Electrical activity and metabolism in cardiac tissue: An experimental and theoretical study

From Bernstein's rheotome to Neher-Sakmann's patch electrode. The action potential

The aim of this review was to provide an overview of the most important stages in the development of cellular electrophysiology. The period covered starts with Bernstein's formulation of the membrane hypothesis and the measurement of the nerve and muscle action potential. Technical innovations make discoveries possible. This was the case with the use of the squid giant axon, allowing the insertion of "large" intracellular electrodes and derivation of transmembrane potentials. Application of the newly developed voltage clamp method for measuring ionic currents, resulted in the formulation of the ionic theory. At the same time transmembrane measurements were made possible in smaller cells by the introduction of the microelectrode. An improvement of this electrode was the next major (r)evolution. The patch electrode made it possible to descend to the molecular level and record single ionic channel activity. The patch technique has been proven to be exceptionally versatile. In its whole-cell configuration it was the solution to measure voltage clamp currents in small cells. See also: https://doi.org/10.14814/phy2.13860 & https://doi.org/10.14814/phy2.13862.

Reduced Na(+) and higher K(+) channel expression and function contribute to right ventricular origin of arrhythmias in Scn5a+/- mice

Brugada syndrome (BrS) is associated with ventricular tachycardia originating particularly in the right ventricle (RV). We explore electrophysiological features predisposing to such arrhythmic tendency and their possible RV localization in a heterozygotic Scn5a+/- murine model. Na(v)1.5 mRNA and protein expression were lower in Scn5a+/- than wild-type (WT), with a further reduction in the RV compared with the left ventricle (LV). RVs showed higher expression levels of K(v)4.2, K(v)4.3 and KChIP2 in both Scn5a+/- and WT. Action potential upstroke velocity and maximum Na(+) current (I(Na)) density were correspondingly decreased in Scn5a+/-, with a further reduction in the RV. The voltage dependence of inactivation was shifted to more negative values in Scn5a+/-. These findings are predictive of a localized depolarization abnormality leading to slowed conduction. Persistent Na(+) current (I(pNa)) density was decreased in a similar pattern to I(Na). RV transient outward current (I(to)) density was greater than LV in both WT and Scn5a+/-, and had larger time constants of inactivation. These findings were also consistent with the observation that AP durations were smallest in the RV of Scn5a+/-, fulfilling predictions of an increased heterogeneity of repolarization as an additional possible electrophysiological mechanism for arrhythmogenesis in BrS.

Alterations of Na+ currents in myocytes from epicardial border zone of the infarcted heart. A possible ionic mechanism for reduced excitability and postrepolarization refractoriness

