Abstracts
2001 Digestive Disease Week

# 1803 Specific Reversible Stimulation of System y⁺ Arginine Transport Activity in Human Intestinal Cells
Ming Pan, Wiley W. Souba Jr., Bruce R. Stevens, Hershey, PA; Gainesville, FLR. Stevens, Hershey,PA; Gainesville, FL

L-arginine, which is intimately involved inL-arginine, which is intimately involved in cellular immune function and nitric acid biology, is transported into cells by the System y⁺ carrier. As the gut epithelium is exposed to various luminal amino acids at any given time, the purpose of this study was to study the regulation of luminal arginine transport by other amino acids. Hypothesis: Luminal System y⁺ substrates acutely and reversibly up-regulate L-arginine transport activity in gut epithelial cells.

Methods: Human intestinal epithelial Caco-2 cells were incubated in an amino acid depleted media and exposed to various concentrations (0 - 10 mM) of amino acids over periods of time (30 s to 48 hrs) at 37C. [³H] L-arginine (0.1 mM - 10 mM) transport was measured in Caco-2 monolayers.

Results: Addition of L-arginine and/or other System y⁺ substrates (lysine and ornithine) to the depleted medium specifically up-regulated arginine transport activity 2-fold (138 ± 10 pmole/mg protein /minute vs. 58 ± 5 pmole/mg protein/minute, P < 0.01) after 1 hr, a response noted as early as 5 min. Non-system y⁺ substrates (proline and alanine) did not affect L-arginine absorption. Kinetic analysis indicated that L-arginine exposure increased both System y⁺ K_m (30 ± 4 mmole vs. 81 ± 6 mmole, p < 0.01) and V_max (2.75 ± 0.2 nmole/mg protein/min vs. 0.25 ± 0.02 nmole/mg protein/min, p < 0.01). Neither cycloheximide nor actinomycin affected this stimulation, indicating the regulation did not involve transcription or translation. Similar results were observed with chronic L-arginine exposure (up to 48 hours). The System y⁺ substrate activation effect was reversible. L-arginine transport activity returned to baseline level within 3 hrs when cells were re-incubated in amino acid free media.

Conclusions: System y⁺ arginine transport activity is acutely and reversibly activated by System y⁺ substrates via a mechanism consistent with transmembrane stimulation that involves both an increase in transporter affinity (Km) and an increase in the functional activity of existing membrane bound System y⁺ carriers. This reversible stimulation did not involve de novo protein synthesis. These findings identify the mechanism by which luminal nutrients up-regulate arginine uptake by the gut lumen.