Abstract

The closely related G protein-coupled receptor kinases GRK2 and GRK3 are both expressed in cardiac myocytes. Although GRK2 has been extensively investigated in terms of regulation of cardiac β-adrenergic receptors, the substrate specificities of the two GRK isoforms at G protein-coupled receptors (GPCR) are poorly understood. In this study, the substrate specificities of GRK2 and GRK3 at GPCRs that control cardiac myocyte function were determined in fully differentiated adult cardiac myocytes. Concentration-effect relationships of GRK2, GRK3, and their respective competitive inhibitors, GRK2ct and GRK3ct, at endogenous endothelin, α₁-adrenergic, and β₁-adrenergic receptor-generated responses in cardiac myocytes were achieved by adenovirus gene transduction. GRK3 and GRK3ct were highly potent and efficient at the endothelin receptors (IC₅₀ for GRK3, 5 ± 0.7 pmol/mg of protein; EC₅₀ for GRK3ct, 2 ± 0.2 pmol/mg of protein). The α₁-adrenergic receptor was also a preferred substrate of GRK3 (IC₅₀,7 ± 0.4 pmol/mg of protein). GRK2 lacked efficacy at both endothelin and α₁-adrenergic receptors despite massive overexpression. On the contrary, both GRK2ct and GRK3ct enhanced β₁-adrenergic receptor-induced cAMP production with comparable potencies. However, the potency of GRK3ct at β₁-adrenergic receptors was at least 20-fold lower than that at endothelin receptors. In conclusion, this study demonstrates distinct substrate specificities of GRK2 and GRK3 at different GPCRs in fully differentiated adult cardiac myocytes. As inferred from the above findings, GRK2 may play its primary role in regulation of cardiac contractility and chronotropy by controlling β₁-adrenergic receptors, whereas GRK3 may play important roles in regulation of cardiac growth and hypertrophy by selectively controlling endothelin and α₁-adrenergic receptors.