Erythromycin contracts rabbit colon myocytes via occupation of motilin receptors

Erythromycin stimulates gastroduodenal motility via action on motilin receptors. We evaluated erythromycin as a colonic muscle motilin agonist using in vitro rabbit colon studies. Isolated myocytes contracted to erythromycin with a half-maximal effective concentration of 2 pM and peak shortening of 22.4 +/- 2.5% at 1 nM, which was superimposable with the response to motilin. 125I-labeled motilin binding to colon muscle homogenates was saturable and specific with a dissociation constant (Kd) of 0.39 nM and maximal binding (Bmax) of 41 +/- 3 fmol/mg protein. Motilin displaced specifically bound 125I-motilin, with a Kd of 0.31 nM. Erythromycin displaced 125I-motilin but was less potent, with an inhibitory constant of 84.0 nM. Bmax values from displacement studies were similar to the Scatchard data. Motilin receptor protection from alkylation by N-ethylmaleimide preserved contraction to motilin and erythromycin but not acetylcholine or cholecystokinin, whereas protection with erythromycin preserved contraction to motilin but not other agonists. In conclusion, erythromycin binds to colon muscle motilin receptors present in densities similar to reported values for the upper gut. Furthermore, erythromycin contracts colonic myocytes via specific action on motilin receptors. Thus erythromycin may have colonic motor-stimulating properties by action on motilin receptors.