T-2 toxemia and brain prostaglandins

T-2 toxin is a trichothecene mycotoxin which is a member of a family of closely related sesquiterpenoids. It was recently shown that T-2 toxemia is associated with elevated plasma levels of eicosanoids. To study further the effect of T-2 on the cyclooxygenase pathway of arachidonate we examined the release of PGE₂, TXB₂ and 6-keto-PGF_1α from brian tissue exposed to T-2 toxin or . Administration of T-2 toxin (0.75 or 2 mg/kg) to conscious rats caused a transient increase in the rate of the release of 6-kto-PGF_1α and TXB₂ from brain slices taken from the cortex (C); no effect was found in the hypothalamus (HT) or the nucleus tractus solitarius (NTS) region of the medulla oblogata. PGE₂ showed time and dose related increments (over 5 folds) in both the C and Ht but not in the NTS. Incubation of cortical or hypothalamic slices in oxygenated Krebs buffer with a wide range of T-2 toxin concentrations (10⁻⁹–10⁻³ M) demonstrated a complex repsonse: stimulation of PGE₂ and TXB₂ release from C slices at 10⁻⁷ M (>40%, p<0.01 and 20%, p<0.05, respectively) and inhibition at high concentrations (>10⁻⁴ M) of all PGs studied. Hypothalamic slices showed decrease in all PGs released by very low (10⁻⁹–10⁻⁸) or very high (10⁻⁴ M) concentrations of T-2.These studies are consistent with the possibility that the arachidonate cascade in the central nervous system might have a role in the pathophysiology of trichothecene mycotoxicosis.     

Effect of toxins on arachidonic acid metabolism in rat cultured pulmonary alveolar macrophages

The action of a trichothecene (T-2), microcystin-LR and saxitoxin on arachidonic acid metabolism in cultured rat alveolar macrophages was studied. Pulmonary macrophages exposed to T-2 trichothecene were stimulated to synthesize and release large amount of thromboxane B2 (TxB2) and 6-Keto F1 alpha. Microcystin-LR induced significant release of prostaglandins F2 alpha (140%), PGE2 (175%) and TxB2 (169%) compared to controls. Saxitoxin induced TxB2 release by 37%. Arachidonic acid release was stimulated by all three toxins. The release of arachidonic acid and its metabolites in alveolar macrophages exposed to T-2 toxin was partially blocked by fluocinolone (1 microM). These results suggest that macrophages synthesize and release inflammatory mediators in response to toxin exposure, and fluocinolone may protect against T-2 toxicosis.     

Gamma-linolenic acid improves the constancy of contractions in uteri from sprayed rats and is metabolized to prostaglandin E1 but not to bisenoic prostanoids

The effects of gamma-linolenic acid (GLA) on the time-dependent constancy of spontaneous contractions (isometric developed tension = IDT and frequency of contractions = FC) in uterine strips isolated from spayed rats, were explored. Moreover, the influence of the unsaturated fatty acid on the basal generation and release of tissue prostaglandins (PGs) as well as the conversion of labelled GLA into prostanoids by the uterine tissue and the effects of p-bromo-phenacyl-bromide (BPB), were also studied. GLA (10(-7)M), attenuated significantly the spontaneous decrement of contractile constancy exhibited by control preparations during a period of 180 min of activity in isolation, whereas BPB (10(5) M) resulted in an augmented and faster decrement of inotropic constancy. Spontaneous changes in the constancy of uterine motility as time progressed involved similarly both IDT and FC. After 180 min of activity in isolation a basal generation and release of PGs E and F of the series 1 and 2, were detected. The challenge with 10(-7) M GLA (delivered immediately after isolation) enhanced significantly the output of PGE1 but did not influence the generation and release of PGE2 or PGF2 alpha. BPB (10(-5) M) had no significant effect on the basal output of PGE1, PGE2 or PGF2 but completely prevented the enhancing action of GLA on the synthesis and release of PGE1. Labelled GLA was mainly converted to PGE1 by rat uterine segments and negligible counts in the 2-series of prostanoids, were observed. In presence of BPB (10(-5) M) the conversion of 1-14C-GLA, to PGE1 was almost completely abolished. The foregoing evidences suggest that exogenous GLA is metabolized by the spayed rat uterus via an elongase, forming di-homogamma-linolenic acid (DHLA), which in turn is substrate for cyclo-oxygenase peroxidase reactions yielding finally PGE1. No evidence of a delta 5-desaturase activity, converting DHLA into arachidonate and further derivatives, was detected. Coincidently, exogenous GLA was able to support a better contractile constancy as a function of time than that evidenced in untreated uterine strips isolated from castrated rats.     

