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

Hydrocortisone (10 μg/ml) had no effect on the basal outputs and A23187-stimulated outputs of PGF_2α, PGE₂ and 6-keto-PGF_1α from the Day 15 guinea-pig uterus superfused . These findings indicate that the high output of PGF_2α from the guinea-pig uterus during the last one-third of the oestrous cycle is not modulated by the adrenal glucocorticoid hormones. Progesterone (10 gmg/ml) had no effect on the A23187-induced increases in PG output from the Day 15 guinea-pig uterus. However, oestradiol (10 gmg/ml but not 1 μg/ml) significantly reduced the increases in outputs of PGF_2α, PGE₂ and 6-keto-PGF_1α 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 μg/ml). The addition of oestradiol (10 μg/ml) and progesterone together (10 gmg/ml) produced the same effects on the Day 15 guinea-pig uterus as oestradiol alone. Oestradiol (10 μg/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 gmg/ml) reduced the increases in outputs of PGF_2α, PGE₂ and 6-keto-PGF_1α 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.     

Modulation by prostaglandins of contractions in guinea-pig ileum

A high concentration of indomethacin (40μg/ml) substantially reduced contractions of guinea-pig isolated ileum in Krebs solution to nerve stimulation with electrical pulses or nicotine. Responses to acetylcholine and histamine were also inhibited, but to a smaller extent. Low concentrations of prostaglandin E₂ (2 or 4ng/ml) mainly restored all the excitatory responses. Using a modified bathing solution (lacking in phosphate and with some other changes) indomethacin 0.36μg/ml selectively inhibited nerve-mediated contractions. The results explain differences in various reports, and support the possibility that prostaglandins modulate the response to cholinergic nerve activity.     

Effect of indomethacin and 7-oxa-13-prostynoic acid on luteinization of transplanted rat ovarian follicles induced by luteinizing hormone and prostaglandin E2

The ability of prostaglandin E₂ (PGE₂) to initiate luteinization was demonstrated using a system of in vitro incubation of ovarian follicles followed by transplantation. Follicles from diestrous rats were incubated with 0.05 to 50 μg/ml PGE₂, 10 μg/ml luteinizing hormone (LH), or alone in Krebs-Ringer bicarbonate buffer plus glucose for 2 hr. Then follicles were transplanted under the kidney capsules of hypophysectomized recipients, with follicles exposed to PGE₂ on one side and those exposed to LH or buffer only on the other side. As determined at autopsy 4 days later and confirmed by histological examination, follicles exposed to PGE₂ at concentrations of 0.5 μg/ml or greater, or to LH, transformed into corpora lutea, but control follicles regressed. Incubation of follicles with LH in the presence of indomethacin, an inhibitor of prostaglandin synthesis, significantly reduced the incidence of luteinization. Prostaglandin E₂ (10 μg/ml) was able to override the inhibition of luteinization by indomethacin (150 μg/ml). The prostaglandin analogue 7-oxa-13-prostynoic acid (100 μg/ml) failed to prevent luteinization in response to either 5 μg/ml LH or 1 μg/ml PGE₂. Results with PGE₂ and with indomethacin suggest a role for prostaglandins in the luteinizing action of LH.We have reported previously that in vitro exposure of diestrous rat follicles to luteinizing hormone (LH) will result in transformation of the follicles to corpora lutea following transplantation under the kidney capsules of hypophysectomized rats. Dibutyryl cyclic AMP (DBC) mimics this effect of LH, and transplants produce progesterone in measurable amounts after both LH and DBC exposure when prolactin is administered in vivo to recipients.Kuehl et al. have suggested that prostaglandins may act as obligatory intermediates in the effect of LH on the ovary, acting between LH and adenylate cyclase. Preliminary results indicated that prostaglandin E₂ (PGE₂) could induce luteinization in our system. The extent of prostaglandin involvement in luteinization was further investigated in this work, using two reported antagonists of prostaglandin action, indomethacin and 7-oxa-13-prostynoic acid. Indomethacin has been found to inhibit synthesis of prostaglandins E₂ and F_2α; 7-oxa-13-prostynoic acid, which acts as a competitive antagonist of prostaglandins, prevented the effect of LH and prostaglandins E₁ and E₂ on cyclic AMP production in mouse ovaries.     

