Na+-dependent alanine transport in plasma membrane vesicles from late-pregnant rat livers

Liver plasma membrane vesicles retaining Na+-dependent L-alanine transport were purified from either virgin or 21-d pregnant rats. Kinetic analysis of L-alanine uptake data revealed that vesicles from pregnant rats show a slight increase in Vmax without significantly affecting Km. Plasma membrane marker recoveries were used to calculate alanine utilization indexes. These data showed that the overall liver capacity to take up alanine was increased 2-fold in late pregnant rats. When considering utilization indexes in the physiologic range of L-alanine concentrations in blood (0.1-2 mM), we found a highly significant enhancement of the liver capacity to take up this amino acid. It is concluded that, at late pregnancy, when amino acid requirements by the fetus are high, amino acid disposal by the maternal liver is not limited by a low transport capacity; thus, other mechanisms should be contributing to the nitrogen sparing at late pregnancy.