Mario Vassalle

Electrogenic suppression of automaticity in sheep and dog purkinje fibers.

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 positive2, 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.

Calcium Overload and Cardiac Function

The changes in cardiac function caused by calcium overload are reviewed. Intracellular Ca²⁺ 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 Ca²⁺ 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⁺-Ca²⁺ exchange and a role of Ca²⁺ in mechanical toxicity. Ca²⁺ 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 Ca²⁺ load (e.g. catecholamines). The Ca²⁺ overload is enhanced by fast rates. Purkinje fibers are more susceptible to Ca²⁺ overload than myocardial fibers, possibly because of their greater Na⁺ load. If the SR is predominantly Ca²⁺ overloaded, V_os and fast discharge are induced through an oscillatory release of Ca²⁺ in diastole from the SR; if the cytoplasm is Ca²⁺ overloaded, the non-oscillatory V_ex tail is induced at negative potentials. The decrease in contractile force by Ca²⁺ 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 Ca²⁺ through the Na⁺-Ca²⁺ exchange. I_os is an oscillatory current due to an oscillatory release of Ca²⁺ in early diastole from the Ca²⁺-overloaded SR, and I_ex is a non-oscillatory current due to the extrusion of Ca²⁺ from the Ca²⁺-overloaded cytoplasm. I_os and I_ex can be present singly or simultaneously. An increase in [Ca²⁺]_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, [Ca²⁺]_i may increase to overload levels and contribute to cardiac failure. Experimental evidence suggests that clinical concentrations of digitalis increase force in Ca²⁺-overloaded cardiac cells by decreasing the inhibition of the Na⁺-K⁺ pump by Ca²⁺, thereby leading to a reduction in Ca²⁺ overload and to an increase in force of contraction.

An oscillatory current in sheep cardiac Purkinje fibers.

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 (Io<) requires a previous depolarization to = −20 mV or beyond and a repolarization to = −40 mV or to more negative potentials. The I0 requires a minimum duration of the depolarizing clamp and becomes larger with longer clamps. With repolarization to more negative potentials (= 90 mV), Io. becomes smaller and may disappear. Also, Io, 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 Ixi and pacemaker current), it can be demonstrated that Io. 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 Io» or may make it appear. In fact, these interventions may extend the voltage range over which Io» appears. These interventions include lowering potassium, increasing calcium, trains of depolarizing clamps, and administration of norepinephrine and of strophanthidin. It is concluded that I0» 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.

An Analysis of Arrhythmias Induced by Ouabain in Intact Dogs

The results of this study show that ouabain administration quite often causes idioventricular automaticity to undergo an initial depression and a subsequent enhancement. When the electrocardiogram shows normal sinus rhythm these changes in ventricular automaticity can be unmasked by vagal stimulation which causes either sinus arrest or atrioventricular block. On occasion, the depression of idioventricular automaticity is permanent; if ouabain produces complete atrioventricular block at this time, ventricular arrest ensues. When, on the contrary, the automaticity of Purkinje fibers is progressively enhanced, a stage is reached in which the idioventricular rhythm becomes faster than the sinus rhythm. Eventually the basic ventricular rhythm exhibits pauses during which electrocardio-graphic complexes of different form become apparent. The second class of arrhythmias comprises extrasystoles which may or may not show a constant coupling to sinus beats but which are characterized by a dependence on an initiating beat. It was demonstrated in this investigation that these extra beats appear during slow intravenous administration of ouabain and that they disappear not only on suppression but also on slowing of the sinus rhythm. Furthermore, it was shown that bigeminal rhythm may be transformed into extrasystoles occurring after every second sinus beat by means of vagal stimulation which slows the sinus rate. If, however, the heart was driven at faster rate, several extrasystoles followed one atrial beat. At a more advanced stage of ouabain intoxication these extrasystoles lose their fixed relationship with the sinus beat and runs of self-sustaining tachycardia appear. Such runs of ventricular tachycardia are unaffected by vagal stimulation but have a tendency to subside, spontaneously, and start anew after another initiating sinus beat. If vagal stimulation inhibits the initiating beat, the run of tachycardia also is suppressed. At an advanced stage of intoxication, however, when polymorphic ventricular complexes are present, vagal stimulation no longer produces any apparent effects. Finally, this investigation has shown that, during vagal stimulation which produces sinus arrest or A-V block, atrial fibrillation can be initiated by retrograde transmission of ventricular escape beats even though atrial escape beats are ineffective in eliciting the same phenomenon.

