Platelet-activating factor stimulates prostaglandin synthesis in cultured cells

Effect of epinephrine on platelet-activating factor-stimulated human vascular smooth muscle cells

BACKGROUND

Animal and human data show that platelet-activating factor (PAF) mediates the life-threatening manifestations of anaphylaxis. Although administration of epinephrine is the mainstay of therapy of acute anaphylaxis, the interaction between epinephrine and PAF has not been studied. In particular, the effect of the timing of epinephrine administration on the action of PAF has not been examined.

OBJECTIVE

Using human vascular smooth muscle cells (HVSMCs), we examined the effect of timing of epinephrine addition on the action of PAF.

METHODS

The effect of epinephrine on PAF-mediated prostaglandin E(2) (PGE(2)) release from human aortic smooth muscle cells was examined. Epinephrine was added at various times before and after PAF stimulation.

RESULTS

HVSMCs stimulated with PAF released PGE(2) in a concentration- and time-dependent manner. Whereas preincubation of HVSMCs with epinephrine before the addition of PAF suppressed PGE(2) release, treatment with epinephrine after PAF stimulation was less effective with time after PAF stimulation. PGE(2) release was suppressed by means of preincubation with 8-bromo-cyclic AMP and forskolin.

CONCLUSIONS

PAF induced PGE(2) release from HVSMCs in a concentration- and time-dependent manner, and early addition of epinephrine was essential for the control of PAF-induced PGE(2) release. Epinephrine was most effective when administered before stimulation with PAF but was progressively less effective with time after PAF stimulation.

Differential signaling pathways in platelet-activating factor-induced proliferation and interleukin-6 production by rat vascular smooth muscle cells

Vascular smooth muscle cells (SMCs) can be induced to proliferate in response to several cytokines and growth factors, including interleukin (IL)-6. Platelet-activating factor (PAF) also has been shown to induce SMC proliferation. Because PAF can stimulate IL-6 production in monocytes, macrophages, and endothelial cells, our study was undertaken to determine whether PAF could induce IL-6 production by SMCs and to define the underlying signaling pathways. Exposure of rat aortic SMCs to picomolar concentrations of PAF resulted in enhanced production of IL-6. The effect was concentration dependent, selective for the active form of PAF, and mediated by specific PAF receptors. Pretreatment of the cells with Bordatella pertussis toxin (PTX) prevented the effect of PAF, suggesting the involvement of alpha i-type subunits of G proteins in the signal-transduction pathway. PAF-dependent IL-6 production was also prevented by inhibition of tyrosine kinases with genistein or erbstatin. Inhibition of eicosanoid production by blocking either phospholipase A2 or cyclooxygenase also abrogated the effect of PAF on IL-6 production. Moreover, inhibition of Ca2+-calmodulin activity with W7 or blocking of calcium channels with verapamil or nifedipine prevented PAF-mediated enhancement of IL-6 production. Whereas PAF-induced signal-transduction pathways leading to IL-6 production and SMC proliferation were partially common, they appeared to diverge downstream of PLA2 activation: inhibition of cyclooxygenase had no effect on proliferation, whereas augmentation of cyclic adenosine monophosphate (cAMP) levels or activation of protein kinase A inhibited proliferation, in contrast to IL-6 production. Our findings suggest a role for PAF in modulating vascular function by stimulating local production of IL-6 by SMCs and promoting their proliferation. The two effects are, however, associated with partially divergent signaling pathways and may not be causally related.

