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Norepinephrine increases the intracellular level of adenosine 3',5'-monophosphate (cyclic AMP) in clonal cell lines of nerve, glia, smooth muscel, and skeletal muscle. The largest response is in skeletal muscle, where the cyclic nucleotide concentration is elevated more than 500-fold. Glia and muscle cells, but not nerve cells, respond to dopamine with increased cyclic AMP accumulation. This response appears to be mediated through a beta-adrenoreceptor.

The regulation of the specific activities of adenylate kinase (EC 2.7.4.3) and creatine kinase (EC 2.7.3.2) in myogenic cell lines is independent of cell fusion. The observed increases in enzyme specific activities are cell density dependent, and may be further broken down into contributions from an increase in enzyme activity per cell and a decrease in protein per cell. Only the former appears to be affected by medium conditioning.There is an increase in the specific activities (enzyme activity per unit of total cellular protein) of adenylate kinase (EC 2.7.4.3; ATP:AMP phosphotransferase) and creatine kinase (EC 2.7.3.2; ATP: creatine N-phosphotransferase) temporally associated with cell fusion during myogenesis in vivo (1) and in vitro (2, 3). These post-fusion increases in enzymatic activities could either be dependent upon myotube formation, or not be functionally coupled with cell fusion. Since myogenic cells normally fuse to form multinucleate fibers, it has been difficult to experimentally dissociate the regulation of "differentiated" muscle metabolism and the morphological changes associated with myogenesis. In an attempt to answer the question of whether myotube formation is required for the expression of differentiated functions in myogenic cells, myotube formation has been inhibited in vitro by calcium deprivation (4) and by growth in 5-bromodeoxyuridine (BrdU) (2). These conditions also inhibited the normal increase in specific activities of creatine kinase and adenylate kinase. However, the possibility exists that both BrdU and calcium deprivation alter cellular functions other than those involved in the fusion process, resulting in a direct inhibition of the normal increase in adenylate kinase and creatine kinase activities.Another approach to the question of enzyme regulation in myogenic cells is through the selection of nonfusing variants from clonal myogenic cell lines, and the use of these lines to separate the temporally associated events of cell fusion and increased adenylate kinase and creatine kinase activities. A nonfusing variant of Yaffe's L6 myogenic cell line (5) has recently been obtained (6). This paper describes experiments in which both L6 and its nonfusing variant were used to examine the regulation of adenylate kinase and creatine kinase in myogenic cells. MATERIALS AND METHODSThe myogenic cell line L6 was derived from rat thigh muscle (3,5,7 (Fig. 1). With L6 cells, the increase began during the exponential phase of growth and continued during myotube formation. However, M3A showed an increase in the specific activities of adenylate kinase and creatine kinase synchronous with L6, but in the absence of cell fusion. These increases in enzyme activities occurred regardless of whether M3A cells were cultured in suspension or attached.A number of other clonal cell lines, including fibroblasts and hepatomas, were examined for similar changes during their growth cycles. The enzymatic activities of adenylate kinase and creatine kinase in these cells were less than one-tenth ...

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