The mechanism of enhancement by fatty acid hydroperoxides of anaphylactic mediator release

Fragments of chopped lung from indomethacin treated guinea-pigs had an anti-aggregating effect when added to human platelet rich plasma (PRP), probably due to the production of prostacyclin (PGI₂) since the effect was inhibited by 15-hydroperoxy arachidonic acid (15-HPAA, 10 μg ml⁻¹). Both 15-HPAA (1–20 μg ml⁻¹ min⁻¹) and 13-hydroperoxy linoleic acid (13-HPLA, 20 μg ml⁻¹ min⁻¹) caused a marked enhancement of the anaphylactic release of histamine, slow-reacting substance of anaphylaxis (SRS-A) and rabbit aorta contracting substance (RCS) from guinea-pig isolated perfused lungs. This enhancement was not reversed by the concomitant infusion of either PGI₂ (5 μg ml⁻¹ min⁻¹) or 6-oxo-prostaglandin F_1α (6-oxo-PGF_1α, 5 μg ml⁻¹ min⁻¹). Anaphylactic release of histamine and SRS-A from guinea-pig perfused lungs was not inhibited by PGI₂ (10 ng - 10 μg ml⁻¹ min⁻¹) but was inhibited by PGE₂ (5 and 10 μg ml⁻¹ min⁻¹). Antiserum raised to 5,6-dihydro prostacyclin (PGI₁) in rabbits, which also binds PGI₂, had no effect on the release of anaphylactic mediators. The fatty acid hydroperoxides may enhance mediator release either indirectly by augmenting thromboxane production or by a direct effect on sensitized cells. Further experiments to distinguish between these alternatives are described in the accompanying paper (27).     

PGE2 and dibutyryl-cAMP enhance the response of rat granulosa cells to FSH

Granulosa cells isolated from immature Sprague-Dawley rat ovaries produce progesterone (31.7 pg/μg cell protein) in response to an acute FSH stimulus (5 μg/ml NIH-FSH-S11, 2 h). After culture for 48 h in the absence of hormones (control culture), progesterone production by the granulosa cells in response to FSH is significantly reduced (2.9 pg/μg cell protein). Cells cultured with prostaglandin E₂ (PGE₂, 1 μg/ml) or dibutyryl-cAMP (dbcAMP, 1 mM) exhibited a discernibly greater steroidogenic response to FSH (12.5 and 53.4 pg/μg cell protein, respectively) than that of control cultures. Therefore the presence of PGE₂ or dbcAMP in the culture medium helps to maintain the steroidogenic capacity of granulosa cells in culture. It is probable that this capacity is maintained at a locus distal to the production of cAMP by FSH.Paradoxically, granulosa cells cultured with PGE₂ produce less cAMP in response to FSH stimulation than cells in control cultures (15.9 250.3 fm/μg cell protein). This may be due to a suppressive effect of prior exposure to PGE₂ on the subsequent activity of adenylate cyclase when the FSH is introduced and a concomitant elevation of phosphodiesterase activity.     

Prostaglandin E2 enhances uterine stromal cell alkaline phosphatase activity

Prostaglandins have been implicated in the process of uterine decidualization , but sites of action are uncertain. Since one of the earliest changes in endometrial stroma following induction of decidualization is an increase in alkaline phosphataseactivity, we have investigated the effects of PGs on stromal cell alkaline phosphatase activity . Immature rats were pretreated with hormones to sensitize their uteri for the decidual cell reaction. Endometrial stromal cells were isolated and cultured for up to 4 days with PGE₂ (0–10 μg/ml) or PGF₂ (0–10 μg/ml) Analysis of variance revealed a highly significant interaction between day of culture and concentration of PGE₂ in medium (P<0.01). Stromal cell alkaline phosphatase activity decreased significantly with increasing culture duration (P<0.01). In the presence of PGE₂, alkaline phosphatase activity was significantly higher (P<0.01) regardless of day of culture. In contrast, PGF_2α had only a small and inconsistent effect. These data indicate that PGs, and in particular PGE₂, can act directly upon stromal cells.     

