Regulation of Cell Membrane Permeability in Skeletal Muscle

The mechanism of the stimulatory action of insulin on the transport of sugar across the plasma membrane of skeletal muscle cells was studied by kinetic analysis of the penetration of 3-O-methyl-d-glucose into isolated frog sartorius muscles. As part of this evaluation, the effects of trypsin and insulin were compared. Phospholipase C exerted a much smaller effect on permeability than either of these agents. Trypsin enhanced the initial rate of entry of 3-O-methylglucose into muscle cells. The changes in permeability that occurred during exposure of muscles to trypsin and during subsequent washing of the muscles with Ringer's solution followed a distinctly different time course from the pattern of response to insulin. Trypsin released nondialyzable sialic acid-containing material, tentatively assumed to be glycopeptides derived from the surface of the muscle cells, whereas insulin did not have this effect. The optimal concentration of trypsin was 2 µg per ml; higher concentrations tended to cause contractures. Trypsin did not increase permeability to d-mannitol. Despite differences in their mode of interaction with the cell, trypsin and insulin caused approximately the same maximal change in permeability to sugar, and their effects were not additive. Therefore, these agents appear to exert a selective stimulatory effect on the same transport system for sugar. Trypsin, like insulin, produced a marked increase in the Vmax of sugar transport without significantly altering the apparent Km for 3-O-methylglucose. Furthermore, the apparent Ki for phlorizin, a competitive inhibitor of 3-O-methylglucose penetration, was approximately the same whether or not the cells had been exposed to insulin. Thus, the intrinsic molecular properties of the carrier seem to be the same in stimulated and unstimulated cells. When the rate of entry of sugar was studied at different temperatures in control muscles and in muscles that had first been exposed to either insulin or trypsin under standarized conditions, the ratio of the rate of entry in stimulated muscles to the rate in control muscles fell as the temperature was raised. On the basis of these observations it has been postulated that insulin and trypsin may alter the structure of the membrane in a manner that permits sugar carriers to move more rapidly within the membrane.