Transport of NG-Nitro-L-Arginine Across Intestinal Brush Border Membranes by Na+-Dependent and Na+ Independent Amino Acid Transporters

Purpose. To clarify the transport mechanism of N^G-nitro-L-arginine (L-NNA), a potent NO-synthase inhibitor, across intestinal brush border membranes (BBM). Methods. Dog intestinal BBM vesicles were used. Results. The time course of L-NNA uptake showed a Na⁺-dependent overshoot phenomenon. Concentration-dependence curves of L-NNA initial uptake were saturable in the presence and absence of Na⁺, indicating participation of Na⁺-dependent and Na⁺-independent carrier-mediated transport systems. The calculated kinetic parameters of L-NNA initial uptake indicate that the former is a low-affinity high-capacity system and the latter is a high-affinity low-capacity one, similar to those in neutral amino acid transport. Neutral and basic amino acids showed cis-inhibitory and trans-stimulatory effects on L-NNA uptake in the presence or absence of Na⁺. N^G-Nitro-L-arginine methyl ester, another potent NO-synthase inhibitor, also had both effects, which were smaller than with amino acids. Conclusions. The present study clearly indicates that transport of L-NNA across the intestinal BBM occurs in the same manner as neutral amino acid transport. However, it is affected by both neutral and basic amino acids in the presence or absence of Na⁺ differently from that across plasma membranes of nonepithelial cells, because B^{0, +} and b^{0, +} amino acid transporters function partly in L-NNA transport across intestinal BBM.