Cyclic strain induces reorganization of integrin alpha 5 beta 1 and alpha 2 beta 1 in human umbilical vein endothelial cells

Cyclic strain has been shown to modulate endothelial cell (EC) morphology, proliferation, and function. We have recently reported that the focal adhesion proteins focal adhesion kinase (pp125FAK) and paxillin, are tyrosine phosphorylated in EC exposed to strain and these events regulate the morphological change and migration induced by cyclic strain. Integrins are also localized on focal adhesion sites and have been reported to induce by tyrosine phosphorylation of pp125FAK under a variety of stimuli. To study the involvement of different integrins in signaling induced by cyclic strain, we first observed the redistribution of alpha and beta integrins in EC subjects to 4 h cyclic strain. Human umbilical vein endothelial cells (HUVEC) seeded on either fibronectin or collagen surfaces were subjected to 10% average strain at a frequency 60 cycles/min. Confocal microscopy revealed that beta 1 integrin reorganized in a linear pattern parallel with the long axis of the elongated cells creating a fusion of focal adhesion plaques in EC plated on either fibronectin (a ligand for alpha 5 beta 1) or collagen (a ligand for alpha 2 beta 1) coated after 4 h exposure to cyclic strain. beta 3 integrin, which is a vitronectin receptor, did not redistribute in EC exposed to cyclic strain. Cyclic strain also led to a reorganization of alpha 5 and alpha 2 integrins in a linear pattern in HUVEC seeded on fibronectin or collagen, respectively. The expression of integrins alpha 5, alpha 2, and beta 1 did not change even after 24 h exposure to strain when assessed by immunoprecipitation of these integrins. Cyclic strain-induced tyrosine phosphorylation of pp125FAK occurred concomitant with the reorganization of beta 1 integrin. We concluded that alpha 5 beta 1 and alpha 2 beta 1 integrins play an important role in transducing mechanical stimuli into intracellular signals.

Activation of the adenylyl cyclase/cyclic AMP/protein kinase A pathway in endothelial cells exposed to cyclic strain

The aim of this study was to assess the involvement of the adenylyl cyclase/cyclic AMP/protein kinase A pathway (AC) in endothelial cells (EC) exposed to different levels of mechanical strain. Bovine aortic EC were seeded to confluence on flexible membrane-bottom wells. The membranes were deformed with either 150 mm Hg (average 10% strain) or 37.5 mm Hg (average 6% strain) vacuum at 60 cycles per minute (0.5 s strain; 0.5 s relaxation) for 0-60 min. The results demonstrate that at 10% average strain (but not 6% average strain) there was a 1.5- to 2.2-fold increase in AC, cAMP, and PKA activity by 15 min when compared to unstretched controls. Further studies revealed an increase in cAMP response element binding protein in EC subjected to the 10% average strain (but not 6% average strain). These data support the hypothesis that cyclic strain activates the AC/cAMP/PKA signal transduction pathway in EC which may occur by exceeding a strain threshold and suggest that cyclic strain may stimulate the expression of genes containing cAMP-responsive promoter elements.

Activation of diacylglycerol in cultured endothelial cells exposed to cyclic strain

Confluent bovine aortic endothelial cells (EC) were grown on flexible membranes and subjected to 10% average strain at 60 cycles/min for up to 500 s. A biphasic increase in diacylglycerol (DAG) occurred, with an initial transient peak at 10 s followed by sustained elevation to 500 s. The early peak corresponded to the transient formation of inositol 1,4,5-trisphosphate, demonstrating hydrolysis of L-alpha-phosphatidylinositol (PI) by PI-specific phospholipase C. To determine the origin of the sustained DAG phase, we incubated confluent bovine aortic EC with 1 microCi/ml [14C]myristate overnight and subjected them to cyclic strain. There was a decrease in phosphatidylcholine (PC) and a corresponding increase in DAG at 10 s and 250 s, suggesting PC hydrolysis with the generation of DAG at both an early (10 s) and a late (250 s) phase. [14C]phosphatidylethanol, a specific product of phospholipase D (PLD) in the presence of 1% ethanol, was measured in EC preincubated with [14C]myristate. Cyclic strain led to an immediate and sustained activation of PLD. Increased ethanol concentration led to a consistent decrease in DAG. Furthermore, when EC were pretreated with 1% ethanol, the strain-induced proliferative response was attenuated.

