Background & aims: To examine the involvement of EP(3) receptors in physiological regulation of duodenal HCO(3)(-) secretion, we disrupted the gene encoding EP receptors in mice by homologous recombination and evaluated acid-induced HCO(3)(-) secretion, which is physiologically important in the mucosal defense against acid injury, using EP(1)- and EP(3)-receptor knockout mice.
Methods: The experiments were performed in the following 3 groups of mice after 18 hours of fasting: wild-type [WT (+/+)] mice, EP(1)-receptor knockout [EP(1) (-/-)] mice, and EP(3)-receptor knockout [EP(3) (-/-)] mice. Under urethane anesthesia, the proximal duodenal loop was perfused with saline that was gassed with 100% O(2), heated at 37 degrees C, and kept in a reservoir, and HCO(3)(-) secretion was measured at pH 7.0 using a pH-stat method and by adding 5 mmol/L HCl.
Results: The duodenum of WT (+/+) mice increased HCO(3)(-) secretion in response to luminal perfusion of prostaglandin E(2) and forskolin as well as mucosal acidification. The latter effect was significantly inhibited by prior administration of indomethacin. HCO(3)(-) response to acid was observed in EP(1) (-/-) mice but disappeared totally in EP(3) (-/-) animals, although the acidification increased mucosal PGE(2) generation by similar degrees in all groups. The HCO(3)(-) stimulatory action of PGE(2) was also absent in EP(3) (-/-) but not EP(1) (-/-) mice, but forskolin effect was observed in both groups of animals, similar to WT (+/+) mice. Perfusion of the duodenum with 20 mmol/L HCl for 4 hours caused severe damage in EP(3) (-/-) mice and WT (+/+) animals pretreated with indomethacin, but not in EP(1) (-/-) mice.
Conclusions: The presence of EP(3)-receptors is essential for maintaining duodenal HCO(3)(-) secretion and mucosal integrity against luminal acid.