Prostaglandin effects upon cyclic AMP levels, corticosterone output, and aldosterone output in the adrenal of the frog, Rana berlandieri forreri.

The mechanism of prostaglandin action in the adrenal of the frog (Rana berlandieri forreri) was evaluated in vitro. The prostaglandins evoked transient (PGA₂, PGB₁, PGE₂), continued (PGA₁, PGB₂, PGF_1α, PGF_2α), or no effects (PGE₁) upon cyclic AMP (cAMP) levels. Further, the cAMP levels were depressed (PGA₁, PGB₁, PGB₂), elevated (PGA₂, PGE₂, PGF_1α, PGF_2α), or unchanged (PGE₁) compared to the controls. Prostaglandins regulate cAMP levels in the frog adrenal. In addition, specific prostaglandins evoke specific effects in this regard. The frog adrenal corticosterone and aldosterone outputs are modulated by the prostaglandins. The modulations produced differ with the prostaglandins tested. The greatest adrenocortical sensitivity is to PGB₂ which evoked about a 13-fold increase in corticosterone output and a 5-fold increase in aldosterone output at 16 min. Only PGF_1α inhibited steroid output; the other prostaglandins stimulated steroid outputs to varying degrees. A close correlation of the prostaglandin-evoked corticosterone and aldosterone output responses with the cAMP changes was not present in the frog adrenal. The prostaglandins differentially affected cAMP levels and when compared to adrenocorticoid outputs, different prostaglandins produce different effects. The responses of the frog adrenal indicate that the mechanisms controlling adrenocortical function are more complex than originally visualized. In this regard, the site(s) of action (plasma membrane, mitochondrion, etc.) of a given factor must also be considered. Thus, the current concepts must be broadened to include a number of interacting factors among which are the cyclic nucleotides and the prostaglandins.     

Microcirculatory effects of prostaglandins E1, E2, and A1 in the rat mesentery and cremaster muscle

The present investigation was carried out to determine the site(s) of vasomotor activity of Prostaglandins E₁ (PGE₁), E₂ (PGE₂), and A₁ (PGA₁) in the terminal vascular bed of the rat mesentery and cremaster (skeletal) muscle. Studies were also performed to determine the effects of PGE₁ and PGA₁ on arteriolar responsiveness to vasoconstrictor agents. In vivo changes in microvascular diameters in response to topical application of prostaglandins and the vasoconstrictor agents were determined quantitatively with an image shearing television microscope and recording system. In the mesentery, PGE₁ elicited arteriolar dilator responses, while similar doses of PGE₂ and PGA₁ were ineffective. The responses of mesenteric arterioles to PGE₁ were not entirely dose-dependent. In the cremaster muscle, the three prostaglandins studied produced a dose-dependent dilation of all muscular microvessels; the order of potency was PGE₁ > PGE₂ > PGA₁.Mesenteric and cremasteric arteriolar responses to epinephrine, norepinephrine, angiotensin, and vasopressin were inhibited by PGE₁, and this inhibition was demonstrable after the dilator action of PGE₁ had terminated. PGA₁ did not alter vascular responsiveness in either tissue. It was concluded that prostaglandins, by virtue of their vasodilator actions and inhibitory effects on microvascular responsiveness, may contribute to local control of blood flow.     

Shell gland responsiveness to prostaglandins F2α and E1 and to arginine vasotocin during the laying cycle of the domestic hen (Gallus domesticus)

Contractile responses of isolated shell gland (SG) strips from laying hens displayed no significant differences 6 hrs before oviposition, at oviposition, and 6 hrs after oviposition when stimulated with arginine vasotocin (AVT), prostaglandin E₁ (PGE₁), or prostaglandin F_2α (PGF_2α). Dose-response curves show that the sensitivity of the SG to these agents, in vitro, is: AVT > PGF₂ > PGE₁. PGF_2α, however, produces the largest contractile response, while PGE₁ appears to be a poor agonist of contractile activity in vitro. These results are discussed in relation to the known hormonal patterns during the ovulatory cycle of the hen and the physiological roles attributed to these oxytocics in the control of oviposition.     

