Carrier-mediated transport of N(G)-nitro-L-arginine, a nitric oxide synthase inhibitor, in the pigmented rabbit conjunctiva

In this study, the transport mechanism of N(G)-nitro-L-arginine (L-NA), a nitric oxide synthase inhibitor that may be useful for alleviating intraocular inflammation, was characterized in the pigmented rabbit conjunctiva. L-NA, when applied to the mucosal side of the conjunctiva, led to dose-dependent increases in the short-circuit current (I(sc)) at 37°C but not at 4°C or under the Na⁺-free condition. Serosally added 1 mM L-NA did not elicit any change in the I(sc). Mucosally added 1 mM L-NA elicited a net absorptive Na⁺ flux of 0.09 μEq/(cm²·hr), comparable with the I(sc) change. L-NA transport at 0.1 mM in the mucosal-to-serosal (ms) direction was 22 times greater than that in the serosal-to-mucosal direction. There was a good correlation between the ms flux of L-NA and the I(sc) changes elicited by L-NA under the same experimental conditions. L-NA transport was saturable, with a K(m) of 0.35 mM and a maximal flux of 290 pmol/(cm²·min). Hill analysis of L-NA flux observed at 0.1 mM L-NA in response to varying Na⁺ concentrations in the mucosal bathing fluid yielded a Hill coefficient of 0.98, suggesting a 1:1 coupling between Na⁺ and L-NA. Moreover, ms ³H-L-NA transport was inhibited by basic amino acids (L-Arg and L-Lys) and a neutral amino acid (L-Leu), but not by an acidic amino acid (L-Glu) and the D- stereoisomer of L-NA. In the case of L-Arg, inhibition was competitive with a K(l) of 0.034 mM. Taken together, the above findings are consistent with the involvement of the L-Arg transport system B^0.+ in the conjunctival transport of L-NA.