Involvement of nitric oxide in inhibitory neuromuscular transmission in equine jejunum

OBJECTIVES: To evaluate the role of nitric oxide (NO), vasoactive intestinal peptide (VIP), and a transmitter acting through an apamin-sensitive mechanism in mediating inhibitory transmission in the equine jejunal circular muscle, and to determine the distribution of VIP-and NO-producing nerve fibers in the myenteric plexus and circular muscle. PROCEDURE: Circular muscle strips were suspended in tissue baths containing an oxygenated modified Krebs solution and attached to isometric force transducers. Responses to electrical field stimulation (EFS), tetrodotoxin, the NO antagonists L-N-nitro-arginine-methyl-ester (L-NAME) and N-nitro-L-arginine, apamin, VIP, authentic NO, and the NO donar sodium nitroprusside were tested. Immunostaining for VIP-like and NADPH diaphorase histochemical staining were performed on paraformaldehyde fixed tissue. RESULTS: Subpopulations of myenteric neurons and nerve fibers in the circular muscle were positive for NADPH diaphorase and VIP-like staining. EFS caused a frequency-dependent inhibition of contratile activity. Tetrodotoxin prevented the EFS-induced inhibition of contractions. L-NAME (200 microM) and apamin (0.3 microM) significantly (P < 0.01) reduced EFS-stimulated inhibition of contractile activity at most frequencies tested. The effects of L-NAME and apamin were additive. In their combined presence, EFS induced excitation instead of inhibition (196.7% increase at 5 Hz, n = 28, P < 0.01). Inhibition of contractile activity by EFS was mimicked by sodium nitroprusside. Authentic NO (3-6 microM) abolished contractile activity. VIP induced a dose-dependent inhibition of contractile activity (89.1 +/- 6.3% reduction at approximately 0.3 microM, n = 16). Antagonism of NO synthesis did not alter the response to VIP. CONCLUSION: NO, VIP, and a substance acting through an apamin-sensitive mechanism appear to comediate inhibitory transmission in the equine jejunal circular muscle. CLINICAL RELEVANCE: These findings may suggest new therapeutic targets for motility disorders, such as agents that inhibit the synthesis or actions of NO.