Receptor-mediated increase in cytosolic calcium in LLC-PK1 cells by platelet activating factor and thromboxane A2

Several studies indicate an important role of platelet activating factor (PAF) and thromboxane A2 (TXA2) in glomerular pathophysiology. However, the potential role of PAF or TXA2 in renal tubular pathophysiology has received little attention, and the presence of functional receptors for these autacoids in renal tubular epithelium has not been previously studied. We examined the effects of PAF and the TXA2 analogue, ONO11113, on the cytosolic free calcium concentration [( Ca2+]i) in cultured LLC-PK1 cell line using a fluorescent probe, fura-2. In these cells, the addition of PAF or ONO11113 caused a significant increment in [Ca2+]i in a dose-dependent manner: both agonists (10(-7) M) increased [Ca2+]i from 148 +/- 16 to 288 +/- 39 nM and from 130 +/- 8 to 240 +/- 18 nM, with the values of EC50 for PAF and ONO11113 being 17 +/- 4 and 17 +/- 2 nM, respectively. These effects were both rapid and transient, returning to baseline in two minutes. The effect of PAF was selectively blocked by PAF receptor antagonist BN50730, but not by TXA2 receptor antagonist L657925. Similarly ONO11113 response was abolished by L657925, but not by BN50730. PAF- or ONO11113-challenged cells did not respond to a second addition of the same agent and showed heterologous desensitization to the other agonist. The initial peaks of [Ca2+]i as well as the sustained elevations in [Ca2+]i induced by PAF or ONO11113 were reduced following the chelation of extracellular Ca2+ by 10 mM ethylene glycol-bis(beta-aminomethyl ether)-N,N,N',N'-tetraacetic acid (EGTA).(ABSTRACT TRUNCATED AT 250 WORDS)

Inositol phospholipid turnover in PAF transmembrane signalling

In a variety of cells and tissues, platelet activating factor (PAF) stimulates phospholipase C catalyzed breakdown of phosphoinositides. This results in the generation of the second messengers, inositol trisphosphate and diglyceride. This process occurs independently of extracellular Ca2+. A number of PAF structural analogues, receptor antagonists and drugs have been utilized to pharmacologically probe the activation of phospholipase C. PAF stimulation of the phosphoinositide turnover was shown to be sensitive to pertussis toxin in some systems, but not in others. The involvement of guanine nucleotide binding protein(s) and tyrosine kinase(s) in this process have also been postulated. These developments give new insights into PAF-receptor function at the molecular level, and also provide leads towards a better understanding of the cellular responses to PAF.

Anticardiolipin antibody-positive serum enhances endothelial cell platelet-activating factor production

Circulating antiphospholipids have been linked to recurrent pregnancy loss by a mechanism involving placental and decidual thrombosis. We hypothesized that platelet-activating factor, an autacoid synthesized by vascular endothelium, might mediate this phenomenon through its ability to promote platelet aggregation and fibrin deposition. Alternatively, antiphospholipid antibodies might exert a procoagulant effect by inhibiting the synthesis of prostacyclin. To evaluate these theories, endothelial cells (harvested from human umbilical veins) were grown to confluence and incubated for 48 hours with 20% concentrations of anticardiolipin antibody-positive and -negative human sera as well as fetal bovine serum. After incubation culture wells were stimulated with 10 mumol/ml calcium ionophore A23187 (an agonist of platelet-activating factor and prostacyclin synthesis). Intracellular platelet-activating factor was measured by tritiated acetate incorporation, phospholipid extraction, thin-layer chromatography, and scintillation spectrophotometry. Enhanced platelet-activating factor synthesis was identified in cultures incubated with anticardiolipin antibody-positive serum (25,544 +/- 2604 disintegrations per minute, mean +/- SD) when compared with anticardiolipin antibody-negative serum (18,600 +/- 3316 dpm) or fetal bovine serum (19,014 +/- 4233 dpm; analysis of variance, p = 0.033). In similar experiments, prostacyclin synthesis was determined by measuring its primary metabolite, 6-keto-prostaglandin F1 alpha, in culture supernatants. No differences between anticardiolipin antibody-positive and control cultures were observed (analysis of variance, p = 0.90). We conclude that in this endothelial cell model, anticardiolipin antibody-positive serum enhances ionophore-mediated platelet-activating factor synthesis but has no apparent effect on the production of prostacyclin. These findings suggest a potential role for platelet-activating factor in anticardiolipin antibody-mediated vascular thrombosis.

