Na+-Ca2+ exchange) rather than in systole (via the L-type Ca2+ current).

The control of Ca release from the cardiac sarcoplasmic reticulum: regulation versus autoregulation

This review discusses the mechanism and regulation of Ca release from the cardiac sarcoplasmic reticulum. Ca is released through the Ca release channel or ryanodine receptor (RyR) by the process of calcium-induced Ca release (CICR). The trigger for this release is the L-type Ca current with a small contribution from Ca entry on the Na-Ca exchange. Recent work has shown that CICR is controlled at the level of small, local domains consisting of one or a small number of L-type Ca channels and associated RyRs. Ca efflux from the s.r. in one such unit is seen as a 'spark' and the properties of these sparks produce controlled Ca release from the s.r. A major factor controlling the amount of Ca released from the s.r. and therefore the magnitude of the systolic Ca transient is its Ca content. The Ca content depends on both the properties of the s.r. and the cytoplasmic Ca concentration. Changes of s.r. Ca content and the Ca released affect the sarcolemmal Ca and Na-Ca exchange currents and this acts to control cell Ca loading and the s.r. Ca content. The opening probability of the RyR can be regulated by various physiological mediators as well as pharmacological compounds. However, it is shown that, due to compensatory changes of s.r. Ca, modifiers of the RyR only produce transient effects on systolic Ca. We conclude that, although the RyR can be regulated, of much greater importance to the control of Ca efflux from the s.r. are effects due to changes of s.r. Ca content.

Can continuous intraperitoneal infusion of 125I-iothalamate and 131I-hippuran be used for measurement of GFR in conscious rats?

We previously described a method to measure GFR in conscious spontaneously voiding rats. This method circumvents the need for anesthesia and for bladder instrumentation. It's main principle is the correction of renal 125I-iothalamate clearance for incomplete urine collection by the ratio of plasma and renal clearance of co-infused 131I-Hippuran. A disadvantage of this technique is the requirement of an intra-arterial catheter for infusion of the renal function tracers. We therefore tested whether intraperitoneal infusion of 125I-iothalamate and 131I-Hippuran can be used for such a GFR measurement in conscious spontaneously voiding rats. We found that during intraperitoneal administration, stable plasma levels of 131I-Hippuran could be obtained. However, urinary recovery of 131I-Hippuran was incomplete (66 +/- 32%), leading to a significant overestimation of GFR by 140 +/- 13% in comparison with the GFR measured by the intra-arterial technique. Thus intraperitoneal infusion of renal function tracers cannot replace intra-arterial infusion.

The control of the contraction of myocytes from guinea-pig heart by the resting membrane potential

1. The influence of different holding potentials (-120 to -70 mV) on the contraction of enzymatically dispersed myocytes from guinea-pig hearts was evaluated. Contractions were elicited by repetitive depolarizations to 0 mV at 0.5 Hz. 2. While ineffective at 140 and 5 mmol l-1 [Na+]o and pipette Na+, respectively, depolarization of the resting membrane with the holding potential increased myocyte shortening at reduced Na+ gradients ([Na+]o 70 or [Na+]i 10-15 mmol l-1). Elevated intracellular Na+ after Na(+)-pump inhibition with ouabain 1-10 mumol l-1 was similarly effective with regard to the inotropic response to different holding potentials. 3. At -70 mV holding potential, reduction of [Na+]o from 140 to 70 mmol l-1 increased myocyte shortening and induced an inwardly directed component of the holding current which peaked at -44 +/- 10 pA and declined thereafter in parallel with the inotropic effect. The relation of this inward current to [Ca2+]i was confirmed by experiments at high Ca2+ buffer capacity where [Na+]o reduction induced a Ni(2+)-insensitive, outwardly directed component (36 +/- 15 pA) of the holding current. The observed inward current is suggested to reflect the extrusion of [Ca2+]i in exchange for [Na+]o as a counter-regulatory mechanism which limits the increase of [Ca2+]i. 4. The interventions which increased the strength of the contraction also enhanced the transient tail current after repolarization, suggesting its close relation to [Ca2+]i. This finding confirmed the pattern found with cell shortening. 5. It is concluded that under certain conditions, voltage-dependent and Na(+)-dependent Na(+)-Ca2+ exchange during the interval between the contractions is relevant to the diastolic concentration of [Ca2+]i which in turn determines the accumulation of Ca2+ in the sarcoplasmic reticulum and the magnitude of the subsequent contraction.

Cardiovascular effects of iodinated contrast agents

The ideal intravascular contrast agent would be biologically inert and have no pharmacologic actions. Pharmacologic actions of currently used radiographic contrast agents are determined principally by 3 physicochemical properties of the iodine-bearing molecule and its formulation: osmolality, sodium concentration and calcium-binding properties. Within this framework, the calcium-binding 1.5 ratio agents have the most marked effects, and the 3.0 ratio nonionic agents the least, with the noncalcium-binding formulations of 1.5 ratio agents and ioxaglate (the only 3.0 ratio ionic agent) in between. Differences in hemodynamic effects are predominantly related to osmolality with the 3.0 agents causing less hemodynamic disturbance. The magnitude of difference is small enough that the 3.0 ratio agents have no important clinical advantage when used in patients with good or moderately impaired left ventricular function. However, the difference may be important in patients with severely impaired circulatory performance. The principal electrophysiologic differences are between the calcium-binding 1.5 ratio agents (which are associated with a clear-cut greater frequency of ventricular fibrillation during coronary injection than the noncalcium-binding 1.5 ratio agents) and the 3.0 ratio agents. There is no justification for the use of calcium-binding 1.5 ratio agents, since noncalcium-binding formulations of the same molecule are available at the same price. The circulatory reserve of most patients makes the differences between 3.0 ratio agents and noncalcium-binding 1.5 ratio agents clinically unimportant. In view of the substantial price disparity between 1.5 ratio and 3.0 ratio agents, noncalcium-binding 1.5 ratio agents are appropriate for patients with good circulatory performance and 3.0 ratio agents are best reserved for patients with impaired circulatory performance.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective block of calcium current by lanthanum in single bullfrog atrial cells

