Sodium efflux in rabbit myocardium: relationship to sodium-calcium exchange

Ionic mechanisms of regional action potential heterogeneity in the canine right atrium

Atrial action potential heterogeneity is a major determinant of atrial reentrant arrhythmias, but the underlying ionic mechanisms are poorly understood. To evaluate the basis of spatial heterogeneity in canine right atrial repolarization, we isolated cells from 4 regions: the crista terminalis (CT), appendage (APG), atrioventricular ring (AVR) area, and pectinate muscles. Systematic action potential (AP) differences were noted: CT cells had a "spike-and-dome" morphology and the longest AP duration (APD; value to 95% repolarization at 1 Hz, 270+/-10 ms [mean+/-SEM]); APG and pectinate muscle cells had intermediate APDs (180+/-3 and 190+/-3 ms, respectively; P<0.001 versus CT for each), with APG cells having a small phase 1; and AVR cells had the shortest APD (160+/-4 ms, P<0.001 versus other regions). The inward rectifier and the slow and ultrarapid delayed rectifier currents were similar in all regions. The transient outward K+ current was significantly smaller in APG cells, explaining their small phase 1 and high plateau. L-type Ca2+ current was greatest in CT cells and least in AVR cells, contributing to their longer and shorter APD, respectively. The E-4031-sensitive rapid delayed rectifier K+ current was larger in AVR cells compared with other regions. Voltage- and time-dependent current properties were constant across regions. We conclude that myocytes from different right atrial regions of the dog show systematic variations in AP properties and ionic currents and that the spatial variation in ionic current density may explain AP differences. Regional variation in atrial ionic currents may play an important role in atrial arrhythmia generation and may present opportunities for improving antiarrhythmic drug therapy.

Evidence for the presence of M cells in the guinea pig ventricle

INTRODUCTION

Recent studies have described the presence of M cells in the deep layers of the canine and human ventricle displaying electrophysiologic and pharmacologic features different from those of epicardial (EPI) and endocardial (ENDO) cells. The M cell is distinguished electrophysiologically by the ability of its action potential to prolong disproportionately to that of other myocardial cells with slowing of the stimulation rate and pharmacologically by its unique sensitivity to Class III antiarrhythmic agents. The present study was designed to test the hypothesis that similar cells are present in the guinea pig ventricle.

METHODS AND RESULTS

We used a dermatome to obtain-thin strips of left ventricular free wall from the hearts of guinea pigs (8 to 14 weeks old) and standard microelectrode techniques to record transmembrane activity. Action potential duration measured at 90% repolarization (APD90) was significantly longer in mid-myocardial (MID) cells than in surface EPI or ENDO cells at all basic cycle lengths (BCLs) tested. At a BCL of 300 msec, APD90 was 102 +/- 21,136 +/- 9, and 95 +/- 15 msec in EPI, MID, and ENDO cells (mean +/- SD; n = 12). At a BCL of 5000 msec, APD90 was 133 +/- 14, 185 +/- 24, and 135 +/- 13 msec in EPI, MID, and ENDO cells ([K+]o = 4 mM). Thus, APD-rate relations were more pronounced in the MID cells. MID cells were also more sensitive to agents with Class III actions (e.g., d,I-sotalol: 10 to 100 microM), exhibiting a greater APD prolongation than EPI or ENDO. d,I-Sotalol also induced early afterdepolarizations in MID cells but not in EPI or ENDO cells. The rate of rise of the action potential upstroke (Vmax) was significantly greater in MID cells: 129 +/- 13, 240 +/- 42, and 192 +/- 28 V/sec in EPI, MID, and ENDO cells (n = 10 to 18).

CONCLUSION

Our results demonstrate the existence of important transmural electrical heterogeneity in guinea pig ventricular myocardium. The study provides data in support of the existence of M cells in the mid-myocardial layers of the guinea pig ventricle exhibiting longer APDs and a greater sensitivity to agents with Class III antiarrhythmic action.

Nuclear magnetic resonance studies of sodium/calcium exchange in frog perfused, beating hearts

This study explores the effect of extracellular Ca2+ concentration ([Ca2+]o), on the intracellular Na+ concentration ([Na+]i), in frog intact hearts using nuclear magnetic resonance spectroscopy, which allows for the measurement of [Na+]i in perfused, beating hearts. Decreases in [Ca2+]o yielded marked increases in [Na+]i. A similar effect was seen during inhibition of the Na+/K+ pump and was fully reversible. This sensitivity of [Na+]i to [Ca2+]o, previously observed using microelectrodes, supports a crucial physiological role for Na+/Ca2+ exchange in frog intact, beating hearts.

