Polyamines in rat adipocytes: their localization and their effects on the insulin receptor binding

Effects of polyamines on the insulin binding in isolated rat adipocytes were studied. In addition, the concentration of polyamines in adipose tissue was determined, and their localization revealed by fluorescence cytochemistry and immunocytochemistry. Spermine (0.1-10 mM) dose-dependently enhanced the insulin receptor binding; at a concentration of 5-10 mM spermine the insulin binding was enhanced by 100% above control values. Spermidine had a weaker effect than spermine whereas putrescine had no effect on insulin binding. Competition curves with unlabelled insulin indicated that spermine increased the insulin binding capacity without significantly affecting the binding affinity. Fluorescence and immunocytochemistry on adipose tissue localized the polyamines spermidine and/or spermine almost exclusively to the thin layer of adipocyte cytoplasm surrounding the lipid droplet. In addition, chemical analysis for polyamines in the adipocyte medium and in the cells indicated that a differential release of polyamines may have occurred from the cells. The possibility that polyamines may act as intracellular or intercellular (autocrine) regulators, modulating insulin binding, is discussed.