Previously, we have shown abnormalities in Vmax and in the recovery of Vmax in myocytes dispersed from the epicardial border zone (EBZ) of the 5-day infarcted canine heart (myocytes from the EBZ [IZs]). Thus, we sought to determine the characteristics of the whole-cell Na+ current (INa) in IsZs and compare them with the INa of cells from noninfarcted hearts (myocytes from noninfarcted epicardium [NZs]). INa was recorded using patch-clamp techniques under conditions that eliminated contaminating currents and controlled INa for measurement (19 degrees C, 5 mmol/L [Na+]zero). Peak INa density (at -25 mV) was significantly reduced in IZs (4.9 +/- 0.44 pA/pF, n = 36) versus NZs (12.8 +/- 0.55 pA/pF, n = 54; P < .001), yet the half-maximal activation voltage (V0.5), time course of decay, and time to peak INa were no different. However, in IZs, V0.5 of the availability curve (I/Imax curve) was shifted significantly in the hyperpolarizing direction (-80.2 +/- 0.48 mV in NZs [n = 45] versus -83.9 +/- 0.59 mV in IZs [n = 27], P < .01). Inactivation of INa directly from a depolarized prepotential (-60 mV) was significantly accelerated in IZs versus NZs (fast and slow time constants [T1 and T2, respectively] were as follows: NZs [n = 28], T1 = 71.5 +/- 5.6 ms and T2 = 243.7 +/- 17.1 ms; IZs [n = 21], T1 = 36.3 +/- 2.4 ms and T2 = 153 +/- 11.3 ms; P < .001). Recovery of INa from inactivation was dependent on the holding potential (VH) in both cell types but was significantly slower in IZs. At (VH) = -90 mV, INa recovery had a lag in 18 (82%) of 22 IZs (with a 17.6 +/- 1.5-ms lag) versus 2 (9%) of 22 NZs (with 5.9- and 8.7-ms lags); at VH = -100 mV, T1 = 60.9 +/- 2.6 ms and T2 = 352.8 +/- 28.1 ms in NZs (n = 41) versus T1 = 76.3 +/- 4.8 ms and T2 = 464.4 +/- 47.2 ms in IZs (n = 26) (P < .002 and P < .03, respectively); at VH = -110 mV, T1 = 33.4 +/- 1.8 ms and T2 = 293.5 +/- 33.6 ms in NZs (n = 21) versus T1 = 44.3 +/- 2.9 ms and T2 = 388.4 +/- 38 ms in IZs (n = 18) (P < .002 and P < .07, respectively). In sum, INa is reduced, and its kinetics are altered in IZs. These changes may underlie the altered excitability and postrepolarization refractoriness of the ventricular fibers of the EBZ, thus contributing to reentrant arrhythmias in the infarcted heart.

Historical overview. The calcium channel of the heart

The present historical paper concentrates on the roots of the pharmacodynamic concept of Ca++ antagonism, and on the various therapeutic consequences of transmembrane Ca++ entry inhibition, i.e., normalization of hyperkinetic cardiac disorders, suppression of arterial and arteriolar spasms, relief of systemic arterial hypertension, stopping of cardiac dysrhythmias. Obviously in all these cases, medicine makes use of the different manifestations of one and the same fundamental action, that is to say, dose-dependent restriction of transmembrane inward Ca++ movements in active myocardium, vascular smooth muscle, or cardiac pacemaker cells. Interestingly, the origin of the principle of Ca++ antagonism and the discovery of drugs that possess Ca++-antagonistic potencies preceded the detection of the "slow Ca++ channels" by some years. However, the subsequent identification of the "slow channels" (or analogous Ca++ transport systems) as the decisive site of action of specific Ca++ antagonists has to be considered a keystone of the actual concept. The present paper does not treat tissue protection by Ca++ antagonists which is provided against intracellular Ca++ overload and its histopathological sequelae, as for instance Ca++-induced myofibrillar or mitochondrial disintegration. However, the inclusion of morphological topics, such as preservation of myocardial and vascular integrity by Ca++ antagonists, would exceed the limits of this article.

Inward-rectifying channels in isolated patches of the heart cell membrane: ATP-dependence and comparison with cell-attached patches

Inward rectifying potassium single-channel currents were studied in the membrane of guinea pig cardiac myocytes. In isolated inside-out patches two different channels were observed: a channel of 25 pS conductance ([K+]o = 147 mM, T = 21 degrees C), if the solution at the cytoplasmic face of the patch contained 4 mM ATP and a channel of 80 pS conductance without ATP. The 25-pS-channel was also regularly seen in cell-attached patches (Sakmann and Trube 1984a,b), but the 80-pS-channel appeared only after inhibiting cellular metabolism by DNP. The percentage of time which the 25-pS-channel spent in the open state was 3.3 times larger in isolated patches compared to cell-attached patches. However, both types of single channel currents disappeared several minutes after the isolation of the patches. In contrast to the 25-pS-channel, the 80-pS-channel, which is activated by the lack of ATP, carried measurable outward currents saturating at 1.5 pA (inward rectification). It is suggested that the 80-pS-channel mediates part of the increase in potassium current during metabolic inhibition. The openings of this channel appeared in bursts. The mean open time was 1.6 ms and the mean duration of the gaps within bursts 0.33 ms at -80 mV.