Interrelationships among prostaglandins, vasopressin and cAMP in renal papillary collecting tubule cells in culture

To determine the influence of prostaglandins on cAMP metabolism in renal papillary collecting tubule (RPCT) cells, intracellular cAMP levels were measured after incubating cells with prostaglandins (PGs) alone or in combination with arginine vasopressin (AVP). PGE1, PGE2 and PGI2, but not PGD2 or PGF2 alpha, increased intracellular cAMP concentrations. At maximal concentrations (10(-5) M) the effects of PGE2 plus PGI2 (or PGE1), but not of PGI2 plus PGE1, were additive suggesting that at least two different PG receptors may be present in RPCT cell populations. Bradykinin treatment of RPCT cells caused an accumulation of intracellular cAMP which was blocked by aspirin and was quantitatively similar to that observed with 10(-5) M PGE2. PGs, when tested at concentrations (e.g. 10(-9) M) which had no independent effect on intracellular cAMP levels, did not inhibit the AVP-induced accumulation of intracellular cAMP in RPCT cells. These results indicate that PGs do not block AVP-induced accumulation of intracellular cAMP in RPCT cells at concentrations of PGs which have been shown to inhibit the hydroosmotic effect of AVP on perfused collecting tubule segments. However, at higher concentrations of PGs (e.g. 10(-5) M), the effects of AVP plus PGE1, PGE2, PGI2 or bradykinin on intracellular cAMP levels were not additive. Thus, under certain conditions, there is an interaction between PGs and AVP at the level of cAMP metabolism in RPCT cells.     

Effects of platelet-activating factor on the release of arachidonic acid and prostaglandins by rabbit iris smooth muscle. Inhibition by calcium channel antagonists.

Addition of physiological concentrations (10(-12)-10(-8)M) of platelet-activating factor (PAF) to rabbit iris muscle induced a rapid release (in 15s) of prostaglandin (PG)E2 and 6-oxo-PGF1 alpha, measured by radioimmunoassay and rapid release of 14C-labelled arachidonate and PGE2 in muscle prelabelled with [14C]arachidonic acid, measured by radiochromatography. These PAF actions are concentration- and time-dependent. The effect of PAF on PG release is not mediated through the cyclo-oxygenase pathway. The studies on the properties and mechanism of arachidonate release from phosphatidylinositol and other phospholipids in prelabelled irides by PAF suggest the involvement of a phospholipase A2. This conclusion is supported by the findings: (a) that both the removal of arachidonate and formation of lysophosphatidylinositol, from phosphatidylinositol, by PAF occur concomitantly in a time-dependent manner, (b) that Ca2+ is required for the agonist-induced release of arachidonate and PGE2, and (c) that in contrast to the rapid release of [3H]myo-inositol phosphates by carbachol and other Ca2+-mobilizing agonists previously reported in the iris muscle [Akhtar & Abdel-Latif (1984) Biochem. J. 224, 291-300], PAF (10(-12)-10(-8)M) did not appreciably enhance the release of [14C]myo-inositol phosphates and 32P labelling of phosphatidate and phosphatidylinositol in this tissue. Ca2+-channel antagonists, such as nifedipine, verapamil, diltiazem and manganese inhibited PAF-induced arachidonate and PGE2 release in a dose-dependent manner. K+ depolarization, which causes influx of extracellular Ca2+ in smooth muscle, did not increase the release of arachidonate and PGE2. The ability of Ca2+ antagonists to inhibit arachidonate release by PAF in this tissue probably reflects interference with PAF binding to its receptor. The PAF-induced release of arachidonate and PGE2 occur independently of the cyclo-oxygenase and lipoxygenase pathways. Whether the PAF-induced release of arachidonate and PG in the iris muscle is involved in the pathogenesis of inflammatory and/or physiological reactions in the eye, and how much the inhibitory effects of Ca2+-entry blockers on the PAF actions contribute to the therapeutic use of these drugs, remain to be established.     

