Plasma insulin response to various secretagogues in insulinoma

Since the development of radioimmunoassay for insulin, the diagnosis of insulinoma has been made easily. However, it has been assumed that insulinoma is heterogenous in the histological structure as well as in clinical findings. Therefore, the present study was performed to investigate the insulin response to various stimuli and to evaluate the various insulin response tests in 19 patients with insulinoma. The fasting blood glucose was 19 to 90 mg/100 ml in insulinoma and 81 +/- 5 (mean +/- S.D.) mg/100 ml in normal controls. Plasma insulin (IRI) in insulinoma ranged from 10 to 255 microU/ml, while in the control it was 14 +/- 9 microU/ml. However, insulin/blood glucose ratio increased in insulinoma (0.2-11.2) compared with the normal control (0.18 +/- 0.11). In oral glucose tolerance tests, plasma IRI increased and reached peak levels of 48-244 microU/ml, remaining elevated in most cases. In the intravenous tolbutamide test, plasma IRI increased conspicuously to 82-1,330 microU/ml and hypoglycemic coma was provoked in 54%. Plasma IRI was elevated in the intravenous glucagon test and reached the peak levels of 85-400 microU/ml, which exceeded those of the control group. Plasma IRI increased to more than 100 microU/ml after arginine infusion and formed bizarre curves. There were no correlations between plasma IRI response to various stimuli and malignancy, type of B-granule or insulin content of insulinoma tumors. It is concluded that fasting plasma IRI, insulin/glucose ratio, tolbutamide test and glucagon test are highly valuable for the diagnosis of insulinoma.