2 COMPONENTS OF INWARD CURRENT IN MYOCARDIAL MUSCLE FIBERS

The membrane potential of trabecula of the sheep ventricle was clamped. In response to a depolarizing step a transient inward current lasting 100 ms was elicited. This transient current could be separated into: (1) A rapid component which was strongly dependent on the holding potential prior to depolarization. The reversal-potential agreed with the sodium equilibrium-potential. It is concluded that this component is carried by sodium ions. (2) A slower component which could not be inactivated by a variation in the holding potential and which could also be observed in sodium free solutions. The amplitude of the current depended on the extracellular Ca++-concentration. The slower transient inward component is possibly carried by calcium and/or sodium ions. when the extracellular sodium was 25 mM/l, the flow of components (1) and (2) caused two distinct peaks in the net current. The second, slower current is related to the positive plateau level of the myocardial action potential. © 1969 Springer-Verlag.