Abstract 1227 Neonatal Disease Oriented Research: Intestinal Development Platform, Saturday, 5/1

The mdr1a isoform of P-glycoprotein (PgP) is an ATP dependent integral plasma membrane efflux pump. Pgp is expressed in brush border epithelial cells of the intestinal mucosa where it limits the absorption and oral bioavailability of a wide variety of unrelated lipophilic compounds. Intestinal Pgp also facilitates the secretion of substrates into the intestinal lumen from enterocytes. There are no data on the developmental expression of intestinal Pgp. We therefore determined the postnatal expression of Pgp in FVB mouse small intestine brush border membranes using RT-PCR and Western immunoblotting techniques. Studies were performed on brush border membrane from small intestine (duodenum. jejunum, ileum, in toto) isolated from day 0 (D-0), 7 (D-7), 21 (D-21) and adult mice. For RT-PCR (n=3 each age group), cDNA was synthesized by reverse transcription from 2 ug of mouse small intestine RNA and 0.02 ug of bovine retinal RNA. Bovine rhodopsin and mdr1 a primers were utilized for PCR amplification of the cDNA. PCR products were separated on a 5% polyacrylamide gel and radioactivity quantitated by phosphorimaging. For Western immunoblots (n=4 each age group), 50 ug of brush border membrane protein was electrophoresed on 7% PAGE, transferred overnight to nitrocellulose membranes, and incubated with the monoclonal mdr1 a antibody C219. Relative Pgp expression across age groups was assessed using scanning densitometry and indexed as a percent of adult levels (mean±SD). Small intestine Pgp mRNA levels were significantly lower at D-0 (22 ±4) and D-7 (28±4) compared with adults (P<0.05). Pgp protein expression was similarly low at D-0 (16 ±4) and D-7 (26±4) compared with adults (P<0.05). Both mRNA and protein levels increased significantly to adult levels at D-21 (P<0.05). We conclude that small intestine Pgp expression increases markedly during postnatal development. We speculate that low intestinal Pgp in newborns may enhance the oral bioavailability of Pgp substrates. Given evidence that unconjugated bilirubin is a substrate for Pgp, we further speculate that low intestinal Pgp expression in neonates might contribute to the enterohepatic circulation of bilirubin in newborns.

Supported by Irene McClenahan Young Investigator Research Award (BM) and NIH RO3 HD-35847 (JFW).