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

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Abstract
Objective—To evaluate effects of erythromycin, lidocaine, and metoclopramide on smooth muscle of the pyloric antrum (PA), proximal portion of the duodenum (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 isometric force transducers. Once stable spontaneous contractions were observed, agents were added. Isometric stress responses were compared with the amplitude of spontaneous contractions.Results—A single dose of erythromycin to the PA increased contractile amplitude (CA) for the longitudinal smooth muscle (mean ± SEM, 76 ± 16 g/cm2) but decreased CA for circular smooth muscle (–79 ± 23 g/cm2). The inhibitory effect was decreased by tetrodotoxin, NG-nitro-L-arginine methyl ester, and a vasoactive intestinal peptide antagonist. Erythromycin increased CA for the MJ, which was maximal at 10–4M(171 ± 36 g/cm2). Lidocaine increased CA for the PD, which was maximal at 10–4M (60 ± 5 g/cm2). Metoclopramide increased the CA, which was maximal at 10–4Mfor the PA (75 ± 26 g/cm2), PD (279 ± 33 g/cm2), and MJ (456 ± 59 g/cm2).Conclusions—Regional differences in responses to erythromycin, lidocaine, and metoclopramide were evident in the gastrointestinal tract of horses. Metoclopramide increased CA in all tissues used, whereas erythromycin inhibited CA in circular smooth muscle but stimulated CA in longitudinal smooth muscle from the PA. Inhibition is caused by stimulation of inhibitory nerves and is mediated, in part, by nitric oxide and vasoactive intestinal peptide. (Am J Vet Res2000;61:413–419)