Failure of exogenous insulin to inhibit C-peptide release from the perfused rat pancreas

In order to ascertain whether insulin secretion is inhibited by insulin per se, the effect of exogenous rat insulin on basal and stimulated rat C-peptide reactivity (CPR) release was studied in the isolated perfused rat pancreas. The pancreas was perfused with a medium containing 20 or 200 ng/ml of rat insulin for a 20 min equilibration period and successively for a 25 to 30 min test period during which time glucose (300 mg/100 ml), tolbutamide (500 micrograms/ml), glucagon (500 ng/ml) or arginine (10 mM) was added as a secretagogue. The concentration of glucose in the basal medium was 60 mg/100 ml. Exogenous insulin did not significantly suppress the basal CPR secretion, nor did it cause a suppression of the peak value or incremental area of CPR while the pancreas was stimulated, but it rather augmented them. No inhibitory effect of exogenous insulin on the basal or stimulated CPR release was noticed in the present study.