In vitro effects of erythromycin, lidocaine, and metoclopramide on smooth muscle from the pyloric antrum, proximal portion of the duodenum, and middle portion of the jejunum of horses

Objective-To evaluate effects of erythromycin, lidocaine, and metoclopramid e on smooth muscle of the pyloric antrum (PA), proximal portion of the duod enum (PD), and middle portion of the jejunum (MJ) of horses. Sample Population-Strips of smooth muscle from 7 horses. Procedure-Isolated muscle strips were suspended in a bath and attached to i sometric force transducers. Once stable spontaneous contractions were obser ved, agents were added. Isometric stress responses were compared with the a mplitude of spontaneous contractions. Results-A single dose of erythromycin to the PA increased contractile ampli tude (CA) for the longitudinal smooth muscle (mean +/- SEM, 76 +/- 16 g/cm( 2)) but decreased CA for circular smooth muscle (-79 +/- 23 g/cm(2)). The i nhibitory effect was decreased by tetrodotoxin, N-G-nitro-L-arginine methyl ester, and a vasoactive intestinal peptide antagonist. Erythromycin increa sed CA for the MJ, which was maximal at 10(-4)M (171 +/- 36 g/cm(2)). Lidoc aine increased CA for the PD, which was maximal at 10(-4)M (60 +/- 5 g/cm(2 )). Metoclopramide increased the CA, which was maximal at 10(-4)M for the P A (75 +/- 26 g/cm(2)), PD (279 +/- 33 g/cm(2)), and MJ (456 +/- 59 g/cm(2)) . Conclusions-Regional differences in responses to erythromycin, lidocaine, a nd metoclopramide were evident in the gastrointestinal tract of horses. Met oclopramide increased CA in all tissues used, whereas erythromycin inhibite d CA in circular smooth muscle but stimulated CA in longitudinal smooth mus cle from the PA. Inhibition is caused by stimulation of inhibitory nerves a nd is mediated, in part, by nitric oxide and vasoactive intestinal peptide.