Intracellular potassium and membrane potential in rat muscles during malnutrition and subsequent refeeding

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Abstract
Nutrition alters total body potassium (TBK) and muscle potassium but little is known about in vivo intracellular K+. We measured free intracellular potassium-ion activity (aKi+ ), membrane potential (Em), and total potassium (Kt) and calculated intracellular potassium concentration [K+]i in predominantly slow- (soleus) and fast- (extensor digitorum longus) twitch muscles in rats undergoing underfeeding and subsequent refeeding. After underfeeding, aKi+ and Em decreased (P< 0.025 and P< 0.006, respectively) only in soleus muscle with restoration after refeeding, whereas [K+]i decreased in both muscles (P< 0.005) and remained low after refeeding. K+ supplementation did not significantly change these indices or the ratio of free to total intracellular potassium (γKi+ ). The data show that aKi+ behaves differently from [K+]i during malnutrition and that changes in aKi+ occur especially in slow-twitch fibers, suggesting that previously observed changes in TBK and muscle function are the result of fundamental alterations in muscle-cell energetics and membrane functions, not just mass.

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