The mechanical activity of chick embryonic myocardial cell aggregates

Predicting the onset of period-doubling bifurcations in noisy cardiac systems

Biological, physical, and social systems often display qualitative changes in dynamics. Developing early warning signals to predict the onset of these transitions is an important goal. The current work is motivated by transitions of cardiac rhythms, where the appearance of alternating features in the timing of cardiac events is often a precursor to the initiation of serious cardiac arrhythmias. We treat embryonic chick cardiac cells with a potassium channel blocker, which leads to the initiation of alternating rhythms. We associate this transition with a mathematical instability, called a period-doubling bifurcation, in a model of the cardiac cells. Period-doubling bifurcations have been linked to the onset of abnormal alternating cardiac rhythms, which have been implicated in cardiac arrhythmias such as T-wave alternans and various tachycardias. Theory predicts that in the neighborhood of the transition, the system's dynamics slow down, leading to noise amplification and the manifestation of oscillations in the autocorrelation function. Examining the aggregates' interbeat intervals, we observe the oscillations in the autocorrelation function and noise amplification preceding the bifurcation. We analyze plots--termed return maps--that relate the current interbeat interval with the following interbeat interval. Based on these plots, we develop a quantitative measure using the slope of the return map to assess how close the system is to the bifurcation. Furthermore, the slope of the return map and the lag-1 autocorrelation coefficient are equal. Our results suggest that the slope and the lag-1 autocorrelation coefficient represent quantitative measures to predict the onset of abnormal alternating cardiac rhythms.

A common mechanism for concurrent changes of diastolic muscle length and systolic function in intact hearts

Mechanical properties of the myocardium at end diastole have been thought to be dominated by passive material properties rather than by active sarcomere cross-bridge interactions. This study tested the hypothesis that residual cross-bridges significantly contribute to end-diastolic mechanics in vivo and that changes in end-diastolic cross-bridge interaction parallel concurrent changes in systolic cross-bridge interaction. Open-chest anesthetized pigs were treated with intracoronary verapamil (n = 7) or 2,3-butanedione monoxime (BDM; n = 8). Regional left ventricular external work and end-diastolic pressure (EDP) versus end-diastolic segment length (EDL) relations were determined in the treated and untreated regions of each heart. Both agents reduced external work of treated regions to 31-38% of baseline and concurrently shifted EDP versus EDL relations to the right (i.e., greater EDL at a given EDP) by an average of 5% (P < 0.05). During washout of the drugs, EDP versus EDL returned to baseline in parallel with recovery of external work. Sarcomere length, measured by transmission electron microscopy in BDM-treated and untreated regions of the same hearts after diastolic arrest and perfusion fixation, was 8% greater in BDM-treated regions (P < 0.01). We concluded that residual diastolic cross-bridges significantly and reversibly influence end-diastolic mechanics in vivo. Alterations of end-diastolic and systolic cross-bridge interactions occur in parallel.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

The Lack of Toxicity of Potassium ChannelActivators in Heart Cell Cultures

High doses of the potassium channel activators (KCAs) BRL 44269, levcromakalin and pinacidil in a number of laboratory animal species cause a profound reduction in blood pressure which results in reflex tachycardia, ischaemia and myocardial necrosis. Thus, it is considered that the in vivo cardiac pathology seen with KCAs is an indirect effect as a consequence of excessive pharmacological effects rather than direct myocardial toxicity. This hypothesis was tested, in vitro, in the chick embryonic myocardial myocyte reaggregate (MMR) model system. Changes in spontaneous beating activity (SBA), leakage of lactate dehydrogenase (LDH) and cell morphology by light and transmission electron microscopy were used to assess toxicity. The MMRs were cultured for up to 24hr in a series of different concentrations of the three KCAs in the range 1-10,000mum. In addition to an untreated control, allylamine (50mum), a known direct acting myocardial cytotoxin, was used as a positive control. Incubation with allylamine caused clear evidence of toxicity and permanent cessation of SBA. In contrast, KCAs caused changes in SBA and significant toxicity was only seen at the highest concentration (10,000mum) of BRL 44269. These results are supportive of the view that KCA-induced cardiac pathology in vivo is due to an indirect pharmacological action rather than a direct, cytotoxic mechanism.

Study of the effects of cardiovascular drugs in heart cell cultures

Compounds that produce myocardial pathology in vivo can be separated into two main classes-those that are directly toxic to the myocardium and those that are considered to act by way of an indirect vascular or neurologically based mechanism. An in vitro model of myocardium without nervous or systemic influences can be used to differentiate between direct myocardial cytotoxic effects and indirect cardiac pathology arising in vivo from exaggerated vascular or neural pharmacological effects of a number of drugs. In this study direct-acting cardiotoxic compounds are distinguished from those causing cardiac pathology by indirect mechanisms by their different pattern of effects in chick embryonic myocardial myocyte reaggregates (MMRs) cultures. The toxicity of the direct-acting cardiotoxic drugs allylamine (positive control, 50 mum) and doxorubicin were compared with digoxin and isoprenaline, which show both direct and indirect mechanisms in vivo, and the indirectly acting hydralazine and pinacidil. Changes in spontaneous beating activity (SBA), leakage of lactate dehydrogenase (LDH) and cell morphology by light and transmission electron microscopy were used to assess toxicity. The MMRs were cultured for up to 24 hr in a series of concentrations of the five compounds in the range 0.1 to 10,000 mum. Allylamine, doxorubicin, digoxin and, to a lesser extent, isoprenaline were highly toxic to the MMRs, as shown by alterations in SBA, LDH leakage and cellular morphology. In contrast, hydralazine showed a very mild degree of toxicity at the highest concentrations in the absence of LDH leakage; treatment with pinacidil did not show any evidence of morphological degeneration but did cause a dose-related inhibition of SBA. These results are consistent with the view that doxorubicin and digoxin are directly toxic to myocardial cells and also suggests that this is an important mechanism in vivo for isoprenaline. The absence of a significant degree of toxicity with hydralazine and pinacidil is consistent with an indirect toxic mechanism.