A single suction microelectrode voltage-clamp technique was used to study the actions of lanthanum ions (La3+) on ionic currents in single cells isolated from bullfrog right atrium. La3+, added as LaCl3, blocked the "slow" inward Ca2+ current (ICa) in a dose-dependent fashion; 10(-5) M produced complete inhibition. This effect was best fitted by a dose-response curve that was calculated assuming 1:1 binding of La3+ to a site having a dissociation constant of 7.5 x 10(-7) M. La3+ block was reversed (to 90% of control ICa) following washout and, in the presence of 10(-5) M La3+, was antagonized by raising the Ca2+ concentration from 2.5 to 7.5 mM (ICa recovered to 56% of the control). However, the latter effect took approximately 1 h to develop. Concentrations of La3+ that reduced ICa by 12-67%, 0.1-1.5 x 10(-6) M, had no measurable effect upon the voltage dependence of steady state ICa inactivation, which suggest that at these concentrations there are no significant surface-charge effects of La3+ on this gating mechanism. Three additional findings indicate that doses of La3+ that blocked ICa failed to produce nonspecific effects: (a) 10(-5) M La3+ had no measurable effect on the time-independent inwardly rectifying current, IK1; (b) the same concentration had no effect on the kinetics, amplitude, or voltage dependence of a time- and voltage-dependent K+ current, IK; and (c) 10(-4) M La3+ did not alter the size of the tetrodotoxin-sensitive inward Na+ current, INa, or the voltage dependence of its steady state inactivation. Higher concentrations (0.5-1.0 mM) reduced both IK1 and IK, and shifted the steady state activation curve for IK toward more positive potentials, presumably by reducing the external surface potential. Our results suggest that at a concentration of less than or equal to 10(-5) M, La3+ inhibits ICa selectively by direct blockade of Ca channels rather than by altering the external surface potential. At higher concentrations, La3+ exhibits nonspecific effects, including neutralization of negative external surface charge and inhibition of other time- and voltage-dependent ionic currents.

Low-sodium contractures indicating sarcolemmal Na/Ca-exchange in the frog heart

1. In the frog heart, Ca2+ enters the cell by the slow inward current (Isi) and by an electrogenic, carrier-mediated, and passive Na-out/Ca-in-exchange. 2. The latter reverses to Na-in/Ca-out-exchange during depolarization and thereby controls relaxation. 3. The exchange ratio is 3 Na+ for 1 Ca2+. 4. The Na/Ca-exchange is not inhibited by organic Ca-antagonists in frog myocardium, indicating that the initiation of the heart beat may mainly depend on Isi. 5. This is not necessarily in contradiction with the Na-Ca-antagonism, since there also exists an antagonism between Na+ and Ca2+ in the slow channel. 6. However, the contractures caused by a decrease of NaO+ are mediated by the Na/Ca-exchange.

Tension activation and relaxation in frog atrial fibres. Evidence for direct effects of divalent cations (Ca2+, Sr2+, Ba2+) on contractile proteins and Na-Ca exchange

The effect of alkali-earth cations (Ca2+, Sr2+, Ba2+) on the excitation-contraction coupling events of the frog atrial fibres were studied using a double mannitol gap voltage clamp technique coupled with a mechano-electric transducer. Photoremoval of the suppressive effect of nifedipine on the calcium channels allowed to obtain rapid transient Ca2+, Sr2+ or Ba2+ ions current jumps. The effect on the amplitude of the associated contraction was proportional to the current jumps. These results together with the correlation established between the estimated increase in the internal concentration of divalent cations and the amplitude of the phasic tension suggest that the essential source of divalent cations for activation of contraction is the extracellular space. Also Ba2+ ions reduced the tonic tension and strongly slowed the relaxation of the phasic component whereas Sr2+ exhibited smaller effects. Sr2+ ions could be more efficient than Ba2+ ions in substituting for Ca2+ ions in the Na+-Ca2+ exchange mechanism known to regulate these two mechanical events. The conclusions are that the order of effectiveness of these ions (Ca2+ greater than Sr2+ greater than Ba2+) is the same with regard to transarcolemmal exchange for Na+ ions, presumed uptake by a "second relaxing system", activation of contraction, and inactivation of the slow inward current.

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)

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.

Biphasic contractions induced by milrinone at low temperature in ferret ventricular muscle: role of the sarcoplasmic reticulum and transmembrane calcium influx

The effects of milrinone were studied in ferret papillary muscle stimulated at various rates and temperatures from 23 degrees to 36 degrees C. In voltage-clamp experiments, 50 micrograms/ml (0.237 mM) milrinone induced a 2.1-fold increase in calcium current at 28 degrees or 36 degrees C. At 50 micrograms/ml, milrinone transiently increased contractility in all muscles at 28 degrees C, but its steady-state effect was either increased (+50%) or decreased (-24.7%) steady-state twitch amplitude. A negative inotropic effect always occurred below 27 degrees C. Milrinone decreased the total twitch duration and split the twitch into two components (P1 and P2) in the absence of any evidence of aberrant conduction. Increasing milrinone concentration from 50 to 300 micrograms/ml decreased P1 and increased P2. Ryanodine (100 mM) or caffeine (10 mM) suppressed P1. Contractions elicited after 30 seconds of rest were also biphasic in the presence of milrinone, but not in its absence. P2 of post-rest contraction was increased by caffeine or calcium (10 mM) and decreased by cobalt (2 mM) when drugs were applied at the beginning of the rest. Ryanodine and caffeine also suppressed P1 of post-rest contraction. The evidence suggests that P1 may be caused by Ca release from the sarcoplasmic reticulum and P2 by increased Ca influx during the action potential via the calcium channel. It is also suggested that P2 may be present under control conditions, but to a lesser extent, and masked by a large P1.

