Ca 2+-ions as coupling agents in enzymatic differentiation and carbohydrate metabolism of cultured muscle cells

Abstract

Ca 2+-ions prove to play an essential role in terminal differentiation of primary embryonic muscle cell cultures. Two different sites of action are suggested to be involved in triggering myoblast fusion and in the induction of synthesis of muscle-specific enzyme patterns. With regard to fusion, Ca 2+ interacts specifically with compounds of the outer surface of the plasma membrane. Induction of enzyme synthesis depends on a minimum exposure period of about 15 hr to 1.4 mM Ca 2+ and can be suppressed by actinomycin D. As shown for glycogen phosphorylase, elaboration of the muscle-specific enzyme profile may occur also under conditions in which Ca 2+-triggered fusion is counteracted by Mg 2+. With regard to the established role of insulin on myotube development, the involvement of Ca 2+ in the action of insulin on myotubes has been investigated. Studies on glucose uptake and glycogen synthetase interconversion reveal that the respective insulin responses depend on the presence of Ca 2+ in the extracellular space. The ionophore A 23187 which is known to increase cytosolic free Ca 2+ was shown to mimic these effects of insulin. Furthermore, it was shown that insulin modifies Ca 2+-transport parameters in myotubes. These data lead to the suggestion that the primary effect of insulin binding consists of an enhanced Ca 2+-permeability of the plasma membrane. Additional steps in the intracellular signal transduction of insulin are suggested from the finding that cytochalasin B inhibits insulin-induced interconversion of glycogen synthetase but does not interfere with the insulin-dependent changes in Ca 2+-pools and transport. It is suggested that the observed effect of Ca 2+-ions on the expression of the muscle-specific enzyme pattern during myotube maturation is explained by the coupling function of Ca 2+-ions as demonstrated for insulin. The data presented cannot exclude that other factors than insulin are also involved in myotube maturation.