Increase in intracellular sodium ion activity during stimulation in mammalian cardiac muscle

Potassium inhibition of sodium-activated potassium (K(Na)) channels in guinea-pig ventricular myocytes

N(a+)-activated potassium channels (K(Na) channels) were studied in inside-out patches from guinea-pig ventricular myocytes at potentials between -100 and +80 mV. External K(+) (K(+)(o)) was set to 140 mM. For inwardly directed currents with 105 mM internal K(+) (K(+)(1)), the unitary current-voltage relationship was fitted by the constant field equation with a potassium permeability coefficient, P(K), of 3.72 x 10(-13) cm(3) s(-1). The slope conductance (-100 to -10 mV) was 194 +/- 4.5 pS (mean +/- s.d., n = 4) with 105 mM K(+)(i) (35 mM Na(+)(i)) but it decreased to 181 +/- 5.6 pS (n = 5) in 70 mM K(+)(i) (70 mM Na(+)(i)). K(Na) channels were activated by internal Na(+) in a concentration-dependent fashion. With 4 mM K(+)(i), maximal activation was recorded with 100 mM Na(+)(i) (open probability, P(o), about 0.78); half-maximal activation required about 35 mM Na(+)(i). When K(+)(i) was increased to 70 mM, half-maximal activation shifted to about 70 mM Na(+)(i). With Na(+)(i) set to 105 mM, channel activity was markedly inhibited when K(+)(i) was increased from 35 to 105 mM. Channel openings were abolished with 210 mM K(+)(i). The inhibitory effect of internal K(+) was also observed at more physiological conditions of osmolarity, ionic strength and chloride concentration. With 35 mM Na(+)(i) and 4 mM K(+)(i), P(o) was 0.48 +/- 0.10 (n = 6); when K(+)(i)was increased to 35 mM, P(o) was reduced to 0.04 +/- 0.05 (n = 7, P < 0.001). The relationship between P(o) and Na(+)(i) concentration at different levels of K(+)(i) is well described by a modified Michaelis-Menten equation for competitive inhibition; the Hill coefficients were 4 for the P(o)-Na(+)(i) relationship and 1.2 for the P(o)-K(+)(i) relationship. It is suggested that Na(+) and K(+) compete for a superficial site on the channel's permeation pathway. K(Na) channels would be most likely to be activated in vivo when an increase in Na(+)(i) is accompanied by a decrease of K(+)(i).

Three-dimensional (87)Rb NMR imaging and spectroscopy of K(+) fluxes in normal and postischemic pig hearts

K(+) uptake rates were measured in the anterior (An) and posterior (Pos) LV walls of pig hearts before and after regional ischemia and reperfusion using Rb(+) as a K(+) congener and 3D (87)Rb NMR imaging and spectroscopy as detection methods. The hearts were perfused by the Langendorff method with Krebs-Henseleit (KH) buffer and loaded with Rb(+) (4.7 mM, Rb-KH) after 120-min ischemia and 60-min reperfusion. A second protocol involved Rb(+) loading prior to ischemia. Ischemia was produced by occlusion of the left anterior descending artery, which after 110 min of reperfusion resulted in infarction in the An wall (24 +/- 6% of the LV mass) determined by triphenyltetrazolium chloride staining. At the end of reperfusion pressure-rate product and oxygen consumption rate decreased to 58 +/- 10 and 74 +/- 4% of their preischemic values, respectively. Phosphocreatine, ATP, and intracellular pH (pHi), measured by (31)P NMR spectroscopy in the infarcted area, decreased to 59 +/- 17, 32 +/- 6%, and 6.7 +/- 0.36 (from 7.05 +/- 0.13), respectively. Serial (87)Rb images were acquired according to both protocols. Rate constants (k x 10(3), min(-1)), relative amount of intracellular Rb(+) (A, %) and relative fluxes (F = kA, %/min) for the An and Pos walls were determined from the images. Before ischemia, F and k were comparable in the Pos and An walls. Ischemia + reperfusion decreased F in the An wall (from 4.4 +/- 0.3 to 1.4 +/- 0.85) due to a decrease in A (20 vs. 73) and increased F in Pos wall (from 3.2 +/- 0.6 to 6.6 +/- 0.23) due to an increase in k (from 42 +/- 3 to 93 +/- 6). The intensities of the Rb images correlated with the Rb(+) content measured in tissue samples. Magn Reson Med 44:83-91, 2000. Published 2000 Wiley-Liss, Inc.

Action potential and contractility changes in [Na(+)](i) overloaded cardiac myocytes: a simulation study

Sodium overload of cardiac cells can accompany various pathologies and induce fatal cardiac arrhythmias. We investigate effects of elevated intracellular sodium on the cardiac action potential (AP) and on intracellular calcium using the Luo-Rudy model of a mammalian ventricular myocyte. The results are: 1) During rapid pacing, AP duration (APD) shortens in two phases, a rapid phase without Na(+) accumulation and a slower phase that depends on [Na(+)](i). 2) The rapid APD shortening is due to incomplete deactivation (accumulation) of I(Ks). 3) The slow phase is due to increased repolarizing currents I(NaK) and reverse-mode I(NaCa), secondary to elevated [Na(+)](i). 4) Na(+)-overload slows the rate of AP depolarization, allowing time for greater I(Ca(L)) activation; it also enhances reverse-mode I(NaCa). The resulting increased Ca(2+) influx triggers a greater [Ca(2+)](i) transient. 5) Reverse-mode I(NaCa) alone can trigger Ca(2+) release in a voltage and [Na(+)](i)-dependent manner. 6) During I(NaK) block, Na(+) and Ca(2+) accumulate and APD shortens due to enhanced reverse-mode I(NaCa); contribution of I(K(Na)) to APD shortening is negligible. By slowing AP depolarization (hence velocity) and shortening APD, Na(+)-overload acts to enhance inducibility of reentrant arrhythmias. Shortened APD with elevated [Ca(2+)](i) (secondary to Na(+)-overload) also predisposes the myocardium to arrhythmogenic delayed afterdepolarizations.

Opposite change with ischaemia in the antifibrillatory effects of class I and class IV antiarrhythmic drugs resulting from the alteration in ion transmembrane exchanges related to depolarization

It is known that class I antiarrhythmic drugs lose their antifibrillatory activity with severe ischaemia, whereas class IV antiarrhythmic drugs acquire such activity. Tachycardia, which is also a depolarizing factor, has recently been shown to give rise to an alteration of ion transmembrane exchanges which is particularly marked in the case of calcium. This leads one to wonder if the change in antifibrillatory activity of antiarrhythmic drugs caused by ischaemia depends on the same process. The change in antifibrillatory activity was studied in normal conditions ranging to those of severe ischaemia with a class I antiarrhythmic drug, flecainide (1.00 mg·kg-1 plus 0.04 mg·kg-1·min-1), a sodium channel blocker, and a class IV antiarrhythmic drug, verapamil (50 µg·kg-1 plus 2 µg·kg-1·min-1), a calcium channel blocker. The experiments were performed in anaesthetized, open-chest pigs. The resulting blockade of each of these channels was assessed at the end of ischaemic periods of increasing duration (30, 60, 120, 180, 300, and 420 s) by determining the ventricular fibrillation threshold (VFT). VFT was determined by means of trains of diastolic stimuli of 100 ms duration delivered by a subepicardial electrode introduced into the myocardium (heart rate 180 beats per min). Ischaemia was induced by completely occluding the left anterior descending coronary artery. The monophasic action potential was recorded concurrently for the measurement of ventricular conduction time (VCT). The monophasic action potential duration (MAPD) varied with membrane polarization of the fibres. The blockade of sodium channels by flecainide, which normally raises VFT (7.0 ± 0.4 to 13.8 ± 0.8 mA, p < 0.001) and lengthens VCT (28 ± 3 to 44 ± 5 ms, p < 0.001), lost its effects in the course of ischaemia. This resulted in decreased counteraction of the ischaemia-induced fall of VFT and decreased aggravation of the ischaemia-induced lengthening of VCT. The blockade of calcium channels, which normally does not alter VFT (between 7.2 ± 0.6 and 8.4 ± 0.7 mA, n.s.) or VCT (between 30 ± 2 and 34 ± 3 ms, n.s.), slowed the ischaemia-induced fall of VFT. VFT required more time to reach 0 mA, thus delaying the onset of fibrillation. Membrane depolarization itself was opposed as the shortening of MAPD and the lengthening of VCT were also delayed. Consequently there is a progressive decrease in the role played by sodium channels during ischaemia in the rhythmic systolic depolarization of the ventricular fibres. This reduces or suppresses the ability of sodium channel blockers to act on excitability or conduction, and increases the role of calcium channel blockers in attenuating ischaemia-induced disorders.Key words: pigs, ion transmembrane exchanges, myocardial ischaemia, sodium channel, calcium channel.

Phospholamban: a major determinant of the cardiac force-frequency relationship

The cardiac force-frequency relationship has been known for over a century, yet its mechanisms have eluded thorough understanding. We investigated the hypothesis that phospholamban, a potent regulator of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), determines the cardiac force-frequency relationship. Isolated left ventricular papillary muscles from wild-type (WT) and phospholamban knockout (KO) mice were stimulated at 2 to 6 Hz. The force-frequency relationship was positive in WT but negative in KO muscles, i.e., it was inverted by ablation of phospholamban (P < 0.01, n = 6 mice). From 2 to 6 Hz, relaxation accelerated considerably (by 10 ms) in WT muscles but only minimally (by 2 ms) in KO muscles (WT vs. KO: P < 0. 0001, n = 6). To show that the lack of frequency potentiation in KO muscles was not explained by the almost maximal basal contractility, twitch duration was prolonged in six KO muscles with the SERCA inhibitor cyclopiazonic acid to WT values. Relaxation still failed to accelerate with increased frequency. In conclusion, our results clearly identify phospholamban as a major determinant of the cardiac force-frequency relationship.