"Creep currents" in single frog atrial cells may be generated by electrogenic Na/Ca exchange

The objective of these experiments was to test the hypothesis that the "creep currents" induced by Na loading of single frog atrial cells (Hume, J. R., and A. Uehara. 1986. Journal of General Physiology. 87:833) may be generated by an electrogenic Na/Ca exchanger. Creep currents induced by Na loading were examined over a wide range of membrane potentials. During depolarizing voltage-clamp pulses, outward creep currents were observed, followed by inward creep currents upon the return to the holding potential. During hyperpolarizing voltage-clamp pulses, creep currents of the opposite polarity were observed: inward creep currents were observed during the pulses, followed by outward creep currents upon the return to the holding potential. The current-voltage relations for inward and outward creep currents in response to depolarizing or hyperpolarizing voltage displacements away from the holding potential all intersect the voltage axis at a common potential, which indicates that inward and outward creep currents may have a common reversal potential under equilibrium conditions and may therefore be generated by a common mechanism. Measurements of inward creep currents confirm that voltage displacements away from the holding potential rapidly alter equilibrium conditions. Current-voltage relationships of inward creep currents after depolarizing voltage-clamp pulses are extremely labile and depend critically upon the amplitude and duration of outward creep currents elicited during preceding voltage-clamp pulses. An optical monitor of mechanical activity in single cells revealed (a) a similar voltage dependence for the outward creep currents induced by Na loading and tonic contraction, and (b) a close correlation between the time course of the decay of the inward creep current and the time course of mechanical relaxation. A mathematical model of electrogenic Na/Ca exchange (Mullins, L.J. 1979. Federation Proceedings. 35:2583; Noble, D. 1986. Cardiac Muscle. 171-200) can adequately account for many of the properties of creep currents. It is concluded that creep currents in single frog atrial cells may be attributed to the operation of an electrogenic Na/Ca exchange mechanism.

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.

Effect of cyanide and verapamil on sodium-free contractures in the frog heart

Sodium-free contractures were studied in myocardial strips from R. pipiens with extracellular sodium (Na0+) replaced by choline chloride and extracellular calcium (Ca20+) varied with EGTA buffer. At calculated Ca02+ below 2.8 X 10(-7) mol/l, no contracture occurred in most of the experiments, even in the presence of cyanide. When Ca02+ was above 2.8 X 10(-7) mol/l, relatively short tension transients of up to 80 sec duration could be avoided if the myocardial strip was previously equilibrated for 20 min in a Na+-Ca2+-free solution. Instead, contractures developed slowly within one to several hours. The maximum contracture was dependent on Ca02+ in a dose-response-like pattern. The time-course of contracture development was not affected by verapamil, but KCN significantly increased the rate of resting tension increase. In solutions with normal Na+-Ca2+ content and even in a Na+-Ca2+-free milieu, the cellular ultrastructure was normal. Development of contracture after addition of Ca2+ to the Na+-free solution was combined with ultrastructural damage of the ventricular strip. It is concluded that Na+-free contractures depend on transsarcolemmal net-Ca2+ uptake as a sum of Na-Ca-exchange-dependent Ca2+ uptake and active sequestering of intracellular free calcium Ca2+ mediated by sarcolemmal and probably intracellular Ca2+-ATPases. The negative inotropic effect of the Ca blocker verapamil seems not to be mediated by the Na-Ca exchange.

Submaximal sodium-lack contractures in rapidly perfused frog ventricular strips

Rapid perfusion of strips of frog ventricular muscle (width less than or equal to 8 mm) by sodium-free Ringer yielded contractures with very fast rates of rise (half time approximately equal to 2 s) and flat plateaus. Tension in lithium Ringer was 0.64 (0.38-0.77) of the maximal tension the muscle could produce [i.e., for an extracellular potassium concentration ([K]o) of 160 mM, and an [Na]o of 0 mM at less than 15 degrees C], showing that intracellular calcium concentration ([Ca]i) in zero [Na]o and normal [K]o was insufficient to saturate the myofilaments. Tension was related to [Ca]o in an S-shaped manner (apparent Km = 1.3 microM), was rapidly increased by electrical depolarization, high potassium, or low temperature, but was unaffected by up to 10(-5) M dihydroouabain. From these data was derived the working hypothesis that the value of [Ca]i during the contracture plateau is a steady-state value due to influx through a sodium-dependent mechanism and calcium uptake or efflux via a sodium-independent mechanism. Also, 1 microM dihydroouabain increased peak twitch tension by 60% but did not potentiate tension during the plateaus of contractures obtained by partial replacement of Nao. This result supports the working hypothesis and suggests that the potentiating effect of raising Nai may depend on the membrane potential (D. A. Eisner, W. J. Ledener, and R. D. Vaughan-Jones. J. Physiol. Lond. 335: 723-743, 1983).

Cellular mechanisms of digitalis action

Although the therapeutic actions of digitalis glycosides have been known for over 200 years, their direct inotropic actions on the heart were not established until the last 50 years. Digitalis has undergone intense research, particularly with respect to its mechanisms of action. Many authors have claimed to have found the true mechanism of action, compounding the complexity of literature. Recent subcellular studies have pointed to specific areas of action of the digitalis glycosides. Each discovery has been dependent on the greater understanding of the electrophysiologic characteristics of cardiac muscle and excitation-contraction coupling. The current hypothesis suggests that digitalis specifically inhibits Na-K ATPase. This produces an elevation in intracellular sodium level that in turn produces an increase in the intracellular calcium level. The increased quantities of calcium available to the contractile elements of cardiac muscle provide the observed increased inotropy.

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.

Na-Ca exchange: stoichiometry and electrogenicity

This review discusses the evidence concerning the stoichiometry of Na-Ca exchange. In particular we consider whether the Na-Ca exchange has been shown to transport more than two Na+ ions per Ca2+ ion and therefore whether it generates an electric current. The first part of this review discusses both direct and indirect evidence concerning the stoichiometry of the exchange and its possible voltage dependence. We find that, although there is some evidence suggesting that more than two Na+ ions may exchange for each Ca2+ ion, most of the available evidence is equivocal and cannot fix the stoichiometry precisely. Furthermore, using a simple and explicit circulating carrier model for the Na-Ca exchange, we show that the effect of membrane potential on the Na-Ca exchange may be considerably more complicated than is generally believed. In particular we find that both electrogenic and electroneutral exchanges will be affected by membrane potential. We therefore conclude that the demonstration of the voltage dependence of the Na-Ca exchange does not necessarily imply that it is electrogenic. Additionally, this analysis shows that, apart from a restricted range near thermodynamic equilibrium, it is impossible to predict either the magnitude or the direction of the effects of membrane potential on the exchange. In the second part of the review we consider whether any known membrane currents may be attributed to Na-Ca exchange. We show, in contrast to previous suggestions, that the Na-Ca exchange can theoretically produce a current that appears to be activated by intracellular Ca and that has a reversal potential. However, the experimental demonstration that a given current is produced by Na-Ca exchange is hampered by the existence of other Ca- and Na-dependent currents. In conclusion, we feel that there is no evidence that allows any particular membrane current to be unambiguously identified with the Na-Ca exchange.

