Calcium depletion in frog muscle tubules: the decline of calcium current under maintained depolarization.

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Abstract
Ca2+ currents in frog skeletal muscle fibers were studied with a voltage-clamp technique. Under membrane depolarization maintained for several s, Ca2+ current declined with time constants of 0.2-2 s when [Ca2+]0 = 10 mM. Ca2+ currents are diminished by nifedipine, D-600 [.alpha.-isopropyl-.alpha.-[N-methyl-N-homoveratryl-.gamma.-aminopropyl]-3,4,5-trimethoxyphenylacetonitrile], tetracaine and Ni2+. When peak current is diminished by making the membrane potential positive, by block with drugs or by substituting the relatively less permeant Mn2+ for Ca2+, the rate of decline is diminished. When peak current is increased by recording at relatively negative membrane potentials or by substituting for Ca2+ the more permeant ions Ba2+ or Sr2+, the rate of decline is increased in proportion. Evidently the size of the current determines the rate of decline. Decline of current is greatly slowed in isotonic Ca2+ saline or when the [Ca2+]0 is buffered by the organic anion malate. The decline of current apparently arises from Ca2+ depletion in an extracellular compartment, probably the transverse tubules. On this basis, an analysis of Ca2+ current decline and recovery leads to the following conclusions. Ca2+ current flows almost entirely across the membranes of the transverse tubules. After allowing for the tortuosity of the tubular network, the apparent diffusion coefficient for Ca2+ in the transverse tubules is about 2.6 .times. 10-6 cm2/s, 3 times less than the diffusion coefficient for K+ in the transverse tubules and about 3 times less than the diffusion coefficient for Ca2+ in free solution. The transverse tubule lumen does not appear to have a large Ca2+-buffering capacity in the mM range. At [Ca2+]0 = 10 mM the tubule lumen binds less than 0.6 dissociable Ca2+ ions for every free ion.

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