Elevated ratio of summed serum proinsulin to insulin response after oral glucose load in Type 2 diabetes decreases following sulfonylurea treatment

Abstract

We showed previously that the disproportionate elevation of serum proinsulin at fasting and after glucose ingestion in Type 2 diabetes is reduced to nearly normal after improvement of glycemic control by diet therapy. In this study, we investigated the effect of sulfonylurea (SU) treatment on serum proinsulin levels and proinsulin/insulin ratio ( PI I ) during oral glucose tolerance test in patients with Type 2 diabetes. Thirteen diabetic patients (age 56 ± 9 years, body mass index 22.4 ± 1.9 kg/m 2, mean ± SD) were examined by 75 g oral glucose tolerance test (OGTT) before and after glycemic control by SU therapy. Mean interval of two OGTTs was 126 days. Serum proinsulin was measured by the radioimmunoassay using a human proinsulin-specific antiserum. When glycemic control improved after SU therapy (mean fasting plasma glucose 11.5 and 6.0 mmol/l, before and after SU treatment), fasting insulin, proinsulin and PI I ratio did not change significantly. Insulin response during OGTT markedly increased after SU therapy. Summed value of insulin (ΣI) increased from 634 to 1064 pmol/l after SU ( P < 0.01), whereas summed proinsulin (ΣPI) did not change significantly (146 and 159 pmol/l), resulting a significant decrease in ΣPI ΣI (23.6 − 15.1%, P < 0.05). We conclude that the disproportionate elevation of proinsulin during OGTT in patients with Type 2 diabetes can be reduced after glycemic control by SU treatment, chiefly by a selective increase in insulin response.