Barium-Calcium Antagonism in the Electrical and Mechanical Activity in Guinea-Pig Ventricular muscle

Experiments were performed to investigate the mechanism underlying the inhibitory effect of Ca ions on the Ba-dependent electrical and mechanical activity in the myocardium. In isolated strips of guinea-pig ventricular muscle perfused with Ca-free, K-rich (27 mM) depolarizing solution, addition of 0.6-1 mM Ba to the bath induced the Badependent slow action potentials. These action potentials were blocked by Co ions in concentrations much lower than those required to block the Ca-dependent action potentials. Development of the Ba-dependent action potentials was accompanied by a rise in resting tension (contracture). Addition of low concentrations (0.05-0.2 mM) of Ca to the depolarizing solution inhibited the action potentials induced by Ba and the contracture was suppressed. Further increase in Ca concentration enhanced the action potentials and these had a property characteristic of the Ca-dependent potential. The effect of Ca was accompanied by little change in the membrane resistance, determined using the sucrose gap method. The results suggest that Ca ions inhibit the Ba current through the slow channel and hence exert a relaxing effect on the Ba-induced contractu re.