Control of enzyme activities in individual myotubes cultured without nerve.

The activities of lactate dehydrogenase, malate dehydrogenase, phosphorylase, and adenylate kinase were measured in single myotubes dissected from primary cultures of rat skeletal muscle. For a given enzyme, activities among the spontaneously contracting cells varied as much as eightfold. When the myotubes were paralyzed with tetrodotoxin, the variability in enzyme levels was markedly decreased. These and other findings suggest that differences in enzyme levels among individual myotubes may arise as a result of differences in their pattern of contractile activity.