Effects of adrenaline on membrane inward currents during the cardiac action potential.

Abstract

1. The effects of adrenaline on the inward currents flowing through the cardiac membranes were investigated in trabeculae of frog atria, by voltage clamp experiments. TTX was used as an inhibitor of the fast sodium channel and Mn as an inhibitor of a slow calcium-sodium channel. 2. The fact that adrenaline increases the amplitude of the plateau of the cardiac action potentials has been confirmed. This substance does not appreciably modify the current flowing through the fast channel but considerably increases the inward current which, according to previous investigations, is carried by Ca++ and Na+. For this reason, adrenaline can be considered as an activator of the slow channel. The increase of an inward calcium current by adrenaline can partly explain the positive inotropic effect of the substance. 3. Besides increasing the amplitude of the slow response, adrenaline decreases its threshold. This effect can explain that adrenaline is able to restore impaired conduction due to the action of TTX on fast sodium channel, as other workers observed.