The effect of althesin, ketamine or pentothal on renal function in saline loaded rats

Rats were prepared with chronic cannulae in the carotid artery, jugular vein and urinary bladder; they were then kept 2 days to allow recovery from surgery. A steady-state continuous saline diuresis was established, then various anaesthetic agents were injected and changes in the urine flow, sodium and potassium excretion rates, GFR, ERPF, ERBF and blood pressure were measured. Five groups of rats were studied: (1) control animals given saline in place of any anaesthetic agent, all parameters measured remained constant; (2) althesin (1.2 mg/kg), all parameters remained constant except for the blood pressure which decreased slightly for 5 min; (3) althesin (12 mg/kg); (4) ketamine (50 mg/kg); (5) pentothal (50 mg/kg). The above anaesthetics altered all of the measured parameters except urine output. Various mechanisms for these anaesthetic agents are discussed. In althesin anaesthetized rats, all the retarded renal parameters recovered within 30 min. It is concluded that for the study of renal function, if a brief anaesthesia is needed, althesin is a more suitable anaesthetic agent.

A model of cardiac electrical activity incorporating ionic pumps and concentration changes

Equations have been developed to describe cardiac action potentials and pacemaker activity. The model takes account of extensive developments in experimental work since the formulation of the M.N.T. (R. E. McAllister, D. Noble and R. W. Tsien, J. Physiol., Lond. 251, 1-59 (1975)) and B.R. (G. W. Beeler and H. Reuter, J. Physiol., Lond . 268, 177-210 (1977)) equations. The current mechanism i K2 has been replaced by the hyperpolarizing-activated current, i f . Depletion and accumulation of potassium ions in the extracellular space are represented either by partial differential equations for diffusion in cylindrical or spherical preparations or, when such accuracy is not essential, by a three-compartment model in which the extracellular concentration in the intercellular space is uniform. The description of the delayed K current, i K , remains based on the work of D. Noble and R. W. Tsien ( J. Physiol., Lond . 200, 205-231 (1969 a )). The instantaneous inward-rectifier, i K1 , is based on S. Hagiwara and K. Takahashi’s equation ( J. Membrane Biol . 18, 61-80 (1974)) and on the patch clamp studies ofB. Sakmann and G. Trube ( J. Physiol., Lond . 347, 641-658 (1984)) and of Y. Momose, G. Szabo and W. R. Giles ( Biophys. J . 41, 311a (1983)). The equations successfully account for all the properties formerly attributed to i K2 , as well as giving more complete descriptions of i K1 and i K . The sodium current equations are based on experimental data of T. J. Colatsky ( J.Physiol., Lond. 305, 215-234 (1980)) and A. M. Brown, K. S. Lee and T. Powell ( J.Physiol., Lond. , Lond. 318, 479-500 (1981)). The equations correctly reproduce the range and magnitude of the sodium ‘window’ current. The second inward current is based in part on the data of H. Reuter and H. Scholz ( J. Physiol., Lond . 264, 17-47 (1977)) and K. S. Lee and R. W. Tsien ( Nature, Lond . 297,498-501 (1982)) so far as the ion selectivity is concerned. However, the activation and inactivation gating kinetics have been greatly speeded up to reproduce the very much faster currents recorded in recent work. A major consequence of this change is that Ca current inactivation mostly occurs very early in the action potential plateau. The sodium-potassium exchange pump equations are based on data reported by D. C. Gadsby ( Proc. natn. Acad. Sci. U. S. A. 77, 4035-4039 (1980)) and by D. A. Eisner and W. J. Lederer ( J. Physiol., Lond . 303, 441-474 (1980)). The sodium-calcium' exchange current is based on L. J. Mullins’ equations ( J. gen.. Physiol. 70, 681-695 (1977)). Intracellular calcium sequestration is represented by simple equations for uptake into a reticulum store which then reprimes a release store. The repriming equations use the data of W. R. Gibbons & H. A. Fozzard ( J. gen. Physiol . 65, 367-384 (1975 b )). Following Fabiato & Fabiato’s work ( J. Physiol., Lond. 249, 469-495 (I975)), Ca release is assumed to be triggered by intracellular free calcium. The equations reproduce the essential features of intracellular free calcium transients as measured with aequorin. The explanatory range of the model entirely includes and greatly extends that of the M.N.T. equations. Despite the major changes made, the overall time-course of the conductance changes to potassium ions strongly resembles that of the M.N.T. model. There are however important differences in the time courses of Na and Ca conductance changes. The Na conductance now includes a component due to the hyperpolarizing-activated current, i r , which slowly increases during the pacemaker depolarization. The Ca conductance changes are very much faster than in the M.N.T. model so that in action potentials longer than about 50 ms the primary contribution of the fast gated calcium channel to the plateau is due to a steady-state ‘window’ current or non-inactivated component. Slower calcium or Ca-activated currents, such as the Na-Ca exchange current, or Ca-gated currents, or a much slower Ca channel must then play the dynamic role previously attributed to the kinetics of a single type of calcium channel. This feature of the model in turn means that the repolarization process should be related to the inotropic state, as indicated by experimental work. The model successfully reproduces intracellular sodium concentration changes produced by variations in [Na]0, or Na-K pump block. The sodium dependence of the overshoot potential is well reproduced despite the fact that steady state intracellular Na is proportional to extracellular Na, as in the experimental results of D. Ellis J. Physiol., Lond . 274, 211-240 (1977)). The model reproduces the responses to current pulses applied during the plateau and pacemaker phases. In particular, a substantial net decrease in conductance is predicted during the pacemaker depolarization despite the fact that the controlling process is an increase in conductance for the hyperpolarizing-activated current. The immediate effects of changing extracellular [K] are reproduced, including: (i) the shortening of action potential duration and suppression of pacemaker activity at high [K ]; (ii) the increased automaticity at moderately low [K ]; and (iii) the depolarization to the plateau range with premature depolarizations and low voltage oscillations at very low [K]. The ionic currents attributed to changes in Na-K pump activity are well reproduced. It is shown that the apparent K m for K activation of the pump depends strongly on the size of the restricted extracellular space. With a 30% space (as in canine Purkinje fibres) the apparent K m is close to the assumed real value of 1 mM . When the extracellular space is reduced to below 5% , the apparent K m increases by up to an order of magnitude. A substantial part of the pump is then not available for inhibition by low [K] b . These results can explain the apparent discrepancies in the literature concerning the K m for pump activation.

