Differential regulation of von Willebrand factor exocytosis and prostacyclin synthesis in electropermeabilized endothelial cell monolayers

Synaptotagmin 5 regulates Ca2+-dependent Weibel-Palade body exocytosis in human endothelial cells

Elevations of intracellular free Ca²⁺ concentration ([Ca²⁺]_i) are a potent trigger for Weibel-Palade body (WPB) exocytosis and secretion of von Willebrand factor (VWF) from endothelial cells; however, the identity of WPB-associated Ca²⁺-sensors involved in transducing acute increases in [Ca²⁺]_i into granule exocytosis remains unknown. Here, we show that synaptotagmin 5 (SYT5) is expressed in human umbilical vein endothelial cells (HUVECs) and is recruited to WPBs to regulate Ca²⁺-driven WPB exocytosis. Western blot analysis of HUVECs identified SYT5 protein, and exogenously expressed SYT5-mEGFP localised almost exclusively to WPBs. shRNA-mediated knockdown of endogenous SYT5 (shSYT5) reduced the rate and extent of histamine-evoked WPB exocytosis and reduced secretion of the WPB cargo VWF-propeptide (VWFpp). The shSYT5-mediated reduction in histamine-evoked WPB exocytosis was prevented by expression of shRNA-resistant SYT5-mCherry. Overexpression of SYT5-EGFP increased the rate and extent of histamine-evoked WPB exocytosis, and increased secretion of VWFpp. Expression of a Ca²⁺-binding defective SYT5 mutant (SYT5-Asp197Ser-EGFP) mimicked depletion of endogenous SYT5. We identify SYT5 as a WPB-associated Ca²⁺ sensor regulating Ca²⁺-dependent secretion of stored mediators from vascular endothelial cells.

Temperature-dependence of Weibel-Palade body exocytosis and cell surface dispersal of von Willebrand factor and its propolypeptide

BACKGROUND

Weibel-Palade bodies (WPB) are endothelial cell (EC) specific secretory organelles containing Von Willebrand factor (VWF). The temperature-dependence of Ca(2+)-driven WPB exocytosis is not known, although indirect evidence suggests that WPB exocytosis may occur at very low temperatures. Here we quantitatively analyse the temperature-dependence of Ca(2+)-driven WPB exocytosis and release of secreted VWF from the cell surface of ECs using fluorescence microscopy of cultured human ECs containing fluorescent WPBs.

PRINCIPAL FINDINGS

Ca(2+)-driven WPB exocytosis occurred at all temperatures studied (7-37°C). The kinetics and extent of WPB exocytosis were strongly temperature-dependent: Delays in exocytosis increased from 0.92 s at 37°C to 134.2 s at 7°C, the maximum rate of WPB fusion decreased from 10.0±2.2 s(-1) (37°C) to 0.80±0.14 s(-1) (7°C) and the fractional extent of degranulation of WPBs in each cell from 67±3% (37°C) to 3.6±1.3% (7°C). A discrepancy was found between the reduction in Ca(2+)-driven VWF secretion and WPB exocytosis at reduced temperature; at 17°C VWF secretion was reduced by 95% but WPB exocytosis by 75-80%. This discrepancy arises because VWF dispersal from sites of WPB exocytosis is largely prevented at low temperature. In contrast VWF-propolypeptide (proregion) dispersal from WPBs, although slowed, was complete within 60-120 s. Novel antibodies to the cleaved and processed proregion were characterised and used to show that secreted proregion more accurately reports the secretion of WPBs at sub-physiological temperatures than assay of VWF itself.

CONCLUSIONS

We report the first quantitative analysis of the temperature-dependence of WPB exocytosis. We provide evidence; by comparison of biochemical data for VWF or proregion secretion with direct analysis of WPB exocytosis at reduced temperature, that proregion is a more reliable marker for WPB exocytosis at reduced temperature, where VWF-EC adhesion is increased.

