Actions of Ba++ on ionic currents of the guinea-pig taenia coli.

The actions of Ba++ have been studied on short segments of the guinea-pig taenia coli voltage-clamped by a double sucrose-gap method. Ba++ slows the rising phase of the action potential, increases the peak amplitude and delays the repolarization with a long plateau. Ba++ carries charges through the normally Ca++ channel and directly influences both the magnitude (Ia) and the reversal potential (Ea) of the early current. The activation is slowed slightly, but the inactivation is slowed markedly. The voltage-conductance relation is shifted toward more negative voltages in Ba++, and the maximum conductance, ga, is increased significantly. Ba++ displaces the late outward current toward more positive voltages. The voltage-conductance relation is shifted by some 20 mV in 2.5 mM, and increasingly larger amounts with increasing [Ba++]o. The maximum conductance, gb, and the reversal potential (Eb) are not altered. Co++ blocks the inward Ba++ current and reverses the effects of Ba++ on the potassium conductance. Ca++ competes with Ba++. The apparent Kd for Ba++ is 6 mM and the apparent Kd for Ca++ is 0.6 to 1.0 mM. When Ca++ is added, the plateau of the long action potential in Ba++ is shortened by one-half. The effects of Ba++ are ascribed to some screening effect on fixed negative charges on the inside and outside surfaces of the membrane.