Synthesis and metabolism of prostaglandins E2, F2α and D2 by the rat gastrointestinal tract. Stimulation by a hypertonic environment in vitro

Whole cell preparations of rat stomach corpus, jejunum, and colon were incubated and the released prostaglandin E₂ (PGE₂), PGF_2α, PGD₂, 15 keto-13,14 dihydro PGE₂, and 15 keto-13,14 dihydro PGF_2α were measured by combined gas chromatography-mass spectrometry. All regions made PGD₂ and possessed a high capacity for producing 15 keto-13,14 dihydro derivatives of both PGE₂ and PGF_2α. Hypertonic sucrose solutions resulted in concentration-dependent increases in prostaglandin release, particularly of PGE₂ and its metabolite. It is suggested that PG's may play a role in the local effects of luminal hyperosomolarity on digestive tract functions.     

Sperm output and serum testosterone in rabbits given prostaglandin F2α or E2

Two experiments were conducted, the first to compare sperm output and the second to determine serum testosterone in rabbits given PGF₂α or PGE₂. In the first, six rabbits were ejaculated twice each Monday, Wednesday and Friday for 5 weeks. Each rabbit was given subcutaneously (sc) each of the following treatments five times: 1) saline, 2) 5 mg PGF₂α and 3) 5 mg PGE₂. Treatments were given, half at 4 hr and half at 2 hr before first ejaculations. Both PGF₂α and PGE₂ caused increased (50% and 84%) sperm content of first ejacula, without significantly altering characteristics of second ejacula. The extra sperm in first ejacula was a function of increased sperm density, because seminal volume was unaltered.In the second experiment, 15 rabbits were bled at 0.5-hr intervals for 9 hr and given (sc): 1) saline at 1 and 3 hr (n=4), 2) 2.5 mg PGF₂α at 1 and 3 hr (n=4), 3) 2.5 mg PGE₂ at 1 and 3 hr (n=4) or 4) 5 mg PGF₂α at 1 hr after the onset of blood sampling. In saline-treated controls, episodic surges of testosterone occurred on the average every 5 hours. After the injection of 2.5 or 5.0 mg PGF₂α, serum testosterone began to rise at 0.5 hr, peaked (8 to 13 ng/ml) at 1 hr and approached a nadir (0.5 ng/ml) within 4 hours. The second injection of 2.5 mg PGF₂α failed to significantly affect serum testosterone. PGE₂ treatment was followed by significantly depressed serum testosterone; only 1 of these 4 rabbits had any surge of testosterone for the 8 hr after treatment. In conclusion, PGF₂α and PGE₂ both increased sperm output, but PGF₂α increased serum testosterone while PGE₂ depressed serum testosterone. Thus, the sperm output effect of these prostaglandins probably is independent of the acute changes in testosterone secretion.     

Effects of indomethacin, prostaglandin E2, prostaglandin F2α and 6-keto-prostaglandin F1α on hatching of mouse blastocysts

The present study has been performed to investigate how PGs would participate the hatching process. Effects of indomethacin, an antagonist to PGs biosynthesis, on the hatching of mouse blastocysts were examined in vitro. Furthermore, it was studied that prostaglandin E₂ (PGE₂), prostaglandin F_2α (PGF_2α) or 6-keto-prostaglandin F_1α (6-keto-PGF_1α) were added to the culture media with indomethacin. (1) The hatching was inhibited by indomethacin yet the inhibition was reversible. (2) In the groups with indomethacin and PGE₂, no improvement was seen in the inhibition of hatching and the inhibition was irreversible. (3) In the groups with indomethacin and PGF_2α, inhibition of hatching was improved in comparison with the group with indomethacin. (4) In the groups with indomethacin and 6-keto-PGF_1α, no improvement was seen. The above results indicated that PGF_2α possibly had an accelerating effect on hatching and a high concentration of PGE₂ would exert cytotoxic effect on blastocysts.     

