Stimulation of prostaglandin synthesis in WI-38 human lung fibroblasts following inhibition of phospholipid acylation by p-hydroxymercuribenzoate

The release of arachidonic acid and its metabolites, prostaglandin E2 and thromboxane A2, from WI-38 human lung fibroblasts was modulated by p-hydroxymercuribenzoate. Exposure to the inhibitor resulted in a dose-dependent decrease in [1-14C]arachidonic acid uptake and incorporation into phospholipids and neutral lipid pools. Activities of lung fibroblast arachidonyl-CoA synthetase and lysolecithin acyltransferase were inhibited by 100 microM p-hydroxymercuribenzoate. [14C]Arachidonic acid labelled fibroblasts exhibited an increased release of [14C]arachidonate and [14C]prostaglandin E2 of 54% and 112%, respectively, when exposed to 100 microM of inhibitor. The stimulatory effects of 8.0 microM delta 1-tetrahydrocannabinol on arachidonate release and prostaglandin E synthesis (Burstein, S., Hunter, S.A., Sedor, C. and Shulman, S. (1982) Biochem. Pharmacol. 31, 2361-2365) were modified by the inclusion of inhibiting agent, resulting in a 608% stimulation in arachidonic acid release, while prostaglandin E2 and thromboxane A2 synthesis increased 894% and 390%, respectively, over levels obtained by untreated cells. The levels of arachidonate metabolites were altered by inhibitor when compared to cells treated with cannabinoid alone. No significant inhibition by delta 1-tetrahydrocannabinol was found on arachidonic uptake in these cells. In unlabelled studies, p-hydroxymercuribenzoate resulted in a profound, dose-dependent stimulation of prostaglandin E synthesis of 1490% at 150 microM inhibitor concentration. These results provide evidence that free arachidonate is reincorporated via acylation, thereby implicating this pathway as a possible control mechanism for the synthesis of arachidonic acid metabolites.     

Mechanism of the thrombin-mediated burst in oxygen consumption by human platelets.

We present evidence that added thrombin stimulates release of endogenous arachidonic acid by suspensions of human platelets. We also show that added arachidonic acid causes a burst in O2 consumption that mimics one of the well described effects of thrombin on these cells. Further, added aspirin, a known inhibitor of the burst in O2 consumption caused by thrombin, also blunted the stimulatory effect of arachidonate on O2 consumption, and eicosatetraynoate, a known inhibitor of arachidonate oxygenation, blunted the burst in O2 consumption initiated by both thrombin and arachidonate. We conclude that rapid oxygenation of endogenously released arachidonic acid accounts for the thrombin-mediated burst in oxygen consumption by platelets.     

Modulation of Cell-Substrate Adhesion by Arachidonic Acid: Lipoxygenase Regulates Cell Spreading and ERK1/2-inducible Cyclooxygenase Regulates Cell Migration in NIH-3T3 Fibroblasts

Adhesion of cells to an extracellular matrix is characterized by several discrete morphological and functional stages beginning with cell-substrate attachment, followed by cell spreading, migration, and immobilization. We find that although arachidonic acid release is rate-limiting in the overall process of adhesion, its oxidation by lipoxygenase and cyclooxygenases regulates, respectively, the cell spreading and cell migration stages. During the adhesion of NIH-3T3 cells to fibronectin, two functionally and kinetically distinct phases of arachidonic acid release take place. An initial transient arachidonate release occurs during cell attachment to fibronectin, and is sufficient to signal the cell spreading stage after its oxidation by 5-lipoxygenase to leukotrienes. A later sustained arachidonate release occurs during and after spreading, and signals the subsequent migration stage through its oxidation to prostaglandins by newly synthesized cyclooxygenase-2. In signaling migration, constitutively expressed cyclooxygenase-1 appears to contribute approximately 25% of prostaglandins synthesized compared with the inducible cyclooxygenase-2. Both the second sustained arachidonate release, and cyclooxygenase-2 protein induction and synthesis, appear to be regulated by the mitogen-activated protein kinase extracellular signal-regulated kinase (ERK)1/2. The initial cell attachment-induced transient arachidonic acid release that signals spreading through lipoxygenase oxidation is not sensitive to ERK1/2 inhibition by PD98059, whereas PD98059 produces both a reduction in the larger second arachidonate release and a blockade of induced cyclooxygenase-2 protein expression with concomitant reduction of prostaglandin synthesis. The second arachidonate release, and cyclooxygenase-2 expression and activity, both appear to be required for cell migration but not for the preceding stages of attachment and spreading. These data suggest a bifurcation in the arachidonic acid adhesion-signaling pathway, wherein lipoxygenase oxidation generates leukotriene metabolites regulating the spreading stage of cell adhesion, whereas ERK 1/2-induced cyclooxygenase synthesis results in oxidation of a later release, generating prostaglandin metabolites regulating the later migration stage.     

