DOES MOTILIN STIMULATE THE GASTROINTESTINAL MOTILITY OF THE PIG - IN-VITRO STUDY USING SMOOTH-MUSCLE STRIPS AND DISPERSED MUSCLE-CELLS

To clarify the physiological role of motilin in the pig gastrointestin al (GI) tract, effect of Leu(13)-porcine motilin (LMT) on the contract ility of GI smooth muscle was investigated in studies using isolated m uscle strips and dispersed muscle cells, LMT produced no contraction i n either longitudinal muscle (LM) or circular muscle (CM) of the stoma ch (fundus, corpus, antrum), duodenum, ileum and colon even at 1 mu M. Pretreatment with LMT (1 nM-1 mu M) did not potentiate the contractil e response to acetylcholine (ACh) in each muscle strip. Dispersed cell s from the duodenum responded to ACh in a concentration dependent mann er (EC(50)=10 pM), but not to LMT even at a high concentration (10 mu M). Electrical field stimulation (EFS) caused a frequency dependent (0 .2-10 Hz) contraction of the duodenal LM that was almost completely in hibited by atropine or tetrodotoxin. EFS caused the relaxation of duod enal CM in a frequency dependent manner (0.1-10 Hz), This relaxation w as not inhibited by atropine, propranolol, phentolamine or guanethidin e, indicating the involvement of noncholinergic, nonadrenergic (NCNA) nerves. N-G-nitro L-arginine methylester (L-NAME, 100 mu M) attenuated the EFS-induced relaxation and the inhibition at low frequency was la rger than that at high frequency. L-Arginine prevented the inhibition by L-NAME but D-arginine did not. LMT (1 nM-1 mu M) had no influence o n EFS induced cholinergic contraction of LM and EFS induced NCNA relax ation of CM layer. The present in vitro studies indicate that motilin is ineffective in producing contraction and in modulating the autonomi c neuroeffector transmission of the pig GI smooth muscle, and suggest that pig GI smooth muscle lacks functional mortilin receptors.