N.L. Poyser

The control of prostaglandin production by the endometrium in relation to luteolysis and menstruation

Oestradiol acting on a progesterone-primed uterus stimulates prostaglandin (PG) F2 alpha synthesis by the endometrium. In some species (notably the sheep, cow and goat) oxytocin released from the ovary also forms part of the physiological stimulus for increased endometrial PGF2 alpha production. The corpus luteum contains high concentrations (> 1 microgram/g tissue) of this peptide in these species. The intracellular mechanisms by which these three hormones control endometrial PGF2 alpha synthesis and release are far from clear. Oxytocin stimulates the synthesis of inositol phosphates and diacylglycerol in the endometrium of some species, but whether this pathway is involved in endometrial PGF2 alpha synthesis is still open to question. There is evidence that increased endometrial PGF2 alpha synthesis is dependent upon increased endometrial protein synthesis but, apart from the recorded effects of steroid hormones on the concentrations of phospholipase A2, prostaglandin H synthase and oxytocin receptors, it is not known what other endometrial proteins are involved. Some disorders of menstruation are associated with abnormal PG production by the endometrium, but the reasons for this abnormality are not clear. During early pregnancy an increase in PGF2 alpha synthesis by the endometrium is prevented, except in the pig where the PGF2 alpha produced is directed from the venous drainage to the uterine lumen. In those species in which endometrial PGF2 alpha synthesis is dependent upon oxytocin secreted by the ovary, the conceptus secretes an interferon-tau (previously named trophoblast protein-1) which prevents oestradiol and oxytocin acting on a progesterone-primed uterus from stimulating endometrial PGF2 alpha synthesis. The identities of the factors produced by the conceptus which prevent endometrial PGF2 alpha synthesis during early pregnancy in other species are not known, although it is clear that they are not interferons.

The identification of trihydroxyeicosatrienoic acids as products from the incubation of arachidonic acid with washed blood platelets

Arachidonic acid is converted by washed platelets from man, horse and dog into a mixture of 8,9,12-trihydroxyeicosa-5,10,14-trienoic acid and 8,11,12-trihydroxyeicosa-5,9,14-trienoic acid (termed 8,9,12-THETA and 8,11,12-THETA respectively and THETA collectively). Gas chromatographic--mass spectrometric evidence of structure is discussed.

Effects of oestradiol, progesterone, hydrocortisone and oxytocin on prostaglandin output from the guinea-pig endometrium maintained in tissue culture

The effects of oestradiol, oxytocin, progesterone and hydrocortisone in vitro on prostaglandin (PG) output from guinea-pig endometrium, removed on days 7 and 15 of the oestrous cycle and maintained in tissue culture for 3 days, have been investigated. Oestradiol (3.7 to 3700 nM) and oxytocin (2 to 200 pM) did not stimulate endometrial PGF2 alpha output, thus not confirming the findings of a previous report (Leaver & Seawright, 1982), nor did they stimulate the outputs of PGE2 and 6-keto-PGF1 alpha. In fact, oestradiol (3700 nM) inhibited the outputs of PGF2 alpha, PGE2 and, to a lesser extent, 6-keto-PGF1 alpha. Progesterone (3.2 to 3200 nM) inhibited the outputs of PGF2 alpha and PGE2; hydrocortisone (2.8 to 2800 nM) had no effect on endometrial PG output. These findings indicate that the inhibitory effect of progesterone on endometrial PG synthesis and release in the guinea-pig is not due to progesterone having a glucocorticoid-like action. Furthermore, progesterone had no effect on 6-keto-PGF1 alpha output, suggesting that the mechanisms controlling endometrial PGI2 synthesis (as reflected by measuring 6-keto-PGF1 alpha) are different from those controlling endometrial PGF2 alpha and PGE2 synthesis.

