Abstract 1349 Neonatal Disease Oriented Research: Molecular Events and Brain Injury Poster Symposium, Tuesday, 5/4

Unconjugated hyperbilirubinemia is the most common clinical condition in the newborn period and when severe, can lead to kernicterus. The pathogenesis of kernicterus remains unclear, but central to its development is the passage of bilirubin across the blood brain barrier. Studies suggest that unconjugated bilirubin is a substrate for P-glycoprotein (Pgp), an ATP dependent integral plasma membrane efflux pump, expressed in abundance on the luminal aspect of brain capillary endothelium, where it limits the influx and CNS retention of many lipophilic compounds. We have recently demonstrated that brain bilirubin uptake is increased in Pgp deficient transgenic null mutant mice as compared to their wild type Pgp sufficient counterparts (Peds Res 44: 763, 1998). Alleged risk factors associated with kernicterus, including hypoxia, acidosis, and perinatal stress, may modulate brain microvessel Pgp expression but little is known regarding cerebral Pgp expression in this milieu. We hypothesized that the expression of Pgp in immature rat brain would be decreased in a milieu associated with uteroplacental insufficiency. This milieu is classically characterized by hypoxia, acidosis, and altered metabolic fuel availability. We performed sham surgery (control) and maternal bilateral uterine artery ligation on Sprague-Dewley pregnant rats on day 19 of gestation (term=21d). 48 hours after surgery, pups were delivered by Caserean section, cerebral tissues were extracted and Pgp mRNA levels were quantitated by RT-PCR (n=6) with an internal control, and Pgp protein levels were quantitated by Western immunoblotting (n=5). RESULTS: Pgp mRNA levels were significantly decreased in cerebral tissue from the uteroplacental insufficiency model: control = 1.0, uteroplacental insufficiency model= 0.56 ± 0.17 (X±SD). Moreover, brain microvessel Pgp protein levels were also reduced: uteroplacental insufficiency model = 0.50 ± 0.04. We conclude that an intrauterine milieu induced by uteroplacental insufficiency is associated with decreased Pgp protein and mRNA expression in immature rat brain. Given that Pgp has been determined to play a role in limiting the net influx of various lipophilic compounds, including bilirubin, across the blood brain barrier, we speculate that the relatively low expression of Pgp in the blood brain barrier of immature rat brain subject to perinatal stress may enhance brain bilirubin uptake.