The Control of the Membrane Potential of Muscle Fibers by the Sodium Pump

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Abstract
Frog sartorius muscles were made Na-rich by immersion in K-free sulfate Ringer''s solution in the cold. The muscles were then loaded with Na24 and the extracellular space cleared of radioactivity. When such Na-rich muscles were transferred to lithium sulfate Ringer''s solution at 20[degree]C, Na efflux was observed to increase with time, to reach a maximum about 15 minutes after the transfer of the muscles to Li2SO4, and then to decline. The decline in efflux from these muscles was proportional to ([Na]i)3 over a considerable range of [Na]1. The membrane potential of Na-rich muscles was about -48 mv in K-free sulfate Ringer''s at 4[degree]C but changed to -76 mv in the same solution at 20[degree]C and to -98 mv in Li2SO4 Ringer''s at 20[degree]C. By contrast, muscles with a normal [Na]i showed a fall in membrane potential when transferred from K-free sulfate Ringer''s to Li2SO4 Ringer''s solution. The general conclusions from this study are (a) that Na extrusion is capable of generating an electrical potential, and (b) that increases in [Na]i lead to reversible increases in PNa of muscle fibers.