The effects of intracellular Na on contraction and intracellular pH in mammalian cardiac muscle

Intracellular Na and pH were measured with recessed-tip ion-selective microelectrodes in voltage-clamped sheep cardiac Purkinje fibers. Intracellular Na activity (aiNa) was elevated by inhibiting the Na/K pump. This produced an increase of twitch tension that had a steep dependence on the increase of aiNa. These effects of aiNa on twitch tension are probably mediated by an Na-Ca exchange. An increase of aiNa also produced a component of tonic tension that appears to be produced directly by the Na-Ca exchange. The dependence of tonic tension and aiNa on membrane potential suggests that this exchange process may be voltage-sensitive. The increase of aiNa is associated with an intracellular acidification that appears to be secondary to an increase of [Ca2+]i produced by Na-Ca exchange. Therefore, as well as affecting [Ca2+]i, Na-Ca exchange can under some circumstances influence pHi indirectly, and this complicates the interpretation of changes in tension, since protons and Ca ions have opposite effects on contractile force.

Na-Ca exchange in cardiac tissues

Our awareness of the importance of Na-Ca exchange in cardiac muscle has progressed from early observations of Na-Ca antagonism in the activation of contractile force. This was followed by demonstrations of actual Na-Ca ion countertransport across cell membranes and later functional studies in which manipulation of intracellular and extracellular Na and Ca concentrations has permitted a better characterization of the exchange process and its contribution to contractile force. The recent development of vesicle preparations from cardiac sarcolemmal membranes has, despite some drawbacks, produced useful information on the electrogenicity of the exchange mechanisms and on the relative affinity of the exchange carrier compared to the ATPase-driven Ca pump. These studies confirmed earlier estimates of the approximate exchange ratio of the Na-Ca countertransport system and have demonstrated its large maximum transport rate capabilities. The application of ion-sensitive microelectrodes in recent years has enabled measurements of the actual ion-activity gradients across the sarcolemmal membrane. These activity gradients together with the membrane potential control the rate and direction of the Na-Ca exchange. Despite the wide range of techniques employed to tackle the problem, the exchange ratio of Na to Ca movement is still in some doubt, with most estimates ranging between 5:2 and 4:1.

Quantitative relation of twitch and tonic tensions to intracellular Na+ activity in cardiac Purkinje fibers

Quantitative relation of twitch and tonic tensions to intracellular Na ion was studied by describing model equations and measuring simultaneously the electrical, mechanical, and intracellular Na ion activities in electrically driven cardiac Purkinje fibers exposed to strophanthidin, tetrodotoxin (TTX), and varied [K+]0. In each experiment a plot of tension (T) vs. intracellular Na ion activity (aiNa) on logarithmic coordinates showed a good fit of the data to a single line described by the equation of T = beta (aiNa) gamma, in which beta and gamma represent the intercept and slope of the log T-log aiNa relation. Implication and average values of beta were presented. The average value of gamma obtained was 6.1 +/- 0.9 (SD, n = 8) in the experiments with strophanthidin (5 X 10(-7) - 10(-6) M) that somewhat depolarized transmembrane potential (Vm). The gamma value was 6.6 +/- 1.4 (n = 6) in the experiments with TTX (10(-6) - 5 X 10(-6) M) that shortened action potential duration. The gamma of 4.3 +/- 0.7 (n = 5) and 4.0 +/- 0.5 (n = 6) were obtained with the low [K+]0 of 1.0 and 2.0 mM, respectively, that hyperpolarized diastolic membrane potential by 12.7 +/- 6.3 mV (n = 5) and 13.7 +/- 2.4 mV (n = 6). The gamma value was 7.0 +/- 1.7 (n = 10) in the experiments with 8.1 mM [K+]0 that depolarized diastolic potential by 10.8 +/- 1.4 mV (n = 10). K+-free solution resulted in the gamma values of 6.0 +/- 0.9 (n = 6) for the twitch-aiNa relation and 5.3 +/- 1.4 (n = 9) for the tonic tension-aiNa relation. Except the experiments with the low [K+]0 the gamma values obtained are reasonably close to the value of 6.0 that is given on the basis of the 3Na-1Ca exchange and Ca2+-tension relation. In the experiments with the low [K+]0, the gamma values lower than 6.0 may be explained by the large hyperpolarization of diastolic membrane potential that could reduce intracellular calcium by means of the Na-Ca exchange. In conclusion, aiNa is a powerful determinant of twitch and tonic tensions which, in most instances, are linear functions of (aiNa)approximately 6.

The quantitative relationship between twitch tension and intracellular sodium activity in sheep cardiac Purkinje fibres

Tension was measured in voltage clamped sheep cardiac Purkinje fibres while simultaneously measuring the intracellular Na activity (aiNa) with a recessed-tip, Na-selective micro-electrode. Inhibiting the Na-K pump either by exposing the preparation to a K-free solution or by adding the cardioactive steroid strophanthidin increased both aiNa and twitch tension and resulted in the development of tonic tension, after-contractions and a transient inward current (ITI). The increase of twitch tension was present at lower aiNa than that required to produce the other phenomena. The relationship between the magnitude of the twitch tension and aiNa was always non-linear. Twitch tension increased steeply with aiNa at first but the relationship flattened off at higher aiNa and tension eventually decreased. Over the steep range, the relationship between tension and aiNa could be represented as: twitch tension = b (aiNa)y where y had a mean value of 3.2. Changing membrane potential or [Ca2+]o changed b but had little effect on y. Mn (2 mmol/l) greatly decreased twitch tension but, at least initially, had little effect on tonic tension. The steep relationship between twitch tension and aiNa was seen, irrespective of whether the Na-K pump was inhibited either by exposure to K-free solution or to strophanthidin and whether the relationship was measured either when aiNa was increasing or after it had reached a steady state. The steep dependence of twitch tension on aiNa observed in the present work means that manoeuvres which produce even small changes of aiNa will have significant effects on contraction.

