¹ From the Department of Medicine, The Johns Hopkins University and Hospital, Baltimore

It was reported previously that insulin hyperpolarized rat skeletal muscle and decreased K⁺ flux in both directions. The observations on K⁺ flux are now extended to take advantage of the greater sensitivity to insulin of hyperphysectomized rats. Insulin caused a shift of water from extracellular to intracellular space if glucose was present, but not in its absence. Insulin caused net gain of muscle fiber K⁺, though not necessarily an increase in K⁺ concentration in fiber water. It probably also decreased intrafiber Na⁺ and Cl^-. Insulin decreased K⁺ efflux. The effect was dose-dependent. Muscles from hypophysectomized rats were more sensitive to the action of insulin on K⁺ flux than were those from normal rats. The effect was demonstrable within the time resolution of the system, suggesting that insulin's action is on cell surfaces. K⁺ influx was also decreased by insulin. Bookkeeping suggests that some K⁺ influx be called active. Insulin seemed to decrease active K⁺ influx and passive K⁺ efflux. It is not resolved whether insulin has a true dual effect or whether it acts only on passive fluxes in both directions (the apparent action on active K⁺ influx being an artefact of incomplete definition of passive flux) or whether a single alteration in the membrane may affect both active and passive fluxes.

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