ACTIONS OF BA++ ON IONIC CURRENTS OF THE GUINEA-PIG TAENIA-COLI

Abstract

The actions of Ba2+ were studied on short segments of guinea-pig tenia coli voltage-clamped by a double sucrose-gap method. Ba2+ slow the rising phase of the action potential, increased the peak amplitude and delayed the repolarization with a long plateau. Ba2+ carried charges through the normally Ca2+ channel and directly influenced both the magnitude (Ia) and the reversal potential (Ea) of the early current. The activation was slowed slightly, but the inactivation was slowed markedly. The voltage-conductance relation was shifted toward more negative voltages in Ba2+, and the maximum conductance, .hivin.ga, increased significantly. Ba2+ displaces the late outward current toward more positive voltages. The voltage-conductance relation was shifted by some 20 mV in 2.5 mM, and increasingly larger amounts with increasing [Ba2+]o Co2+. The maximum conductance, .hivin.gb, and the reversal potential (Eb) were not altered. Co2+ blocked the inward Ba2+ current and reversed the effects of Ba2+ on the K conductance. Ca2+ competed with Ba2+. The apparent Kd for Ba2+ was 6 mM and the apparent Kd for Ca2+ was 0.6-1.0 mM. When Ca2+ was added, the plateau of the long action potential in Ba2+ was shortened by 1/2. The effects of Ba2+ are ascribed to some screening effect on fixed negative charges on the inside and outside surfaces of the membrane.