Role of Na-Ca exchange in the action potential changes caused by drive in cardiac myocytes exposed to different Ca2+ loads

The role of Na-Ca exchange in the membrane potential changes caused by repetitive activity ("drive") was studied in guinea pig single ventricular myocytes exposed to different [Ca2+]o. The following results were obtained. (i) In 5.4 mM [Ca2+]o, the action potentials (APs) gradually shortened during drive, and the outward current during a train of depolarizing voltage clamp steps gradually increased. (ii) The APs shortened more and were followed by a decaying voltage tail during drive in the presence of 5 mM caffeine; the outward current became larger and there was an inward tail current on repolarization during a train of depolarizing steps. (iii) These effects outlasted drive so that immediately after a train of APs, currents were already bigger and, after a train of steps, APs were already shorter. (iv) In 0.54 mM [Ca2+]o, the above effects were much smaller. (v) In high [Ca2+]o APs were shorter and outward currents larger than in low [Ca2+]o. (vi) In 10.8 mM [Ca2+]o, both outward and inward currents during long steps were exaggerated by prior drive, even with steps (+80 and +120 mV) at which there was no apparent inward current identifiable as ICa. (vii) In 0.54 mM [Ca2+]o, the time-dependent outward current was small and prior drive slightly increased it. (viii) During long steps, caffeine markedly increased outward and inward tail currents, and these effects were greatly decreased by low [Ca2+]o. (ix) After drive in the presence of caffeine, Ni2+ decreased the outward and inward tail currents. It is concluded that in the presence of high [Ca2+]o drive activates outward and inward Na-Ca exchange currents. During drive, the outward current participates in the plateau shortening and the inward tail current in the voltage tail after the action potential.Key words: ventricular myocytes, repetitive activity, outward and inward Na-Ca exchange currents, caffeine, nickel.

Sodium-pump potentials and currents in guinea-pig ventricular muscles and myocytes

When guinea-pig papillary muscles were depolarized to ca. -30 mV by superfusion with K+-free Tyrode's solution supplemented with Ba2+, Ni2+, and D600, addition of Cs+ transiently hyperpolarized the membrane in a reproducible manner. The size of the hyperpolarization (pump potential) depended on the duration of the preceding K+-free exposure; peak amplitudes (Epmax) elicited by 10 mM Cs+ after 5-, 10-, and 15-min K+-free exposures were 12.9, 17.7, and 23.2 mV, respectively. Pump potentials were unaffected by external Cl- but suppressed by cardiac glycosides, hyperosmotic conditions, and low-Na+ solution. Using Epmax as an indicator of Na+ pump activation, the half-maximal concentration for activation by Cs+ was 12-16.3 mM. At 6 mM, Cs+ was three times less potent than Rb+ or K+ and five times more potent than Li+. From these findings, and correlative voltage-clamp data from myocytes, we calculate that (i) a pump current of 7.8 nA/cm2 generates an Epmax of 1 mV and (ii) resting pump current in normally polarized muscle (~0.16 µA/cm2) is five times smaller than previously estimated.Key words: sodium pump, cesium, rubidium, sodium pump current.

On the mechanism of cesium-induced voltage and current tails in single ventricular myocytes

The mechanisms by which different concentrations of cesium modify membrane potentials and currents were investigated in guinea pig single ventricular myocytes. In a dose-dependent manner, cesium reversibly decreases the resting potential and action potential amplitude and duration, and induces a diastolic decaying voltage tail (Vex), which increases at more negative and reverses at less negative potentials. In voltage-clamped myocytes, Cs+ increases the holding current, increases the outward current at plateau levels while decreasing it at potentials closer to resting potential, induces an inward tail current (Iex) on return to resting potential and causes a negative shift of the threshold for the inward current. During depolarizing ramps, Cs+ decreases the outward current negative to inward rectification range, whereas it increases the current past that range. During repolarizing ramps, Cs+ shifts the threshold for removal of inward rectification negative slope to less negative values. Cs+-induced voltage and current tails are increased by repetitive activity, caffeine (5 mM) and high [Ca2+]O (8.1 mM), and are reduced by low Ca2+ (0.45 mM), Cd2+ (0.2 mM) and Ni2+ (2 mM). Ni2+ also abolishes the tail current that follows steps more positive than ECa. We conclude that Cs+ (1) decreases the resting potential by decreasing the outward current at more negative potentials, (2) shortens the action potential by increasing the outward current at potentials positive to the negative slope of inward rectification, and (3) induces diastolic tails through a Ca2+-dependent mechanism, which apparently is an enhanced electrogenic Na-Ca exchange.

Effect of isoprenaline, carbachol, and Cs+ on Na+ activity and pacemaker potential in rabbit SA node cells

Effects of isoprenaline, carbachol, and Cs+ on intracellular Na+ activity (a(i)Na) and spontaneous action potentials were studied in multicellular and single cell preparations isolated from rabbit sinoatrial (SA) nodes. a(i)Na was measured with double-barreled Na+-selective microelectrodes and the fluorescent Na+-indicator sodium-binding benzofuran isophthalate (SBFI). In spontaneously beating cells, aiNa measured with Na+-selective microelectrodes and SBFI were 4.5 +/- 1.2 mM (means +/- SD, n = 21) in multicellular preparations and 4.0 +/- 1.1 mM (n = 16) in single cells, respectively. Measurements of a(i)Na with microelectrodes showed that isoprenaline increased a(i)Na from 4.7 +/- 1.2 to 5.5 +/- 1.6 mM (n = 16, P < 0.01) and shortened the action potential cycle length (ACL) from 338 +/- 46 to 269 +/- 35 ms (n = 16, P < 0.01). However, increasing the action potential rate by pacing produced a much smaller increase in a(i)Na. Changes in a(i)Na and ACL produced by isoprenaline were blocked by Cs+. The selective hyperpolarization-activated inward current (If) blocker ZD-7288 decreased a(i)Na from 5.2 +/- 1.0 to 4.6 +/- 1.3 mM (n = 4, P < 0.01) and prolonged ACL from 394 +/- 20 to 553 +/- 68 ms (n = 4, P < 0.01). The If blocker substantially inhibited the increase in a(i)Na produced by isoprenaline. Carbachol and Cs+ decreased aiNa from 4.6 +/- 1.4 to 3.9 +/- 1.2 mM (n = 15, P < 0.01) and from 4.9 +/- 1.0 to 3.9 +/- 1.3 mM (n = 18, P < 0.01), respectively. In addition, carbachol and Cs+ prolonged ACL from 345 +/- 44 to 587 +/- 100 ms (n = 15, P < 0.01) and from 353 +/- 30 to 464 +/- 87 ms (n = 18, P < 0.01), respectively. However, carbachol and Cs+ almost did not change a(i)Na when SA node cells became quiescent in a 25.4 mM extracellular K+ concentration. The results suggest that isoprenaline, ZD-7288, carbachol, or Cs+ might have changed a(i)Na and action potential rate by possibly stimulating or inhibiting If carried by Na+. Measurements of a(i)Na with SBFI showed that isoprenaline, carbachol, and Cs+ produced a(i)Na changes that were similar to those measured with the microelectrodes.

Kinetics of ATP-sensitive K+ channels in isolated rat hearts assessed by 87Rb NMR spectroscopy

An experimental model was developed to evaluate the effects of activators and inhibitors of K(ATP) channels on unidirectional K+ fluxes in the whole heart. Isolated rat hearts perfused in the Langendorff mode were equilibrated with Pi-free Krebs-Henseleit buffer (KH buffer) containing 0.94-2.14 mM RbCl and 3.76 mM KCl (20-36% of K+ substituted by Rb+). Rb+ efflux was initiated by removing Rb+ from the perfusate and 87Rb spectra were acquired continuously with a 1-2 min time resolution. In hearts with normal energetics, the efflux of Rb+ fit a monoexponential function, and the rate constant did not depend on intracellular [Rb+]. Agents depressing excitability and heart rate (HR), such as 0.6 mM lidocaine (Lido), 10 microM carbachol (carb) and 20 mM MgSO4, inhibited Rb+ efflux such that the rate constant, k (10(3)/min), decreased from 50+/-1.2 in the beating heart to 26+/-1, 40+/-1.1 and 19+/-1.2, respectively. In contrast, high [K+] (21 mM) did not affect the k value (50+/-4.5), independently of the presence or absence of bumetanide (Bum, 30 microM) and glibenclamide (Glib, 5 microM). Dinitrophenol (DNP, 0.2 mM) added in the presence of high [K+] + Bum increased k three-fold, to 160+/-5. This effect was associated with a significant decrease in phosphocreatine (PCr, <10% of initial) and ATP ( 15%) levels, and a 7-fold increase in the Pi level, assessed by 31P-NMR spectroscopy. Glib completely reversed the effect of DNP. Pinacidil (Pin, 20-80 microM) did not affect the k value either in beating control hearts or in the presence of Carb or KCl + Bum. Moreover, under conditions of moderate metabolic stress induced by 0.05 mM DNP (PCr, 35%; ATP, 65%), where half-maximal activation of K(ATP) channels occurred, Pin did not further activate Rb+ efflux. We conclude that:(1) heart rate-independent Rb+ efflux accounts for 40-80% of the total Rb+ efflux in beating (300 bpm) rat hearts;(2) DNP-activated Rb+ efflux is a good model for testing inhibitors of KATP channels in whole hearts; and (3) Pin is not an effective K(ATP) channel opener in the rat heart model.

Mechanisms for spontaneous termination of monomorphic, sustained ventricular tachycardia: results of activation mapping of reentrant circuits in the epicardial border zone of subacute canine infarcts

OBJECTIVES

The objective of this study was to determine why sustained ventricular tachycardias (VT) sometimes stop without outside intervention.

BACKGROUND

Sustained, monomorphic VT in patients with ischemic heart disease is often caused by reentrant excitation. These tachycardias can degenerate into rapid polymorphic rhythms or occasionally terminate spontaneously.

METHODS

Sustained VT was induced by programmed stimulation in dog hearts 4 to 5 days after ligation of the left anterior descending coronary artery. Activation in reentrant circuits in the epicardial border zone of the infarct was mapped using 192 to 312 bipolar electrodes.

