Turnover of lipid-bound arachidonate and biosynthesis of prostanoids in the endometrium and myometrium of the laying hen.

In vitro incorporation of [3H]arachidonate into various lipid fractions of minced avian endometrium and myometrium has shown that (i) phospholipids and triacylglycerols are labeled preferentially, whereas labeling of cholesteryl esters is only marginal; (ii) acylation reactions with arachidonoyl residues are very rapid in these tissues; (iii) incorporation of arachidonate into phosphatidylcholine and phosphatidylethanolamine seems to be a one-step reaction, whereas the transfer of arachidonate to phosphatidylinositol and phosphatidylserine proceeds after a time lag, suggesting that intermediate reaction steps are involved; and (iv) under conditions where tissue differences in free arachidonate levels are diminished by adding radioinert arachidonate to the incubation medium, endometrium incorporates more than twice as much [3H]arachidonate/unit of phospholipids than does the myometrium, whereas there is no such tissue-specific difference in the labeling of triacylglycerols. Furthermore, after a 90-min incubation phosphatidylserine and phosphatidylinositol not only are labeled markedly higher than either phosphatidylcholine or phosphatidylethanolamine in both tissues but the tissue-specific difference is also the highest (2.4- to 2.8-fold in the case of these phospholipids). Prostanoid synthesis from [3H]arachidonate in a crude membrane preparation in vitro has demonstrated that the myometrium possesses a significantly higher prostanoid-synthesizing capacity than the endometrium. This difference is particularly apparent (about 2.5-fold) in the case of prostaglandin E2. The results suggest that the endometrium is endowed with a special capacity to respond to signals with a rapid alteration of the level of free arachidonic acid, whereas the myometrium has the specific capacity to amplify its own contractions by an increased production of prostaglandins.