Phospholipid metabolism in cardiomyopathic hamster heart cells

We demonstrated that the activities of phosphatidylinositide-specific phospholipase C, inositol 1,4,5-trisphosphate (IP3) kinase, and IP3 phosphatase were enhanced in cardiomyopathic hamster hearts (BIO 14.6 and BIO 53.58) in comparison to control hamsters (F1b). Release of both arachidonic acid and prostacyclin was markedly enhanced by norepinephrine in the cardiomyopathic hamsters. Phospholipase C in heart has high substrate specificity to phosphatidylinositol. IP3 production was markedly enhanced in the cardiomyopathic hamsters. We also determined the intracellular calcium concentration, which was higher in BIO 53.58 hamsters than in BIO 14.6 hamsters at 5-20 weeks of age. There was no significant difference in the intracellular calcium level between F1b and BIO 14.6 hamsters at 5 weeks of age. These results suggest that phosphatidylinositol turnover stimulated by norepinephrine may produce high intracellular calcium levels in both BIO 14.6 and BIO 53.58 myocytes. In addition, in BIO 53.58 hamsters, some mechanism such as the sarcoplasmic reticulum, which controls the intracellular calcium level, may deteriorate in function. We concluded from these results that a prolonged high intracellular calcium level may lead to the death of BIO 53.58 myocytes.

Phospholipid metabolism and prostacyclin synthesis in hypoxic myocytes

We observed that in hypoxic myocardial cells prostacyclin and arachidonic acid release increased and that during hypoxia phospholipid degradation also occurred. In order to clarify the mechanism of phospholipid degradation, we determined the activity of phospholipases A2 and C. We found that phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were markedly decreased and that lysophosphatidylcholine and lysophosphatidylethanolamine were increased. In contrast, there was only slight phosphatidylinositol degradation and no lysophosphatidylinositol elevation was observed. These results show that phospholipase A2 was activated in hypoxic myocytes and had substrate specificity towards PC and PE. To study phospholipase C activity, membrane phospholipids were labeled with [3H]choline, [3H]inositol or [3H]ethanolamine. The release of inositol was observed, but neither choline nor ethanolamine was released. In hypoxia, myocardial-cell phospholipase C has high substrate specificity towards phosphatidylinositol. The activation of phospholipases is closely related to the intracellular Ca2+ concentration; it is though that inositol polyphosphatides may regulate intracellular Ca2+. We determined how Ca2+ influx occurs in hypoxia. beta-Adrenergic blockade and Ca2+ antagonists markedly suppressed Ca2+ influx, phospholipase A2 activity, phospholipase C activity and cell death. However, the alpha 1-adrenergic blockade was less effective in suppressing these phenomena. These results suggest that in hypoxic myocardial cells Ca2+ influx mediated by beta-adrenergic stimulation activates phospholipases A2 and C, and that phospholipid degradation and prostacyclin release then occur.

Myointimal hyperplasia: pathogenesis and implications. 1. In vitro characteristics

Myointimal hyperplasia (MIH) is an arterial wall smooth muscle cell (SMC) proliferative disorder. This process is responsible for a significant number of early and long-term arterial reconstructive and graft failures. Histopathologically, this process is characterized by a proliferation of SMC in the intima of traumatized arteries resulting in arterial and/or anastomatic stenosis with secondary thrombosis. In vitro studies of cultured SMC have allowed the evaluation of SMC response to factors suspected of being clinically associated with MIH. Principal among these is platelet derived growth factor (PDGF), which is known to be secreted by several cell types including endothelial cells (ECs) and monocytes as well as being stored and secreted by platelets. PDGF, somatomedin-C, epithelial growth factor, insulin, and other factors have been found to significantly increase SMC replication in vitro. Lipoproteins may be important substrates for SMC proliferation in contrast to heparin, which may directly inhibit SMC protein synthesis. Unlike SMCs, whose continued growth in culture is dependent on various growth factors and nutrients, ECs essentially cease to proliferate after the cells have formed a monolayer over the available surface. Extracellular matrix proteins, polypeptide mitogens, and heparin have been shown to modify EC migration and proliferation in vitro. Wounding of EC monolayers by scratching results in increased replication and migration, processes which require plasma factors that remain poorly defined. However, two general forms of EC growth factor have been isolated from many body tissues, are potent stimulators of capillary endothelial growth, and appear important both for normal EC monolayer homeostasis and for the response to injury. Cultured ECs produce mitogens for SMC. Production of the principal mitogen, PDGF, is significantly increased in sparse versus confluent cell cultures as well as by toxic agents such as endotoxin and phorbol esters. Acetyl low density lipoprotein as well as omega-3 fatty acids may significantly and selectively inhibit EC PDGF production, a finding with potentially profound implications for the clinical control of MIH in vivo.