Effects of third ventricle injection of prostaglandins on gonadotropin secretion in goldfish,Carassius auratus

The effects of injection of prostaglandin (PG) E₁, PGE₂, and PGF_2α into the third ventricle on serum gonadotropin (GTH) concentrations in the goldfish were tested. Blood samples were taken at 30 min postinjection for radioimmunoassay of serum GTH. PG dosages of 0.5 and 1.0 μg were ineffective. However, PGE₂ and PGF_2α at the 2.0-μg dosage significantly suppressed serum GTH. PGE₁ at a 2.0-μg dosage had no effect. There were no effects on serum GTH when 2.0 μg of PGE₁, PGE₂, or PGF_2α were injected intraperitoneally. The results indicate that PGE₂ and PGF_2α suppress gonadotropin secretion by some action, presumably on the hypothalamus. However, action of PGE₂ and PGF_2α by diffusion from the site of injection to some other brain site or the pituitary cannot be eliminated in the present study.     

Prostaglandins of the one,two, and three series affect blood pressure in the American bullfrog, Rana catesbeiana

The effects of prostagandins (PGs), and three potential prostaglandin precursors, were studied on blood pressure and heart rate of the American bullfrog, Rana catesbeiana. Bullfrogs were chronically cannulated with a T cannula in the right sciatic artery. The mean systemic arterial blood pressure (SAP) prior to infusion was 20.4 ± 1.1 mm Hg. Mean preinfusion systolic and diastolic pressures were 23.9 ± 1.4 and 17.1 ± 0.9 mm Hg, respectively. Mean preinfusion heart rate was 41.1 ± 0.5 beats/min. Of the PGs tested, PGI₂ was a potent hypotensive agent, with effects at 0.03 μg/kg bw. PGE₂ was more potent than PGE₃, and PGE₁. PGA₁ and PGA₂ were the least potent, and were ineffective at doses below 100 μg/kg bw. PGF_2α was the most potent hypertensive agent tested, with thromboxane B₂ less potent. All compounds tested elevated heart rate, with PGE₂ the most effective. The prostaglandin precursors, eicosatrienoic acid, arachidonic acid, and eicosapentaenoic acid (2000 μg/kg bw) all decreased blood pressure by approximately 25%. The decrease was attenuated by indomethacin (4 mg/kg bw). These results indicate that the bullfrog utilizes all three hypotensive activity. The ability of the bullfrog to utilize several substrates makes it a good choice for comparative studies on prostaglandin synthesis.     

Prostaglandin synthesis by adult heart myocytes

Adult cardiac myocytes, prepared by proteolytic disaggregation of left ventricular tissue of rats or dogs, were incubated with [1-¹⁴C]-arachidonic acid for periods up to 90 min at 37°C. Ethyl acetate extracts of the acidified media containing carrier prostaglandins were subjected to thin layer silicic acid chromatography for separations of prostacyclin and thromboxane derivatives and classical prostaglandings (PG). Synthesis of prostaglandins was evaluated by gas and thin layer radiochromatography. Unperturbed rat heart myocytes synthesized prostaglandins linearly during the 90 min incubation and these included predominantly PGE₂ and its metabolites (PGA₂-PGB₂) and equal levels of prostacyclins (determined as 6-keto PGF_1α) thromboxane B₂ and PGF_2α. Dog heart myocytes, in contrast, produced largely 6-keto PGF_1α and PGD₂ with lesser amounts of other PGs. Incorporation of labeled arachidonate into these PGs inhibited by indomethacin or eicosatetryenoic acid, and was enhanced by incubation of cells with methylprednisolone, a phospholipase A₂ inhibitor, or by using cells from essential fatty acid-deficient rats.     

Microcirculatory effects of prostacyclin (PGI2) in the hamster cheek pouch.

The effects of prostacyclin (PGI₂) on small blood vessels in the hamster cheek pouch microcirculation were studied microscopically. Prostacyclin applied systemically or locally caused an increase in the diameter of precapillary arterioles (10–50 μm diameter) with inherent or induced tone. It was more potent prostaglandin PGE₁, PGE₂, or bradykinin. In animals treated with indomethacin (4 mg/kg po), the relative vasodilator potency of prostacyclin to PGE₁ and PGE₂ was increased. The cyclic endoperoxide precursor of prostaglandins, PGH₂, and the stable chemical breakdown product of prostacyclin, 6-oxo-PGF_1α, were also dilators but were less potent than prostacyclin itself. In some experiments, responses of arterioles to PGH₂ were biphasic, a long-lasting dilatation being preceded by a short-lasting vasoconstriction. The postcapillary venules (20–50 μm diameter) in this preparation were inactive and did not respond by dilatation or constriction to PGI₂, PGE₁, PGE₂, PGH₂, 6-oxo-PGF_1α, bradykinin, or noradrenaline.     

