1. Ca2+ currents in frog skeletal muscle fibres were studied with a voltage‐clamp technique. Under membrane depolarization maintained for several seconds, Ca2+ current was found to decline with time constants of 0.2‐2 sec when [Ca2+]o = 10 mM. 2. Ca2+ currents are diminished by nifedipine, D‐600, tetracaine and Ni2+. 3. 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+ then the rate of decline is diminished also. When peak current is increased by recording at relatively negative membrane potentials or by substituting for Ca2+ the more permeant ions Ba2+ or Sr2+, then the rate of decline is increased in proportion. Evidently, the size of the current determines the rate of decline. 4. Decline of current is greatly slowed in isotonic Ca2+ saline or when the [Ca2+]o is buffered by the organic anion malate. These findings indicate that the decline of current arises from Ca2+ depletion in an extracellular compartment, most probably the transverse tubules. On this basis, an analysis of Ca2+ current decline and recovery leads to the following conclusions. 5. Ca2+ current flows almost entirely across the membranes of the transverse tubules. 6. After allowing for the tortuosity of the tubular network, the apparent diffusion coefficient for Ca2+ in the transverse tubules is about 2.6 X 10(‐6) cm2/sec, three times less than the diffusion coefficient for K+ in the transverse tubules and about three times less than the diffusion coefficient for Ca2+ in free solution. 7. The transverse tubule lumen does not appear to have a large Ca2+‐buffering capacity in the millimolar range. At [Ca2+]o = 10 mM, the tubule lumen binds less than 0.6 dissociable Ca2+ ions for every free ion.
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