The mechanisms of preterm labour; the interaction between amnion cells and leukocytes in the metabolism of arachidonic acid.

Chorioamnionitis is frequently associated with preterm labour. We have used a cell culture model system to examine the effects of leukocytes upon the metabolism of endogenous arachidonic acid from within amnion cells. We have demonstrated that activated leukocytes release substances which increase the overall release and metabolism of endogenous arachidonic acid within amnion cells causing an increase in prostaglandin E2 production as well as a smaller increase in non-cyclo-oxygenase metabolism. When amnion cells and leukocytes are cultured together, in addition to prostaglandin E2 production by amnion cells, arachidonic acid released by the amnion cells appears to be metabolised by leucocytes to prostaglandin F2 alpha, prostacyclin and thromboxane A2. Prostaglandins E2 and F2 alpha are the principal cyclo-oxygenase products of this interaction. We postulate that chorioamnionitis stimulates preterm labour not only by causing an increase in prostaglandin E2 synthesis by amnion cells but by metabolism of amnion derived arachidonic acid to the powerfully oxytocic prostaglandin F2 alpha by leukocytes.     

Lysophosphatidic acid potentiates the thrombin-induced production of arachidonate metabolites in platelets

Concentrations (1 to 20 microM) of 1-oleoyl-lysophosphatidic acid which alone do not affect platelet metabolism of arachidonic acid, do augment the effects of suboptimal concentrations of thrombin on the formation of [14C]phosphatidic acid and the production of [14C]arachidonate metabolites from platelets prelabeled with [14C]arachidonate. The effect on [14C]phosphatidate occurs with concentrations of thrombin (0.1 unit/ml) which are lower than those (0.2 unit/ml) needed to observe the effects on [14C]arachidonate metabolites. The effect of 1-oleoyl-lysophosphatidic acid (10 microM) plus thrombin (0.2 unit/ml) on the formation of phosphatidic acid temporally precedes the production of arachidonate metabolites consistent with a sequential activation of phosphatidylinositol-specific phospholipase C and phospholipase A2 activities. Preincubation of platelets with (32P)orthophosphate shows that the phosphatidic acid formed by 1-oleoyl-lysophosphatidic acid (10 microM) plus thrombin (0.2 unit/ml) is derived from phosphatidylinositol. The Ca2+-ionophoretic properties of lysophosphatidic acid might explain the accumulation of phosphatidic acid since Ca2+ prevents the conversion of phosphatidic acid to phosphatidylinositol. That effect of lysophosphatidic acid is inhibited by prostacyclin, possibly through a cyclic-AMP-mediated effect on calcium homeostasis.     

Radioimmunologic identification of prostaglandins produced by serum-stimulated mouse embryo palate mesenchyme cells

High-performance liquid chromatography and radioimmunoassay were used to identify the prostaglandins synthesized by mouse embryo palate mesenchyme cells. Serum stimulated the release of several different metabolites of arachidonic acid including 6-ketoprostaglandin F1 alpha (the stable product of prostacyclin, prostaglandin I2), prostaglandin E2 and prostaglandin F2 alpha. Compared to control cells, the serum-stimulated cells produce elevated levels of prostaglandin E2 (36-fold), 6-ketoprostaglandin F1 alpha (15-fold) and prostaglandin F2 alpha (7-fold). The acetylenic analogue of arachidonic acid, 5,8,11,14-eicosatetraynoic acid prevented this accelerated synthesis.     

