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 i n Ussing flux chambers. The competitive NO-synthase inhibitors N-G-met hyl-L-arginine (L-NMA), and N-G-nitro-L-arginine (L-NNA) and the effec ts of NO released from acidified sodium nitrite solution were evaluate d in tissues pretreated with guanethidine and atropine. Serosal L-NMA or L-NNA (10-300 mu M), but not N-G-methyl-D-arginine (D-NMA), produce d 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 mu Amps/cm(2), respectively; mucosal appli cation of L-NMA or L-NNA produced transient increases in I-sc. The A(5 0) (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) mu M, respectively. L-Arginine (0.1-10 mM), but not D-arginine, produced both a concentration-related reversal of L-NM A or L-NNA-induced increases in I-sc. Additionally, pretreatment with L-arginine blocked the L-NMA or L-NNA effects, suggesting a competitiv e interaction. L-NMA-mediated increases in I-sc were unaffected by bic arbonate-free buffer, whereas replacement of chloride ions with glucon ate ions almost completely attenuated the response to L-NMA. Further, the effects of L-NMA or L-NNA were blocked by tetrodotoxin or chloriso ndamine, 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 i n I-sc and PD, with an A(50) (95% CL) value of 47 (36-62) mu M. This s odium 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 mu M), an inhibitor of soluble guanylate cyclas e. Pretreatment with superoxide dismutase (25 U/ml), which prevents me tabolism of NO by superoxide anion, prolonged the activity of sodium n itrite (0.3 mM) from 5.3 +/- 0.2 to 9.4 +/- 0.7 min. These results sug gest that NO produces a net proabsorptive effect on ion transport in m ouse ileum, and that release of NO from nonadrenergic, noncholinergic nerves is involved in the tonic regulation of basal ion transport in t his tissue.