Cell-cell interaction underlies formation of fluid in the male reproductive tract of the rat

The epithelia lining the epididymides of many species consists of several cell types. We have provided evidence that the basal cells are essential to the integrated functions of the epithelium. Basal cells, but not principal cells, and other cells in the epididymis express TRPC3 and COX-1. We have isolated basal cells from intact rat epididymis using antibody-coated Dynabeads and subjected them to whole-cell patch-clamp measurement of nonselective cation channel activity, a feature of TRPC3 protein, and Fluo-3 fluorescence measurement of intracellular Ca2+ concentration. The results show that a nonselective cation current blockable by La3+ (0.1 mM), Gd3+ (0.1 mM), or SKF96365 (20 microM) could be activated by lysylbradykinin (200 nM). In cells loaded with Fluo-3, addition of lysylbradykinin (100 nM) caused a sustained increase of intracellular Ca2+. This effect was blocked by Gd3+ (0.1 mM) or SKF96365 (20 microM) and was not observed in Fluo-3-loaded principal cells. Stimulation of basal cell/principal cell cocultures with lysylbradykinin (200 nM) evoked in principal cells a current with CFTR-Cl- channel characteristics. Isolated principal cells in the absence of basal cells did not respond to lysylbradykinin but responded to PGE2 (100 nM) with activation of a CFTR-like current. Basal cells, but not principal cells, released prostaglandin E2 when stimulated with lysylbradykinin (100 nM). The release was blocked by SKF96365 (20 microM) and BAPTA-AM (0.05 or 0.1 mM). Confluent cell monolayers harvested from a mixture of disaggregated principal cells and basal cells responded to lysylbradykinin (100 nM) and PGE2 (500 nM) with an increase in electrogenic anion secretion. The former response was dependent on prostaglandin synthesis as piroxicam blocked the response. However, cell cultures obtained from principal cells alone responded to PGE2 but not to bradykinin. These results support the notion that basal cells regulate principal cells through a Ca2+ and COX signaling pathway.

Regulation of epididymal principal cell functions by basal cells: role of transient receptor potential (Trp) proteins and cyclooxygenase-1 (COX-1)

The epithelia lining the epididymides of many species including the human are known to consist of several cell types. Among them, the principal cells are the most abundant and their functions most extensively studied. There are other cell types such as the narrow cells, clear cells, halo cells and basal cells which are scattered along the duct in lesser number. Although these minority cell types have not been studied to the same extent as the principal cells, it is conceivable that their presence are essential to the integrated functions of the epididymis. In the intact epididymis, basal cells can be seen adhering to the basement membrane forming close contact with the principal cells above them. Work in our laboratory has provided evidence that through local formation of prostaglandins, basal cells may regulate electrolyte and water transport by the principal cells. This regulatory process involves two proteins which are exclusively expressed by the basal cells. They are the transient receptor potential (Trp) proteins, which serve as transmembrane pathways for Ca(2+) influx, and cyclooxygenase 1 (COX-1), a key enzyme in the formation of prostaglandins. The role of the two proteins in the integrated functions of the basal cells as humoral regulators of principal cells is discussed.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Differential exocytosis from human endothelial cells evoked by high intracellular Ca(2+) concentration