Electrophysiological effects of the salicylates on isolated atrial muscle of the rabbit

1 Intracellular recordings were made from cells of the sinoatrial (S-A) node region and from atrial muscle fibres of rabbit hearts. The effects of sodium salicylate and 5-bromo salicylate on various parameters of the membrane action potential were studied.2 5-Bromo salicylate (30-100 muM) and sodium salicylate (300-500 muM) caused a dose-dependent decrease in the frequency of discharge of the SA node cells. Applications of atropine (2.6 muM) with propranolol (3.3 muM) did not affect the negative chronotropic effect, whereas adrenaline (5 muM) reversed it.3 Depolarization and shortening of the action potential duration were found in atrial muscle fibres after the application of 5-bromo salicylate (60-100 muM). The reduction of the action potential duration (APD) was not affected by atropine (2.6 muM).4 Higher concentrations of 5-bromo salicylate (> 100 muM) also caused a dose-dependent reduction in the action potential amplitude (APA), in the overshoot (OS) of the action potential and in the maximum rate of rise of the action potential (V(max)). All these effects were completely reversed on washing.5 Substitution of the NaCl of the bathing Tyrode solution by an equimolar concentration of Na isethionate did not affect the plateau depression induced by the salicylates in atrial muscle fibres.6 After increasing the K concentration to 27 mM in the presence of isoprenaline (1 muM), ;slow responses' were obtained upon stimulation. 5-Bromo salicylate (20-60 muM) and sodium salicylate (100 muM) decreased reversibly the amplitude and the rate of rise of the ;slow response'.7 A four fold increase in Ca concentration of the standard Tyrode solution did not antagonize the plateau depression of atrial muscle fibres or the negative chronotropism induced by salicylates.8 Addition of CsCl (10 mM) to the Tyrode solution did not affect the shortening of the APD induced by the salicylates in atrial muscle fibres.9 When the K concentration in the Tyrode solution was increased from 2.7 mM to 5.4 mM, the effects of 5-bromo salicylate on the APA, OS and V(max) were potentiated. However, a significant reduction in the shortening of the APD produced by the salicylate was observed.10 It is suggested that the salicylates possibly depress the slow inward current in both S-A node cells and atrial muscle fibres of the rabbit heart. In atrial muscle fibres, a concomitant increase in the outward potassium current is probably involved.

Electrophysiological effects of lactates in mammalian ventricular tissues

Effects of excessive lactates on electrophysiological parameters of Purkinje and ventricular muscle fibers, excised from dog hearts, were studied by the microelectrode technique. The application of 60mM-lactic acid produced the following changes in membrane potentials in both tissues: slight depolarization of resting potentials of 7 to 8 mV; decrease in the max.dV/dt of action potentials by an average of 10% from the control value; and shortening of action potential durations as well as of effective refractory periods by 17%. These effects appeared earlier in the Purkinje fibers than in the ventricular muscle fibers. The voltage dependence, max.dV/dt, was not changed after the lactate perfusion. This pointed to the depolarization of the resting potential as a cause of the decrease in the max.dV/dt of action potentials. In the Purkinje fibers, slow diastolic depolarization became manifest after the lactate application. Excessive lactates moderately depressed the slow response activity in both tissues. The perfusion of 60 mM-Na lactate had essentially similar effects in both tissues. In the ventricular muscle fibers, the voltage clamp experiments revealed that excessive lactate decreased the slow inward current and barely affected the steady-state K current. The results indicate that excessive lactates promote conditions likely to lead to a re-entry into the ventricle and, at the same time, to enhance the ectopic impulse formations in ventricular conducting tissues.