Culture of mesothelial cells from bovine pericardium and characterization of their arachidonate metabolism

Cultures of mesothelial cells from bovine pericardium were established and their arachidonate metabolism was characterized. The identification of the cultured cells was based on morphological observations, and by electrophoretic analysis of cytoskeletal proteins, which demonstrated a pattern previously reported for mesothelial cells. Factor VIII-related antigen was present by indirect immunofluorescence, but the cells had no thrombomodulin activity. The cultured pericardial cells metabolized arachidonic acid to 6-ketoprostaglandin F1 alpha and a small amount of prostaglandin E2. The same metabolites were produced by pieces of intact parietal pericardium but not by pieces from which mesothelium had been removed. The cultured mesothelial cells produced 94.6 +/- 60.4 (mean +/- S.D.) ng/mg (n = 3) cell protein of 6-ketoprostaglandin F1 alpha in response to the calcium ionophore A23187, 117.3 +/- 13.6 ng/mg (n = 3) with exogenous arachidonic acid, 18.3 +/- 11.3 ng/mg (n = 5) with bradykinin, 8.4 +/- 4.3 ng/kg (n = 4) with histamine and 11.2 +/- 9.7 ng/mg (n = 5) with thrombin. All of these values were significantly higher (P less than 0.05) than the control (2.1 +/- 1 ng/mg; n = 5). From these results, we conclude that the mesothelial cells account for the arachidonate metabolism in the pericardium. The production of prostaglandin I2 occurs in response to physiological or pathological, agonists, and is substantial. That is, it is approximately the same as endothelial cells. The release of eicosanoids by mesothelial cells into the pericardial space may have a significant role in cardiac physiology and pathology.     

Stimulatory effects of cholera toxin on arachidonic acid metabolism in Chinese hamster ovary cells

Cholera toxin (CT) stimulated the release of arachidonic acid (AA) from Chinese hamster ovary cells with no apparent lag period. CT-induced release of [3H]AA or its metabolites was dose dependent during a 4-hr period of toxin exposure with a minimum effective dose of 0.1 ng/ml. CT-induced release of [3H]AA metabolites began within 15 min of toxin addition and became maximal after approximately 5 hr. Neither CT-A subunit nor CT-B subunit alone caused [3H]AA release. Furthermore, [3H]AA release was not caused by addition of dibutyryl cAMP to the culture medium, indicating that the observed effect of CT on arachidonate metabolism appeared to be independent of cAMP. The effect of CT on AA metabolism is proposed as a possible mechanism leading to the synthesis of prostaglandin E and fluid secretion during cholera.     

Release of arachidonate and reduction of oxygen. Independent metabolic bursts of the mouse peritoneal macrophage

Diverse particulate and soluble stimuli trigger two metabolic bursts in mouse peritoneal macrophages important in the inflammatory and/or cytotoxic actions of the cells: release, oxygenation, and further metabolism of arachidonic acid from endogenous phospholipids and reduction of molecular oxygen to reactive intermediates. We tested the hypothesis that the release of arachidonic acid or formation of its metabolites are obligatory intermediate steps in triggering the NADPH oxidase that reduces O2 to O-2. With phorbol diesters as stimuli, the following inhibitors of phospholipase A2 and lipoxygenase suppressed release of H2O2 at nontoxic concentrations (microM range): p-bromophenacyl bromide, quinacrine, eicosatetraenoic acid, nordihydroguaiaretic acid, and phenidone. Indomethacin and acetylsalicylic acid were ineffective. However, the suppressive effect of the first five agents on H2O2 release could be attributed to their suppression of macrophage glucose uptake at the same concentrations, a previously unrecognized effect of these compounds. Further, concanavalin A, wheat germ agglutinin, and thrombin each stimulated abundant arachidonate release without H2O2 release. Finally, noncytolytic concentrations of cycloheximide and/or emetine suppressed arachidonate release without affecting H2O2 secretion triggered either by phorbol esters or zymosan. Release and metabolism of arachidonic acid and secretion of reactive oxygen intermediates appear to be two frequently coincident but mutually independent metabolic pathways in the mouse peritoneal macrophage.     

Differential Involvement of the Arachidonic Acid Cascade on the α1-Adrenergic Potentiation of Vasoactive Intestinal Peptide-Versus β-Adrenergic-Stimulated Cyclic AMP and Cyclic GMP Accumulation in Rat Pinealocytes

