The differentiation of avian skeletal muscle in culture: Changes in responsiveness of adenylyl cyclase to prostaglandin E1 and adrenergic agonists

The responsiveness of adenylyl cyclase during avian myogenesis in vitro has been examined. Measurements of cyclic AMP generation in intact cells revealed that the precursor myoblast is highly responsive to prostaglandin E1 (11‐fold maximum stimulation): whereas its response to isoproterenol is much smaller (2‐fold). From the onset of terminal differentiation, responsiveness to the β‐adrenergic agonist increases progressively, reaching a 5.5‐fold maximum response by 96 hr of culture. In contrast, there is little change in the cell population's responsiveness to prostaglandin E1. The rise in catecholamine responsiveness is consistent with previously reported increases in β‐receptors accompanying differentiation. DL‐propanolol blocks the response of myoblasts and myotubes to 10−6 M isoproterenol with the same half maximal inhibition value of 1 × 10−8 mol. The results also suggest a change in the adrenergic character of the receptors and/or coupling to adenylyl cyclase as myoblasts differentiate. First the α‐adrenergic antagonist phentolamine (10−7−10−4 mol) inhibits the myoblast's response but enhances that of the myotube. Second, the potency ratios for the responses to isoproterenol, epinephrine, and norepinephrine shift from 1.1:1.0:1.0 in the myoblast to 3.3:2.1:1.0 in the myotube. The findings are discussed with reference to (1) the role of prostaglandins in the positive control of muscle differentiation and (2) the changes in the catecholamine‐responsive adenylyl cyclase system as an aspect of the expression of the muscle phenotype.