MEMBRANE CURRENTS RESPONSIBLE FOR CONTRACTION AND RELAXATION OF THE BULLFROG VENTRICLE

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.