Independent control of selected insulin-sensitive cell membrane and intracellular functions—the linkage of cell membrane and intracellular events controlled by insulin

Summary

The effects of pH, oxidation reduction compounds and trypsin on insulin binding, hexose transport, and activation of glycogen synthase were studied utilizing rat adipocytes. In this paper the effect of pH is examined; while in the subsequent two papers the effects of glutathione and trypsin are examined.

Increase in pH from 6 to 8.5 increased labelled glucose oxidation, 2-deoxyglucose transport as well as labelled insulin binding to the receptor. Enhanced insulin binding was due to an increased rate of association k₊₁ with no effect the rate of dissociation k₋₁ resulting in a decreased equilibrium dissociation constant K_D. Glycogen synthase activity was unaffected by increase in pH when adipocytes were incubated with or without glucose. Insulin in contrast to pH was effective in increasing the activity of glycogen synthase. With 2-deoxyglucose, % glycogen synthaseI activity was increased by an increase in pH. Glycogen synthase activity was thus stimulated by insulin by the direct mechanism, previously termed mechanism 1, involving the formation of a chemical mediator, and clearly distinguishable from the activation of hexose transport, previously termed mechanism 2(1). Increase in labelled glucose oxidation and in 2-deoxyglucose transport with increased pH, as well as insulin stimulation, was abolished by preincubation with trypsin, or cytochalasin B; suggesting that trypsin-sensitive and cytochalasin B-binding protein(s) presumably in the plasma membrane are involved in these effects of pH. Since increase in pH alone activates cell membrane-mediated hexose transport and insulin receptor binding under conditions where glycogen synthase is not activated, increase in pH acts presumably by a non-mediator mechanism. Insulin acts at the membrane to enhance further the effects of increased pH, via a mediator mechanism.