Prostacyclin as neuromodulator in the sympathetically stimulated rabbit heart

Prostaglandin E₂ (PGE₂) has previously been shown to inhibit sympathetic neurotransmission in different organs and species. Based on this inhibitory effect and on its reversal by cyclo-oxygenase inhibitors, PGE₂ has been claimed to be a physiological modulator of in vivo release of norepinephrine (NE) from sympathetic nerves. It is now recognized that prostacyclin (PGI₂) is the main cyclo-oxygenase product in the heart. We therefore addressed the question whether PGI₂, within the same preparation, is formed in increased amounts during sympathetic nerve stimulation and has neuromodulatory activity.The effluent from isolated rabbit hearts subjected to sympathetic nerve stimulation or to infusion of NE or adenosine (ADO) was collected, and its content of PGE₂ and 6-keto-PGF_1α (dehydration product of PGI₂) was analyzed using gas chromatography/mass spectrometry, operated in the negative ion/chemical ionization mode. Other hearts were infused with PGI₂ and nerve stimulation induced outflow of endogenous NE into the effluent was analyzed using HPLC with electrochemical detection. Nerve stimulation at 5 or 10 Hz (before but not after adrenergic receptor blockade), as well as infusion of NE (10⁻⁶–10⁻⁵M) or ADO (10⁻⁴M) increased the cardiac outflow of 6-keto-PGF_1α. Basal and nerve stimulation induced efflux of 6-keto-PGF_1α was approximately 5 times higher than the corresponding efflux of PGE₂. PGI₂ dose-dependently inhibited the outflow of NE from sympathetically stimulated hearts, the inhibition at 10⁻⁶M being approximately 40%.On the basis of these observations we propose that PGI₂ is a more likely candidate than PGE₂ as a potential modulator of neurotransmission in cardiac tissue in vivo.     

Effects of all CIS-5, 8,11,14,17 eicosapentaenoic acid and PGH3 on platelet aggregation

In human platelet-rich plasma (PRP) eicosapentaenoic acid (EPA) inhibited platelet aggregation induced by a stable analogue of PGH₂ (U46619), arachidonic acid, collagen or ADP. EPA was more potent than oleic, linoleic, α-linolenic or γ-linolenic acids. In aspirin-treated platelets, aggregation induced by U46619 was inhibited to a similar extent by arachidonic acid or by EPA over a range of concentrations of 0.05–0.3 mM. EPA incubated with PRP did not induce the generation of a thromboxane (TXA)-like activity; indeed it prevented the formation of TXA₂ induced by arachidonic acid or by collagen. The anti-aggregatory activity of EPA was not influenced by inhibitors of cyclo-oxygenase and lipoxygenase. The anti-aggregatory action of EPA may be caused by a rapid occupancy by EPA of TXA₂/PGH₂ “receptors” on platelet membrane as well as by a slower displacement of arachidonic acid from platelet phospholipids by chemically unchanged molecules of EPA.Not all samples of PRP were irreversibly aggregated by PGH₂, but in those that were, PGH₃ also induced an immediate dose-dependent but reversible aggregation. After a 4 min incubation of non-aggregating doses of PGH₂ or PGH₃ (100–300 nM) with PRP a stable anti-aggregatory compound was detected. The inhibitory activity produced from PGH₃ was apparently more potent (ca 10 times) than that obtained from PGH₂. The anti-aggregating compounds were identified by TLC and GLC-MS as PGD₂ and PGD₃. The apparent difference of potency between PGD₂ and PGD₃ was attributed to the concurrent production of PGE₂ and PGE₃. PGE₂ prevented the inhibitory effect of PGD₂ whereas PGE₃ did not affect the activity of PGD₃.It is concluded that one of the reasons for the low incidence of myocardial infarction in Eskimos could be that the pro-aggregatory arachidonic acid is replaced in their phospholipids by the anti-aggregatory EPA.     

