Five guinea-pigs actively immunized against a prostaglandin F2alpha(PGF2alpha)-bovine serum albumin conjugate showed elongated oestrous cycles. During these, corpora lutea were maintained in a functional secretory state as indicated by plasma progesterone levels. The results are compatible with the view that the PGF2alpha antibodies neutralized the PGF2alpha released from the uterus and thus prevented its normal luteolytic effect. Similar patterns of progesterone secretion were observed in two hysterectomized animals and in two animals with intra-uterine implants of indomethacin.

The supply of free arachidonic acid from phospholipids is generally regarded as the rate-limiting step for prostaglandin (PG) synthesis by tissues. Two enzymes involved in arachidonic acid uptake into, and release from, phospholipids are acyl-CoA:lysophospholipid acyltransferase (ACLAT) and phospholipase A2 (PLA2), respectively. PGF2 alpha produced by the endometrium induces luteolysis in several species including guinea-pigs. Thimerosal, an inhibitor of ACLAT, and aristolochic acid, an inhibitor of PLA2, both reduced, in a concentration-dependent manner, the output of PGF2 alpha from guinea-pig endometrium cultured for 24 h on days 7 and 15 of the oestrous cycle. This study showed that the continual production of PGF 2 alpha by guinea-pig endometrium is not only dependent upon the activity of PLA2 for releasing free arachidonic acid for PGF2 alpha synthesis, but also on the incorporation of arachidonic acid into the phospholipid pool by the activity of ACLAT. The inhibitory effects of thimerosal and aristolochic acid on the outputs of PGE2 and 6-keto-PGF1 alpha were less marked, particularly on day 7 when the low output of PGE2 was unaffected and the output of 6-keto-PGF1 alpha was increased at the lower concentrations of thimerosal. This finding indicates that there are different pools of arachidonic acid bound as phospholipid for the syntheses of PGF2 alpha and 6-keto-PGF1 alpha by guinea-pig endometrium.

Summary. Melittin, an activator of phospholipase (PL) A-2, increased the outputs of prostaglandin (PG) F-2\g=a\and 6-keto-PGF-1\g=a\, but not of PGE-2, from Day-7 guineapig uterus superfused in vitro. Reducing the extracellular calcium concentration (by omitting calcium chloride from the superfusing fluid) partially inhibited the stimulatory effect of melittin on uterine PG production. (an intracellular calcium antagonist) completely prevented the stimulation of PGF-2\g=a\and 6-keto-PGF-1\g=a\ output by melittin, although the production of both PGs tended to increase after stopping the melittin and TMB-8 treatments. TMB-8 also inhibited the increases in outputs of PGF\x=req-\ 2\g=a\,6-keto-PGF-1\g=a\ and PGE-2 and prevented contraction of the uterus induced by exogenous PLA-2. Trifluoperazine (a calmodulin antagonist) had no inhibitory effect on the increases in outputs of PGF-2\g=a\and 6-keto-PGF-1\g=a\ produced by melittin; it potentiated the stimulatory effect of melittin on 6-keto-PGF-1\g=a\ output and allowed melittin to increase PGE-2 output. When melittin was applied twice to the superfused uterus with an interval of 1 h between each treatment, partial refractoriness of the responses to melittin was seen: the magnitudes of the increases in PGF-2\g=a\and 6\ x =r eq-\ keto-PGF-1\g=a\ outputs were 40\p=n-\50%less after the second treatment than after the first treatment. These results show that melittin stimulates the synthesis of PGF-2\g=a\and PGI-2 (measured as 6-keto-PGF-1\g=a\) in guinea-pig uterus by mechanisms which are calcium dependent. The results are compatible with there being a protein of similar functional activity to melittin in the guinea-pig uterus, which may be involved in the stimulation of endometrial PGF-2\g=a\ synthesis.