Membrane currents responsibile for contraction and relaxation of the bullfrog ventricle.

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1) The membrane potentials, currents, and contractile responses were measured in the bullfrog ventricular trabeculae under voltage clamped conditions by means of the double glycerol-gap technique, and the membrane currents responsible for initiation of contraction and relaxation were studied.2) For the initiation of twitch contraction, the fast Na current was found to play a minor role, producing less than 6% of the maximum tension, whereas the slow current system was most important.3) Under constant presence of tetrodotoxin (10-7g/ml), 50% Na-depletion produced a marked augmentation of the active slow inward current as well as contractile tension and a conspicuous decrease in the threshold voltages, indicating the presence of Ca-Na antagonism in this slow current system. These effects, however, became temporary when external Na concentration was further decreased to 25% or totally eliminated.4) Na depletion experiments also revealed that a steady outward current due to elimination of leaky Na inward current was accompanied by a sustained tonic contracture, while reintroduction of Na ions elicited a steady inward current and a relaxation of contracture.5) Kinetics of tension development due to rectangular depolarization and relaxation due to repolarization appeared quite different, the former showing a complication and the latter a simple exponential curve in Nadeficient condition. Na ions accelerated the relaxation by decreasing the time constant and appeared necessary for rapid relaxation.6) These results suggest a presence of more than two types of excitationcontraction systems in the bullfrog ventricle; the fast and slow inward currents responsible for the initiation of phasic contraction and some Na-Ca exchange mechanism responsible for that of tonic contraction and for the relaxation of phasic and tonic contractions.