Prostanoids and hemostasis in chickens: Anti-aggregating activity of the prostaglandins E1 and E2, but not of prostacyclin and prostaglandin D2

Aggregation of chicken thrombocytes was studied in whole blood using an electronic aggregometer. Serotonin (5-hydroxytryptamine, 5HT), arachidonic acid (AA) and collagen, but not adenosinediphosphate (ADP) induced aggregation. Prostaglandin (PG) endoperoxides were essential for arachidonic acid-induced aggregation, but were not involved in 5HT-induced aggregation, as indicated by inhibitory studies with indomethacin. Similar experiments indicated that biosynthesis of endogenous PG endoperoxides contributed to the aggregation induced by low concentrations of collagen, but was of little importance when high collagen doses were employed. PGE₁ and PGE₂ could abolish all types of aggregation studied, whereas prostacyclin (PGI₂) and PGD₂ were without any anti-aggregatory activity at 1 μg/ml. Between 1 and 100 ng/ml PGE₁ and PGE₂ inhibited arachidonic acid- and 5HT-induced aggregation dose-dependently.The lack of any hemostatic function of PGI₂ in chickens was also indicated by the absence of biosynthesis of endogenous PGI₂ in chicken aorta. PGI₂ was assessed as anti-aggregating activity, released by aortic fragments stirred in rabbit platelet rich plasma. Still, the presence of chicken aortic tissue i chicken whole blood inhibited 5HT-, but not arachidonic acid-induced aggregation. This inhibition was not affected by pretreatment of the aortic fragments with indomethacin or pargyline.     

Effect of prolonged prostaglandin exposure on prostaglandin synthesis by human lung fibroblasts

Pretreatment of human lung fibroblasts with PGE₂ but not PGF_2α enhanced synthesis of prostaglandins (PGs). The effect of the pretreatment on PG synthesis was related to the concentration of PGE₂ that was added to the culture medium. Pretreatment with PGE₂ at 5 × 10⁻¹²M did not enhance PG synthesis whereas pretreatment with PGE₂ at 5 × 10⁻⁶M induced a maximal effect. Production of PGs was increased following 1 day of pretreatment with PGE₂ and was increased further following 3 days of pretreatment. The PGE₂ treated cells showed only a slight increase in the bradykinin-induced release of radioactivity from cells prelabeled with [³H]arachidonic acid but showed a dramatic increase in the bradykinin-induced synthesis of radio-labeled PGs. The conversion of free arachidonate to PGs in both intact cells and in a cell-free preparation was increased by PGE₂ pretreatment. The presence of cyclohexamide during the pretreatment did not inhibit the PGE₂-induced activation of PG synthesis. Taken together, the results indicate that pretreatment of cells with PGE₂ increased PG synthesis by augmenting the conversion of arachidonate to PGs.     

Prostacyclin production of rat aorta “in vitro” is increased by the combined action of dipyridamole plus pentoxifylline

The present study evaluates the effect of dipyridamole and pentoxifylline, individually and in combination, on PGI₂-like production and arachidonic acid metabolism of rat aorta “in vitro”. Pentoxifylline 100 μM and dipyridamole 92 and 184 μM increased PGI₂-like activity, as measured by the platelet aggregation inhibitory capacity of the aortic ring incubates, by 71%, 46% and 60% respectively; a greater increase in PGI₂-like activity was observed with the combination of the drugs than when they were used separately. This effect was observed even at the lowest doses assayed. In fact, dipyridamole 9.2 μM plus pentoxifylline 1 μM increased the PGI₂-like activity by 30% while the individual increase was 4.5% and 10.6% respectively. To obtain more information on the effect of the dipyridamole-pentoxifylline combination on arachidonic acid metabolism, arteries were incubated with (1-¹⁴C) arachidonic acid, and the 6-keto-PGF_1α and PGE₂ quantified. Dipyridamole 92 μM plus pentoxifulline 1 and 10 μM increased 6-keto-PGF_1α and PGE₂ production by about 30% and 48% respectively while combination with pentoxifylline 100 μM increased the 6-keto-PGF_1α 76.5% and the PGE₂ 50%. The possible biological effect and therapeutic implications of increased PGI₂ production by the arteries due to the dipyridamole-pentoxifylline combination remains to be ascertained.     

