INVOLVEMENT OF NITRIC-OXIDE IN INHIBITORY NEUROMUSCULAR-TRANSMISSION IN EQUINE JEJUNUM

Objectives-To evaluate the the role of nitric oxide (NO), vasoactive i ntestinal peptide (VIP), and a transmitter acting through an apamin-se nsitive mechanism in mediating inhibitory transmission in the equine j ejunal circular muscle, and to determine the distribution of VIP-and N O-producing nerve fibers in the myenteric plexus and circular muscle. Procedure-Circular muscle strips were suspended in tissue baths contai ning an oxygenated modified Krebs solution and attached to isometric f orce transducers. Responses to electrical field stimulation (EFS), tet rodotoxin, the NO antagonists L-N-nitro-arginine-methyl-ester (L-NAME) and N-nitro-L-arginine, apamin, VIP, authentic NO, and the NO donar s odium 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 fi bers in the circular muscle were positive for NADPH diaphorase,and VIP -like staining. EFS caused a frequency-dependent inhibition of contrac tile activity. Te trodotoxin prevented the EFS-induced inhibition of c ontractions. L-NAME (200 mu M) and apamin (0.3 mu M significantly (P < 0.01) reduced EFS-stimulated inhibition of contractile activity at mo st frequencies tested. The effects of L-NAME and apamin were additive. in their combined presence, EFS-induced excitation instead inhibition (196.7% increase at 5 Hz, n = 28, P < 0.01). inhibition of contractil e activity by EFS was mimicked by sodium nitroprusside. Authentic NO ( 3-6 mu M) abolished contractile activity. VIP induced a dose-dependent inhibition of contractile activity (89.1 +/- 6.3% reduction at approx imately 0.3 mu M, n = 16). Antagonism of NO syn thesis did not alter t he response to VIP. Conclusion-NO, VIP, and a substance acting through an apamin-sensitive mechanism appear to comediate inhibitory transmis sion in the equine jejunal circular muscle. Clinical Relevance-These f indings may suggest new therapeutic targets for motility disorders, su ch as agents that inhibit the synthesis or actions of NO.