Tension fall after contraction of bullfrog atrial muscle examined with the voltage clamp technique.

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1) Tension fall after termination of a depolarization was studied on the bullfrog atrial muscle under voltage clamp by the double glycerol-gap technique.2) The tension fall was composed of three different phases:(a) initial retardation, (b) principal rapid relaxation, and (c) final slow relaxation.3) The initial and second phases appeared exponential with time after repolarization. At 17°C, the time constant of the former was 0.07-0.08sec and that of the latter, 0.3-0.4sec for 25 to 80mV depolarizations.4) The tension and the rate of tension fall during the second principal phase appeared almost proportional to the maximal tension at the end of depolarization, and strongly depended on the membrane potential levels before and after repolarization.5) The principal tension fall during relaxation at different potential levels was also exponential, and the time constant increased in a hyperbolic manner with depolarization.6) The relationship between the logarithm of late tension around 1 sec after repolarization and the membrane potential level during relaxation was linear for the depolarizing side. For the hyperpolarizing side the tension, becoming independent of the potential, appeared as a mere function of time.7) These results are discussed in connection with possibilities that the initial retardation of tension fall relates to probable Ca influx immediately after repolarization and the second principal phase of relaxation, to the Na-Ca exchange mechanism.