Platelet aggregation induced by platelet-activating factor is suppressed by cystine protease inhibitor

The effect of NCO-700, a cystine protease inhibitor, on platelet-activating factor-induced platelet aggregation was determined. A newly synthesized cystine protease inhibitor (calcium-activated neutral protease and cathepsin B inhibitor), NCO-700 (bis[ethyl (2R, 3R)-3-[(S)-methyl-1-[4-(2,3,4- trimethoxyphenylmethyl) piperazine-1-ylcarbonyl]butylcarbonyl]oxiran-2-carboxy late]sulfate), inhibited platelet-activating factor-induced platelet aggregation. The inhibition was dependent on the preincubation time with NCO-700 and on the concentration of the inhibitor. The release of serotonin was also inhibited almost completely by the 20-min preincubation with 10(-4) M NCO-700. Leupeptin also inhibited platelet-activating factor-induced platelet aggregation. But calcium-activated neutral protease inhibitor did not inhibit it. These observations suggest that NCO-700-sensitive protease(s) such as cystine protease may contribute to platelet aggregation induced by platelet-activating factor.

Effect of streptokinase on prostacyclin synthesis and phospholipase activity in cultured pulmonary artery endothelial cells

In this study, we examined the effects of streptokinase on arachidonic acid release and prostacyclin biosynthesis in cultured bovine pulmonary artery endothelial cells. When intact cells were incubated with streptokinase, a significant stimulatory effect on prostacyclin biosynthetic activity in cells was evident without any cellular damage at all concentrations used (1-10,000 units/ml). Streptokinase also caused a marked release of arachidonic acid. It induced rapid phospholipid hydrolysis, resulting in the release of up to 15% of incorporated [3H]arachidonic acid into the medium. After the addition of streptokinase, degradation of phosphatidylcholine and phosphatidylethanolamine was observed and lysophosphatidylcholine and lysophosphatidylethanolamine were produced. We also observed a transient rise in diacylglycerol after the addition of streptokinase. To test for phospholipase C activity, the release of incorporated [3H]choline, [3H]inositol and [3H]ethanolamine into the culture medium was determined. The level of radioactive inositol showed an increase, but the changes in choline and ethanolamine were comparatively small. An increase in inositol was detectable within 1 min after streptokinase addition and peaked after 15 min. Inositol phosphate and inositol trisphosphate were released, and these releases were suppressed by the addition of neomycin (50 microM). These results suggest that streptokinase stimulates phospholipase A2 and C activity, and that prostacyclin biosynthesis is subsequently increased in cultured endothelial cells.

Endothelin stimulates angiotensin I to angiotensin II conversion in cultured pulmonary artery endothelial cells

We studied the effects of endothelin on the conversion of angiotensin I to angiotensin II in pulmonary artery endothelial cells. Endothelin had a novel effect on angiotensin I conversion. When endothelin was added to pulmonary artery endothelial cells, the conversion of angiotensin I to angiotensin II was enhanced about two-fold. The maximum stimulation was achieved at 10(-8) M of endothelin. This stimulatory effect was suppressed by angiotensin converting enzyme inhibitors such an enalapril. When the calcium antagonist, nifedipine, was incubated with 10(-8) M of endothelin, the conversion of angiotensin I to angiotensin II stimulated with endothelin was slightly suppressed by nifedipine. Enalapril (10(-6) M) completely inhibited the conversion of angiotensin I to angiotensin II in the presence of endothelin. These results suggest that endothelin may play an important role in regulating vascular tone by modulating the conversion of angiotensin I to angiotensin II.