In vitro synthesis of prostaglandins and related lipids by populations of human peripheral blood mononuclear cells

This report focuses on the identification of the human peripheral blood mononuclear cells that do or do not produce prostaglandins (PGs) and related arachidonic acid metabolites. Our results, using two different assay systems, indicate that the monocyte/macrophage (MØ) is the major and possibly sole source of thromboxane (TXB₂) and prostaglandin E₂ (PGE₂) among peripheral blood mononuclear cells. Adherent peripheral blood monocytes (> 95% esterase positive) produced substantial amounts of these compounds. Quantitation of products which had incorporated exogenous ¹⁴C-arachidonic acid and radioimmunoassay of adherent cell culture fluids demonstrated that the amount of TXB₂ produced by these cells was appreciably greater than the amount of PGE₂ produced. Additional confirmation of TXB₂ synthesis was shown by abolishing the TXB₂ peak on TLC and TXB₂ activity detected by RIA by treating cells with a specific inhibitor of thromboxane synthetase. In contrast, non-adherent T cells failed to synthesize either PGE₂ or TXB₂. Non-adherent B cells (95% Ig positive) incubated with ¹⁴C-arachidonic acid produced a small peak of radioactivity co-chromatographing with TXB₂, and no PGE₂. All three cell populations incorporated similar amounts of ¹⁴C-arachidonic acid into hydroxy-fatty acids. We were unable to detect 6-keto-F_1α, the hydrolysis product of prostacyclin (PGI₂) in any of the cell types tested. The absence of PG synthesis among normal peripheral blood T and B cells was also noted among established human lymphoid cell lines. Neither a human T (CCRF), nor a human B-cell line (GM-130), produced PGE₂ or TXB₂. Three murine macrophage cell lines, P388D₁, J774.2, and WHI-3 produced PGE₂ and the latter TXB₂ as well.     

Effects of prostaglandins E2, I2 and F2α, arachidonic acid and indomethacin on pressor responses to norepinephrine in conscious rats

The effects of prostaglandins E₂ (PGE₂), I₂ (PGI₂) and F₂α (PGF₂α), arachidonic acid and indomethacin on pressor responses to norepinephrine were examined in conscious rats. Intravenously infused PGE₂ (0.3, 1.25 μg/kg/min), PGI₂ (50, 100 ng/kg/min), PGF₂α (1.8, 5.4 μg/kg/min) and arachidonic acid (0.7, 1.4 mg/kg/min) did not change the basal blood pressure. Both PGE₂ and PGI₂ significantly attenuated pressor responses to norepinephrine, whereas PGF₂α significantly potentiated them. Arachidonic acid, a precursor of the prostaglandins (PGs), significantly attenuated pressor responses to norepinephrine. Since the attenuating effect of arachidonic acid was completely abolished by the pretreatment with indomethacin (5 mg/kg), arachidonic acid is thought to exert an effect through its conversion to PGs. On the contrary, intravenously injected indomethacin (0.2–5.0 mg/kg) facilitated pressor responses to norepinephrine in a dose-related manner without any direct effect on the basal blood pressure. These results suggest that endogenous PGs may participate in the regulation of blood pressure by modulating pressor responses to norepinephrine in conscious rats.     