The synthesis and biological activities of some 12-Aza-prostaglandin analogues

12-Aza-prostaglandin (PG) analogues containing the pyrrolidine-2,4-dione ring system have been synthesized from ,2-disubstituted glycine esters via cyclisation of their -ethoxycarbonylacetyl derivatives. 5-(6-Carboxyhexyl)-1-octylpyrrolidine-2,4-dione (5) had little or no PG-like activity on superfused intestinal or vascular smooth muscle preparations but it selectively antagonised smooth muscle responses to PGE₂, PGE₁, PGF_2α and PGA₂in vitro. At a concentration of 10⁻⁵ g/ml it reduced responses of the rat stomach strip to PGE₂ by over 80% but did not affect responses of this tissue to acetylcholine, 5-hydroxytryptamine (5-HT) or bradykinin. Polyphloretin phosphate (PPP), the known PG antagonist, had a similar effect at the same concentration (10⁻⁵ g/ml).5-(6-Carboxyhexyl)-1-(3-hydroxyoctyl)pyrrolidine-2,4-dione (12) had the same profile of activity on superfused smooth muscle preparations as PGE₂ or PGA₂. On intravenous injection into anaesthetised rats it caused dose-dependent falls in arterial blood pressure with associated tachycardias, which is typical of the response to PGE₂. The smooth muscle activity of (12) was not reduced by passage through isolated perfused guinea-pig lungs nor was its potency as a vasodepressor increased when given intra-arterially to rats. These results suggest that, unlike PGE₂, this analogue is not removed by the pulmonary circulation.     

Effects of PGE1 or PGE2 on luteal function in pseudopregnant rats

Effects of PGE₁ or PGE₂ on luteal function were studied in 163 pseudopregnant rats. PGE₁ (10, 100, or 300μg) given intrauterine every 6 hr did not shorten pseudopregnancy (P < 0.05), however, the same doses of PGE₂ given intrauterine every 6 hr advanced luteolysis (P < 0.05). PGE₁ (100 or 300μg) given every 4 hr intramuscular maintained levels of progesterone in peripheral blood above controls (P < 0.05) while 100 or 300μg of PGE₂ hastened the decline in progesterone (P < 0.05). The antiluteolytic effect of PGE₁ was not via an inhibition of PGF secretion (P < 0.05) by the uterus or by induction of ovulation in treated animals. Moreover, PGE₁ (100, 200, or 500μg) given intramuscular every 4 hr from day 4 of pseudopregnancy until the next proestrus delayed luteal regression around 3 days (P < 0.05). PGE₂ at doses of 100, 200, or 500μg every 4 hr given intramuscular consistently shortened pseudopregnancy (P < 0.05). Lower doses were without effect (P < 0.05). Based on the above data it is concluded that PGE₂ is consistently luteolytic whereas PGE₁ is not luteolytic in pseudopregnant rats and that PGE₁ may be an antiluteolysin.     