Mechanism of increased angiotensin-converting enzyme activity stimulated by platelet-activating factor

We studied the effects of platelet activating factor (PAF) on angiotensin-converting enzyme (ACE). PAF (1 x 10(-10) to 1 x 10(-6) M) had a novel effect on angiotensin I conversion. Pulmonary artery endothelial cells converted 1 nmol/dish of 125I-angiotensin I to angiotensin II in the absence of PAF. ACE activity was increased to 2.5 nmol/dish by the addition of 1 x 10(-6) M of PAF. To clarify the mechanism of this stimulatory effect of PAF on ACE, Ca2+ influx and inositol 1,4,5-trisphosphate (IP3) release in pulmonary artery endothelial cells were determined. PAF stimulated Ca2+ influx in a dose-dependent manner. PAF also stimulated phospholipase C (PLC) activity and released IP3. To study the relationship between PLC activity and ACE activity, neomycin was added. The Ca2+ influx and IP3 release stimulated by 10(-6) M of PAF were suppressed by about 60-70%. ACE activity was also inhibited up to 70% in the presence of PAF (10(-10) - 10(-6) M) by 50 M of neomycin. These results suggest that ACE was stimulated by PAF, and that its activity in endothelial cells may be mediated by the PI-turnover pathway via changes in PLC activity and IP3-mediated Ca2+ release from intracellular stores.

Interaction of platelet-activating factor with endothelial and vascular smooth muscle cells in coculture

Platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine [PAF]) is a vasoactive ether lipid produced by activated blood cells. To examine the molecular traffic and sites of metabolism of PAF released in the vascular wall, we used a coculture system in which endothelial cells are grown on micropore filters suspended over confluent cultures of vascular smooth muscle cells. The endothelial cells took up PAF 5-7 times more readily from the apical than from the basolateral surface, converting it to 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine (2-acyl-PAF) and other minor metabolites. Intact endothelial monolayers effectively shielded the underlying smooth muscle cells from PAF present in the apical fluid; after a 30-min incubation with [3H]-PAF, only 1% of the radioactivity was transferred to the interstitial fluid. By contrast, PAF readily entered the interstitial fluid when the endothelial monolayers were injured by exposure to xanthine and xanthine oxidase. PAF did not significantly increase the permeability of endothelial monolayers to albumin. Smooth muscle cells took up and metabolized interstitial PAF more quickly and more completely than did endothelial cells; 65% was converted to 2-acyl-PAF in 15 min by the smooth muscle cells. PAF enhanced the proliferative effect of PDGF on smooth muscle cells, as assessed by [3H]-thymidine incorporation. These findings suggest that endothelial cells form a barrier to PAF released at the luminal surface, but PAF released in the vascular intima interacts primarily with smooth muscle cells, possibly stimulating proliferation in these cells.

Plasmenylethanolamine is the major storage depot for arachidonic acid in rabbit vascular smooth muscle and is rapidly hydrolyzed after angiotensin II stimulation

