Desensitization of Chick Embryo Ventricle to the Physiological and Biochemical Effects of Isoproterenol Evidence for Uncoupling of the ft Receptor-Adenylate Cyclase Complex

To determine if cardiac tissue is capable of modulating its response to a stimulating hormone, we studied desensitization to the positive inotropic effect of catecholamines on embryonic chick ventricular tissue using a phase contrast microscope-video motion detector system and correlated the contractility findings with concurrent observations of /?-adrenergic receptor properties and adenylate cyclase activity. Incubation for 30 minutes with 1 /IM isoproterenol produced a diminution in the subsequent inotropic response to 0.1 /IM isoproterenol to 35 ± 8% (mean ± SEM) of the initial response. This desensitization to the positive inotropic effect of isoproterenol was catecholamine-specific and was not accompanied by alteration in the inotropic response to Ca. To investigate the mechanism of desensitization, we studied properties of the /3-adrenergic receptor in homogenates of chick embryo ventricle using [H]dihydroalprenolol as a ligand. Thirty minutes of incubation with 1 pM isoproterenol produced no change in )8-adrenergic receptor density (92.8 ± 5.1 fmol/mg protein) and only a small change in receptor affinity (KD = 5.2 ± 0.3 DM VS. 7.0 ± 0.3 nM; P < 0.01). Receptor affinity for isoproterenol, as judged by [H]dihydroalprenolol displacement, was not changed significantly. The adenylate cyclase stimulation by isoproterenol in similarly prepared tissue, however, was reduced to 29% of the control value after 30 minutes of exposure to 1 /IM isoproterenol. Adenylate cyclase sensitivity was restored by guanosine 5'-(/?,y-imino)triphosphate. Thus, desensitization of physiological responsiveness of ventricular tissue to a /f-adrenergic agonist was accompanied by little change in fiadrenergic receptor properties but by marked diminution in adenylate cyclase responsiveness. These observations suggest that uncoupling of the y3-adrenergic receptor-adenylate cyclase complex may be the mechanism of short-term desensitization of ventricular muscle to the positive inotropic effects of isoproterenol. Circ Res 47: 493-501, 1980