Insulin resistance and glucose transporter expression during the euglycemic hyperinsulinemic clamp in the lamb.

Three- to six-day-old lambs infused with 100 mU x kg(-1) x min(-1) insulin required greater amounts of glucose to maintain euglycemia during a euglycemic hyperinsulinemic clamp compared with 31- to 35-day-old insulin-infused lambs (15.87 +/- 3.47 vs. 4.30 +/- 1.11 mg x kg(-1) x min(-1), P < 0.05, respectively). Endogenous glucose production persisted in both groups; however, the percent decrease compared with age-matched lambs receiving no insulin was greater in the younger group compared with the older group (53%, P < 0.001, vs. 34%, P < 0.01). The younger animals showed greater glucose utilization compared with the older animals (215 vs. 96%, respectively, P < 0.01). No effect of insulin was noted on GLUT-4 protein expression in either group. GLUT-2 expression was increased in older vs. younger lambs. Older insulin-infused lambs showed lower GLUT-2 expression than older 0 insulin-infused lambs [0.94 +/- 0.07 vs. 1.64 +/- 0.10 (OD) units, P < 0.005]. Increased sensitivity to insulin in the younger animals was not related to acute changes in GLUT-4 expression. Increased GLUT-2 expression with age, as well as decreased expression with hyperinsulinemia, is consistent with the development of an insulin-resistant state in the adult.

Insulin counterregulatory hormones are ineffective in neonatal hyperinsulinemic hypoglycemia.

Insulin counterregulatory hormones play a major role in the maintenance of glucose homeostasis. To evaluate the hypothesis that the reported imprecise control of glucose production by insulin is mirrored by a corresponding lack of response to the various insulin counterregulatory hormones, 30 spontaneously delivered mixed-breed term lambs weighing 4.9 +/- 0.5 kg (mean +/- SD) were studied at 5.0 +/- 0.7 days after birth following administration of 100 microCi D-[6-(3)H2]glucose in 0.9% NaCl by the primed-constant infusion technique to measure glucose kinetics. Infusion of 2.0 mU kg(-1) x min(-1) insulin produced hyperinsulinemic hypoglycemia and was combined with 1.0 mg x kg(-1) x min(-1) somatostatin (SRIF) to block insulin, glucagon, and growth hormone release. Infusion of 2 ng x mg(-1) x min(-1) glucagon or 10 microg x kg(-1) x h(-1) growth hormone with SRIF and insulin isolated the glucagon or growth hormone effect, respectively. The addition of metyrapone blocked cortisol release. Controls received only the isotope. In toto, the data can be interpreted to suggest that insulin has a greater effect on glucose uptake than on glucose production, and that neither glucagon, growth hormone, nor cortisol appreciably influenced the endogenous glucose production rate (Rp) during hyperinsulinemic hypoglycemia. The imprecise effect of these insulin counterregulatory hormones on neonatal glucose production mirrors the previously documented imprecise control by insulin.

The contribution of glucose to neonatal glucose homeostasis in the lamb.

A multiplicity of substrates and hormones interact to influence neonatal glucose homeostasis. Based on prior studies in our laboratory, we hypothesized that glucose alone does not tightly control neonatal glucose homeostasis. Fifteen spontaneously delivered, mixed-breed term lambs, weighing 4.7 +/- 0.9 kg (mean +/- SD) were studied at 4.7 +/- 0.6 days after birth following administration of 100 microCi D[6,6-(3)H2] glucose in 0.9% NaCl by the prime plus constant infusion technique to measure glucose production. After a baseline period, five lambs received 8.5 mg x kg(-1) x min(-1) glucose in combination with the following to isolate the contribution of glucose: somatostatin to block insulin, glucagon, and growth hormone release; metyrapone to block cortisol release; phentolamine to block alpha-adrenergic release; and propranolol to block beta-adrenergic release (glucose + blockade). Five lambs received the above without the glucose infusion (blockade). Five lambs continued to receive 0.9% NaCl alone as the diluent for the isotopic tracer throughout the study (control). The glucose + blockade group had a significant increase in plasma glucose (P < .0001) and a significant increase in total glucose appearance (P < .0001) compared with both the blockade and control groups. Under conditions of glucose infusion at a rate 49% greater than the basal rate, the endogenous glucose production rate persisted such that there was only an evanescent decrease compared with that of the control group, which was not statistically different over time. As a substrate, glucose does not tightly control neonatal glucose homeostasis.

Persistent glucose production and greater peripheral sensitivity to insulin in the neonate vs. the adult.

Insulin resistance has been reported to partially explain the clinical appearance of neonatal hyperglycemia. To determine the relative resistance to insulin of glucose production vs. glucose utilization, the euglycemic hyperinsulinemic clamp technique was employed for the first time in the human neonate and was combined with stable isotopic determination of glucose production and glucose utilization. The basal rates of glucose production and glucose utilization were determined, after which each neonate was clamped at his or her own euglycemic glucose concentration while receiving regular human insulin at one rate of 0.2, 0.5, 1.0, 2.0, or 4.0 mU. kg-1.min-1. Persistent glucose production (> or = 1 mg.kg-1.min-1) during the clamp was recorded for all groups. A significant increase in the glucose infusion rate (P < 0.001) and in percent glucose utilization (P < 0.01) occurred in the 2.0 and 4.0 mU.kg-1.min-1 insulin groups. Metabolic clearance rate of insulin was significantly greater in the neonate compared with the adult at the 2.0 mU.kg-1.min-1 insulin infusion rate (P = 0.036). Our results indicate that, in contrast to the adult, the neonate has persistent glucose production (P = 0.001) and greater peripheral sensitivity to insulin (P = 0.015).