Mary E. Mako

Proinsulin, insulin, and C-peptide concentrations in human portal and peripheral blood.

Concentrations of insulin, proinsulin, and C-peptide were measured in portal and peripheral venous blood in six nondiabetic, nonobese subjects. Portal vein samples were obtained by umbilical vein catheterization. Three subjects were studied with intravenous infusion of 25 g glucose, and three with 30 g arginine. Insulin and proinsulin were determined in the insulin immunoassay after separation by gel filtration, and C-peptide was measured by direct immunoassay. With both glucose and arginine stimulation, portal vein levels of all three peptides peaked at 90-120 s after the onset of the stimulus. Relative increases in insulin concentration were greater than those of proinsulin or C-peptide. In peripheral venous blood, maximal levels of the three peptides were observed later (2-5 min), and the increase in insulin relative toproinsulin and C-peptide was not as great. At the time of peak secretion, portal vein insulin and C-peptide approached equimolar concentrations, and proinsulin, as measured against an insulin standard, comprised approximately 2.5% of the total immunoreactive insulin. After stimulation by glucose or arginine, portal insulin, proinsulin and C-peptide levels were not correlated with the concentrations measured in simultaneously drawn peripheral samples. At all sampling times, however, significant correlation was found between insulin and C-peptide in both peripheral and portal blood. The results indicate that under the conditions studied, insulin and C-peptide are secreted in equimolar concentrations in man, and that proinsulin is secreted in the same proportion to insulin as found in the pancreas. Consideration of the relative secretory and metabolic rates of the three beta cell peptides explains their peripheral concentrations. The data further support the use of plasma C-peptide as an indicator of beta cell secretory function.

Factitious hypoglycemia. Diagnosis by measurement of serum C-peptide immunoreactivity and insulin-binding antibodies.

In seven patients with factitious hypoglycemia due to the surreptitious injection of insulin, we made the diagnosis by measurements of plasma insulin and C-peptide immunoreactivity (in seven patients), facilitated by the finding of circulating insulin-binding antibodies (in two patients). The simultaneous demonstration of low plasma glucose, high immunoreactive insulin and suppressed C-peptide immunoreactivity represents a triad of results pathognomonic of exogenous insulin administration. Determination of plasma free C-peptide and free insulin permitted patients with high titers of insulin antibodies, including those with a history of insulin-treated diabetes, to be studied and diagnosed in a way similar to that in subjects who had no circulating insulin antibodies.

Circulating Proinsulin in Patients with Maturity Onset Diabetes

The contribution of proinsulin to the total serum immunoreactive insulin (IRI) was measured in 59 patients with maturity onset diabetes (23 being treated with diet alone and 36 with oral sulfonylurea agents) and compared to that in 44 control subjects. The percentage of proinsulin was increased in 11 patients and correlated with plasma glucose, but not with IRI. There was no difference between the drug-treated group and diet-treated group, or between patients taking different sulfonylurea agents. Sequential studies in one patient showed normalization of the proportion of proinsulin following lowering of the plasma glucose level. It is probably that the increased circulating proportion of proinsulin in hyperglycemic diabetic patients is secondary to beta cell exhaustion with release of less mature granules.

Diabetes Due to Secretion of an Abnormal Insulin

A 51-year-old, nonobese man with diabetes mellitus had marked hyperinsulinemia (70 to 120 muU per milliliter; 502 to 860 pmol per liter) and fasting hyperglycemia (140 to 170 mg per 100 ml; 7.8 to 9.4 mmol per liter). Plasma proinsulin, glucagon, growth hormone, and cortisol levels were normal; insulin antibodies and insulin-receptor antibodies were not detected. The patient showed relatively normal insulin sensitivity, and insulin receptors on circulating monocytes were within the normal range. Insulin from the patients serum bound to IM-9 lymphocytes and rat adipocytes approximately 40 per cent as well as insulin standards. Its biologic activity on rat adipocytes averaged 15 per cent of that expected from its immunologic concentration. The impaired biologic activity of this patients circulating insulin was probably due to a structural abnormality. Subsequent studies of the patients insulin (fortuitously obtained from his pancreas during a laparotomy for a pancreatic cyst) have confirmed this conclusion. (N Engl J Med 302:129-135, 1980).

Familial hyperproinsulinemia. An autosomal dominant defect.

We describe a genetic defect in a kindred in whom proinsulin or a proinsulin-like material constitutes the major fraction of circulating insulin immunoreactivity in both the fasting and stimulated states. The defect, familial hyperproinsulinemia, affects eight males and 10 females in four generations of the kindred, with an autosomal dominant mode of transmission. Familial hyperproinsulinemia is asymptomatic in the affected progeny, with no apparent relation to hypoglycemia or to the development of diabetes mellitus. This genetic defect may represent either a deficiency in the proinsulin cleaving enzyme (or enzymes) within the beta cell, or more probably, an abnormal species of proinsulin.

