Insulin responses to intravenous glucose and the hyperglycemic clamp in cystic fibrosis patients with different degrees of glucose tolerance.

The relationship between altered insulin secretion and impaired glucose tolerance was studied in 32 cystic fibrosis patients, 16 men and 16 women, aged 8-26 y, using oral and i.v. glucose tolerance tests and a hyperglycemic glucose clamp (10 mmol/L). Seven of these subjects were already being treated with insulin; seven had fasting blood glucose levels below 7.2 mmol/L but satisfied diabetic criteria at the oral glucose tolerance test; glucose tolerance was impaired in 13 subjects and normal in five. The insulin responses to the two i.v. glucose stimuli were inversely correlated with the plasma glucose levels (60 and 120 min) and the area under the curve of the oral glucose tolerance test. However, the acute insulin response to i.v. glucose was severely altered in patients with impaired glucose tolerance, whereas plasma insulin levels during the hyperglycemic clamp did not differ from those of healthy subjects. The responses to the two stimuli were dramatically low in the diabetic patients. These results suggest that cystic fibrosis patients with normal or impaired glucose tolerance retain their capacity to secrete insulin. Alterations in the acute phase of glucose-stimulated insulin secretion seem to be principally responsible for the early impairment in glucose tolerance.