RESULTS

Spontaneous termination of sustained VT always occurred when the reentrant wave front blocked in the central common pathway in reentrant circuits with a figure-of-eight configuration. Two major patterns of termination were identified from activation maps of the circuits that were not distinguishable from each other on the surface electrocardiogram: 1) Abrupt termination was not preceded by any change in the pattern of activation or cycle length. It could occur at different locations within the central common pathway, was not related to the directions of the muscle fiber orientation and was not caused by a short excitable gap. 2) Termination caused by premature activation (after a short cycle) either resulted from shortening of the functional lines of block around which the reentrant impulse circulated or was caused by wave fronts originating outside the reentrant circuit. In only one episode were oscillations of cycle length associated with termination.

CONCLUSIONS

The mechanisms for termination of reentry in functional circuits causing VT are different from those in anatomic circuits where oscillatory behavior precedes termination.

Extracellular adenosine 5'-triphosphate induces Ca2+ efflux from freshly isolated adult rat cardiomyocytes: possible involvement of Na+/Ca2+ exchange mechanism

In the present study, we examined the effect of extracellular adenosine 5'-triphosphate (ATP) on Ca2+ efflux from freshly isolated adult rat cardiomyocytes. ATP at 1 mM caused a release of 3.6+/-0.08% of the total cellular content. The 45Ca2+ efflux from the cells was also stimulated by adenosine-5'-O-(3-thiotriphosphate) (ATP-gamma s), alpha, beta-methylene-ATP and adenosine 5'-diphosphate (ADP), but not by adenosine 5'-monophosphate (AMP) or adenosine. The effect of ATP was inhibited by a known purinergic P2-receptor antagonist, but not by a P1-receptor antagonist. From these results, it is conceivable that the effect of ATP on Ca2+ efflux from cardiomyocytes is mediated through P2-purinoceptors. It was also observed that ATP caused a rise in [Ca2+]i to almost 200 nM. The ATP-stimulated 45Ca2+ efflux was not affected by removal of extracellular Ca2+, but was dependent on the presence of extracellular Na+. Moreover, ATP caused a 22Na+ influx into the cells of about 2.0-fold over the basal value. These result suggest that ATP stimulates extracellular Na+-dependent 45Ca2+ efflux from freshly isolated adult rat cardiomyocytes, probably through its stimulatory effect on plasma membrane P2-purinoceptors which may couple to Na+/Ca2+ exchange.

[Status of digitalis in therapy of acute and chronic heart failure]

Although supported by more than 200 years of experience and anecdotal clinical evidence, the efficacy of digitalis in the management of heart failure has been questioned until the past decade. The idea to improve contractility of the diseased myocardium with an inotropic agent is fundamental in the management of left ventricular dysfunction. The majority of clinical trials published since 1980, most of which examined patients with mild to moderate heart failure, indicate that digitalis alone or in combination with vasodilators may improve the clinical outcome particular in those patients with more advanced symptoms and poorer left ventricular function. Aside from its action as an inotropic drug the pharmacology and the mechanisms by which digitalis influence the diseased myocardium and peripheral circulation in heart failure has gained more complexity within the last years, raising the idea of other mechanisms that might be involved in its action. Particular for ACE inhibition multiple clinical trials have conclusively demonstrated its impact on survival and morbidity in congestive heart failure. Improvement of clinical outcome as measured in terms of fewer hospitalizations and improvement of symptoms in patients receiving digitalis seems to be comparable to patients receiving beta-blockers additional to diuretics and ACE inhibitors, an entirely different approach to the treatment of heart failure. Despite initial improvement of hemodynamics it now appears that there is no survival benefit found for digitalis in the management of heart failure.

Overdrive Excitation in the Guinea Pig Sinoatrial Node Superfused in High [K(+)](o)

The aim of the present experiments was to study the characteristics and mechanisms of the rhythm induced by overdrive ('overdrive excitation', ODE) in the sinoatrial node (SAN) superfused in high [K(+)](o) (8-14 mM). It was found that: (1) overdrive may induce excitation in quiescent SAN and during a slow drive; (2) in spontaneously active SAN, overdrive may accelerate the spontaneous discharge; (3) immediately after the end of overdrive, a pause generally precedes the onset of the induced rhythm; (4) during the pause, an oscillatory potential (V(os)) may be superimposed on the early diastolic depolarization (DD); (5) during the subsequent late DD, a different kind of oscillatory potential appears near the threshold for the upstroke (ThV(os)) which is responsible for the initiation of spontaneous activity; (6) once started, the induced rhythm is fastest soon after overdrive; (7) faster drives induce longer and faster spontaneous rhythms; (8) the induced action potentials are slow responses followed by DD with a superimposed V(os), but ThV(os) is responsible for ODE; (9) the induced rhythm subsides when ThV(os) miss the threshold and gradually decay; (10) low [Ca(2+)](o) abolishes ODE; (11) in quiescent SAN, high [Ca(2+)](o) induces spontaneous discharge through ThV(os) and increases its rate by enhancing V(os) and shifting the threshold to more negative values, and (12) tetrodotoxin abolishes ODE as welll as the spontaneous discharge induced by high [Ca(2+)](o). In conclusion, in K(+)-depolarized SAN, ODE may be present in the apparent absence of calcium overload, is Ca(2+)- and Na(+)-dependent and is mediated by ThV(os) and not by V(os). Copyright 1997 S. Karger AG, Basel

Kinetic properties of unitary Na+-dependent K+ channels in inside-out patches from isolated guinea-pig ventricular myocytes

1. Single Na+-activated K+ channels (K(Na)) were investigated by means of the inside-out patch clamp technique in ventricular myocytes isolated from the guinea-pig heart. 2. Na+-activated K+ channels were observed at very low density (< 9% of patches). In symmetrical (60/60 mM) K+ solutions, K(Na) channels had a mean slope conductance of 75 pS and in asymmetrical (150/70 mM; outside/inside) K+ solutions, they had a mean slope conductance of 220 pS. The reversal potentials obtained under these two ionic conditions were close to the equilibrium potential for K+, suggesting K+ selectivity. 3. In high (98 mM) [Na+]i, the channel showed two open states and up to four closed states, and K(Na) channels also displayed long closures (of the order of seconds). The opening probability (Po) was not voltage dependent. Transient sublevels between 8 and 86% of the main state were identified and appeared to be a common feature of K(Na) channels. 4. Decreasing the activating [Na+]i, reduced Po and this was associated with both an increase in mean closed times and a decrease in mean open times. Lowering [Na+]i also increased the longer closed-time constants and their relative proportions. The first open-time constant was more sensitive to alterations in [Na+]i. 5. Distributions of burst duration, between burst duration and openings within bursts were best described by the sum of two exponentials. Lowering [Na+]i decreased the burst duration and the duration of openings within burst. 6. These observations show that the Na+-activated K+ channel from guinea-pig ventricular myocytes has complex gating and bursting behaviour.

Mechanisms underlying the shortening of the action potential at high and low stimulus rates in sheep Purkinje fibres

The shortening of the action potential of sheep Purkinje fibres at high and low rates of stimulation has been investigated. The shortening of the action potential at high rates can be entirely accounted for by incomplete recovery of the plateau conductances between beats. When sufficient time is allowed for membrane recovery, a prolongation of the action potential, rather than a shortening, occurs at high frequencies. The effect on electrical activity of increasing the stimulus frequency is similar to decreasing the bathing K concentration. The possibility of a reduction in the cleft K concentration at high frequencies is discussed. The shortening of the action potential at low rates is unaffected by 4-amino pyridine (a blocker of the transient outward current, i to ) is abolished by D600 (a blocker of the second inward current, i st ) and by a rise in the bathing Ca concentration. It is concluded that i si rather than i to is involved in action potential shortening at low rates. Action potential shortening at low rates is closely associated with declines in the maximum diastolic potential and the pacemaker potential; all of these changes are abolished by ouabain (a blocker of the Na-K pump). It is concluded that the shortening of the action potential at low rates may be the result of a decline in i si , which in turn is dependent on a decline in [Na] i . It is suggested that the rate-dependent changes in the maximum diastolic potential, pacemaker potential and tension are also related to [Na] i .

Activity-dependent changes in the electrical behaviour of sheep cardiac Purkinje fibres

Rate-dependent changes in the electrical activity of sheep Purkinje fibres maintained at 37 °C have been investigated. The duration of the action potential is maximal at a frequency of about 60 min -1 . When the rate is increased above 60 min -1 there is a substantial shortening of the action potential; this occurs abruptly in the first beat at the higher rate although subsequently there can be further changes in duration and these can result in a small prolongation, no change, or a small further shortening of the action potential and can take up to 10 min to reach a steady-state. When the rate is reduced from 60 min -1 there is also a shortening of the action potential but it occurs gradually over several hundred seconds. Action potential duration reaches a minimum value at a rate of about 6 min -1 . 70% of preparations studied showed an increase in duration again at rates below 6 min -1 but duration is always constant at frequencies below about 0.1 min -1 . The maximum diastolic potential is more negative and the pacemaker potential is larger at higher rates of stimulation. When the frequency is raised these variables increase over a time course lasting several hundred seconds. At rates below 60 min -1 the slow changes in action potential duration, maximum diastolic potential and pacemaker potential, after a change in the stimulus frequency, all have similar monoexponential time courses (Ƭ ≈ 3 min) and are accompanied by slow changes in tension production over a similar time course. In Purkinje fibres that exhibit spontaneous activity, rapid stimulation results in overdrive excitation: an acceleration of spontaneous activity when stimulation is ceased.

