Dopamine-1 Receptor G-Protein Coupling and the Involvement of Phospholipase A2 in Dopamine-1 Receptor Mediated Cellular Signaling Mechanisms in the Proximal Tubules of SHR

Abstract

Dopamine-induced natriuretic response which results from the activation of tubular dopamine1 (DA1) receptors is diminished in spontaneously hypertensive rats (SHR). This may be a result of alterations occurring at the receptor level and within the cellular signaling pathway which ultimately causes inhibition of Na+,K+-ATPase. There have been reports showing that DA, receptor induced inhibition of Na+,K+-ATPase is abolished in SHR which is due to a decreased activation of PLC and PKC by dopamine. Of the mechanisms, adenylyl cyclase and phospholipase C are two known enzymes linked to DA1 receptors via G proteins. Furthermore, the involvement of phospholipase A2 (PLA2) has also been reported in this process. However, the site of defect in DA1 receptor signaling pathway in SHR is still not well understood. This report will (i) review the coupling of DA1 receptor with G proteins and their levels in Wistar Kyoto (WKY) rats and SHR and (ii) discuss studies dealing with the role of PLA2 in dopamine-induced inhibition of Na+,K+-ATPase in WKY rat and SHR kidneys. Fenoldopam, DA1 receptor selective agonist stimulated [35S]GTPγ S binding in a concentration (10−9-10−4 M)-dependent manner in WKY rats which was attenuated in SHR. Fenoldopam (10 μM)-induced stimulation of [35S]GTPγ S binding was significantly reduced by a DA1 receptor selective antagonist, SCH 23390 suggesting the involvement of DA, receptor. Furthermore, the specific antipeptides Gsa, and Gq/l la significantly blocked fenoldopam-stimulation of [35S]GTPrS binding suggesting the coupling of DA, receptor with both the G proteins. Western analysis revealed a significant decrease in Gq/lla but no changes in Gsa in SHR compared to WKY rats. Dopamine inhibited Na′,K+-ATPase activity in a concentration (lo5 M)-dependent manner in WKY rats while it failed to inhibit the enzyme activity in SHR. Dopamine (10 &I)-induced inhibition in Na′,K'′ATPase activity was significantly blocked by mepacrine (a PLA2 inhibitor) suggesting the involvement of PLA2 in dopamine-mediated inhibition of Na′,K′-ATPase. Arachidonic acid (AA), a PLA2 product, inhibited Na′,K′-ATPase in a concentration (1-100 &I)-dependent manner in WKY rats while the inhibition in SHR was significantly attenuated (IC5,: 7.5, A4 in WKY and 80 &I in SHR). Furthermore, lower concentration (1 pM) of AA stimulated the enzyme activity in SHR. This suggests a defect in the metabolism of AA in SHR. Proadifen (10 pM), 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 (i) the reduced activation of G proteins following DA, receptor stimulation, (ii) reduced amount of Gq/l l a and (iii) a defect in the AA metabolism may be responsible for the reduced dopaminergic inhibition of sodium pump activity and a diminished natriuretic response to dopamine in SHR.