Gene regulation by mechanical forces

Endothelial cells are subjected to various mechanical forces in vivo from the flow of blood across the luminal surface of the blood vessel. The purpose of this review was to examine the data available on how these mechanical forces, in particular cyclic strain, affect the expression and regulation of endothelial cell function. Studies from various investigators using models of cyclic strain in vitro have shown that various vasoactive mediators such as nitric oxide and prostacyclin are induced by the effect of mechanical deformation, and that the expression of these mediators may be regulated at the transcription level by mechanical forces. There also seems to be emerging evidence that endothelial cells may also act as mechanotransducers, whereby the transmission of external forces induces various cytoskeletal changes and second messenger cascades. Furthermore, it seems these forces may act on specific response elements of promoter genes.

Protein phosphatase 2A in stretch-induced endothelial cell proliferation

We previously proposed that activation of protein kinase C is a key mechanism for control of cell growth enhanced by cyclic strain [Rosales and Sumpio (1992): Surgery 112:459-466]. Here we examined protein phosphatase 1 and 2A activity in bovine aortic endothelial cells exposed to cyclic stain. Protein phosphatase 2A activity in the cytosol was decreased by 36.1% in response to cyclic strain for 60 min, whereas the activity in the membrane did not change. Treatment with low concentration (0.1 nM) of okadaic acid enhanced proliferation of both static and stretched endothelial cells in 10% fetal bovine serum. These data suggest that protein phosphatase 2A acts as a growth suppressor and cyclic strain may enhance cellular proliferation by inhibiting protein phosphatase 2A as well as stimulating protein kinase C.

Amplitude-dependent modulation of brush border enzymes and proliferation by cyclic strain in human intestinal Caco-2 monolayers

Little is known about the effects of repetitive deformation during peristaltic distension and contraction or repetitive villus shortening on the proliferation and differentiation of the intestinal epithelium. We sought to characterize the effects of repetitive deformation of a physiologically relevant magnitude and frequency on the proliferation and differentiation of human intestinal epithelial Caco-2 cells, a common cell culture model for intestinal epithelial biology. Human intestinal epithelial Caco-2 cells were cultured on collagen-coated membranes deformed by -20 kPa vacuum at 10 cycles/minute, producing an average 10% strain on the adherent cells. Proliferation was assessed by cell counting and 3H-thymidine incorporation. Alkaline phosphatase and dipeptidyl dipeptidase specific activity were measured in cell lysates. Since cells at the membrane periphery experience higher strain than cells in the center, the topography of brush border enzyme histochemical and immunohistochemical staining was analyzed for strain-dependence. Cyclic strain stimulated proliferation compared to static cells. Proliferation was highest in the membrane periphery where strain was maximal. Strain also modulated differentiation independently of its mitogenic effects, selectively stimulating dipeptidyl dipeptidase while inhibiting alkaline phosphatase. Strain-associated enzyme changes were also maximal in areas of greatest strain. The PKC inhibitors staurosporine and calphostin C ablated strain mitogenic effects while intracellular PKC activity was increased by strain. The strain-associated brush border enzyme changes were attenuated but not blocked by PKC inhibition. Thus, strain of a physiologically relevant frequency and magnitude promotes proliferation and modulates the differentiation of a well-differentiated human intestinal epithelial cell line in an amplitude-dependent fashion. PKC may be involved in coupling strain to increased proliferation.

Tyrosine phosphorylation of pp125FAK and paxillin in aortic endothelial cells induced by mechanical strain

The objective of this study was to determine whether focal adhesion proteins pp125FAK (focal adhesion kinase) and paxillin are phosphorylated on tyrosine and might play a role in the morphological change and cell migration induced by strain. Bovine aortic endothelial cells (EC) were subjected to 10% average strain at 60 cycles/min. Cyclic strain increased the tyrosine phosphorylation of pp125FAK at 30 min (3.4-fold) and 4 h (5.9-fold) and the tyrosine phosphorylation of paxillin at 4 h (2.0-fold). Confocal microscopy showed that, after 4-h exposure to strain, EC began to elongate and F-actin, pp125FAK, and paxillin aligned, although they randomly distributed in static condition. Tyrosine kinase inhibitor tyrphostin A25 (100 microM) inhibited not only the tyrosine phosphorylation of pp125FAK and paxillin but also the redistribution of pp125FAK and paxillin, morphological change, and migration of EC induced by strain. These data demonstrate that cyclic strain induced tyrosine phosphorylation and reorganization of pp125FAK and paxillin and suggest that these focal adhesion proteins play a specific role in cyclic strain-induced morphological change and migration.

