Abstract 286 Poster Session I, Saturday, 5/1 (poster 144)

Glucose supply to the fetus is derived from the maternal circulation via placental transport of substrate. This transport process is mediated by two major facilitative glucose transporter protein isoforms, Glut 1 (Km 2 to 5 mM) and Glut 3 (Km 1.8 mM). Previously, we demonstrated a time-dependent decline in ovine placental Glut 1 protein concentrations secondary to chronic hyperglycemia and hypoglycemia (Am. J. Physiol. 274:R339-R347, 1998). In the present study we examined the time-dependent effect of hyperglycemia and hypoglycemia upon ovine placental Glut 3 protein levels and localization using the recently characterized rabbit anti-ovine Glut 3 IgG (Am. J. Physiol. 273:R1132-R1141, 1997) in the well established chronically catheterized pregnant sheep model. Maternal and fetal chronic hyperglycemia (HG) were achieved by a continuous 50% dextrose infusion (n=17) at a variable rate to ensure maternal serum glucose concentrations that were two-fold above the control, while chronic hypoglycemia (LG) was accomplished by a constant infusion of insulin (30-60 pmol.kg-1 .min-1) at a variable rate to maintain a maternal glucose concentration that was 50% of control (n=15). The control group (C) consisted of gestational age-matched euglycemic, normoinsulinemic ewes (n=10+11). Western blot analysis revealed no change in placental Glut 3 (50 & 80 kD bands) levels at 2-24 hr (n=7), 10d (n=3) and 13-15d (n=4), but a 60% decine at 17-20d (n=3) (p<0.01) of HG vs C. In contrast, LG led to no change at 4-48 hr (n=4), 5-15d (n=4), and 25-41d (n=7). Immunolocalization (n=4) revealed Glut 3 in trophoblastic cells and in certain fetal vessels found in the epitheliochorial ovine placental caruncle. We conclude that ovine placental Glut 3 protein is 1] downregulated by HG in a time-dependent manner, 2] not affected by LG ranging from 2-24 hr to 17-20d, and 3] localized at the trophoblastic cell layer. We speculate that placental Glut 3 is the important isoform that mediates materno-fetal transfer of substrate. While a downregulation of Glut 3 with HG constitutes a protective effect against fetal "glucose toxicity", no change with LG is suggestive of a low Km isoform maximizing fetal glucose availability by ensuring complete saturability of the most efficient transport system in the presence of limited substrate.

[Supported by NIH-HD33997, HD25024, HD28794, & HD20761].