On the Cause of Ventricular Asystole during Vagal Stimulation

In anesthetized dogs the vagus nerve was stimulated for 2 min; during the first minute the ventricles were driven at a rate higher than the control sinus rate; after discontinuation of the drive, the duration of asystole was prolonged. When the ventricles were driven at a rate lower than the sinus rate during vagal stimulation the subsequent asystole was shortened. Slowing sinus activity by graded vagal stimulation before maximal vagal stimulation led to a shorter asystole. Driving the ventricles at a rate higher than the sinus node rate before vagal stimulation resulted in longer asystole. In animals with chronic atrioventricular block, “overdriving” the ventricles resulted in subsequent temporary inhibition of ventricular pacemakers. In dogs with atrioventricular block, coronary sinus plasma potassium increased during the period of ventricular overdriving, and the magnitude of the rise was a function of the driving rate. These results support the concept that ventricular asystole results from the suppressive action of the fast rate imposed by the sinus node upon the slowly discharging ventricular pacemakers. Suppression of sinus node activity by the vagus reveals the rate-dependent inhibition of ventricular pacemakers. The mechanism of inhibition may be related to changes in ionic concentration gradients.

Norepinephrine and potassium fluxes in cardiac Purkinje fibers

SummaryDog ventricular Purkinje fibers, loaded with42K, were mounted in a tissue bath located over a beta-detector and washed in Tyrode solution. Tissue radioactivity was measured at intervals and transmembrane potential recorded by means of microelectrodes. Relative potassium influx was estimated by re-exposing the fiber to radioactive solution for fixed periods and measuring the tissue radio-activity increase at the end of the uptake period. Potassium efflux was estimated by measuring tissue and effluent radioactivities. After control recordings, administration of norepinephrine caused the following changes during the period it was applied: 1. increase in potassium influx; 2. increase in potassium efflux in nonstimulated fibers; 3. a smaller increase in potassium efflux in fibers stimulated electrically at a constant rate; 4. increase in potassium content in fibers loaded to equilibrium with42K; 5. a smaller increase in K uptake in fibers driven at a high rate; and 6. enhancement of potassium uptake even in the presence of high [K]0. It is concluded that norepinephrine increases both influx and efflux in Purkinje fibers. The larger enhancement of the influx with respect to the efflux and the increase in potassium content suggests that norepinephrine stimulates active transport of potassium. This stimulatory action can be dissociated from the stimulation of automaticity.

On the Sympathetic Control of Ventricular Automaticity THE EFFECTS OF STELLATE GANGLION STIMULATION

The effects of stellate ganglion stimulation on idioventricular automaticity were studied in anesthetized dogs. An idioventricular rhythm was obtained in one series of animals by ligating the His bundle and in the other by stimulating the vagus. On isolation of the left stellate ganglion, the idioventricular rate fell by 5 beats/min which is compatible with a rate of “tonic” discharge of about 3 impulses/sec. On stimulation of the stellate ganglia at increasingly higher frequencies, the idioventricular rate accelerated progressively and the frequency-response curve was sigmoid. When the stimulation frequency was increased above 10 to 15 pulses/sec, there was no further enhancement of ventricular automaticity. The ventricular rates attained during sympathetic stimulation rarely exceeded 70/min. The interval between the initiation of sympathetic stimulation and the maximal ventricular acceleration was shorter at higher stimulation frequencies. The effects of right and left stellate stimulation on the rate of idioventricular pacemakers were comparable. Reflex vagal inhibition had little influence on the acceleratory action of the sympathetic nerves on the idioventricular pacemakers since in dogs with atrioventricular block the results before and after vagotomy were similar. Furthermore, the idioventricular rhythm during vagally induced block increased on sympathetic stimulation to values comparable to those obtained in dogs with surgically induced block.