Prostaglandin biosynthesis by the rat uterus during the oestrus cycle, temporal correlation with plasma oestradiol and progesterone concentrations, identification of 6 keto PGF1α as the major metabolite

Prostaglandin biosynthesis was studied in the rat uterus during the oestrous cycle. Uterine homogenates were incubated for 20 minutes in the presence of exogenous substrate (2.10⁻⁵M). PGF_2α and PGE₂ were measured by R.I.A.. A sharp peak PGF_2α and a smaller peak of PGE₂ were observed at prooestrus, 20 h. Another small PGE₂ peak occurred at dioestrus II, 15 h. The lowest values of both PGs were found on dioestrus, 15 h. Plasma oestradiol concentration were highest at proestrus, 15 h and 20 h. A sharp progesterone peak occurred at prooestrus, 20 h. The PGF_2α peak is next to the oestradiol peak and is superimposable or lags slightly beyond the progesterone peak.Incubation with ¹⁴C arachidonic acid and subsequent analysis of extracts by TLC and scanning showed that the major metabolite is PGI₂, identified as 6 keto PGF_1α. The conversion rate of arachidonic acid into 6 keto PGF_1α is 5 times higher than into PGF_2α. 6 keto PGF_1α was further identified by GC/MS. No significant difference was observed between 6 keto PGF_1α production during oestrus and dioestrus.     

Arachidonate metabolism in bovine gallbladder muscle

Incubation of [1-¹⁴C]arachidonic acid (AA) with homogenates of bovine gallbladder muscle generated a large amount of radioactive material having the chromatographic mobility of 6-keto-PGF_1α (stable product of PGI₂) and smaller amounts of products that comigrated with PGF_2α and PGE₂. Formation of these products was inhibited by the cyclooxygenase inhibitor indomethacin. The major radioactive product identified by thin-layer chromatographic mobility and by gas chromatography - mass spectrometric analysis was found to be 6-keto-PGF_1α. The quantitative metabolic pattern of [1-¹⁴C]PGH₂ was virtually identical to that of [1-¹⁴C]AA. Incubation of arachidonic acid with slices of bovine gallbladder muscle released labile anti-aggregatory material in the medium, which was inhibited by aspirin or 15-hydroperoxy-AA.These results indicate that bovine gallbladder muscle has a considerable enzymatic capacity to produce PGI₂ from arachidonic acid.     

Binding to protaglandin (PG) receptors and activation of adenyl cyclase by 20-isopropylidene-PGS in rabbit platelet membranes

20-Isopropylidene-PGE₁ (Isop-PGE₁) was about 10 times more potent than PGE₁ in inhibition of thrombin-induced aggregation of rabbit washed platelets. Likewise, 20-isopropylidene-17(R)-methyl-carbacyclin (CS-570), a stable PGI₂ analogue, was more potent than carbacyclin in the anti-aggregatory activity. In order to define the platelet-prostaglandin interactions, a binding assay was done using platelet membranes with [³H]-PGE₁ as a radioligand. Isop-PGE₁ (IC₅₀μM) bound to the PG receptors more potently than PGE₁ (IC₅₀ = 2.1 μM). CS-570 (IC₅₀=0.39 μM) was more potent than carbacyclin (C₅₀=1.9 μM). These indicate that introduction of an isopropylidene group to the carbon 20 of PGs increases the binding ability to the receptors. These PGE₁ and PGE₂ analogues activated platelet membrane adenyl cyclase and increased intracellular cAMP levels with the same potency series obtained in the binding experiments. All these results suggest that the binding to the receptors by these PGs is coupled to the activation of adenyl cyclase, followed by the increase in cAMP levels in platelets and the inhibition of platelet aggregation. Thus, the increased anti-aggregatory activity of 20-isop-PGs may be explained by their increased affinity for the PG receptors and stimulation of adenyl cyclase.15-Epimeric-20-isopropylidene-PGE₁ (15-Epi-isop-PGE₁), which has an unnatural configuration of the 15-hydroxyl group, was much less potent than isop-PGE₁ in the binding experiment and the other three investigations. This indicates that the configuration of the 15-hydroxyl group is important for the binding to the PG receptors and the consequent activities in platelets.     

