Christian Binder

Characterization of Seven C-peptide Antisera

The plasma C-peptide immunoreactivity (CPR) in 10 normal subjects varied considerably when measured with different antisera in parallel assays. The CPR level correlated with the blank “CPR” value measured in plasma devoid of C-peptide and to a lesser degree with the sensitivity of the standard curves obtained with the individual antisera. Storage of plasma samples at different temperatures and for different lengths of time before the analyses were carried out resulted In further variation in the CPR results. This was caused by a time- and temperature-dependent fall in CPR, which was more pronounced with some antisera than with others. This sensitivity to storage of plasma did not correlate with the antigenk characteristics of the antisera as determined by their reactivity with 11 specific fragments of the C-peptide molecule. The contribution of human proinsulin to the CPR concentration in normal subjects was considered to be negligible even though the relative immunoreactivity of human proinsulin and C-peptide ranged from 11 to 143 per cent among these antisera. These results suggest that differences in C-peptide antisera are a major reason for the variation in the concentration of circulating CPR as measured in different, C-peptide immunoassays.

HYPERINSULINISM OF HEPATIC CIRRHOSIS: DIMINISHED DEGRADATION OR HYPERSECRETION?

The breakdown of proinsulin in the pancreatic beta cell yields insulin and C-peptide which are secreted in equimolar amounts. Unlike insulin, C-peptide is not degraded significantly by the liver, so that its measurement should give a better assessment of insulin secretion than estimation of peripheral insulin levels alone; particularly in the presence of hepatic dysfunction. Plasma C-peptide and insulin response to an oral glucose load have therefore been assessed in 14 cirrhotic and 7 normal subjects. Cirrhotic patients were divided into hyperinsulinaemic and normoinsulinaemic groups based on fasting plasma-insulin concentrations. Fasting blood-blucose and plasma-C-peptide concentrations were the same in normal and cirrhotic subjects, suggesting that basal pancreatic insulin secretion was the same in all subjects. Thus the C-peptide/insulin ratio was significantly decreased in hyperinsulinaemic subjects (2-13 +/- 0-31, compared with 4-63 +/- 0-48 in controls). After oral glucose, the two groups of cirrhotic patients showed the same glucose intolerance. C-peptide concentrations were also the same but insulin concentrations were markedly increased in the hyperinsulinaemic group. It is suggested that pancreatic insulin secretion is not increased in cirrhosis and that the peripheral hyperinsulinism is due solely to decreased hepatic insulin degradation secondary to either spontaneous portal-systemic shunting or to parenchymal damage.

Role of residual insulin secretion in protecting against ketoacidosis in insulin-dependent diabetes.

The role of preserved beta-cell function in preventing ketoacidosis in type I insulin-dependent diabetes was assessed in eight patients with and seven patients without residual beta-cell function as determined from C-peptide concentrations. After 12 hours of insulin fatty-acid, and glycerol concentrations were all significantly higher in patients without beta-cell function than in those with residual secretion. Mean blood glucose concentrations reached 17.2 +/- SE of mean 1.3 mmol/l (310 +/- 23 mg/100 ml) in the first group compared with 8.8 +/- 1.4 mmol/l (159 +/- 25 mg/100 ml) in the second (P less than 0.01), while 3-hydroxybutyrate concentrations rose to 5.5 +/- mmol/l (57 +/- 5 mg/100 ml) and 1.4 +/- 0.3 mmol/l (15 +/- 3 mg/100 ml) in the two groups respectively (P less than 0.01). Individual mean C-peptide concentrations showed a significant inverse correlation with the final blood glucose values (r = -0.91; P less than 0.02). These findings strongly suggest that even minimal residual insulin secretion is important for metabolic wellbeing in diabetes and may prevent the development of severe ketoacidosis when insulin delivery is inadequate.

