Prostaglandin (PG) F output from the guinea pig uterus increases after
Day 11 of the oestrous cycle and oestradiol (whose output from the ovary increases
after Day 10) acting on a progesterone-primed uterus is probably the physiological
stimulus for this increase in uterine PGF„ release. The availability of free Act
arachidonic acid in the endometrium is the rate-limiting step in PGF synthesis.
The aim of the present studies was to determine the source of arachidonic acid
for endometrial PGF synthesis. Initially, the endometrium ^as maintained in tissue culture in the presence of tritiated arachidonic acid ( H-AA) and the
radioactive contents of the various lipids was subsequently measured. The
uptake of H-AA into phosphatidylcholine (PC) and phosphatidylethanolamine (PE),
was significantly higher on Day 15 (day of high PGF^ synthesis) than on Day 7
(day of low PGF„ synthesis) . Arachidonic acid couid be ir.coroorated into these Act
phospholipids by an increase in the mass amounts of phospholipids. However
results indicated that there was no significant difference in the amount of
phospholipids present on Day 7 and Day 15. Arachidonic acid could be incorporated
into the phospholipids either by an increase in de novo synthesis or by the
addition to lysophospholipid. The rates-of PC synthesis and phosphatidylinositol
synthesis did not differ on Days 7 and 15, indicating that the measured uptake
of arachidonic acid into phospholipid is not due to increased de novo synthesis
but rather to increased addition to lysophospholipid. Oestradiol may be the
stimulus for this latter process