Effects of amiloride on metabolism and contractility during reoxygenation in perfused rat hearts

Myocardial recovery after hypoxia may be determined not only by the extent of metabolic depression during the hypoxic period but also by changes in cation contents as well. Calcium overload during reoxygenation, mediated in part by Na-Ca exchange and supported by the rise in cell sodium during hypoxia, may be one factor. The effects of amiloride (0.1 mM), a diuretic that inhibits Na(+)-H+ and Na-Ca exchanges in cardiac sarcolemma and mitochondria preparations, were studied during hypoxia-reoxygenation in the isovolumic, isolated rat heart. During hypoxia, cell sodium, measured using potassium ethylenediamine tetraacetate cobaltate as an extracellular marker, increased in amiloride and amiloride-free hearts, but there was no increase in cell calcium (3.3 +/- 0.3 vs. 3.6 +/- 0.9 mumol/g dry wt; p = NS). Amiloride did not alter developed pressure (DP), end-diastolic pressure (EDP), pH, or integrated areas of adenosine triphosphate (ATP) and phosphocreatine (PCr) (determined by phosphorus-31-nuclear magnetic resonance spectroscopy) during hypoxia or normal perfusion conditions. Forty minutes after reoxygenation, however, cell calcium was significantly lower in the amiloride (5.1 +/- 1.3 mumol/g dry wt) than in the amiloride-free group (10.4 +/- 1.8 mumol/g dry wt; p less than 0.001), and there was improved recovery of DP (percent of initial) (72 +/- 12% vs. 41 +/- 12%; p less than 0.001), PCr (99 +/- 9% vs. 70 +/- 14%; p less than 0.001), and pH (7.17 +/- 0.17 vs. 6.88 +/- 0.16; p less than 0.001) in the amiloride group. To determine whether this dose of amiloride inhibits the manifestations of sodium-mediated calcium gain in the same model during normoxia, the metabolic and functional sequelae of lithium-substituted low sodium (50 mM) perfusion were studied. Amiloride significantly limited the manifestations of sodium-mediated calcium gain as indexed (all expressed as percent of control) by a lower peak DP (221 +/- 25% vs. 284 +/- 20%) at 3 minutes, improved preservation of PCr (85 +/- 10% vs. 68 +/- 9%) and ATP (104 +/- 12% vs. 84 +/- 9%), lower rise in inorganic phosphate (201 +/- 74% vs. 332 +/- 106%), and a smaller fall in intracellular pH (7.01 +/- 0.04 vs. 6.70 +/- 0.15, p less than 0.05) for all metabolic parameters during a 20-minute period.(ABSTRACT TRUNCATED AT 400 WORDS)

A constraint on possible stoichiometries of myocardial sodium-calcium exchange

We have examined the sodium-calcium exchange stoichiometry in Langendorff-perfused rabbit hearts using gamma-emitting tracers under conditions of sodium pump inhibition. Following a 60-min perfusion with 10(-5) acetylstrophanthidin, and extracellular concentrations [Na]o = 70 mM and [Ca]o = 300 microM, intracellular sodium rose to 59.2 mM. At this point an increase in extracellular calcium [Ca]o = 1.52 mM) caused a net efflux of sodium, but an increase in sodium [Na]o = 105 mM) caused no measurable change. When sodium and calcium were simultaneously increased according to the ratio [Na]o)n/[Ca]o = [Na]'o)n/[Ca]'o, a sodium efflux is observed when n = 4, but not when n = 3. These results are consistent with an exchange stoichiometry of 3 Na+ for each Ca2+ ion, but not values of 4 or more.

Comparison of ATP-dependent calcium transport and calcium-activated ATPase activities of cardiac sarcoplasmic reticulum and sarcolemma from rats of various ages