OKY-046 inhibits thromboxane synthesis with no effect on brain edema and neurological status in head traumatized rats

Head trauma (HT) was induced in the left hemisphere of rats by a weight drop device. Edema was maximal 24 h after HT in the injured zone, and PGE2, TXB2 and 6-keto-PGF1 alpha were elevated in both the injured and remote areas. The effect of a specific thromboxane synthetase inhibitor, OKY-046, on the outcome of HT was studied. OKY-046, 100 mg/kg, was given to rats immediately and 8 h after HT. The neurological severity score (NSS) was evaluated at 1 h after HT, and at 24 h, just prior to sacrifice. Specific gravity (SG) of both hemispheres was measured after decapitation. Prostaglandins (PGs) were extracted from the site of injury and from the frontal lobes, remote from the injury, and assayed by RIA. Basal levels of PGE2 and 6-keto-PGF1 alpha were not reduced by the drug while basal TXB2 levels were lowered. However, the increased production due to HT of all PGs, was inhibited by OKY-046, especially that of TXB2. The ratio of TXB2/6-keto-PGF1 alpha, known to affect vascular tone, was reduced by OKY-046 treatment as a result of TXA2 synthesis inhibition. Still, no effect was found on the neurological outcome (as evaluated by the NSS), or on edema formation (expressed by reduced SG). Thus, based on the present findings increased TXA2 synthesis cannot be implicated in the pathophysiology of cerebral edema or dysfunction following HT.     

Does protein kinase C activation mediate thrombin-induced arachidonate release in human platelets?

Thrombin stimulated rapid formation of diacylglycerol, inositol 1,4,5-trisphosphate (IP3) and thromboxane B2 (TXB2) in human platelets. Formation of diacylglycerol and IP3 appeared to precede that of TXB2. Activation of protein kinase C by diacylglycerol combining with Ca+2 mobilization by IP3 has been implicated in mediating arachidonate release. However, addition of the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) to platelet suspension did not inhibit thrombin-stimulated arachidonate release and TXB2 synthesis, whereas addition of the Ca+2 antagonist, 3,4,5-trimethoxybenzoic acid 8-(diethylamino) octyl ester (TMB-8) or the calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) abolished arachidonate release. The correlation of IP3 production with arachidonate release on increasing the concentrations of thrombin was further examined. IP3 production reached near maximum at 0.2 U/ml, whereas TXB2 synthesis continued to increase at 1 U/ml. These results suggest that protein kinase C activation may not mediate arachidonate release and that Ca+2 mobilization by IP3 may only partially account for arachidonate release in platelets stimulated with relatively high concentrations of thrombin.     

Expression of enzymes of cyclooxygenase pathway and secretion of prostaglandin E2 and F2alpha by porcine myometrium during luteolysis and early pregnancy

Past studies of uterine prostaglandin (PGs) and pig reproduction have focused on endometrial rather than myometrial PGs. This study documents the synthesis and secretion of myometrial prostaglandins (PGs) in pigs and the involvement of oxytocin (OT) in these processes. Cyclooxygenase-2 (COX-2) expression was similar in myometrial explants from cyclic and pregnant pigs (days 14-16) and OT (10(-7) M) in vitro significantly increased COX-2 protein regardless of reproductive state. Basal expression of prostaglandin E2 synthase (PGES) was higher during pregnancy than during luteolysis. Conversely, prostaglandin F synthase (PGFS) was highest during luteolysis and lower in myometrium from gravid animals. OT had no influence on the expression of PGES and PGFS. In another tissue culture experiment, myometrial slices produced more PGE2 than PGF2alpha regardless of reproductive state of the female. OT stimulated PGE2 production in myometrium harvested during luteolysis and increased PGF2alpha production in all tissues examined. Progesterone (P4; 10(-5) M) blocked stimulatory effect of OT on myometrial PG release. Myometrial OTr mRNA was higher (P=0.03) during luteolysis than during pregnancy. In conclusion: (1) oxytocin increases myometrial COX-2 expression, but does not influence the expression of terminal enzymes of PGs synthesis (PGES and PGFS); (2) porcine myometrium preferentially produces PGs during early pregnancy and secretes more PGE2 than PGF2alpha; (3) myometrial OT and OTr support secretion of PGs from myometrium during luteolysis.     

