No abstract

Driving Purkinje fibers at a fast rate is followed by a temporary suppression of spontaneous activity, "overdrive suppression." The mechanism of this suppression was studied in Purkinje fibers perfused in vitro and stimulated for variable periods of time at selected rates. The overdrive procedures caused an initial decrease in maximum diastolic potential below control followed by a late increase above control. After the overdrive, the slope of diastolic depolarization was decreased and the threshold voltage more positive* these changes were responsible for the temporary suppression of spontaneous activity. After the cessation of 2-minute overdrive, the increase in maximum diastolic potential subsided gradually over a period of 2 to 5 minutes. At higher [K] o , the effect of overdrive on membrane potential was reduced. Substitution of Na + with Li + or exposure to 2,4-dinitrophenol abolished the late hyperpolarization during overdrive. The membrane resistance was not altered after an overdrive period. The results suggest that driving ventricular Purkinje fibers at a rate higher than their intrinsic rate causes an initial loss of K + and the activation of an electrogenic Na + pump. The activation of the electrogenic pump is the major mechanism responsible for overdrive suppression.

The changes in cardiac function caused by calcium overload are reviewed. Intracellular Ca(2+) may increase in different structures [e.g. sarcoplasmic reticulum (SR), cytoplasm and mitochondria] to an excessive level which induces electrical and mechanical abnormalities in cardiac tissues. The electrical manifestations of Ca(2+) overload include arrhythmias caused by oscillatory (V(os)) and non-oscillatory (V(ex)) potentials. The mechanical manifestations include a decrease in force of contraction, contracture and aftercontractions. The underlying mechanisms involve a role of Na(+) in electrical abnormalities as a charge carrier in the Na(+)-Ca(2+) exchange and a role of Ca(2+) in mechanical toxicity. Ca(2+) overload may be induced by an increase in [Na(+)](i) through the inhibition of the Na(+)-K(+) pump (e.g. toxic concentrations of digitalis) or by an increase in Ca(2+) load (e.g. catecholamines). The Ca(2+) overload is enhanced by fast rates. Purkinje fibers are more susceptible to Ca(2+) overload than myocardial fibers, possibly because of their greater Na(+) load. If the SR is predominantly Ca(2+) overloaded, V(os) and fast discharge are induced through an oscillatory release of Ca(2+) in diastole from the SR; if the cytoplasm is Ca(2+) overloaded, the non-oscillatory V(ex) tail is induced at negative potentials. The decrease in contractile force by Ca(2+) overload appears to be associated with a decrease in high energy phosphates, since it is enhanced by metabolic inhibitors and reduced by metabolic substrates. The ionic currents I(os) and I(ex) underlie V(os) and V(ex), respectively, both being due to an electrogenic extrusion of Ca(2+) through the Na(+)-Ca(2+) exchange. I(os) is an oscillatory current due to an oscillatory release of Ca(2+) in early diastole from the Ca(2+)-overloaded SR, and I(ex) is a non-oscillatory current due to the extrusion of Ca(2+) from the Ca(2+)-overloaded cytoplasm. I(os) and I(ex) can be present singly or simultaneously. An increase in [Ca(2+)](i) appears to be involved in the short- and long-term compensatory mechanisms that tend to maintain cardiac output in physiological and pathological conditions. Eventually, [Ca(2+)](i) may increase to overload levels and contribute to cardiac failure. Experimental evidence suggests that clinical concentrations of digitalis increase force in Ca(2+)-overloaded cardiac cells by decreasing the inhibition of the Na(+)-K(+) pump by Ca(2+), thereby leading to a reduction in Ca(2+) overload and to an increase in force of contraction.

Transmembrane potentials were recorded from mammalian Purkinje fibers. Adding saccharose to the bathing solution slowed the spontaneous rate, probably as a result of cell shrinkage and an increase in the intracellular K concentration. An opposite result was found with hypotonic medium. In solutions containing 5.4 mm K the fibers were quiescent. Lowering K to 2.7 mm left the membrane resting potential unchanged but decreased the membrane conductance to half. There was only a minor effect of extracellular K on membrane conductance during the plateau of the action potential. Spontaneous firing regularly started when extracellular K was reduced to or below 2.7 mm. This was preceded by subthreshold oscillations which increased in amplitude. A low K conductance associated with a sizeable difference between membrane potential and potassium equilibrium potential seem to be essential for spontaneous activity to occur in cardiac tissue.

