The effects of calcium on outward membrane currents in the cardiac Purkinje fibre.

Abstract

Properties of outward membrane currents in Purkinje fibers from sheep hearts were studied with particular reference to the effects of external Ca (CaO). Altering CaO shifts the potential-dependence of channel activation. The effects suggest that Ca ions bind to and neutralize negative charges at the external face of the membrane; the different magnitudes of the effects of low CaO on the pace-maker and plateau currents suggest that the affinities of Ca-binding sites adjacent to each channel type are widely different. Raising CaO causes a positive shift in the pace-maker current reversal potential, EK2, which may reflect a small elevation in the K concentration (Kc) in the restricted cleft space immediately outside the membrane. Other possible causes of the shift in EK2 are discussed. Raising CaO has effects on the plateau and pace-maker current rectifier relations and on the time-independent membrane current similar to those of a small increase in extracellular Kc. Possible mechanisms for an increase in Kc in elevated CaO are discussed. Positive shifts in EK2 can be seen even when the membrane current becomes more inward. Apparently the increase in Kc results from an activation of K channels by Ca ions. Increases in Ca may partially inhibit the Na:K exchange pump. The maximum transient outward current elicited by strong depolarizing steps is not affected by moderate reductions in CaO. Reducing NaO [external Na] depresses the pace-maker current rectifier relation with little shift in the activation curve. Apparently the effects of CaO on outward currents are due to shifts in the potential-dependence of channel activation; others result from a small increase in Kc. No evidence for a direct effect of Ca on K channels was found.