LOSARTAN-SENSITIVE AII RECEPTORS LINKED TO DEPOLARIZATION-DEPENDENT CORTISOL SECRETION THROUGH A NOVEL SIGNALING PATHWAY

In bovine adrenal zona fasciculata (AZF) cells, angiotensin II (AII) m ay stimulate depolarization-dependent Ca2+ entry and cortisol secretio n through inhibition of a novel potassium channel (I-AC), which appear s to set the resting potential of these cells, Aspects of the signalin g pathway, which couples AII receptors to membrane depolarization and secretion, were characterized in patch clamp and membrane potential re cordings and in secretion studies. AII mediated inhibition of I-AC, me mbrane depolarization, and cortisol secretion were all blocked by the AII type I (AT(1)) receptor antagonist losartan, These responses were unaffected by the AT(2) antagonist PD123319, Inhibition of I-AC by AII was prevented by intracellular application of guanosine 5'-O-2-(thio) diphosphate but was not affected by pre-incubation of cells with pertu ssis toxin, Although mediated through an AT(1) receptor, several lines of evidence indicated that AII inhibition of I-AC occurred through an unusual phospholipase C (PLC)-independent pathway. Acetylcholine, whi ch activates PLC in AZF cells, did not inhibit I-AC. Neither the PLC a ntagonist neomycin nor PLC-generated second messengers prevented I-AC expression or mimicked the inhibition of this current by AII, I-AC exp ression and inhibition by AII were insensitive to variations in intrac ellular or extracellular Ca2+ concentration. AII-mediated inhibition o f I-AC was markedly reduced by the non-hydrolyzable ATP analog adenosi ne 5'-(beta,gamma-imino)triphosphate and by the non-selective protein kinase inhibitor staurosporine. The protein phosphatase antagonist oka daic acid reversibly inhibited I-AC in whole cell recordings, These fi ndings indicate that AII-stimulated effects on I-AC current, membrane voltage, and cortisol secretion are linked through a common AT(1) rece ptor, Inhibition of I-AC in AZF cells appears to occur through a novel signaling pathway, which may include a losartan-sensitive AT(1) recep tor coupled through a pertussis-insensitive G protein to a staurospori ne-sensitive protein kinase, Apparently, the mechanism linking AT(1) r eceptors to I-AC inhibition and Ca2+ influx in adrenocortical cells is separate from that involving inositol trisphosphate-stimulated Ca2+ r elease from intracellular stores, AII-stimulated cortisol secretion ma y occur through distinct parallel signaling pathways.