Primary insulin antagonism of glucose transport in muscle from the older-obese rat.

Effects of insulin (1 mU/ml) on diaphragms removed from older-obese (70--110 days, 350--520 g) male Sprague-Dawley rats were compared to responses on muscle removed from younger-lean (27--36 days, 80--150 g) animals. Insulin antagonism on glucose transport (2DG uptake), glucose uptake, glycogen synthesis, glycolysis (lactate production), and glucose oxidation was demonstrated in tissue from the older-obese rats. Extracellular water spaces (measured with inulin-H3) were significantly decreased in these tissue. To determine if insulin antagonism of glucose transport could be secondary to inhibition of a rate-limiting reaction in the Embden-Meyerhof pathway with a subsequent negative feedback on transport, both tissue levels of glycolytic intermediates and oxidation of intracellular lipids were measured. No free intracellular glucose was found in diaphragms from either group of rats. Levels of G-6-P, F-6-P, F-1, 6-diP, PEP, and pyruvate were all lower in muscle from the older-obese animals. Incorporation of C14-FFA into tissue TG was slightly, but significantly, lower in this same tissue. Oxidation of intracellular TG and PL was similar in the two groups. In conclusion, diaphragms from older-obese rats manifest insulin antagonism of glucose transport that is probably responsible for the diminished hormonal effect on glucose uptake and the intracellular pathways of glycogen synthesis, glycolysis, and glucose oxidation. This inhibition of insulin action cannot be accounted for by changes in glycolytic intermediates causing a negative feedback on transport or enhanced lipid oxidation and therefore should be considered primary. The relative effects of age and obesity will need to be evaluated in future studies.