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

Background: Hypothermia may induce hypokalemia and increase intracellular Ca 2+ by affecting serum K + and Ca 2+ 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 Ca 2+ fluxes by Ca 2+ channel blockade with verapamil. Methods: A conventional microelectrode method was used to determine the effects of hypothermia (32±0.5 o C and 28±0.5 o 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% (APD 50 ) and at 95% (APD 50 ) repolarization in isolated canine cardiac Purkinje fibers. To evaluate the contribution of the slow inward Ca 2+ current to action potential changes in hypothermia, the experiments were repented in the presence of the Ca 2+ -channel antagonist verapamil (1 μM). Results: Variations of [K + ] o induced the expected shifts in maximum diastolic potential, and hypothermia (28 o 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 APD 50 and the APD 95 . The effects of verapamil on APD were temperature and [K + ] o dependent; between 37 o C and 28 o C with 2.3 mM [K + ] o in the superfusate, verapamil did not affect APD. At 28 o C in the presence of verapamil, the APD 50 and APD 95 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