Effect of euglycemic hyperinsulinemia on neonatal canine hepatic and muscle metabolism

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 resistance and the transcription of the glucose-6-phosphatase gene in newborn dogs

In the present report changes in the mRNA level of glucose-6-phosphatase (G6Pase; EC 3.1.39) in newborn and adult dogs in vivo were studied to further test the hypotheses that neonatal hyperglycemia may be due to unsuppressed gluconeogenesis by insulin and that the antidiabetic role of insulin-like growth factor-1 (IGF-1) may be intact in newborn dogs who have consistently demonstrated insulin resistance. Our results were the following: (i) Both renal and hepatic G6Pase mRNA were expressed at birth and increased with time during a 24-h period of fasting after birth. (ii) The renal G6Pase mRNA levels in newborn dogs did not respond to either insulin or epinephrine. (iii) Hyperinsulinemia lowered the liver G6Pase mRNA by only 16.3% in newborn dogs, but reduced the liver G6Pase mRNA to an undetectable level in adult dogs. (iv) Hyperglycemia decreased the hepatic G6Pase mRNA by 14.3% in newborn dogs under hyperinsulinemia. (v) Infused epinephrine did not elevate the hepatic G6Pase mRNA level in newborn dogs in the presence of hyperglycemia and hyperinsulinemia. (vi) In newborn dogs, hyper-IGF-1 rapidly reduced the hepatic G6Pase mRNA level by 50%, and hypoglycemia was unable to elevate the hepatic G6Pase mRNA level under the hyper-IGF-1. We concluded that the reduced rate of suppression of transcription of the liver G6Pase gene by insulin in newborn dogs may reflect the unsuppressed neonatal hepatic gluconeogenesis due to insulin resistance and that the physiological roles of IGF-1 seemed to be intact in newborn dogs and may be not responsible for neonatal hyperglycemia.

Developmental aspects of transcription of fructose-1,6-bisphosphatase in newborn dogs

Our previous investigations demonstrated that unsuppressed gluconeogenesis under hyperinsulinemia in newborn dogs may be a mechanism of neonatal hyperglycemia. In the present study, the transcription of the gene for fructose-1,6-bisphosphatase (fru-1,6-P2ase; E 3.1.3.11) of newborn dogs was studied under various metabolic perturbations (age, suckling, fasting, and hyperinsulinemia). Total RNAs isolated from livers and kidneys were hybridized with a rat fru-1,6-P2ase cDNA probe. We observed that (i) fru-1,6-P2ase mRNA was expressed in both kidney and liver at birth and was about 40 and 80% of those in kidney and liver of adult dog, respectively; (ii) suckling decreased the kidney fru-1,6-P2ase mRNA level to 77.8 +/- 1.7% (24 h) from 100.0 +/- 8.0% (4 h), but increased liver mRNA to 158.6 +/- 11.4% (24 h) from 100.0 +/- 2.3% (4 h); (iii) during a 24-h period of fasting, the kidney fru-1,6-P2ase mRNA level did not change in the first 10 h and then increased 18.5% at 24 h, whereas the liver fru-1,6-P2ase mRNA increased ca. 20% during the first 10 h and then up to 161.1 +/- 18.0% at 24 h compared to that at 100.0 +/- 11.4% (0 h); (iv) euglycemic hyperinsulinemia did not change the renal fru-1,6-P2ase mRNA level, but lowered the hepatic fru-1,6-P2ase mRNA level to 56.0 +/- 8.7 from 100.0 +/- 11.8% (fasted controls) in newborn dogs, which was identical to that in adult dogs. These data suggest that the fru-1,6-P2ase in liver may play a more important role in glucose homeostasis of newborn dogs than that in kidney during the first day of their lives and that the incomplete suppression of transcription of the hepatic fru-1,6-P2ase gene by insulin in newborn dogs may not contribute to neonatal hyperglycemia due to insulin resistance.