Spontaneous Electrical Activity Induced by Depolarizing Currents in Canine Ventricular Myocardium

Abstract

Spontaneous action potential (AP) discharge could be induced by an application of long (5-10 s) depolarizing currents in 68% of canine ventricular myocardium tested in the voltage range between about -65 and -10 mV. The constant currents of various intensities were applied across a sucrose gap, and intracellular potentials were recorded with a microelectrode. The firing rate of the AP was voltage-dependent and ranged between 0.7-2.5 Hz. The AP was dependent on both [Ca2+]0 and [Na+]0. Increase of [Na+]0 from 37-149 mM increased the firing rate, maximum rate of rise and overshoot of the AP, while increase of [Ca2+]0 from 0.45 mM-1.8 and 7.2 mM increased the firing rate and maximum rate of rise, but did not change the amplitude of overshoot. AP discharges were not blocked by tetrodotoxin (10-5 g/ml), but were effectively blocked by verapamil (6 .times. 10-6 g/ml). Adrenaline (5 .times. 10-6 g/ml) initiated AP in otherwise quiescent preparations. Spontaneous AP may be produced by inflowing of slow Na+ and Ca2+ currents modified by an underlying change of K+-permeability. Possible clinical significance of this phenomenon is discussed in relation to ventricular arrhythmia.