Hyperpolarization of mammalian skeletal muscle fibers in K-free media.

Time course of changes in resting potential following removal of K ions from Krebs solution was studied in the isolated rat soleus muscle. In K+-free Krebs solution the muscle fibers hyperpolarized to a peak within 60 min followed by a variable, gradual depolarization over the next few hours. A significant decrease of the intracellular K+ concentration ([K]i) occurred within 30 min in K+-free Krebs. Compensatory increase of the intracellular Na+ concentration ([Na]i) lagged behind [K]i change. Both hyperpolarization and successive depolarization were modified in K+-free medium containing different [Na]o, [Ca]o, or osmotic strength. A part of the hyperpolarization was reduced by adding ouabain. Therefore, ouabain-sensitive potential of the fibers in K+-free medium was attributed to the activation of electrogenic Na pump by K+ leaked out continuously from the fibers and accumulated just outside of cell membranes but not to changes of membrane Na+ and Cl- permeabilities. It is concluded that hyperpolarization of muscles exposed to K+-free medium is the sum of the diffusional K+ potential and ouabain-sensitive metabolic potential. Interestingly, the time course of changes of membrane response to K+-free krebs was identical for contralateral soleus muscles in the same rat, though it varied among different rats.