Roles of prostaglandin E2-EP1 receptor signaling in regulation of gastric motor activity and emptying

It is widely accepted that the inhibition of gastric motor activity as well as the maintenance of gastric mucosal blood flow and mucous secretion are important for the homeostasis of the gastric mucosa. The present study was performed to ascertain whether or not endogenous PGs, which can protect the stomach from noxious stimuli, affect gastric motor activity and emptying. The myoelectrical activity of rat gastric smooth muscle was increased at intragastric pressures of over 2 cmH(2)O. Replacement of intragastric physiological saline with 1 M NaCl solution significantly increased PGI(2) and PGE(2) in stomach and suppressed the myoelectrical activity under a pressure of 2 cmH(2)O by 70%. Indomethacin inhibited the suppression of myoelectrical activity by 1 M NaCl. The myoelectrical activity under a pressure of 2 cmH(2)O was suppressed by continuous infusion of a selective EP1 agonist (ONO-DI-004, 3-100 nmol·kg(-1)·min(-1)) into the gastric artery in a dose-dependent manner, but not by that of the PGI receptor agonist beraprost sodium (100 nmol·kg(-1)·min(-1)). Suppression of myoelectrical activity with 1 M NaCl was inhibited by continuous infusion of a selective EP1 antagonist (ONO-8711, 100 nmol·kg(-1)·min(-1)) into the gastric artery. Furthermore, gastric emptying was tested in EP1 knockout mice and their wild-type counterparts. Gastric emptying was strongly suppressed with intragastric 1 M NaCl in wild-type mice, but this 1 M NaCl-induced suppression was not seen in EP1 knockout mice. These results suggest that PGE(2)-EP1 signaling has crucial roles in suppression of myoelectrical activity of gastric smooth muscles and inhibition of gastric emptying and that EP1 is an obvious target for drugs that control gastric emptying.