Mechanism of the negative force-frequency relationship in physiologically intact rat ventricular myocardium--studies by intracellular Ca2+ monitor with indo-1 and by 31P-nuclear magnetic resonance spectroscopy

We studied the subcellular mechanisms of the negative force-frequency relationship in rat myocardium by measuring 1) intracellular Ca2+ transients by indo-1 fluorometry and 2) intracellular pH (pHi) and phosphate compounds with 31P-nuclear magnetic resonance (NMR). The data were compared with those from guinea pig hearts, which show a positive force-frequency relationship. By increasing the pacing rate from 3 Hz to 5 Hz, the peak positive first derivative of left ventricular pressure (LVdP/dt) in rat heart decreased by 10 +/- 1% (n = 6). In contrast to this negative inotropic response, simultaneously measured peak Ca2+ transients increased by 6 +/- 1%. Guinea pig heart (n = 6) showed an increase in peak positive LVdP/dt (33 +/- 1%) which was associated with an increase in peak Ca2+ transients (8 +/- 1%). Under equivalent experimental conditions in an NMR spectrometer, this increase in the pacing rate did not affect intracellular levels of phosphate compounds in either rat (n = 6) or guinea pig heart (n = 6). In contrast, pHi showed a decrease of 0.031 +/- 0.006 pH units in rat heart, while no changes were observed in guinea pig heart. These results suggest that in physiological rat myocardium, pHi is susceptible to changes in the stimulus frequency and may affect the Ca(2+)-responsiveness of contractile proteins, which results in the negative force-frequency relationship.

Action potential duration and force-frequency relationship in isolated rabbit, guinea pig and rat cardiac muscle

The effect of action potential duration and elevated cytosolic sodium concentration on the force-frequency relationship in isolated rabbit, guinea pig and rat papillary muscle preparations was studied. Shortening of action potential duration in guinea pig and rabbit from 150-200 ms to values characteristic of rat (20-40 ms), using the K(ATP) channel activator levkromakalim (15 mumol.l-1), markedly reduced the force of contraction and converted the positive force-frequency relationship into negative one at longer pacing cycle lengths. This conversion was greatly enhanced in the presence of acetylstrophanthidin (0.2-1 mumol.l-1), an inhibitor of the Na-K pump. Acetylstrophanthidin (1 mumol.l-1) alone, however, had no effect on the force-frequency relationship. Prolongation of action potential duration in rat with inhibitors of cardiac K channels (4-aminopyridine [10 mmol.l-1] plus tetraethylammonium [2 mmol.l-1) increased the force of contraction and abolished the negative force-frequency relationship observed in rat at longer pacing-cycle lengths. It is concluded that both action potential duration and cytosolic sodium concentration are major determinants of the force-frequency relationship in mammalian myocardium.

Effect on the fura-2 transient of rapidly blocking the Ca2+ channel in electrically stimulated rabbit heart cells

1. We used a rapid solution switcher technique to investigate mechanisms that might trigger intracellular Ca2+ release in rabbit ventricular myocytes. The study was carried out at 36 degrees C, intracellular Ca2+ (Ca2+i) was monitored with fura-2, and myocytes were electrically stimulated. 2. In patch-clamped cells, using the switcher to apply 20 microM nifedipine (an L-type Ca2+ current (ICa,L) blocker) 4 s before a depolarization to +10 mV reduced the amplitude of ICa,L to 10.25 +/- 2.25% of control (mean +/- S.E.M., n = 7 cells). 3. In externally stimulated cells, a rapid switch to 20 microM nifedipine 4 s before a stimulus reduced the amplitude of the fura-2 transient to 64.01 +/- 2.09% of control (mean +/- S.E.M., n = 19 cells). Using an in vivo calibration curve for fura-2, this was equivalent to a reduction in the Ca2+ transient to 50% during nifedipine application. Since an identical nifedipine switch reduced ICa,L to 10.25%, it would seem that blocking a large fraction of ICa,L inhibited only half the Ca2+ transient. 4. The Na(+)-Ca2+ exchanger is inhibited by 5 mM nickel. Switching to 20 microM nifedipine +5 mM nickel 4 s before a stimulus abolished the fura-2 transient completely, consistent with the hypothesis that Ca2+ entry via reverse Na(+)-Ca2+ exchange might trigger a fraction of the fura-2 transient that remained during nifedipine. 5. After the Na(+)-K+ pump was inhibited by strophanthidin to increase intracellular Na+ (Na+i), a switch to 20 microM nifedipine became progressively less effective in reducing the fura-2 transient. This suggests that as Na+i rose, other mechanisms (perhaps reverse Na(+)-Ca2+ exchange) appeared able to substitute for ICa,L in triggering the Ca2+ transient. 6. In cells depleted of Nai+ to inhibit the triggering of sarcoplasmic reticulum (SR) Ca2+ release by reverse Na(+)-Ca2+ exchange, a nifedipine switch reduced the fura-2 transient to 10.9 +/- 4.19% (mean +/- S.E.M., n = 7; equivalent to 6.5% of the Ca2+ transient). 7. A switch to Na(+)-free (Li+) solution 100 ms before an electrical stimulus caused an increase in the fura-2 transient of 12.2 +/- 1.5% (mean +/- S.E.M., n = 7; equivalent to a 22% increase in the Ca2+ transient). 8. The results confirm that ICa,L is an important trigger for SR Ca2+ release and the resulting Ca2+ transient. However, since 50% of the Ca2+ transient remained when ICa,L was largely inhibited, it would seem likely that other SR trigger mechanisms might exist in addition. These data are consistent with the idea that Ca2+ entry via reverse Na(+)-Ca2+ exchange during the upstroke of the normal cardiac action potential might trigger a fraction of SR Ca2+ release and the resulting Ca2+ transient.

Electrophysiologic changes in ischemic ventricular myocardium: I. Influence of ionic, metabolic, and energetic changes

Myocardial ischemia leads to significant changes in the intracellular and extracellular ionic milieu, high-energy phosphate compounds, and accumulation of metabolic by-products. Changes are measured in extracellular pH and K+, and intracellular pH, Ca2+, Na+, Mg2+, ATP, ADP, and inorganic phosphate. Alterations of membrane currents occur as a consequence of these ionic changes, adrenergic receptor stimulation, and accumulation of lactate, amphipathic compounds, and adenosine. Changes in the volume of the extracellular and intracellular spaces contribute further to the ultimate perturbations of active and passive membrane properties that underlie alterations in excitability, abnormal automaticity, refractoriness, and conduction. These characteristic changes of electrophysiologic properties culminate in loss of excitability and failure of impulse propagation and form the substrate for ventricular arrhythmias mediated through abnormal impulse formation and reentry. The ability to detail the changes in ions, metabolites, and high-energy phosphate compounds in both the extracellular and intracellular spaces and to correlate them directly with the simultaneously occurring electrophysiologic changes have greatly enhanced our understanding of the electrical events that characterize the ischemic process and hold promise for permitting studies aimed at developing interventions that may lessen the lethal consequences of ischemia.

Cesium, Na(+)-K(+) Pump and Pacemaker Potential in Cardiac Purkinje Fibers

The mechanisms of the hyperpolarizing and depolarizing actions of cesium were studied in cardiac Purkinje fibers perfused in vitro by means of a microelectrode technique under conditions that modify either the Na(+)-K(+) pump activity or I(f). Cs(+) (2 mM) inconsistently increased and then decreased the maximum diastolic potential (MDP); and markedly decreased diastolic depolarization (DD). Increase and decrease in MDP persisted in fibers driven at fast rate (no diastolic interval and no activation of I(f)). In quiescent fibers, Cs(+) caused a transient hyperpolarization during which elicited action potentials were followed by a markedly decreased undershoot and a much reduced DD. In fibers depolarized at the plateau in zero [K(+)](o) (no I(f)), Cs(+) induced a persistent hyperpolarization. In 2 mM [K(+)](o), Cs(+) reduced the undershoot and suppressed spontaneous activity by hyperpolarizing and thus preventing the attainment of the threshold. In 7 mM [K(+)](o), DD and undershoot were smaller and Cs(+) reduced them. In 7 and 10 mM [K(+)](o), Cs(+) caused a small inconsistent hyperpolarization and a net depolarization in quiescent fibers; and decreased MDP in driven fibers. In the presence of strophanthidin, Cs(+) hyperpolarized less. Increasing [Cs(+)](o) to 4, 8 and 16 mM gradually hyperpolarized less, depolarized more and abolished the undershoot. We conclude that in Purkinje fibers Cs(+) hyperpolarizes the membrane by stimulating the activity of the electrogenic Na(+)-K(+) pump (and not by suppressing I(f)), and blocks the pacemaker potential by blocking the undershoot, consistent with a Cs(+) block of a potassium pacemaker current. Copyright 1995 S. Karger AG, Basel

The role of the Na(+)-Ca2+ exchanger in the rate-dependent increase in contraction in guinea-pig ventricular myocytes

1. The intracellular sodium activity (alpha Na1), contraction and membrane current were recorded simultaneously in voltage-clamped guinea-pig ventricular myocytes. 2. Increasing the frequency (from 0.5 to 3 Hz) of voltage clamp pulses to 0 mV from a holding potential of -80 mV led to an increase in both alpha Na1 and contraction. The rate-dependent increase in contraction was reduced by 25 microM tetrodotoxin (TTX) and abolished with a holding potential of -40 mV. There was no rate-dependent rise in alpha Na1 with a holding potential of -40 mV. These results suggest an important role for alpha Na1 and in particular Na+ influx via Na+ channels during rate-dependent changes in contraction. 3. After an increase in frequency from 0.5 to 3 Hz, membrane current at the end of voltage clamp pulses became progressively more outward and the tail current upon at repolarization became progressively more inward compared with those recorded at 0.5 Hz. TTX reduced the magnitude of both the outward and inward rate-dependent shifts of current. 4. The addition of extracellular CsCl blocked the inward rectifier potassium current (IK.1) and the delayed rectifier (IK), but did not change the rate-dependent shift in current. 5. The difference between current-voltage relationships at 0.5 and 3 Hz showed that the rate-dependent outward shift of current at the end of voltage clamp pulses was small at potentials negative to -20 mV, was larger at more positive potentials and was reduced by TTX at most potentials. The TTX-sensitive component reversed at -47 mV. 6. These results are consistent with a net increase in outward Na(+)-Ca2+ exchange current during a voltage clamp pulse in response to the rise of alpha Na1. The increase in outward current (resulting from either enhanced Ca2+ influx or reduced Ca2+ efflux) will augment the Ca2+ load of the cell and contribute to the rate-dependent increase in contraction.