Metabolism of [14C] arachidonic acid in the isolated rabbit spleen

Infusion of [¹⁴C] arachidonic acid (AA) into the isolated, Tyrode perfused rabbit spleen resulted in the release of a substance into the venous effluent with the musculotropic activity and chromatographic properties of prostaglandin (PG)E₂. Smaller amounts of radioactive materials with the chromatographic properties of PGF_2α, 6-keto-PGF_1α, and PGD₂ were also released. The radiolabeled material released in largest amounts from the spleen was identified as PGE₂ on the basis of: 1) Co-chromatography with PGE₂ in three solvent systems, 2) Conversion of the radioactive material and of authentic [³H] PGE₂ to similar products by treatment with sodium borohydride and with potassium hydroxide, and 3) Stability of the musculotropic activity in Tyrode solution at 37°C. Release of the major and minor radioactive products was inhibited by pretreatment of the spleen with either indomethacin or 5,8,11,14-eicosatetraynoic acid.     

Regulation of pancreatic β-cell function and mass dynamics by prostaglandin signaling

Prostaglandins (PGs) are signaling lipids derived from arachidonic acid (AA), which is metabolized by cyclooxygenase (COX)-1 or 2 and class-specific synthases to generate PGD₂, PGE₂, PGF_2α, PGI₂ (prostacyclin), and thromboxane A₂. PGs signal through G-protein coupled receptors (GPCRs) and are important modulators of an array of physiological functions, including systemic inflammation and insulin secretion from pancreatic islets. The role of PGs in β-cell function has been an active area of interest, beginning in the 1970s. Early studies demonstrated that PGE₂ inhibits glucose-stimulated insulin secretion (GSIS), although more recent studies have questioned this inhibitory action of PGE₂. The PGE₂ receptor EP3 and one of the G-proteins that couples to EP3, Gα_Z, have been identified as negative regulators of β-cell proliferation and survival. Conversely, PGI₂ and its receptor, IP, play a positive role in the β-cell by enhancing GSIS and preserving β-cell mass in response to the β-cell toxin streptozotocin (STZ). In comparison to PGE₂ and PGI₂, little is known about the function of the remaining PGs within islets. In this review, we discuss the roles of PGs, particularly PGE₂ and PGI₂, PG receptors, and downstream signaling events that alter β-cell function and regulation of β-cell mass.     

Mechanism of action of suprofen, a new peripheral analgesic, as demonstrated by its effects on several nociceptive mediators

Suprofen is a new potent, orally effective non-narcotic analgesic agent having a potent inhibitory action on prostaglandin (PG) biosynthesis. Recent experiments have shown that suprofen inhibits uterine hyperactivity induced by the physiological substances, arachidonic acid, bradykinin (BK) and PGF_2α. The present study explores the possibility that the analgesic activity of suprofen may involve multiple mechanisms of interaction with PGs, inhibiting synthesis at low doses and with higher doses possibly directly interacting with PGs and other physiological mediators of nociception at a common site. Experiments in mice have shown that suprofen antagonizes abdominal stretching induced by the physiological precursor of PG release, arachidonic acid (ED₅₀ = 0.07 mg/kg, p.p.), and by the nociceptive agents acetylcholine (ACh) (ED₅₀ = 1.7 mg/kg, p.o), BK (ED₅₀ = 65 mg/kg, p.o.) acetic acid (HAC) (H⁺ ion; ED₅₀ = mg/kg, p.o), and PGE₂, itself (ED₅₀ = 20.2 mg/kg, i.p.). In rabbits, i.a. administered suprofen (ED₅₀ = 0.98 mg/kg) blocked the reflex discharge of spinal sensory neurons evoked by BK (2 to 8 μg, i.a). The analgesic activity of suprofen may involve multiple mechanism of interaction with PGs and other mediators, including BK; suprofen blocks the nociceptive actions of PGs by inhibiting their formation, via the cyclooxygenase pathway, and possibly at PG sites of action, probably at peripheral nerve endings.     