Effects of chronic mitral regurgitation on diastolic function in isolated cardiocytes

We have previously shown that chronic mitral regurgitation (MR) increases the rate of left ventricular early diastolic filling. These changes in chamber diastolic function were felt to be secondary to alterations in left ventricular loading conditions. Therefore, cellular diastolic function measured in cardiac muscle cells (cardiocytes) isolated from animals with chronic MR (absent alterations in loading conditions) was expected to be normal. However, chronic MR caused a decrease in sarcomere lengthening rate. The purpose of the current study was to define the mechanisms causing this decreased sarcomere lengthening rate in chronic MR cardiocytes and to explain the apparent dichotomy between chamber and cellular diastolic properties. Accordingly, sarcomere motion was measured using laser diffraction techniques in enzymatically isolated cardiocytes from seven control dogs and 11 dogs with chronic MR (produced by closed-chest transection of the mitral chordae). In the MR cardiocytes, there were abnormalities in cellular systolic function (decreased extent and velocity of shortening) and in cellular diastolic function (decreased velocity of sarcomere lengthening). Because studies in papillary muscles have shown that there is a direct relation between abnormal diastolic function (decreased velocity of muscle lengthening) and abnormal systolic function (decreased extent of muscle shortening), it was unclear whether the changes in cellular relaxation rate observed in chronic MR merely reflected a concomitant decrease in the extent of shortening or instead reflected an impairment in intrinsic relaxation properties. To make this distinction, the relation between relaxation velocity (measured as peak sarcomere lengthening rate) and sarcomere shortening extent was examined in MR cardiocytes and compared with that in control cardiocytes. There was a direct relation between sarcomere relaxation velocity and sarcomere shortening extent in both control and MR cardiocytes. Over a wide range of shortening extent, the slopes and y intercepts of this relation were similar in control and MR cardiocytes (slope, 27.7 sec-1 in control cells versus 28.1 sec-1 in MR cells; y intercept, -1.1 microns/sec in control cells versus -1.7 microns/sec in MR cells; p = NS). At any common shortening extent, relaxation velocity was the same in control and MR cardiocytes. To prove that this relation could detect abnormalities in the intrinsic myocardial relaxation process, interventions known to produce primary alterations in the intrinsic myocardial relaxation process were examined: the effects of hypothermia (30 degrees C) and isoproterenol (10(-6) M) on the relaxation velocity-shortening extent relation were studied in normal and MR cardiocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Cytosolic calcium and myofilaments in single rat cardiac myocytes achieve a dynamic equilibrium during twitch relaxation

1. Single isolated rat cardiac myocytes were loaded with either the pentapotassium salt form or the acetoxymethyl ester (AM) form of the calcium-sensitive fluorescent probe, Indo-1. The relationship of the Indo-1 fluorescence transient, an index of the change in cytosolic calcium [Ca2+]i concentration, to the simultaneously measured cell length during the electrically stimulated twitch originating from slack length at 23 degrees C was evaluated. It was demonstrated that even if the Ca2+ dissociation rate from Indo-1 was assumed to be as slow as 10 s-1, the descending limb ('relaxation phase') of the Indo-1 fluorescence transient induced by excitation under these conditions is in equilibrium with the [Ca2+]i transient. Additionally, the extent of Indo-1 loading employed did not substantially alter the twitch characteristics. 2. A unique relationship between the fluorescence transient and cell length was observed during relaxation of contractions that varied in amplitude. This was manifest as a common trajectory in the cell length vs. [Ca2+]i phase-plane diagrams beginning at the time of cell relengthening. The common trajectory could also be demonstrated in Indo-1 AM-loaded cells. The Indo-1 fluorescence-length relation defined by this common trajectory is steeper than that described by the relation of peak contraction amplitude and peak fluorescence during the twitch contractions. 3. The trajectory of the [Ca2+]i-length relation elicited via an abrupt, rapid, brief (200 ms) pulse of caffeine directly onto the cell surface or by 'tetanization' of cells in the presence of ryanodine is identical to the common [Ca2+]i-length trajectory formed by electrically stimulated contractions of different magnitudes. As the [Ca2+]i and length transients induced by caffeine application or during tetanization in the presence of ryanodine develop with a much slower time course than those elicited by electrical stimulation, the common trajectory is not fortuitous, i.e. it cannot be attributed to equivalent rate-limiting steps for the decrease of [Ca2+]i and cell relengthening. 4. The [Ca2+]i-length relation defined by the common trajectory shifts appropriately in response to perturbations that have previously been demonstrated to alter the steady-state myofilament Ca2+ sensitivity in skinned cardiac fibres. Specifically, the trajectory shifts leftward in response to an acute increase in pH or following the addition of novel myofilament calcium-sensitizing thiadiazinone derivatives; a rightward shift occurs in response to an acute reduction in pH or following the addition of butanedione monoxime.(ABSTRACT TRUNCATED AT 400 WORDS)

