Stimulatory role for endogenous opioid peptides on postexercise insulin secretion in rats

Endogenous opioid peptides (EOP) and prior exercise may modulate the stimulatory effect of glucose on insulin secretion. To gain insights into these relationships, we studied male Wistar rats (187-245 g) during sustained hyperglycemia by use of the glucose clamp technique. Four groups of sedentary fed rats (n = 8/group) either ran (Ex) at 24 m/min, 0% grade, or rested (R) for 40 min. Thirty minutes after Ex or R, arterial blood glucose was elevated to and maintained at 11 mM for 2 h by a variable glucose infusion. At the start of Ex or R rats had saline (Sal) or naloxone (Nal, an opioid antagonist) intravenous infusions for 160 min (40 min Ex + 30 min R + 90 min of a 120-min glucose clamp). Steady-state glucose infusion rates (SSGIR) were approximately 55 mg.kg-1.min-1 at the start of the clamp and declined significantly over the 2nd h to approximately 45 mg.kg-1.min-1. No significant differences existed in SSGIR between groups. R-Sal and Ex-Sal groups did not differ in their insulin response to hyperglycemia. In contrast, when all groups were compared at the end of the Nal or Sal infusion, Ex-Nal had the lowest insulin concentration (749 +/- 174 pmol/l), whereas the R-Nal group had the highest (1,581 +/- 216 pmol/l, P less than 0.05). These data suggest a stimulatory role for EOP on insulin secretion that is expressed after a prior stress (Ex). Thus one function of exercise-induced activation of EOP may be to regulate insulin secretion in the immediate postexercise period.