ANGIOTENSIN-II STIMULATION OF NA-H ANTIPORTER ACTIVITY IS CAMP-INDEPENDENT IN OKP CELLS

Angiotensin II has been reported to stimulate the proximal tubule Na-H antiporter by inhibition of adenylyl cyclase, and possibly by an adenosine 3',5'-cyclic monophosphate (cAMP)-independent mechanism. We examined the effect of angiotensin II on Na-H antiporter activity (J(Na-H)) in opossum kidney (OKP) cells, a proximal tubule-like cell line, whose Na-H antiporter resembles that of the proximal tubule apical membrane. We found that angiotensin II regulates J(Na-H) in a concentration-dependent manner similar to the proximal tubule, with angiotensin II concentrations < 10(-8) M stimulating and > 10(-8) M inhibiting J(Na-H) The stimulatory effect of angiotensin II was blocked by 10(-8) M losartan and was pertussis toxin sensitive, suggesting mediation through an angiotensin II (AT(1)) receptor coupled to a pertussis toxin-sensitive G protein. Acute treatment with 10(-4) M 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) inhibited J(Na-H) by 30% and blocked angiotensin II-induced stimulation. However, angiotensin II (10(-12)-10(-6) M) did not inhibit basal, dopamine-stimulated or forskolin-stimulated cAMP production measured in the presence of 3-isobutyl-1-methylxanthine (IBMX). In addition, angiotensin II had no effect on cAMP levels measured in the absence of IBMX. We conclude that angiotensin II at physiological concentrations stimulates J(Na-H) in OKP cells via a cAMP-independent mechanism mediated by an AT(1) receptor and a pertussis toxin-sensitive G protein.