A colorimetric technique for detecting trichothecenes and assessing relative potencies

We tested a novel colorimetric toxicity test, based on inhibition of beta-galactosidase activity in the yeast Kluyveromyces marxianus, for sensitivity to a range of mycotoxins. A variety of trichothecene mycotoxins could be detected. The order of toxicity established with this bioassay was verrucarin A > roridin A > T-2 toxin > diacetoxyscirpenol > HT-2 toxin > acetyl T-2 toxin > neosolaniol > fusarenon X > T-2 triol > scirpentriol > nivalenol > deoxynivalenol > T-2 tetraol. The sensitivity of detection was high, with the most potent trichothecene tested, verrucarin A, having a 50% effective concentration (concentration of toxin causing 50% inhibition) of 2 ng/ml. Other mycotoxins (cyclopiazonic acid, fumonisin B1, ochratoxin A, patulin, sterigmatocystin, tenuazonic acid, and zearalenone) could not be detected at up to 10 micrograms/ml, nor could aflatoxins B1 and M1 be detected at concentrations up to 25 micrograms/ml. This test should be useful for trichothecene detection and for studies of relevant interactions-both between trichothecenes themselves and between trichothecenes and other food constituents.     

Stimulatory and inhibitory effects of prostaglandins on the gonadotropin-sensitive adenylate cyclase in the monkey corpus luteum

Detailed analysis of the action of prostaglandins (PGs) on the corpus luteum in primate species is very limited. In this study we examined the response of the adenylate cyclase system to PGs in homogenates prepared from the corpus luteum of rhesus monkeys at midluteal phase of the menstrual cycle. The conversion of [alpha 32p] ATP to [32p] cyclic AMP (cAMP) was assessed in the absence (control activity; 50 microM GTP) and presence of various concentrations of seven PGs and arachidonic acid, either alone or in combination with 250 nM hCG. Cyclic AMP production increased up to three-fold in the presence of PGD2, PGE2, PGI2 or PGF2 alpha; however PGA2, PGB2, 13, 14-dihydro-15-keto PGE2 and arachidonic acid alone did not alter cAMP levels. In dose-response studies, adenylate cyclase was 10 and 100-fold more sensitive to PGD2 (Vmax at 1 X 10(-5) M) than to PGE2 or to PGI2 and PGF2 alpha, respectively. Activity in the presence of hCG plus either PGD2, PGE2, PGI2 or PGF2 alpha did not differ from that for hCG (or the PG) alone. In contrast, addition of PGA2 or arachidonate inhibited (p less than 0.05) hCG-stimulated cAMP production by 50 and 100 percent. We conclude that the gonadotropin-sensitive adenylate cyclase of the macaque corpus luteum is also modulated by several PGs. These factors may either mimic (e.g., PGD2, PGE2, PGI2) or suppress (PGA2) gonadotropin-stimulated cAMP production and possibly cAMP-mediated events in luteal cells.     

Role of prostaglandins in calcium-induced corticosteroid secretion by isolated frog interrenal gland

The role of prostaglandins (PGs) in calcium-induced corticosteroid secretion by frog adrenal (interrenal) gland has been examined in vitro using a perifusion technique. Increasing concentrations of CaCl2 (4-10 mM) stimulated in a dose-dependent manner aldosterone, PGE2 and 6-keto-PGF1 alpha production, whereas TXB2 was not affected. The kinetics of the adrenal response to CaCl2 indicated that the increase in PG output always preceded that of steroid. Administration of cobalt (4 mM), a calcium-channel inhibitor, blocked the calcium-induced stimulation of PGs and corticosteroids. Infusion of indomethacin (5 X 10(-6) M), a specific cyclooxygenase inhibitor, significantly decreased the basal production of PGs and steroids, and prevented the stimulatory effect of CaCl2 (6 mM). Infusion of the calcium ionophore A 23187 (10(-6) M), for 20 min, induced a marked stimulation of PG and steroid production. Taken together, these data support the notion that biosynthesis of prostaglandins is associated with calcium-induced corticosteroid secretion in frog adrenal cells.     

Prostaglandin inhibition of testosterone production induced by luteinizing hormone, dibutyryl cyclic AMP or 3-isobutyl-1-methyl xanthine in dispersed rat testicular interstitial cells.