Age-associated decline in the Ca2+ pump function of cardiac sarcoplasmic reticulum (SR), and increase in the Ca2+ pump activity of sarcolemma (SL) were suggested by my previous study which compared the ATP-energized in vitro Ca2+ transport activities of these membranes from young (3-4-month-old) and aged (24-25-month-old) rat myocardium (Biochim. Biophys. Acta, 678 (1981) 442-459). In the present study, ATP-dependent Ca2+ transport and Ca2+ sensitive ATPase activities of SR and SL derived from the myocardium of rats aged 3 (young), 6 (young adult), 12 (adult), 18 (aging) and 24 (aged) months were determined so as to further characterize age-related changes in the Ca2+ transport function of these membranes. The rates of ATP-dependent Ca2+ accumulation by SR from 3- and 6-month-old rats were virtually similar whereas the rates of Ca2+ accumulation by this membrane from 12-, 18- and 24-month-old rats were significantly lower when compared to 3- or 6-month-old rats; the magnitude of this age-related decline amounted to approx. 18, 45 and 50%, respectively, for SR from 12-, 18- and 24-month-old animals. In contrast to the above findings with SR, SL from 18- and 24-month-old rats displayed substantially higher rates (approx. 45 and 80% increase, respectively, at 18 and 24 months of age) of ATP-dependent Ca2+ accumulation than SL preparations from 3-, 6- and 12-month-old rats; no significant age-related difference was evident between the latter three age groups. The divergent age-related changes in the Ca2+ accumulating activities of SR and SL were seen at varying Ca2+ concentrations (0.54-25.2 microM). With either membrane, kinetic analysis showed that the velocity of Ca2+ transport, but not the apparent affinity of the transport system for Ca2+ underwent age-related changes. The Ca2+-stimulated ATPase activities of SR and SL were not altered significantly with increasing age from 3 to 24 months. Comparison of the 'combined Ca2+ transport activity' of SR and SL from rats of various ages showed a significant overall age-related decline in the rates of Ca2+ transport via the ATP-driven membrane Ca2+ pumps; this decrement in membrane function was moderate at 12 months of age (approx. 16%) and became pronounced with advancing age thereafter (approx. 35 and 38%, respectively, at 18 and 24 months of age). Similar progressive age-related decline was observed in the ATP-dependent Ca2+ sequestering activity of cardiac homogenates.(ABSTRACT TRUNCATED AT 400 WORDS)

Sodium/calcium exchange and intracellular calcium buffering in ferret myocardium: an ion-sensitive micro-electrode study

Measurements of the intracellular activity of Ca (aiCa), Na (aiNa) and H (pHi) ions have been made with resin-filled ion-sensitive micro-electrodes in ferret ventricular trabeculae. The mean values in quiescent muscle at 30 degrees C were: aiNa, 11.1 +/- 1.0 mM; aiCa, 58.4 +/- 6.4 nM, and pHi, 7.20 +/- 0.11. The relation between aiNa and extracellular Na activity (aoNa) is not linear and is sensitive to temperature: the Q10 for the change in aiNa in normal Tyrode solution is 1.3 +/- 0.5 and rises to 3.5 +/- 0.5 when aoNa is reduced to 1.1 mM. The addition of CN to the bathing fluid causes little or no change in aiNa or aiCa but pHi rises to 7.38 +/- 0.10, yet in some preparations resting tension increases. Similar results are seen with carbonyl cyanide m-chlorophenyl hydrazone. On lowering [Na]o, the fall in aiNa is very much greater than the rise in aiCa and the pHi is generally unchanged. When [Na]o is lowered in the presence of a respiratory inhibitor, the fall in aiNa is reduced, the rise in aiCa and the contracture tension are increased while pHi falls. The apparent coupling ratio for the Na/Ca exchange varies between 3 and 4 depending on the experimental conditions. These results suggest that an intracellular process, with a high Q10 and which depends upon respiration and aiNa, is able to remove Ca2+ from the sarcoplasm and thereby interact with the sarcolemmal Na/Ca exchange. This process could be the increase in the energy-dependent accumulation of Ca2+ within mitochondria that will occur when the Ca efflux from these organelles is progressively inhibited as aiNa falls.

Calcium-dependent enhancement of myocardial diastolic tone and energy utilization dissociates systolic work and oxygen consumption during low sodium perfusion

The relationships and correlations among functional, metabolic, and ionic consequences of low sodium perfusion were studied in isovolumic, retrograde-aortic perfused working rat hearts by 31P nuclear magnetic resonance, oxygen consumption, and atomic absorption spectrometry. Reduction of perfusate sodium from 144 to 74, 51, 39, and 25 mM in four separate groups of hearts via lithium substitution for 15 minutes decreased cell sodium to mean values of 62, 51, 43, and 36 mumol/g dry weight, respectively (P less than 0.001 vs. control of 107). There was a transient rise and then a fall in developed pressure and a decline in phosphocreatine and adenosine triphosphate, all of which were graded and correlated with perfusate sodium (P less than 0.01 for all parameters vs. perfusate sodium). This was accompanied by a 2- to 7-fold elevation of diastolic pressure while oxygen consumption remained near control levels. All parameters except adenosine triphosphate returned toward baseline values when normal perfusate sodium was reintroduced. Although cell calcium as measured by atomic absorption spectrometry did not differ among the groups, the functional and metabolic changes did not occur if the sodium steps were performed in reduced perfusate calcium (0.08 mM). In hearts in which systolic function was obliterated by verapamil, exposure to zero sodium caused a 4-fold increase in oxygen consumption, an increase in diastolic pressure, and a reduction of high energy phosphates. In the presence of ryanodine, a specific inhibitor of sarcoplasmic reticulum calcium release, the metabolic changes did not occur, and the excess oxygen consumption in zero sodium was substantially reduced. Thus, the effect of lowered perfusate sodium in beating hearts, i.e., to dissociate oxygen consumption and systolic function, and to increase diastolic pressure and its effect in arrested hearts to increase oxygen consumption, are calcium dependent, energy consuming, and modulated by sarcoplasmic reticulum calcium cycling.

