1546 / 3 - Adrenergic Receptor - G Protein - Adenylate Cyclase Complex in Experimental Canine Congestive Heart Failure Produced by Rapid Ventricular Pacing

in an experimental canine model of low-output heart failure produced by chronic rapid ventricular pacing. The contractile response occurring after exposure to the P-adrenergic agonist dobutamine, measured as peak left ventricular +dP/dt, was decreased after 3 weeks of pacing. To further characterize the diminished functional responsiveness to 3-adrenergic receptor stimulation, ,3-adrenergic receptor-adenylate cyclase coupling was investigated using membranes prepared from both control and paced animals. The density of J3-adrenergic receptors was decreased by 40% with a selective downregulation of the P3-subtype. The affinity of the receptor for the antagonist radioligand [1251]iodocyanopindolol remained unchanged. A defect in coupling was suggested by a decreased ability of isoproterenol, fluoride, and forskolin to stimulate adenylate cyclase in membranes prepared from failing hearts. Determination of the levels of Gia (the a-subunit of G;) by immunoblotting and pertussis toxin labeling revealed modest increases of -30%. Furthermore, Mn2' and purified G, failed to stimulate adenylate cyclase in membranes prepared from failing hearts, indicating an impairment in the catalytic moiety of adenylate cyclase itself or in the ability of adenylate cyclase to couple to G,. In contrast, complementation assay did not reveal differences in the functional activity ofQ. (the