Characterization of prostaglandin E2 receptors (EP2, EP4) in the horse oviduct

Biological effects of prostaglandin E2 are mediated via one of four receptors designated EP1, EP2, EP3 and EP4 which are encoded by separate genes. In general, EP1 and EP3 induce smooth muscle contraction whereas EP2 and EP4 induce smooth muscle relaxation. The objective of the current study was to characterize the expression of the genes for PGE2 receptors (EP2 and EP4) in the horse oviduct based upon immunohistochemistry (IHC) and quantitative PCR (qPCR). Oviductal tissue was collected from mares at estrus (n = 5), at 5 days post-ovulation (n = 4), and from prepubertal mares (n = 5). Isthmic and ampullar regions of the oviduct were fixed for IHC or preserved for RNA isolation. Prostaglandin E2 receptors EP2 and EP4 were strongly expressed by the luminal epithelium of both the isthmic and ampullar regions of the horse oviduct with a lesser immuno-expression noted within the smooth muscle in both regions of the oviduct. Based upon qPCR, relative amounts of EP2 or EP4 mRNA did not differ across estrous cycle stage or from prepubertal mares. However, across region and estrous cycle stage, relative amount of EP2 was greater (P < 0.05) than EP4, and relative amount of EP2 mRNA was greater (P < 0.001) in the ampullar compared with the isthmic oviduct. The relative roles of these receptors in regulating oviduct function in the mare remains to be determined.