AT1Receptor Gene Regulation in Cardiac Myocytes and Fibroblasts

The regulation of the AT1receptor gene was studied in neonatal cardiomyocytes and fibroblastsin vitro. Incubation with angiotensin II (Ang II) resulted in a time-dependent and dose-dependent decrease in AT1mRNA levels in both cardiomyocytes and fibroblasts. Co-incubation with Ang II and the specific AT1antagonist losartan prevented the decrease in AT1mRNA whereas the AT2antagonist PD123319 was ineffective in preventing the decrease in AT1mRNA. Because Ang II is known to decrease cAMP levels in cardiomyocytes, the role of cAMP in the regulation of the AT1gene was examined. Treatment with the adenylyl cyclase stimulant forskolin or the cAMP stereoisomer Sp-cAMPS increased AT1mRNA levels or prevented the Ang II mediated decrease in AT1mRNA levels. The role of calcium in the regulation of the AT1gene was determined by incubation with the calcium ionophores A23187 and ionomycin (0.0625–1μ ) which resulted in a profound, dose-dependent decrease in AT1mRNA levels. Treatment with BAPTA, an intracellular chelator of calcium, prevented the Ang II-mediated decrease in AT1mRNA. Therefore Ang II is a potent negative regulator of the AT1gene in cardiomyocytes and fibroblasts via the AT1receptor. This Ang II mediated decrease in AT1mRNA is mediated by two complementary mechanisms involving cAMP and intracellular calcium.