Greater ATP dependence than sodium dependence of radiocalcium efflux in bullfrog ventricle

45Ca efflux was studied in intact bullfrog ventricles following a 2-h period of loading with radiocalcium-containing Ringer solution. The cannulated ventricle was placed in a closed air-filled container to which were applied rhythmic, electronically timed, positive- and negative-pressure pulsations, which induced ventricular volume excursions. The mechanical arrangement and timing circuitry made it possible for each period to be as short in duration as 15 s. By use of this technique, penetration of the extracellular space by [14C]inulin was found to be complete within 30 s, and recovery of the inulin proceeded with a time constant of 17-24 s, indicating a completeness of recovery of 98% within 90 s. Washout of added 45Ca was quantitatively quite close to that of inulin, and in addition the estimated rate of sequestration of the isotope was slow enough to introduce only a small error into the experimental results. 45Ca efflux was only slightly (15%) sensitive to replacement of extracellular sodium but was profoundly sensitive to the inhibitors of ATP synthesis, cyanide and 2-deoxy-glucose.

Effects of amiloride on calcium uptake by myocytes isolated from adult rat hearts

Amiloride at high concentrations inhibits the uptake of Ca by rat heart myocytes containing elevated levels of intracellular Na and retards the development of Ca-dependent hypercontracture in these cells. In contrast, amiloride enhances the net uptake of Ca in Ca-tolerant myocytes containing normal levels of Na. The results suggest that amiloride may inhibit Na-Ca exchange across the sarcolemma of cardiac myocytes.

ATP-dependent Na+ transport in cardiac sarcolemmal vesicles

Although the enzyme (Na+ + K+)-ATPase has been extensively characterized, few studies of its major role, ATP-dependent Na+ pumping, have been reported in vesicular preparations. This is because it is extremely difficult to determine fluxes of isotopic Na+ accurately in most isolated membrane systems. Using highly purified cardiac sarcolemmal vesicles, we have developed a new technique to detect relative rates of ATP-dependent Na+ transport sensitively. This technique relies on the presence of Na+-Ca2+ exchange and ATP-driven Na+ pump activities on the same inside-out sarcolemmal vesicles. ATP-dependent Na+ uptake is monitored by a subsequent Nai+-dependent Ca2+ uptake reaction (Na+-Ca2+ exchange) using 45Ca2+. We present evidence that the Na+-Ca2+ exchange will be linearly related to the prior active Na+ uptake. Although this method is indirect, it is much more sensitive than a direct approach using Na+ isotopes. Applying this method, we measure cardiac ATP-dependent Na+ transport and (Na+ + K+)-ATPase activities in identical ionic media. We find that the (Na+ + K+)-ATPase and the Na+ pump have identical dependencies on both Na+ and ATP. The dependence on [Na+] is sigmoidal, with a Hill coefficient of 2.8. Na+ pumping is half-maximal at [Na+] = 9 mM. The Km for ATP is 0.21 mM. ADP competitively inhibits ATP-dependent Na+ pumping. This approach should allow other new investigations on ATP-dependent Na+ transport across cardiac sarcolemma.

Excitation- and rest-dependent shifts of Ca in guinea-pig ventricular myocardium

The rest- and excitation-dependent shifts of Ca and 45Ca in the isolated, perfused ventricles of guinea-pig hearts were investigated. As much as 50% of the total Ca content (2.2 mmol/kg ww) found in the ventricular muscle stimulated at a steady rate of 60/min, was released into perfusate during 4 min of rest. In the preparations perfused with 45Ca containing solution during the 4 min of rest or during the last 20 s of rest only, a single beat resulted in extra uptake of 0.359 and 0.287 mmol of labelled calcium (45Ca) per kg ww, respectively. Single post-rest excitation evoked in the ventricles which were previously perfused with radioactive solution for 64 min, resulted in increase in tissue 45Ca content by 0.229 mmol/kg ww. In these preparations, the gain in 45Ca is equivalent to the net Ca uptake. Continued post-rest stimulation at the rate of 60/min resulted in recovery of pre-rest content of 45Ca and of total Ca. Gain of 45Ca was paralleled by recovery of contractile force. Uptake of 45Ca in the preparations stimulated at the steady rate of 60/min was 0.137 mmol/kg ww and its value did not depend on the number of beats during exposure to the isotope. Thus 45Ca uptake over a number of steady-state beats may be regarded as equal to the uptake in a single beat. This uptake is by orders of magnitude larger than reported previously by other authors. It is proposed that contraction is triggered by Ca influx into the excited cells (Ca1), and that the response of contractile proteins to this trigger is controlled by a large intracellular Ca2 fraction whose volume is rate-dependent.

