Tonic regulation of mouse ileal ion transport by nitric oxide

The possible role of nitric oxide (NO) in the regulation of intestinal ion transport was studied in isolated sheets of mouse ileum mounted in Ussing flux chambers. The competitive NO-synthase inhibitors N(G)-methyl-L-arginine (L-NMA), and N(G)-nitro-L-arginine (L-NNA) and the effects of NO released from acidified sodium nitrite solution were evaluated in tissues pretreated with guanethidine and atropine. Serosal L-NMA or L-NNA (10-300 μM), but not N(G)-methyl-D-arginine (D-NMA), produced a sustained concentration-related increase in short-circuit current (I(sc)) and potential difference (PD) with maximal I(sc) increases of 50.8 ± 8.2 and 45.5 ± 5.8 μAmps/cm², respectively; mucosal application of L-NMA or L-NNA produced transient increases in I(sc). The A₅₀ (and 95% CL) values for serosal L-NMA and L- NNA were 25.6 (15.7-41.9) and 8.7 (5.1-14.9) μM, respectively. L-Arginine (0.1-10 mM), but not D-arginine, produced both a concentration-related reversal of L-NMA or L-NNA-induced increases in I(sc). Additionally, pretreatment with L-arginine blocked the L-NMA or L-NNA effects, suggesting a competitive interaction. L-NMA-mediated increases in I(sc) were unaffected by bicarbonate-free buffer, whereas replacement of chloride ions with gluconate ions almost completely attenuated the response to L-NMA. Further, the effects of L-NMA or L-NNA were blocked by tetrodotoxin or chlorisondamine, suggesting neural actions involving ganglionic transmission. Serosal application of acidified sodium nitrite solution (0.03-1.0 mM) produced a concentration- dependent and transient (3-6 min) decrease in I(sc) and PD, with an A₅₀ (95% CL) value of 47 (36-62) μM. This sodium nitrite effect on ileal I(sc) was blocked by the pretreatment of tissue with hemoglobin (100 nM), which is known to trap free NO avidly, or methylene blue (5 μM), an inhibitor of soluble guanylate cyclase. Pretreatment with superoxide dismutase (25 U/ml), which prevents metabolism of NO by superoxide anion, prolonged the activity of sodium nitrite (0.3mM) from 5.3 ± 0.2 to 9.4 ± 0.7 min. These results suggest that NO produces a net proabsorptive effect on ion transport in mouse ileum, and that release of NO from nonadrenergic, noncholinergic nerves is involved in the tonic regulation of basal ion transport in this tissue.