Is there an A-type K+ current in guinea pig ventricular myocytes?
A unique transient outward K+ current (Ito) has been described to result from the removal of extracellular Ca2+ from ventricular myocytes of the guinea-pig (Inoue and Imanaga Am. J. Physiol. 264: C1434-C1438, 1993). This study addressed the question of whether this current represented K+-selective Ito or the efflux of K+ via L-type Ca2+ channels. This outward current was inhibited by Cd2+, Ni2+, Co2+ and La3+. It was inhibited by nifedipine. All of these compounds were equally effective inhibitors of the L-type Ca2+ current. It was not inhibited by 4-AP. 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