Uptake of [3H]-arachidonic acid by human endometrium. Differences between normal and menorrhagic tissue

Human proliferative and secretory endometrium from normal women and from menorrhagic patients was maintained in culture for up to 24 h in the presence of [3H]-arachidonic acid (3H-AA). This prostaglandin (PG) precursor was incorporated into endometrial neutral lipids and phospholipids in a time-dependent manner. Uptake of 3H-AA into phospholipids was significantly higher in normal secretory endometrium than in normal proliferative endometrium. However, this increased uptake of 3H-AA into phospholipids between the 2 phases of the cycle did not occur in menorrhagic endometrium. In contrast, uptake of 3H-AA into neutral lipids (especially triglyceride) was approximately 2-fold higher in menorrhagic endometrium compared to normal endometrium at both stages of the cycle, particularly during the proliferative phase. Abnormalities apparently exist in menorrhagic endometrium in the uptake processes which control arachidonic acid (AA) turnover. These abnormalities may be responsible, in part for abnormal PG production by menorrhagic endometrium.

Elongation of oestrous cycle in the guinea-pig following active immunisation against prostaglandin F2α

Abstract Five guinea-pigs actively immunised against a PGF2α-bovine serum albumin conjugate showed elongated cycles, whereas 5 control animals injected with a mixture of PGF2α and bovine serum albumin showed no change in cycle length. These results are compatible with the hypothesis that the uterine luteolytic hormone in the guinea-pig is PGF2α.

Release of prostaglandins I2 and E2 from the perfused mesenteric arterial bed of normotensive and hypertensive rats. Effects of sympathetic nerve stimulation and norepinephrine administration

The spontaneous output of prostaglandin (PG) I2 from the perfused mesenteric arterial bed in vitro was significantly higher in hypertensive rats than in normotensive rats. Sympathetic nerve stimulation (at 1OHz) of the mesenteric arterial bed from normotensive rats caused a rapid and short-lived (less than 4 min) two-fold increase in PGI2 output and a smaller increase in PGE2 output. Sympathetic nerve stimulation (at 1OHz) of the mesenteric arterial bed from hypertensive rats failed to increase PGI2 and PGE2 output. It is not possible to conclude whether this lack of response is a cause or a result of hypertension. Surprisingly, norepinephrine administration stimulated PGI2 and PGE2 release from the mesenteric arterial bed of both normotensive and hypertensive rats. Obviously, differences exist in the responsiveness of rat mesenteric arteries to endogenous and exogenous norepinephrine concerning PG release between the normotensive and hypertensive states.

Tissue levels of prostaglandins and what do they mean? Studies on the guinea-pig uterus.

The ratios of the concentrations of PGF2 alpha, PGE2 and 6-keto-PGF1 alpha in guinea-pig uterine horns, which were removed and placed in ethanol in 1.5 to 2 min, were 0.3:1.0:0.6 on day 7 and 13.8:1.0:0.8 on day 15 of the oestrous cycle. Adding indomethacin (10 micrograms/ml) to the ethanol had no significant effect on the tissue levels observed. These ratios were similar to the ratios of the outputs of PGF2 alpha, PGE2 and 6-keto-PGF1 alpha from the guinea-pig uterus (0.6:1.0:0.9 on day 7 and 7.6:1.0:1.5 on day 15), but were different (particularly on day 7, but only for 6-keto-PGF1 alpha on day 15) to the ratios of the amounts of the three PGs synthesized by homogenates of the guinea-pig uterus (7.2:1.0:2.4 on day 7 and 11.7:1.0:3.3 on day 15). Consequently, the measurement of tissue levels of PGs in the guinea-pig uterus reflects PG synthesis by intact tissue and changes in this synthesis, rather than PG synthesis by homogenates (broken cell preparations). Therefore, it appears meaningful to measure levels of PGs in the guinea-pig uterus since they reflect uterine PG output. Separation of the endometrium from the myometrium, which involved handling and mild trauma, stimulated uterine PG levels, but the ratio of the levels of PGF2 alpha, PGE2 and 6-keto-PGF1 alpha in the endometrium was still similar to that found in the non-separated uterus.

