Regulation of thick ascending limb ion transporter abundance in response to altered acid/base intake

Changes in ammonium excretion with acid/base perturbations are dependent on changes in medullary ammonium accumulation mediated by active NH4+ absorpt ion by the medullary thick ascending limb. To investigate whether alteratio ns in the abundance of medullary thick ascending limb ion transporters, nam ely the apical Na+/K+ (NH4+)/2Cl(-)-cotransporter (BSC-1), the apical Na+/H +-exchanger (NHE3), and the Na+/K+-ATPase alpha 1-subunit, may be responsib le in part for altered medullary ammonium accumulation, semiquantitative im munoblotting studies were performed using homogenates from the inner stripe of the rat renal outer medulla. After 7 d of NH4Cl (7.2 mmol/220 g body wt per d) loading (associated with increased medullary ammonium accumulation) , neither BSC-1 nor Na+/K+-ATPase protein expression was altered, but NHE3 protein abundance was significantly increased. On the other hand, both BSC- 1 and Na+/K+-ATPase protein abundance was increased significantly in rats f ed NaHCO3 (7.2 mmol/220 g body wt per d) for 7 d. Rats fed a high-NaCl diet (7.7 mEq Na+/220 g body wt per d) for 5 d also showed marked increases in both BSC-1 and Na+/K+-ATPase expression. The expression level of NHE3 prote in did not change with either NaHCO3 or high NaCl intake. None of these thr ee transporters showed a significant difference in abundance between the gr oups fed equimolar (7.2 mmol/220 g body wt per d for 7 d) NaHCO3 or NaCl. I t is concluded that outer medullary BSC-1 and Na+/K+-ATPase alpha 1-subunit protein abundance is increased by chronic Na+ loading but not by acid/base perturbations and that outer medullary NHE3 protein abundance is increased by chronic NH4Cl loading.