Inhibitory effect of a calcium antagonist (diltiazem) on aortic and coronary contractions in rabbits

This report deals with the effect of a slow calcium channel blocker, diltiazem [6] on the isometric contractions induced by certain prostaglandins, PGF_2α and PGE₂, 5-hydroxytryptamine (5-HT) and nicotine on isolated rabbit coronary artery and aortic strips. While the effect of certain prostaglandins, particularly PGH₂ metabolites on the contraction of isolated spiral strips of beef and human coronary arteries has been previously documented [3, 4] no studies have been reported on the effect of slow calcium channel blockers on these compounds. Likewise, no data are available on the effect of slow calcium channel blockers on nicotine or on 5-HT induced isometric contractions in rabbit aortas and coronary arteries. We offer evidence that a slow calcium channel blocker diltiazem selectively inhibits contraction, induced by 5-HT, the prostaglandins PGF_2α and PGE₂ in coronary arteries, but not in aortic strips. The contractions induced by nicotine, which was effective in aortic smooth muscle only, are also markedly inhibited by diltiazem as well as by prazosin an α₁ adrenergic inhibitor. Rauwolscine, an α₂ inhibitor, has little effect.     

Does of the local myocardial release of prostaglandin E2 or F2α contribute to the early consequences of acute myocardial ischaemia?

The possible role of prostaglandins in acute myocardial ischaemia was investigated in anaesthetized greyhounds which were subjected to either short (3 min) occlusions or permanent ligation of the left anterior descending coronary artery. Plasma PGE₂ and PGF_2α concentrations were measured by radioimmunoassay in blood from the aorta, the coronary sinus (draining the essentially normal myocardium) and from a local coronaryvein (draining the area rendered ischaemic by coronary artery ligation). In the permanent ligation studies PGE₂ and PGF_2α were measured before, and 2, 10 and 30 min post-ligation. At 2 and 10 min post ligation there were no significant changes in the concentrations of either prostaglandin in blood from the essentially normal myocardium or from the acutely ischaemic myocardium. After 30 min of coronary artery ligation there was a significant increase in PGE₂ in the local coronary vein. This release of PGE₂ from the ischaemic myocardium was not related to the occurrence of cardiac dysrhythmias but may reflect the onset of changes in cellular integrity in the developing area of infarction. Despite electrocardiographic and metabolic evidence of acute myocardial ischaemia there was no increase in PGE₂ or PGF_2α values in either the coronary sinus or the local coronary vein following the release of a 3-min coronary artery occlusion. Since neither PGE₂ nor PGF_2α is released from the acutely ischaemic myocardium during the first 10 min post-ligation or during reperfusion following 3-min occlusions, it is unlikely that either of these prostaglandins is involved in the early consequence of coronary artery ligation.     

Cyclic AMP levels in cultured myocardial cells under the influence of chronotropic and inotropic agents.

In an attempt to determine whether cAMP is involved in the chronotropic and inotropic response produced by glucagon, catecholamines and prostaglandins, the intracellular cAMP levels of beating myocardial cells were determined after pretreatment with the above agents. It was found that the catecholamines and glucagon did increase cAMP levels, the order of the effect being epinephrine> norepinephrine>glucagon. The catecholamine response could be blocked by practolol, a potent β-adrenergic blocking agent. Prostaglandins E₁ and F_1α also increased cAMP levels to a similar degree to that observed with glucagon, whereas PGE₂ and PGF_2α produced no detectable effects.     

Urinary Excretion of Prostaglandin F2α and 6-Keto-Prostaglandin F1αa during Volume Expansion in Patients with Glomerulonephritis

ABSTRACT. Thirteen patients with active IgA glomerulonephritis (IgA GN), ten patients with a history of Henoch-Schönlein glomerulonephritis (HS GN) and nine healthy controls were studied during hydropenia (HP) and 3% volume expansion (VE) with isotonic saline. Clearance of inulin and para-aminohippurate, urinary excretion of Na, immunoreactive prostaglandin F_2α (PGF_2α) and 6-keto-prostaglandin F_1α (6-keto-PGF_1α) were determined. The patients with a history of HS GN had normal blood pressure and renal function. As in the controls, the urinary excretion of PGF_2α decreased and the excretion of 6-keto-PGF_1α increased during VE. In the patients with IgA GN the glomerular filtration rate (GFR) was normal, markedly reduced and supernormal. Five patients had hypertension and an increased N A excretion in relation to the GFR during VE. As a group, the patients with IgA GN increased their urinary excretion of 6-keto-PGF_1α during VE, while the excretion of PGF_2α did not change. In relation to the GFR, the urinary excretion of PGF_2α, and 6-keto-PGF_1α was markedly increased in two patients with low GFR, which implies that these substances play a role in advanced renal disease. VE had little effect on PG excretion in these patients. In the hypertensive patients the urinary excretion of PGF_2α and 6-keto-PGF_1α was the same as in those with normal blood pressure. PGs are therefore not likely to mediate the increased natriuretic response to VE in hypertension.     

