Effects of hypothermia, potassium, and verapamil on the action potential characteristics of canine cardiac Purkinje fibers

Abstract

Background: Hypothermia may induce hypokalemia and increase intracellular Ca2+ by affecting serum K+ and Ca2+ fluxes across the cell membrane. These ionic alterations may significantly change the electrophysiologic characteristics of the cardiac action potential and may induce cardiac arrhythmias. The current study was undertaken to determine whether electrophysiologic changes in Purkinje fibers induced by hypothermia could be reversed by manipulating the extracellular K+ and transmembrane Ca2+ fluxes by Ca2+ channel blockade with verapamil. Methods: A conventional microelectrode method was used to determine the effects of hypothermia (32 ± 0.5°C and 28 ± 0.5°C) and various external K+ concentrations ([K+](o)) (2.3, 3.8, and 6.8 mM) on maximum diastolic potential, maximum rate of phase 0 depolarization (V(max)), and action potential duration (APD) at 50% (APD50) and at 95% (APD95) repolarization in isolated canine cardiac Purkinje fibers. To evaluate the contribution of the slow inward Ca2+ current to action potential changes in hypothermia, the experiments were repeated in the presence of the Ca2+-channel antagonist verapamil (1 μM). Results: Variations of [K+](o) induced the expected shifts in maximum diastolic potential, and hypothermia (28°C) induced moderate depolarization, but only when [K+](o) was ≥3.9 mM (P < 0.05). Hypothermia decreased V(max) at all [K+](o) studied (P < 0.05). Regardless of the temperature, V(max) was not affected by verapamil when [K+](o) was <3.9 mM, but at 6.8 mM [K+](o) in hypothermia V(max) was significantly lower in the presence of verapamil. Hypothermia increased both the APD50 and the APD95. The effects of verapamil on APD were temperature and [K+](o) dependent; between 37°C and 28°C with 2.3 mM [K+](o) in the superfusate, verapamil did not affect APD. At 28°C in the presence of verapamil, the APD50 and APD95 decreased only if the [K+](o) was >3.9 mM. Conclusions: Verapamil and K+ supplementation in hypothermia may exert an antiarrhythmic effect, primarily by reducing the dispersion of prolonged APD.