1. The purpose of the present study was to examine the ability of insulin to inhibit its own secretion in type 2 diabetes independently of the prevailing plasma glucose concentration.
2. The responses of the plasma C-peptide concentration to sustained hyperinsulinaemia were assessed during a 200 min isoglycaemic clamp study in 14 patients with type 2 diabetes and seven age-and weight-matched control subjects. The arterialized venous plasma glucose concentration was clamped at ∼ 0.3 mmol/l below each subject's own basal level and was not permitted to rise above the basal level.
3. In the fasting state, the plasma C-peptide concentration was slightly, but not significantly, higher in the diabetic patients than in the control subjects (667 versus 413 pmol/l, respectively, P = 0.07), but it remained significantly higher in the diabetic patients during the clamp studies in absolute terms (minimum plasma C-peptide concentration 400 pmol/l in diabetic patients versus 151 pmol/l in control subjects, P <0.05) and was suppressed to a lesser extent when expressed as a percentage change from basal (35.8% in diabetic patients versus 59.4% in control subjects, P <0.01).
4. In order to investigate whether a high plasma glucose concentration was maintaining the plasma C-peptide concentration in the diabetic patients, six of these patients underwent a second clamp study at euglycaemia (plasma glucose concentration 5.2 mmol/l). Under these conditions, the plasma C-peptide concentration was suppressed to the same extent as in the control subjects (from 623 to 195 pmol/l, a change of 62.7%).
5. The results suggest that the insulin-β feedback loop is intact in type 2 diabetes when euglycaemia is acutely restored, but that basal hyperglycaemia maintains elevated insulin secretion rates and may, therefore, blunt the ability of insulin to regulate its own secretion in this condition.