Prostaglandin controls Neuromuscular Transmission in Guinea-pig Vas Deferens

PROSTAGLANDINS of the E type (PGE₁, PGE₂) inhibit sympathetic neurotransmission in several tissues and species^1–4. On the basis of their natural occurrence and availability for release, as well as observations on the pharmacological actions of the PGs, endogenous PGE₁ and PGE₂ are postulated to operate on sympathetic neurotransmission by a feedback mechanism and thereby modulate the effector responses to nerve activity^{1, 5}. Inhibition by 5,8,11,14-eicosatetraynoic acid (ETA) of PG synthesis in the cat spleen and in the rabbit heart increases the release of noradrenaline (NA) in response to nerve stimulation, thus strongly supporting the hypothesis^{6, 7}. We report here that guinea-pig vas deferens releases PG in response to nerve stimulation and that the neuromuscular transmission is facilitated after inhibition of PG synthesis. PG synthesis was irreversibly inhibited using ETA⁸.     

Metabolism of [C] arachidonic acid in the isolated rabbit spleen

Infusion of [¹⁴C] arachidonic acid (AA) into the isolated, Tyrode perfused rabbit spleen resulted in the release of a substance into the venous effluent with the musculotropic activity and chromatographic properties of prostaglandin (PG)E₂. Smaller amounts of radioactive materials with the chromatographic properties of PGF_2α, 6-keto-PGF_1α, and PGD₂ were also released. The radiolabeled material released in largest amounts from the spleen was identified as PGE₂ on the basis of: 1) Co-chromatography with PGE₂ in three solvent systems, 2) Conversion of the radioactive material and of authentic [³H] PGE₂ to similar products by treatment with sodium borohydride and with potassium hydroxide, and 3) Stability of the musculotropic activity in Tyrode solution at 37°C. Release of the major and minor radioactive products was inhibited by pretreatment of the spleen with either indomethacin or 5,8,11,14-eicosatetraynoic acid.     

Comparison of the inhibitory effect of E-prostaglandins in human and rabbit platelet-rich plasma and washed platelets

1. The anti-aggregatory potency of a number of E-type PGs was compared in human and rabbit platelet-rich plasma (PRP) and washed platelets. The potency of 13,14-dihydro-PGE₁ and 5,6-dihydro-PGE₃ is significantly higher in human than in rabbit washed platelets, while the potency of 15-keto-13,14-dihydro-PGE₁ is higher in rabbit.2. The potency of PGEs in rabbit PRP is very similar to that of washed platelets, with the exception of 1a,1b-dihomo-PGE₂, which is of a significantly lower potency in PRR.3. In human, 5,6-trans-PGE₂, PGE₃, and 15-keto-13,14-dihydro-PGE₁ are more potent in PRP than in washed platelets.4. The results indicate that the potency of E-type PGs in human and rabbit platelets is different and plasma can essentially influence the anti-aggregatory effect of PGEs; plasma can either decrease or increase potency.     

Effect of essential polyunsaturated fatty acid modifications on prostaglandins production by MDCK canine kidney cells

Supplementation of growing MDCK canine kidney tubular epithelial cultures with linoleic acid produced a 3.6- to 4.9-fold increase in bradykinin-stimulated PGE₂ release as measured by radioimmunoassay. Under these conditions the cell phospholipids contained 3.9-times more linoleic acid and 5.6-times more arachidonic acid, with the inositol, ethanolamine and choline phosphoglycerie fractions becoming enriched in arachidonic acid. By contrast, supplementation with arachidonic acid did not enhance bradykinin-stimulated PGE₂ release even though the arachidonic acid content of the cell phospholipids was increased 8.8-fold. The distribution of radioactive prostaglandin products was unchanged by these fatty acid enrichments, with PGE₂ accounting for 55 to 68% of the total output from [1-¹⁴C]arachidonic acid. Linoleic acid supplementation also produced a 2.5-fold increase in PGE₂ formation stimulated by extracellular arachidonic acid, whereas supplementation during culture with arachidonic acid caused a 55 to 80% inhibition. This difference cannot be accounted for by changes in the ability of the cells to incorporate extracellular arachidonic acid. it is suggested that at least some of the effects of linoleate supplementation on prostaglandin production are due to the resulting enrichment of the intracellular phospholipid substrate pools with arachidonic acid. In addition, it appears that prolonged exposure to arachidonic acid during culture has an overriding inhibitory effect on prostaglandin production even though the total cell lipids bocome highly enriched in arachidonate.     

