Effects of Fasting on Insulin Binding, Glucose Transport, and Glucose Oxidation in Isolated Rat Adipocytes RELATIONSHIPS BETWEEN INSULIN RECEPTORS AND INSULIN ACTION

glucose oxidation were studied in isolated adipocytes obtained from fasting rats. Fasting led to an inicrease in the overall binding affinity for insulini, wlhile the numiiber of receptor sites per cell remainiedl constanit. Glucose oxidation was markedly attenuated during fasting. Basal rates of oxidation decreased 1)w about 50%, while insulin-stimulated rates decreased 6 to 10-fold. Glucose transport was assessed by measturinig initial uiptake rates of 2-deoxy-glucose. Fatstinig led to a 40-50 % decrease in the apparent miaximiial transpoit capacity (Vmax) of 2-deoxy-glucose uptaike with no change in apparent K,,. A progressive decrease in basal and insulin-stimulated rates of 2-deoxy-glucose uptake was seen from 24-72 h of starvation anld a sig-nificant correlation (r = 0.85, P < 0.001) existed be-tween basal and maximiial insulin-stimllulated uptake rates in individual animals. When 2-deoxy-glucose uptake was plotted as a function of insulini bound, due to the decrease in mnaximal uptake capacity, cells from fasting animals took up less hexose for any acmount of insulin bound. When the insulin bound was plotted as a function of the percent instlin effect on uptake, control cells and cells from 24-h-fasted rats gave comparable results, while cells from 48- and 72-h-fasted animals still took Up less hexose for any amount of bound insulin. The effects of fast-ing on 3-0-methyl glucose uptake were comparable to the 2-deoxy-gltucose data. In conclusion: (a) insulin binding is increased durinig fasting due to ain inereased Dr. Olefsky is a Clinical Investigator with the Veterans Administration Hospital. Received for publication 20 May 1976 and in revised for