β-Arrestin2-mediated inotropic effects of the angiotensin II type 1A receptor in isolated cardiac myocytes

The G protein-coupled receptor kinases (GRKs) and β-arrestins, families of molecules essential to the desensitization of G protein-dependent signaling via seven-transmembrane receptors (7TMRs), have been recently shown to also transduce G protein-independent signals from receptors. However, the physiologic consequences of this G protein-independent, GRK/β-arrestin-dependent signaling are largely unknown. Here, we establish that GRK/β-arrestin-mediated signal transduction via the angiotensin II (ANG) type 1A receptor (AT_1AR) results in positive inotropic and lusitropic effects in isolated adult mouse cardiomyocytes. We used the "biased" AT_1AR agonist [Sar¹, Ile⁴, Ile⁸]-angiotensin II (SII), which is unable to stimulate G_αq-mediated signaling, but which has previously been shown to promote β-arrestin interaction with the AT_1AR. Cardiomyocytes from WT, but not AT_1AR-deficient knockout (KO) mice, exhibited positive inotropic and lusitropic responses to both ANG and SII. Responses of WT cardiomyocytes to ANG were dramatically reduced by protein kinase C (PKC) inhibition, whereas those to SII were unaffected. In contrast, cardiomyocytes from β-arrestin2 KO and GRK6 KO mice failed to respond to SII, but displayed preserved responses to ANG. Cardiomyocytes from GRK2 heterozygous knockout mice (GRK2^+/-) exhibited augmented responses to SII in comparison to ANG, whereas those from GRK5 KO mice did not differ from those from WT mice. These findings indicate the existence of independent G_αq/PKC- and GRK6/β-arrestin2-dependent mechanisms by which stimulation of the AT_1AR can modulate cardiomyocyte function, and which can be differentially activated by selective receptor ligands. Such ligands may have potential as a novel class of therapeutic agents.