Effects of exogenous and endogenous prostaglandins on the fast phase of contraction of the guinea-pig vas deferens produced by electrical field stimulation

The initial, fast phase of contraction of the guinea-pig vas deferens produced by electrical field stimulation (10 pulses) was dose-dependently and completely inhibited by prostaglandin (PG) E2, sulprostone and, at high concentrations, by cicaprost. Sulprostone was more potent than PGE2 indicating that the EP3 receptor was involved. Cicaprost (a PGI2 analogue) apparently had weak EP3 receptor against activity. At low concentrations, cicaprost potentiated the contractions of the vas deferens, presumably by acting on an IP receptor. Exogenous arachidonic acid also dose-dependently and completely inhibited contractions of the guinea-pig vas deferens. The action of arachidonic acid was delayed when compared to PGE2 and was inhibited by indomethacin, suggesting that the arachidonic acid was converted to PGE2 by the vas deferens. Indomethacin (1.4 to 6.0 microM) had no significant, potentiating effect on the contractions of the guinea-pig vas deferens which suggests that endogenous PGs do not normally inhibit this fast phase of contraction. In higher concentrations, the contractions were reduced by indomethacin. The fast phase of concentration of the guinea-pig vas deferens consisted of 3 components. PGE2, sulprostone and arachidonic acid inhibited all components. The order of inhibition of the components was component 2, then component 3, followed by component 1.

Is the inhibitory effect of progesterone on endometrial prostaglandin F2α production due to an inhibition of protein synthesis

Progesterone and a high concentration of oestradiol (i) reduced the outputs of prostaglandin (PG) F2 alpha and, to a lesser extent, PGE2 from Day-7 and Day-15 guinea-pig endometrium in culture, but had little or no effect on the output of 6-keto-PGF1 alpha, (ii) prevented the increase in PGH synthase concentrations which normally occur in Day-7 and Day-15 guinea-pig endometrium during culture, and (iii) reduced the synthesis of secreted proteins by Day-15 guinea-pig endometrium in culture. These findings suggest that the inhibitory effect of progesterone and of high concentrations of oestradiol on endometrium PGF2 alpha synthesis is due to an inhibition of the syntheses of proteins involved in PGF2 alpha production.

Effects of hydrocortisone, oestradiol and progesterone on A23187-stimulated prostaglandin output from the guinea-pig uterus superfused in vitro

Hydrocortisone (10 micrograms/ml) had no effect on the basal outputs and A23187-stimulated outputs of PGF2 alpha, PGE2 and 6-keto-PGF1 alpha from the Day 15 guinea-pig uterus superfused in vitro. These findings indicate that the high output of PGF2 alpha from the guinea-pig uterus during the last one-third of the oestrous cycle is not modulated by the adrenal glucocorticoid hormones. Progesterone (10 micrograms/ml) had no effect on the A23187-induced increases in PG output from the Day 15 guinea-pig uterus. However, oestradiol (10 micrograms/ml but not 1 microgram/ml) significantly reduced the increases in outputs of PGF2 alpha, PGE2 and 6-keto-PGF1 alpha induced by A23187 from the Day 15 guinea-pig uterus, without affecting basal PG outputs. The increase in uterine tone induced by A23187 in the Day 15 guinea-pig uterus was reduced by 20-50% by oestradiol (10 micrograms/ml). The addition of oestradiol (10 micrograms/ml) and progesterone together (10 micrograms/ml) produced the same effects on the Day 15 guinea-pig uterus as oestradiol alone. Oestradiol (10 micrograms/ml) also reduced the A23187-induced increases in PG output from the Day 7 guinea-pig uterus, but did not reduce the increase in uterine tone. Oestradiol (10 micrograms/ml) reduced the increases in outputs of PGF2 alpha, PGE2 and 6-keto-PGF1 alpha induced by exogenous arachidonic acid from the Day 7 and Day 15 guinea-pig uterus. Previous studies have shown that oestradiol is not a cyclo-oxygenase inhibitor. The present findings suggest that oestradiol, at a relatively high concentration, may interfere with the access of arachidonic acid to the cyclo-oxygenase enzyme. This action of oestradiol may explain its anti-luteolytic action when administered to guinea-pigs in large doses after Day 9 of the cycle.