HEPATIC AMINO-ACID-UPTAKE IS DECREASED IN LACTATING RATS - IN-VIVO AND IN-VITRO STUDIES

To study the redistribution of amino acids to the mammary gland during lactation we used lactating and virgin rats fed liquid diets. Virgin rats were divided in two groups: one group was fed daily a diet containing the same amount of protein that was consumed the previous day by lactating rats (high protein diet-fed rats), and the other virgin group was fed the normal liquid diet (control). The hepatic availability of amino acids was significantly higher in the lactating rats than in the other two groups, but the uptake and fractional extraction of amino acids by the liver were lower in lactating rats than in the high protein-fed virgin controls. When primary hepatocyte cultures were used, the uptake of 2-amino-[1-C-14]isobutyric acid (AIB) and the activity of system A were found to be significantly higher in the hepatocytes from virgin rats fed the high protein diet than in those obtained from the lactating and control virgin groups. No difference was observed between the control virgin rats and the lactating rats. The kinetics of AIB showed that the V-max/K-m ratio was significantly lower in hepatocytes from lactating rats than in those from the high protein diet-fed virgin rats. Addition of prolactin to the incubation medium decreased the uptake of AIB in hepatocytes from both groups of virgin rats. Moreover, uptake of AIB was greater in bromocriptine-treated lactating rats and in lactating rats that had had their pups removed for the preceding 24 h compared with values for the lactating rats. These data suggest that liver amino acid uptake is decreased during lactation, redistributing these amino acids from the liver to the lactating mammary gland, and that prolactin is a potential signal for this intertissue flux of amino acids.