Selective depression of 2,4-dinitrophenol depolarized canine Purkinje fibers by lidocaine

Concentrations of lidocaine which minimally decrease conduction and excitability of normal canine Purkinje fibers markedly decrease these parameters in fibers depolarized by metabolic depression (DNP superfusion). The decrease in Vmax and the slowing of the post-stimulation recovery of Vmax by lidocaine are also more marked during DNP superfusion than during superfusion with a DNP-free salt solution. Since these effects could be reversed by a lidocaine-free DNP superfusion, it is concluded that lidocaine selectivity depresses the electrical activity of tissue depolarized by the metabolic inhibitor DNP. This selective depressive action of lidocaine is similar to that observed in cardiac tissues depolarized by other means.

Comparison of myocardial potassium and thallium flux as studied by tracer methods

Although myocardial scintigraphy with 201thallium is widely applied in humans, the behavior of thallium at the cellular level is still under discussion. We compared the transmembrane fluxes of potassium and thallium in the isolated papillary muscle of guinea pigs. A qualitative conformity exists between potassium and thallium fluxes with respect to heart rate and temperature. Quantative comparison revealed a decreased flux rate of thallium for efflux when compared with potassium. The time dependence of thallium influx indicates that thallium scintigraphy of the myocardium reflects mainly an extracellular distribution.

Effect of 2-4-dinitrophenol on intercellular communication in mammalian cardiac fibres

The effect of 2-4-dinitrophenol (DNP) on cell communication, in canine Purkinje fibres, was investigated. It was found that DNP (0.5 MM) suppressed the electrical coupling in about 10 min. This effect of DNP was largely due to an increment in intracellular longitudinal resistance. The longitudinal movement of fluorescein (mol. wt. 320) along Purkinje strands, followed with the cut-end method, was also suppressed by DNP (0.5 mM). The decoupling action of DNP was related to release of Ca from intracellular stores and increase in free (Ca)i. The intracellular injection of EDTA reestablished the electrical coupling of Purkinje cells previously uncoupled by DNP. The results described in this paper indicate that cell communication in heart fibres is greatly dependent on the synthesis of high energy phosphate bonds.

Influence of low extracellular pH upon the Ca inward current and isometric contractile force in mammalian ventricular myocardium

In isolated papillary muscles of cats the changes in Ca inward current and isometric contractile force following a decrease of extracellular pH from 7.4 to 5.5 were studied. The Ca current was analyzed (a) by measuring the upstroke velocity of Ca-mediated action potentials and (b) in voltage clamp experiments using the double sucrose gap technique. 1. At a pH of 5.5 the upstroke velocity of the Ca-mediated action potential decreased to 65% of the control, while overshoot and action potential duration remained almost unchanged. Furthermore, the relative refractory period was prolonged and in some cases, a "Wenckebach-like" phenomenon occurred. In voltage clamp experiments, the slow inward current was found to be diminished to 50-60% of the initial control value and over a broad voltage range the current voltage relationship curve was shifted to weaker currents. Acidosis did not influence the steady state inactivation but altered the kinetics of inactivation of the slow inward current and induced an increase of tauinactivation and taurecovery. This indicates that acidosis exerts a complex effect on the slow membrane channel. 2. The normal response of the Ca current towards variations of the extracellular Ca concentration (0.5-4 mM) or towards the addition of the beta-stimulating compound isoproterenol (2 mg/l) was not altered by the lowered extracellular pH. 3. In the acid medium, isometric contractile force declined to 40% of the control value within 25 min and, thus, reacted stronger than the Ca current. This indicates that those forms of acidosis used in the present experiments caused their negative inotropic effect not exclusively via a depression of the Ca current. Rather an additional intracellular effect has to be assumed which finally leads to a reduced activity of the contractile system. 4. At pH 5.5 excess Ca (4 mM) induced the same quantitative response of the contractile system as obtained at normal pH. In contrast, the positive-inotropic effect of 2 mg/l isoproterenol was more pronounced, whilst the sensitivity of the Ca inward current towards this beta-stimulating compound remained unchanged.