Prostaglandins E1 and E2 stimulate the proliferation of pulmonary artery smooth muscle cells

Prostaglandins E₁ and E₂ are thought to be inhibitors of the growth of systemic vascular smooth muscle cells (SMC). However, their effect on the proliferation of SMC from the pulmonary artery (PA) has not been described and was the subject of this investigation. Cultures of bovine PA SMC were exposed to PGE₁ and PGE₂ under various conditions and their growth was assessed. PGE₁ and PGE₂ did not inhibit the growth of PA SMC in 10% fetal calf serum (FCS), but instead caused a dose dependent (10 ng - 1 μg/ml) increase in [³H]-thymidine incorporation when added to cultures containing 0.5% FCS; the highest doses resulted in 95% and 75% increases in [³H]-thymidine uptake at 24 hours with PGE₁ and PGE₂ respectively. This was accompanied by a modest increase in actual cell numbers (e.g., 20% with 1 μg/ml PGE₁). Furthermore, PGE₁ could mimic insulin-like growth factor (IGF-1) by potentiating the stimulation of SMC growth by fibroblast growth factor, suggesting that PGE₁ may act as a progression factor in the growth cycle of these cells. There was, however, no effect of PGE₁ on the proliferation of bovine aortic SMC. We conclude that, contrary to most reported effects on systemic SMC, PGE₁ and PGE₂ do not inhibit the proliferation of PA SMC but rather stimulate it.     

Corticosteroids inhibit prostaglandin release from perfused mesenteric blood vessels of rabbit and from perfused lungs of sensitized guinea pig

Infusion of norephinephrine (NE) (1 – 3 μg/ml/min) into the isolated mesenteric vascular preparation of rabbit resulted in a rise in perfusion pressure, which was associated with the release of a prostaglandin E-like substance (PGE) at a concentration of 2.81 ± 0.65 ng/ml in terms of PGE₂. Indomethacin (3 μg/ml) abolished the NE-induced release of PGE. Arachidonic acid (0.2 μg/ml) in the presence of indomethacin did not restore the NE-induced release of PGE. Hydrocortisone (10 – 30 μg/ml) and dexamethasone (2 – 5 μg/ml) also inhibited the NE-induced release of PGE. The inhibitory action of both corticosteroids was abolished by arachidonic acid (0.2 μg/ml). Antigen-induced release of a prostaglandin-like substance(PGs) (43.1 ± 3.8 ng/ml in terms of PGE₂ and a rabbit aorta contracting substance (RCS) from perfused lungs of sensitized guinea pigs was completely abolished by indomethacin (5 μg/ml) or by hydrocortisone (100 μg/ml). Indomethacin, however, increased histamine release up to 280% of the control level, which was 470 ± 54 ng/ml, while hydrocortisone diminished histamine release down to 30% of the control level. A superimposed infusion of arachidonic acid (1 μg/ml) into the pulmonary artery reversed the hydrocortisone-induced blockade of the release of RCS and PGs. It may be concluded that corticosteroids neither inhibit prostaglandin synthetase nor influence prostaglandin transport through the membranes but they do impair the availability of the substrate for the enzyme.     

Corticosteroids inhibit prostaglandin release from perfused mesenteric blood vessels of rabbit and from perfused lungs of sensitized guinea pig

Infusion of norephinephrine (NE) (1 – 3 μg/ml/min) into the isolated mesenteric vascular preparation of rabbit resulted in a rise in perfusion pressure, which was associated with the release of a prostaglandin E-like substance (PGE) at a concentration of 2.81 ± 0.65 ng/ml in terms of PGE₂. Indomethacin (3 μg/ml) abolished the NE-induced release of PGE. Arachidonic acid (0.2 μg/ml) in the presence of indomethacin did not restore the NE-induced release of PGE. Hydrocortisone (10 – 30 μg/ml) and dexamethasone (2 – 5 μg/ml) also inhibited the NE-induced release of PGE. The inhibitory action of both corticosteroids was abolished by arachidonic acid (0.2 μg/ml). Antigen-induced release of a prostaglandin-like substance (PGs) (43.1 ± 3.8 ng/ml in terms of PGE₂ and a rabbit aorta contracting substance (RCS) from perfused lungs of sensitized guinea pigs was completely abolished by indomethacin (5 μg/ml) or by hydrocortisone (100 μg/ml). Indomethacin, however, increased histamine release up to 280% of the control level, which was 470 ± 54 ng/ml, while hydrocortisone diminished histamine release down to 30% of the control level. A superimposed infusion of arachidonic acid (1 μg/ml) into the pulmonary artery reversed the hydrocortisone-induced blockade of the release of RCS and PGs. It may be concluded that corticosteroids neither inhibit prostaglandin synthetase nor influence prostaglandin transport through the membranes but they do impair the availability of the substrate for the enzyme.     