Paradoxal pharmacologic effects observed with beta-blocker agents on cardiac cells in culture

The direct effects of propranolol and metoprolol were studied on cultured neonatal rat ventricular myocytes by recording cellular contraction with a video signal analyzer of cells movements. The experiments were performed on 3-d-old cultures forming a synchronously beating monolayer. The spontaneous beating frequency of preparations depended on cultures and varied from 100 to 330 beats/min with a peak for the interval 140 to 149 beats/min. Propranolol and metoprolol were tested at 1, 5, 10 microM and 10, 50, 100 microM, respectively. At these doses the two beta-blockers induced chronotropic and inotropic effects in the same range. The two agents induced rapid and short-lasting negative inotropic effects, which were dose-dependent and delayed negative chronotropic effects even with the lowest doses. In some preparations paradoxal effects were evidenced: an increase of the amplitude or frequency of the contractions was observed. The results obtained with 5 microM propranolol or 50 microM metoprolol could be separated in two groups depending on the basal beating rate (less than or greater than to 150 beats/min). In cultures with a rapid frequency, the drugs had a marked negative chronotropic effect and, surprisingly, a positive inotropic effect was observed. These findings confirm the interdependence of the two parameters frequency and amplitude of contraction on this model, and evidence the interest of taking into account the control parameters before any interpretation.

Effect of platelet release products on cytosolic calcium in cardiac myocytes

The effect of platelet release products on cytosolic calcium [( Ca++]i) was examined by monitoring the fluorescence of chick embryonic heart cells loaded with the fluorescent calcium indicator indo-1 AM. Cell free filtrate of platelet release products was obtained from rabbit platelets activated with thrombin or collagen. This filtrate caused a rapid increase in both systolic and diastolic [Ca++]i in a dose-dependent manner. The effect was not blocked by pretreating the platelets with aspirin or a thromboxane synthetase inhibitor. It was not mimicked by a thromboxane analog, or by several substances known to be released from platelets including ADP, serotonin, or platelet activating factor. Apyrase or ATP-gamma S had no effect on the activity. The responsible product was heat-sensitive, trypsin-sensitive, and partitioned into the aqueous phase of a chloroform suspension. It has a low molecular weight (less than 3kD) and is sensitive to 2-mercaptoethanol. Protease inhibitor appears to prolong the activity. These results suggest that trypsin-sensitive peptide(s) released from activated platelets can increase [Ca++]i in cardiac cells.

Effect of thrombin on calcium homeostasis in chick embryonic heart cells. Receptor-operated calcium entry with inositol trisphosphate and a pertussis toxin-sensitive G protein as second messengers

Thrombin increases intracellular calcium ([Ca++]i) in several cell types and causes a positive inotropic effect in the heart. We examined the mechanism of the thrombin-induced [Ca++]i increase in chick embryonic heart cells loaded with the fluorescent calcium indicator, indo-1. Thrombin (1 U/ml) increased both systolic and diastolic [Ca++]i from 617 +/- 62 and 324 +/- 46 to 1041 +/- 93 and 587 +/- 38 nM, respectively. An initial rapid [Ca++]i increase was followed by a more sustained increase. There were associated increases in contraction strength, beat frequency, and action potential duration. The [Ca++]i increase was not blocked by tetrodotoxin or verapamil, but was blocked by pretreatment with pertussis toxin (100 ng/ml). The thrombin-induced [Ca++]i increase was partly due to intracellular calcium release, since it persisted after removal of external calcium. The [Ca++]i increase in zero calcium was more transitory than in normal calcium and was potentiated by 10 mM Li+. Thrombin also induced influx of calcium across the surface membrane, which could be monitored using Mn++ ions, which quench indo-1 fluorescence when they enter the cell. Thrombin-induced Mn++ entry was insensitive to verapamil, but was blocked by 2 mM Ni++. Thrombin increased inositol trisphosphates by 180% at 90 s and this effect was also blocked by pretreatment with pertussis toxin.

CONCLUSION

thrombin promotes calcium entry and release in embryonic heart cells even when action potentials are inhibited. Both modes of [Ca++]i increase may be coupled to the receptor by pertussis toxin-sensitive G proteins.

RS 30026: a potent and effective calcium channel agonist

1. A series of dihydropyridine derivatives has been evaluated for calcium channel agonist activity using reversal of nisoldipine-induced inhibition of beating of aggregates of embryonic chick myocytes. This test appears to be specific for calcium channel agonists since isoprenaline and cardiac glycosides are inactive. 2. RS 30026 was the most potent of the series, was significantly more potent than CGP 28392 and of similar potency to Bay K 8644 (pEC50 = 7.45, 6.16 and 7.20, respectively). RS 30026 increased edge movement of individual aggregates, in the absence of nisoldipine, by 50% at 2 nM. 3. Compounds were also evaluated for their effects on guinea-pig papillary muscle and porcine coronary artery rings. RS 30026 displayed positive inotropism at concentrations between 10(-9) and 10(-6) M (pEC200 = 8.21), but was a much more powerful inotrope than Bay K 8644, increasing contractility to 1300% of control at 10(-6) M (compared to 350% of control for Bay K 8644). RS 30026 caused vasoconstriction at concentrations between 10(-10) and 10(-7) M. 4. Calcium channel currents in single embryonic chick myocytes were recorded by whole-cell voltage clamp techniques. RS 30026 (100 nM-500 nM) produced large increases in peak current amplitude and shifted the voltage for threshold and maximal currents to more negative values. RS 30026 (500 nM) also produced large increases in the inward tail currents evoked upon repolarization. The effects of Bay K 8644 (50 and 500 nM) were much less marked. 5. Analysis of the activation characteristics of currents showed parallel shifts in the activation curve to more negative potentials in the presence of 50 nm Bay K 8644, with a much smaller shift in the presence of 500nm Bay K 8644. RS 30026 (100 and 500nM) caused concentration-dependent shifts in the activation of the calcium channel currents with an increase of the slope of the curve. 6. RS 30026 appears to be the most potent and effective calcium channel agonist described to date.

