Spectrophotometric assay of renal ouabain-resistant Na+-ATPase and its regulation by leptin and dietary-induced obesity.

Apart from Na+,K+-ATPase, a second sodium pump, Na+-stimulated, K+-independent ATPase (Na+-ATPase) is expressed in proximal convoluted tubule of the mammalian kidney. The aim of this study was to develop a method of Na+-ATPase assay based on the method previously used by us to measure Na+,K+-ATPase activity (Acta Biochim Polon.; 2002, 49: 515-27). The ATPase activity was assayed as the amount of inorganic phosphate liberated from ATP by isolated microsomal fraction. Na+-ATPase activity was calculated as the difference between the activities measured in the presence and in the absence of 50 mM NaCl. Na+-ATPase activity was detected in the renal cortex (3.5 ± 0.2 μmol phosphate/h per mg protein), but not in the renal medulla. Na+-ATPase was not inhibited by ouabain or an H+,K+-ATPase inhibitor, Sch 28080, but was almost completely blocked by 2 mM furosemide. Leptin administered intraperitoneally (1 mg/kg) decreased the Na+,K+-ATPase activity in the renal medulla at 0.5 and 1 h by 22.1% and 27.1%, respectively, but had no effect on Na+-ATPase in the renal cortex. Chronic hyperleptinemia induced by repeated subcutaneous leptin injections (0.25 mg/kg twice daily for 7 days) increased cortical Na+,K+-ATPase, medullary Na+,K+-ATPase and cortical Na+-ATPase by 32.4%, 84.2% and 62.9%, respectively. In rats with dietary-induced obesity, the Na+,K+- ATPase activity was higher in the renal cortex and medulla by 19.7% and 34.3%, respectively, but Na+-ATPase was not different from control. These data indicate that both renal Na+-dependent ATPases are separately regulated and that up-regulation of Na+-ATPase may contribute to Na+ retention and arterial hypertension induced by chronic hyperleptinemia.