BACKGROUND: The immunosuppressive drugs tacrolimus and sirolimus may have a stimulatory influence on gastric emptying, in view of their macrolide structure. The aim of this study was to investigate in vitro the possible interaction of tacrolimus and sirolimus with motilin receptors in the rabbit antrum and duodenum. METHODS: Rabbit duodenum strips were mounted under a load of 1 g in 10 ml organ baths containing Krebs solution. Erythromycin, tacrolimus, cyclosporin, and sirolimus were added to the bathing solution in a cumulative way; in a second series, after incubation with cyclosporin (10(-7) mol/l), tacrolimus (10(-8) mol/l), or sirolimus (3 x 10(-8) mol/l), a cumulative concentration-response curve to erythromycin was obtained. The effect of cumulatively added tacrolimus and nle(13)-porcine motilin on the contractile response to electrical field stimulation was tested in strips from the rabbit gastric antrum. Displacement by tacrolimus of (125)I-nle(13)-porcine motilin bound to its receptor was tested with crude homogenates of the smooth-muscle layer of the rabbit antrum. RESULTS: In rabbit duodenum strips, carbachol (10(-5) mol/l) induced a stable and reproducible contraction. Erythromycin induced concentration-dependent contractions. Expressed as a percentage of the response to carbachol, the maximal attained effect was 78%; the EC(50) was 4.8 x 10(-7) mol/l. Tacrolimus (10(-8) to 10(-5) mol/l), cyclosporin (10(-8) to 10(-5) mol/l), and sirolimus (10(-8) to 3 x 10(-5) mol/l) had no influence. The response to erythromycin (10(-8) to 3 x 10(-5) mol/l) in the presence of cyclosporin (10(-7) mol/l), tacrolimus (10(-8) mol/l), or sirolimus (3 x 10(-8) mol/l) did not differ from that obtained with erythromycin alone, except for a decrease of the EC(50) in the presence of tacrolimus (2.2 x 10(-7) mol/l) (P<0.05 vs erythromycin alone). Motilin enhanced the response to electrical field stimulation of rabbit antral strips, but tacrolimus had no stimulatory effect. Tacrolimus weakly displaced motilin bound to its receptor. The pIC(50) was 4.97. CONCLUSIONS: As opposed to erythromycin, neither tacrolimus nor sirolimus showed a contractile effect in the rabbit duodenum. Tacrolimus did not activate the neural motilin receptor of the rabbit gastric antrum and had low affinity for the smooth-muscle motilin receptor. It is thus unlikely that these macrolide immunosuppressive drugs possess gastroprokinetic effects via motilin agonism.