Follicle stimulating hormone and prolactin release induced by prostaglandins in rat

Ten to 60 minutes following a single i.v. injection of PGE₂ (500 μg/rat) into male rats of 30 to 35 days of age FSH concentration in the serum was raised significantly. The rise in FSH was maintained from 10 to 60 minutes after treatment, then at 90 minutes FSH had declined and was not significantly different from that of the control before treatment. Prostaglandin E₁, E₂ or F_2α (670μg/rat) significantly increased the serum prolactin level 10 to 60 minutes after a single i.v. injection in spayed rats primed with estrogen and progesterone. And, rats primed with estrogen and progesterone. And, increases in prolactin in the serum were observed with as little as 2μg of PGE₁ or E₂, and 20μg of PGF_2α. Twenty μg of PGE₂, and 200μg of PGE₁ or F_2α gave the maximum stimulation. These results indicate that release of pituitary hormones is affected by prostaglandins.Prostaglandins (PGs) are widely distributed in mammalian tissues, and they have been reported to have an almost equally wide variety of endocrine and metabolic effects. It was recently postulated that PGs may be involved in the process of ovulation because ovulation was blocked by inhibitors of PG synthesis (1–5).     

Is the epithelium-derived inhibitory factor in guinea-pig trachea a prostanoid?

To examine further the possible prostanoid involvement in the influence of the epithelium on guinea-pig tracheal smooth muscle responsiveness, we have analyzed the effects of LTD₄, methacholine and histamine on the level of airway smooth muscle tone and on the amounts of PGE_2α and PGI₂ (determined by radioimmunoassay) in the presence and absence of the epithelium. Removal of the epithelium increased the sensitivity of guinea-pig trachea to the contractile effects of LTD₄, methacholine and histamine. LTD₄ (3–100 nM), methacoline (0.1–10 μM) or histamine (0.3–30 μM) did not increase prostanoid release above control values in either the presence or absence of the epithelium. The unstimulated release of PGE₂ and PGF_2α but not PGI₂, was decreased in tissues lacking epithelium. Indomethacin (1 μM) reduced the baseline tone to a smaller extent in the absence of epithelium. In the presence but not the absence of the epithelium, indomethacin increased the sensitivity of preparations to the contractile effect of methacholine. The results support the postulate of an epithelium-derived inhibitory factor modulating guinea-pig tracheal smooth muscle responsiveness. The identity of this factor is not known but is not PGI₂ and is unlikely to be PGF_2α or PGE₂. However, the possibility remains that the basal release of PGE₂ and/or PGF_2α derived from the epithelium may markedly affect the responsiveness of guinea-pig tracheal smooth muscle. Furthermore, the epithelium is a significant source of PGE₂ and PGF_2α which may be involved in the maintenance of baseline tone.     

Reversal of indomethacin-induced inhibition of tubuloglomerular feedback by prostaglandin infusion

Experiments were performed in rats to study the effect of infusion of PGI₂, PGE₂, and PGF_2α on tubuloglomerular feedback responses (i.e. the change of SNGFR in response to a change of loop of Henle flow rate) in the presence and absence of simultaneous inhibition of endogenous PG synthesis with indomethacin. Infusion of PGI₂ or PGE₂ at rates that did not alter arterial blood pressure did not significantly modify the magnitude of feedback responses (PGI₂) 8.5 μg/hr, PGE₂ 85 μg/hr). Some inhibition of feedback responses was seen when PGI₂ and PGE₂ were administered at higher rates were associated with a reduction of blood pressure (PGI₂ 20 μg/hr, PGE₂ 200 μg/hr). PGI₂ (8.5 μg/hr) and PGE₂ (85 μg/hr) largely prevented feedback inhibition induced by indomethacin. When given subsequent to indomethacin PGI₂ and PGE₂ restored feedback responsiveness almost to normal. In contrast, PGF_2α did not influence feedback inhibition caused by indomethacin. Infusion of PGI₂ induced partial restoration of feedback responses in DOCA-salt treated animals in which the feedback system is virtually completely inactive. Our results indicate that availability of PGI₂ or PGE₂ is necessary for the normal operation of the tubuloglomerular feedback mechanism for control of nephron filtration rate.     