Role of Intracellular Sodium Activity in the Control of Contraction in Cardiac Purkinje Fibers

The role of intracellular sodium activity (a(i)(Na)) in the control of force was studied in sheep cardiac Purkinje fibers exposed to norepinephrine (NE) and high [Ca](o) in the absence and presence of overdrive or of a low concentration of strophanthidin. Both NE and high [Ca](o) decrease a(i)(Na) and increae force, while overdrive increases and low strophanthidin decreases both parameters. In the presence of NE, overdrive increases a(i)(Na) less than force and is followed by a more pronounced undershoot in a(i)(Na) and force. In contrast, in high [Ca](o) overdrive increases a(i)(Na) more than force and is followed by a less pronounced undershoot in a(i)(Na) and force than in NE. High [Ca](o) increases force to a peak, but then the decreasing a(i)(Na) reduces force. In all these conditions, a(i)(Na) determines force changes during recovery from overdrive. NE and high [Ca](o) decrease a(i)(Na) less and increase force more in low strophanthidin. Thus, changes in a(i)(Na) modulate the increase in force due to increased Ca influx and control force development when Ca influx is either unchanged (low strophanthidin) or has reached a steady state (high [Ca](o), recovery from overdrive). Copyright 1994 S. Karger AG, Basel

Effects of the Anemonia sulcata toxin (ATX II) on intracellular sodium and contractility in rat and guinea-pig myocardium

The effects of the Anemonia sulcata toxin ATX II on action potentials and contractility of isolated papillary muscles and single myocytes from rat and guinea-pig hearts have been studied. ATX II prolonged the action potential in both rat and guinea-pig papillary muscle. Although it produced a positive inotropic effect in guinea-pig papillary muscle, it failed to do so in rat papillary muscle. However, in single rat and guinea-pig ventricular cells, it both prolonged the action potential and had a positive inotropic effect. We suggest that ATX II does not cause a positive inotropic effect in rat papillary muscle, because it induces Ca2+ overload. In single cells the positive inotropic effect was reduced by approximately 50% when the contractions were triggered by voltage clamp pulses of constant duration rather than by action potentials. This suggests that the inotropic effect of ATX II is in part the result of the prolongation of the action potential. The intracellular Na+ activity (a(i)Na) in single ventricular cells was measured with the Na(+)-sensitive fluorescent dye SBFI. After exposure of the cells to ATX II, a(i)Na was increased by a maximum of 1.9 +/- 0.3 and 2.2 +/- 0.3 mM in rat and guinea-pig cells, respectively. It is suggested that the positive inotropic effect of ATX II is also in part the result of the rise in a(i)Na.

Effects of electrical pacing to the preischemic rate during rewarming after hypothermic ischemia in the rat heart

The effects of electrical pacing during the early reperfusion following hypothermic global ischemia (60 min, at 25 degrees C) was studied in the isolated working rat heart model. The hearts were divided into three groups. Hearts in Group I (n = 8) were control without hypothermia, ischemia or pacing. Hearts in Group II (n = 16) were paced with ventricular rate at 300 beats/min with 1 mVolt for 10 min during the Langendorff mode after an initial 5 min of reperfusion. Hearts in Group III (n = 14) were not paced. The recovery of aortic flow (both absolute and percent) was significantly better in Group II than in Group III, but was significantly lower in both groups than in control. No significant differences were noted, however, in heart rate, aortic pressure or coronary flow between Group II and III. In contrast, the tissue concentration of adenosine triphosphate (ATP) in Groups II and III decreased significantly by the end of reperfusion relative to Group I, but no difference in ATP existed between Group II and III. Myocardial ATP concentrations did not correlate with percent recovery of aortic flow. The myocardial concentration of calcium in Groups II and III increased by the end of reperfusion as compared with Group I, but no difference in calcium existed between Group II and III. The myocardial concentration of calcium demonstrated a significant correlation with percent recovery of aortic flow (r = 0.71, n = 30, p < 0.005). Our results indicate that an electrical pacing during early reperfusion in the myocardium improves functional recovery of aortic flow.

Postextrasystolic potentiation. Do we really know what it means and how to use it?

Postextrasystolic potentiation (PESP), the increase in contractility that follows an extrasystole, is an interesting phenomenon that has been known for almost 100 years. The literature on this effect is reviewed. It is found that there is significant evidence that the phenomenon is independent of muscle loading and represents a distinct property of the myocardium. Examination of the literature pertaining to the cause of the effect suggests that calcium shifts within the sarcoplasmic reticulum are responsible, although there are some conflicts with this conclusion. Regarding the utility of PESP as a diagnostic test of latent viability of ischemic myocardium, the literature review reveals contradictions and conflicts with several methodological problems of the experiments. Finally, concerning the utility of continuous PESP (paired-pacing) to augment ventricular function in the failing ventricle, the studies again are inconclusive and methodologically suspect. Conditions for the proper analysis of the PESP response are reported, and suggestions for future studies are introduced.

Twitch-dependent SR Ca accumulation and release in rabbit ventricular myocytes

Using caffeine-induced contractures (Ccaf) and thapsigargin (TG), we estimated the fraction of sarcoplasmic reticulum (SR) Ca released at one twitch and also the number of twitches required to reload a Ca-depleted SR. Similar results were obtained for twitches or intracellular Ca (Cai) transient with the fluorescent indicator, indo 1. Sustained exposure to 10 mM caffeine completely depletes the SR of Ca in < 5 s (as assessed by a second Ccaf). After such Ca depletion, four to five twitches are necessary to reload the SR to the steady-state level (with a twitch constant, tau = 1.6 twitches). We also determined the time required for complete inhibition of the SR Ca-adenosinetriphosphatase (ATPase) by TG. After SR Ca depletion, 5 microM TG was applied for different periods of time before a train of "reloading" twitches. A TG exposure period of 90 s was sufficient to completely prevent Ccaf after these reloading twitches. When SR is Ca depleted, the twitch is larger in the presence of TG, indicating that the SR Ca-ATPase can limit the ability of Ca influx to activate contraction. To assess SR Ca released at one twitch in cells with normally Ca-loaded SR, 5 microM TG was applied for 90 s to prevent SR Ca reuptake. Then one or several twitches were activated (causing SR Ca release, but with reuptake completely blocked). After the twitch (or train), a Ccaf was used to assess remaining SR Ca.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular sodium activity and its regulation in guinea-pig atrial myocardium

1. Intracellular Na+ activity (aNai) and membrane resting potential were studied in quiescent guinea-pig atrial and papillary muscles by means of Na(+)-sensitive and conventional microelectrodes. The effects of the cardioactive steroid dihydroouabain (DHO) on aiNa, force of contraction and sarcolemmal Na+, K(+)-ATPase activity were also investigated. 2. In thirty atria and twenty-two papillary muscles, aNai amounted to 8.0 +/- 0.2 and 4.7 +/- 0.3 mM, respectively (mean +/- S.E.M.). When both tissues were from the same animal, with the same ion-sensitive microelectrode mean aNai values of 7.9 +/- 0.2 and 5.1 +/- 0.5 mM (P < 0.01) were obtained from eight atrial and eight papillary muscles, respectively. 3. Membrane resting potentials (Em) were significantly (P < 0.001) more negative in the papillary muscles (-83.5 +/- 0.7 mV; n = 8) than in the atrium (-78.1 +/- 0.5 mV; n = 8). Deviation of Em from EK (determined by K(+)-sensitive microelectrodes) was 3.0 +/- 0.2 mV in ventricular (P < 0.05) and 6.1 +/- 0.3 mV in atrial preparations (P < 0.05). 4. Inhibition of the Na+ pump by DHO increased aNai of the atrium within 10 min by 0.6 +/- 0.1 (n = 7), 1.3 +/- 0.1 (n = 5) and 3.2 +/- 0.2 mM (n = 5) at 5, 10 and 30 microM, respectively. In the papillary muscle, 10 microM DHO was without effect while aNai rose by 1.0 +/- 0.1 (n = 5) and 2.9 +/- 0.2 mM (n = 6) at 30 and 120 microM DHO. 5. Consistent with the aNai measurements, the potency of DHO to increase force of the isometric contraction was three times higher in atrium than in papillary muscle (stimulation frequency 0.2 Hz). 6. Hydrolytic activity of sarcolemmal Na+,K(+)-ATPase isolated from atria amounted to only one third of that detected in ventricles (0.07 +/- 0.01, n = 6, versus 0.2 +/- 0.01 mumol phosphate released min-1 (g tissue)-1, n = 5). The inhibitory potencies of DHO on sarcolemmal Na+,K(+)-ATPase preparations were found to be identical in the enzymes from either tissue. 7. It is concluded that a lower Na+ pump density is responsible for the higher aNai and for the lower resting membrane potential in atrial as compared to ventricular cells. The regulation of cellular Na+ homeostasis in atrial muscle appears to be closer to the limits of its capacity than in ventricle, explaining the higher sensitivity of the atrium to interventions which impede Na+ pump activity.

Functional dissociation of cellular activation as a mechanism of Mobitz type II atrioventricular block

BACKGROUND

Several mechanisms have been advanced to explain Mobitz type II atrioventricular block in the ischemically damaged His-Purkinje system. Only recently, however, has an animal model been developed to study this form of conduction defect in vivo and in vitro.

