Maternal hyperphenylalaninemia: an experimental model in rats.

Experimental maternal hyperphenylalaninemia produced in pregnant F344 rats by the combined use of p-Chloro-DL-phenylalanine and L-phenylalanine reduced fetal birth weight in comparison to saline-injected controls. Offspring who experiences hyperphenylalaninemia in utero died within 5 days after birth. Fetal plasma phenylalanine levels were several times higher than maternal plasma phenylalanine levels, indicating that the placenta actively concentrates maternal phenylalanine. Fetal brain phenylalanine levels rose in direct proportion to elevations in fetal plasma phenylanaline, whereas maternal brain phenylalanine levels remained low during maternal plasma phenylalanine elevation; the contrast suggests that the maternal brain is better able than the fetal brain to screen itself against high circulating plasma phenylalanine levels.