Prostanoids and the Cough Reflex

Prostanoids such as prostaglandin (PG) D₂, PGE₂, PGF_2α, prostacyclin (PGI₂), and thromboxane (Tx) A₂ act via five classes of receptors named DP, EP, FP, IP, and TP, respectively, and mediate a diverse range of physiological effects. Prostanoids are commonly associated with many diseases as a proinflammatory mediator; however, in the lung, prostanoids, particularly PGE₂, seem to have a protective role. Inhaled PGE₂ has been shown to be anti-inflammatory and a bronchodilator but causes cough. This has hindered the development of prostanoids for the treatment of airway inflammatory diseases. We discuss here the extensive research into the role of prostanoids in the airways and their modulation of the cough reflex.     

Platelet-activating factor stimulates eicosanoid production in cultured feline tracheal epithelial cells

The effect of platelet-activating factor (PAF) on eicosanoid generation and release in cultured feline tracheal epithelial cells was investigated by measuring a wide range of lipoxygenase and cyclooxygenase pathway products. Subconfluent epithelial cell cultures were stimulated by PAF and eicosanoid production was determined by high performance liquid chromatography (HPLC) of [³H]-labeled arachidonic acid (AA) metabolites and by radioimmunoassay (RIA) following HPLC separation. The HPLC chromatograms revealed that PAF augmented the release of prostaglandin (PG)E₂, PGF_2α, 12-hydroxyeicosatetraenoic acid (HETE), and AA. Among these eicosanoids, PGE₂ predominated under baseline conditions and following PAF exposure. RIAs of the nonradiolabeled HPLC elution corresponding to various eicosanoid standards demonstrated that PAF increased the production of 6-keto-PGF_1α, thromboxane B₂ (TXB₂), PGD₂, 5-HETE, and 15-HETE, as well as PGE₂, PGF_2α, and 12-HETE. The PAF-induced eicosanoid augmentation was dose-dependent and occurred within 1 hour with a prompt decline following termination of PAF exposure. This stimulating effect of PAF on eicosanoid release was blocked by two PAF receptor antagonists, Ro 19-3704 and WEB 2086. The PAF-induced increase in eicosanoid release was similar in magnitude to the increase caused by calcium ionophore (Ca-ionophore) A23187, a potent known stimulus for eicosanoid release. Cells of different culture durations (3 and 6 days) showed similar capacity for eicosanoid production. We conclude that PAF stimulates the production of cyclooxygenase and lipoxygenase pathway products from airway epithelial cells via PAF receptors, and that these epithelium-derived eicosanoids may be responsible for some of the PAF-induced respiratory physiological and pathophysiological effects.     

Synthesis of prostaglandins by cultured rat hearts myocytes and cardiac mesenchymal cells

Ischemia, trauma and hormonal stimulation elicit the release of prostaglandins (PGs) from the heart. Although PGI₂ is synthesized by coronary arteries, the capacities for PG synthesis of individual types of cells comprising the heart have not been elucidated. Accordingly, synthesis of prostaglandins by cultured rat cardiac myocytes and mesenchymal cells was evaluated by radiochromatography of products obtained by incubating cells with [1-¹⁴C]arachidonate, and verified by assessing the effects of cell incubation medium on platelet aggregation. Cultured mesenchymal cells synthesized PGs E₂, F_2α and 6-keto-F_1α, a metabolite of PGI₂ (2076 ± 183, 1284 ± 158 and 1194 ± 152 dpm/mg protein/30 min, respectively). Medium from mesenchymal cells inhibited platelet aggregation, an effect abolished by preincubating the cells with indomethacin, further indicating that these cells synthesized PGI₂. Cardiac myocytes synthesized PGE₂ and PGF_2α (952 ± 227 and 287 ± 104 dpm/mg protein/30 mins, respectively), but no PGI₂. Medium from myocytes did not inhibit platelet aggregation. Prostaglandins D₂, A₂ and thromboxane were not synthesized by either type of cell. Thus, PGI₂ is synthesized by cardiac mesenchymal cells and the hitherto uncharacterized sources of PGE₂ and PGF_2α found in coronary sinus effluent may include cardiac myocytes as well as mesenchymal cells.     

