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 patient's 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 patient's circulating insulin was probably due to a structural abnormality. Subsequent studies of the patient's 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: partial characterization of circulating proinsulin-like material.

Familial hyperproinsulinemia is an autosomal dominant defect that is associated with strikingly elevated levels of serum proinsulin-like material. Our studies show that trypsin converts familial hyperproinsulinemia proinsulin to insulin more slowly than it converts a 131I-labeled porcine proinsulin marker. Molar yields of insulin indicated that the material may be an intermediate proinsulin. Studies with two human C-peptide antisera that differ in their relative immunoreactivity with human C-peptide and proinsulin showed that the two antisera reacted equally with familial hyperproinsulinemia proinsulin, suggesting that it is a partially cleaved proinsulin intermediate. Sulfitolysis of highly purified material to break the inter- and intra-chain disulfide bridges and subsequent adsorption on a specific B-chain antibody covalently bound to Sepharose beads showed that the C-peptide was still connected to the B-chain. These data indicate that familial hyperproinsulinemia proinsulin is normally cleaved at the C-peptide-A-chain linkage site. A structural abnormality appears to underlie familial hyperproinsulinemia proinsulin, which impairs its cleavage at the B-chain-C-peptide linkage site.

C-Peptide and insulin secretion in Pima Indians and Caucasians: constant fractional hepatic extraction over a wide range of insulin concentrations and in obesity.

Peripheral serum insulin and C-peptide concentrations during oral glucose tolerance tests were measured in 10 nondiabetic Pima Indians and 10 nondiabetic Caucasians with varying degrees of obesity. Although both insulin and C-peptide levels were elevated in the Indians compared to the Caucasians (p less than 0.05), hepatic insulin extraction, measured by comparing the C-peptide to insulin ratios, was similar over a wide range of insulin concentrations in both groups. The ratios of C-peptide to insulin were independent of the degree of obesity. These studies indicate that the peripheral hyperinsulinemia in Pima Indians and obese subjects is due in general to pancreatic hypersecretion rather than to diminished hepatic extraction of insulin.

Insulin resistance due to a defect distal to the insulin receptor: demonstration in a patient with leprechaunism.

We have studied a 2-year-old girl with acanthosis nigricans, glucose intolerance, marked hyperinsulinemia, and somatic features characteristic of the leprechaunism syndrome. Circulating plasma insulin levels were increased up to 50-fold and the patient showed a blunted hypoglycemic response to an injection of exogenous insulin (0.2 units/kg), indicating the presence of severe insulin resistance. Insulin purified from the patient's plasma was normal on the basis of chromatographic, electrophoretic, and immunologic criteria. Furthermore, the purified insulin competed effectively with (125)I-labeled insulin for binding to insulin receptors on cultured IM-9 lymphocytes and rat fat cells and also exhibited normal biological potency when tested on rat fat cells. Anti-insulin receptor and anti-insulin antibodies were not detected in the patient's plasma, and plasma levels of glucagon, growth hormone, and cortisol were normal. Insulin binding to the patient's circulating monuclear leukocytes was only slightly depressed into the low normal range and could not account for the severe insulin resistance. Studies on the patient's fibroblasts revealed normal levels of insulin receptors but a total absence of insulin's ability to accelerate glucose transport. Because rates of glucose transport and metabolism were normal in the basal state in the absence of insulin, we conclude that this patient's insulin resistance is due to an inherited cellular defect in the coupling mechanism between occupied insulin receptors and the plasma membrane glucose transport system.

Measurement of serum proinsulin-like material: cross-reactivity of porcine and human proinsulin in the insulin radioimmunoassay.

The variation in cross-reactivity of human and porcine insulins and proinsulins was determined with a number of insulin antisera. The relative immunoreactivity of insulin and proinsulin with these antisera ranged from a ratio of 1.08 to 5.7 on a molar basis. The displacement curves of human insulin and proinsulin were not parallel, and the relationship of porcine and human proinsulin varied with different antisera. Sera with varying concentrations of PLM (proinsulin and its intermediate components) and serum PLM and insulin fractions (separated by gel filtration) were measured in two assays using the antisera which reacted most differently with human insulin and proinsulin standards. With the use of these two antisera, measurement of serum PLM against a human insulin standard gave different results, which were only partially corrected by reading the values from a human proinsulin standard. The magnitude of the serum IRI differences with these antisera was related to the proinsulin/insulin ratio in each sample. These results indicate the necessity for each laboratory to critically evaluate the reaction of their insulin antiserum with human proinsulin. In addition, measurement of serum PLM in terms of a insulin standard will give different results depending upon the particular antiserum used.

C-peptide in conditions other than diabetes mellitus.

