Electrophysiological characterization of a motilin agonist, GM611, on rabbit duodenal smooth muscle

Abstract

Effects of motilin and a newly synthesized erythromycin derivative, GM611, on membrane potential and currents of rabbit duodenal smooth muscle have been investigated by intracellular potential recording and whole cell patch-clamp technique and compared with results from contractile experiments. Motilin and GM611 (0.01-100 nM) dose dependently produced slowly sustained depolarizations (half-maximal effective dose = 0.15 and 3.9 nM for motilin and GM611, respectively) but exhibited biphasic effects on spike activities superimposed on slow waves. With small depolarizations, the number of spike discharges increased, whereas larger depolarizations markedly reduced spike amplitude. Motilin-induced (or GM611-induced) depolarization appeared to be associated with the activation of monovalent cation-selective channels, and the reduction in the spike amplitude appeared mainly to be associated with inhibition of voltage-dependent Ca2+ channels. Furthermore, data from patch-clamp experiments suggested that Ca2+ release occurred from heparin-sensitive internal stores upon stimulation of motilin receptors by these agonists. Possible implications of these electrophysiological effects in motilin- or GM611-induced tonic and phasic contractions have been discussed.

In vitro Pharmacological Characterization of Mitemcinal (GM-611), the First Acid-Resistant Non-Peptide Motilin Receptor Agonist, in Smooth Muscle of Rabbit Small Intestine

“…This also indicates that the action of mitemcinal, as well as motilin, is not mediated by neural activities but by directly mediated effects on intestinal smooth muscle motilin receptors in rabbits. These findings are consistent with previous reports [12,13,23,24] in which, by experiments in the isolated rabbit small intestine, the contractile response to pMTL or some EMA derivatives was found not to be affected by TTX, atropine, hexamethonium, mepyramine (H 1 blockade), naloxone or CP-99994 (NK 1 blockade). These results suggested that contractions evoked by pMTL or EMA derivatives in the isolated rabbit small intestine are directly mediated through motilin receptors on smooth muscle cells, not by the intramural nervous system or by muscarinic, nicotinic, histamine, opiate or neurokinin receptors.…”

“…The EC 50 values of mitemcinal and pMTL in the present study were 14.33 8 2.55 nmol/l (n = 6) and 4.33 8 0.43 nmol/ l (n = 10). The result is consistent with the previous report [13] that both of mitemcinal and pMTL induced the dosedependent tonic contraction in the rabbit duodenal muscle strip with EC 50 values 11.9 nmol/l for mitemcinal and 3.0 nmol/l for pMTL, respectively. The selective motilin antagonist, GM-109, shifted the concentration-dependent curves for mitemcinal and pMTL right in a concentration-dependent manner.…”

“…These results suggested that contractions evoked by pMTL or EMA derivatives in the isolated rabbit small intestine are directly mediated through motilin receptors on smooth muscle cells, not by the intramural nervous system or by muscarinic, nicotinic, histamine, opiate or neurokinin receptors. It is well known that the contraction induced by pMTL in the isolated rabbit small intestine is strongly dependent on extracellular Ca 2+ [13,[23][24][25] . In the present study, contractile responses to mitemcinal and pMTL were markedly diminished by pretreatment with verapamil, which blocks the voltage-dependent L-type Ca channel, or by removing Ca 2+ from the medium.…”

“…7 a). The previous report [13] demonstrated that 10 mol/l of mitemcinal was high enough to strongly inhibit L-type Ca channel current in the rabbit duodenum, whereas a wide range concentration of mitemcinal (0.1-100 nmol/l) could not enhance L-type Ca channel directly in the same preparation.…”

“…Before the experiments, each strip was subjected to repeated stimulation with 100 mol/l ACh until a reproducible response was obtained. In isolated rabbit small intestinal muscle strips, contractions induced by EMA and its derivatives as well as pMTL consisted of tonic contractions (basal tone of the muscle) and phasic contractions (oscillatory components) [13] ( fig. 2 a, b).…”