AJP - Regulatory, Integrative and Comparative Physiology, Vol 252, Issue 1 78-R84, Copyright © 1987 by American Physiological Society
Valine entry into rat brain after diet-induced changes in plasma amino acids
J. K. Tews, J. Greenwood, O. E. Pratt and A. E. Harper
Passage of amino acids across the blood-brain barrier is assumed to be
modified by amino acid composition of the blood. To gain a better
understanding of the effects of protein intake on brain amino acid uptake,
we examined associations among diet, plasma amino acid patterns, and the
rate of entry of valine into the brain. Rats were fed (8 h/day for 7-10
days) diets containing 6, 18, or 50% casein before receiving one meal of a
diet containing 0, 6, 18, or 50% casein. After 4-7 h, they were
anesthetized and infused intravenously with [14C]valine for 5 min before
plasma and brain samples were taken for determination of radioactivity and
content of individual amino acids. As protein content of the meal was
increased from 0 to 50% casein, plasma and brain concentrations of valine
and most other large neutral amino acids (LNAA) increased severalfold; also
the ratio of [14C]valine in brain to that in plasma decreased by greater
than 50%, and the rate of valine entry into the brain increased 3.5-fold.
The increase in valine flux slowed as plasma levels of LNAA, competitors
for valine transport, increased. The results were far more dependent on
protein content of the final meal than on that of the adaptation diet; thus
changes in protein intake, as reflected in altered plasma amino acid
patterns, markedly altered valine entry into the brain.