Amplitude-dependent modulation of brush border enzymes and proliferation by cyclic strain in human intestinal Caco-2 monolayers

Little is known about the effects of repetitive deformation during peristaltic distension and contraction or repetitive villus shortening on the proliferation and differentiation of the intestinal epithelium. We sought to characterize the effects of repetitive deformation of a physiologically relevant magnitude and frequency on the proliferation and differentiation of human intestinal epithelial Caco-2 cells, a common cell culture model for intestinal epithelial biology. Human intestinal epithelial Caco-2 cells were cultured on collagen-coated membranes deformed by -20 kPa vacuum at 10 cycles/minute, producing an average 10% strain on the adherent cells. Proliferation was assessed by cell counting and 3H-thymidine incorporation. Alkaline phosphatase and dipeptidyl dipeptidase specific activity were measured in cell lysates. Since cells at the membrane periphery experience higher strain than cells in the center, the topography of brush border enzyme histochemical and immunohistochemical staining was analyzed for strain-dependence. Cyclic strain stimulated proliferation compared to static cells. Proliferation was highest in the membrane periphery where strain was maximal. Strain also modulated differentiation independently of its mitogenic effects, selectively stimulating dipeptidyl dipeptidase while inhibiting alkaline phosphatase. Strain-associated enzyme changes were also maximal in areas of greatest strain. The PKC inhibitors staurosporine and calphostin C ablated strain mitogenic effects while intracellular PKC activity was increased by strain. The strain-associated brush border enzyme changes were attenuated but not blocked by PKC inhibition. Thus, strain of a physiologically relevant frequency and magnitude promotes proliferation and modulates the differentiation of a well-differentiated human intestinal epithelial cell line in an amplitude-dependent fashion. PKC may be involved in coupling strain to increased proliferation.

Amplitude-dependent modulation of brush border enzymes and proliferation by cyclic strain in human intestinal Caco-2 monolayers

Little is known about the effects of repetitive deformation during peristaltic distension and contraction or repetitive villus shortening on the proliferation and differentiation of the intestinal epithelium. We sought to characterize the effects of repetitive deformation of a physiologically relevant magnitude and frequency on the proliferation and differentiation of human intestinal epithelial Caco-2 cells, a common cell culture model for intestinal epithelial biology. Human intestinal epithelial Caco-2 cells were cultured on collagen-coated membranes deformed by -20 kPa vacuum at 10 cycles/minute, producing an average 10% strain on the adherent cells. Proliferation was assessed by cell counting and 3H-thymidine incorporation. Alkaline phosphatase and dipeptidyl dipeptidase specific activity were measured in cell lysates. Since cells at the membrane periphery experience higher strain than cells in the center, the topography of brush border enzyme histochemical and immunohistochemical staining was analyzed for strain-dependence. Cyclic strain stimulated proliferation compared to static cells. Proliferation was highest in the membrane periphery where strain was maximal. Strain also modulated differentiation independently of its mitogenic effects, selectively stimulating dipeptidyl dipeptidase while inhibiting alkaline phosphatase. Strain-associated enzyme changes were also maximal in areas of greatest strain. The PKC inhibitors staurosporine and calphostin C ablated strain mitogenic effects while intracellular PKC activity was increased by strain. The strain-associated brush border enzyme changes were attenuated but not blocked by PKC inhibition. Thus, strain of a physiologically relevant frequency and magnitude promotes proliferation and modulates the differentiation of a well-differentiated human intestinal epithelial cell line in an amplitude-dependent fashion. PKC may be involved in coupling strain to increased proliferation.