Coregulation of calcium channels and beta-adrenergic receptors in cultured chick embryo ventricular cells

To examine mechanisms whereby the abundance of functional Ca channels may be regulated in excitable tissue, Ca channel number was estimated by binding of the dihydropyridine (DHP) antagonist 3H (+)PN200-110 to monolayers of intact myocytes from chick embryo ventricle. Beta adrenergic receptor properties were studied in cultured myocytes using [3H]CGP12177, an antagonist ligand. Physiological correlates for alterations in DHP binding site number included 45Ca uptake and contractile response to (+)BAYk 8644, a specific L-type Ca channel activator. All binding and physiological determinations were performed in similar intact cell preparations under identical conditions. 4-h exposure to 1 microM isoproterenol reduced cell surface beta-adrenergic receptor number from 44 +/- 3 to 17 +/- 2 fmol/mg (P less than 0.05); DHP binding sites declined in number from 113 +/- 25 to 73 +/- 30 fmol/mg (P less than 0.03). When protein kinase A was activated by a non-receptor-dependent mechanism, DHP binding declined similarly to 68% of control. Exposure to diltiazem, a Ca channel antagonist, for 18-24 h had no effect on number of DHP binding sites. After 4-h isoproterenol exposure, 45Ca uptake stimulated by BAYk 8644 declined from 3.3 +/- 0.2 nmol/mg to 2.9 +/- 0.3 nmol/mg (P less than 0.01) and BAYk 8644-stimulated increase in amplitude of contraction declined from 168 +/- 7 to 134 +/- 11% (P = 0.02). Thus, elevation of [cAMP] in myocytes is associated with a time-dependent decline in Ca channel abundance as estimated by DHP binding and a decline in physiological responses that are in part dependent on abundance of Ca channels. Binding of a directly acting Ca channel antagonist for 18-24 h does not modulate the number of DHP binding sites.

Interaction of palmitoyl carnitine with calcium antagonists in myocytes

1. Beating of aggregates of embryonic chick myocytes, in primary culture, was quantified by use of a motion-detector and video-recorder technique. Interactions of palmitoyl carnitine, a putative endogenous ligand at Ca2+ channels, with calcium antagonists were investigated. 2. Bay K 8644 (1-100 nM) and palmitoyl carnitine (0.2-30 microM) increased edge movement of the aggregates; beats fused so that there was an increase in baseline 'tone'. The concentrations required to produce a 50% increase in edge movement were 2.5 nM for Bay K 8644 and 2 microM for palmitoyl carnitine. Higher concentrations (20-30 microM) of palmitoyl carnitine caused tachycardia of abrupt onset but resulted in cessation of beating. The effects of palmitoyl carnitine were not stereo-selective in that the (+)- and (-)-isomers were equieffective. Lysophosphatidyl choline (LPC) had no effect in concentrations up to 10 microM but higher concentrations caused tachycardia followed by cessation of beating. High concentrations of both palmitoyl carnitine and LPC (100 microM) caused break-up of the aggregates, presumably as a result of detergent effects. 3. Palmitoyl carnitine (1-100 microM) reversed the inhibitory effects of nisoldipine (0.3 microM), diltiazem (10 microM) and verapamil (1 microM). Ouabain was ineffective in reversing the effects of nisoldipine, differentiating the effects of palmitoyl carnitine from those of Na+/K+ ATPase inhibition. In contrast, palmitoyl carnitine did not reverse the inhibitory effects of pimozide (2 microM) or lidoflazine (7 microM); palmitoyl carnitine showed a similar profile to Bay K 8644 in this respect. 4. These findings indicate that the effects of palmitoyl carnitine closely resemble those of Bay K 8644 and can be differentiated from those of lysophospholipids. As palmitoyl carnitine accumulates in the sarcolemma during myocardial ischaemia, the mode of action in the Ca2 + channel may have clinical relevance for the use of calcium antagonists in ischaemia.

