CAMP-INDEPENDENT, G-PROTEIN-LINKED INHIBITION OF NA-BORDER BY D1 DOPAMINE AGONISTS( H+ EXCHANGE IN RENAL BRUSH)

When D1 dopamine agonists are incubated with renal cortical tissue, Na +/H+ exchange activity is inhibited, presumably due to D1 receptor-med iated stimulation of adenylyl cyclase and subsequent increase in prote in kinase A activity. Although the role of adenosine 3',5'-cyclic mono phosphate (cAMP) and cAMP-dependent protein kinase in the regulation o f Na+/H+ exchange activity is well established, receptors functionally coupled to adenylyl cyclase can regulate Na+/H+ exchange activity ind ependently of changes of cAMP accumulation. The current studies were d esigned to determine whether D1 agonists can inhibit Na+/H+ exchange a ctivity independently of changes of cAMP accumulation and also to dete rmine the role of G proteins in this process. The D1 agonist, fenoldop am, inhibited Na+/H+ exchange activity in a time-related and concentra tion-dependent manner. The 50% inhibitory concentration was 5-34 muM. Occupation of the renal D1 receptor mediates this action, since the D1 antagonist, SKF 83742, partially blocks the effect. This action, howe ver, was independent of adenylyl cyclase, protein kinase A, and protei n kinase C activity. Inhibition of adenylyl cyclase with dideoxyadenos ine or inhibition of protein kinase A and C with the isoquinolines N-( 2-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride (H-4) and 1- (5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) did not block the ef fect of fenoldopam on the exchanger. The action of fenoldopam is not d ue to an amiloride-like action on the exchanger, because kinetic analy sis of the inhibitory action was noncompetitive and the effect of feno ldopam was time dependent. The process involved G proteins, since guan osine 5'-O-(2-thiodiphosphate) prevented while guanosine 5'-O-(3-thiot riphosphate) increased the inhibitory effect of fenoldopam.