Calcium‐dependent electrical activity and contraction of voltage‐clamped frog single muscle fibres.

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Abstract
The electrical and mechanical activities of isolated frog muscle fibers have been simultaneously recorded under conditions which allow the development of a Ca permeability (chloride-free solution containing 72 mM calcium levulinate instead of 115 mM NaCl). Long-lasting Ca action potentials and large and sustained contraction occur without inhibition of the K permeability. The relationship observed between the slow inward Ca current and the amplitude of the contraction, under voltage-clamp conditions, resembles that found between IBa [ionic strength of Ba] and tension in frog skeletal muscle and between ICa [ionic strength of Ca] and the contraction in frog heart. A part of the mechanical response which is not abolished by Mn seems to be potential-dependent. Integration of the current traces shows that the threshold of the contraction occurs at [Ca]i near to 2 .cntdot. 10-6 M and that the contractile proteins are fully activated at [Ca]i near to 10-4 M, values which are in good agreement with those proposed by others. The minimum Ca influx necessary to induce a detectable contraction is close to 4.8 pmol/cm2. This value is 10-20 times larger than the Ca influx measured during a normal action potential. On detubulated fibers the Ca permeability fails to develop; this current originates from the T-system. The close correspondence between ICa and the contraction, and between the time course of the contraction at the end of the depolarizing steps suggests that the potential of the tubular membrane is better clamped than in normal physiological conditions.