The effects of calcium antagonists on calcium overload contractures in embryonic chick myocytes induced by ouabain and veratrine

1. The protective effects of some calcium antagonists against different forms of calcium overload contracture were investigated in embryonic chick cardiac myocytes. 2. Tetrodotoxin-sensitive sodium currents were recorded from the myocytes by the whole-cell voltage-clamp technique. Although the peak current was attenuated by veratrine, the inactivation process was markedly inhibited, resulting in a large increase in the total inward current. Action potentials were prolonged by veratrine, automaticity was inhibited and the membrane potential depolarized from -79 to around -45 mV. 3. Measurements of contraction were made from aggregates of myocytes using a video edge detection technique which quantified edge movement. Veratrine caused an initial positive inotropism then inhibited automaticity of aggregates with subsequent development of a tonic contracture to around 300% of the twitch contraction. 4. Veratrine-induced contractures were not significantly affected by 10 microM diltiazem or verapamil. Nifedipine (5 microM), nimodipine (5 microM) and ryanodine (5 microM) also had little effect whilst nicardipine and flunarizine caused a concentration-dependent inhibition of veratrine-induced contractures with IC50s of 3 microM and 2 microM respectively. 5. Veratrine-induced contractures were found to be very sensitive to extracellular calcium concentration with an EC50 of 32 microM. Edge movement associated with beating of the myocytes was much less sensitive to calcium (EC50 = 1 mM). Submaximal veratrine contractures in 20-50 microM extracellular calcium were not potentiated by 1 microM Bay K 8644. 6. Tetrodotoxin also inhibited veratrine-induced contractures but did not affect contractions induced by ouabain in the presence of 10 microM diltiazem. 7. Ouabain-induced contractures were also inhibited by nicardipine and flunarizine indicating that these drugs can protect against calcium overload in embryonic chick heart by a mechanism independent of the normal form of voltage-sensitive sodium or calcium channels.

Effect of Bay K8644 on cytosolic calcium transients and contraction in embryonic cardiac ventricular myocytes

Cytosolic calcium transients were recorded from spontaneously beating chick embryonic myocardial cell aggregates loaded with the fluorescent [Ca2+]i indicator, indo-1. Calcium transients rose rapidly from an end-diastolic [Ca2+]i of 230 +/- 18 nM to a peak systolic [Ca2+]i of 619 +/- 34 nM (n = 21). Relaxation of the transients was slow, and continued throughout diastole. Bay K8644 (0.5 microM) markedly prolonged the action potential and caused similar prolongation of the calcium transients. Calcium transients in the presence of Bay K8644 had an inflection on their rising phase, which was followed by a more gradual increase that continued until the membrane had repolarized to a negative potential of -15 to -30 mV. Bay K8644 caused marked elevation of peak systolic [Ca2+]i to 955 +/- 56 nM (P less than 0.002), with concomitant elevation of end-diastolic [Ca2+]i to 400 +/- 36 nM (P less than 0.002). Optical recordings of contraction showed changes similar to those in the calcium transient: the initial upstroke of the contraction was followed by a more gradual second component, which gave the contraction a "half-dome" appearance. The time to peak [Ca2+]i and the time to peak contraction increased linearly with action potential duration (APD50). The effects of Bay K8644 were simulated, in part, by CsCl (7.5 mM), which produced equivalent prolongation of the action potential and calcium transients. However, CsCl did not elevate diastolic [Ca2+]i. To determine the mechanism of the diastolic [Ca2+]i increase, Bay K8644 was applied to aggregates rendered quiescent by tetrodotoxin. Bay K8644 caused a graded increase in [Ca2+]i, which was followed by resumption of spontaneous beating.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of calcium channel antagonists and facilitators on beating of primary cultures of embryonic chick heart cells

1. Primary aggregate cultures of embryonic chick heart have been used to investigate the effects of calcium channel antagonists and facilitators on myocardial contractility. 2. The number of aggregates showing movement was inhibited in a concentration-dependent manner by calcium antagonists from different subgroups with negative log concentrations inhibiting movement in 50% of aggregates as follows: Class 1-nisoldipine (7.20); Class 2-verapamil (6.36), diltiazem (5.83); Class 3-lidoflazine (5.68), pimozide (6.25). 3. The effects of the dihydropyridine facilitators Bay K 8644 and CGP 28392 on aggregate beating were investigated by evaluating the interaction between calcium channel facilitators and antagonists from the three subgroups of calcium antagonists. Concentrations of antagonists that inhibited beating in 85% of aggregates were used. Both Bay K 8644 and CGP 28392 reversed nisoldipine-, diltiazem- or verapamil-induced inhibition of beating. 4. Bay K 8644 was approximately 10 times more potent than CGP 28392 in reversing nisoldipine-, diltiazem- or verapamil-induced inhibition of beating. 5. For each facilitator the concentrations causing 50% reversal of inhibition of aggregate beating against the three antagonists were similar. There was little evidence for differential modulation by verapamil or diltiazem of the action of the dihydropyridine facilitators. 6. Bay K 8644 did not reverse lidoflazine- or pimozide-induced inhibition of beating, indicating that these drugs may act at a site distinct from the dihydropyridine site on the calcium channel.

Cytosolic calcium staircase in cultured myocardial cells

To elucidate the mechanism of the tension staircase, chick embryonic myocardial cell aggregates were loaded with the fluorescent cytosolic calcium indicator, indo 1. Fluctuations in indo 1 fluorescence were compared with recordings of cell edge movement during spontaneous beating and during stimulation by intracellular current pulses. Indo 1-loaded aggregates exhibit fluorescence transients during each transmembrane action potential. The rising phase of the transients is rapid, but the decaying phase is slow (several hundred msec) and is similar in time course to the pandiastolic relaxation seen in the optical recordings of cell edge movement. Acceleration of beat frequency by brief depolarizing current pulses produces an ascending staircase in both edge movement and systolic [Ca2+]i. There is a similar staircase in the diastolic [Ca2+]i, which is also reflected by diastolic cell edge movement. The existence of a diastolic [Ca2+]i staircase may provide new insight into the mechanism of the force-frequency relation in the heart.

