Altered arachidonic acid metabolism contributes to the failure of dopamine to inhibit Na+,K+-ATPase in kidney of spontaneously hypertensive rats

Dopamine decreases tubular sodium reabsorption in part by inhibition of Na+,K+-ATPase activity in renal proximal tubules. The signaling mechanism involved in dopamine-mediated inhibition of Na+,K+-ATPase is known to be defective in spontaneously hypertensive animals. The present study was designed to evaluate the role of phospholipase A2 (PLA2) and its metabolic pathway in dopamine-induced inhibition of Na+,K+-ATPase in renal proximal tubules from Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). Renal proximal tubular suspensions were prepared and Na+,K+-ATPase activity was measured as ouabain-sensitive adenosine triphosphate hydrolysis. Dopamine inhibited Na+,K+-ATPase activity in a concentration (1 nM - 10 mu M)-dependent manner in WKY rats while it failed to inhibit the enzyme activity in SHR. Dopamine (10 mu M)-induced inhibition of Na+,K+-ATPase activity in WKY rats was significantly blocked by mepacrine (10 mu M), a PLA2 inhibitor, suggesting the involvement of PLA2 in dopamine-mediated inhibition of Na+,K+-ATPase. Arachidonic acid (a product released by PLA2 action) inhibited Na+,K+-ATPase in a concentration-dependent (1-100 mu M) manner in WKY rats while the inhibition in SHR was significantly attenuated (IC50: 7.5 and 80 mu M in WKY rats and SHR, respectively). Furthermore, lower concentrations of arachidonic acid stimulated (30% at 1 mu M) Na+,K+-ATPase activity in SHR. This suggests a defect in the metabolism of arachidonic acid in SHR. Proadifen (10 mu M), an inhibitor of cytochrome P-450 monoxygenase (an arachidonic acid metabolizing enzyme) significantly blocked the inhibition produced by arachidonic acid in WKY rats and abolished the difference in arachidonic acid inhibition of Na+,K+-ATPase between WKY rats and SHR. These data suggest that PLA2 is involved in dopamine-induced inhibition of Na+,K+-ATPase and altered arachidonic acid metabolism may contribute to reduced dopaminergic inhibition of Na+,K+-ATPase activity in spontaneously hypertensive rats.