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

1. The electrical and mechanical activities of isolated frog muscle fibres have been simultaneously recorded under conditions which allow the development of a calcium permeability (chloride-free solution containing 72 mM-calcium levulinate instead of 115 mM-NaCl). 2. Long-lasting calcium action potentials and large and sustained contraction occur without inhibition of the potassium permeability. 3. The relationship observed between the slow inward calcium current and the amplitude of the contraction, under voltage-clamp conditions, resembles that found between IBa and tension in frog skeletal muscle and between ICa and the contraction in frog heart. A part of the mechanical response which is not abolished by manganese seems to be potential-dependent. 4. Integration of the current traces shows that the threshold of the contraction occurs at [Ca]1 near to 2.10(-6) M and that the contractile proteins are fully activated at [Ca]1 near to 10(-4) M, values which are in good agreement with those proposed by other authors. 5. The minimum calcium influx necessary to induce a detectable contraction is close to 4.8 p-moles/cm2. This value is 10-20 times larger than the calcium influx measured during a normal action potential. 6. On detubulated fibres the calcium permeability fails to develop: this indicates that 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.