Hypoxia increases potassium efflux from mammalian myocardium

Hypoxia with or without simultaneous depletion of extracellular glucose increases 42K-efflux in cat and guinea-pig papillary muscles and bovine Purkinje fibres. The change observed in K efflux may be the result of an increase in K conductance at rest.

[Transmembrane inward currents during excitation of the heart (author's transl)]

During excitation of the myocardial cell 2 transmembrane inward currents occur. The initial fast Na current is responsible for the upstroke of the normal action potential. The slow inward current is triggered at a threshold potential of about -40 mV and causes the plateau phase of action potential. Under physiological conditions Ca ions are the main charge carriers of the slow inward current. Both inward currents are mediated by 2 membrane channels which are independent from each other. The normal excitability of the myocardial cell depends upon the availability of the fast Na channel but the transmembrane Ca supply will be determined by the Ca conductance of the slow channel. After inactivation of the fast Na channel the excitability of the myocardial cell does not disappear completely. In this situation the slow inward current can mediate action potentials (so called Ca action potentials). The slow inward current can be considered as the predominant mediator of the excitation process in the pacemaker cells of the sinoatrial node and the av node. Specific inhibitors of the slow membrane channel (verapamil, D 600, Ni, Co, and Mn ions) block the transmembrane Ca current leading to excitation contraction uncoupling. The excitation process will be impaired only if it is carried by the slow inward current alone. Specific inhibitors of the fast Na channel reduce the Na-dependent excitability of the myocardial cell without significant changes of the Ca current. The existence of 2 separate channels in the ventricular myocardium allows selective alteration of contractility without concomitant changes of the Na-dependent excitation process or, conversely, the reduction of excitability whereas the Ca current remains unchanged.

Mitochondrial uncoupling agents. Effects on membrane permeability of molluscan neurons

Agents which uncouple oxidative phosphorylation in mitochondria were applied to identified neurons in an isolated ganglion of the marine mollusc Navanax inermis. Aromatic monocarboxylic acids, acetanilides, benzamides, benzaldehydes and phenols all caused a rapid, reversible, dose-dependent increase in the membrane potential and conductance of the neurons tested. These events were due primarily to an increase in the membrane's conductance to potassium, relative to chloride. All active compounds also produced a reversible, dose-dependent decrease in the permeability of alkali-cations relative to potassium. The relative activity of congeners in each group of substances was directly correlated with the octanol-water partition coefficients of the various compounds, indicating that hydrophobicity was important in determining drug effect and suggesting that steric requirements were minimal. The results suggest that the observed changes in membrane electrical properties and cation selectivity are due to an increase in the membrane's anionic field strength caused by the hydrophobic interaction of anionic and nonionic agents with the neuronal membrane.

Kinetics of inactivation and recovery of the slow inward current in the mammalian ventricular myocardium

In order to study the kinetics of inactivation and recovery of the slow inward current in the mammalian ventricular myocardium voltage clamp experiments using the double sucrose gap technique were performed on isolated trabeculae and papillary muscles of cats. The separation of the slow inward current from the fast Na current was achieved by use of the conditioning clamp procedure. 1. The decay of the Ca current reflects the inactivation which develops due to depolarization. The rate of inactivation depends upon the membrane potential. Excess Ca (8.8 mM) accelerates the inactivation speed indicating that Ca ions not only act as charge carrier of the slow inward current but might influence in addition the kinetics of the slow membrane channel. In the presence of a lowered temperature a deceleration of inactivation (Q10 2.3) occurs. 2 If the membrane is repolarized a recovery process takes place restoring the availability of the slow membrane channel. As the inactivation the recovery rate depends upon the membrane potential. Excess Ca causes an acceleration whereas a decrease in temperature diminishes the recovery speed (Q10 2.3). Consequently, the Ca supply to the myocardial cell can be modified not only by changes of the transmembrane Ca concentration gradient or by an alteration of the Ca conductance of the slow channel but also by changes in the degree of recovery after a preceding Ca current. 3. Compared with the inactivation the recovery proceeds very slowly. Assuming that this slow recovery represents an inherent kinetic feature of the slow channel the kinetics of inactivation and removal of inactivation are not describable by a single inactivation variable (called as f by Reuter, 1973) which is of the Hodgkin-Huxley type. If a second inactivation variable (called as l) would be introduced additionally a formulation of the inactivation-recovery process of the slow membrane channel on the basis of the Hodgkin-Huxley model becomes feasible.

