1. Preincubation of the immature rat uterus under physiological conditions was found to increase its subsequent ability to transport α-aminoisobutyric acid, l-proline, l-alanine and 1-aminocyclopentanecarboxylic acid. Uptakes of l-valine, l-phenylalanine and l-leucine were not affected. With α-aminoisobutyric acid, a doubling of the uptake could be obtained after a 3–5h preincubation period. Uteri from oestradiol-primed rats gave increases similar to those found in tissues from untreated animals. In both cases the preincubation increased the Vmax. of α-aminoisobutyric acid uptake but did not affect the Km. 2. The conditions during the preincubation period determined the increase in subsequent uptake of α-aminoisobutyric acid. No increase in uptake was found if the preincubation was carried out at 1°C, in the presence of cyanide or dinitrophenol, under anaerobiosis or with a concentration of puromycin that depressed incorporation of l-leucine into protein by 95%. The puromycin was also shown to prevent the increase in Vmax. normally found after the preincubation period. In addition, no increase was found if Na+was omitted from the preincubation medium. Other inorganic ions had smaller effects. 3. The uptake of α-aminoisobutyric acid by uteri before and after a preincubation period showed the same general patterns of sensitivity to competitive inhibitors, K+, pH, temperature and 2,4-dinitrophenol. 4. The results suggest that the preincubation leads to an increase in a protein component of the ‘A system’ for amino acid transport in the uterus, and that metabolic energy is required for the reactions involved.

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