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

Dopamine-induced natriuretic response which results from the activatio n of tubular dopamine(1) (DA(1)) receptors is diminished in spontaneou sly hypertensive rats (SHR). This may be a result of alterations occur ring at the receptor level and within the cellular signaling pathway w hich ultimately causes inhibition of Na+,K+-ATPase. There have been re ports showing that DA(1) 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 DA(1) receptors via G proteins. Fur thermore, the involvement of phospholipase A2 (PLA2) has also been rep orted in this process. However, the site of defect in DA(1) receptor s ignaling pathway in SHR is still not well understood. This report will (i) review the coupling of DA(1) receptor with G proteins and their l evels in Wistar Kyoto (WKY) rats and SHR and (ii) discuss studies deal ing with the role of PLA2 in dopamine-induced inhibition of Na+,K+-ATP ase in WKY rat and SHR kidneys. Fenoldopam, DA(1) receptor selective a gonist stimulated [S-35]GTP gamma S binding in a concentration (10(-9) -10(-4) M)-dependent manner in WKY rats which was attenuated in SHR. F enoldopam (10 mu M)-induced stimulation of [S-35]GTP gamma S binding w as significantly reduced by a DA(1) receptor selective antagonist, SCH 23390 suggesting the involvement of DA(1) receptor. Furthermore, the specific antipeptides Gs alpha, and Gq/11 alpha significantly blocked fenoldopam-stimulation of [S-35]GTP gamma S binding suggesting the cou pling of DA(1) receptor with both the G proteins. Western analysis rev ealed a significant decrease in Gq/11 alpha but no changes in Gs alpha in SHR compared to WKY rats. Dopamine inhibited Na+,K+-ATPase activit y in a concentration (10(-9)-10(-5) M)-dependent manner in WKY rats wh ile it failed to inhibit the enzyme activity in SHR. Dopamine (10 mu M )-induced inhibition in Na+,K+-ATPase activity was significantly block ed 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 m u M)-dependent manner in WKY rats while the inhibition in SHR was sign ificantly attenuated (IC50: 7.5 mu M in WKY and 80 mu M in SHR). Furth ermore, lower concentration (1 mu M) of AA. stimulated the enzyme acti vity in SHR. This suggests a defect in the metabolism of AA in SHR. Pr oadifen (10 mu M), an inhibitor of cytochrome P-450 monoxygenase (an a rachidonic acid metabolizing enzyme) significantly blocked the inhibit ion produced by arachidonic acid in WKY rats and abolished the differe nce in arachidonic acid inhibition of Na+,K+-ATPase between WKY rats a nd SHR. These data suggest that (i) the reduced activation of G protei ns following DA(1) receptor stimulation, (ii) reduced amount of Gq/11 alpha and (iii) a defect in the AA metabolism may be responsible for t he reduced dopaminergic inhibition of sodium pump activity and a dimin ished natriuretic response to dopamine in SHR.