Amino acid absorption by the uterus of the rat during the estrous cycle and pregnancy



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The tissue accumulation technique (in vitro incubation of uterine tissue segments) was used to study the altering accumulation rate of amino acids by the rat uterus during the estrous cycle and pregnancy. Dual-labeled D- mannitol and a-aminoisobutyric acid or L-proline were used to determine passive uptake of amino acid or total accumulation of amino acid, respectively. The uptake by active processes was estimated by subtracting the total accumulation of D-mannitol from the total uptake of the test amino acid. Although data were expressed on the basis of unit wet weight, interpretation was extended to uptake by the total uterus. Results obtained during the estrous cycle indicate that absorption via all three modes studied (passive, active, and total) was least during late estrus and greatest during proestrus on the basis of total uterine weight. In gravid uteri, the uptake via passive processes per unit wet weight may have been maximal on Day 7, although uptake per uterus was maximal on Days 14 and 20. The total uptake and uptake by active processes were maximal on Days 14 and 20 on either basis. Thus, the uterine uptake of amino acids varies significantly during both the estrous cycle and pregnancy. The degree of uptake appears to reflect the changing nutritional demands imposed upon the uterine tissues for its own growth and/or for the growth of the feti.