Involvement of nitric oxide in inhibitory neuromuscular transmission in equine jejunum.
OBJECTIVE
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.


METHODS
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.


CONCLUSIONS
NO, VIP, and a substance acting through an apamin-sensitive mechanism appear to comediate inhibitory transmission in the equine jejunal circular muscle.


CONCLUSIONS
These findings may suggest new therapeutic targets for motility disorders, such as agents that inhibit the synthesis or actions of NO.
Rakestraw
P C
PC
Department of Surgical and Radiological Sciences, University of California, Davis 95616, USA.
Snyder
J R
JR
Woliner
M J
MJ
Sanders
K M
KM
Shuttleworth
C W
CW
D000818:Animals; D001030:Apamin / Q000494:pharmacology; D004558:Electric Stimulation; D006651:Histocytochemistry; D006736:Horses; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000187:drug effects; D007583:Jejunum / Q000187:drug effects / Q000294:innervation / Q000502:physiology*; D009130:Muscle, Smooth / Q000187:drug effects / Q000294:innervation / Q000502:physiology*; D009252:NADPH Dehydrogenase / Q000032:analysis; D019331:NG-Nitroarginine Methyl Ester / Q000494:pharmacology*; D009469:Neuromuscular Junction / Q000187:drug effects; D009569:Nitric Oxide / Q000494:pharmacology / Q000502:physiology*; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors; D019335:Nitroarginine / Q000494:pharmacology*; D009599:Nitroprusside / Q000494:pharmacology; D009435:Synaptic Transmission / Q000187:drug effects; D014660:Vasoactive Intestinal Peptide / Q000494:pharmacology
D009599:Nitroprusside; D019335:Nitroarginine; D001030:Apamin; D009569:Nitric Oxide; D014660:Vasoactive Intestinal Peptide; D019001:Nitric Oxide Synthase; D009252:NADPH Dehydrogenase; D019331:NG-Nitroarginine Methyl Ester