ENDOGENOUS OPIOID-PEPTIDES STIMULATE POSTEXERCISE INSULIN-RESPONSE TOGLUCOSE IN RATS

Exercise is associated with profound changes in glucose metabolism and insulin secretion. Endogenous opioid peptides may be involved in thes e metabolic adaptations. To gain insights into this hypothesis, we stu died the effects of the opioid antagonist naloxone on the insulin resp onse to glucose after a 2.5 h exercise bout, either by means of an int ravascular glucose tolerance test in male Wistar rats or from rat isle ts of Langerhans isolated just after exercise. There was a tenfold inc rease in plasma beta-endorphin concentrations (9.8 +/- 2.1 vs. 114.2 /- 22.0 fmol/ml, p < 0.001) in animals killed immediately after exerci se. The in vivo post-exercise peak insulin response to glucose was mar kedly reduced compared to resting controls (p < 0.01). Interestingly, naloxone (10 mg/kg) still further decreased the insulin response compa red to saline injected exercised rats (p < 0.05), but did not alter th e response from resting animals. The post-exercise islet insulin respo nse to 8.3 mM glucose was significantly reduced compared to resting ra t islets (p < 0.05) and was further inhibited when naloxone (10 mu M) was added to the culture medium (p < 0.05). In another experiment, we also tested the effect of 10(-8) and 10(-6) M beta-endorphin on contro l islets. Both concentrations of beta-endorphin significantly increase d the islet insulin response to 8.3 mM glucose (p < 0.05) and this eff ect was completely blocked by naloxone. These results suggest that end ogenous opioid peptides participate in the physiological adaptation to exercise stress in maintaining post-exercise insulin response to gluc ose.