Immunoreactive methionine-enkephalin secretion by porcine uterus

The current study examined the presence of immunoreactive methionine-enkephalin (ir-MENK) in porcine uterine fluid and endometrial extracts, characterized ir-MENK biochemically, and investigated the effect of ovarian steroids on uterine secretion of ir-MENK. Porcine uterine fluid was collected by flushing the uterine lumen with saline. Endometrial tissues were extracted with acetic acid. Both uterine fluid and endometrial extracts exhibited inhibition curves parallel to that of authentic MENK in the MENK RIA system. Sephadex G-15 gel filtration chromatographic profiles indicated that both concentrated uterine fluid and endometrial extracts contained two peaks of ir-MENK, a major peak which coeluted with standard MENK, and a minor peak eluting near the void volume (Vo). Reverse phase-HPLC chromatographic profiles also demonstrated two peaks of ir-MENK for concentrated uterine fluid and endometrial extracts, a major peak which coincided with standard MENK, plus a highly hydrophilic peak. The effect of ovarian steroids on the uterine secretion of ir-MENK was examined by measuring ir-MENK in uterine fluids from cyclic and pregnant gilts as well as ovariectomized, ovarian steroid-treated gilts. Day effects (P less than 0.01) were detected for cyclic and pregnant gilts, since values for ir-MENK increased between days 8 and 14 after onset of estrus. In ovariectomized gilts, treatment with progesterone (P4) increased the uterine secretion of ir-MENK (202 +/- 9 vs. 65 +/- 4 pg/ml for control, P less than 0.05). The combined treatment of P4 and estradiol did not further enhance secretion of ir-MENK, while treatment with estradiol did not alter ir-MENK levels relative to values for control gilts. These results indicate the presence of ir-MENK in porcine uterine fluid and endometrium, and suggest that uterine secretion of ir-MENK is regulated primarily by P4.