The influence of metabolic inhibitors upon the transmembrane slow inward current in the mammalian ventricular myocardium

The effect of metabolic alterations of isolated trabeculae and papillary muscles of cats upon the transmembrane slow inward current was studied in voltage clamp experiments using the double sucrose gap technique. The slow inward current which is mainly carried by Ca ions was separated from the fast Na current by applying the conditioning clamp technique. 1. After inhibition of the oxidative phosphorylation of the myocardial cell caused by cyanide (100 mg/l), the transmembrane slow inward current decreased by 25% on average within 25 min. The same diminution appeared after poisoning with 2.4 dinitrophenol (40 mg/l) which amounted to 46% on an average. In both cases the extrapolated reversal potential of the slow inward current was shifted to less positive values. The reduction of the slow inward current seems to reflect a diminished Ca driving force due to a metabolically evoked increase in intracellular free Ca. But these metabolic inhibitors might exert some direct effects upon the membrane since after the treatment with cyanide and 2.4 dinitrophenol, respectively, a slight delay of current inactivation could be observed. Nevertheless, the kinetics of the recovery from inactivation remained unaffected. 2. Metabolic poisoning abolished the normal response of the slow inward current to an increase of the extracellular Ca concentration or to the addition of Sr ions. Excess Ca failed to augment the current, and in some cases even a slight diminution occurred. If at all 5.5 mM Sr caused an increase by not more than 20%. These unusual reactions may also result from a reduced driving force for Ca or Sr, respectively. 3. Catecholamines still exerted their promoting effects upon the slow inward current after poisoning with cyanide or 2,4 dinitrophenol. The addition of 5 mg/l isoproterenol not only neutralized the effect of these metabolic inhibitors but caused an increase of the slow inward current even surpassing the initial control values.

The effect of 2,4-dinitrophenol on electrical and mechanical activity, metabolism and ion movements in guinea-pig ventricular muscle

1. DNP (2,4-dinitrophenol) reduced the duration of the action potential of guinea-pig ventricular muscle at a greater rate than did anoxia. The effect was dose-dependent and was modified by the concentration of glucose in the medium. DNP (0.1 mM) reduced the amplitude of the action potential of muscles incubated with 5 mM glucose; on raising the glucose concentration to 50 mM the effect was reversed.2. A large dose-dependent loss of K(+) occurred within 15 min of incubation with DNP and was attributed to increased efflux. K(+) loss was not related to Na(+) gain during the first 60 min of incubation; during the first 30 min DNP-treated muscle did not gain any Na(+). Although the shortening of the action potential by DNP during aerobic incubation was similar to that of muscles incubated under anaerobic conditions in glucose-free medium, the anaerobic incubation was not associated with increased (42)K efflux.3. It was concluded that the reduction in duration of the action potential was not necessarily the result of an increased K(+) efflux. The effect of DNP on (42)K efflux is considered to result from a direct effect on the cell membrane; the effect on electrical activity may be a combination of the increase in K(+) efflux and a reduction in the inward current due to Na(+) and Ca(++) previously assumed to be dependent on the glycolytic production of ATP.4. Electrogenic Na(+) pumping may contribute to the maintenance of resting potential in K(+)-depleted, DNP-treated cardiac muscle.