Measurement of serum and urinary C-peptide has been shown to be of value in several conditions other than diabetes mellitus. It is particularly useful because it can distinguish endogenous beta cell secretion from exogenously administered insulin and because circulating insulin-binding antibodies do not interfere with its measurement. Because the liver removes little, if any, C-peptide, peripheral blood values may more accurately reflect beta cell secretion than do peripheral insulin levels. Clinically, serum C-peptide has been most useful in diagnosing hypoglycemic disorders. Diagnosis of insulinomas is facilitated in both diabetic and nondiabetic patients, and surreptitious insulin injection is readily detected. In studies of insulin regulation, circulating C-peptide has been used to demonstrate suppression of endogenous insulin secretion by exogenous insulin. Peripheral insulin and C-peptide levels have been compared in studies of the role of the liver in states of altered insulin homeostasis. Because of its higher urinary clearance, determination of urinary C-peptide is preferable to urinary insulin measurement in situations where frequent blood sampling is impossible or difficult to accomplish.

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.

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.

C-peptide suppression test for insulinoma.

During hypoglycemia induced by an infusion of porcine insulin, impaired suppression of endogenous insulin secretion as measured by C-peptide was demonstrated in 11 of 12 patients with insulinoma. During hypoglycemia (plasma glucose less than or equal to 40 mg/dl) the mean C-peptide immunoreactivity (CPR) of normal subjects was less than or equal to 1.2 ng/ml, whereas 11 of 12 insulinoma patients had a mean CPR of larger than or equal to 1.9 ng/ml. One patient showed normal CPR suppression by these criteria but may have shown impaired CPR suppression for glucose less than or equal to 30 mg/dl. Impaired CPR suppression during insulin-induced hypoglycemia may prove to be a useful test for insulinoma.

Clinical correlations of serum proinsulin-like material in islet cell tumours.

To examine the possibility that the concentration of circulating proinsulin-like material (PLM) might be helpful in evaluating the therapeutic response of patients with islet cell tumours, serum levels of PLM in three patients with islet cell tumours were correlated with hypoglycaemic symptoms and plasma glucose concentrations before and after treatment. In two patients ranges of fasting PLM concentration were 0.21-0.29 and 0.91-0.93 ng/ml, respectively, before treatment. After surgical excision of their islet cell adenomas, PLM concentrations decreased to 0.06-0.09 and 0.03-0.05 ng/ml. Insulin concentrations were low preoperatively in both patients and were unchanged postoperatively. The resulting relief from hypoglycaemia was paralleled by a reduction of PLM, with no significant change in insulin. In a third patient, treatment with streptozotocin resulted in marked symptomatic improvement, a 65% reduction in PLM concentration, but no significant change in insulin levels. Relapse was associated with increasing frequency of hypoglycaemic symptoms and increasing PLM concentrations. These findings suggest that changes in the levels of serum PLM may prove to be a sensitive indicator of the response of islet cell tumours to therapy.

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.

Peripheral metabolism of insulin, proinsulin, and C-peptide in the pregnant rat.

To clarify alterations in carbohydrate metabolism which occur in pregnancy, metabolic clearance rates of insulin, proinsulin, and C-peptide were measured by the constant infusion technique in term-pregnant rats and in virgin littermates. In addition, placental permeability to these peptides was evaluated by simultaneous determination of their concentration in fetal blood, amniotic fluid, and maternal arterial blood and the renal extraction and excretion of insulin and C-peptide were determined during simultaneous studies of renal hemodynamics. The metabolic clearance rate (MCR) of insulin was higher (P less than 0.005) in pregnant animals (61.5+/-1.7 ml/min per kg nonconceptus body weight) than in virgin littermates (51.5+/-2.2 ml/min per kg). Insulin disappearance from the circulation after both single injection and discontinuance of a constant infusion was also faster in gravid animals. In contrast, the MCR of proinsulin and C-peptide, and the disappearance of C-peptide from the circulation were similar in pregnant and control rats. The placenta was virtually impermeable to each of the three polypeptides since their mean levels in both fetal blood and amniotic fluid did not exceed 2.5 ng/ml and were only minimally influenced by pharmacological concentrations as high as 60 ng/ml in the maternal circulation. The renal clearance of insulin (renal arteriovenous insulin difference X renal plasma flow) was lower, and its contribution to insulin MCR was less in pregnant animals than in controls (19.4+/-1.5% vs. 28.7+/-3.7%, P less than 0.05), whereas the renal clearance and renal clearance/MCR of C-peptide were similar in pregnant rats and virgin littermates. These results indicate that the peripheral metabolism of insulin is accelerated in pregnancy, while that of pro-insulin and C-peptide is unaffected. Since transplacental passage of insulin is negligible and its renal clearance is not increased, the enhanced MCR of insulin in pregnancy is due to increased metabolism at an extrarenal site probably within the placenta itself.

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.

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.

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.