Hypoglycemia in IDDM

Hypoglycemia causes substantial morbidity and some mortality in insulin-dependent diabetes mellitus (IDDM). It is often the limiting factor in attempts to achieve euglycemia. The prevention or correction of hypoglycemia normally involves both dissipation of insulin and activation of glucose counterregulatory systems. Among the latter, glucagon plays a primary role initially, whereas epinephrine is not critical, although it becomes critical when glucagon is deficient. Growth hormone and cortisol play demonstrable roles in recovery from prolonged hypoglycemia. Glucose autoregulation may be involved in defense against severe hypoglycemia. With respect to pathophysiology, counterregulatory systems are involved in at least five clinical glucoregulatory syndromes. Defective glucose counterregulation is associated with, and best attributed to, combined deficiencies of the glucagon and epinephrine responses to plasma glucose decrements. Almost assuredly in concert with hypoglycemia unawareness, it results in a markedly increased frequency of severe hypoglycemia, at least during intensive therapy of IDDM. Defined as a night to morning increase in plasma glucose concentration, the dawn phenomenon is thought to result from dissipation of insulin plus the effects of nocturnal growth hormone secretion. Despite a sound rationale, the clinical relevance of the Somogyi phenomenon has been recently questioned. The clinical impression of altered glycemie thresholds for symptoms, i.e., patients with poorly controlled IDDM suffer symptoms of hypoglycemia at relatively high plasma glucose levels, whereas those with very well-controlled IDDM often tolerate subnormal glucose levels, has received experimental support. Clearly, hypoglycemia in IDDM is a problem that needs to be solved. Numerous issues need to be addressed through both basic and clinical research. Fundamentally, pending the prevention or cure of IDDM, we must learn to deliver insulin in a much more physiological fashion or to prevent, correct, or compensate for compromised glucose counterregulation if we are to achieve euglycemia safely in most patients with IDDM.

Glycaemic threshold for changes in electroencephalograms during hypoglycaemia in patients with insulin dependent diabetes

The relation between blood glucose concentration, the symptoms and signs of hypoglycaemia, and electroencephalographic changes in diabetic patients is not known. The effect of hypoglycaemia on brain function was studied in 13 patients with insulin dependent diabetes. During a gradual fall in blood glucose concentration induced by a bolus injection of insulin followed by an intravenous infusion of insulin, during 60 minutes of biochemical hypoglycaemia, and after restoration of normoglycaemia with intravenous glucose electroencephalograms were evaluated continuously by period-amplitude analysis; blood samples were taken every 10 minutes throughout. No changes were seen in electroencephalograms when the blood glucose concentration was above 3 mmol/l. At a median blood glucose concentration of 2·0 (95% confidence interval 1·7 to 2·3) mmol/l alpha activity decreased abruptly in the electroencephalograms concomitant with an increase in theta activity, reflecting neuronal dysfunction in the cortex. When the blood glucose concentration was further lowered changes were observed in the electroencephalograms indicating that deeper brain structures were affected. A normal electroencephalogram was re-established at a blood glucose concentration of 2·0 (1·8 to 2·1) mmol/l. There was no significant correlation between the blood glucose concentration at the onset of changes in the electroencephalograms and age, duration of diabetes, insulin dose, haemoglobin A1c concentration, initial blood glucose concentration, rate of fall in blood glucose concentration, and appearance of symptoms and signs of hypoglycaemia. Changes in electroencephalograms during hypoglycaemia appear and disappear at such a narrow range of blood glucose concentrations that the term threshold blood glucose concentration for the onset of such changes seems justified.

Nocturnal hypoglycaemia in patients receiving conventional treatment with insulin

The prevalence of nocturnal biochemical hypoglycaemia--that is, blood glucose concentrations below 3 mmol/l (55 mg/100 ml)--was evaluated in a random sample of 58 insulin dependent diabetics receiving twice daily insulin. Seventeen patients had at least one blood glucose value below 3 mmol/l (55 mg/100 ml) and five a value below 2 mmol/l (36 mg/100 ml) during the night. Both bedtime (2300) and fasting morning (0700) blood glucose concentrations were significantly lower in the group with nocturnal hypoglycaemia compared with the group without (p less than 0.00001). If the bedtime blood glucose concentration was below 6 mmol/l (108 mg/100 ml) the risk of nocturnal hypoglycaemia was 80% (95% confidence limits 51-96%). If the bedtime blood glucose concentration was above 6 mmol/l the likelihood of hypoglycaemia not occurring during the night was 88% (74-96%). The mean glycosylated haemoglobin A1c (HbA1c) concentration in the group with nocturnal biochemical hypoglycaemia (8.2 (range 5.0-12.4)%) was significantly lower than that in the group without (9.4(7.0-14.2)%) (p less than 0.02). The prevalence of nocturnal hypoglycaemia in the patients receiving twice daily insulin (29%) was compared with that in 15 patients receiving thrice daily insulin (47%) and was not found to be significantly different. The likelihood of this risk being greater with thrice daily insulin was, however, 88%. No patient with nocturnal biochemical hypoglycaemia woke up during the night with symptomatic hypoglycaemia. Nocturnal biochemical hypoglycaemia is common during twice daily treatment with insulin, and low values of HbA1c might be associated with a higher risk of such hypoglycaemia. The blood glucose concentration at bedtime is a significant predictor of nocturnal biochemical hypoglycaemia, and HbA1c values might be of help in identifying patients at risk.