The effect of adrenergic stimulation on urinary prostaglandin E2 and 6 keto PGF1α in man

To evaluate the details of the adrenergic stimulation of urinary prostaglandins in man, ten normal volunteers were given various agonists and antagonists. The effect of 4 hour IV infusions of norepinephrine (NE), NE + phentolamine (PHT), NE + phenoxybenzamine (PHB), NE + prazosin (PZ), isoproterenol (ISO), and PHT alone on urinary PGE₂ and PGI₂ (6 keto PGF_1α) were determined. PGE₂ and 6 keto PGF_1α were measured by radioimmunoassay from 4 hour urine samples. NE stimulated both PGE₂ (196±40 to 370±84 ng/4 hrs/g creatinine and 6 keto PGF_1α(184±30 to 326±36), both p<0.01. In contrast, ISO had no effect on either PGE₂ or 6 keto PGF_1α excretion. Alpha blockade with PHT. PHB, or PZ inhibited the NE induced systemic pressor effect. However, the effect of the alpha blockers on the NE induced stimulation of PGE₂ and 6 keto PGF_1α varied. PHT did not alter the NE stimulated PGE₂ or 6 keto PGF_1α release (370±84 vs. 381±80) PGE₂ and (326±50 vs. 315±40) 6 keto PGF_1α, both p>0.2). PHT alone stimulated only 6 keto PGF_1α. PHB and the specific α₁ antagonist PZ similarly eliminated the NE induced prostaglandin release. These results suggest that adrenergically mediated urinary prostaglandin release in man is via an alpha receptor with α₁ characteristics.     

Effects of prostaglandins E1 and F2α on serum luteinizing hormone concentration and on some sexual functions in male rabbits

PGE₁(50μg/animal) and PGF_2α (250 μg/animal) caused a transient in serum LH at 5 min after injection. PGE₁ (250 μg/animal) had a biphasic effect on serum LH. A small peak was obtained at 5 min, and a second, larger peak at 60 min after injection. It is suggested that the first peak is a result of the stress associated with injection of the PGs, whereas the second peak represents a physiological effect of PGE. Subcutaneous injection of PGE₁ (1 mg in arachis oil b.i.d.) for 10 days did not affect the concentration of LH in serum, the function of the accessory sexual glands or the sexual activity. PGF_2α, given at the same dose and in the same manner, increased the sexual activity but left all other variables unaffected. The pituitary responsiveness to LH-RH was unaltered by the treatment with PGE₁ and PGF_2α.     

Prostaglandin E1 and F2α specific binding in bovine corpora lutea: Comparison with luteolytic effects

Preliminary characterization indicated the presence of separate prostaglandin (PG)E₁ and (PG)F_2α binding sites in membrane fractions prepared from bovine corpora lutea. These differ in the rate and temperature dependence of the specific binding. Equilibrium binding data indicate the apparent dissociation constants as 1.32 × 10⁻⁹M and 2.1 × 10⁻⁸M for PGE₁ and PGF_2α, respectively. Competition of several natural prostaglandins for the PGE₁ and PGF_2α bovine luteal specific binding sites indicates specificity for the 9-keto or 9α-hydroxyl moiety, respectively. Differences in relative ability to inhibit ³H-PG binding were found due to sensitivity to the absence or presence of the 5,6-cis-double bond as well.Bovine luteal function was affected following treatment of heifers with 25 mg PGF_2α as measured by reduced estrous cycle length, decreased corpus luteum size and significantly decreased plasma progesterone levels. In contrast, treatment with 25 mg PGE₁ resulted in cycle lengths comparable to those of non-treated herdmates with no apparent modification in corpus luteum size. However, plasma progesterone levels were increased significantly following PGE₁ treatment compared to pretreatment values. In so far as data obtained on PGF_2α relative binding affinity to the bovine CL can be compared to data obtained independently on PGF_2α induced luteolysis in the bovine, PGF_2α relative binding to the CL and luteolysis appeared to be associated. By similar reasoning, there was no apparent relationship between PGE₁ relative binding affinity in the luteal fractions and luteolysis in estrous cyclic cattle.     

Prostaglandin E1 and F2α specific binding in bovine corpora lutea: Comparison with luteolytic effects

