Characterization of acute inhibition of Na/H exchanger NHE-3 by dopamine in opossum kidney cells

Background. Dopamine (DA) is a principal natriuretic hormone that defends e xtracellular fluid volume from a Na load. Natriuresis is effected partly th rough inhibiting the proximal tubule Na/H exchanger NHE-3. Changes in NHE-3 phosphorylation is one mechanism by which NHE-3 activity is regulated. Methods. We used opossum kidney (OK) cells to characterize the differential and synergistic effects of DA receptor subtype-1 (DA(1)) and -2 (DA(2)) ag onists and the effect of blockade of protein kinase A (PKA) or protein kina se C (PKC) on NHE-3 activity and phosphorylation. Results. DA and DA(1) agonists inhibited NHE-3 activity, and DA(1) antagoni st blocked the effect of either DA or DA(1) agonist. DA(1) agonist alone ha d no effect, but DA(2) antagonist reduced the DA effect on NHE-3 activity. DA(1) and DA(2) agonists together were more potent than DA(1) alone. PKA in hibition eliminated the effect of DA(1) agonist and partially blocked the e ffect of DA on NHE-3 activity. PKC inhibition did not block the DA effect. DA1 agonist and PKA activation phosphorylated NHE3 on identical sites. Desp ite lack of effect on NHE-3 activity, DA(2) agonists increased NHE-3 phosph orylation. DA-induced NHE-3 phosphorylation was distinct from DA1 and PKA b ut closely resembled DA(2). Conclusion. We postulate the following: (1) DA modifies NHE-3 phosphorylati on by activating PKA through DA1 and by other kinases/phosphatases via DA(2 ). (2) DA(1) is sufficient to inhibit NHE-3, while DA(2) is insufficient bu t plays a synergistic role by altering NHE-3 phosphorylation.