Amino Acid Transport by the Rat Uterus During the Estrous Cycle and Pregnancy

Active and passive components of α-aminoisobutyric acid (AIB) uptake were measured in the rat uterus during the estrous cycle and pregnancy. An in vitro procedure was employed, using dual-labeled AIB and D-mannitol to measure total amino acid uptake and passive entry, respectively. The difference between the latter values was considered an estimate of the active transport of AIB. Active uptake per g of tissue during the estrous cycle and all components of the uptake/uterus varied with changing uterine weight, allowing the nutritional contribution of the transport process to vary with uterine and fetal needs. During the estrous cycle passive uptake/g varied independently, with highest levels in metestrus and diestrus. During gestation passive uptake/g did not change, whereas active uptake/g was maximal on day 14, tending to decrease slightly on day 20. Temporal correlations to known hormonal events suggest that, in addition to the previously hypothesized role of estrogen in uterine transport. progesterone may play important roles in controlling and/or modulating estrogen control of amino acid transport mechanisms in the uterus.