Increased adrenergic signaling is responsible for decreased glucose-stimulated insulin secretion in the chronically hyperinsulinemic ovine fetus

Abstract

Insulin may stimulate its own insulin secretion and is a potent growth factor for the pancreatic β-cell. Complications of pregnancy, such as diabetes and intrauterine growth restriction, are associated with changes in fetal insulin concentrations, secretion, and β-cell mass. However, glucose concentrations are also abnormal in these conditions. The direct effect of chronic fetal hyperinsulinemia with euglycemia on fetal insulin secretion and β-cell mass has not been tested. We hypothesized that chronic fetal hyperinsulinemia with euglycemia would increase glucose-stimulated insulin secretion (GSIS) and β-cell mass in the ovine fetus. Singleton ovine fetuses were infused with iv insulin to produce high physiological insulin concentrations, or saline for 7-10 days. The hyperinsulinemic animals also received a direct glucose infusion to maintain euglycemia. GSIS, measured at 133 ± 1 days of gestation, was significantly attenuated in the hyperinsulinemic fetuses (P < .05). There was no change in β-cell mass. The hyperinsulinemic fetuses also had decreased oxygen (P < .05) and higher norepinephrine (1160 ± 438 vs 522 ± 106 pg/mL; P < .005). Acute pharmacologic adrenergic blockade restored GSIS in the hyperinsulinemic-euglycemic fetuses, demonstrating that increased adrenergic signaling mediates decreased GSIS in these fetuses.

Figure 1.. Schematic representation of experimental study design.

Diagrams for the chronic 7- to 11-day experiment with the primed continuous variable-rate hyperglycemic clamp (A), and the chronic fetal hyperinsulinemic-euglycemic clamp with the fetal insulin secretion study ± adrenergic blockade (B). ASIS, arginine-stimulated insulin secretion.

Figure 2.. Fetal insulin and glucose infusion rates and concentrations.

Fetal insulin (A) and glucose (B) infusion rates in the INS group. Fetal arterial plasma insulin (C) and glucose (D) concentrations. Open circles, INS (n = 8); solid squares, CON (n = 9). *, significant difference between INS and CON, P < .05 by mixed models ANOVA. Values are mean ± SE. For these graphs, data from days 8–11 have been reduced to 1 point, as reflected by error bars in the x-axis.

Figure 3.. Fetal oxygenation.

Fetal arterial pO₂ (A), O₂ content (B), and SaO₂ (C) are shown as mean ± SE. Open circles, INS (n = 8); solid squares, CON (n = 9). *, significant difference between INS and CON, P < .05 by mixed models ANOVA. Values are mean ± SEM. For these graphs data from days 8–11 have been reduced to 1 point, as reflected by error bars in the x-axis.

Figure 4.. Fetal insulin secretion at the end of the treatment period.

Mean ± SE fetal arterial plasma glucose (A) and insulin (B) concentrations are shown relative to the start of the primed continuous variable-rate hyperglycemic clamp (time, 0 min). Open circles, INS (n = 8); solid squares, CON (n = 9). *, significant difference between INS and CON, P < .05 by mixed models ANOVA.

Figure 5.. GSIS is restored with adrenergic blockade in INS fetuses.

Mean ± SE baseline and hyperglycemic clamp fetal insulin concentrations during the primed continuous variable-rate hyperglycemic clamp are shown for CON fetuses and for INS fetuses with and without adrenergic blockade. *, significant difference between baseline and hyperglycemic clamp insulin concentrations, P < .05 by mixed models ANOVA.

Figure 6.. Glucokinase is lower and GLUT2 and IGF-I are higher in INS fetal pancreases.

Glucokinase, GLUT2, and IGF-I were measured by Western blotting in fetal pancreases in CON (n = 8) and INS (n = 8) groups. Western blottings are shown with densitometry quantification. There was significantly less (**, P < .01) glucokinase and more (*, P < .05) GLUT2 and IGF-I in INS pancreases. IGF-1, insulin like growth factor 1.