PROXIMAL TUBULAR DOPAMINE PRODUCTION REGULATES BASOLATERAL NA-K-ATPASE

Regulation of proximal tubular Na-K-adenosine-triphosphatase (ATPase), brush-border membrane Na+-H+ antiporter and Na+-P(i) symporter activity by endogenously produced dopamine was examined in Wistar rats. Na-K-ATPase was measured in basolateral membrane (BLM) fractions permeabilized with alamethicin or sodium dodecyl sulfate (SDS). Carbidopa (5 mg/kg) injected 18 h before removal of kidneys increased maximal activity (V(max)) noncompetitively in cortical BLM but not in other membrane fractions or outer medullary BLM (-2 +/- 4%). Chronic renal denervation did not alter the response. Carbidopa stimulated Na-K-ATPase in cortical BLM from rats eating a normal salt diet with and without 1% saline to drink (+ 18 +/- 4% and + 22 +/- 4%, respectively; P > 0.001). Carbidopa did not increase V(max) of BLM Na-K-ATPase from rats eating a low-salt diet (+ 1.5 +/- 4%); however, when the low-salt diet was supplemented with 1 mM dihydroxyphenylalanine (dopa) to drink for 1 day carbidopa, increased V(max) by 18 +/- 3% (P = 0.018). Carbidopa did not alter the Michaelis constant (K(m)) for Na or K or inhibitory constant (K(i)) for ouabain. Injection of the DA1 antagonist Sch 23390 (2 mg/kg) also increased Na-K-ATPase (18 +/- 4%; P = 0.014). Western blots using a monoclonal alpha-subunit antibody revealed a 22 +/- 8% increase following carbidopa treatment (P = 0.033; n = 19 pairs). Carbidopa had no effect on Na+-H+ antiporter activity (Na-22 uptake) or on Na+-P-32i cotransport in brush-border membrane vesicles. These results indicate that dopamine produced in proximal tubules tonically reduces Na-K-ATPase V(max) by decreasing the number of alpha-subunits associated with the BLM.