Preliminary characterization indicated the presence of separate prostaglandin (PG)E₁ and (PG)F_2α binding sites in membrane fractions prepared from bovine corpora lutea. These differ in the rate and temperature dependence of the specific binding. Equilibrium binding data indicate the apparent dissociation constants as 1.32 × 10⁻⁹M and 2.1 × 10⁻⁸M for PGE₁ and PGF_2α, respectively. Competition of several natural prostaglandins for the PGE₁ and PGF_2α bovine luteal specific binding sites indicates specificity for the 9-keto or 9α-hydroxyl moiety, respectively. Differences in relative ability to inhibit ³H-PG binding were found due to sensitivity to the absence or presence of the 5,6-cis-double bond as well.Bovine luteal function was affected following treatment of heifers with 25 mg PGF_2α as measured by reduced estrous cycle length, decreased corpus luteum size and significantly decreased plasma progesterone levels. In contrast, treatment with 25 mg PGE₁ resulted in cycle lengths comparable to those of non-treated herdmates with no apparent modification in corpus luteum size. However, plasma progesterone levels were increased significantly following PGE₁ treatment compared to pretreatment values. In so far as data obtained in vitro on PGF_2α relative binding affinity to the bovine CL can be compared to data obtained independently in vitro on PGF_2α induced luteolysis in the bovine, PGF_2α relative binding to the CL and luteolysis appeared to be associated. By similar reasoning, there was no apparent relationship between PGE₁ relative binding affinity in the luteal fractions and luteolysis in estrous cyclic cattle.     

Stereospecificity of enzymatic reduction of prostaglandin E2 to F2α

Antibodies directed toward PGF_2β were prepared in rabbits. The serologic specificity of the immune reaction was determined by inhibition of sodium borohydride-reduced (³H) PGE₂ anti-PGF_2β binding by several prostaglandins. The antibodies to PGF_2β recognize the β-hydroxyl configuration in the cyclopentane ring of PGF_2β. With the use of both anti-PGF_2α and anti-PGF_2β, the product of PGE₂ reduction by 9-ketoreductase purified from chicken heart was identified as PGF_2α. Guinea pig liver and kidney homogenates were examined for PGE 9-ketoreductase activity. Although enzyme activity was present, no evidence of PGF_2β production was found.     

The effects of prostaglandins PGE1, PGE2, PGF1a, and PGF2a on canine synovial perfusion

In these experiments we have examined the effects of PGE₁, PGE₂, PGF_1α and PGF_2α on synovial perfusion in the normal canine synovial microcirculation. The effects of the drugs on synovial perfusion were determined indirectly from the changes produced in the rate of clearance of ¹³³Xenon from the joint by their intra-articular injection. Prostaglandins PGE₁ and PGE₂ were found to be strongly vasodilator with PGE₁ being the more active. PGF_1α appeared to have little or no vasoactive properties in doses up to 1 ugm. (2.8 × 10^−5M) in our I preparation while PGF_2α was vasodilator at this high dosage only. Neither SC19920 nor diphloretin phosphate antagonised the effects of PGE₁ in these experiments.     

PGE2 is a more potent vasodilator of the lamb ductus arteriosus than is either PGI2 or 6 keto PGFα

It has been shown in vitro that the lamb ductus arteriosus forms prostaglandins PGE₂, PGF_2α, 6 keto PGF_1α (and its unstable precursor PGI₂). In this study the relative potencies of these endogenous prostaglandins were investigated on isolated lamb ductus arteriosus preparations contracted by exposure to elevated PO₂ and indomethacin. All the prostaglandins (except PGF_2α) relaxed the vessel. This is consistent with the hypothesis that endogenous prostaglandins inhibit the tendency of the vessel to contract in response to oxygen. Only PGE₂, however, relaxed the vessel at concentrations below 10⁻⁸M. PGI₂ and 6 keto PGF_1α had approximately 0.001 and 0.0001 times the activity of PGE₂. Although PGE₂ has been observed to be a minor product of prostaglandin production in the lamb ductus arteriosus, the tissue's marked sensitivity to PGE₂ might make it the most significant prostaglandin in regulating the patency of the vessel.     

Urinary excretion of prostaglandin E2 and prostaglandin F2α in potassium-deficient rats

Potassium-deficiency was induced in rats by dietary deprivation of potassium. The animals became polyuric and urine osmolality decreased more then three-fold compared to controls. Urinary excretion of prostaglandin E₂ (PGE₂) and prostaglandin F_2α (PGF_2α) did not increase during 2 weeks of potassium depletion. Partial inhibition of renal prostaglandin synthesis by meclofenamate did not increase the urine osmolality after water deprivation. These results make unlikely the hypothesis that the polyuria of potassium-deficiency, is the result of enhanced renal synthesis of prostaglandins with subsequent antagonism of the hydro-osmotic effect of vasopressin. Male animals consistently excreted less PGE₂ than female animals.     

