Cardiac-specific Overexpression of AT1 Receptor Mutant Lacking G Alpha Q/G Alpha I Coupling Causes Hypertrophy and Bradycardia in Transgenic Mice

Ang II type 1 (AT1) receptors activate both conventional heterotrimeric G protein-dependent and unconventional G protein-independent mechanisms. We investigated how these different mechanisms activated by AT1 receptors affect growth and death of cardiac myocytes in vivo. Transgenic mice with cardiac-specific overexpression of WT AT1 receptor (AT1-WT; Tg-WT mice) or an AT1 receptor second intracellular loop mutant (AT1-i2m; Tg-i2m mice) selectively activating G(alpha)q/G(alpha)i-independent mechanisms were studied. Tg-i2m mice developed more severe cardiac hypertrophy and bradycardia coupled with lower cardiac function than Tg-WT mice. In contrast, Tg-WT mice exhibited more severe fibrosis and apoptosis than Tg-i2m mice. Chronic Ang II infusion induced greater cardiac hypertrophy in Tg-i2m compared with Tg-WT mice whereas acute Ang II administration caused an increase in heart rate in Tg-WT but not in Tg-i2m mice. Membrane translocation of PKCepsilon, cytoplasmic translocation of G(alpha)q, and nuclear localization of phospho-ERKs were observed only in Tg-WT mice while activation of Src and cytoplasmic accumulation of phospho-ERKs were greater in Tg-i2m mice, consistent with the notion that G(alpha)q/G(alpha)i-independent mechanisms are activated in Tg-i2m mice. Cultured myocytes expressing AT1-i2m exhibited a left and upward shift of the Ang II dose-response curve of hypertrophy compared with those expressing AT1-WT. Thus, the AT1 receptor mediates downstream signaling mechanisms through G(alpha)q/G(alpha)i-dependent and -independent mechanisms, which induce hypertrophy with a distinct phenotype.