METHODS AND RESULTS

Conduction defects were induced in anesthetized dogs by ischemic damage to the proximal His-Purkinje system after anterior septal artery ligation. Stable 2:1 atrioventricular block, localized within the His bundle or in the proximal bundle branches, was obtained in each dog by atrial pacing at an average rate of 239 +/- 20 beats per minute (n = 12). In vitro studies were then performed from the same hearts. Action potentials and electrograms were simultaneously recorded from the His bundle and the proximal right bundle branch at the site of damage. At slow rates of pacing (40-60 beats per minute), the action potential amplitude was 85 +/- 4 mV, and some cells (10 +/- 3%) showed dissociation from the electrical activity in the bundle. At fast rates (149 +/- 11 beats per minute), during 1:1 conduction, the frequency of cellular dissociation increased to 57 +/- 6% (p < 0.001), and the action potential amplitude decreased (-31 +/- 4%, p < 0.001). The frequency of dissociation closely correlated with the reduction in action potential amplitude (r = 0.87, p < 0.001). These changes were markedly attenuated once 2:1 block developed. The site of block was not constant but rather showed a dynamic behavior with spatial shifting in response to changes in pacing rate or the introduction of extrastimuli.

CONCLUSIONS

These results indicate that in the ischemically damaged proximal His-Purkinje system, an increase in rate leads to reduced and asynchronous cellular activation before 2:1 block. The latter provides a more stable activation pattern, because the frequency of dissociation is markedly reduced.

Contribution of sarcolemmal sodium-calcium exchange and intracellular calcium release to force development in isolated canine ventricular muscle

The aim of this work was to determine the relationship between peak twitch amplitude and sarcoplasmic reticulum (SR) Ca2+ content during changes of stimulation frequency in isolated canine ventricle, and to estimate the extent to which these changes were dependent upon sarcolemmal Na(+)-Ca2+ exchange. In physiological [Na+]o, increased stimulation frequency in the 0.2-2-Hz range resulted in a positive inotropic effect characterized by an increase in peak twitch amplitude and a decrease in the duration of contraction, measured as changes in isometric force development or unloaded cell shortening in intact muscle and isolated single cells, respectively. Action potentials recorded from single cells indicated that the inotropic effect was associated with a progressive decrease of action potential duration and a marked reduction in average time spent by the cell near the resting potential during the stimulus train. The frequency-dependent increase of peak twitch force was correlated with an increase of Ca2+ uptake into and release from the SR. This was estimated indirectly using the phasic contractile response to rapid (less than 1 s) lowering of perfusate temperature from 37 degrees C to 0-2 degrees C and changes of twitch amplitude resulting from perturbations in the pattern of electrical stimulation. Lowering [Na+]o from 140 to 70 mM resulted in an increase of contractile strength, which was accompanied by a similar increase of apparent SR Ca2+ content, both of which could be abolished by exposure to ryanodine (1 x 10(-8) M), caffeine (3 x 10(-3) M), or nifedipine (2 x 10(-6) M). Increased stimulation frequency in 70 mM [Na+]o resulted in a negative contractile staircase, characterized by a graded decrease of peak isometric force development or unloaded cell shortening. SR Ca2+ content estimated under identical conditions remained unaltered. Rate constants derived from mechanical restitution studies implied that the depressant effect of increased stimulation frequency in 70 mM [Na+]o was not a consequence of a decreased rate of refilling of a releasable pool of Ca2+ within the cell. These results demonstrate that frequency-dependent changes of contractile strength and intracellular Ca2+ loading in 140 mM [Na+]o require the presence of a functional sarcolemmal Na(+)-Ca2+ exchange process. The possibility that the negative staircase in 70 mM [Na+]o is related to inhibition of Ca(2+)-induced release of Ca2+ from the SR by various cellular mechanisms is discussed.

The relationship between contraction and intracellular sodium in rat and guinea-pig ventricular myocytes

1. The contraction, measured optically, and the intracellular Na+ activity (aNai), measured with the Na(+)-sensitive fluorescent dye SBFI, have been recorded simultaneously in rat and guinea-pig ventricular myocytes. 2. In rat and guinea-pig ventricular myocytes at rest, aNai was 7.8 +/- 0.3 mM (n = 4) and 5.1 +/- 0.3 mM (n = 16), respectively. 3. When both rat and guinea-pig ventricular myocytes were stimulated at 1 Hz after a rest there was usually a gradual increase in twitch shortening (referred to as a 'staircase') over several minutes accompanied by an increase in aNai over a similar time course. Twitch shortening increased by 21 +/- 3% (n = 6) and 20 +/- 4% (n = 16) (of steady-state twitch shortening during 1 Hz stimulation) per millimolar rise in aNai in rat and guinea-pig ventricular myocytes, respectively. 4. When rat and guinea-pig ventricular myocytes were exposed to strophanthidin to block the Na(+)-K+ pump, there were increases in twitch shortening and aNai over similar time courses. Twitch shortening increased by 24 +/- 4% (n = 5) and 20 +/- 3% (n = 10) (of control twitch shortening) per millimolar rise in aNai in rat and guinea-pig ventricular myocytes respectively. 5. The inotropic effect of cardiac glycosides, such as strophanthidin, is widely regarded to be principally the result of the rise in aNai. The similarity of the relation between twitch shortening and aNai during the staircase and on application of strophanthidin suggests that the progressive increase in the strength of contraction during the staircase was also linked to the rise in aNai. 6. In guinea-pig, but not rat, ventricular myocytes there was hysteresis in the relation between twitch shortening and aNai on application and wash-off of strophanthidin. This indicates that strophanthidin has another inotropic action in guinea-pig ventricular myocytes. 7. A computer model of excitation-contraction coupling has been developed to simulate the staircase and the action of cardiac glycoside and to account for the relation between contraction and intracellular Na+.

Modeling the dynamic features of the electrogenic Na,K pump of cardiac cells

The purpose of this paper is to examine the dynamic features of the electrogenic Na,K pump of cardiac cells, based on a comparative analysis of a mechanistic model and an ad hoc mathematical description of the Na,K pump. Both representations are incorporated into a modified version of the Beeler-Reuter model for the ventricular membrane, and the resulting action potential models are studied under conditions of repetitive stimulation at steady rates between 0 and 3 Hz. The two Na,K pump representations have nearly identical steady-state characteristics of sensitivity to internal Na+ concentration, external K+ concentration, and membrane potential. Rapid voltage-dependent transient pump currents are present in the mechanistic model, while they are absent in the ad hoc mathematical description we used. The stimulation results show that a sizable peak of pump current caused by the action potential upstroke in the mechanistic model affects phase 1 repolarization, and that this effect is relatively independent of the stimulation rate. The pump current generated by our ad hoc mathematical description is constant during the action potential and does not affect directly the repolarization time course. While the two Na,K pump models show similar pumping efficiency at low stimulation rates, the mechanistic pump is more efficient at high rates of activity. In essence, the distinctive features of the mechanistic model are due to an energy barrier expressing the voltage dependence of the translocation step of the mechanism, and to the redistribution of the intermediates of the biochemical reactions during activity. In comparison, the ad hoc mathematical description exhibits a fixed dependence of the pump current on voltage and ionic concentrations.

Control of the Na-Ca exchanger in isolated heart cells. II. Beat-dependent activation in normal cells by intracellular calcium

Electrical stimulation of isolated adult rat heart cells in suspension at 4 Hz resulted in a fourfold increase in the rate of sodium influx and efflux across the sarcolemma, with no change in total cell sodium, as measured with 22Na. The magnitude of stimulation-dependent sodium fluxes under these conditions averaged 17 nmol/min/mg protein. The increased rate of efflux was inhibited by tetrodotoxin, verapamil, or dichlorobenzamil and required extracellular calcium. The inhibition by tetrodotoxin was overcome by Bay K 8644. The basal rate of 22Na efflux in cells at rest was inhibited only slightly by dichlorobenzamil. The stimulation-induced efflux was not inhibited by ouabain, but in the presence of ouabain, stimulation increased the rate of accumulation of total sodium by 4 nmol/min/mg. This increase was inhibited by tetrodotoxin or verapamil. A calcium-dependent increase in rate of 22Na influx and efflux could also be induced by KCl addition. This was inhibited by verapamil and dichlorobenzamil but not by tetrodotoxin and was reversed by EGTA, but only after a delay. We conclude the following. 1) The Na-Ca exchanger in cells at rest is no more than 10% activated. 2) The exchanger becomes activated directly or indirectly by calcium that enters the cell through calcium channels during excitation. 3) In this preparation the major part of excitation-induced sodium fluxes are mediated by the Na-Ca exchanger, with only a relatively small direct participation of sodium channels. These channels participate indirectly by promoting calcium channel activation. 4) If all the calcium-dependent sodium fluxes were Na-Ca exchange, then calcium flux through the exchanger per beat would be about sevenfold larger than that through the calcium channels. An undetermined part of the calcium-dependent sodium fluxes, however, could be a direct Na-Na exchange through the activated Na-Ca exchanger.

Aging: stimulation rate on cardiac intracellular Na+ activity and developed tension

Previous reports suggested that Na,K-ATPase activity and Na(+)-pump capacity decrease with senescence in left atrial myocardium of F344 rats. Current experiments were designed to determine if this reduction in the Na(+)-pump affects free intracellular Na+ levels. Mean intracellular Na+ ion activity (aiNa) was measured with Na-selective microelectrodes in left atrial muscle isolated from hearts of 4-, 14- and 25-month-old F344 rats. Preparations were stimulated randomly at frequencies between 0 and 12 h. There were no age-associated differences in aiNa measured at any frequency or in the decay of Na+ activity following discontinuation of electrical stimulation. These data indicate that the aging-related decline in Na,K-ATPase does not result in elevated aiNa even at extremely high stimulation frequencies, thus suggesting that other routes of Na+ influx and efflux are also altered in atrial muscle.

Evolution of rhythms during periodic stimulation of embryonic chick heart cell aggregates

During periodic stimulation of spontaneously beating chick heart cell aggregates, there is often an evolution of coupling patterns between the stimulator and the aggregate action potential. For example, at rapid stimulation frequencies, a rhythm that is initially 1:1 (stimulus frequency:aggregate frequency) can evolve to other rhythms such as 5:4 and 4:3. Time-dependent effects generated during periodic stimulation are characterized by three types of experiments to determine 1) the effect of periodic stimulation on the intrinsic cardiac beat rate (overdrive suppression), 2) the effect of periodic stimulation on the phase resetting properties of the aggregate, and 3) the time-dependent changes in the coupling patterns between the stimulator and the aggregate during periodic stimulation. The protocols involved variations in the duration and rate of periodic stimulation. A mathematical model is developed in the form of a two-dimensional finite difference equation based on the data from experiments 1 and 2. The model is used to predict the data generated by experiment 3. There is good correspondence with the experiments in that the theory reproduces complex transitions between various rhythms and displays irregular rhythms similar to those observed experimentally. These results have implications for the evolution of cardiac arrhythmias such as atrioventricular heart block and modulated parasystole.