Calcium-induced release of calcium from the cardiac sarcoplasmic reticulum

The hypothesis of a Ca2+-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR) is supported by experiments done in skinned cardiac cells (sarcolemma removed by microdissection). According to this hypothesis, the transsarcolemmal Ca2+ influx does not activate the myofilaments directly but through the induction of a Ca2+ release from the SR. The stimulus gating CICR is not a small change in free Ca2+ concentration (delta[free Ca2+]) outside the SR but a function of the rate of this change (delta[free Ca2+/delta t]). The initial relatively fast component of the transsarcolemmal Ca2+ current would trigger Ca2+ release; the subsequent slow component, perhaps corresponding to noninactivating Ca2+ channels, would load the SR with an amount of Ca2+ available for release during subsequent beats. Inactivation of CICR is caused by the large increase of [free Ca2+] outside the SR resulting from Ca2+ release, which inhibits further release. This negative feedback helps to explain that CICR is not all or none. During relaxation the Ca2+ reaccumulation in the SR is backed up by the Ca2+ efflux across the sarcolemma through Na+-Ca2+ exchange and the sarcolemmal Ca2+ pump. Computations of the Ca2+ buffering in the mammalian ventricular cell and of the systolic transsarcolemmal Ca2+ influx do not support the alternative hypothesis that this influx of Ca2+ is large enough to activate the myofilaments directly. Yet the hypothesis of a CICR can be challenged because of many problems and uncertainties related to the preparations and methods used for skinned cardiac cell experiments.

The regulation of the Na+ -Ca2+ exchanger of heart sarcolemma

The Na+/Ca2+-exchange of calf-heart sarcolemma is activated by a treatment with ATP, Mg2+, and Ca2+, and deactivated by a treatment with phosphorylase phosphatase. The effect of the latter can be substituted by a treatment with Mg2+, Ca2+, and calmodulin. The activating treatment does not require added calmodulin, but is inhibited by calmodulin antagonists. Evidently, endogenous calmodulin is required and sufficient. Activation is half-maximal at about 2 microM Ca2+. Added calmodulin, however, decreases the Km (Ca2+) of the activating process to about 0.8 microM. Deactivation is half-maximal, at optimal calmodulin concentrations, at about 1.5 microM Ca2+. Experiments with adenosine 5'-[gamma-thio]triphosphate have shown that the activating treatment is mediated by a kinase and the deactivating treatment by a phosphatase. The concerted operation of the two enzymes is made possible by their different Ca2+ affinity. At saturating Ca2+ concentrations, the level of ATP may also influence the balance of the two enzymes.

Sodium tracer kinetics and transmembrane flux in tissue-cultured chick heart cells

Considerable difficulty has been encountered in defining the physiological significance of sodium tracer kinetic measurements in cardiac muscle. In this study, 24Na+ efflux experiments were performed by directly monitoring tissue radioactivity during the superfusion of growth-oriented embryonic chick heart cells in tissue cultured. The cellular 24Na+ efflux from contractile preparations exhibited at least two exponential components whereas noncontractile, fibroblastlike preparations had a single efflux component similar in rate to the slower component of the contractile preparations. We concluded that the slow component represents efflux from nonmuscle cells, whereas the faster component reflects the muscle cell compartment. The mean Na+ efflux rate constants for contractile preparations (beating 150 min-1) were 3.1 and 0.35 min-1. Intracellular Na+ concentrations, as determined by isotope uptake and by flame photometry, were 18 and 16 mM for contractile and nonmuscle preparations, respectively. The steady-state, transmembrane fluxes are 98 and 5 pmol . cm-2 . s-1 for muscle and nonmuscle cells, respectively. The Na+ efflux kinetics in 10(-4) M ouabain were reduced by approximately 16% from the control value. These findings indicate that the greater part of the steady-state Na+ efflux in cultured heart cells is due to mechanisms other than the Na+-K+ pump.