Absorption of isophane (NPH) insulin and its clinical implications.

Absorption of 125I-NPH insulin (125I-isophane insulin) (40 IU/ml) was studied in eight diabetics given 50% and 150% of their normal daily dose of insulin. Insulin absorption correlated with plasma insulin (r = 0.97, p less than 0.001) and blood glucose (r = -0.87, p less than 0.01) concentrations. Absorption was slower at higher doses, so that trebling the insulin dose only doubled the amount absorbed over the first 24 hours. The plasma elimination half time (t12) of insulin was about five minutes. Thus, the disappearance of radiolabelled insulin is a reliable and quantitative index of insulin absorption; subcutaneous degradation, if present, is minimal and constant. Changes in dise of intermediate-acting insulin further increases the large variation in insulin absorption. This implies that minor adjustments of intermediate insulin dosage are probably futile.

Follow-Up of Women With Previous GDM: Insulin, C-Peptide, and Proinsulin Responses to Oral Glucose Load

Gestational diabetes mellitus (GDM) is a strong predictor of glucose intolerance later in life. Former GDM (n = 145) and control (n = 41) subjects were studied 3–4 yr after the index pregnancy. They were subjected to a 75-g oral glucose tolerance test (OGTT) with measurements of insulin, C-peptide, and proinsulin in the basal state and every 30 min for 180 min. In the former GDM group, 5 subjects (3.4%) had developed non-insulin-dependent diabetes mellitus (NIDDM), and 32 (22%) had developed impaired glucose tolerance (IGT; by World Health Organization criteria). In the control group, 2 (4%) had IGT. In the GDM group, IGT or NIDDM was significantly associated with obesity (body mass index [BMI] ≥25 kg/m2) and earlier diagnosis of GDM during pregnancy (P < 0.001). Nonobese (BMI <25 kg/m2) GDM subjects with normal glucose tolerance at follow-up had significantly higher mean glucose(P < 0.01), insulin (P < 0.05), and proinsulin (P < 0.001) values during the OGTT than control subjects, whereas there was no significant difference in C-peptide values. A comparison between control subjects with normal OGTT and BMI <25 kg/m2 (n = 39) and GDM subjects (n = 39) selected to have a comparable area under the glucose curve, BMI, and age demonstrated no group differences in glucose, C-peptide, or insulin levels, whereas the proinsulin levels were significantly higher (P < 0.001) during the glucose load. The molar ratio between proinsulin and insulin was also significantly higher among the former GDM subjects. We speculate that an elevated proinsulin-insulin ratio in women with previous GDM could be a marker for later development of NIDDM.

Kinetics of Human C-peptide in Man

A review of studies concerned with the metabolism of human C-peptide showed that the kidney is the major organ in C-peptide removal. The turnover rate of C-peptide was similar to that of proinsulin but considerably slower than that of insulin. An analysis of corresponding C-peptide and insulin concentrations in normal subjects after the administration of oral glucose or intravenous glucagon was used to define the relationships between the two peptides. These results show that different peripheral vein concentrations of insulin may correspond to identical C-peptide concentrations, depending on sampling conditions.

Residual Beta-cell Function and Its Metabolic Consequences

Improvement in the sensitivity and specificity of the C-peptide immunoassay and studies of larger groups of patients have increased our knowledge of the importance of residual beta-cell function and its metabolic consequences in insulin-treated diabetic patients. During the first five to 10 years after the onset of diabetes mellitus residual beta-cell function is demonstrable in the majority of insulin-treated patients irrespective of the severity of the initial symptoms and only partly dependent on the patients age at diagnosis. Residual beta-cell function facilitates good control. Stable patients have a higher C-peptide concentration in plasma than unstable ones, but unmeasurable C-peptide is not always associated with poor control. More data are needed before the full significance of an even minimal reserve of beta-cell function is elucidated. It also remains to be shown whether the reduction in beta-cell function in diabetic patients has a qualitative as well as quantitative component.