The Stimulation of Adipocyte Plasma Membrane Magnesium Ion-stimulated Adenosine Triphosphatase by Insulin and Concanavalin A

Abstract

SUMMARY These studies were designed to determine whether insulin affected the ATPase system of the adipocyte plasma mem- brane. The hormone was found to stimulate the Mg2+- ATPase significantly at concentrations as low as 5 microunits per ml and reaching maximum stimulation between 50 to 100 microunits per ml. The effect occurred at the earliest time point measured (2.0 min) and was linear over a 20-min incubation. The maximal mean dBerence in specific activity caused by 100 microunits of insulin per ml was 1.10 f 0.11 pmoles of Pi per mg of protein per 10 min in 25 experiments and was a 12.1% increase over the basal mean specific activity of 9.82 f 0.38. Insulin did not stimulate the sodium and potassiti ion-activated Mg2+ATPase of the plasma membrane nor the Mg2+ATPase of either the adipo- cyte mitochondrial or microsomal fractions. Epinephrine had no effect on the plasma membrane Mg2+ATPase. Pre- incubation of adipocytes with insulin prior to preparation of the plasma membranes or preincubation of the membranes prior to assay did not prevent the insulin stimulation of the Mg2+ATPase. The basal enzyme activity of insulin-pre- treated membranes or of membranes from insulin-pre- treated cells was signi&antly lower than the corresponding controls. Oligomycin and sodium azide significantly lowered the basal Mg2+ATPase specific activity and prevented the insulin stimulation of the enzyme but neither agent blocked the binding of insulin to the plasma membranes. Con- canavalin A mimicked the effect of insulin on the plasma membrane Mg2+ATPase but produced a greater magnitude of change and could overcome the inhibition caused by oligo- mycin and sodium azide. The data suggest that insulin stimulates the Mg2+ATPase after interacting with the mem- brane receptor but the role of this phenomenon in the mech- anism of insulin action remains to be determined. Insulin is known to cause a set