Is there an A-type K+ current in guinea pig ventricular myocytes?

Abstract

A unique transient outward K+ current ( I to) has been described to result from the removal of extracellular Ca2+ from ventricular myocytes of the guinea pig (15). This study addressed the question of whether this current represented K+-selective I to or the efflux of K+ via L-type Ca2+ channels. This outward current was inhibited by Cd2+, Ni2+, Co2+, and La3+ as well as by nifedipine. All of these compounds were equally effective inhibitors of the L-type Ca2+ current. The current was not inhibited by 4-aminopyridine. Apparent inhibition of the outward current by extracellular Ca2+ was shown to result from the displacement of the reversal potential of cation flux through L-type Ca2+ channels. The current was found not to be K+ selective but also permeant to Cs+. The voltage dependence of inactivation of the outward current was identical to that of the L-type Ca2+ current. It is concluded that extracellular Ca2+ does not mask an A-type K+current in guinea pig ventricular myocytes.