Insulin stimulation of adipocyte membrane glucose transport: A graded biologic response insensitive to bilayer lipid disordering

Abstract

Aspects of the mechanism by which insulin stimulates the membrane glucose transport system were examined by (1) assessing the influence of the bilayer lipid structure on transport stimulation characteristics, and (2) considering the form of the insulin dose-response curve. We tested the effects of membrane lipid perturbation on the insulin stimulation process. Benzyl alcohol, at concentrations (25 mM) that grossly fluidize lipids forming the adipocyte membrane bilayer matrix, caused 50% inhibition of intrinsic transporter activity. However, this membrane perturbation had no significant effect on either the insulin dose-response curve (conducted at 37°) or the time-course of the insulin stimulation of hexose transport (conducted at 32°). These data are difficult to rationalize in terms of a model in which transport stimulation involves interaction of transporters and hormone-bound receptors that is limited by lateral diffusion of these proteins in the fluid lipid bilayer. Curve-fitting experimental insulin dose-response data for stimulation of 2-deoxy- d-glucose and d-glucose uptake provided an estimate of an insulin “association constant” for transport regulation that may be compared with recent insulin receptor binding data. Similar magnitude constants were obtained whether estimated directly from plots of transport velocity versus arithmetic hormone dose, or by extrapolation from linear segments of sigmoidal velocity versus log dose plots, or from inverse (Lineweaver-Burk-type) plots of the insulin dose-response data. Insulin apparently regulates transport by associating with a binding site, having an apparent dissociation constant which is determinable through kinetic measurements of hexose uptake ( K D app approx. 17−40 pM). This is in good agreement with the dissociation constant, K D , determined from Scatchard plots of recent binding data to adipocytes, for a class of receptors representing the “high affinity” binding sites for insulin. Insulin dose-response curve simulations also indicated that the stimulation process may be classified in pharmacologie terms as a typical graded biologic response and may involve insulin association with a site that regulates transport rates in a manner kinetically analogous to allosteric modulation of a V-series enzyme by a noncompetitive ligand. From the results we suggest that (1) a relatively close association occurs between transport and receptor proteins in the membrane, where the relative activation of transport depends on the fractional occupancy of functional high affinity receptors by insulin, and (2) the insulin stimulation of transport involves regions of the membrane that are not influenced significantly by disordering the membrane lipid matrix.