Keeping serum K+ within narrowly defined limits is critical for the maintenance of life. K+ homeostasis is an essential need of the body, but the means by which K^+ is conserved are poorly understood. K+-sparing is an important homeostatic component in response to volume depletion as may occur in such conditions as diarrhea, diuretic use, and congestive heart failure. The kidney adapts to volume depletion by reabsorbing more NaCI but the means by which K+ is conserved when NaCI is reabsorbed is not understood. The overall objective of this project is to elucidate the renal K+ conserving mechanisms involved in the adaptation to hypovolemia. I propose that K reabsorption occurs in volume depletion via a mechanism involving H+-K+-ATPase proteins present in the intercalated cells of the cortical collecting duct and these proteins are responsible for preserving K+-balance under conditions of volume depletion. Further, I propose that the apical Cl-/HCO-3 exchanger in the base secreting intercalated cell subtype, in cortical collecting duct, is the mechanism by which Cl-reabsorption accompanies Na+ reabsorption. Specific Aim I will examine the effect of volume depletion in in vitro studies on the activities of the H+-K+ ATPase and CI/HCO-3 exchanger in intercalated cells of cortical collecting tubules, and H+-K+-ATPase abundance and localization in renal cortex homogenate and in fixed kidney from control and volume depleted rats. In vivo studies will be performed on the effects of angiotensin, part of the renin-angiotensin-aldosterone system, which is stimulated in voluime depletion, on H+-K+-ATPase activity and Cl/HCO-3 exchange rate. Specific Aim II will investigate the role of angiotensin in acutely regulating the function of H+-K+-ATPases and the Cl-/HCO-3 exchanger in intercalated cells in vitro. Two signal transduction pathways associated with the actions of angiotensin will be studied in order to determine how angiotensin exerts its effects on intercalated cell transport. The overall premise for conducting these experiments is that if we understand the physiological basis by which the body normaIly conserves K+, then we will also have identified a target that potentially can be exploited for therapy of disorders of K+ metabolism.