Transgenic mice were created with cardiac-specific overexpression of the beta 2-adrenergic receptor. This resulted in increased basal myocardial adenylyl cyclase activity, enhanced atrial contractility, and increased left ventricular function in vivo; these parameters at baseline in the transgenic animals were equal to those observed in control animals maximally stimulated with isoproterenol. These results illustrate a useful approach for studying the effect of gene expression on cardiac contractility. Because chronic heart failure in humans is accompanied by a reduction in the number of myocardial beta-adrenergic receptors and in inotropic responsiveness, these results suggest a potential gene therapy approach to this disease state.

Stimulation of Gj-coupled receptors leads to the activation of mitogen-activated protein kinases (MAP kinases). In several cell types, this appears to be dependent on the activation of p21ras (Ras). Which G-protein subunit(s) (Ga or the Gpy complex) primarily is responsible for triggering this signaling pathway, however, is unclear. We have demonstrated previously that the carboxyl terminus of the -adrenergic receptor kinase, containing its Gp-rbinding domain, is a cellular Gpy antagonist capable of specifically distinguishing G.-and Gprmediated processes. Using this Gpy inhibitor, we studied Ras and MAP kinase activation through endogenous G,-coupled receptors in Rat-i fibroblasts and through receptors expressed by transiently transfected COS-7 cells. We report here that both Ras and MAP kinase activation in response to lysophosphatidic acid is markedly attenuated in Rat-i cells stably transfected with a plasmid encoding this GP, The a2-adrenergic receptor (a2-AR) and the receptor for lysophosphatidic acid (LPA) have been shown to transduce cellular signals through G1 proteins, resulting in cell proliferation (1-3). These responses resemble those evoked by growth factor tyrosine kinase receptors, in which the signaling cascade has been more thoroughly delineated. Signaling via growth factors such as epidermal growth factor' (EGF) involves phosphorylation of tyrosine residues and a series of protein-protein interactions, mediated via Src homology 2 and 3 (SH2/SH3) domains, leading to serial activation of p21ras (Ras), Raf-1 kinase and mitogen-activated protein kinases (MAP Recently, several G1-coupled receptors, including the a2-AR, LPA receptor, M2 muscarinic acetylcholine receptor (AChR), and platelet-activating factor (PAF) receptor, have been shown to stimulate MAP kinase activity in various cell types (7-13). The signaling pathways by which G1-coupled receptors activate MAP kinase, however, remain poorly understood, but there is evidence for both Ras-dependent (7-12) and Ras-independent (5, 13, 14) activation of MAP kinases. The recently discovered MAP kinase kinase kinase (MEK kinase or MEKK) (14) is a possible link in a Ras/Rafindependent G protein-coupled MAP kinase activation cascade.Agonist activation of G protein-coupled receptors results in the dissociation of heterotrimeric (a/3y) G proteins into activated GaGTP and Gj,, subunits (15,16