[Studies on the question of a possible autoregulation of insulin secretion (author's transl)]

Insulin augmentation of glucose-stimulated insulin secretion is impaired in insulin-resistant humans

Type 2 diabetes (T2D) is characterized by insulin resistance and pancreatic β-cell dysfunction, the latter possibly caused by a defect in insulin signaling in β-cells. We hypothesized that insulin's effect to potentiate glucose-stimulated insulin secretion (GSIS) would be diminished in insulin-resistant persons. To evaluate the effect of insulin to modulate GSIS in insulin-resistant compared with insulin-sensitive subjects, 10 participants with impaired glucose tolerance (IGT), 11 with T2D, and 8 healthy control subjects were studied on two occasions. The insulin secretory response was assessed by the administration of dextrose for 80 min following a 4-h clamp with either saline infusion (sham) or an isoglycemic-hyperinsulinemic clamp using B28-Asp-insulin (which can be distinguished immunologically from endogenous insulin) that raised insulin concentrations to high physiologic concentrations. Pre-exposure to insulin augmented GSIS in healthy persons. This effect was attenuated in insulin-resistant cohorts, both those with IGT and those with T2D. Insulin potentiates glucose-stimulated insulin secretion in insulin-resistant subjects to a lesser degree than in normal subjects. This is consistent with an effect of insulin to regulate β-cell function in humans in vivo with therapeutic implications.

Insulin infusion during normoglycemia modulates insulin secretion according to whole-body insulin sensitivity

OBJECTIVE

Glucose is the major stimulus for insulin release. Time course and amount of insulin secreted after glycemic stimulus are different between type 2 diabetes mellitus (T2DM) patients and healthy subjects. In rodents, it was demonstrated that insulin can modulate its own release. Previous studies in humans yielded contrasting results: Insulin was shown to have an enhancing effect, no effect, or a suppressive effect on its own secretion. Thus, we aimed to evaluate short-term effects of human insulin infusion on insulin secretion during normoglycemia in healthy humans and T2DM subjects of both sex.

RESEARCH DESIGN AND METHODS

Hyperinsulinemic-isoglycemic clamps with whole-body insulin-sensitivity (M) and C-peptide measurements for insulin secretion modeling were performed in 65 insulin-sensitive (IS) subjects (45 ± 1 year, BMI: 24.8 ± 0.5 kg/m(2)), 17 insulin-resistant (IR) subjects (46 ± 2 years, 28.1 ± 1.3 kg/m(2)), and 20 T2DM patients (56 ± 2 years, 28.0 ± 0.8 kg/m(2); HbA(1c) = 6.7 ± 0.1%).

RESULTS

IS subjects (M = 8.8 ± 0.3 mg · min(-1) · kg(-1)) had higher (P < 0.00001) whole-body insulin sensitivity than IR subjects (M = 4.0 ± 0.2) and T2DM patients (M = 4.3 ± 0.5). Insulin secretion profiles during clamp were different (P < 0.00001) among the groups, increasing in IS subjects (slope: 0.56 ± 0.11 pmol/min(2)) but declining in IR (-0.41 ± 0.14) and T2DM (-0.87 ± 0.12, P < 0.00002 IR and T2DM vs. IS) subjects. Insulin secretion changes during clamp directly correlated with M (r = 0.6, P < 0.00001).

CONCLUSIONS

Insulin release during normoglycemia can be modulated by exogenous insulin infusion and directly depends on whole-body insulin sensitivity. Thus, in highly sensitive subjects, insulin increases its own secretion. On the other hand, a suppressive effect of insulin on its own secretion occurs in IR and T2DM subjects.

Suppression of basal, but not of glucose-stimulated insulin secretion by human insulin in healthy and obese hyperinsulinemic subjects

To evaluate the suppressive effect of biosynthetic human insulin (BHI; 2.5 U/m2 . h) on basal and glucose-stimulated insulin secretion in healthy and obese hyperinsulinemic subjects, the plasma C-peptide response was measured during maintenance of euglycemia and hyperglycemia by means of the glucose clamp technique. In five healthy subjects in whom arterial insulin concentration was increased to 94 +/- 8 microU/mL, but euglycemia was maintained at the fasting level. C-peptide concentration fell from 1.3 +/- 1.0 ng/mL by 21 +/- 8% (P less than 0.05). When hyperglycemia of 7 mmol/L above basal was induced by a variable glucose infusion, the C-peptide response was similar in the control (5.0 +/- 0.6 ng/mL) and BHI experiments (4.7 +/- 0.6 ng/mL) and was paralleled by an identical increase in plasma insulin above the prevailing insulin concentration. In seven obese patients plasma C-peptide fell from 3.5 +/- 0.4 to 2.8 +/- 0.5 ng/mL (P less than 0.05) when BHI was infused at the same rate of euglycemia maintained as in the lean subjects. As in healthy subjects, however, the plasma C-peptide response to the hyperglycemic stimulus (8.7 +/- 0.9 ng/mL) was not altered by BHI (7.9 +/- 0.8 ng/mL). Glucose utilization as determined by the glucose infusion rate necessary to maintain the desired glucose level was reduced by half in the obese patients compared with that of normal subjects. From these data we conclude that in healthy as well as obese hyperinsulinemic subjects, insulin at concentrations capable of suppressing its basal secretion fails to suppress its glucose-stimulated secretion.