A release of prostaglandins may be responsible for acute tolerance to norepinephrine infusions

A chick isolated rectum pretreated with atropine and indomethacin and superfused with the oxygenated mixed venous blood of anaesthetized cats, was selectively contracted by PGE₁ and PGE₂ at concentrations of <1 ng/ml. Intravenous infusion of norepinephrine (0.2 – 8.0 μg/kg/min) into the cats resulted in a contraction of the blood-bathed chick rectum. This was matched by contractions produced by PGE₂ (0.4 – 7 ng/ml) infused directly over the assay organ. The appearance of a chick rectum contracting substance in the venous blood was paralleled by a decline in the pressor response to norepinephrine. A single injection of indomethacin (3 – 10 mg/kg) prevented both the formation of the prostaglandin-like material and the acute tolerance to the pressor response to norepinephrine. Both effects could then be reproduced by an intra-arterial infusion of PGE₂ at a rate 0.125 – 0.5 μg/kg/min. β-Adrenoceptor blockade had no influence on the response of chick rectum and arterial blood pressure to an infusion of norepine phrine, but α-adrenoceptor blockade abolished both responses. It is postulated that the acute tolerance to norepinephrine infusions is the result of a release of PGE-like material from the contracting vascular bed.     

Selective antagonism of the systemic vasodepressor response to PGE1 by d,1–11, 15-bisdeoxy PGE1

Several bisdeoxy PGE₁ analogs are potent, competitive antagonists of PGE₁-induced colonic contractions in the gerbil. The efficacy of these analogs in antagonizing PGE₁-mediated systemic vasodepression has not been previously demonstrated. In this study, serial doses of PGs were administered before, during and after infusion of d,1–11, 15-bisdeoxy PGE₁. Bolus injections of PGE₁ (3.0 μk/kg), PGE₂ (3.0 μg/kg) and PGI₂ (0.3 μg/kg) were administered via the right external jugular vein to male Wistar rats. PGE₁, PGE₂ and PGI₂ decreased systemic arterial pressure 41%, 38% and 38%, respectively. The PGE₁ analog was infused (200 μg/kg/min) through the right common carotid artery. The analog itself had no effect on mean systemic arterial pressure, but maximum reversible inhibition (51%) of PGE₁-mediated vasodepression occurred following a 50 minute infusion. No significant effect of the PGE₁ analog was observed on PGE₂ or PGI₂-mediated vasodepression. These data demonstrate the ability to antagonize PGE₁-mediated vasodepression, and to differentiate the vascular responses to PGE₁ and PGE₂ or PGI₂.     

Biosynthesis of prostaglandins and thromboxane B2 by fetal lung homogenates

The conversion of arachidonic acid to prostaglandins (PG's) and thromboxane B₂ (TXB₂) was investigated in homogenates from fetal and adult bovine and rabbit lungs. Adult bovine lungs were very active in converting arachidonic acid (100 μg/g tissue) to both PGE₂ (10.7 μg/g tissue) and TXB₂ (6.2 μ/g tissue). Smaller amounts of PGF_2α (0.9 μ/g) and 6-oxoPGF_1α were formed. Homogenates from fetal calf lungs during the third trimester of pregnancy were quite active in converting arachidonic acid to PGE₂, but formed very little TXB₂, PGF_2α or 6-oxoPGF_1α. Homogenates from rabbit lungs converted arachidonic acid (100 μg/g) mainly to PGE₂, both before and after birth. The amount of PGE₂ formed increased during gestation to a maximum of about 6 μg/g tissue at 28 days of gestation. It then decreased to a minimum (1.5 μg/g) which was observed 8 days after birth, followed by an increase to about 4 μg/g in older rabbits.     

