Effects of some inhibitors of ionic permeabilities on ventricular action potential and contraction of rat and guinea-pig hearts

Abstract

The effect of tetrodotoxin (TTX), which blocks Na conductance increase, tetraethylammonium (TEA), which reduces K conductance, and manganese (Mn), which suppresses Ca conductance, on the ventricular intracellular action potential (AP) and contraction was studied in isolated, perfused rat and guinea-pig hearts. In the rat, TTX produces an important decrease of the AP amplitude (initial component) and a shortening of the plateau without marked modification of repolarization. TEA produces resting depolarization and an elevation and lengthening of the plateau. Mn ions diminish the amplitude and duration of the plateau. In the guinea-pig, TTX produces only a very small decrease in AP amplitude, but the first part of the ascending phase decreases markedly, the second part being on the other hand only slightly modified. Under the action of TEA the cells are depolarized, the amplitude of the first part of the plateau decreased, and the repolarization delayed. Mn leads to marked slowing of the second part of the ascending phase and to a lengthening of the plateau. In both species, the contraction is very slightly modified by TTX, reduced or suppressed by TEA and Mn. Initial effects of low-Ca media are similar to Mn effects, but changes in contraction and AP during potentiation are opposite. Differences between rat and guinea-pig AP and the possible participation of Ca ions in the genesis of the ventricular AP are discussed.