Motilin was isolated from the extract of the porcine duodenal mucosa as a gastrointestinal (GI) motility-stimulating peptide. Although responsiveness of motilin in GI tract is different depending on experimental conditions (in vivo, in vitro), GI regions and species, it has been demonstrated that motilin is a physiological mediator of phase III of migrating motor complexes (MMCs) in the stomach, at least in humans, dogs, house musk shrews (suncus), monkeys, and opossums. Since the discovery of motilin, rabbits have been used for in vitro research. Although small intestinal MMCs in rabbits have been observed in vivo, their characteristics are different from those in humans and dogs probably due to the lack of hunger periods and it is still unknown whether motilin regulates intestinal MMCs or not. The density of motilin receptors is highest in colonic enteric neurons and the responsiveness of motilin is highest in the colon. These findings are own characteristics of rabbits and it is possible that motilin regulates colonic motility related to defecation or caecotrophy, but this possibility has not been clarified at present. Further in vivo functional studies with measurements of colonic motility and plasma motilin are needed to understand the physiological role of motilin in regulation of GI motility in rabbits.

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The present study was designed to investigate the oral pharmacokinetics and bioavailability of roxithromycin, a macrolide class antimicrobial drug, in broiler chickens at a single dose of 20 mg/kg body weight. Plasma concentrations of roxithromycin were measured by High Performance Liquid Chromatography (HPLC) method using UV (ultraviolet) detector. The pharmacokinetic (PK) parameters were calculated from plasma concentrations versus time data using ‘PK Solver 2.0’ software. Following single intravenous administration, the volume of distribution at steady state (V_ss) and total body clearance (Cl) were 3.04 L/kg and 0.45 L/hr/kg, respectively. Following oral administration of roxithromycin at the dose rate of 20 mg/kg body weight in broilers, a characteristic short lag phase of 15 minutes in absorption was observed and thereafter mean maximal plasma concentration (C_max: 3.60 μg/ml) was achieved at 2 hr. The mean values of elimination half-life (t_1/2β) and mean resident time (MRT) were 8.30 and 9.57 hr, respectively. Calculated oral bioavailability (F) ranged from 51.95 to 62.57% with a mean value of 56.86%. Based on pharmacokinetic-pharmacodynamic (PK-PD) integration efficacy predictor, oral dose at 20 mg/kg body weight of roxithromycin twice a day, is predicted to be effective for treating the susceptible bacterial infections in the broiler chickens with a MIC value ≤ 1.0 μg/ml.

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