In the rat pineal gland, alpha 1-adrenergic agonists, which stimulate arachidonic acid release, also potentiate vasoactive intestinal peptide (VIP)- or beta-adrenergic-stimulated cyclic AMP (cAMP) and cyclic GMP (cGMP) accumulation. In this study, the possible involvement of the arachidonic acid pathway in the potentiation mechanism was examined in dispersed rat pinealocytes using two inhibitors of the arachidonic acid cascade, indomethacin and nordihydroguaiaretic acid. These two inhibitors appeared to have differential effects on the alpha 1-adrenergic potentiation of VIP- or beta-adrenergic-stimulated cAMP and cGMP responses. Whereas nordihydroguaiaretic acid was effective in suppressing both the alpha 1-adrenergic potentiation of VIP- or beta-adrenergic-stimulated cAMP and cGMP responses, indomethacin inhibited selectively the VIP-mediated cAMP and cGMP responses. The role of arachidonic acid metabolites was further determined using several prostaglandins--A2, I2, E2, and F2 alpha--and leukotrienes--B4, C4, and D4. Of the seven compounds tested, prostaglandins E2 and F2 alpha stimulated basal cAMP but not cGMP accumulation. The prostaglandin E2- and F2 alpha-stimulated cAMP responses were additive to those stimulated by VIP or beta-adrenergic receptors. The other five compounds had no effects on basal or VIP- or beta-adrenergic-stimulated cAMP or cGMP accumulation. Taken together, these findings indicate that the arachidonic acid cascade is likely involved in the alpha 1-adrenergic potentiation of VIP- or beta-adrenergic-stimulated cAMP and cGMP accumulation. However, the specific arachidonic acid metabolite involved in the potentiation mechanisms of VIP- versus beta-adrenergic-stimulated cyclic nucleotide responses may be different.     

Decreased prostaglandin production by cholesterol-rich macrophages

The regulation of prostaglandin production by macrophages enriched in cholesterol was examined. Mouse peritoneal macrophages were incubated for 18 h with 25 micrograms/ml of human acetyl-LDL (low density lipoprotein) and trace amounts of labeled arachidonic acid. After cholesterol enrichment, the cells were incubated with phorbol 12-myristate 13-acetate (PMA), calcium ionophore, or zymosan to stimulate endogenous arachidonic acid metabolism. A high performance liquid chromatography profile of the eicosanoids released revealed no qualitative differences between unmodified and modified macrophages. Cholesterol-rich cells, however, released less prostacyclin (PGI2) and prostaglandin E2 (PGE2) compared to unmodified cells, and products from the lipoxygenase pathway became the predominant metabolites. A decrease in the synthesis of PGI2 and PGE2 by cholesterol-rich macrophages was confirmed by radioimmunoassay and radiolabeled experiments. The activity of prostaglandin synthetase was modestly increased in the cholesterol-modified macrophages compared to controls. As an estimation of phospholipase activity, the release of labeled arachidonic acid from membrane phospholipids, however, was significantly decreased in cholesterol-rich macrophages. The phosphatidylinositol fraction was particularly resistant to arachidonate release in response to calcium ionophore and PMA in the modified cells. The measurement of membrane phospholipid fatty acid composition before and after calcium ionophore supported the observation that less arachidonate was released by cholesterol-enriched cells in response to the ionophore. Based on these observations, we propose that prostaglandin synthesis from endogenous arachidonate stores is decreased in the cholesterol-rich macrophage. A decrease in agonist-induced activation of the phospholipase activity is proposed as a mechanism for this effect.     

Prostaglandins and renin release: I. Stimulation of renin release from rabbit renal cortical slices by PGI2.

Prostaglandins have been shown to be involved in the mechanism of renin secretion in a variety of situations. Both arachidonic acid and prostaglandin endoperoxide have been shown to release renin from cortical slices and to be converted to PGI2 by cortical microsomes. In the present studies PGI2 was found to cause a time dependent increase in renin release from rabbit renal cortical slices, a system isolated from any indirect effects that result from the administration of prostaglandins in vivo. The stimulation was linear up to 30 minutes and effective over a range of concentrations from 10(7 M to 10(-5) M. At similar concentrations 6-keto-prostaglandin F1alpha was not active on these slices. Thus, it is proposed that PGI2 exerts a direct effect on the release of renin from cortical cells and may be the mediator of arachidonate or prostaglandin endoperoxide stimulated renin secretion.     