Involvement of rho p21 in cyclic strain-induced tyrosine phosphorylation of focal adhesion kinase (pp125FAK), morphological changes and migration of endothelial cells

The molecular mechanisms by which endothelial cells sense and respond to physical forces remain to be elucidated. Recently we reported that cyclic strain-induced morphological change and migration of EC were regulated by the tyrosine phosphorylation of focal adhesion kinase (pp125FAK) and paxillin. The aim of the present study was to clarify the role of the small GTP-binding protein rho p21 in EC exposed to cyclic strain. Bovine aortic endothelial cells (EC) were subjected to 10% average strain at 60 cycle/min. Clostridium botulinum C3 transferase (C3) was used as a specific inhibitor of rho p21. Preincubation of EC with C3 inhibited ADP-ribosylation of rho (94%) and inhibited the morphological change, reorganization of actin filaments, and migration induced by cyclic strain. Moreover, C3 inhibited the cyclic strain-induced tyrosine phosphorylation of pp125FAK and paxillin. These results demonstrate that rho downregulates the tyrosine phosphorylation of pp125FAK and paxillin and can modulate the morphological changes and migration induced by cyclic strain.

Matrix-specific effect of endothelial control of smooth muscle cell migration

PURPOSE

Smooth muscle cell (SMC) migration is a critical element in the development of intimal hyperplasia. The effect of endothelial cells (ECs) on SMC migration and the modulation of this cell-to-cell interaction by extracellular matrix is not well understood.

METHODS

To examine this relationship SMCs and ECs were cocultured on opposite sides of a semipermeable membrane and were compared with SMCs cultured alone. To assess migration SMCs were plated at confluent density into the center of the membrane with a steel fence. After the fence was removed, SMCs were treated for 2 hours with mitomycin C (20 micrograms/ml) to assess migration independent of proliferation. Cell migration was measured with morphometry. Experiments were performed on plastic and membranes coated with fibronectin or type I collagen (n > or = 8/group). Cell adhesiveness was quantitated by cell attachment and spreading assays.

RESULTS

ECs stimulated SMC migration by 187% when compared with SMCs cultured alone on plastic and by 160% when cultured on fibronectin (p < 0.01). Type I collagen stimulated migration of SMCs cultured alone and prevented EC stimulated migration in cocultured SMCs (p < 0.01). Cell adhesiveness was significantly increased in cocultured SMCs compared with SMCs cultured alone regardless of whether cells were cultured on plastic (EC/SMC, 13.5 +/- 0.6 SMCs/high power field vs SMC, 8.9 +/- 0.5, p < 0.01), fibronectin (16.3 +/- 0.8 vs 12.3 +/- 0.7, p < 0.01) or type I collagen (15.5 +/- 1.0 vs 12.4 +/- 0.6, p < 0.01). ECs increased SMC cell spreading on plastic and fibronectin when compared with SMCs cultured alone. No difference in SMC cell spreading was seen in the presence or absence of ECs when cells were cultured on type I collagen. EC-SMC contact was not required; EC-conditioned media alone increased SMC migration by 75% when compared with SMCs cultured alone. Our data suggest that ECs increase SMC migration by a diffusable molecule that may also alter SMC adhesion molecule expression. Extracellular matrix composition can attenuate these effects.

Transcriptional regulation of endothelin

Three isoforms of endothelin (ET) exist, ET-l, ET-2,and ET-3.Nucleotide sequences for the three human ET genes are highly conserved. ET-l exactly matches the sequence ofET originally isolated from the condi- tioned medium of cultured bovine aortic endothelial cells (BAECs)."All three forms have been found in vascular, neural, adrenal, and kidney tissue, but are expressed in different proportions. Endothelial cells ex- clusively produce ETAll three isoforms have different vasoconstrictive potencies but are otherwise quali- tatively similar. ET-2 is the most potent vasoconstrictor, followed by ET-l and ET.

Cyclic strain causes heterogeneous induction of transcription factors, AP-1, CRE binding protein and NF-kB, in endothelial cells: species and vascular bed diversity