Changes in external Na induce a membrane current related to the Na-Ca exchange in cesium-loaded frog heart cells

The effects of transient alterations in Nao were investigated under voltage clamp conditions in frog heart cells previously loaded with Cs. Tetrodotoxin and Cs were used to inhibit Na and K currents. On applying a Na-poor solution (39.2 mM), an outward current was generated during both depolarizations and hyperpolarizations. The current amplitude described a U-shaped function of the membrane potential. On reapplying the standard solution after 15 min equilibration, an inward current was then induced that exhibited a bell-shaped function of the membrane potential. Current amplitude was sensitive to the external Ca concentration. Increasing pHi by 10 mM NH4Cl enhanced this current, while the internal acidification that occurred on switching back to the control solution greatly reduced it. Variations in the amplitude of this current during repetitive stimulations or long pauses are best explained by subsequent alterations in Nai and pHi; no evidence for a time dependence was found. This current was inhibited by La3+, Co2+, and D600, and was sensitive to adriamycin, quinidine, and disopyramide; lidocaine, another local anesthetic, and nifedipine had no effect. These observations extend previous work on intact heart cells and sarcolemmal vesicles. They suggest that the Na-Ca exchange may generate a current that is outward when Ca ions are moving into the cell.

Factors influencing free intracellular calcium concentration in quiescent ferret ventricular muscle

The photoprotein aequorin was injected into cells of ferret papillary muscles to monitor the resting intracellular free Ca concentration [( Ca2+]i). Increasing the external Ca concentration [( Ca2+]o) increased both resting [Ca2+]i and resting tension. The tension and [Ca2+]i both rose to a peak and then declined to a steady-state level which was higher than the control. Qualitatively similar, but larger, effects were observed if [Ca2+]i was first elevated with strophanthidin. The increase of [Ca2+]i was accompanied by the development of spontaneous oscillations of [Ca2+]i. When a steady level of [Ca2+]i had been reached in high [Ca2+]o, [Ca2+]o was reduced back to the control level for a brief period. A subsequent increase of [Ca2+]o produced a rise of [Ca2+]i to the same steady level as that previously found in the high [Ca2+]o but the initial peak and subsequent decline were absent. It is suggested that the decline of [Ca2+]i from the initial peak is mediated by a fall of intracellular Na concentration [( Na+]i) limiting Ca entry on a Na-Ca exchange. Increasing external K concentration [( K+]o) from 5 to 30 mmol/l had no detectable effect on [Ca2+]i under control conditions. However, if [Ca2+]i was first increased either by applying strophanthidin or by increasing [Ca2+]o, increasing [K+]o produced a transient rise of [Ca2+]i and tension. This rise was unaffected by D600. It is suggested that the secondary decline of [Ca2+]i after the initial rise may, again, be produced by a fall of [Na+]i acting on an Na-Ca exchange. Acidification produced by increasing [CO2] had no detectable effect on [Ca2+]i under control conditions. However, if [Ca2+]i was increased by strophanthidin, acidification produced a rise of [Ca2+]i. This rise of [Ca2+]i was partly transient even when the intracellular acidification was presumably maintained (raising CO2 at constant [HCO3-]). Acidification in Na-free solutions had qualitatively similar effects to those in Na-containing solutions. In Na-free solutions (Na replaced by K) the [Ca2+]i could be maintained at a low level for at least several hours. Increases of [Ca2+]o in Na-free solutions led to a decrease of [Ca2+]i, and similarly decreasing [Ca2+]o led to an increase in [Ca2+]i. These anomalous effects of [Ca2+]o on [Ca2+]i could be abolished by Mn ions or D600. It is suggested that changes in [Ca2+]o may have reciprocal effects on Ca permeability and hence on [Ca2+]i. The application of the mitochondrial uncoupler FCCP in Na-free solutions led to an increase of resting tension followed, after a substantial delay, by an increase of [Ca2+]i.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for sodium-calcium exchange in the guinea-pig ureter

The effects of high-K and low-Na solutions on the smooth muscle of the guinea-pig ureter have been examined in both normal tissues, and tissues in which the Na pump had been blocked by exposure to K-free solutions or ouabain (high-Na tissues). Tension recording, membrane potential measurements and ion analysis were used. High-K solutions depolarize normal tissues, leading to action potential generation and phasic contractions followed, at concentrations greater than 20-30 mM, by cessation of action potentials and the development of a biphasic contracture which declines slowly during continuous exposure. The contracture is abolished by Ca-antagonist drugs, procaine and Ca-free solutions. Short exposures of normal tissues to Na-free solutions do not result in tension development. Longer exposures may initiate tension, depending on the Na substitute used. Sucrose causes depolarization of the cells and spike development associated with phasic contractions, superimposed on a small contracture; Li depolarizes the cells but causes no tension generation; Tris hyperpolarizes the cells and a small increase in basal tone may be seen. On exposure to K-free solutions or ouabain, the tissues do not develop significant tone but their response to short application of high-K solutions grows with time. The tissues also develop the ability to contract on short applications of low-Na solutions. The low-Na contractures are resistant to concentrations of Ca antagonists that abolish the K responses of normal tissues, but are abolished in Ca-free solutions. The ability of the tissues to contract in Na-free solutions is accompanied by an increase in intracellular Na and loss of intracellular K. Even after several hours' exposure to ouabain, however, the tissues still contain significant amounts of K and the membrane potential is the same as, or more negative than that in normal tissues. Therefore it appears that another mechanism, apart from the Na pump, can regulate intracellular Na. On continuous exposure to Na-free solutions, the contracture declines rapidly. The decline is associated with a loss of intracellular Na. The Na-free contracture is larger when K rather than Tris is used as the substitute. This difference persists in the presence of a concentration of Mn that abolishes the K contracture of normal tissues but is abolished by high concentrations (10 mM) of procaine.(ABSTRACT TRUNCATED AT 400 WORDS)

Sodium/calcium exchange in mammalian ventricular muscle: a study with sodium-sensitive micro-electrodes