Differential effects of detergents and dimethylsulfoxide on membrane prostaglandin E1 and F2α receptors

Pretreatment of membranes for 1 hr at 4° with up to 0.1% Triton X-100 (TX-100) and sodium desoxycholate (SDC), resulted in a greater loss of [³H] prostaglandin (PG)F₂α binding compared to E₁ binding. Lubrol WX (LWX) tended to cause a greater loss of [³H]PGF₂α than E₁ binding. However, the differential loss was not as marked as with TX-100 or SDC. Triton X-305 was relatively ineffective, but loss of [³H]PGE₁ binding was greater than for PGF₂α. Increasing concentrations of dimethylsulfoxide (DMSO) progressively inhibited PGF₂α binding without affecting PGE₁ binding. The detergent, but not DMSO, induced losses of membrane PG binding were due to solubilization of the receptors. Greater amounts of membrane protein and phospholipids were solubilized at detergent (TX-100 and SDC) concentrations that solubilized 100% of PGE₁ receptors compared to 100% solubilization of F₂α receptors. Neither the duration of preincubation nor the amount of membrane protein chosen were responsible for differential PGE₁ and F₂α receptor losses. These differential membrane PG receptor losses raise the possibility of differences in PGE₁ and F₂α receptors association with the membrane structure.     

Relation between prostaglandin E2, F2α, and cyclic nucleotides levels in rat brain and induction of convilsions

Fully convulsant doses of pentamethylenetetrazole cause marked increase in rat brain cortical PGF_2α, PGE₂, cGMP and cAMP during seizures, whereas subconvulsant doses cause an increase of rat brain cortical PGF_2α without affecting the other biochemical parameters considered. Rat cerebellar prostaglandins were not modified by the convulsant agent at either dosage.     

Prostaglandins and renin release: III. Effects of PGE1, E2 F2α and D2 on renin release from rabbit renal cortical slices

We have investigated the direct effects of prostaglandins E₁, E₂, F_2α and D₂ on renin release from rabbit renal cortical slices. Prostaglandin E₁ (PGE₁) was the most potent stimulant of renin release, while PGE₂ was 20–30 fold less active. PGF_2α was found not to be an inhibitor of renin release as reported by others, but rather a weak agonist. PGD₂ up to a concentration of 10 μg/ml had no activity in this system. That the stimulation of renin release by PGE₁ is a direct effect is supported by the finding that PGE₁-induced release is not blocked by L-propranolol or by Δ^5,8,11,14-eicosatetraynoic acid (ETYA), a prostaglandin synthesis is inhibitor. The fatty acid precursor of PGE₁, Δ^8,11,14-eicosatrienoic acid, also stimulated renin release, an effect which was blocked by ETYA. In addition to the above findings, ethanol, a compound frequently used to dissolve prostaglandins, was shown to inhibit renin release.     

Effects of prostaglandin E2 and F2α on the absorption and portal transport of sugar and on the local intestinal circulation

With the isolated jejunum loop technique investigations of prostaglandin E₂ and F_2α were made on canine intestinal absorption and transport of glucose and on the circulation of the intestinal loop. These compounds decreased glucose absorption; intra-arterial prostaglandin administration decreased the portal transport of glucose, but intraluminal administration caused an increase. PGE₂ enhanced the circulation of the intestinal loop; intra-arterial PGF_2α diminished this circulation, whereas on intraluminal PGF_2α had no significant effect.     

Prostaglandin E1 inhibits the pulmonary vascular pressor response to hypoxia and prostaglandin F2α

In the anesthetised dog an infusion of exogenous prostaglandin E₁ (100μG/min) inhibits the pulmonary vascular pressor response to hypoxia. Both 25 and 100 μG/min PGE₁ can reduce the transient pulmonary hypertension caused by a bolus of prostaglandin F_2α. This suggests that hypoxia and PGF₂α may share a final common pathway in producing pulmonary vasoconstriction. These results may help to explain the mechanism by which endotoxin inhibits the pulmonary vascular response to hypoxia. This effect is probably achieved by stimulating the production of an endogenous dilator prostaglandin. Exogenous PGE₁ can mimic this effect.     

