Effects of Insulin on Fasting and Meal-Stimulated Somatostatin-Like Immunoreacitivity in Noninsulin-Dependent Diabetes Mellitus: Evidence for More than One Mechanism of Action*
We assessed the effects of insulin and normalization of blood glucose on plasma levels of somatostatin-like immunoreactivity (SLI) in patients with noninsulin-dependent diabetes mellitus (NIDDM). In one series of experiments, normalization of blood glucose was achieved by Biostator-controlled feedback infusion of insulin.This procedure reduced plasma SLI levels by 34% [from 17.1 ± 2.1 (±SEM) to 11.3 ± 1.9 pg/ml; P < 0.05], concomitant with a significant reduction in plasma glucagon and C-peptide and an evanescent decrease in plasma gastric inhibitory peptide (GIP) levels. An ensuing mixed meal elicited a rise in SLI that reached the same levels during infusion of insulin as during uncontrolled hyperglycemia; the incremental increase was, however, 45% higher (P < 0.005) during insulin infusion. Furthermore feedback insulin infusion enhanced GIP and decreased C-peptide responses, but did not affect the glucagon response to the meal. To further evaluate the influence of insulin on SLI levels, we compared the effects of normo- and hyperglycemia during constant hyperinsulinemia by varying the rate of glucose infusion(glucose clamping). Basal SLI levels decreased significantly only during the normoglycemic clamp. The SLI response to a meal was more pronounced during the normoglycemic than the hyperglycemic clamp. The patterns of glucagon and GIP were similar during the two clamp conditions, while both basal and stimulated C-peptide levels were lower during the normoglycemic clamp. To investigate the temporal relationship between changes in blood glucose and SLI levels, patients were studied during a prolonged (270-min) period of normoglycemic clamp and fasting. After attaining normoglycemia, SLI levels continued to decline for 150 min, whereas glucagon and GIP levels did not change. We conclude that in patients with NIDDM, 1) insulin significantly lowers basal SLI levels if normoglycemia is concomitantly attained; this action of insulin was partially dissociated from its hypoglycemic action; 2) hyperglycemia per se inhibits a meal-induced SLI response, and 3) insulin effects on SLI are not secondary to changes in glucagon or GIP levels.