The effect of prostaglandins on cultured lapine articular chondrocytes

The effect of 8 prostaglandins (PG) on growth and sulfate incorporation by monolayer and spinner-cultured rabbit articular chondrocytes has been measured. PGA₁, PGB₁, PGE₁ and PGE₂ reduced synthesis of sulfated glycosaminoglycans (GAG) but the PGF series did not. PGA₁ was the most potent, being effective at a concentration of 2.5 μg/ml [6.8 μM] while the others required 25 μg/ml. These compounds had no effect on degradation of GAG. All 8 PGs augmented growth slightly but significantly at 2.5 μg/ml. At the higher concentration, PGA₁ was highly cytotoxic, and PGB₁ as well as PGE₂ reduced cell growth. The cytotoxicity of PGA₁ was also observed in two additional types of cultured connective tissue cells, but the inhibition of sulfated-GAG synthesis by PGA₁ and PGB₁ was confined to the chondrocytes. The response of cultured chondrocytes to exogenous PGs, albeit at apparently unphysiologically high concentrations, together with other evidence, suggests that these compounds may conceivably play a direct role in cartilage metabolism in vivo.     

Effect of modulators of cytoskeletal function on desensitization and recovery of PGE2-responsive ovarian adenylate cyclase

Exposure of cultured Graafian follicles to PGE₂ for 20 h resulted in a loss of the cyclic AMP response to fresh hormone. This desensitization was prevented by addition to the medium of D₂O (25–50%) or Li⁺ (0.6 – 6 mM), agents believed to stabilize microtubules, as well as by phalloidin (1.0 – 10 μM), believed to stabilize the polymerized state of actin, in a dose-dependent manner.The spontaneous recovery of responsiveness to PGE₂ upon incubation of refractory follicles for 6 h in hormone-free medium was prevented by addition to the medium of cytochalasin B (CB; 3 μg/ml) or of the actin-binding myosin subfragment HMM S-1 (80 μg/ml) or of anti-actin serum; viz. by agents likely to interfere with microfilament function. D₂O (50%) caused morphological damage to the inner layer of the membrana granulosa and severe depression of protein synthesis. The other drugs used (phalloidin, LiCl and cytochalasin B) had no such effects. Resensitization of refractory follicles was also prevented by cycloheximide (10 μg/ml) and by actinomycin D (10 μg/ml). It is speculated that the recovery process may involve the insertion of a newly synthesized protein, such as PG-receptor, into the membrane by a mechanism dependent on microfilament action.     

Effect of methylated prostaglandin E2 analogue on insulin secretion in man

Previous studies of the effect of E series prostaglandins /PGs/ on insulin secretion gave conflicting results in animals and little information in man. This study was designed to determine the effect of methylated PGE₂ analogue /15/S/-15-methyl PGE₂ methyl ester/, given orally, intraduodenally or intravenously, on insulin secretion, both under basal conditions and in response to intraduodenal or intravenous administration of glucose in 22 male volunteers. Methylated PGE₂ kept basal serum insulin level unchanged, but significantly reduced insulin response by 15 ± 6 μU/ml to intravenous glucose pulse injection /0.1 g/kg/ or by 45 ± 11 μU/ml to intraduodenal glucose infusion /0.5 g/kg-hr/. Blood glucose level was unaffected in tests with intraduodenal methylated PGE₂, but in tests with intravenous administration it was significantly reduced. These studies demonstrate that methylated PGE₂ analogue given orally, intraduodenally or intravenously results in a potent suppression of insulin response to glucose challenge.