Analysis of prostaglandins formed from endogenous and exogenous arachidonic acid in homogenates of human reproductive tissues

Isotope-labelled arachidonic acid has been used to study in vitro formation of prostaglandins and other products in mammalian tissue. Quantitative conclusions about cyclooxygenase activity have been drawn from such studies. However, arachidonic acid is present in all tissues, free and esterified, and therefore it can be expected that endogenous arachidonate would interfere with transformation of the radioactive exogenous substrate. (1-14C)-labelled arachidonate was, therefore, incubated with homogenates of various human tissues (amnion, chrorion, placenta and myometrium), and the two molecular forms, 12C and 14C, of arachidonic acid as well as of prostaglandin E2 and prostaglandin F2 alpha were quantitated, before and after 30 min of incubation, using gas chromatography-mass spectrometry with multiple ion detection. The results demonstrate a substantial release of arachidonic acid into the medium during incubation. There was no correlation between either the initial concentration of [12C]arachidonic acid and initial concentration of [12C]prostaglandin E2 or the percent increase of those compounds during incubation. The net formation of [12C]prostaglandin E2 and [14C]prostaglandin E2 from endogenous and exogenous precursor, respectively, were also very different. The study shows that by simply incubating (1-14C)-labelled arachidonic acid in tissue homogenates and measuring the amount of radioactivity transformed into various prostaglandins only qualitative conclusions can be drawn.     

Effects of lipoxygenase and cyclooxygenase inhibitors on glucose-stimulated insulin secretion from the isolated perfused rat pancreas

Some of the metabolites of arachidonic acid formed in the lipoxygenase and cyclooxygenase pathways stimulate insulin release. We studied the relative importance of each of these pathways in the modulation of glucose-induced insulin release by using inhibitors of arachidonate metabolism. Perfusion of the isolated rat pancreas with two chemically different inhibitors of cyclooxygenase, flurbiprofen and sodium salicylate, markedly inhibited prostaglandin E2 release, but had little effect on glucose-induced insulin release or on potentiation of insulin release caused by prior exposure to glucose. On the other hand, nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, not only inhibited both phases of glucose-induced insulin release but also abolished the potentiation effect. These effects of NDGA prevailed, when it was administered together with flurbiprofen, which caused profound inhibition of prostaglandin E2 release. We conclude that 1) lipoxygenase pathways play a dominant role in glucose-stimulated insulin release, and 2) endogenous lipoxygenase metabolites influence the potentiating effect of glucose on the release of insulin in response to a subsequent stimulation.     

Mechanism of arachidonic acid-induced Ca2+ mobilization from rat liver microsomes

Recent evidence indicates that unesterified arachidonic acid functions as a mediator of intracellular Ca2+ mobilization by inducing Ca2+ release from the endoplasmic reticulum of pancreatic islet beta cells in a manner closely similar to that of inositol 1,4,5-trisphosphate. To test the generality and explore the mechanism of this phenomenon we have examined the effects of arachidonic acid on calcium accumulation and release by hepatocyte subcellular fractions enriched in endoplasmic reticulum (microsomes). At concentrations above 0.017 mumol/mg microsomal protein, arachidonate induced rapid (under 2 min) 45Ca2+ release from microsomes that had been preloaded with 45Ca2+. Arachidonate also suppressed microsomal 45Ca2+ accumulation when present during the loading period, as reflected by reduction both of 45Ca2+ accumulation at steady state and of the rate of uptake. Neither the cyclooxygenase inhibitor indomethacin nor the lipoxygenase/cyclooxygenase inhibitor BW755C suppressed arachidonate-induced 45Ca2+ release, indicating that this effect was not dependent upon oxygenation of the fatty acid to metabolites. The long-chain unsaturated fatty acids oleate and linoleate were less potent than arachidonate in inducing 45Ca2+ release, and the saturated fatty acid stearate did not exert this effect. Albumin prevented 45Ca2+ release by arachidonate, presumably by binding the fatty acid. As is the case for inositol 1,4,5-trisphosphate, the ability of arachidonate to induce 45Ca2+ release was dependent on the ambient free Ca2+ concentration. Arachidonate did not influence microsomal membrane permeability or Ca2+-ATPase activity and may exert its effects on microsomal Ca2+ handling by activation of a Ca2+ extrusion mechanism or by dissociating Ca2+ uptake from Ca2+-ATPase activity.     

