Studies on the potential role of secretin in the islet cell tumor diarrheogenic syndrome

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Enhancement of insulin release to acute glycaemic stimulation with depression of basal insulin production rates in insulinoma following diazoxide administration

Single and double load oral glucose tolerance tests were performed repetitively both before and during administration of diazoxide to a 15-year old girl who had an insulin secreting islet cell tumour. Plasma insulin concentrations increased above baseline values by a greater magnitude in response to a single acute oral glycaemic stimulus following diazoxide treatment, compared to the increases resulting from comparable prediazoxide glucose tolerance tests, and plasma insulin either attained higher values or sustained elevations for a longer duration during the early part (first hour) of the single load tests. This provides evidence that diazoxide does not prevent the normal insulin release response to a glycaemic stimulus, and that enhanced insulin secretion rates may occur with insulinomas under the study conditions employed.

Fasting plasma insulin concentrations were lower during the period of diazoxide administration which indicates that insulin biosynthesis was depressed under fasting steady-state conditions. Considering that the first part of the glucose tolerance curve reflects primarily insulin release, our data is consistent with the view that insulin storage within the insulinoma cells is preserved under the study conditions employed and may even be enhanced by diazoxide. Consequently, depression of insulin biosynthesis is considered to be a resultant effect and not a primary action of diazoxide. These results suggest a possible basis for »distinguishing« types of insulinomas should additional perspective reveal that glycaemic-induced enhancement of insulin secretion rates cannot be made to occur uniformly in diazoxide treated patients having insulinomas.

Blood sugar, serum insulin and free fatty acid interrelationships during intravenous tolbutamide testing in normal young adults and in patients with insulinoma

Blood sugar, serum immunoreactive insulin and free fatty acids were determined in three young male patients with islet cell tumors and in thirty young adult controls before and after intravenous injection of tolbutamide. The serum insulin response was compatible with the diagnosis in two of the patients who had high fasting insulin levels, but was of no value in the one patient with normal fasting insulin concentrations. Evaluation of the fasting blood sugar and free fatty acid concentrations in relation to the fasting insulin levels (FBS/IRI and FFA/IRI) made the diagnosis more apparent in all three insulinoma patients. The FBS/ IRI for the thirty male controls was 5.6 ± 1.2 (mean ± 2 S.E.M.). Only three of these values were below 2.5. In contrast, not one of the thirty-four preoperative FBS/IRI values from the three patients was above 2.5. All seventeen postoperative FBS/IRI values were above 2.5. The FFA/IRI for the thirty male controls was 30.6 ± 9.4. Only four of these values were below 15. In the two patients where data were available, twelve of the fourteen values were below 15.

Plasma insulin studies in the diagnosis of insulinoma

Plasma insulin levels were measured in 8 patients of whom 5 were surgically proven to have, and 3 had strongly suggestive evidence of, a functioning islet cell tumour.

The fasting plasma insulin level was elevated in only 3 patients, despite repeated sampling.

The administration of tolbutamide produced an excessive insulin response in 5 of the 8 patients, glucagon produced an excessive insulin response in 4 of 6 patients tested, 1-leucine produced a rise in plasma insulin in 1 of 2 subjects tested and the only subject studied after 50g. oral glucose had a high insulin response. The insulin response was highest after tolbutamide in all but one patient and this would appear to be the single most valuable test. However false negatives may occur and therefore the whole range of tests may have to be performed to establish the diagnosis.

Two patients were studied during surgery and in one the insulin content of the tumour was assayed after extraction with acid alcohol.

Four patients were studied again after treatment by surgery (2) and diazoxide (2) when normal insulin levels were found.