SYNERGISTIC EFFECT OF DEXAMETHASONE AND ISOPROTERENOL ON THE EXPRESSION OF ANGIOTENSINOGEN IN IMMORTALIZED RAT PROXIMAL TUBULAR CELLS

To investigate whether the expression of angiotensinogen (ANG) in rat kidney proximal tubules is stimulated by dexamethasone and isoproteren ol, immortalized rat proximal tubular cells (IRPTC) were cultured in a monolayer. Immunoreactive rat ANG (IR-rANG) in the culture medium was measured by a specific radioimmunoassay (RIA) for rANG. This RIA was developed by employing rabbit antiserum against the purified recombina nt rat ANG (rANG). The purified rANG from plasma and the iodinated rAN G were used as the hormone standard and tracer, respectively. The RIA is specific for rat ANG and it has no cross-reactivity with other pitu itary hormone preparations or other rat plasma proteins. The sensitivi ty of detection of the RIA is approximately 2 ng of rANG. The levels o f IR-rANG in the culture media of IRPTC ranged from 2 to 5 ng/ml/24 hr /10(6) cells. The addition of dexamethasone (10(-13) to 10(-5) M) stim ulated the expression and secretion of rANG from IRPTC in a dose-depen dent manner, whereas the addition of isoproterenol alone had no effect . However, a combination of both dexamethasone and isoproterenol syner gistically stimulated the expression and secretion of rANG by IRPTC. T he synergistic effect of dexamethasone and isoproterenol was blocked b y the presence of RU 486 (a glucocorticoid receptor antagonist) or pro pranolol (beta-adrenoceptor blocker). These studies suggest that the a ddition of dexamethasone and isoproterenol acts synergistically to sti mulate the expression and secretion of ANG protein in rat proximal tub ules in vivo.