Proinsulin, insulin, and C-peptide in cystic fibrosis after an oral glucose tolerance test.

The beta-cell response to an oral glucose load was studied in 22 patients with cystic fibrosis (CF) by means of insulin, C-peptide and proinsulin, and the results compared with those from 20 healthy sex and age matched controls. According to WHO criteria two had diabetes mellitus, eight had impaired glucose tolerance and twelve had normal glucose tolerance. All patients showed lower insulin and C-peptide levels than the controls at 30 minutes. However the insulin and C-peptide responses were sustained so that the areas under the curves were comparable between controls, CF patients with normal glucose tolerance, and CF patients with impaired glucose tolerance. By contrast, the area under the proinsulin curve was significantly higher in the CF patients with impaired glucose tolerance compared with both controls (p less than 0.05) and with the CF normal glucose tolerance group (p less than 0.02). In the CF patients with impaired glucose tolerance the proinsulin level was significantly elevated at 120 minutes (median 50 pmol/l) and at 180 minutes (29 pmol/l) as compared with the controls (24 and 19 pmol/l p less than 0.01) and with the CF patients with normal glucose tolerance (21 and 16 pmol/l p less than 0.01). These data confirm that impaired glucose tolerance and diabetes is frequent in cystic fibrosis. The elevated proinsulin immunoreactive material in CF patients with impaired glucose tolerance may partly compensate for the relative insufficient insulin response found in these patients.