Prostaglandin E1 or E2 (PGE1, PGE2) prevents premature luteolysis induced by progesterone given early in the estrous cycle in ewes

The objective of this study was to determine whether prostaglandin E₁ (PGE₁) or prostaglandin E₂ (PGE₂) prevents premature luteolysis in ewes when progesterone is given during the first 6 days of the estrous cycle. Progesterone (3 mg in oil, im) given twice daily from Days 1 to 6 (estrus = Day 0) in ewes decreased (P < 0.05) luteal weights on Day 10 postestrus. Plasma progesterone concentrations differed (P < 0.05) among the treatment groups; toward the end of the experimental period, concentrations in jugular venous blood decreased (P < 0.05) compared with the other treatment groups. Plasma progesterone concentrations in ewes receiving PGE₁ or PGE₁ + progesterone were greater (P < 0.05) than in vehicle controls or in ewes receiving PGE₂ or PGE₂ or PGE₂ + progesterone. Chronic intrauterine treatment with PGE₁ or PGE₂ prevented (P < 0.05) decreases in plasma progesterone concentrations, luteal weights, and the proportion of luteal unoccupied and occupied LH receptors on Day 10 postestrus in ewes given exogenous progesterone, but did not affect (P > 0.05) concentrations of PGF_2α in inferior vena cava blood. Progesterone given on Days 1 to 6 in ewes advanced (P < 0.05) increases in PGF_2α in inferior vena cava blood. We concluded that PGE₁ or PGE₂ prevented progesterone-induced premature luteolysis by suppressing loss of luteal LH receptors (both unoccupied and occupied).     

Prostaglandin E2: Midterm abortion in rats using Silastic-PVP tubes

The results of the present study establish that 1.5 mg PGE₂ (lyophilized sodium salt) incorporated in one cm long open-ended Silastic-polyvinylpyrrolidone (PVP) tube when inserted into 10 day pregnant rats induced abortion within 70–72 hours in all the treated rats. A combined treatment of PGE₂ and 17β-estradiol failed to increase the abortion inducing effect of a Silastic-PVP-PGE₂ tube. It is observed that PGE₂ is about 4 times less potent than PGF_2α in inducing midterm abortion in rats. It is suggested that either PGE₂ exerts luteolytic effect after being converted to PGF_2α, although how it occurs is not clear; or PGE₂ causes expulsion of the fetuses by its uterine stimulating property. 17β- estradiol increases the uterine synthesis of PGF_2α as described earlier but seems not affecting the production of PGE₂ by the uterus. The release rate of ³H-PGE₂ from Silastic-PVP tube and is also described.     

Levuglandin E2 crosslinks proteins

Levuglandin E₂ (LGE₂), a γ-ketoaldehyde produced by rearrangement of the prostaglandin endoperoxide PGH₂ under the aqueous conditions of its biosynthesis, causes extensive intermolecular crosslinking of ovalbumin at pH 6 or pH 7 and 37°C. The time dependence of protein oligomerization is monitored by SDS-PAGE. Effects of pH and concentration on the extent of LGE₂-induced crosslinking are examined. The efficacy of LGE₂ for inducing crosslinking is compared with other oxidative metabolites of arachidonic acid (AA), including the prostaglandins PGE₂, PGD₂, PGA₂, PGB₂, and PGF_2α, as well as malondialdehyde and E-4-hydroxy-non-2-enal. LGE₂ is orders of magnitude more effective in crosslinking protein than any other cyclooxygenase or lipoxygenase metabolite of AA tested.     

Different responses to prostaglandin F2α and E2 in human extra- and intramyometrial arteries

Tubal segments of the ascending uterine arteries and of intramyometrial arteries were obtained from 18 women who underwent hysterectomy at various phases of the menstrual cycle. Ring preparations of the vessels were mounted in organ baths and isometric tension was recorded. In extramyometrial arteries (outer diameter 2–3 mm) prostaglandin (PG) F_2α most potently, but also PGE₂ caused concentration-related contractions. In contrast, the contractant effects of both PGs on intramyometrial arteries (outer diameter 0.5–0.6 mm) were negligible. Both extra- and intramyometrial vessels were relaxed to a moderate degree (10–25%) by low concentrations of PGF_2α and PGE₂. No significant differences between the responses to vasopressin and noradrenaline were found between the vessel preparations. Thus human uterine arteries seem to change their responses to PGF_2α and PGE₂ as they enter the myometrium and decrease in diameter, and the results raise doubt about the view that direct vasoconstrictor effects of these PGs contribute to the regulation of myometrial blood flow. Such effects of vasopressin and noradrenaline cannot be excluded.