Influence of insulin and environmental factors on glucose uptake in epididymal fat tissue

Abstract

An 8- to 10-fold stimulation by insulin of glucose uptake by epididymal fat tissue was obtained in serial incubations of this tissue in a simplified medium composed of tris (hydroxymethyl) aminomethane chloride, CO 2, and glucose. The replacement of chloride ions in the incubation medium by acetate or other anions resulted in an increase in the glucose uptake by epididymal fat tissue in the absence of insulin to a level approaching the rate obtained in the medium in the presence of insulin. This effect is not readily reversed by addition of chloride ions to the medium after the incubation period. Chloride ions, therefore, appear necessary for the maintenance of insulin responsiveness of the system. During incubation in tris chloride, there is a rapid appearance of significant quantities of sodium and potassium into the incubation medium. This ionic redistribution did not influence significantly the normal basal glucose uptake or its stimulation by insulin. The presence of the hormone also did not influence the rate of appearance of these ions in the incubation medium. It is suggested that the sodium ions appearing in the incubation medium are derived primarily from the intercellular space. The stimulation of glucose uptake by amorphous (zinc-low) insulin in a zinc-free tris chloride medium was similar to that observed in a zinc-containing system. The uptake of zinc ions into the tissues was not markedly influenced by insulin. As the pH of the incubation medium is increased from 6 to 7, glucose uptake by insulin-treated tissue increased while that of tissue without insulin remained constant. In the region from pH 7 to 8, the uptake by insulin-stimulated tissue remained maximal while the uptake by tissue without insulin increased to this level, thus abolishing the insulin effect. The effects of pH on the basal glucose uptake were not reversed by readjusting the pH after the incubation period. There was no marked difference in the rate of utilization of α- and β-glucose by epididymal fat tissue in the presence or absence of insulin under conditions where two- or threefold differences in rate would have been expected if one of the forms was specifically utilized by the system. Lineweaver-Burk plots of glucose uptake as a function of glucose concentration in the presence and absence of insulin indicate the action of insulin is primarily in increasing the uptake of glucose by epididymal fat tissue at low glucose concentrations. The apparent K m of the system in the absence of insulin is about 60 m M, and in the presence of this hormone is 7 m M. The maximal glucose uptake, about 2 μmoles/100 mg. tissue per hour, is obtained at high levels of glucose and is not significantly affected by insulin. These data are consistent with a role of insulin on a rate-limiting step in the process (probably glucose transport). It is suggested that various treatments of the tissue which are reported here to increase the glucose uptake in the absence of insulin probably affect the permeability of the cells.