A method for mounting and rapidly perfusing small ventricular trabeculae (diameter around 250 micron) from either ferret or guinea-pig is described. Tension, membrane potential (Em) and intracellular Na activity (aiNa) were measured. aiNa was measured using Na-sensitive micro-electrodes. At room temperature (22-26 degrees C), [Na]o 155 mmol/l and [Ca]o 5.4 mmol/l, aiNa was 10.9 mmol/l +/- S.D. 4.2 mmol/l (n = 148). When [Na]o was reduced from 155 to 1.5 mmol/l contractures developed. These were about twitch height in guinea-pig but less than the twitch height in ferret. Associated with the development of the contracture there was a decrease in aiNa. The aiNa halved within 30 s. The decrease in aiNa was not influenced by changing pHo from 7.4 to 9.5, K-free solution or strophanthidin 50 mumol/l and was not passive since, even when the [Na]o was 1.5 mmol/l, the driving force for Na ions remained inward. The aiNa decreased if [Ca]o was increased and [Na]o decreased or vice versa. On the basis of these findings it is concluded that the decrease in aiNa is mainly due to Na/Ca exchange. Despite the large decrease in aiNa the [Ca]i, as monitored by tension changes, hardly increased. Since Ca uptake does occur in Na-free conditions in heart muscle it is proposed that the mitochondria take up Ca ions and so prevent an excessive rise in cytoplasmic Ca. Strophanthidin increased both aiNa and the withdrawal contracture, but collected results from a number of experiments showed no clear correlation between the initial aiNa and contracture amplitude. Strophanthidin may, therefore, have actions additional to increasing aiNa.

Voltage- and frequency-dependent block of diltiazem on the slow inward current and generation of tension in frog ventricular muscle

The effects of a new "Ca2+ -antagonist", diltiazem, were studied in frog ventricular myocardium. The single sucrose gap voltage clamp technique was used to control membrane potential and measure membrane current and tension. Diltiazem had no effect on the resting potential, but reduced the slow inward current (Isi) and increased the net outward current. Membrane conductance measurements suggest that the increase in the total membrane current may be due to the suppression of a maintained inward current. The blocking action of diltiazem is frequency-dependent such that at higher frequencies of stimulation, the steady-state amplitude of twitch is reduced. The diltiazem-induced suppression of tension and Isi could be partially reversed by hyperpolarizing the surface membrane for brief intervals. These results suggest that diltiazem blocks Isi in a voltage- and time-dependent manner. These effects of diltiazem on Isi seem to be, in part, responsible for the tension-suppressant effect of the drug.

The control of tonic tension by membrane potential and intracellular sodium activity in the sheep cardiac Purkinje fibre

Intracellular Na activity (aiNa) was measured with recessed-tip, Na-selective micro-electrodes in voltage-clamped sheep cardiac Purkinje fibres. Tension was measured simultaneously. aiNa was increased reversibly either by exposing the preparation to K-free, Rb-free solution of by adding the cardioactive steroid strophanthidin. An increase of aiNa produced an increase of tonic tension which was larger at depolarized membrane potentials. At sufficiently negative membrane potentials, changes of aiNa (over the range 6-30 mM) had no effect on tonic tension. Therefore, both an increase of aiNa and a depolarization are required to increase tonic tension. It is concluded that either a low level of aiNa or a large negative membrane potential is sufficient to maintain a low intracellular Ca concentration. Tonic tension was measured as a function of aiNa. At a given membrane potential the relationship can be described empirically by an equation of the form: tonic tension = b(aiNa)y, where y is a constant and b depends on membrane potential. In five experiments y was found to be 3.7 +/- 0.7 (mean +/- S.E.M.) over a range of potentials from -60 to -10 mV. Tonic tension was measured as a function of membrane potential. At a given aiNa the relationship can be described approximately as: tonic tension = k exp (aV), where a is a constant and k depends on aiNa. In five experiments a was found to be 0.06 +/- 0.01 mV-1 (mean +/- S.E.M.). A depolarization of 10 mV increases tonic tension by the same amount as does an increase of aiNa that is equivalent to a 3.7 mV change of the Na equilibrium potential, ENa. Hence ENa is nearly 3 times more effective than membrane potential in controlling tonic tension. During a prolonged depolarization (several minutes) the initial increase of tonic tension decays gradually. This is associated with a fall of aiNa. The relationship between tonic tension and aiNa is similar to that seen when aiNa is increased by inhibiting the Na pump. It is concluded that the fall of aiNa is responsible for the decay of tonic tension. The changes of tonic tension reported in this paper are consistent with the effects of aiNa and membrane potential on a voltage-dependent Na-Ca exchange. The possibility that a voltage-dependent Ca channel contributes to tonic tension is also discussed.

Calcium efflux during the cold-induced contraction of mammalian striated muscle fibres

The efflux of 45Ca from mammalian slow twitch muscle fibres has been studied to provide a measure of the concentration of free Ca2+ in the sarcoplasm. The kinetically complex early phases of washout of the isotope are succeeded by a prolonged slower phase which exhibits first-order kinetics. This later phase is accelerated by caffeine, by preventing oxidative phosphorylation and also during an isometric contraction, whether this contraction is produced by lowering the temperature or by electrical stimulation. The local anaesthetic tetracaine abolishes the contraction caused by cold and in this case the rate constant for efflux is progressively lowered as the temperature is reduced (Q10 value of 2.3). The removal of external Na+ and Ca2+ reduces the efflux rate constant. Caffeine, sodium removal and the inhibition of oxidative phosphorylation, all potentiate the cold contraction and the associated extra 45Ca efflux. Ca removal causes the cold contraction to become phasic. It appears that caffeine, sodium removal, the inhibition of oxidative phosphorylation and a decrease in temperature to below 10 degrees C are all treatments which, like electrical stimulation, increase the sarcoplasmic free calcium concentration to varying degrees.

Intracellular calcium and sodium activity in sheep heart Purkinje fibres. Effect of changes of external sodium and intracellular pH

Intracellular Ca, Na and H selective microelectrodes were used to study the effects of reduction of the extracellular Na concentration, [Na]0, on the free intracellular Ca concentration, [Ca]i, Na activity, (aiNa), and intracellular pH (pHi) in sheep heart Purkinje fibres. 1. Reduction of [Na]0 from 140 mM to 14 mM produced a decrease of aiNa, and increase of free [Ca]i, and normally an increase of resting tension. 2. Inhibition of the Na-K pump by 10(-5) M acetyl-strophanthidin produced a slow rise of [Ca]i and resting tension. 3. The magnitude of the increase of free [Ca]i (and tension) produced by [Na]0 reduction was greatly enhanced when the Na-K pump is inhibited by either acetylstrophanthidin or K-free solutions. 4. We suggest that this enhanced rise of free [Ca]i in the presence of Na-K pump inhibition is due to Ca loading of intracellular Ca buffering systems during the pump inhibition. 5. Addition of NH4Cl produced a transient decrease of free [Ca]i that accompanied an alkaline change in pHi. Removal of NH4Cl (which produced a transient intracellular acidification) produced a transient increase of free [Ca]i. We conclude that a close relationship exists between the control of free [Ca]i and pHi which may be due to competition at, or common use of, intracellular buffering systems.