Cytosolic calcium transients from the beating mammalian heart

To elucidate the role of cytosolic calcium, [Ca2+]i, in the physiology of the normal and ischemic heart, we have developed a method for recording [Ca2+]i transients from the epicardial surface of the rabbit ventricle after arterial perfusion with the cell-permeant cytosolic calcium indicator indo-1 AM. Hearts were illuminated at 360 nm, and fluorescence was recorded simultaneously at 400 and 550 nm. The F400/F550 fluorescence ratio was calculated by an analog circuit that allowed cancelation of small movement artifacts that were present at single wavelengths. Clear [Ca2+]i transients were present in the F400/F550 signal and were remarkable for their slow decay. Slow decay of the transients was not due to buffering of [Ca2+]i by indo-1, since there was no associated impairment of contraction or relaxation. The peak amplitude of the [Ca2+]i transients was increased by ouabain, adrenaline, postextrasystolic potentiation, and acetylcholine. The extent to which the transients decayed diminished with shortening of the interbeat interval, but decay of the transients could be further diminished by acetylcholine or caffeine. A major advantage of the intact heart over isolated myocytes is the ability to measure changes in [Ca2+]i during ischemia. Ischemia produced a marked increase in both peak systolic and end-diastolic [Ca2+]i, which was most rapid during the first 30 sec, and approached a plateau value after 90 sec. This increase in [Ca2+]i was associated with a characteristic broadening of the peak of the transient. The increase in [Ca2+]i during ischemia is consistent with a proposed causative role of [Ca2+]i in mediating early electrophysiological abnormalities.

Evidence for distinct calcium channel agonist and antagonist binding sites in intact cultured embryonic chick ventricular cells

To determine whether calcium channel agonists and antagonists bind to distinct pharmacologically active sites, the binding of dihydropyridine calcium channel agonists and antagonists was related to calcium flux and contractile state in primary monolayer cultures of spontaneously contracting chick embryo ventricular cells. Equilibrium binding studies using the antagonist (+)-[3H]PN200-110 demonstrated equilibrium binding to intact, beating cells consistent with a single class of binding sites (KD, 1.1 nM; Bmax, 40 fmol/mg protein). Membrane depolarization of the intact cells by incubation in 30 mM potassium caused a 91% increase in the apparent number of (+)-PN200-110 binding sites (Bmax 76 fmol/mg protein), but no significant change in the KD (1.2 nM). The (+)-PN200-110 produced a concentration-dependent decrease in calcium uptake (IC50 2.2 nM) and contractile amplitude (IC50 5.6 nM). The calcium channel agonist, (+/-)-[3H]BAY k 8644, bound to two distinct binding sites with high affinity (KD 1.0 nM) and low affinity (KD 1.9 microM). The (+/-)-BAY k 8644 produced biphasic modulation of calcium flux and contractile state. At concentrations of 100 nM or less, (+/-)-BAY k 8644 increased calcium flux and contractile amplitude, consistent with drug interaction with the high affinity agonist site. However, at higher concentrations, the stimulatory effect of (+/-)-BAY k 8644 on calcium flux and contractile amplitude was abolished, a finding that is consistent with drug interaction with the low affinity antagonist site.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ryanodine and caffeine on contractility, membrane voltage, and calcium exchange in cultured heart cells

To investigate the mechanisms of action of ryanodine and caffeine, changes in mechanical and electrical activity caused by these agents were correlated with alterations in 45Ca fluxes and cell Ca contents in chick embryo ventricular cell monolayer cultures. Ryanodine (10(-10)-10(-5) M) irreversibly decreased contraction amplitude by 10-70% relative to control in a concentration-dependent manner with minimal effects on electrical activity. Ryanodine caused a slight decrease in rapid 45Ca uptake, but no change in total exchangeable calcium content or rapid 45Ca efflux. Caffeine (1-20 mM) caused a transient (less than 10 seconds) 5-12% increase in contraction amplitude followed by a sustained 9-76% decrease in contraction amplitude and a 10 mV decrease in diastolic membrane voltage. Caffeine caused a decrease in rapid 45Ca uptake, a decrease in total exchangeable calcium content, and an increase in rapid 45Ca efflux. These results suggest that caffeine produces a decrease in sarcoplasmic reticulum (SR) Ca2+ uptake, and/or an increase in SR Ca2+ release that eventually depletes the SR of Ca2+, presumably accounting for the negative inotropic effect. The ryanodine effects on contraction are more difficult to account for solely in terms of alterations of transsarcolemmal Ca2+ fluxes and Ca2+ contents. Our data indicate an important role for the SR in excitation-contraction coupling in cultured chick embryo ventricular cells and suggest that SR Ca2+ is part of the rapidly exchanging Ca2+ compartment noted in 45Ca flux studies.