Endothelial cells secrete a range of procoagulant, anticoagulant and inflammatory proteins by exocytosis to regulate blood clotting and local immune responses. The mechanisms regulating vesicular exocytosis were studied in human umbilical vein endothelial cells (HUVEC) with high-resolution membrane capacitance (C(m)) measurements. The total whole-cell C(m) and the amplitudes and times of discrete femtoFarad (fF)-sized C(m) steps due to exocytosis and endocytosis were monitored simultaneously. Intracellular calcium concentration [Ca(2+)](i) was elevated by intracellular photolysis of calcium-DM-nitrophen to evoke secretion and monitored with the low-affinity Ca(2+) indicator furaptra. Sustained elevation of [Ca(2+)](i) to > 20 microM evoked large, slow increases in C(m) of up to 5 pF in 1-2 min. Exocytotic and endocytotic steps of amplitude 0.5-110 fF were resolved, and accounted on average for ~33 % of the total C(m) change. A prominent component of C(m) steps of 2.5-9.0 fF was seen and could be attributed to exocytosis of von-Willebrand-factor-containing Weibel-Palade bodies (WPb), based on the near-identical distributions of capacitance step amplitudes, with calculated estimates of WPb capacitance from morphometry, and on the absence of 2.5-9.0 fF C(m) steps in cells deficient in WPb. WPb secretion was delayed on average by 23 s after [Ca(2+)](i) elevation, whereas total C(m) increased immediately due to the secretion of small, non-WPb granules. The results show that following a large increase of [Ca(2+)](i), corresponding to strong stimulation, small vesicular components are immediately available for secretion, whereas the large WPb undergo exocytosis only after a delay. The presence of events of magnitude 9-110 fF also provides evidence of compound secretion of WPb due to prior fusion of individual granules.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.

Normal endothelial cell function

Endothelial cell functions, primarily involving regulated mediator secretion or altered surface protein expression, are vital for normal homeostasis. Endothelial cells secrete the potent vasodilator and anti-platelet agent prostacyclin and nitric oxide, and also the potent vasoconstrictor peptide endothelin-1; they control the selective adhesion and emigration of leukocytes from the bloodstream; and they are the source of circulating von Willebrand factor, tissue plasminogen activator and type 1 plasminogen activator inhibitor. The properties of healthy endothelium ensure that an antithrombotic and anticoagulant balance is maintained in the bloodstream, and provide a tonic vasodilator action that controls blood flow and pressure on a minute-to-minute basis. Disturbances of normal endothelial function are strongly implicated in the pathogenesis of atherosclerosis and autoimmune vasculitic diseases including lupus.

Membrane capacitance changes induced by thrombin and calcium in single endothelial cells cultured from human umbilical vein

1. Vesicular secretion from single human umbilical vein endothelial cells (HUVECs) was monitored by changes in membrane capacitance (Cm). Secretion was evoked by dialysis with strongly buffered intracellular free Ca2+ concentrations ([Ca2+]i), flash photolysis of Ca2+-loaded DM-nitrophen or caged InsP3, or by thrombin. [Ca2+]i was monitored spectrofluorimetrically with furaptra. The results show that a large, slowly rising component of vesicular secretion requires prolonged exposure to high [Ca2+]i. 2. Cm increased during intracellular perfusion with [Ca2+] buffered in the range 1.0-20 microM. Changes in Cm comprised an initial slowly rising small component of 0.1-0.5 pF followed by a faster rising larger component of up to approximately 7 pF, seen when [Ca2+]i > 2 microM and which was maximal at 10-20 microM Ca2+. 3. Thrombin evoked rapid initial elevations of [Ca2+]i to a peak of 7.1 +/- 1.5 microM (mean +/- s.e. m., n = 5) that declined within approximately 20-30 s with thrombin present either to resting levels or to a maintained elevated level of 2.0 +/- 0.7 microM (mean +/- s.e.m., range 1.0-3.6 microM, n = 3). Transient [Ca2+]i rises were associated with small, slowly rising increases in Cm of 0.1-0.2 pF, that recovered to pre-application levels over 2-3 min. Maintained elevations of [Ca2+]i caused larger, faster-rising sustained increases in Cm to 1.14 +/- 0.12 pF (mean +/- s.e.m., n = 3). Separate specific enzyme-linked immunosorbent assay (ELISA) showed that 1.0 U ml-1 thrombin produced secretion of von Willebrand factor in HUVEC cultures. 4. Short-lived [Ca2+]i elevations with a peak of 3-25 microM and a duration of approximately 20 s generated by flash photolysis of caged InsP3 or DM-nitrophen produced either no net change in Cm, or small slow increases of approximately 0.1-0.6 pF at up to 5 fF s-1 that recovered to pre-flash levels over 2-3 min. 5. Maintained elevations of [Ca2+]i in the range 1-28 microM produced by flash photolysis of DM-nitrophen caused large increases in Cm, up to approximately 4 pF, corresponding to approximately 25-30 % of the initial cell Cm. The maximum rate of change of Cm was up to 50 fF s-1 at steady [Ca2+] up to 20 microM; Cm recovered towards pre-flash levels only when [Ca2+] had declined.