Force-interval relations of twitches and cold contractures in rat cardiac trabeculae. Effect of ryanodine

The twitch force (Ft)-interval relation of cardiac muscle reflects recovery of calcium release from the sarcoplasmic reticulum (SR). The calcium content of the SR is thought to be reflected by force developed during a contracture (Fc), induced by rapid cooling to near 0 degrees C. In right ventricular trabeculae of rat, under control conditions, the Ft-interval relation consisted of recovery of Ft to steady state (early recovery), followed by a secondary increase of Ft up to a maximum at an interval of approximately 100 seconds (rest potentiation) and a decline of Ft at intervals greater than 100 seconds (rest depression). The mechanisms that may underlie recovery of force after the last twitch at short intervals are 1) time-dependent transport of Ca2+ from the uptake compartment of the SR to the release compartment, 2) recovery of slow inward Ca2+ current during the action potential, and 3) recovery of the Ca2+ release channels in the SR. The Fc-interval relation was similar to the Ft-interval relation in that both a rest potentiation and a rest depression phase were present. However, at short interstimulus intervals (less than 1 second), Fc was independent of time, suggesting that the mechanism underlying early recovery was bypassed. Ryanodine (0.1-10 nM) reduced rest potentiation in a dose-dependent manner and accelerated rest depression of both Ft and Fc. At high ryanodine concentration, a significant Fc could only be induced after short intervals. Significant acceleration of rest depression was observed at low ryanodine concentrations, when Ft at intervals of 5 seconds was kept constant by increasing the stimulus frequency of [Ca2+]o, suggesting that the ryanodine effect was enhanced by increased [Ca2+]i. Ryanodine also increased the rate of decay of postextrasystolic potentiation in a dose-dependent manner. A significant effect was observed in 10 nM ryanodine. The twitch was not prolonged by ryanodine at these concentrations. These results suggest that the small magnitude of the twitch at short intervals is due to the finite time required by SR Ca2+ release channels to fully recover after a twitch. Furthermore, the results offer support for the hypothesis that ryanodine (in the nanomolar range) promotes Ca2+ leak from the SR in a dose-dependent manner and thereby limits Ca2+ accumulation during the interstimulus interval. Therefore, it may be expected that the negative inotropic effect of ryanodine is due to the SR Ca2+ depletion, and it is not necessary to postulate that ryanodine "blocks" the Ca2+ release channels in the SR.

Mechanisms by which calcium modulates diastolic depolarization in sheep cardiac Purkinje fibers

The mechanisms by which calcium modulates diastolic depolarization (DD) in sheep cardiac Purkinje fibers were studied in vitro. Increasing [Ca]o from 2.7 mM to 10.8 mM increased both the slope and amplitude of DD, induced oscillatory potentials (V(os)), and prolonged depolarization (V(ex)). The steepening of DD occurred even in the absence of an obvious V(os). The increase in DD amplitude was due both to an increase in the maximum diastolic potential and to a less negative steady-state level. At constant [Ca]o, increasing the driving rate had effects similar to those induced by increasing [Ca]o. The increase in DD slope and amplitude was least at the slowest rates and leveled off at the fastest rates in high [Ca]o. Lowering [Ca]o decreased DD slope and amplitude, but spontaneous activity could be present during interruption of the drive. In slowly driven fibers, increasing [Ca]o to 10.8 mM initially shifted the maximum diastolic potential and steady state DD to more negative values, and subsequently shifted the latter (but not the former) to less negative values. On recovery, a transient depolarization occurred. Quiescent fibers exposed to high [Ca]o also underwent a transient hyperpolarization and a subsequent depolarization, whereas reciprocal effects occurred when [Ca]o was lowered. It is concluded that [Ca]o modulates DD through several different mechanisms and that most (but not all) modifications induced are brought about by changes in [Ca]i.

Contributions of [Ca2+]i, [Pi]i, and pHi to altered diastolic myocyte tone during partial metabolic inhibition

Ischemia may cause increased or decreased distensibility of the left ventricle, but the cellular mechanisms involved have not been clarified. We examined the possible contributions of changes in intracellular inorganic phosphate, pH, and Ca2+ concentrations to altered diastolic function in cultured myocytes subjected to partial metabolic inhibition. Paced cultured embryonic chick and adult rabbit ventricular myocytes superfused with 20 mM 2-deoxyglucose (2DG) exhibited an increase in end-diastolic intracellular free calcium concentration ([Ca2+]i) and an upward shift in end-diastolic cell position. These results indicate that glycolytic blockade increases diastolic and systolic calcium in paced ventricular myocytes, and that this elevated diastolic calcium influences the extent of diastolic relaxation. In contrast, paced ventricular myocytes superfused with 1 mM cyanide (CN) exhibited a similar increase in end-diastolic [Ca2+]i but a decrease in end-diastolic cell position and amplitude of motion. Although changes in ATP contents were similar in both groups (2DG, -29.9%; CN, -40.1%), alterations of intracellular pH and inorganic phosphate concentrations were different. In 2DG-treated cells, pHi did not decrease significantly (7.18 +/- 0.04 to 7.12 +/- 0.11, n = 14) but in the CN group it decreased markedly within 6 min (7.18 +/- 0.04 to 6.76 +/- 0.11, n = 11, P less than 0.01). Intracellular inorganic phosphate decreased slightly in the 2DG group (-14.8%, NS) but increased in cells exposed to CN (45.7%, P less than 0.02). We conclude that while a prominent increase in diastolic [Ca2+]i occurs in rapidly paced ventricular myocytes exposed to either inhibitors of glycolysis or oxidative phosphorylation, the effects of this increase in [Ca2+]i on diastolic distensibility may be influenced by intracellular accumulation of metabolites that decrease the sensitivity of myofilament to [Ca2+]i.

Effect of norepinephrine and heart rate on intracellular sodium activity and membrane potential in beating guinea pig ventricular muscle

The effect of 3 microM norepinephrine (NE) on intracellular sodium activity (aiNa) and resting membrane potential was studied by continuous intracellular recordings with a conventional and an ion-selective microelectrode. The electrodes were impaled simultaneously in small (diameter, 0.3 mm) superfused trabeculae of the beating guinea pig ventricle at 37 degrees C. In the absence of NE, changes of the beating rate produced an increase of aiNa by 1.5 +/- 0.17 mM (from 0 to 1 Hz) and 1.9 +/- 0.47 mM (from 0 to 2 Hz). In the presence of NE, there was a very small significant increase of aiNa during constant stimulation (1 Hz) and at at [K+]o of 4.7 and 11.5 mM. After 7 minutes of exposure, aiNa increased by 0.5 +/- 0.19 mM (mean +/- SEM, n = 4) at [K+]o of 4.7 mM and by 0.5 +/- 0.22 (n = 6) at [K+]o of 11.5 mM. Resting membrane potential became more positive by 1 mV at both levels of [K+]o. The effect of NE became also clearly manifest from the configurational changes of action potentials (profound increase in plateau height and duration). Stimulation of the Na(+)-K+ pump by NE became manifest from the changes of resting membrane potential and aiNa after abrupt cessation of stimulation. The magnitude and the rate of the decrease in aiNa and the initial rate of hyperpolarization were significantly greater in the presence of NE than in its absence.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of ouabain binding in beating ventricular myocardium from guinea-pigs: effects of lidocaine and monensin

To further elucidate the interdependence between digitalis sensitivity and the cellular Na+ load, the influence of two Na+ load modifying drugs, monensin and lidocaine, on the concentration-dependence of ouabain binding and ouabain effects was studied in beating ventricular strips from guinea-pig heart. Monensin (3 x 10(-6)M), a Na+ ionophore known to elevate Na+ influx, enhanced 3H-ouabain binding (by approximately 40%) as well as the ouabain effect at non-toxic ouabain concentrations, and it shifted the threshold for toxicity towards threefold lower ouabain concentrations. Lidocaine (2 x 10(-4)M), a Na+ channel blocker, lowered ouabain binding by about one third, and it extended the ouabain concentration range tolerated without toxicity by a factor of three. In the concentrations used, neither compound exerted any direct effect on ouabain binding studied in isolated cardiac membranes. The binding data obtained in the muscle strips were well fitted by a mathematical model which quantifies the dependence of ouabain binding on the underlying (Na+ +K+)-ATPase activity. These findings provide evidence for an indirect drug-induced modulation of ouabain binding via the interference with the cellular Na+ load.

NASPE young investigator awardee-1990. Complex rhythms resulting from overdrive suppression in electrically stimulated heart cell aggregates

Spontaneously beating embryonic chick heart cell aggregates were stimulated with current pulses delivered either as periodic trains, or at a fixed delay after each action potential. Following stimulation at fixed rates faster than the intrinsic rate, there was a transient slowing of the spontaneous rhythm. This response, called overdrive suppression, can lead to a complex evolution of rhythms. During periodic stimulation there is a continuum of dropped beat patterns, and during fixed delay stimulation bursting activity appears. This study provides a conceptual basis for understanding analogous rhythms in the intact heart.