Testicular interstitial cells were utilized to study the effects of prostaglandins (PG) on in vitro testosterone production and to examine the role of cyclic adenosine-3',5'-monophosphate (cAMP) in this process. Testosterone production was assessed after 3 hour incubations while cAMP accumulation was examined after a 0.5 hour incubation period. Testosterone and cAMP were measured by radioimmunoassay. None of the PGs tested (PGA, PGA2, PGB1, PGE1, PGE2, PGF1alpha PGF2alpha) altered basal testosterone production when present in incubates at concentrations of 1.3 X 10(-8) M to 1.3 X 10(-4). However, at concentrations of 1.3 X 10(-4) M all of these PGs were capable of decreasing Luteinizing Hormone (LH; 100ng)-induced testosterone production. The inhibition of LH-induced testosterone production by the B, E and F series PGs was less pronounced than that for the A series. PGA1 and PGA2 exhibited 80% and 95% inhibition, respectively, at 1.3 X 10(4) M. The inhibitory action of 4 X 10(5) M PGA1 or PGA2 was evident within 30 minutes. Preincubation of interstitial cells with indomethacin (10(-5) or 10(-6) M) for 30 minutes did not alter subsequent basal or LH (100ng)-induced testosterone production. Accumulation of cAMP was stimulated by LH (10 microgram) or by PGs (1.3 X 10(-4) M PGA1, PGA2, PGB1, PGE1 or PGF2alpha). The PG-induced cAMP accumulation thus occurred at concentrations where LH-stimulated testosterone production was inhibited. Furthermore, PGA1 and PGA2 (1.3 X 10(-4) M) inhibited testosterone production induced by either 3-isobutyl-1-methyl xanthine (MIX; 10(-4) M or 10(-3) M) or dibutyryl cAMP (dbcAMP; 10(-4) M or 10(-3) M). These results indicate that PGs can block testosterone production by a direct effect on testicular interstitial cells and suggest that PGs exert their inhibitory action distal to stimulation of cAMP formation. PGs do not appear to play a role in the mechanism of LH action.     

Mass quantitation of agonist-induced arachidonate release and icosanoid production in a fibrosarcoma cell line. Effect of time of agonist stimulation, amount of cellular arachidonate, and type of agonist

The mass of total arachidonate released from phospholipids upon agonist stimulation of the cell and the fraction of released arachidonate which is converted to icosanoids are two parameters of arachidonate metabolism which have been difficult to quantitate because the mass of arachidonate released upon cell stimulation is very low. We have been able to quantitate both of these parameters under a variety of experimental conditions using a unique essential fatty acid-deficient mouse fibrosarcoma cell line (EFD-1), which when repleted with arachidonate, produces prostaglandin E2 (PGE2). Because there is no endogenous pool of arachidonate in these cells, the specific activity of exogenous arachidonate does not change upon incorporation into cells, an advantage which permits mass determination of very small quantities of arachidonate directly from radioactive counts. EFD-1 cells were incubated with various concentrations of [14C]arachidonate (for release studies) or unlabeled arachidonate (for PGE2 radioimmunoassays) for 24 h and then stimulated with bradykinin. The time courses for arachidonate release and PGE2 production demonstrated that free arachidonate was rapidly converted to PGE2 with plateau levels attained for both parameters within 240 s of agonist exposure for 2 microM and for 10 microM arachidonate-repleted cultures. There was a linear relationship (r = 0.94) between the mass of arachidonate in the cell and the mass of arachidonate released upon stimulation, up to a cellular concentration of 11 nmol of arachidonate/10(6) cells, a concentration 10-20% above normal for the parent mouse fibrosarcoma cell line (HSDM1C1) which is not essential fatty acid-deficient. Importantly, the percent of released arachidonate which was converted to PGE2 decreased from 90 to 15% with increasing concentrations of cellular arachidonate, because PGE2 production plateaued at greater than or equal to 6 nmol of arachidonate/10(6) cells, but total arachidonate release continued to rise. Finally, we demonstrated that agonist stimulation with thrombin, A23187, and bradykinin all showed the same percent conversion of released arachidonate to PGE2, implying that the determination of this fraction is not a function of the mechanism of release. These studies with our unique cell line indicate that, when the concentration of arachidonate in the cell is not elevated above amounts normally found in our HSDM1C1 cell line, released arachidonate is rapidly and almost quantitatively converted to PGE2, independent of the agonist used to stimulate the cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Comparative effects of trichothecene mycotoxins on bovine platelet function: Acetyl T-2 toxin,a more potent inhibitor than T-2 toxin

