Utilization of glutamine in the developing rat jejunum.

The activity of glutaminase was measured in the jejunum of rats during the suckling period. Activity increased significantly (P less than 0.05) from 2.37 +/- 0.31 mol glutamate/(mg jejunal protein.h) (X +/- SD) in the first week to 3.50 +/- 0.99 in the second week and 4.75 +/- 0.96 during the third week. The quantity of the glutaminase protein, measured with a dot immunobinding assay, remained constant during the first (592 +/- 174 cpm bound/g protein) and second (599 +/- 125) weeks and then increased significantly by the third week (784 +/- 270) after birth. These results indicate that the activity of glutaminase is regulated by alterations in both the quantity and activity of glutaminase protein. The oxidation of [U-14C]glutamine to 14CO2 in vitro also increased during the suckling period, with significantly (P less than 0.05) higher rates of oxidation observed by the third week after birth. The capacity of the developing rat jejunum to utilize both glutamine and -hydroxybutyrate (BHB) as fuel sources to support [methyl-3H]thymidine (3HTdR) incorporation was also determined. Addition of glutamine to jejunal homogenates in vitro resulted in a significantly (P greater than 0.05) higher rate of 3HTdR incorporation than was observed with either glucose or BHB as a fuel source during the early suckling period. In the late suckling period, the addition of BHB and glutamine together resulted in significantly (P greater than 0.05) higher rates of 3HTdR incorporation than that found with glucose as a fuel source. These data suggest that both glutamine and BHB are important fuel sources in the jejunum during the suckling period.