The present study demonstrates that rabbit aortic intimal smooth muscle cells contain the majority of their endogenous arachidonic acid mass in plasmenylethanolamine molecular species. To demonstrate the potential significance of these plasmenylethanolamines as substrates for the smooth muscle cell phospholipases that are activated during agonist stimulation, aortic rings were prelabeled with [3H]arachidonic acid and stimulated with angiotensin II. Although the specific activities of the choline and inositol glycerophospholipid pools were similar after the labeling interval, ethanolamine glycerophospholipids had a specific activity of only 20% of the specific activity of choline and inositol glycerophospholipids. Despite the marked disparity in the specific activities of these three phospholipid classes, angiotensin II stimulation resulted in similar fractional losses (35-41%) of [3H]arachidonic acid from vascular smooth muscle choline, ethanolamine, and inositol glycerophospholipid classes. Reverse-phase HPLC demonstrated that greater than 60% of the [3H]arachidonic acid released from ethanolamine glycerophospholipids after angiotensin II stimulation originated from plasmenylethanolamine molecular species. Taken together, the results demonstrate that the major phospholipid storage depot for arachidonic acid in vascular smooth muscle cells are plasmenylethanolamine molecular species which are important substrates for the phospholipase(s) that are activated during agonist stimulation.

Effect of various plasminogen activators on prostacyclin synthesis in cultured vascular cells

In this study, we examined the effects of various plasminogen activators on arachidonic acid release and prostacyclin biosynthesis in cultured rat aortic smooth muscle cells and bovine pulmonary artery endothelial cells. Prostacyclin was the major product formed from arachidonic acid in aortic smooth muscle cells and endothelial cells. When intact cells were incubated with streptokinase, one of the plasminogen activators, a significant stimulatory effect on prostacyclin biosynthetic activity in cells was evident without any cellular damage at all concentrations used (1-5,000 units/ml). Streptokinase also caused a marked release of arachidonic acid. However, when it was incubated with cell-free homogenates and [3H]arachidonic acid, it did not show any effects on prostacyclin biosynthesis. The addition of urokinase and tissue-type plasminogen activator had no effect on prostacyclin biosynthesis. Urokinase stimulated the release of arachidonic acid from cells, but it did not show any effect on prostacyclin release at any concentration of urokinase (1-5,000 units/ml). The release of arachidonic acid and the increased prostacyclin synthesis were not observed when tissue-type plasminogen activator was added. These results indicate that, among various plasminogen activators investigated, only streptokinase causes the release of arachidonic acid and prostacyclin. This could be a beneficial effect in thrombolytic therapy.

Prostacyclin biosynthesis and phospholipase activity in hypoxic rat myocardium

Phospholipid metabolism was studied in rat myocardial slices that were incubated under normoxia or hypoxia for up to 24 hours. Phospholipid degradation was prominent in hypoxic myocardium, particularly phosphatidylcholine, which markedly decreased after 24 hours of hypoxia. In contrast, lysophosphatidylcholine increased. The mechanism of phospholipid degradation in hypoxic myocardium was studied. The highest activity for phospholipase A2 among subcellular fractions was found in microsomal fraction. In hypoxic myocardium, this phospholipase A2 activity markedly increased and had substrate specificity toward phosphatidylcholine and phosphatidylethanolamine. Phosphatidylcholine was slightly hydrolyzed in control myocardium, but it was markedly hydrolyzed in hypoxic myocardium. Phospholipase C activity was found in cytosol and had a high substrate specificity toward phosphatidylinositol. In hypoxic myocardium, its activity gradually decreased during hypoxic incubation. Prostacyclin biosynthesis was also determined. The synthesis of prostacyclin in hypoxic myocardial microsomes did not increase. These results suggest that hypoxia causes phospholipid degradation and activates phospholipase A2 activity.

