Insulin assay in insulinomas

The discovery of peptides used for the treatment of diabetes/obesity: A personal journey

The aim of this personal reminiscence is to acquaint the reader with seminal workwork carried out in 1960 s and 1970 s that made possible the subsequent development of highly effective long-acting GLP-1R agonists and GLP-1R/GIPR co-agonists that are now in clinical practice for the treatment of Type 2 diabetes and obesity. The article highlights the particular contributions of the author's collaborators Ellis Samols and Desmond Turner in elucidating the nature and significance of gut glucagon-like immunoreactivity (enteroglucagon) and GIP. The potent incretin GLP-1(7-36)amide identified in the 1980 s met the criteria for a glucagon-like-substance with incretin like properties postulated to exist by Samols and others in 1966.

Rebirth of the Incretin Concept: Its conception and early development

This paper describes the resurrection of the Incretin Concept in the early 1960s. It began with the more or less simultaneous discovery by three groups working independently in London. Dupre demonstrated that secretin given intravenously with glucose increased its rate of disappearance from the blood, McIntyre and co-workers established that hyperglycaemia evoked by oral glucose stimulated more insulin secretion than comparable hyperglycaemia produced by intravenous glucose and Marks and Samols established the insulinotropic properties of glucagon. The concept evolved with the discovery by Samols and co-workers that oral glucose stimulated the release of immunoreactive glucagon-like substances from the gut mucosa and the subsequent isolation of glucagon immunoreactive compounds, most notably oxyntomodulin and glicentin, and of gastic inhibitory polypetide (GIP). It concluded with the isolation and characterisation of glucagon-like peptide 1 (7-36) amide.

Diagnostic interpretation of the intravenous tolbutamide test for insulinoma

The plasma glucose and insulin responses to intravenous administration of tolbutamide in patients with insulinoma and healthy control subjects were compared to determine the sensitivity and specificity of the intravenous tolbutamide test for the diagnosis of insulinoma. The records of 406 healthy persons without concurrent disease known to affect glucose homeostasis or insulin concentrations and 41 patients with histologically confirmed insulinoma who underwent standard intravenous tolbutamide testing during the period from 1976 to 1986 were reviewed. The 5th percentile of the mean of plasma glucose levels at the 120-, 150-, and 180-minute points (G120-180) after injection of tolbutamide was 55 mg/dl for lean and 62 mg/dl for obese control subjects. With 95% specificity, the sensitivity of these criteria was 95% for lean and 100% for obese patients with insulinoma. Minimal differences were observed between men and women. At 95% specificity, the sensitivities were less for the ratio of the 180-minute plasma glucose to fasting plasma glucose level (64% in lean and 75% in obese patients), for mean of plasma insulin at the 120-, 150-, and 180-minute points (IRI120-180) (53% in lean and 36% in obese patients), for maximal insulin concentration (27% in lean and 31% in obese patients), for increase in insulin concentration above basal (18% in lean and 23% in obese patients), and for the combined criteria of the 5th percentile of G120-180 and the 95th percentile of IRI120-180 (47% in lean and 73% in obese patients).(ABSTRACT TRUNCATED AT 250 WORDS)

The reduction of peripheral insulin concentrations in obese subjects following a hypocaloric diet: reduced pancreatic secretion or increased hepatic extraction?

The fasting hyperinsulinaemia characteristic of obesity is due to pancreatic hypersecretion and is accompanied by a corresponding increase in plasma C-peptide concentrations. The exaggerated plasma insulin and C-peptide responses to oral glucose in obese subjects which is greater for insulin than C-peptide suggests that the stimulated hyperinsulinaemia is due to both pancreatic hypersecretion and decreased fractional hepatic extraction of insulin. In obese subjects fasting and stimulated insulin concentrations, but not those of C-peptide, are lower after 3 weeks of energy restriction than before it. This reduction in fasting and stimulated insulin levels is, therefore, due to an increase in the fractional extraction of insulin by the liver rather than to a reduction in pancreatic secretion as had previously been supposed.

Insulinoma: diagnosis, surgical management and long-term follow-up. Review of 41 cases

The clinical features, diagnosis and surgical considerations in 41 patients with insulinoma treated at Columbia-Presbyterian Medical Center are presented along with a review of the literature. One hundred percent of patients surviving surgery for benign functional beta cell tumors were cured of hypoglycemia. A detailed long-term follow-up study in the majority of patients after surgical cure documented a statistically significant incidence of neuropsychiatric aberration, adult-onset diabetes mellitus and peptic ulcer disease. Patients who have undergone surgery for endogenous hyperinsulinism should be followed up carefully for the development of these disorders, and their overall prognosis should be considered cautiously.

Insulinoma with hypoglycemia and normal immunoreactive insulin but with an insulin-like activity restricted to the portal vein

In a 46-year old Caucasian woman, the authors report a B-cell adenoma with plasma immunoreactive insulin (IRI) ranging from 10 to 32 microunits/ml, despite severe spontaneous hypoglycemia. In a peroperative sample withdrawn from the portal vein, normal IRI (40 micromicron/ml) in the presence of high insulin-like activity (290 microunits/ml) was observed by using a biological assay performed on rat epididymal fat tissue. Furthermore, this material did not cross-react with insulin antibodies and was undetectable in systemic venous samples. Although further identification by chromatographic extraction was not performed, the substance secreted by the tumor is probably identical to the non-suppressible insulin-like activity (NSILA) isolated by Froesch and responsible for hypoglycemia in a few cases of extrapancreatic tumors. The absence of this material in systemic samples indicates an immediate removal by a single passage through the liver.

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.

Islet cell tumors and hypoglycemia

Images

Glucagon test for insulinoma: a chemical study in 25 cases

The capillary blood glucose response to lmg of intramuscular glucagon was determined in 13 patients with insulinoma and in 33 normal controls; the insulinoma patients showed a normal initial rise, but this was followed by an abnormally large fall, reaching hypoglycaemic levels between 90 and 180 minutes in every case. In 14 insulinoma patients the response of venous blood glucose and also plasma insulin to lmg of intravenous glucagon was compared with 10 normal controls; there was an abnormally large rise of plasma insulin in 10 of the 14 patients, and in the majority the venous blood glucose was below normal throughout the test. In these 14 patients the plasma insulin response was also determined after oral and intravenous glucose, after oral leucine, and after intravenous tolbutamide, and the value of these tests in the recognition and differential diagnosis of insulinoma was compared with that of the intravenous glucagon test.