Thyroxine-Induced Changes in the Development of Neutral ?-Amino Acid Transport Systems of Rat Brain

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Abstract
Transport of representative neutral .alpha.-amino acids was measured in brain slices after injecting thyroxine into donor rats of various ages from 1-23 days old. The hormone did not alter uptake in slices from 1 day old rats even when treatment was begun on pregnant rats as much as 10 days before delivery. Injecting thyroxine until age 6 days, however, decreased the activity of transport system A (the major Na-dependent system in most mammalian cells) and caused appearance of a new transport system used by the model amino acids, 2-aminoisobutyric acid and 2-(methylamino)isobutyric acid. Uptake at 6 days was similar to slices from older, untreated rats (e.g., those 14 days old). One action of thyroxine is to accelerate the development of neutral .alpha.-amino acid transport systems of brain over the first 5 days after birth. Thyroxine treatment of rats from birth to age 14 days also appears to increase the activities of both system A and the 2nd transport system used by the 2 model amino acids in brains from 14 day old rats.