The effects of the trichothecene mycotoxins (acetyl T-2 toxin, T-2 toxin, HT-2 toxin, palmityl T-2 toxin, diacetoxyscirpenol (DAS), deoxynivalenol (DON), and T-2 tetraol) on bovine platelet function were examined in homologous plasma stimulated with platelet activating factor (PAF). The mycotoxins inhibited platelet function with the following order of potency: acetyl T-2 toxin > palmityl T-2 toxin = DAS > HT-2 toxin = T-2 toxin. While T-2 tetraol was completely ineffective as an inhibitor, DON exhibited minimal inhibitory activity at concentrations above 10×10^?4M. The stability of the platelet aggregates formed was significantly reduced in all mycotoxin treated platelets compared to that of the untreated PAF controls. It is suggested that the increased sensitivity of PAF stimulated bovine platelets to the more lipophilic mycotoxins may be related to their more efficient partitioning into the platelet membrane compared to the more hydrophilic compounds.     

Evolution of Streptozotocin–Pancreatic Damage in the Rat: Modulatory Effect of Endothelins on the Nitridergic and Prostanoid Pathway

Many lines of evidence indicate that an increased pancreatic production of nitric oxide (NO) and prostaglandins (PGs) is found in the pancreas of streptozotocin-diabetic rats and that endothelins (ETs) are closely related to the nitridergic and prostanoid pathway in several tissues. In the present study the relationship between NO, ETs, and PGs has been explored in isolated pancreatic tissue from streptozotocin-diabetic rats. Pancreatic ET levels are higher in pancreatic tissues from diabetic (D) rats compared to control (C) animals. The addition of nitric oxide synthase (NOS) inhibitors (1 mM N(G)-nitro-l-arginine methyl ester, 600 microM N(G)-monomethyl-l-arginine) in the incubating medium reduces and NO donors (SIN-1, 300 microM spermine suppress, NONOate 100 microM) increases ET levels in pancreatic slices from C and D animals. PGE(2) (10(-7) M) increases and indomethacin (10(-6) M) decreases ET pancreatic production only in D but not in C tissues when added into the incubating bath. When tissues are incubated in the presence of endothelin 1 (ET-1) (10(-7) M), NOS activity is higher in C pancreas, while the ET-receptor antagonist bosentan (B) decreases NOS levels in D but not in C tissues. When pancreatic arachidonic acid (AA) conversion to prostaglandins was explored, ET-1 increased PGF(2alpha), PGE(2), and TXB(2) levels in C but not in D tissues. B abolishes TXB(2) increment due to the diabetic state, but failed in modulating AA conversion to 6-keto PGF(1alpha), PGF2(alpha) and PGE(2) in D pancreas. Our results show an alteration in AA metabolism, ET production, and NO increment associated with pancreatic damage due to streptozotocin.     

Histamine alters prostaglandin output from diestrous rat uteri. Involvement of H2-receptors and 9-keto-reductase

The effects of exogenous histamine (H) on prostaglandin (PG) generation and release in uteri isolated from diestrous rats and the influences of H2-receptors blockers (cimetidine and metiamide) on the output of uterine PGs, were explored. Moreover, the action of H on the uterine 9-keto-reductase, was also studied. Histamine (10(-4) M) failed to alter the basal output of PGE1 but reduced significantly the generation and release of PGE2 and augmented the output of PGF2 alpha. On the other hand, cimetidine (10(-5) M) enhanced the basal release of PGE2 but had no action on the outputs of PGs E1 or F2 alpha. The enhancing effect of H on the production and release of PGF2 alpha was abolished in the presence of cimetidine. Also, the antagonist reversed the influence of H on the output of PGE2. Metiamide, another H2-receptor antagonist, did not alter the basal control generation and release of uterine PGs, but antagonized the augmenting influence of H on PGF2 alpha uterine output, as much as cimetidine did, and prevented the depressive action of H on the release of PGE2 from uteri. Histamine (10(-4) M) significantly stimulated uterine formation of cyclic-adenosine monophosphate, an action which was antagonized by the presence of cimetidine (10(-5) M), a blocker of H2 receptors. Also, histamine (10(-5) M) and dibutyrylcyclic-adenosine monophosphate (DB-cAMP) at 10(-3) M, enhanced significantly the formation 3H-PGF2 alpha from 3H-PGE2. Results presented herein demonstrate that H is able to diminish the generation of PGE2 in uteri from rats at diestrus augmenting the synthesis of PGF2 alpha, apparently via the activation of H2-receptors, enhancing adenylate-cyclase. These effects appear to increase uterine 9-keto-reductase activity which transforms PGE2 into PGF2 alpha. Relationships between the foregoing results and those evoked by estradiol, are also discussed.     

