Suppression of endogenous insulin secretion by exogenous insulin in patients with insulinoma

OBJECTIVE Previous studies have demonstrated that endogenous insulin secret ion is not suppressed by exogenous insulin in patients with insulinoma. In this study we examined whether insulin secretion in insulinoma patients is suppressed by exogenous insulin during hypoglycaemia. SUBJECTS AND METHODS Sixteen insulinoma patients (5 men and 11 women) and 1 0 normal subjects were studied. Hyperinsulinaemic glucose clamp studies wer e performed at both euglycaemia (4.5 mmol/l glucose) and hypoglycaemia (2.5 mmol/l glucose). RESULTS In normal subjects, plasma C-peptide levels were suppressed by 66% during the euglycaemic hyperinsulinaemic clamps (P < 0.01). In contrast, in insulinoma patients, plasma C-peptide levels increased by 25% during the c lamps (P < 0.05). In the hypoglycaemic hyperinsulinaemic clamps, plasma C-p eptide levels were nearly completely (91%) suppressed in normal subjects an d partially (39%) suppressed in patients with insulinoma (P < 0.01). The de crease in C-peptide levels during the hypoglycaemic clamps was > 30% in 12 (75%) of 16 insulinoma patients and > 50% in 8 (50%) patients. CONCLUSIONS This study demonstrated that in patients with insulinoma, insul in secretion was not suppressed by exogenous insulin during euglycaemia but was suppressed during hypoglycaemia, although the degree of suppression wa s less than that in normal subjects. Our results suggest that the feedback regulation of insulin secretion by exogenous insulin is partially retained in patients with insulinoma.