Alanine uptake by liver at midpregnancy in rats.

The participation of the liver to the increase in alanine utilization seen at midpregnancy was studied in 9- and 12-day pregnant rats. Liver fractional extraction of alanine was assessed in vivo from the changes in concentration in afferent and efferent vessels. Hepatic active transport of alanine was determined in vitro using isolated plasma-membrane vesicles. Compared with nonpregnant controls, alanine fractional extraction was significantly increased on day 12 but not on day 9 of pregnancy. Vesicles isolated from 9- and 12-day pregnant animals had a greater capacity for Na+-dependent transport than those from controls. Eadie-Hofstee plotting showed that this increase was due to an increase in Vmax with no change in Km. Both A and ASC systems contributed to the Vmax increase. These results indicate that, although by day 9 the liver has developed an increased capacity for alanine uptake, the actual extraction is seen only by day 12 of pregnancy. At this stage the liver participates actively in the turnover of alanine and the development of hypoalaninemia.