Autocrine regulation of endothelial exocytosis: von Willebrand factor release is induced by prostacyclin in cultured endothelial cells

Vascular endothelial cells respond to external stimuli by altering the secretion of several bioactive molecules, including von Willebrand factor (vWf), prostacyclin (PGI2) and nitric oxide (NO). The release of all three molecules is regulated by a rise in cytosolic calcium ([Ca2+]i). In the present study we investigated whether cAMP-dependent signaling provides differential regulation of these effector systems by modulating the effect of [Ca2+]i in cultured human endothelial cells. The stable PGI2 analog iloprost, like other cAMP-raising agents (forskolin and adenosine), caused an acute dose-dependent increase in vWf release and potentiated the secretory response to thrombin. In contrast, iloprost, forskolin and adenosine failed to induce PGI2 release and inhibit thrombin-induced release. Our findings indicate cAMP-raising agents have opposite effects on [Ca2+]i-mediated vWf secretion and PGI2 release. PGI2 may potentiate vWf release and inhibit its own release in an autocrine manner.

Kinetics of Ca2+ release by InsP3 in pig single aortic endothelial cells: evidence for an inhibitory role of cytosolic Ca2+ in regulating hormonally evoked Ca2+ spikes

1. The role of the InsP3 receptor and its interaction with Ca2+ in shaping endothelial Ca2+ spikes was investigated by comparing InsP3-evoked intracellular Ca2+ release with hormonally evoked Ca2+ spikes in single endothelial cells. 2. InsP3 was generated by flash photolysis of intracellular caged InsP3. InsP3 at 0.2 microM or higher released Ca2+ from stores with a time course comprising a well-defined delay, a fast rise of free [Ca2+] to a peak where net flux into the cystosol is zero, and a slow decline to preflash levels. InsP3-evoked Ca2+ flux into unit cytosolic volume was measured as the rate of change of free [Ca2+]i during the fast rise, d[Ca2+]i/dt (mol s-1 l-1). 3. The mean delay decreased from 433 ms at 0.2 microM to 30 ms at 5 microM. At very high InsP3 concentrations, 78 microM, the delay was shorter, < 10 ms. At low InsP3 concentration the delay was reduced by approximately 30% by prior elevation of free [Ca2+]i, supporting a co-operative action of free [Ca2+] and InsP3 in activation. 4. Both Ca2+ flux and peak free [Ca2+]i increased with InsP3 concentration within each cell. Maximal activation was at > 5 microM, 50% maximum Ca2+ flux was at 1.6 microM InsP3 and the Hill coefficient was between 3.6 and 4.3. A large variation of Ca2+ flux and peak [Ca2+]i was found from cell to cell at the same InsP3 concentration. 5. Strong inhibition of InsP3-evoked flux was produced by an immediately preceding response, with complete inhibition at peak free [Ca2+]i due to the first pulse. InsP3 sensitivity returned over 1-2 min, with 50% recovery at approximately 25 s. The recovery of InsP3 sensitivity may determine the minimum interval between hormonally evoked spikes. 6. Ca2+ flux due to a pulse of InsP3 terminated rapidly, in the continued presence of InsP3, producing a well-defined peak [Ca2+]. A reciprocal relation was found between the duration and the rate of Ca2+ flux, such that high Ca2+ flux was of brief duration. The rate of termination of flux measured as the reciprocal of the 10-90% rise time of free [Ca2+]i showed a linear correlation with Ca2+ flux over a large range in all cells. A systematic deviation from linearity at low InsP3 concentration showed a greater rate of termination at low InsP3 concentration than at high for the same flux. 7. Elevating cytosolic free [Ca2+] by 0.1-2.5 microM strongly inhibited Ca2+ release by InsP3, and buffering free [Ca2+] to low levels greatly prolonged Ca2+ release. Both results support the idea that Ca2+ flux quickly produces locally high free [Ca2+] which inhibits the receptor and terminates Ca2+ release. 8. Hormonally evoked Ca2+ spikes showed a similar reciprocal relation between rise time and Ca2+ flux, seen in the initial Ca2+ spike evoked by extracellular ATP in porcine aortic endothelial cells and by acetylcholine in rat aortic endothelial cells in situ, supporting the idea that the same mechanism of cytosolic Ca2+ inhibition determines the duration of hormonally and InsP3-evoked Ca2+ spikes.