Recent studies demonstrate that cyclic strain stimulates protein kinase C in bovine aortic endothelial cells (BAEC) as well as the induction of immediate early genes and the transcription factor activator protein-1 (AP-1) in human umbilical vein endothelial cells (HUVEC). The objective of this study was to determine whether transcriptional factor induction in endothelial cells (EC) exposed to strain is the same with regard to the species and vascular bed they are derived from. Evidence for a heterogeneous response for growth, orientation and prostacyclin secretion has been obtained for a variety of EC exposed to cyclic strain. In this study, we investigated cyclic strain mediated induction of transcription factors, AP-1, cAMP response element binding protein (CRE) and nuclear factor kB (NF-kB) in cultured EC from HUVEC, human aorta (HAEC), and BAEC. EC were exposed to 10% average strain at 60 cpm for up to 24 h. At varying time points, nuclear protein was extracted and analyzed for production of AP-1, CRE and NF-kB by electromobility shift assay. The results demonstrate that EC exposure to cyclic strain leads to a significant induction of AP-1, CRE and NF-kB in HAEC and HUVEC, but not in BAEC. Furthermore, these findings are in marked contrast to the previously described shear stress induced activation of AP-1 and NF-kB in BAEC. There was also a temporal difference in their response such that stretch-induced activation of AP-1 and NF-kB peaked at 4 h, whereas CRE increased in a biphasic manner at 15 min and 24 h. These results may partially explain the divergent effects of cyclic strain on EC gene expression and phenotype in EC from different vascular beds and species and underscore the difference in EC response to cyclic strain and shear stress.

Cyclic strain upregulates nitric oxide synthase in cultured bovine aortic endothelial cells

In vivo, endothelial cells (EC) are subjected to hemodynamic forces which may influence the production of nitric oxide. This study was designed to examine the effect of cyclic strain on the expression of endothelial nitric oxide synthase (eNOS) in cultured bovine aortic EC. EC were grown on flexible membranes which were subjected to deformation at 60 cycles/min with -5 or -20 kPa of vacuum. This results in an average strain of 6 and 10%, respectively, which is transmitted to the attached cells. Northern blot analysis of total cytosolic RNA demonstrated an increase in eNOS gene expression with both strain regimens but the increase with 10% average strain was greater than that at 6%. Nuclear runoff transcription assays confirmed the induction of eNOS transcripts. Western blot analysis showed an increase in eNOS level after 24 h of cyclic 10% average strain compared with controls or 6% average strain. Immunohistochemical staining of EC for eNOS was increased in the high strain periphery (7-24% strain) of membranes deformed with -20 kPa vacuum. These results demonstrate that cyclic strain upregulates the expression of eNOS transcripts and protein levels in bovine aortic EC thus emphasizing the importance of hemodynamic forces in the regulation of eNOS in vivo.

Endothelin-1 increases intracellular calcium in human monocytes and causes production of interleukin-6

OBJECTIVE

To define whether endothelin-1, a peptide produced by injured/ischemic endothelium, has any effect on monocyte intracellular calcium and the production of interleukin (IL)-1 and IL-6.

DESIGN

Prospective controlled laboratory study. Human monocytes from healthy donors were assayed for intracellular calcium by fluorometry and were stimulated for 24 hrs in tissue culture with purified endotoxin or endothelin.

SETTING

Veterans Affairs Medical Center surgical critical care basic science laboratory.

MEASUREMENTS AND MAIN RESULTS

Endothelin-1 increased the intracellular calcium concentration in fura-2 loaded human monocytes to a mean value of 37 +/- 4 nmol. Phytohemagglutinin increased intracellular calcium in control monocytes to a mean value of 97 +/- 12 nmol (n = 15; p < .001). Endothelin had no effect on neutrophil or lymphocyte intracellular calcium. Monocytes incubated with 10(-9) M endothelin significantly increased IL-6 production to values nearly as high as the lipopolysaccharide controls, but did not increase IL-1 production (n = 8; p < .01).

CONCLUSION

Endothelin-1 increased intracellular calcium in monocytes and caused production of IL-6.

Splenomegaly and variceal bleeding--hemodynamic basis and treatment implications

Splenectomy and splenic embolization have been advocated as definitive therapy in cirrhotic patients bleeding from varices. While splenomegaly is commonly associated with portal hypertension, no clear hemodynamic link between portal pressure and splenic enlargement has yet been established. In an effort to clarify the hemodynamic significance of splenomegaly in portal hypertensive patients the relationship between spleen size and portal pressure was retrospectively reviewed and the contribution of splenic inflow to portal hypertension prospectively studied. In 50 consecutive cirrhotic variceal bleeders studied angiographically, there was no correlation between spleen size and corrected sinusoidal pressure. Portal pressure was then prospectively measured before and after splenic vein clamping in 12 cirrhotic patients undergoing distal splenorenal shunt. No significant pressure drop occurred following elimination of splenic venous flow. On the basis of these data, there would appear to be no firm hemodynamic basis for splenectomy or splenic embolization alone in the unselective management of cirrhotic patients with variceal bleeding.