Starvation enhances the ability of insulin to inhibit its own secretion

To examine whether decreased insulin secretion during starvation is related to a change in the ability of insulin to inhibit its own secretion, plasma C-peptide was measured after plasma insulin levels were acutely raised by intravenous (IV) insulin infusion in a dose of 40 and 80 mU/M2/min in obese subjects before and after a 72 hour fast. Plasma glucose concentration was maintained +/- 4% of basal levels by a variable glucose infusion. During the 80 mU infusion, at plasma insulin levels of 200 microU/mL, plasma C-peptide fell by 0.17 pmol/mL in the fed state. In the fasted state, despite basal levels that were 36% lower, C-peptide decreased by 0.21 pmol/mL. Highly significant increases in percent suppression after fasting were noted during both 40 mU and 80 mU studies. The plasma C-peptide response was related to the insulin infusion dose in both the fed and fasted state. In contrast, alpha cell suppression by insulin, as determined by plasma glucagon levels, was not altered by fasting. It is concluded that enhanced inhibitory influences of insulin on the beta cell during starvation may be a physiologically important mechanism for diminished insulin secretion during the transition from the fed to the fasting state.

Feedback inhibition by biosynthetic human insulin of insulin release in healthy human subjects

To determine the impact of biosynthetic human insulin (BHI) on endogenous insulin release, splanchnic output and arterial concentrations of C-peptide were measured in eight healthy men after intravenous administration of 0, 0.5, 1.25, U BHI . m-2 . h-1 for 70 min each. Euglycemia was maintained by a variable glucose infusion. Arterial levels of serum insulin were 48 +/- 6 pmol/liter before and 135 +/- 12, 265 +/- 18, and 593 +/- 47 pmol/liter after BHI infusion. Splanchnic C-peptide output was reduced by BHI infusion from 88 +/- 10 pmol/min before to 50 +/- 9, 28 +/- 10, and 18 +/- 16 pmol/min (P less than 0.0025). Simultaneously, arterial concentrations of C-peptide fell from 539 +/- 54 pmol/liter by 29 and 43% when 1.25 and 2.5 U . m-2 . h-1 of BHI were administered. Hepatic insulin uptake was directly related with BHI infusion rate (r = 0.88) and rose during BHI administration from a basal value of 58 +/- 7 to an uptake of 265 +/- 31 pmol/min when 2.5 U . m-2 . h-1 were infused (P less than 0.0005). Basal hepatic insulin clearance was 4.75 +/- 0.60 ml . kg-1 . min-1 and remained unchanged after BHI infusion as did hepatic fractional extraction of insulin, which was 61 +/- 4% in the basal state. Metabolic clearance rate of immunoreactive insulin (MCRi) was dose-dependently reduced by BHI infusion, whereas the relative share of hepatic insulin clearance in total MCRi rose simultaneously (P less than 0.01). We conclude that feedback inhibition of endogenous insulin release may play an important role in vivo. Furthermore, it appears that nonhepatic insulin degradation is a saturable phenomenon as total MCRi fell in the presence of its unchanged hepatic clearance rate after the infusion of large amounts of BHI.

Feedback inhibition of insulin secretion by insulin: relation to the hyperinsulinemia of obesity

We investigated the possible existence of a negative short-loop feedback of circulating insulin on the parent beta cell in 10 lean Caucasians, 10 obese Caucasians, and 10 obese Pima Indians. Plasma insulin levels were raised acutely by 100 microunits per milliliter for 90 minutes, and plasma glucose was maintained by the "clamp" technique. C-peptide levels were suppressed in all groups to approximately 50 per cent of basal values. However, the obese groups had absolute C-peptide levels much higher than those of the lean group. During the hour after infusion, the rate and magnitude of C-peptide recovery in the obese groups were higher than in the lean group. Thus, negative short-loop insulin-beta-cell feedback was operative in both the lean and obese states. Despite this suppression, the insulin-secretion rate in obese subjects was still greater than that in non-obese subjects. Inadequate feedback suppression may account in part for the prevailing hyperinsulinemia of the obese.