Effect of Dexamethasone on Prostaglandin Synthesis in Various Areas of the Rat Brain

Glucocorticoid hormones are known to inhibit the production of prostaglandins in many cell types and tissues. The effect of these hormones on the biosynthesis of brain tissue is not yet clear. In the present study we investigated the effect of dexamethasone on the release of prostaglandin E2 (PGE2), thromboxane B2 (TXB2), and 6-keto-PGF1 alpha from various brain areas of male rats. Slices from cortex, hippocampus, hypothalamus, and striatum taken from rats pretreated with dexamethasone 4 mg/kg body weight or vehicle, 18 h and 2 h prior to killing, were incubated in Krebs-Ringer-bicarbonate for 1 h. The accumulation of PGs in the medium was determined by radioimmunoassay. Pretreatment with dexamethasone significantly reduced the release of all PGs from the cortex by 40-50%. In the striatum and hippocampus only TXB2 was reduced by approximately 40%. In the hypothalamus the effect of dexamethasone was not significant. When slices of the same brain areas from intact rats were incubated for 1 h in the presence of 40 microM dexamethasone, only the release of PGE2 from the cortex was reduced (by 30%). These results suggest that glucocorticoids can inhibit PG synthesis in brain tissue, and that the cortex is the most sensitive area to the inhibitory effect of the hormone.     

Mode of action of erythropoietin and glucocorticoids on the hepatic erythroid precursor cells: role of prostaglandins

The hypothesis that prostaglandins, and especially PGE2, are the second messengers of erythropoietin (Ep) and that glucocorticoids inhibit Ep action by inhibiting PG synthesis was tested on the erythroid cell line from fetal rat liver. The optimal (10(-9) M) stimulatory concentration of PGE2 did not reproduce, by far, the maximal effect of Ep on the growth of CFUE erythroid colonies. Ep did not increase PGE2 release in liquid culture media of cell suspensions made of the whole erythroid line or enriched (over 85%) in precursor cells. Ep did not modify the turnover rate of arachidonate. Nevertheless, indomethacin partially inhibited Ep effect on CFUE development, and this inhibition was abolished by PGE2. These results suggest that PGE2 potentiates Ep action but is not its second messenger. Spontaneous PGE2 release in liquid culture media brought about concentrations of the order of 10(-9) M, and 10(-7) M dexamethasone completely inhibited this release. Part of (but not all) the anti-Ep effects of glucocorticoids might thus be mediated this way. Dexamethasone effects required previous protein synthesis.     

Effect of prostaglandins on hypothalamic-pituitary-testicular function in vitro.

The effect of prostaglandins (PG) A1, E1, E2 and F2 alpha in the concentration range of 10(-7)--10(-4) M were studied in vitro on a rat hypothalamic tissue, collagenase-digested isolated anterior pituitary cell and Leydig cell suspension system by measuring the testosterone production of incubated Leydig cells. PGs did not change the testosterone production and the hCG sensitivity of the Leydig cells, nor the LH secretion and the LHRH sensitivity of the anterior pituitary cells. PGE2 at concentrations of 10(-6), 10(-5) and 10(-4) M significantly increased the hypothalamic tissue-induced pituitary-testicular activation, and this stimulatory effect of PGE2 was dose dependent. PGA1, PGE1 and PGF2 alpha did not alter hypothalamic LHRH release measured in vitro. The results suggest that PGE2 has a direct stimulatory effect on hypothalamic LHRH release.