There is a positive association between diets rich in potassium, control of blood pressure, and prevention of stroke. Extracellular [K+] is regulated closely to maintain normal membrane excitability by the concerted regulatory responses of muscle and kidney. Although kidney is responsible for ultimately matching K+ output to K+ intake each day, muscle contains more than 90% of the body's K+ and can buffer changes in extracellular fluid [K+] by either acutely taking up extracellular fluid K+ or releasing intracellular fluid K+ from muscle. It long has been assumed that the changes in muscle K+ transport are a function of sodium pump (Na,K-adenosine triphosphatase [Na, K-ATPasel]) abundance, especially that of the alpha2 isoform, which predominates in skeletal muscle. To test the physiologic significance of changes in muscle Na,K-ATPase expression, we developed the K+ clamp, which measures insulin-stimulated cellular K+ uptake in vivo in the conscious rat. By using the K+ clamp we discovered that significant insulin resistance to cell K+ uptake occurs as follows: (1) early in K+ deprivation before a decrease in muscle sodium pump pool size, and (2) during glucocorticoid treatment, which increases muscle Na,K-ATPase alpha2 levels greater than 50%. We also discovered that adaptation of renal and extrarenal K+ handling to altered K+ balance often occurs without changes in plasma [K+], supporting a feedforward mechanism involving K+ sensing in the splanchnic bed and adjustment of K+ handling. These findings establish the advantage of combining molecular analyses of Na,K-ATPase expression and activity with systems analyses of cellular K+ uptake and excretion in vivo to reveal regulatory mechanisms operating to control K+ homeostasis.