Cyclic strain stimulates dephosphorylation of the 20kDa regulatory myosin light chain in vascular smooth muscle cells

The role of cyclic strain in the regulation of 20 kDa myosin light chain phosphorylation (MLC20) in cultured smooth muscle cells (SMC) is unknown. The objective of this study was to determine whether cyclic strain stimulates the dephosphorylation of MLC20 in serum-fed SMC displaying a high basal level of phosphorylation. Confluent bovine aortic SMC were subjected to 10% average strain at 60 cycles per minute for 30 and 60 minutes. Basal MLC20 phosphorylation (N = non,M = mono,D = di) of serum-fed SMC was as follows: N = 34%:M = 27%:D = 39%. After 60 min of cyclic strain, both mono and diphosphorylated MLC20 were decreased to 21 and 15% respectively. The strain-induced dephosphorylation of MLC20 was partially inhibited by the protein phosphatase 1/2A inhibitor, calyculin A (5 nM). However, phosphorylase a phosphatase activities in Triton-soluble and insoluble fractions of SMC were unaffected by cyclic strain. The data suggest that cyclic strain causes dephosphorylation of MLC20 in SMC which may be partially due to activation of MLC20 phosphatase and/or inhibition of MLC20 phosphorylation.

Inhibition of smooth muscle cell proliferation by visible light-activated psoralen

The present study was designed to evaluate the effect of 8-methoxypsoralen (8-MOP) activated with visible light (419 nm) on the suppression of smooth muscle cell (SMC) proliferation in vitro. We hypothesize that if visible light (VL) instead of UVA is used to photoactivate 8-MOP, cytotoxic 8-MOP-DNA cross-link formation can be minimized. Bovine aorta SMCs (2 x 10(4)/cm2) were incubated with 8-MOP (1 micrograms/mL) for 30 minutes (in the dark) and exposed to a range of VL (2 to 69 J/cm2) to determine the dose of VL that inhibits SMC proliferation with minimal toxicity. The results show that 8-MOP in combination with 2 to 12 J/cm2 VL reversibly inhibited SMC proliferation for up to 5 days after treatment. SMC viability was confirmed by trypan blue exclusion. 8-MOP in combination with 23- or 69-J/cm2 VL irreversibly inhibited SMC proliferation. In cell cycle studies, 12-J/cm2 VL was used to activate 8-MOP. A phase-specific G2 blockade that correlated temporally with recovery of SMC replication was observed. Photoadduct repair studies showed that cell proliferation rates recovered when 60% of the adducts had been removed. These results demonstrate for the first time the possibility of using VL to activate 8-MOP to inhibit cell proliferation and suggest that 8-MOP/VL photochemotherapy can be used to control SMC growth.

Cyclic strain increases endothelial nitric oxide synthase activity

BACKGROUND

Endothelial nitric oxide synthase (eNOS) is an important enzyme that controls the production of a potent vascular smooth muscle relaxing factor, nitric oxide. However, the role of hemodynamic forces (blood pressure, cyclic strain, and shear stress) on the regulation of eNOS has not been fully elucidated. Recently, we showed that cyclic strain increases eNOS gene and protein in cultured bovine aortic endothelial cells (EC). Because an increase in gene transcription and protein synthesis may not necessarily translate into an increase in functional activity, the aim of this study was to determine the effects of cyclic strain on eNOS activity.

METHODS

EC were seeded onto plates with flexible bottoms that can be deformed by vacuum and were then exposed to 60 cycles/minute of either 24% maximum strain (-20 kPa vacuum) or 10% maximum strain (-5 kPa vacuum) for 24 hours. eNOS activity was assessed, and nitric oxide production was determined (as nitrite) by the Greiss reaction.

RESULTS

Twenty-four percent strain, at 60 cycles/min, but not 10% strain significantly increases eNOS activity compared with stationary controls. Both strain regimens increased nitric oxide (as nitrite) in culture media compared with stationary controls, although nitrite in media of EC exposed to high strain were significantly increased compared with the lower strain.

CONCLUSIONS

Cyclic strain increases eNOS activity in cultured bovine aortic EC. These results may indicate the importance of hemodynamic forces in the regulation of eNOS in vivo.