Nicardipine suppresses bronchoconstrictor actions of pharmacologic agents in guinea pigs

We investigated the inhibitory effects of nicardipine on airway smooth muscle constriction in the guinea pig via both in vitro and in vivo preparations. In the in vitro studies, we created dose-response curves of the excised tracheal strip. The increase in respiratory overflow produced by leukotriene (LT) D₄ was measured in the in vivo preparation. In the organ bath, nicardipine (1–12μg/ml) significantly inhibited constriction of the isolated tracheal spiral strip induced by 7 different agonists, acetylcholine (Ach), histamine (HA), serotonin (5-HT), prostaglandin F_2α (PGF_2α), slow-reacting substance of anaphylaxis (SRS-A), and LTC₄ and LTD₄. Compared to nifedipine and verapamil (data in the literature), the inhibitory potency of nicardipine on the constriction of the isolated guinea pig tracheal strip is 100–1,000 times greater. In the in vivo studies, nicardipine (50 and 100μg/kg) significantly attenuated the LTD₄-induced increase in respiratory overflow. These results suggest that nicardipine inhibits bronchoconstrictor-induced constriction of airway smooth muscle both in vitro and in vivo, and that it is a potent bronchodilator. The inhibitory characteristics of nicardipine may prove to be applicable clinically.     

Cardiovascular and Sympathetic Effects of 1-O-Hexadecyl-2-Acetyl-sn-Glycero-3-Phosphocholine in Conscious Shr and Wky Rats

Injections of 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine (alkylacetyl-GPC, 0.2–5.0 nmol/300 g body weight) induced dose-related hypotension and tachycardia in spontaneous hypertensive (SHR) and normotensive control (WKY) rats. The hypotension that developed was more pronounced in SHR than in WKY rats and was unchanged by indomethacin pretreatment. Plasma norepinephrine (NE) and epinephrine (EPI) levels were markedly increased at the time of maximal hypotension (2 min after injection of alkylacetyl-GPC); plasma EPI (but not NE) was higher in the SHR than in WKY animals. Plasma levels of TXB₂, but not 6-keto-PGF_1α, increased in both groups; the increase was more pronounced in SHR than in WKY rats. In pithed SHR rats, alkylacetyl-GPC caused only short lasting hypotension without any effect on heart rate or circulating levels of NE or EPI. These data suggest that there is an increased vascular sensitivity to alkylacetyl-GPC in SHR rats and activation of thromboxane-generating elements in both SHR and WKY rats.     

Effects of oestrogen on progesterone synthesis and arachidonic acid metabolism in human luteal cells

OBJECTIVE Locally produced oestrogens and prostaglandins (PGs) are implicated in the regulation of luteal lifespan in the human ovary. This study (1) assesses direct effects of these factors on progesterone synthesis in isolated luteal cells, and (2) explores interactions between luteal age and treatment with gonadotrophin or oestrogen on the metabolism of arachidonic acid (prostaglandin precursor) by steroidogenic luteal cells in vitro. DESIGN Primary monolayer cultures of human luteal cells obtained at different stages of the luteal phase were used to investigate the effect of oestradiol, catechol oestrogens (2- and 4-hydroxyoestradiol), diethylstilboestrol, PGE₂ and PGF_2x on basal and human chorionic gonadotrophin (hCG) stimulated progesterone production in vitro. The role of PGs as modulators of luteal cell function was further investigated by studying the metabolic fate of radioactively labelled arachidonic acid in hormone treated (oestradiol and hCG) and control cultures, assessed by high performance liquid chromatography. ATIENTS Corpora lutea were enucleated from nine women with regular ovulatory cycles undergoing microsurgical reversal of tubal sterilization. Granulosa cell aspirates were obtained from three patients undergoing in-vitro fertilization treatment. RESULTS PGE₂ and PGF_2α, at various concentrations did not have a consistent effect, whereas oestradiol, diethylstilboestrol (and 2-hydroxyoestradiol in early luteal cell cultures) significantly inhibited basal and hCG stimulated progesterone biosynthesis. Evidence for direct inhibition of 3β-hydroxysteroid dehydrogenase enzymic activity by oestradiol was obtained. Both major metabolic pathways of arachidonic acid (lipoxygenase and cyclo-oxygenase) were operative in steroidogenic luteal cells recovered throughout the luteal phase. The ratio of PGE₂ to PGF₂ synthesis in vitro by human luteal cells from endogenously incorporated arachidonic acid did not change significantly with corpus luteum age, with PGE₂ tending to predominate. Oestradiol treatment shifted arachidonic acid metabolism from the lipoxygenase towards the cyclo-oxygenase pathway in cells isolated from ageing corpora lutea. CONCLUSIONS Oestradiol, at relatively high concentrations, is a potent inhibitor of basal and hCG induced luteal cell steroidogenesis in vitro. No support is provided for the concept that luteolysis is mediated by local production of PGF_2α. The putative luteolytic effect of oestradiol may entail reduced metabolism of arachidonic acid to lipoxygenase derived products by luteal cells rather than direct stimulation of prostaglandin production by itself.     

