Membrane Depolarization and the Metabolism of Muscle

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Abstract
The respiration of frog twitch muscles rises markedly when [K]0is raised; respiration is stimulated by levels of [K]0 below the threshold for contracture(Fenn, 1931).Respiration is also stimulated by elevated [Rb]0 and [Cs]0 in direct relation to their ability to depolarize the membrane. Respiration is stimulated even when the anions in the high [K]o solution cannot permeate the membrane.If [K]0is raised to 25 mM there is an increase in respiration which is sustained for hours. If [K]0is 30 mM or above, there is a transitory burst of stimulated respiration followed by a decline back to the basal level. The response to elevated[K]0 can be blocked by divalent cations or by local anesthetics; the blocking agents act rapidly, probably on the cell membrane. Either extracellular calcium or strontium is needed for a prolonged stimulation of respiration. Depolarization seems to increase respiration by causing the release of calcium into the sarcoplasm. Since respiration is increased by a depolarization below the threshold for producing a contracture, respiration is a sensitive indicator of the sarcoplasmic concentration of calcium. A model for the relation between sarcoplasmic [Ca] and membrane potential is proposed. Calcium can be released from a store in the cell, the released Ca++ entering the sarcoplasm. The store is replenished by Ca++ entering the fiber from the extracellular solution. When the membrane is depolarized, the rate of release of calcium in the store is increased; at the same time the rate at which extracellular calcium can replenish the store is decreased.This model accounts well for the data on respiration and also for the contractures of single muscle fibers. Calcium probably acts within the cell to activate an ATP-ase which causes an increase in ADP and hence an increase in respiration. Other investigators have found changes in the activity of certain enzymes and in the permeability of the membrane; possibly these changes are also a direct response to an increase in sarcoplasmic calcium.

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