Na+/Ca2+ exchange in cardiac myocytes. Effect of ouabain on voltage dependence

Sarcolemmal sodium/calcium exchange activity was examined in individual chick embryonic myocardial cell aggregates that were loaded with quin 2. The baseline [Ca2+]i was 68 +/- 4 nM (n = 29). Abrupt superfusion with sodium-free lithium solution produced a fourfold increase in steady-state [Ca2+]i to 290 +/- 19 nM, which was reversible upon sodium restitution. Other methods of increasing [Ca2+]i such as KCl-depolarization or caffeine produced a dose-dependent increase in quin 2 fluorescence, accompanied by sustained contracture. The [Ca2+]i increase in zero sodium was linear, and its half-time (t1/2) of 15.1 +/- 0.1 s was similar to that of the sodium-free contracture (t1/2 = 14.4 +/- 0.5 s) under the same conditions. The sodium-dependent [Ca2+]i increase was not significantly greater when potassium served as the sodium substitute instead of lithium. This suggests that sodium/calcium exchange has little voltage dependence in this situation. However, in aggregates pretreated with ouabain (2.5 microM), the [Ca2+]i increase was almost threefold greater with potassium than with lithium (P less than 0.007). Ouabain therefore potentiated the effect of membrane potential on calcium influx. We propose that elevation of [Na2+]i is a prerequisite for voltage dependence of the sodium/calcium exchange under the conditions studied. Sodium loading will then drastically increase calcium influx during the action potential while inducing an outward membrane current that could accelerate repolarization.

Enkephalins increase cyclic adenosine monophosphate content, calcium uptake, and contractile state in cultured chick embryo heart cells

Peripheral vascular effects of opioid peptides are well known, but direct myocardial effects have not been established. We studied the inotropic response of spontaneously beating cultured chick embryo ventricular cells to the enkephalin analogue [D-Ala2]-enkephalin. Amplitude of cell motion increased in a concentration-dependent manner with 0.53 microM [D-Ala2]-enkephalin producing half-maximal response. The mechanism of this positive inotropic effect was investigated by examining alterations in 45Ca influx, cyclic AMP accumulation and adenylate cyclase activity in response to [D-Ala2]-enkephalin. At maximally inotropic concentrations, the 45Ca influx rate increased 39%, adenylate cyclase was stimulated by 30%, and cyclic AMP content rose more than twofold. Thus, in contrast to neural tissue, receptors for enkephalin in cultured heart cells are coupled to adenylate cyclase in a stimulatory manner. Occupancy of these receptors produces an increase in cyclic AMP levels and exerts a positive inotropic effect via a verapamil-sensitive enhancement of Ca influx.

Paradoxical electromechanical effect of lanthanum ions in cardiac muscle cells

Although lanthanum ions (La+++) block calcium influx in cardiac cells, they may paradoxically accentuate the sodium-free contracture. We have therefore studied the effects of La+++ on the zero sodium response in chick embryonic myocardial cell aggregates. Zero sodium alone causes: (a) A maintained contracture; (b) Asynchronous localized contractions that are selectively inhibited by caffeine or ryanodine, and presumably reflect release of calcium from the sarcoplasmic reticulum; (c) A nonspecific conductance increase that is ascribable to calcium-activated ion channels. Addition of La+++ potentiates the sodium-free contracture, and causes similar potentiation of the localized contractions and the conductance increase. All three phenomena occur 5-10-fold faster in 1 mM La+++ than in sodium-free fluid alone. In contrast, when La+++ is combined with caffeine or ryanodine, the zero sodium response is suppressed. We conclude that the paradoxical effect of La+++ on the contracture is not due to calcium influx, but to enhancement, or disinhibition of intracellular calcium release. Relaxation of normal myocardium may involve control of spontaneous calcium release by lanthanum- and sodium-sensitive calcium transport across the surface membrane.

Mechanisms of beta-adrenergic receptor regulation in cultured chick heart cells. Role of cytoskeleton function and protein synthesis

To examine mechanisms by which cardiac tissue regulates the beta-adrenergic receptor and physiological response to beta-adrenergic agonists, we studied the effects of cytoskeletal disrupting agents and inhibition of protein synthesis on receptor properties and contractile response to isoproterenol in intact cultured ventricular cells from embryonic chick heart. Thirty minutes of exposure of intact cells to 1 microM isoproterenol produced loss of the high-affinity state (KD = 4.5 +/- 1.5 nM) of the receptor found in cell membranes with no loss of total receptor number, whereas there was concomitant decline in the contractile response to 1 microM isoproterenol to 41 +/- 16% (SD) of control. Contractile response recovered within 60 minutes of agonist removal to 78 +/- 11% of initial response. There was concomitant recovery of the high-affinity state of the receptor, so that 1 hour after agonist removal there was 72% of the initial proportion of high-affinity receptors. This desensitization of the contractile response, as well as recovery after agonist removal, was markedly blunted by preincubation with cytochalasin B so that contractile responsiveness to isoproterenol was maintained at 77 +/- 13% of the initial response. Colchicine (10 microM) was without effect on the first 30 minutes of agonist-induced desensitization. More prolonged agonist exposure (1 microM isoproterenol for 24 hours) produced colchicine-sensitive loss of receptors from intact cells to 40% of control levels. Full recovery of receptor number occurred over 72 hours; this was completely blocked by cycloheximide (P less than 0.01). Thus, rapid desensitization and resensitization of the beta-receptor-mediated contractile response is associated with alterations in high-affinity agonist binding and appears to be modulated by microfilaments. Receptor down-regulation is dependent on functional microtubules, and recovery of these receptors after agonist removal requires protein synthesis.

