Catecholamine effects on intracellular sodium activity and tension in dog heart

Because catecholamines have been reported to stimulate the sodium pump and Na⁺-K⁺-ATPase in skeletal and cardiac muscle, we examined the effects of isoproterenol (0.2-1.0 x 10^-6 M) and norepinephrine (2-4 x 10^-7 M) on intracellular sodium activity (a(i)(Na)) and twitch tension in dog Purkinje strands and ventricular muscles, a(i)(Na) was measured with Na⁺-sensitive microelectrodes. During the initial rapid increase in tension induced by catecholamines in Purkinje strands, no changes in a(i)(Na) were found. After 5-10 min a(i)(Na) decreased by about 2mM, coincident with a small decline in twitch tension. When the catecholamine was removed, tension declined rapidly to a level less than control. Recovery of tension to its control level occurred simultaneously with recovery of a(i)(Na). Comparable changes in a(i)(Na) occurred in ventricular muscle, but the biphasic effect of catecholamines on tension was not seen. These results are consistent with sodium pump stimulation in cardiac muscle. In Purkinje strands the resulting changes in a(i)(Na) may alter the drect positive inotropic effect of catecholamines, probably by influencing Na⁺-Ca²⁺ exchange.