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

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 less than 0.025 and P less than 0.006, respectively) only in soleus muscle with restoration after refeeding, whereas [K+]i decreased in both muscles (P less than 0.005) and remained low after refeeding. K+ supplementation did not significantly change these indices or the ratio of free to total intracellular potassium (gamma Ki+). 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.