The effects of synthetic rat C-peptide 1 and C-peptide 2 on plasma insulin and blood glucose concentrations in the rat were studied. Infusion of rat C-peptide (500 micrograms X h-1 X kg-1) diminished glucose induced increase of plasma insulin by 56% (15.2 +/- 0.9 versus 6.6 +/- 0.6 ng/ml, p less than 0.01, mean +/- SEM). Somatostatin infused at a rate of 50 micrograms X h-1 X kg-1 body weight inhibited glucose-induced insulin secretion by 33%. In the presence of a mixture of both C-peptides or somatostatin, blood glucose after intravenous glucose was higher than in the control experiments. In alloxan-diabetic rats, C-peptide (160 micrograms/kg) significantly increased and prolonged the hypoglycaemic effect of exogenous insulin. It is suggested that C-peptide may not be a biologically inert substance.
To study the influence of insulin on its own secretion, collagenase-isolated islets of rat pancreas were prelabelled with [3H]leucine for 2 h. After washing the islets, (pro-)insulin release was stimulated by glucose in the presence or absence of exogenous insulin (up to 2-5 mu./ml). Hormone release was unchanged by the presence of exogenous insulin as judged by determination of both immunoreactive insulin and radioactivity incorporated into the proinsulin and insulin fractions of the medium. No direct feedback mechanism for insulin secretion was apparent from this study.
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