The metabolism of prostaglandin E2 in perinatal rabbit lungs

The inactivation of prostaglandin E₂ (PGE₂) was studied in isolated perfused lungs of fetal and neonatal rabbits. 200 nmol of ¹⁴C-PGE₂ was infused into the pulmonary circulation and the metabolites of PGE₂ were analysed from the nonrecirculating perfusion effluent. The amount of the main metabolite, 13,14-dihydro-15-keto-PGE₂, increased significantly between the 28th and 30th day of fetal life, remained relatively constant at the time of birth and increased again between 1st and 7th postnatal day. In contrast the amount of 15-keto-PGE₂ remained relatively stable during the studied period. The activity of NAD⁺-dependent 15-hydroxyprostaglandin dehydrogenase (15-OH-PGDH) was determined from the 100.000 g supernatant fraction of fetal, neonatal and maternal rabbit lungs using ¹⁴C-PGE₂ (20 μM) as the substrate. In the lungs of late fetal rabbits the activity of 15-OH-PGDH was significantly higher compared to the early postnatal period. Maternal rabbit lungs possessed, however, very high activities compared to the studied perinatal lungs. The results show, that the activity of the pulmonary 15-OH-PGDH is high already during the late fetal period. The inactivation of PGE₂ in isolated perfused lungs seems, however, to increase during the last prenatal days. Thus it seems possible that the uptake mechanism could be the rate limiting step in the metabolism of PGE₂ in rabbit lungs during the perinatal period.     

The inactivation of prostaglandin E2 is decreased by dipyridamole and sulfinpyrazone in isolated rat lungs

The inactivation of prostaglandin E₂ (PGE₂) was decreased in the pulmonary circulation of isolated rat lungs, when either dipyridamole or sulfinpyrazone was infused into the pulmonary artery at the concentration of 20 μM. After pulmonary injection of 7.1 nmoles of ¹⁴C-PGE₂ the amount of 15-oxo-metabolites of PGE₂ in the effluent was 3.91 ± 0.19 nmoles from control lungs and 2.05 ± 0.19 nmoles (2P < 0.001) in that from 20 μM dipyridamole treated lungs. The corresponding values for control and 20 μM sulfinpyrazone lungs were 4.11 ± 0.25 and 3.03 ± 0.14 nmoles (2P < 0.01), respectively. The amounts of unmetabolized PGE₂ were correspondingly increased in the effluents from dipyridamole and sulfinpyrazone (20 μM) lungs. Neither dipyridamole nor sulfinpyrazone had at concentration of 2 μM any significant effect on the amount of 15-oxo-metabolites in the effluent, although the amount of unmetabolized PGE₂ was slightly increased in 2 μM sulfinpyrazone experiments.     

Pulmonary uptake of PGE2 is inhibited by dipyridamole in rat isolated lungs

The metabolism of prostaglandin E₂ (PGE₂) is decreased by dipyridamole (20 μM) in rat isolated perfused lungs. The inhibition of the metabolism is reversible as the decreased metabolism returned to the control level when pulmonary infusion of dipyridamole was abolished. After pulmonary injection of ¹⁴C-PGE₂ (10 nmol) the radioactivity appeared more rapidly in the effluent when dipyridamole was infused into pulmonary circulation. Dipyridamole in vitro did not change the activity of 15-hydroxyprostaglandin dehydrogenase (15-OH-PGDH) in the 100, 000 × g supernatant fraction of homogenized lungs. Thus, the decreased metabolism seems to be due to the inhibition of the uptake of PGE₂ into the lungs. When the rats were pretreated with dipyridamole in drinking water for one week the activity of 15-OH-PGDH in the 100, 000 × g supernatant fraction of the lungs was not changed significantly.