PREFERENTIAL ESTERIFICATION OF ARACHIDONIC-ACID INTO ETHANOLAMINE PHOSPHOLIPIDS IN EPITHELIAL-CELLS FROM OVINE ENDOMETRIUM

In sheep, the pulsatile release of prostaglandin F-2 alpha by the endo metrium is necessary to achieve luteolysis which occurs at the end of the oestrous cycle. The production of prostaglandins is known to depen d upon the availability of arachidonic acid, the fatty acid precursor of prostaglandin biosynthesis. Consequently, the mechanisms controllin g intracellular amounts of arachidonate may be involved in the regulat ion of prostaglandin synthesis. Since arachidonic acid is mostly found in phospholipids and the endometrial epithelium is the primary source of prostaglandin F-2 alpha during luteolysis, the fate of arachidonic acid when incorporated into epithelial cells from the ovine uterus wa s investigated. Endometrial epithelial cells isolated from cyclic ewes at day 15 after oestrus were cultured in the presence of [H-3]arachid onic acid. Incorporation and distribution of the radiolabelled arachid onic acid into the various phospholipid classes were examined using HP LC. We observed that ethanolamine glycerophospholipids contained 61% o f the total tritiated arachidonic acid incorporated into cellular lipi ds, whereas phosphatidylinositols, phosphatidylcholines and phosphatid ylserines contained 17%, 13% and 4.7%, respectively. In addition, the radioactivity measured within phosphatidylethanolamines was preferenti ally detected in the 1-alkenyl-2-acyl (44%) forms of ethanolamine phos pholipids, also called plasmalogens. The kinetic study of arachidonic acid uptake into ethanolamine phospholipids showed that arachidonic ac id was rapidly esterified into the diacyl forms and then uptake decrea sed, whereas the incorporation increased continuously into the plasmal ogen forms for at least 24 h. These results demonstrate that the prima ry pool of esterified arachidonic acid is found in ethanolamine plasma logens of epithelial cells from the ovine endometrium. The high arachi donate content of ethanolamine plasmalogens suggests that these phosph olipids play a crucial role in the control of arachidonic acid availab ility and ultimately in the regulation of prostaglandin synthesis.