Effect of elastase on phospholipase activity in aortic smooth muscle cells

We have studied the effects of elastase on phospholipase activity in aortic smooth muscle cells and have found that when added to cells prelabeled with [3H]arachidonic acid, elastase induced rapid phospholipid hydrolysis, resulting in release of up to 18% of incorporated [3H]arachidonic acid into the medium. Maximum stimulation by elastase without any cellular damage was observed at a concentration of 50 units/ml. At higher concentrations (75-100 units/ml), release of arachidonic acid was still observed, but cells were damaged. After the addition of elastase, degradation of phosphatidylcholine, phosphatidylinositol and phosphatidylethanolamine was observed and it was found that their loss was comparable to the amount of [3H]arachidonic acid released. In aortic smooth muscle cells biosynthetically labeled by the incorporation of [3H]choline, [3H]inositol and [3H]ethanolamine into cellular phosphatidylcholine, phosphatidylinositol and phosphatidylethanolamine, respectively, the amount of phosphatidylcholine and phosphatidylethanolamine hydrolyzed following elastase-treatment was not equal to the amounts of lysophosphatidylcholine and lysophosphatidylethanolamine produced. We also observed a transient rise in diacylglycerol after the addition of elastase. To test for phospholipase C activity, the release of incorporated [3H]choline, [3H]inositol and [3H]ethanolamine into the culture medium was determined. The levels of radioactive choline and ethanolamine showed increases, but the change in inositol was comparatively small. An increase in inositol was detectable within 1 min after elastase addition, and peaked after 15 min, whereas increases in choline and ethanolamine continued for up to 60 min. These results indicate that elastase stimulated the activities of phospholipases A2 and C. Both were shown to be Ca2+-dependent, and it was found that, moreover, elastase enhanced Ca2+ influx. These results suggest increased cell-membrane permeability to Ca2+-stimulated phospholipases A2 and C. Prostaglandin biosynthesis in these cells was also enhanced by elastase.

Effects of platelet-activating factor on conversion of angiotensin I to II

We have studied the effects of platelet-activating factor (alkyl-acetyl-GPC) on conversion of angiotensin I to II. Platelet-activating factor (PAF) had a novel effect on angiotensin I conversion. Pulmonary artery endothelial cells converted 30% of [125I]angiotensin I to angiotensin II in the absence of PAF, but their activity was greatly stimulated in its presence. When PAF was added to pulmonary artery endothelial cells, the conversion of angiotensin I to II was enhanced up to 68%. Maximal stimulation without cellular damage was achieved at 10(-5) M PAF. LysoPAF did not stimulate the conversion of angiotensin I to angiotensin II at any concentration used.

Renal phospholipase C and diglyceride lipase activity in spontaneously hypertensive rats

Phospholipase C activity and diglyceride lipase activity were studied in the renal cortex and medulla of 10- and 40-week-old stroke-prone spontaneously hypertensive rats (SHRSP) and age-matched normotensive Wistar-Kyoto rats (WKY). Enhanced phospholipase C activity was found in the cortical and medullary cytosol of kidney from SHRSP, and microsomal diglyceride lipase in SHRSP also increased. In SHRSP, phospholipase C and diglyceride lipase activities increased with age, but this increase was not evident in WKY. Phospholipase C had high substrate specificity for phosphatidylinositol in renal cytosol of both WKY and SHRSP. The increased activities were accompanied by prostaglandin E2 synthesis in renal medullary microsomes of 10-week-old SHRSP and were also present in the kidney of 40-week-old SHRSP. Total phospholipid and arachidonic acid contents in kidney were markedly high in the medulla of 10-week-old SHRSP, but these lipids were decreased in 40-week-old SHRSP. These results suggest that phospholipids and arachidonic acid in SHRSP may be genetically high and that the activated phospholipase C and diglyceride lipase hydrolyze phospholipids, providing arachidonic acid for prostaglandin synthesis, which results in a decrease of phospholipids and arachidonic acid in the kidney of 40-week-old SHRSP. These studies demonstrate that a phosphatidylinositol-specific phospholipase C-prostaglandin synthetic system may play an important role in the course of hypertension in SHRSP.

Microsomal phospholipase A2 is activated by surfactant toxins in 3T3 mouse fibroblasts

In 3T3 mouse fibroblasts that were treated by surfactant toxins such as staphylococcal delta toxin, melittin from bee venom and lysolecithin, microsomal phospholipase A2 (PLA2) was activated. However, these toxins did not activate PLA2 when added to cell-free homogenates or microsomal preparations. The maximal activation was achieved at 1 mM Ca2+ pH 8.5. The microsomal PLA2 stimulated by these toxins had a high fatty acid specificity (C-2 position) toward arachidonic acid.