AT1 receptor gene regulation in cardiac myocytes and fibroblasts

The regulation of the AT1 receptor gene was studied in neonatal cardiomyocytes and fibroblasts in vitro. Incubation with angiotensin II (Ang II) resulted in a time-dependent and dose-dependent decrease in AT1 mRNA levels in both cardiomyocytes and fibroblasts. Coincubation with Ang II and the specific AT1 antagonist losartan prevented the decrease in AT1 mRNA whereas the AT2 antagonist PD123319 was ineffective in preventing the decrease in AT1 mRNA. Because Ang II is known to decrease cAMP levels in cardiomyocytes, the role of cAMP in the regulation of the AT1 gene was examined. Treatment with the adenylyl cyclase stimulant forskolin or the cAMP stereoisomer Sp-cAMPS increased AT1 mRNA levels or prevented the Ang II mediated decrease in AT1 mRNA levels. The role of calcium in the regulation of the AT1 gene was determined by incubation with the calcium ionophores A23187 and ionomycin (0.0625-1 microM) which resulted in a profound, dose-dependent decrease in AT1 mRNA levels. Treatment with BAPTA, an intracellular chelator of calcium, prevented the Ang II-mediated decrease in AT1 mRNA. Therefore Ang II is a potent negative regulator of the AT1 gene in cardiomyocytes and fibroblasts via the AT1 receptor. This Ang II mediated decrease in AT1 mRNA is mediated by two complementary mechanisms involving cAMP and intracellular calcium.