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 AT₁ 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 μM) 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.