Signal transduction via G protein-coupled receptors (GPCRs) is essential for the regulation of cardiovascular function, including heart rate, growth, contraction, and vascular tone. Pertubations in GPCR signaling have pathophysiological consequences and are major contributors to cardiac disease. Regulators of G protein Signaling (RGS proteins) belong to a diverse protein family that was originally discovered for their ability to accelerate signal termination in response to GPCR stimulation, thereby reducing the amplitude and duration of GPCR effects. Interaction of their common RGS domain with some Gα subunits of heterotrimeric G proteins mediates their biologic regulation of GPCR signaling by limiting the interactions between Gα-GTP and Gβγ with intracellular or membrane effectors or ion channels. Additional functional domains in some RGS protein isoforms can facilitate protein-protein and protein-lipid interactions that confer additional regulatory functions. Several RGS proteins are expressed in the heart and have been implicated in the cardiac remodeling response and heart rate regulation. Changes in RGS protein expression and/or function are believed to participate in the pathophysiology of cardiovascular diseases including hypertrophy, failure, arrhythmias and hypertension. Our investigations have focused primarily on RGS2, a member of the RGS R4 subfamily. RGS2 distinguishes itself from other closely related RGS isoforms by its susceptibility to regulation and signaling specificity. Expression studies in isolated adult rat ventricular cardiomyocytes (CM) and cardiac fibroblasts (CF) revealed that – among the cardiac RGS isoforms – RGS2 is most susceptible to regulation and that this is a highly dynamic process. Short-term activation of the Gq/11 signaling pathway transiently up-regulates RGS2 mRNA in both CM and CF, which likely represents a negative feedback mechanism. Acute β-adrenergic or forskolin stimulation also markedly increases RGS2 mRNA, which points to potential crossregulation and desensitization between Gs- and Gq/11-mediated signaling pathways. Importantly, in contrast to acute stimulation, marked RGS2 down-regulation was discovered in vivo in ventricles subjected to pressure overload, CM from mice expressing constitutively active Gαq as well as CM and CF from rats subjected to prolonged Angiotensin II infusion, which has been implicated in exacerbating cardiac remodeling in stressed or injured hearts. Functionally, using a gain-of-function approach (adenoviral infection), we show that RGS2 is a potent negative regulator of Gq/11 signaling in both CM and CF but has no effect on sympathetic or parasympathetic regulation of adenylate cyclase (AC) or AC itself (as reported for other cell types). Using a loss-of-function approach (RNA interference), we demonstrate that endogenous RGS2 is a functionally important negative regulator of Gq/11-mediated CM hypertrophy and function as well as CF proliferation and collagen production. It was previously shown that mice with generalized targeted deletion of RGS2 display enhanced hypertrophy and fibrosis in response to pressure overload (Takimoto et al., 2009), which is mediated in large part via Gq/11 signaling (Wettschureck et al., 2001). We generated a mouse model with α-myosin heavy chain promoter-driven RGS2 expression to investigate whether RGS2 overexpression in CM can be used to attenuate Gq/11 signaling and hypertrophy in vivo. Transgenic RGS2 was highly functional in negatively regulating Gq/11 signaling in atrial appendages and to a much lesser extent in ventricles. In ventricles (but not atrial appendages), RGS2-mediated modulation of Gq/11 signaling was absent after pressure overload induction by transverse aortic constriction (TAC), which may account for the lack of anti-hypertrophic effect we observed in transgenic mice. Importantly, TAC-induced RGS2 expression changes (down-regulation) did not underlie the functional differences we observed between atria and ventricles. Random insertion effect and changes in the expression of other RGS proteins were excluded as well. Interestingly, our findings suggest that RGS2 function is differentially regulated in atria and ventricles in vivo, which could include differences in posttranslational modifications (such as phosphorylation and palmitoylation) that can modulate the effectiveness of RGS2 in exerting regulatory effects on GPCR signaling. Since transgenic overexpression of RGS2 was associated with highly effective inhibition of Gq/11 signaling in the atria, the new model can be utilized to advance understanding of atrial signaling and to mitigate Gq/11 signaling in the atrium for potential therapeutic purposes. The in vivo role of RGS2 in regulating CF signaling and function in both atria and ventricles under both physiological and pathophysiological conditions also remains to be explored.