An investigation into the role of prostaglandins in zebrafish oocyte maturation and ovulation

This study explored the potential for ovarian-derived prostaglandins (PGs) to be involved in the regulation of oocyte maturation and ovulation in zebrafish. It was demonstrated that cultured vitellogenic follicles have the capacity to produce prostaglandin E₂ (PGE₂) and PGF_2α in response to arachidonic acid (AA) in a concentration-dependent manner, and that AA stimulates the in vitro production of 17β-estradiol (E₂). The production of AA-stimulated PGF_2α was significantly reduced by treatment with the non-selective cyclooxygenase (COX) inhibitor, indomethacin (INDO). Treatment of full-grown follicles with AA did not induce oocyte maturation as assessed by germinal vesicle breakdown, but INDO significantly decreased the rate of spontaneous maturation. Using Real-Time PCR, it was shown that follicles of different developmental size classes (primary growth and pre-vitellogenic, early-vitellogenic, and mid- to full-grown vitellogenic) express enzymes that release (cytosolic phospholipase A₂ (cPLA₂); phospholipase Cγ1) or metabolize (COX-1, COX-2, and prostaglandin synthase-2) AA to PG metabolites. The expression of cPLA₂ was found to be significantly greater in full-grown follicles compared to follicles of the pre- and early-vitellogenic stages. In vivo studies demonstrated that breeding groups of zebrafish exposed to 100 μg/L INDO exhibited reduced spawning rates and clutch sizes compared with control and 1 μg/L INDO exposed fish. In other studies, it was shown that naturally spawning groups of females exhibit increased ovarian levels of PGF_2α, E₂, and 17α,20β-dihydroxy-4-pregnen-3-one (a maturation-inducing hormone in zebrafish) near the time of ovulation compared with non-breeding females. Collectively, these experiments indicate that the AA pathway in zebrafish ovaries is involved in the regulation of oocyte maturation and ovulation and a non-selective inhibitor of COX disrupts these processes.     

Antioxidant inhibition of prostaglandin production by rat renal medulla.

Antioxidant effects upon renal production of both prostaglandins and cAMP were investigated using slices of rat inner medulla. Synthetic antioxidants were more potent inhibitors of prostaglandin production than were naturally occurring antioxidants. Synthetic compounds 2,7-naphthalenediol, and Santoquin^® (Ethoxyquin) caused a 60% inhibition of prostaglandin E₂(PGE₂) synthesis at a concentration of 0.01 mM. Ascorbic acid caused only a 30% inhibition at a concentration of 10 mM. Antioxidant inhibition of prostaglandin production was also observed following arachidonic acid addition. Antioxidants that reduced PGE₂ synthesis also reduced PGF_2α synthesis. Test agents found to reduce prostaglandin synthesis also lowered cAMP content. 2,7-Naphthalenediol elicited a dose-dependent decrease in both prostaglandin synthesis and cAMP content. While PGE₁ did not increase cAMP in control slices, the low cAMP level produced by Santoquin was increased to control values by PGE₁. Furthermore, Santoquin and 2,7-naphthalenediol did not alter arginine vasopressin-stimulated cAMP content. By contrast, inhibition of the arginine vasopressin stimulation by butylated hydroxyanisole suggested additional effects by this agent. These results are consistent with the hypothesis that endogenously produced PGE₂ can exert a hormonelike action in the inner medulla by increasing cAMP content. Advantages of the inner medullary slice system compared to homogenates for investigation of the actions of antioxidants or other agents thought to alter prostaglandin synthesis are discussed.