Evaluation of nitric oxide as an inhibitory neurotransmitter in the equineventral colon

Objective-To determine the role of nitric oxide and an apamin-sensitive non adrenergic noncholingeric inhibitory transmitter on contractility of the ve ntral colon of horses. Sample population-Strips of the circular and longitudinal muscle layers and taenia of the ventral colon from 14 horses. Procedure-Muscle strips were suspended in tissue baths and attached to forc e transducers. Contractile activity of circular, longitudinal, and taenia m uscle strips in response to electrical field stimulation was measured after addition of apamin and a nitric oxide inhibitor, N-nitro-L-arginine methyl ester (L-NAME). Results-Electrical field stimulation reduced contractile activity in the ci rcular muscle layer and taenia but not the longitudinal muscle layer. Addit ion of L-NAME significantly reduced inhibitory contractile activity at all frequencies for the circular muscle layer whereas a significant effect was evident for the taenia only at the highest frequency. The combination of L- NAME and apamin resulted in a significant reduction in inhibition of the ta enia at all frequencies but for circular muscle only at lower frequencies. Conclusions and Clinical Relevance-Nitric oxide and an apamin-sensitive neu rotransmitter appear to mediate a component of inhibitory transmission in t he circular muscle and taenia, but not the longitudinal muscle layer, of th e equine ventral colon. Nitric oxide has a role in regulating contractile a ctivity of the equine ventral colon, and nitric oxide synthase inhibitors m ay be useful in horses with ileus of the large colon.