Effect of potassium depolarization on sodium-dependent calcium efflux from goldfish heart ventricles and guinea-pig atria

1. (45)Ca fluxes were studied in normal and potassium-depolarized goldfish ventricles as a function of the external Na concentration. Some of the experiments were also performed on guinea-pig auricles.2. When the external K concentration was increased from 5.4 to 142 mM, keeping osmolarity constant by adding 137 mM-Li or choline (hyperosmotically) to the low K solution, the (45)Ca efflux was reversibly inhibited, whereas the [(3)H]sucrose efflux was unaffected.3. Goldfish ventricles, which have been depolarized with 142 mM-K for 100 min, repolarized within 20 min, from ca. -15 mV to ca. -70 mV, following the application of 5.4 mM-K. This repolarization was independent of the presence of external Na. During the repolarization the (45)Ca efflux was reactivated. This reactivation, however, depended on the external Na concentration. Comparable results were obtained in guinea-pig atria.4. A similar repolarization and Na-dependent reactivation of (45)Ca efflux was obtained in goldfish ventricles superfused with 10(-6) M-Ca(2+) (4.5 mM-Ca, 5 mM-EGTA, pH 7.1), provided that the (45)Ca washout was started in high K.5. In 10(-6) M-Ca(2+), 137 mM-Na, 5.4 mM-K and 137 mM-choline goldfish ventricles depolarized to about -25 mV within 80 min. If the choline was now replaced by 137 mM-K, the membrane potential moved to ca. -15 mV, and under these conditions the (45)Ca efflux was slightly increased.6. Following Na-free perfusion for 100 min, and at normal external Ca concentrations, the (45)Ca efflux from goldfish ventricles was stimulated by the addition of Na. The curve relating this stimulation to the external Na concentration had a sigmoidal shape and was shifted to the right by K-depolarization. In guinea-pig atria the inhibition of the Na-stimulated Ca efflux by depolarization was of a non-competitive type.7. Following a Na-free incubation of 100 min and a subsequent period of 20 min in 137 mM-Na, the intracellular Na content of goldfish ventricular cells was some 20% lower in K-depolarized cells than in cells at the resting potential.8. (45)Ca influx in goldfish ventricles in the presence of 137 or 68.5 mM-Na was not significantly changed by K-depolarization.9. The results show that the Na-dependent fraction of Ca efflux is inhibited by high external K. The effect is probably due to depolarization, which may be an argument in favour of electrogenic n Na(+)-1 Ca(2+) exchange, with n >/= 3.

Voltage-clamp studies of transient inward current and mechanical oscillations induced by ouabain in ferret papillary muscle

1. We studied the effects of a toxic concentration of ouabain on transmembrane electrical activity and on mechanical behaviour of right ventricular papillary muscles from ferrets in a single sucrose-gap using current clamp and voltage clamp.2. Ouabain (1.4-1.8 muM) induced oscillatory after-potentials and after-concentrations in current-clamp experiments. Voltage clamp showed that the oscillatory after-potential was caused by a transient inward current, similar to that in Purkinje fibres.3. The transient current had a sigmoidal dependence on the preceding (activating) voltage step V1, with a treshold around -13 mV and a plateau between +10 and 20 mV. There was a decline in current amplitude for more positive clamps. When activated by a fixed V1 voltage step, and measured at different repolarization levels V2, the transient current manifested an inverse dependence on V2 between -50 and -10 mV. No outward transient current could be detected. Total replacement of Na in the bathing medium by Tris or by sucrose abolished the transient current.4. Ouabain caused an increase of phasic (twitch) tension responses to voltage steps at all potentials without shifting the curve relating these variables on the voltage axis. The drug evoked an even greater increase in the tonic tension responses.5. After prolonged exposure, oscillatory mechanical responses were frequently recorded during positive voltage steps. Unlike the after-contraction, these mechanical fluctuations were not consistently damped and were not accompanied by detectable synchronous current fluctuations. Catecholamines and dibutyryl cyclic AMP markedly reduced the amplitude of the tonic contraction and the mechanical oscillations but increased their frequency. Caffeine had no effect on the tonic contraction amplitude but abolished the fluctuations.6. These results support the proposal that Ca is transiently released from the overloaded sarcoplasmic reticulum in ouabain-intoxicated muscle and may evoke oscillatory responses in nearby contractile fibrils. When these transient increases of sarcoplasmic free Ca are large enough, they may induce the transient transmembrane current described above.

Parameters affecting the slow inward channel repriming process in frog atrium

1. The time of recovery (from the inactivation) of the slow inward current was studied in the frog atrium, using the double sucrose gap voltage clamp technique. 2. The 'repriming' process was found to be distinct from the current inactivation, and to depend on experimental protocol: double pulses given at low frequencies (at 'rest') gave a faster recovery time when compared to recovery during constant stimulation, with interposed stimuli monitoring the recovery. Longer durations of the clamp pulses led to a faster recovery process. 3. Changing the holding potential of the membrane (with double pulses to the same absolute membrane potential monitoring the recovery process) greatly affect the repriming with depolarized levels slowing down the process. 4. The recovery time was fastest following clamp pulses to intermediate membrane potentials (in the plateau range). This was determined by double pulses, from a constant hold potentials, to different levels. 5. Decreasing extracellular Ca prolonged, and increasing Ca enhanced the recovery process. 6. The recovery process was markedly slowed down in Na or in K-free solutions. 7. The recovery process was enhanced in solutions with a raised concentration of Mg or H ions (lower pH). In higher Mg solutions, the inactivation of the slow inward current was slower. 8. It is proposed that the recovery process is sensitive to alterations in intracellular Ca ions and to variations in extracellular surface charges. The possible implications are discussed.