Strophanthidin inotropy: Role of intracellular sodium ion activity and sodium-calcium exchange

The relation among the ratio of extra- and intra-cellular sodium ion activities (aoNa/aiNa), contractile force and action of strophanthidin was studied in cardiac Purkinje fibers when transmembrane Na+ and Ca2+ gradients were changed. The aiNa, contractile force and action potential were simultaneously measured. Simultaneous reduction of [Na+]o and [Ca2+]o to 80.8 and 1.08 mM respectively, decreased aiNa from 8.0 +/- 1.1 mM (mean +/- S.D., n = 17) to 6.0 +/- 0.9 mM (n = 17) whereas contractile force transiently increased and then recovered toward the level similar to that in Tyrode solution. Reduction of [Ca2+]o alone increased aiNa by 1.7 +/- 0.4 mM (n = 5) and decreased contractile force by 87 +/- 5% (n = 5). Raising osmolarity of Tyrode solution with sucrose increased both aiNa and contractile force. Substitution of sucrose with Na+ (high [Na+] solution) increased aiNa by 1.2 +/- 0.3 mM (n = 5) and decreased contractile force by 31 +/- 9% (n = 5). Strophanthidin (2 X 10(-7) M) increased aiNa by 0.4 +/- 0.1 mM (n = 6) and contractile force by 24 +/- 8 (n = 6) in a low [Na+] - [Ca2+] solution. These changes were smaller than those in Tyrode solution (1.1 +/- 0.3 mM); 96 +/- 32%, n = 6). On the other hand, strophanthidin increased aiNa and contractile force more in a low [Ca2+] (2.7 +/- 0.5 mM; 220 +/- 24%, n = 5) or a high [Na+] (2.3 +/- 0.9 mM; 164 +/- 37%, n = 5) solution than in Tyrode solution. In the solutions containing the altered [Na+]o and/or [Ca2+]o, the increases in aiNa and force by strophanthidin were parallel. Therefore, the parallel increase in aiNa and contractile force due to strophanthidin depends on the initial level of aiNa, suggesting the dependence of digitalis inotropy on the rate of Na+ extrusion by the Na+ -K+ pump. The results also indicate that the ratio of aoNa/aiNa is an important and powerful factor in the control of contractile force. Presumably this is mediated through the Na+ -Ca2+ exchange.

Overdrive excitation: onset of activity following fast drive in cardiac Purkinje fibers exposed to norepinephrine.

SummaryPurkinje fibers perfused in vitro in 5.4 mM K Tyrode remained quiescent in the presence of 8.8×10−7 M norepinephrine. When these fibers were driven at 30/min and then overdriven at 120/min for 2 min, the cessation of drive was followed by the onset of spontaneous activity (“overdrive excitation”). It was found that overdrive excitation: 1. required both overdrive and norepinephrine to occur; 2. occurred when during the overdrive there was an increase in maximum diastolic potential (Emax) but not ifEmax decreased; 3. was caused by a steepening of diastolic depolarization following overdrive; 4. was not inhibited by a burst of fast drive; and 5. subsided through a progressive decrease ofEmax and of the slope of diastolic depolarization.It is proposed that overdrive excitation is due to a faster fall of the slowly changing potassium current to a smaller value. These changes, in turn, are caused by the combined action of: 1. a hyperpolarization due to both overdrive and norepinephrine; and 2. a shift in a depolarizing direction of the relationship between voltage and steady state slow potassium conductance due to norepinephrine.

Effects of potassium on ouabain-induced arrhythmias

Abstract In dogs poisoned with ouabain, potassium chloride was infused intravenously to study its antiarrhythmic properties. The ouabain-enhanced idioventricular automaticity, which leads to ventricular tachycardia, was progressively depressed by administration of potassium. Initially the polymorphic complexes caused by ouabain were replaced by a uniform ventricular tachycardia. As the rate of the idioventricular rhythm decreased, sinus rhythm reappeared intermingled with ventricular beats. When only the sinus rhythm was present, stimulation of the vagus, which provoked sinus arrest or atrioventricular block, could reveal a fast idioventricular rhythm. Later during infusion of potassium, when the P waves were still present on the electrocardiogram, the rate of idioventricular escape fell below control values. Thus, one of the ways potassium counteracts digitalis arrhythmias is the progressive depression of idioventricular automaticity. The possible mechanisms by which potassium depresses idioventricular automaticity are discussed. No data have been collected on the suppression by potassium of “re-entry” type beats. At an advanced stage of administration of potassium, the large diphasic ventricular complexes have been shown to result from disturbed intraventricular conduction of impulses originating from a supraventricular pacemaker; stimulation of the vagus consistently led to their inhibition. When cardiac arrest eventually ensued, direct stimulation of the ventricles elicited electric and mechanical responses. Therefore, cardiac arrest appears to be due either to the cessation of the activity of a supraventricular pacemaker or to failure of conduction, rather than to a depolarization or inexcitability of the ventricular muscle fibers.