The beta-cell response to glucagon and mixed meal stimulation in non-insulin dependent diabetes

Fasting and meal-stimulated serum C-peptide in long-standing type 1 diabetes mellitus

AIMS

This study aims to evaluate the stability of C-peptide over time and to compare fasting C-peptide and C-peptide response after mixed-meal tolerance test (MMTT) at T90 or T120 with C-peptide area under the curve (AUC) in long-standing type 1 diabetes.

METHODS

We included 607 type 1 diabetes individuals with diabetes duration >5 years. C-peptide concentrations (ultrasensitive assay) were collected in the fasting state, and in a subpopulation after MMTT (T0, just prior to, T30-T60-T90-T120, 30-120 min after ingestion of mixed-meal) (n = 168). Fasting C-peptide concentrations (in n = 535) at Year 0 and Year 1 were compared. The clinical determinants associated with residual C-peptide secretion and the correspondence of C-peptide at MMTT T90 / T120 and total AUC were assessed.

RESULTS

A total of 153 participants (25%) had detectable fasting serum C-peptide (i.e ≥ 3.8 pmol/L). Fasting C-peptide was significantly lower at Year 1 (p < 0.001, effect size = -0.16). Participants with higher fasting C-peptide had a higher age at diagnosis and shorter disease duration and were less frequently insulin pump users. Overall, 109 of 168 (65%) participants had both non-detectable fasting and post-meal serum C-peptide concentrations. The T90 and T120 C-peptide values at MMTT were concordant with total AUC. In 17 (10%) individuals, C-peptide was only detectable at MMTT and not in the fasting state.

CONCLUSIONS

Stimulated C-peptide was detectable in an additional 10% of individuals compared with fasting in individuals with >5 years of diabetes duration. T90 and T120 MMTT measurements showed good concordance with the MMTT total AUC. Overall, there was a decrease of C-peptide at 1-year follow-up.

The clinical utility of C-peptide measurement in the care of patients with diabetes

C-peptide is produced in equal amounts to insulin and is the best measure of endogenous insulin secretion in patients with diabetes. Measurement of insulin secretion using C-peptide can be helpful in clinical practice: differences in insulin secretion are fundamental to the different treatment requirements of Type 1 and Type 2 diabetes. This article reviews the use of C-peptide measurement in the clinical management of patients with diabetes, including the interpretation and choice of C-peptide test and its use to assist diabetes classification and choice of treatment. We provide recommendations for where C-peptide should be used, choice of test and interpretation of results. With the rising incidence of Type 2 diabetes in younger patients, the discovery of monogenic diabetes and development of new therapies aimed at preserving insulin secretion, the direct measurement of insulin secretion may be increasingly important. Advances in assays have made C-peptide measurement both more reliable and inexpensive. In addition, recent work has demonstrated that C-peptide is more stable in blood than previously suggested or can be reliably measured on a spot urine sample (urine C-peptide:creatinine ratio), facilitating measurement in routine clinical practice. The key current clinical role of C-peptide is to assist classification and management of insulin-treated patients. Utility is greatest after 3-5 years from diagnosis when persistence of substantial insulin secretion suggests Type 2 or monogenic diabetes. Absent C-peptide at any time confirms absolute insulin requirement and the appropriateness of Type 1 diabetes management strategies regardless of apparent aetiology.

The effect of chronic hyperglycaemia on the islet B-cell responsiveness in newly diagnosed type 2 diabetes

The aim of the present study was to evaluate the effect of chronic hyperglycaemia on the pancreatic B-cell response to stimulation with a standard mixed meal or intravenous glucagon in 7 subjects with newly diagnosed Type 2 diabetes. Stimulation was performed at mean chronic fasting hyperglycaemia of 11.8 +/- 0.7 (SEM) mmol l-1 and at normoglycaemia obtained by an intravenous infusion of regular insulin followed by an insulin wash-out period. The incremental plasma C-peptide area under the curve after stimulation with the meal was similar at normo- and hyperglycaemia. In contrast, prestimulatory plasma C-peptide and the incremental plasma C-peptide area under the curve after stimulation with glucagon were significantly higher at chronic hyperglycaemia than at normoglycaemia (p less than 0.01 and p less than 0.05). In conclusion, chronic hyperglycaemia as seen in newly diagnosed Type 2 diabetes is associated with a complete lack of potentiation of postprandial islet B-cell secretion but a partly preserved potentiation of basal and post-glucagon islet B-cell secretion.

The plasma C-peptide and insulin responses to stimulation with intravenous glucagon and a mixed meal in well-controlled type 2 (non-insulin-dependent) diabetes mellitus: dependency on acutely established hyperglycaemia

The dose-response relationships between acutely established hyperglycaemia and the plasma C-peptide and insulin responses to i.v. stimulation with 1 mg of glucagon and a standard mixed meal were investigated in 10 patients with well-controlled Type 2 (non-insulin dependent) diabetes mellitus. Hyperglycaemia was maintained for 90 min before stimulation using a hyperglycaemic clamp technique. Each test was performed on different steady state blood glucose levels of approximately 6 mmol/l, approximately 12 mmol/l, and approximately 20 mmol/l, respectively. The plasma C-peptide and insulin responses after glucagon and the meal were potentiated markedly at each level of prestimulatory hyperglycaemia. After glucagon injection, the relative glucose potentiation of the insulin response was significantly higher than the relative glucose potentiation of the C-peptide response at each level of hyperglycaemia (p less than 0.01). This difference may be explained by a higher fractional hepatic removal of insulin at normoglycaemia, since the molar ratio between the incremental C-peptide and insulin responses after glucagon stimulation was higher at prestimulatory normoglycaemia (4.85 (3.65-12.05] than at the prestimulatory blood glucose concentrations approximately 12 mmol/l (2.41 (2.05-4.09] (p less than 0.01) and approximately 20 mmol/l (2.24 (1.37-3.62] (p less than 0.01). In conclusion, the islet B-cell responses to glucagon and a standard mixed meal are potentiated to a high degree by acutely established prestimulatory hyperglycaemia in patients with well-controlled Type 2 diabetes. Acute prestimulatory hyperglycaemia is also associated with a markedly reduced incremental C-peptide/insulin ratio after glucagon stimulation in such patients.(ABSTRACT TRUNCATED AT 250 WORDS)