Cerebral blood flow and blood-brain influx of some neutral amino acids in control and hypothyroid 16-day-old rats.

Rats were made hypothyroid by a daily subcutaneous injection of propylthiouracil beginning the first day after birth. CBF, brain plasma volume, blood-brain extraction, and influx of some neutral amino acids were studied in 16-day-old animals. In hypothyroid rats, the brain plasma volume was decreased by approximately 30%. CBF was decreased by greater than 50%. This decrease was the highest in cerebellum. Blood-brain extraction of small neutral amino acids (alanine, serine, cysteine) was greatly enhanced. This greater extraction compensated for the decreased supply of alanine brought about by its decreased plasma concentration and the lower CBF. In contrast, the extraction of the large amino acids tested (leucine, phenylalanine) was hardly increased, and the influx of phenylalanine was slightly decreased. These results suggest an alteration in the maturation of the brain capillary bed and capillary transport for neutral amino acids in hypothyroidism. The differential effect of hypothyroidism on some small and large amino acids is an additional argument for the existence of two systems of transport for neutral amino acids at the luminal membrane of brain capillary endothelial cells of immature rats.