776-5 Mechanism of Decrease of Angiotensin II Receptor Message Through Dual Signal Transduction Pathways in Vascular Smooth Muscle

Abstract

We previously have shown that the angiotensin II receptor, type 1a (AT la R) message transiently decreases during ang II stimulation of cultured rat aortic vascular smooth muscle cells (VSMC); and, it has been previously reported by others that the mechanism of this decrease in cultured kidney cells is through cAMP dependent pathways. In this study we examined the signal transduction pathway of this regulation in quiescent VSMC. Total cellular RNA was harvested after time course exposure to reagents, Northern blotted and probed. AT 1a R message abundance decreased beginning two hours after exposure to ang II (10 nM) and reached a minimum at 4 hours after stimulation of 25% of baseline level. The protein kinase C(PKC) stimulator, PMA (50 nM), caused a decrease in AT 1a R message level with the same time course and to the same extent as ang II. Downregulation of PKC with 24 hour prior exposure to PDBU (500 nM) blocked the decrease in AT 1a R message caused by PMA. However, pretreatment with either PDBU orthe potent PKC inhibitor calphostin C(50 nM) did not block the decrease of AT 1a R message level caused by ang II. In contrast. stimulation of cAMP with forskolin (50 mM), cholera toxin (10 ng/ml) or the soluble analog 8-bromocyclic AM P(1 mM) did not decrease AT 1a R message level. whereas inhibition of PKA with H-8 pretreatment (10 mM) followed by ang II still decreased AT 1a R message abundance. We conclude that the decrease of AT 1a R message level in VSMC is through both PKC and non-PKC dependent pathways, but not through PKA or cAMP dependent pathways.