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+]iin predominantly slow- (soleus) and fast- (extensor digitorum longus) twitch muscles in rats undergoing underfeeding and subsequent refeeding. After underfeeding, aKi+and Emdecreased (P< 0.025 andP< 0.006, respectively) only in soleus muscle with restoration after refeeding, whereas [K+]idecreased 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+]iduring 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.