Origin of the background sodium current and effects of sodium removal in cultured embryonic cardiac cells

Cardiac automaticity is partly due to a diastolic sodium current. Possible mediators of this include tetrodotoxin-sensitive "fast" channels, cesium-sensitive time-dependent pacemaker current channels, calcium-gated nonspecific channels, and electrogenic sodium-calcium exchange. We have studied the effects of abrupt sodium removal on membrane current and conductance in voltage-clamped chick embryonic myocardial cell aggregates, in the presence of various sodium flux inhibitors. Total replacement of sodium by lithium, Tris, or tetraethylammonium ions in aggregates clamped in the pacemaker range caused a brief outward current followed by a sustained net inward current. The outward current reached a peak value of 1.1 +/- 0.5 microA/cm2 at a mean latency of 5.4 +/- 1.2 sec. (n = 6; V = -70.5 +/- 8.9 mV; Tris). Conductance often decreased during the outward current. The inward current developed exponentially (t = 19 +/- 5 sec) and reached a steady state value of -1.6 +/- 0.4 microA/cm2. This current was reversed by depolarization (mean reversal potential = -13 +/- 13 mV), and was accompanied by increased conductance and spontaneous mechanical activity. Neither of the sodium-removal currents was affected by 20 microM tetrodotoxin. Cesium (up to 20 mM) had no effect on the late inward current or the mechanical activity, but decreased the early outward current by 80 +/- 12%. Manganese (25 mM), which blocks sodium-calcium exchange, abolished the late inward current and the mechanical activity. Manganese also reduced the early outward current by 27 +/- 10%. Manganese and cesium together blocked all the effects of sodium removal. We conclude that removal of extracellular sodium interrupts a cesium-sensitive "background" current, that may be related to the time-dependent pacemaker current, If. Sodium removal also causes gradual activation of a nonspecific conductance, which can ultimately depolarize the cells, and which may be gated by cytoplasmic calcium.

Mechanism by which metabolic inhibitors depolarize cultured cardiac cells

To elucidate the means by which metabolic inhibition depolarizes cardiac cells, spontaneously beating chicken embryonic myocardial cell aggregates were voltage clamped during superfusion with 2,4-dinitrophenol and iodoacetic acid. In aggregates continuously clamped in the pacemaker potential range, abrupt exposure to these metabolic inhibitors produced a slow transient inward current. This inward current was not due to an alteration of the pacemaker current, IK2, because it could still be elicited after IK2 was abolished by Cs+ ions. The inward current was increased by hyperpolarization and decreased by depolarization. It became larger and more sustained if intermittent action potentials were allowed during exposure or if the aggregates were pretreated with either 10 mM Ca2+ or 2.7 microM acetylstrophanthidin. The inward current was suppressed by removal of extracellular Na+ or Ca2+. These observations suggest that early depolarization of cultured cardiac cells by metabolic inhibitors involves some of the same mechanisms as the transient inward current of digitalis toxicity--specifically, an effect of intracellular Ca2+ ions on membrane permeability. Similar phenomena could occur during other forms of metabolic inhibition such as myocardial ischemia.

Caffeine induces a transient inward current in cultured cardiac cells

Electrical excitation of cardiac muscle may sometimes be due to initiation of inward current by the presence of Ca2+ ions at the inner surface of the cell membrane. During digitalis toxicity and other conditions that abnormally augment cellular Ca2+ stores, premature release of Ca2+ from the sarcoplasmic reticulum leads to a transient inward current, which is large enough to initiate premature beats and is accompanied by a transient contractile response. This inward current may be mediated either by electrogenic sodium-calcium exchange or by specific Ca2+-activated cation channels that have recently been characterized in tissue cultures of cardiac myocytes. An obvious question raised by these observations is whether release of the sequestered Ca2+ stores during each normal beat exerts a similar influence on membrane potential. To explore this, chick embryonic myocardial cell aggregates were voltage-clamped during abrupt exposure to caffeine, which is known to release Ca2+ from the sarcoplasmic reticulum. The speed of the perfusion system and the relative absence of diffusion barriers in the tissue-cultured cells allowed the effects of caffeine-induced Ca2+ release to be studied on a time scale comparable to that of a single normal beat. We report here that abrupt exposure of the cells to caffeine produced a transient inward current having similar features to that of digitalis toxicity, and which was both large enough and rapid enough to potentially contribute to the action potential.

Time parameters of contraction of inotropically responding cultured heart cells in relation to cellular cyclic AMP levels

Television video microscopy combined with photoelectric recording was used to determine the influence of a number of positive inotropic agents on the amplitude (peak height) and the course of the contraction of electrically paced myocytes in 4-day monolayer cultures derived from the heart ventricles of 1 to 2-day old rats. Cyclic AMP was determined in parallel cultures of the same cell population. Reductions in time to 90% of peak height, 90% of relaxation time, and duration of contraction caused by peak height-augmenting concentrations of isoproterenol, epinephrine, dibutyryl cyclic AMP, and 1-methyl-3-isobutylxanthine, but not of theophylline, correlated with rises in cellular cyclic AMP levels. Ouabain, a rise in extracellular CaCl2, and, in some experiments, phenylephrine in the presence of propranolol increased peak height, but did not change time to 90% of peak height, 90% of relaxation time, duration of contraction, and cyclic AMP content. These responses are compared to those observed by other authors in intact cardiac muscle and are discussed in the light of evidence linking increased myocardial cyclic AMP levels with an abbreviation of systole.