Membrane currents and activation of contraction in rat ventricular fibres.

1. A double sucrose gap technique has been combined with an optical method of measuring the movement of the preparation and applied to rat ventricular trabeculae.2. Experiments performed in normal McEwen and in depleted sodium solutions with and without tetrodotoxin, manganese or predepolarization suggest that the fast inward current is not as important as the slow inward current in determining the contractile strength. Furthermore, there is a marked similarity between potential dependence of the slow inward current and the contraction, particularly in sodium-depleted fluids.3. The appearance of a staircase response associated with a series of depolarizations, in normal McEwen, may mean that there is an intervention of a further, possibly intracellular, stage between inward current and contraction, as suggested by earlier experiments on frog as well as on mammalian heart.