Toxic effects of ouabain on single Purkinje fibers and ventricular muscle fibers were investigated in vitro by microelectrode technique. Toxicity developed much earlier in the specialized conducting fibers and consisted of a progressively increasing rate of diastolic depolarization and a decrease of amplitude and duration of the action potential. The majority of Purkinje fiber preparations developed extrasystoles and rapid spontaneous rhythms. The resting potential was much decreased. The ouabain-induced changes in ventricular muscle fibers occurred much later than did changes in Purkinje fibers and consisted of a decrease in the plateau and in the amplitude of the action and resting potential. Spontaneous depolarization was not observed in muscle fibers. The effect of the rate of stimulation on the development of ouabain toxicity was studied in another series of experiments on driven and quiescent muscles. Signs of toxicity appeared earlier in the driven muscles than in duplicate quiescent muscles and, at faster rates of stimulation, the time required for the toxic changes was shortened.

The inward current ("oscillatory current") which may be present after the end of a depolarizing clamp was studied in sheep cardiac Purkinje fibers by means of a voltage-clamp method. The following results were obtained. In order to appear, the oscillatory current (Ios) requires a previous depolarization to approximately or equal to -20 mV or beyond and a repolarization to approximately or equal to -40 mV or to more negative potentials. The Ios requires a minimum duration of the depolarizing clamp and becomes larger with longer clamps. With repolarization to more negative potentials (approximately or equal to 90 mV), Ios becomes smaller and may disappear. Also Ios can be triggered twice if the potential is clamped to two different levels in succession. By several procedures which modify the other known currents (fast Na+ current, slow inward current, early outward current, plateau current Ix1 and pacemaker current), it can be demonstrated that Ios is not due to their oscillatory behavior and can occur in the absence of any one of them. Interventions which increase the contractile force presumably by increasing intracellular calcium stores enhance the Ios or may make it appear. In fact, these interventions may extend the voltage range over which Ios appears. These interventions include lowering potassium, increasing calcium, trains of depolarizing clamps, and administration of norepinephrine and of strophanthidin. It is concluded that Ios is a physiological event which is enhanced by certain procedures, and it appears to be of much importance in drive-induced arrhythmias under different conditions.

No abstract

It is generally assumed that in cardiac Purkinje fibers the hyperpolarization activated inward current/f underlies the pacemaker potential. Because some findings are at odds with this interpretation, we used the whole cell patch clamp method to study the currents in the voltage range of diastolic depolarization in single canine Purkinje myocytes, a preparation where many confounding limitations can be avoided. In Tyrode solution ([K+]o = 5.4 mM), hyperpolarizing steps from Vh = -50 mV resulted in a time-dependent inwardly increasing current in the voltage range of diastolic depolarization. This time-dependent current (ixda) appeared around -60 mV and reversed near F_~. Small superimposed hyperpolarizing steps (5 mV) applied during the voltage clamp step showed that the slope conductance decreases during the development of this time-dependent current. Decreasing [K+]o from 5.4 to 2.7 mM shifted the reversal potential to a more negative value, near the corresponding EK. Increasing [K+]o to 10.8 mM almost abolished iaaa. Cs + (2 mM) markedly reduced or blocked the time-dependent current at potentials positive and negative to EK. Ba 2+ (4 mM) abolished the timedependent current in its usual range of potentials and unmasked another timedependent current (presumably if) with a threshold of ~-90 mV (>20 mV negative to that of the time-dependent current in Tyrode solution). During more negative steps, /f increased in size and did not reverse. During/f, the slope conductance measured with small (8-10 mV) superimposed clamp steps increased. High [K+]o (10.8 mM) markedly increased and Cs + (2 mM) blocked if. We conclude that: (a) in the absence of Ba 2+, a time-dependent current does reverse near EK and its reversal is unrelated to K § depletion; (b) the slope conductance of that time-dependent current decreases in the absence of K + depletion at potentials positive to F~ where inactivation of/K1 is unlikely to occur. (c) Ba 2+ blocks this time-dependent current and unmasks another time-dependent current (if) with a more negative (>20 mV) threshold and no reversal at more negative values; (d) Cs + blocks both time-dependent currents recorded in the absence and presence of Ba 2+. The data suggest that in the diastolic range of potentials in Purkinje myocytes there is a voltage-and time-dependent K + current (iKad) that can be separated from the hyperpolarization-activated inward current if.