Appearance of beta-adrenergic receptors and catecholamine-responsive adenylate cyclase activity during fusion of avian embryonic muscle cells.

The appearance of P-adrenergic receptors and isoproterenol-stimulated adenylate cyclase during differentiation of embryonic quail muscle cells in tissue culture has been examined. P-Adrenergic receptors first appear during the fusion stage of myogenesis. ['"I]]Iodohydroxybenzylpindolol (['2sI]IHYP) was used as the ligand to characterize &adrenergic receptors. In equilibrium binding studies, mature myotubes contained, per diploid nucleus, approximately 800 specific and sa turable binding sites, which have a dissociation constant ( K d ) for ['251]IHYP of 108 PM. When determined from the association and dissociation rate constants, the kinetically derived Kd is 63 PM. ['2sI]IHYP binding to myotube membranes is blocked stereoselectively by a number of P-adrenergic agonists and antagonists. In contrast, proliferating myoblasts contain fewer than 10 /?-adrenergic receptors per cell. Both myoblast and myotube membranes contain an adenylate cyclase activity which is stimulated by guanine nucleotides and fluoride, and is activated in membranes derived f rom cells treated with cholera toxin. Myotube membrane adenylate cyclase, but not myoblast membrane adenylate cyclase, is stimulated 2to %fold by isoproterenol. Accumulation of CAMP by intact myotubes, but not by myoblasts, is stimulated 3to &fold by isoproterenol. Both cell types accumulate CAMP in response to cholera toxin. These results indicated that as the embryonic muscle cells fuse, they develop specific P-adrenergic receptors that become functionally coupled to a pre-existing adenylate cyclase.