Glucagon acutely inhibits but chronically activates Na(+)/H(+) antiporter 3 activity in OKP cells

Abstract

Background: We previously found that the Na(+)/H(+) exchanger 3 (NHE3) is localized in the apical membrane of the rat renal proximal tubule and thick ascending limb of Henle. In the present study, we examined the direct effect of glucagon on the opossum kidney P (OKP) cell Na(+)/H(+) antiporter, encoded by NHE3.

Methods: Na(+)/H(+) antiporter activity was measured as the rate of cell pH recovery from an acid load using 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. Northern blot and Western blot analyses were performed using OKP NHE3 cDNA and anti-OKP-NHE3 antibodies.

Results: Glucagon (1 ng/ml) acutely (1 h) inhibited, but chronically (24 h) activated NHE3 activity in OKP cells. These effects were blocked by either KT5720 or RpcAMP [protein kinase A (PKA) inhibitors], and mimicked by 10(-4) M dibutyryl-cAMP. Both NHE3 mRNA and protein abundance increased with the 24-hour incubation in glucagon or dibutyryl-cAMP. Cycloheximide did not prevent a significant increase in NHE3 activity at 24 h. We therefore examined NHE3 protein abundance in the surface membrane by the biotinylation method. cAMP or glucagon significantly increased NHE3 protein abundance in the surface membrane when incubated with cycloheximide for 24 h.

Conclusions: Glucagon acutely inhibits but chronically activates NHE3 activity in OKP cells via a PKA-dependent pathway. Both protein-synthesis-dependent and -independent mechanisms play important roles in the chronic activation of NHE3.