Sequential Changes in Beta-Cell Function in Insulin-Treated Diabetic Patients Assessed by C-Peptide Immunoreactivity

Abstract Assessment of beta-cell function in insulin-treated diabetes is hampered by the presence of circulating insulin antibodies, which interfere with the commonly used insulin immunoassay. The development of an immunoassay for C-peptide has enabled beta-cell secretory products, in addition to insulin, to be monitored in the circulation of such patients. Using this assay, we studied the progressive changes in beta-cell secretion in three insulin-treated diabetic patients with hyperglycemia and ketonemia. Serum C-peptide immunoreactivity increased during the remission phase of the diabetic state, suggesting that the improvement in carbohydrate tolerance was due to renewed beta-cell secretion. Eventual clinical relapse was associated with decreasing serum C-peptide immunoreactivity. These results confirm the long held clinical impression that the cause of the remission phase in diabetes is usually resumption of beta-cell secretory activity. (N Engl J Med 288:1144–1148, 1973)

Potentiation of the plasma insulin response to glucose by prior administration of alcohol. An apparent islet-priming effect.

Ethyl alcohol which has been reported to be without effect on insulin secretion apparently modifies beta-cell function nevertheless, as indicated by the plasma insulin responses to glucose loading after prior administration of alcohol. Glucose was injected intravenously in nine young adults on three separate occasions at intervals of at least two weeks. During the twelve hours preceding each test the subjects received ethanol either by mouth or by vein or, as a control, no ethanol. Plasma insulin and glucose concentrations were not noticeably affected by the ethanol alone but alcohol pretreatment was followed in all instances by heightened plasma insulin responses to the glycemic pulse stimulus and by accelerated rates of plasma glucose disappearance. The mean plasma insulin response was increased 50 per cent by the alcohol, irrespective of the route of administration. Unlike recognized insulin secretogogues, therefore, ethanol appears to augment insulin secretion only on demand. The route of administration appeared not to be a factor in determining the magnitude of the alcohol effect. Other alcohol effects included blunting of the plasma pyruvate and exaggeration of the plasma lactate elevations after glucose.

Insulin and the Kidney

Changes in renal function and structure are frequently observed in patients with diabetes mellitus. In the early phases of the disease, alterations in glomerular filtration rate, renal plasma flow, glomerular permeability and tubular capacity for glucose reabsorption occur. In the late stages of juvenile onset diabetes, renal failure is a common cause of death. For this reason, increasing attention is being paid to the possibility of long-term dialysis and renal transplantation in these patients. The kidneys play an important role in regulating insulin metabolism. The renal arteriovenous difference is approximately 30-45% and a linear relationship exists between the arterial insulin level and the renal arteriovenous concentration difference. The renal extraction of insulin is 200 ml/min in man, and it is estimated that 6-8 U are removed and degraded by the kidney in 24 h. The quantity of insulin in urine is small. However, its clearance is relatively constant over a wide range of serum concentrations and is 0.15-0.5 ml/min. The mean basal insulin excretion is 3.6 muU/mg creatinine, and a fourfold rise occurs following a glucose load. The urinary insulin values in neonates, children and patients with diabetes and renal failure are reviewed. In diabetic patients, progressive renal disease is accompanied by decreasing insulin requirements. In contrast, nondiabetic patients who develop renal failure frequently show abnormalities in carbohydrate metabolism, the commonest of which is a pseudodiabetic state.

Differential Plasma Insulin Response to Glucose and Glucagon Stimulation Following Ethanol Priming

The early phase of the insulin secretory response to glucose may not be derived from the total pool of stored insulin, but possibly from a specific and relatively small fraction in a state or locus necessary for its immediate release. In an attempt to deplete this hypothetical fraction, and thereby to support the concept of its existence and obtain an estimate of its size under different circumstances, normal and mildly diabetic subjects were given a series of three large glucose loads followed by glucagon, with and without alcohol “priming”. Results indicate that repetitive glucose or glucagon challenges over a two-hour period failed to exhaust the supply of immediately releasable insulin in either group of subjects as judged by the increments in plasma insulin during the first few minutes after each challenge. Alcohol pretreatment resulted in a larger early insulin secretory response to the first of three glucose loads, but a decreasing response to the subsequent two, possibly indicating that the initially augmented secretory response had resulted in depletion of a finite supply of immediately available insulin. An unexpected finding was the inhibitory effect of alcohol on glucagon-induced insulin release, in direct contrast to its effect when glucose is the insulinogogue, suggesting that glucose and glucagon exert their effects throughdifferent mechanisms or on different fractions of the insulin pool.

Insulinoma with Low Circulating Insulin Levels: The Diagnostic Value of Proinsulin Measurements

Hypoglycemia coexisting with very low plasma immunoreactive insulin concentrations was found in a 78-year-old woman with an insulinoma. However, the absolute proinsulin levels during hypoglycemia were elevated (0.9 to 1.4 ng/ml), accounting for 66.5% of the total circulating immunoreactive insulinlike material. The raised serum proinsulin concentrations together with an abnormal proinsulin:insulin ratio proved to be of critical importance in establishing the diagnosis of an islet-cell tumor in this patient.