Arachidonic acid release is closely related to the Fc gamma receptor-mediated superoxide generation in macrophages

Stimulation of macrophages with IgG2 immune complexes induced dose-dependently the O2- generation and the release of arachidonic acid and its metabolites. This Fc gamma R-mediated O2- generation was inhibited by a phospholipase A2 inhibitor, 4-p-bromophenacyl bromide (4-pBPB), in parallel to the dose-dependent inhibition of arachidonic acid release. The main arachidonic acid metabolites released were shown to be prostaglandin E2 and thromboxane B2 and blocking of the production of these metabolites by indomethacin did not inhibit the O2- generation. Inhibition of the Fc gamma R-mediated O2- generation and the arachidonic acid release by the C-kinase inhibitor, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), was less intense than by 4-pBPB. These results support the previously proposed hypothesis that arachidonic acid acts as an intracellular activator of the Fc gamma R-mediated O2- generation in macrophages. Although the C-kinase activation may also contribute to the activation of the O2--generating system, arachidonic acid release appears to play a major role in Fc gamma R-mediated O2- generation. In contrast, activation of C-kinase seems to be contributing mainly in the induction of both the arachidonic acid release and O2- generation by 12-o-tetradecanoylphorbol 13-acetate (TPA). Furthermore, suboptimal concentrations of TPA and arachidonate were found to act synergistically to stimulate O2- generation and the inhibition study suggested a positive synergism between C-kinase and arachidonic acid release to induce O2- generation. This synergistic action may have general importance in receptor-mediated O2- generation.     

Alpha 1-adrenergic stimulation of arachidonic acid release and metabolism in a rat thyroid cell line. Mediation of cell replication by prostaglandin E2

The rat thyroid cell line, FRTL-5, expresses an alpha 1-adrenergic receptor when exposed to thyrotropin. We have found that occupation of this alpha 1-adrenergic receptor by norepinephrine stimulated the release of [3H]arachidonic acid from prelabeled cells. Arachidonic acid was metabolized primarily to prostaglandin E2 and to much smaller amounts of 11-hydroxy-5,8,11,13-eicosatetraenoic acid, 15-hydroxy-5,8,11,13-eicosatetraenoic acid, prostaglandin D2, and thromboxane B2. Synthesis of all these metabolites was inhibited by the cyclooxygenase inhibitor indomethacin. When FRTL-5 cells were starved of thyrotropin for 24 h, norepinephrine nearly doubled [3H]thymidine uptake into DNA. Cyclooxygenase inhibitors inhibited norepinephrine-stimulated thymidine uptake by 60-70%. Of several arachidonic acid metabolites tested, none was able to stimulate thymidine uptake directly in the presence of indomethacin. Prostaglandin E2, however, was able to restore [3H]thymidine uptake when added together with norepinephrine in the presence of indomethacin. Thus, occupation of an alpha 1-adrenergic receptor in a functional rat thyroid cell line leads to arachidonic acid release. Subsequent metabolism of the arachidonic acid by the cyclooxygenase pathway leads to synthesis of prostaglandin E2, which mediates a norepinephrine-stimulated activity related to cell replication.     

Comparison of arachidonic acid metabolism between myofibroblasts and fibroblasts isolated from rat palatal mucoperiosteum

Myofibroblasts were cultured successfully from experimental wound tissue in rat palatal mucoperiosteum. Arachidonic acid metabolizing activity in cultured myofibroblasts was compared with that in fibroblasts cultured from normal mucoperiosteum. Prostaglandins biosynthesized from [14C]arachidonic acid in cell-free homogenates of both myofibroblasts and fibroblasts were prostaglandins D2, E2 and F2 alpha, and the activity producing each prostaglandin was not significantly different between the myofibroblasts and the fibroblasts, whereas smooth muscle cells, which are histologically similar to myofibroblasts, produced mainly 6-ketoprostaglandin F1 alpha, and relatively small amounts of prostaglandin E2. The release of arachidonic acid from cells prelabeled with [14C]arachidonic acid was compared among three types of cell. The calcium ionophore A23187 strongly enhanced arachidonic acid release in all three cell types. Bradykinin, 5-hydroxytryptamine and prostaglandin F2 alpha affected the stimulation of arachidonic acid release in the fibroblasts but were less or not effective in the myofibroblasts and smooth muscle cells. In addition, prostaglandin E2 biosynthesized in response to several stimuli was measured by radioimmunoassay. The content of prostaglandin E2 correlated closely with arachidonic acid release. In this study, we showed homogeneity between the myofibroblasts and fibroblasts in prostaglandin synthesizing activity and similarity in response to various stimuli between the myofibroblasts and smooth muscle cells, from the standpoint of arachidonic acid metabolism.