Potassium exchange in frog sartorius muscle

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Abstract
Under normal conditions, the fibers of frog sartorius muscle contain two K compartments. About two-thirds of the muscle K is in a compartment which readily exchanges with K in the extracellular fluid; the remainder of the muscle K is relatively nonexchangeable. Compartmentalization of the muscle K is not a result of high phosphate or low [K] in the Ringer's solution or previous equilibration at low temperature, but occurs also in vivo. By means of K42, the average muscle K influx and efflux in 2.5 K-Ringer's solution at 20 C were found to be (in µEq g–1 hr–1) 6.41 and 5.81, respectively (different muscles). In the steady state, K moves into the exchangeable compartment of muscle K by a process that can be described by a single exponential equation, but this mechanism fails when the energy stores of the muscle become low. K influx increases with the [K] of the Ringer's solution but reaches a maximum at high external levels of K. It is inferred from these results that the influx of K in frog muscle involves a saturable intermediate, or carrier, system.

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