Net transsarcolemmal Ca2+ shifts versus Ca/Ca exchange in guinea pig ventricular muscle

We investigated the net transsarcolemmal Ca2+ shifts and Ca/Ca exchange by means of 45Ca in isolated, perfused ventricles of guinea pig heart treated with vanadate to inhibit ATP-driven sarcolemmal Ca2+ pump. The heart was stimulated (at the rate of 60/min) and perfused with a solution containing 45Ca for 60 min. Thereafter stimulation was stopped and either perfusion with radioactive solution was continued or the solution was exchanged for a non-radioactive one. In the first case, tissue 45Ca content (equivalent to the exchangeable Ca2+ content) dropped from 1.960 +/- 0.120 mmol/kg of wet weight (w.w.) to 0.715 +/- 0.049 mmol/kg w.w. and stabilized at this level between 5th and 10th min. In the second case, decrease in 45Ca content continued and within 40 min attained 0.047 +/- 0.004 mmol/kg w.w., despite stabilizing of the total exchangeable Ca2+ content. Drop of 45Ca content in the rested heart perfused (until the end of experiments) with radioactive solution resulted from the net transsarcolemmal Ca2+ shift and it was strongly inhibited by removal of extracellular Na+. The continuing drop in 45Ca content in the heart perfused with non-radioactive solution while total Ca2+ content stabilized must have resulted from Ca/Ca exchange; it was stimulated by removal of extracellular Na+. These experiments separate two modes of 45Ca fluxes and suggest that a common route of these fluxes is the Na/Ca exchanger.

Acidification and intracellular sodium ion activity during stimulated myocardial ischemia

With the use of microelectrodes, intracellular pH (pHi), surface pH (pHs), and intracellular Na+ activity (aiNa) were measured in isolated guinea pig papillary muscles during normal superfusion and during a reversible condition of simulated ischemia. Acid loading by NH+4 prepulse or by CO2-HCO3- addition during superfusion with pH 7.4 solutions caused internal acidification followed by a recovery of pHi, which could be inhibited by amiloride. pHi recovery was associated with an amiloride-sensitive peak rise of aiNa and membrane hyperpolarization, indicative of Na(+)-H+ exchange. Peak increase of aiNa was absent if the pH of the superfusion solution was concomitantly lowered. Imposed ischemia after control superfusion caused membrane depolarization and acidification of pHi and pHs. The change of pHs consistently was larger than that of pHi. aiNa decreased from 5.5 to 4.6 mM after 10-min ischemia. Enlarging the pHi (and pHs) decrease in ischemia by prior reduction of the tissue buffer capacity (CO2-HCO3(-)-free superfusion) was unable to induce a rise of aiNa during the subsequent ischemic period. Amiloride had no significant effect on aiNa during ischemia. It is concluded that the important acidification of pHs reduces the rate of pHi regulatory Na(+)-H+ exchange and thereby contributes to a longer maintenance of the Na+ electrochemical gradient in ischemic cardiac muscle.

Sodium current-induced release of calcium from cardiac sarcoplasmic reticulum

The role of sodium-calcium exchange at the sarcolemma in the release of calcium from cardiac sarcoplasmic reticulum was investigated in voltage-clamped, isolated cardiac myocytes. In the absence of calcium entry through voltage-dependent calcium channels, membrane depolarization elicited release of calcium from ryanodine-sensitive internal stores. This process was dependent on sodium entry through tetrodotoxin-sensitive sodium channels. Calcium release under these conditions was also dependent on extracellular calcium concentration, suggesting a calcium-induced trigger release mechanism that involves calcium entry into the cell by sodium-calcium exchange. This sodium current-induced calcium release mechanism may explain, in part, the positive inotropic effects of cardiac glycosides and the negative inotropic effects of a variety of antiarrhythmic drugs that interact with cardiac sodium channels. In response to a transient rise of intracellular sodium, sodium-calcium exchange may promote calcium entry into cardiac cells and trigger sarcoplasmic calcium release during physiologic action potentials.

Interaction of intracellular ion buffering with transmembrane-coupled ion transport

The role of the Na/Ca exchanger in the control of cellular excitability and tension development is a subject of current interest in cardiac physiology. It has been suggested that this coupled transporter is responsible for rapid changes in intracellular calcium activity during single beats, generation of plateau currents, which control action potential duration, and control of intracellular sodium during Na/K pump suppression, which may occur during terminal states of ischemia. The actual behavior of this exchanger is likely to be complex for several reasons. First, the exchanger transports two ionic species and thus its instantaneous flux rate depends on both intracellular sodium and calcium activity. Secondly, the alteration in intracellular calcium activity, which is caused by a given transmembrane calcium flux, and which controls the subsequent exchanger rate, is a complex function of available intracellular calcium buffering. The buffers convert the ongoing transmembrane calcium fluxes into changes in activity that are a small and variable fraction of the change in total calcium concentration. Using a number of simple assumptions, we model changes in intracellular calcium and sodium concentration under the influence of Na/Ca exchange, Na/K ATPase and Ca-ATPase pumps, and passive sodium and calcium currents during periods of suppression and reactivation of the Na/K ATPase pump. The goal is to see whether and to what extent general notions of the role of the Na/Ca exchanger used in planning and interpreting experimental studies are consistent with its function as derived from current mechanistic assumptions about the exchanger. We find, for example, that based on even very high estimates of intracellular calcium buffering, it is unlikely that Na/Ca exchange alone can control intracellular sodium during prolonged Na/K pump blockade. It is also shown that Na/Ca exchange can contaminate measurements of Na/K pump currents under a variety of experimental conditions. The way in which these and other functions are affected by the dissociation constants and total capacity of the intracellular calcium buffers are also explored in detail.

Effects of alpha-adrenergic stimulation on intracellular sodium activity and automaticity in canine Purkinje fibers

Approximately 60% of adult canine Purkinje fibers respond to alpha 1-adrenergic stimulation with a decrease in automaticity. Recent studies of disaggregated Purkinje myocytes have suggested that this negative chronotropic effect results from alpha 1-adrenergic activation of the Na-K pump. In this study we evaluated 1) whether Na-K pump activation is associated with the negative chronotropic effect of alpha 1-adrenergic stimulation in adult canine Purkinje fibers and 2) if the effect of alpha-agonists on the pump is direct or mediated by an increase in intracellular sodium activity (aNai). We used sodium selective microelectrodes to determine the effects of 5 x 10(-9) and 5 x 10(-8) M phenylephrine on aNai. Phenylephrine decreased automaticity in five of eight Purkinje fibers while an increase occurred in the other three. The rate decrease was always accompanied by a decrease in aNai (-3.9 mM; p less than 0.05), whereas in fibers showing an increase in rate, aNai was unchanged. To evaluate the effect of phenylephrine in the absence of changes in automaticity, 10 Purkinje fibers were studied during pacing. A clear-cut reduction in aNai (-2.8 mM) was present in six fibers; no change was seen in the other four. The effect of phenylephrine was blocked by prazosin but not by propranolol. We conclude that the effect of alpha 1-adrenergic stimulation to reduce aNai is consistent with activation of the Na-K pump. Moreover, this action of alpha 1-adrenergic stimulation is closely linked to its negative chronotropic effect.

Effects of strophanthidin on the slow inward current in guinea-pig isolated ventricular myocytes

1. The effect of strophanthidin on the slow inward current (Isi) and on contractile force were studied in guinea-pig isolated ventricular myocytes and intact papillary muscles, respectively. In myocytes, both low (10 nmol/L) and high (1-10 mumols/L) concentrations had small or no effects in either direction on Isi whereas norepinephrine (10-100 nmol/L) increased it. To determine whether the same results are obtained after decreasing or increasing intracellular calcium or sodium, the same concentrations of strophanthidin were tested in different procedures that are known to (i) increase [Ca]i and decrease [Na]i (high [Ca]o, 3.6-5.4 mmol/L; low [Na]o, 112 mmol/L; (ii) decrease [Ca]i and increase [Na]i (low [Ca]o, 0.45-1 mmol/L; Sr, 1 mmol/L; (iii) decrease [Ca]i and [Na]i (Cd, 0.1-0.2 mmol/L); and (iv) increase [Ca]i and [Na]i (veratridine, 0.2 mumol/L). High [Ca]o and veratridine increased whereas low [Ca]o and Cd decreased Isi. In contrast, during these various procedures, strophanthidin had small and inconsistent effects at a low or high concentration. In intact papillary muscles, low strophanthidin decreased whereas high strophanthidin increased contractile force. It is concluded that strophanthidin has little direct or indirect effect on Isi and that the decrease in force by low and increase in force by high concentrations in intact muscle are probably related to demonstrated decrease and increase, respectively, in intracellular sodium activity.

A model study of the contribution of active Na-K transport to membrane repolarization in cardiac cells

A biochemical model of active Na-K transport in cardiac cells was studied in conjunction with a representation of the passive membrane currents and ion concentration changes. The active transport model is based on the thermodynamic and kinetic properties of a six-step reaction scheme for the Na,K-ATPase. It has a fixed Na:K stoechiometry of 3:2, and its activation is governed by three parameters: membrane potential intracellular Na+ concentration, and interstitial K+ concentration. The Na-K pump current is directly proportional to the density of Na,K-ATPase molecules. The passive membrane currents and ion concentration changes involve only Na+ and K+ ions, and no attempt was made to provide a precise representation of Ca2+ currents or Ca2+ concentration changes. The surface-to-volume ratio of the interstitial compartment is 55 times larger than that of the intracellular compartment. The flux balance conditions are such that the original equilibrium concentration values are re-established at each stimulation cycle. The underlying assumptions of the model were checked against experimental measurements on Na-K pump activity in a variety of preparations. In addition, the qualitative validation of the model was carried out by comparing its behavior following sudden frequency shifts to corresponding experimental observations. The overall behavior of the model is quite satisfactory and it is used to provide the following indications: (1) when the intracellular and interstitial volumes are relatively large, the ion concentration transients are small and the pumping rate depends essentially on average concentration levels. (2) An increase in internal Na+ concentration potentiates the response of the Na-K pump to rapid membrane depolarizations. (3) When the internal Na+ concentration is large enough, the Na-K pump current transient plays an important role in shaping the plateau and repolarization phase of the action potential. (4) A rapid increase in external K+ concentration during voltage clamp in multicellular preparations could saturate the Na-K pump response and lead to a fairly linear dependence of the pump activity on the internal Na+ concentration.