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Regulation of small intestinal glutamine transport by epidermal growth factor
R M Salloum1, B R Stevens, G S Schultz
1Department of Surgery, University of Florida College of Medicine, Gainesville.
Background:
Epidermal growth factor (EGF) stimulates cell replication and increases DNA content of the small intestine, but its effects on mucosal amino acid transport are unknown.
Methods:
To investigate these effects, we treated adult rats with vehicle or EGF (10 micrograms/100 gm body weight subcutaneously every 8 hours for three doses). Jejunal brush border membrane vesicles (BBMVs) from each group were prepared by Mg++ aggregation/differential centrifugation. BBMVs were enriched fifteen-fold in alkaline phosphatase, indicating BBMV purity. Transport of 3H-glutamine and 3H-alanine was studied by a rapid mixing filtration technique. Uptakes were primarily Na+ dependent, occurred in an osmotically active space, exhibited classic overshoots, and had similar 2-hour equilibrium values.
Results:
Glutamine transport by BBMVs more than doubled in rats treated with EGF (16.4 +/- 0.1 pmol glutamine/mg protein/10 sec in EGF vs 7.1 +/- 0.5 pmol glutamine/mg protein/10 sec in controls; p < 0.001). Kinetic studies of the glutamine transporter showed that the increase in transport was the result of a 70% increase in maximal transport velocity (total maximum glutamine uptake = 193 +/- 8 pmol glutamine/mg protein/10 sec in EGF vs 114 +/- 7 pmol glutamine/mg protein/10 sec in controls; p < 0.0001 with no change in transporter affinity (transporter affinity = 224 +/- 6 mumol/L in EGF vs 242 +/- 37 mumol/L in controls; difference, not significant). Alanine uptake by BBMVs was also increased with EGF administration (10.2 +/- 2.0 pmol alanine/mg protein/10 sec in EGF vs 4.5 +/- 0.5 pmol alanine/mg protein/10 sec in controls; p < 0.005). Simultaneously, glucose transport was decreased by 50% in EGF-treated rats, indicating that the Na(+)-dependent glucose cotransporter is regulated independently from and opposite to amino acid transporters.
Conclusions:
We conclude that EGF up-regulates amino acid transport activity in jejunal BBMVs, an event that is most likely caused by an increase in de novo biosynthesis of transporter protein. The increase in amino acid uptake not only may support de novo protein synthesis but, in the case of glutamine, also may be required for energy production and nucleotide biosynthesis.