Abstract 1670 Neonatal Nutrition and Metabolism I Poster Symposium, Saturday, 5/1

Skeletal muscle (SM), adipose tissue, and myocardium constitute the classically insulin responsive tissues, which express two isoforms of the facilitative glucose transporters, Glut 4, the insulin responsive isoform, and Glut 1, the basal isoform. The developmental pattern of expression reveals higher levels of Glut 1 during the postnatal phase, while Glut 4 expression predominates in the adult. This postnatal expression pattern parallels the diminished insulin sensitivity present in the suckling animal. Glut 4 overexpression (10-fold) in the adult transgenic mouse causes an enhanced insulin sensitivity leading to obesity (Pessin et al, J. Biol. Chem. 1993). In contrast, the phenotype of the developing animal remains uncharacterized. To define the role of Glut 4 during development, in the present investigation, we determined the postnatal phenotype of the Glut 4 overexpressing progeny. We employed the neonatal heterozygous human Glut 4 overexpressing transgenic mice (OE; n=6 litters, at each time point) (since maternal Glut 4 OE homozygotes=nonsurvival) and compared them to their aged matched littermates (WT; n=6 litters, at each time point) at d1 and d14. Genotype was confirmed by tail genomic DNA extraction and subsequent PCR using Glut 4 primers. Hind limb SM was subjected to RT-PCR using human and murine Glut 4 specific primers. Human Glut 4 mRNA was present in the OE from day 1, while being absent in the WT. Murine Glut 4 mRNA amounts were no different between OE and WT mice at d1 and d14. This translated into a two-fold increase in Glut 4 protein levels on d1 (p<0.002) and a three-fold increase on d14 in the OE vs WT (p<0.006) by Western blots using an anti-Glut 4 IgG that does not distinguish between human and murine Glut 4 peptides. In contrast, no change in either Glut 1 mRNA or protein was observed in SM obtained from OE vs WT. While glucose levels were no different at d1, a significant decline was noted in the d14 OE vs WT (p<0.003). Body wts. and organ wts, (heart, lung, liver and brain) were not altered at d1 and d14 by OE of Glut 4. We conclude that: 1] Heterozygous overexpression of Glut 4 results in only a two-fold increase in SM Glut 4 expression at d1 and a three-fold increase on d14, 2] No compensatory effect upon SM Glut 1 expression is present, and 3] Overexpression of Glut 4 is not associated with a functional correlate (hypoglycemia) at d1 as opposed to d14. We speculate that the reason for a two fold overexpression of Glut 4 at d1 not causing hypoglycemia is due to an immaturity in the insulin-induced Glut 4 translocating mechanism (He & Devaskar, Ped. Res. 1998) which hinders accessibility of the protein to circulating glucose.