Involvement of calcium and G proteins in the acute release of tissue-type plasminogen activator and von Willebrand factor from cultured human endothelial cells

In this study, we investigated the role of Ca2+ and G proteins in thrombin-induced acute release (regulated secretion) of tissue-type plasminogen activator (TPA) and von Willebrand factor (vWF), using a previously described system of primary human umbilical vein endothelial cells (HUVECs). The acute release of TPA and vWF, as induced by alpha-thrombin, was almost zero after chelation of Ca2+i, showing that an increase in [Ca2+]i was required. It did not matter whether the increase in [Ca2+]i came from an intracellular or extracellular Ca2+ source. Thrombin-induced release of TPA and vWF already started at low [Ca2+]i, around 100 nmol/L. Half-maximal release was found at a [Ca2+]i, of 261 nmol/L for TPA and at 222 nmol/L for vWF. The Ca2+ signal was transduced to calmodulin, as calmodulin inhibitors inhibited TPA and vWF release. The Ca2+ ionophore ionomycin dose dependently released vWF; half-maximal vWF release occurred at a [Ca2+]i of 311 nmol/L. In contrast, no TPA release was found at all below a [Ca2+]i of 500 nmol/L. Thus, below 500 nmol/L [Ca2+]i, an increase in [Ca2+]i alone was sufficient to induce vWF release but not sufficient to induce TPA release. Protein kinase C did not appear to be involved in TPA or vWF release, as neither an activator nor an inhibitor of protein kinase C significantly influenced release. Inhibition of phospholipase A2 also did not reduce thrombin-induced TPA and vWF release. The involvement of G proteins was studied by using both saponin-permeabilized and intact cells. GDP-beta-S, which inhibits heterotrimeric and small G proteins, significantly inhibited thrombin-induced vWF and TPA release from permeabilized cells. AlF-4, which activates heterotrimeric G proteins, induced TPA and vWF release in both intact and permeabilized HUVECs. Preincubation of HUVECs with pertussis toxin significantly inhibited thrombin-induced vWF release, due to inhibition of thrombin-induced Ca2+ influx. Pertussis toxin did not affect ionomycin-induced release. The inhibitory effect of pertussis toxin was less obvious in thrombin-induced TPA release, because it was counterbalanced by a positive effect of the toxin on TPA release. Thus, both inhibitory and stimulatory (pertussis toxin-sensitive) G proteins were involved in TPA release. Therefore, thrombin-induced acute release of TPA and vWF differed in two respects. First, below a [Ca2+]i of 500 nmol/L, an increase in Ca2+ was sufficient for vWF release but not for TPA release. Second, pertussis toxin-sensitive G proteins were differentially involved in acute TPA and vWF release.

