Response of plasma somatostatin-like immunoreactivity (SLI) to a 75 g oral glucose tolerance test in normal subjects and patients with impaired glucose tolerance.

To study the role of somatostatin in the pathophysiology of glucose intolerance in man, plasma somatostatin-like immunoreactivity (SLI) was measured in 8 normal subjects, 6 patients with insulin dependent diabetes mellitus (IDDM), 13 with non-insulin dependent diabetes mellitus (NIDDM), and 9 with hyperthyroidism, by extraction of plasma SLI and radioimmunoassay. The extraction method gave a recovery rate for synthetic somatostatin-14 and somatostatin-28 of 72 +/- 6 and 55 +/- 7%, respectively. No SLI corresponding to somatostatin-28 in human peripheral blood was observed. Incubation of somatostatin-28 in plasma gave a rapid decrease of immunoreactivity, and no conversion to somatostatin-14 was observed. It is speculated that SLI extracted with acid-acetone mainly represents a molecular weight similar to somatostatin-14. After oral administration of glucose (75 g), a clear and sustained rise in plasma SLI was seen in normal subjects from an initial value (+/- SEM) of 29.9 +/- 5.4 pg/ml to a peak value, at 60 min of 93.4 +/- 15.5 pg/ml. The increase of plasma SLI after 75 g glucose was also observed in IDDM and NIDDM. The peak level of SLI was significantly less than that for normal subjects. The extraction of plasma SLI with acetic acid and acetone gave reproducible results and showed a fluctuation of SLI with glucose concentration.