SK&F 96365 inhibits intracellular Ca2+ pumps and raises cytosolic Ca2+ concentration without production of nitric oxide and von Willebrand factor

The effects of the imidazole compound SK&F 96365 on Ca2+ movements and production of nitric oxide (NO) and von Willebrand factor (vWF) have been investigated in human endothelial cells. Changes in cytosolic Ca2+ concentration ([Ca2+]i) were measured with Fura-2. Real-time production of NO was monitored with a porphyrinic microsensor and the release of vWF with an enzyme-linked immunosorbent assay. Irrespective of the transmembrane Ca2+ gradient, 30 microM SK&F 96365 doubled [Ca2+]i suggesting a Ca2+ release from intracellular stores. The SK&F 96365-induced [Ca2+]i rise was not accompanied by detectable NO and vWF production, while 1 microM thapsigargin enhanced [Ca2+]i 2.5 times, doubled the secretion of vWF and increased the NO production to 10 +/- 4 nM (n = 5). Pretreatment with SK&F 96365 prevented thapsigargin from increasing [Ca2+]i, NO production and vWF secretion. To investigate the mechanism by which SK&F 96365 released Ca2+ from internal pools, its effect and that of thapsigargin on the ATP-dependent 45Ca2+ uptake into platelet membrane vesicles were compared. SK&F 96365 as thapsigargin, dose-dependently reduced the initial rate of 45Ca2+ uptake. In conclusion, we demonstrate that, in the absence of Ca2+ entry from the extracellular space, the [Ca2+]i increase elicited by SK&F 96365 or thapsigargin is not sufficient to initiate NO synthesis and vWF secretion. This confirms the important role of Ca2+ influx in endothelial secretion processes.

Protein tyrosine kinases regulate agonist-stimulated prostacyclin release but not von Willebrand factor secretion from human umbilical vein endothelial cells

The rapid synthesis and release of prostacyclin (PGI2) and the exocytotic secretion of von Willebrand Factor (vWF) elicited by activation of G-protein-coupled receptors on endothelium occur via signaling mechanisms which are incompletely defined. Activation of protein tyrosine kinases (PTKs) and modulation of the tyrosine-phosphorylation state of endogenous proteins have been implicated in several cellular processes including arachidonate release and exocytosis. In the present study we have examined the regulatory role of PTKs in agonist-stimulated release of PGI2 and vWF from human umbilical vein endothelial cells (HUVECs) using two chemically and mechanistically dissimilar PTK inhibitors (genistein and ST271). Genistein, but not the less active analogue daidzein, dose-dependently attenuated PGI2 release in response to thrombin and histamine (IC50 approx. 20 microM), and to the thrombin-receptor-activating peptide. A more potent inhibition of thrombin- and histamine-induced PGI2 synthesis was observed in cells exposed to ST271. In contrast, neither genistein nor ST271 modulated agonist-drive vWF secretion. At concentrations that abolished PGI2 release, genistein blocked thrombin- or histamine-evoked tyrosine phosphorylation of a 42 kDa protein. Ca2+ ionophore-induced PGI2 generation, but not vWF secretion, was also inhibited by both genistein and ST271, suggesting that these agents modulate PGI2 synthesis by acting at, or distal to, agonist-induced changes in intracellular CA2+ ([Ca2+]i). In fura-2-loaded HUVECs genistein partially reduced the histamine-induced peak [Ca2+]i but had no effect on the thrombin response. Ca(2+)-induced PGI2 release from electrically permeabilized HUVECs was abolished in the presence of ST271 or genistein, but not diadzein. The generation of PGI2 in response to exogenous arachidonic acid was not modulated by genistein or ST271, suggesting that PTK inhibitors do not directly inhibit cyclo-oxygenase activity. Taken together, these results suggest that PTKs regulate PGI2 synthesis and release in HUVECs by modulating, directly or indirectly, a CA(2+)-sensitive step upstream of cyclo-oxygenase.