Protease-Activated Receptor 2 Controls Vascular Smooth Muscle Cell Proliferation in Cyclic AMP-Dependent Protein Kinase/Mitogen-Activated Protein Kinase Kinase 1/2-Dependent Manner.

INTRODUCTION: Cardiovascular disorders are characterized by vascular smooth muscle (VSM) transition from a contractile to proliferative state. Protease-activated receptor 2 (PAR2) involvement in this phenotypic conversion remains unclear. We hypothesized that PAR2 controls VSM cell proliferation in phenotype-dependent manner and through specific protein kinases. METHODS: Rat clonal low (PLo; P3-P6) and high passage (PHi; P10-P15) VSM cells were established as respective models of quiescent and proliferative cells, based on reduced PKG-1 and VASP. Western blotting determined expression of cytoskeletal/contractile proteins, PAR2, and select protein kinases. DNA synthesis and cell proliferation were measured 24-72 h following PAR2 agonism (SLIGRL; 100 nM-10 µm) with/without PKA (PKI; 10 µm), MEK1/2 (PD98059; 10 µm), and PI3K (LY294002; 1 µm) blockade. RESULTS: PKG-1, VASP, SM22α, calponin, cofilin, and PAR2 were reduced in PHi versus PLo cells. Following PAR2 agonism, DNA synthesis and cell proliferation increased in PLo cells but decreased in PHi cells. Western analyses showed reduced PKA, MEK1/2, and PI3K in PHi versus PLo cells, and kinase blockade revealed PAR2 controls VSM cell proliferation through PKA/MEK1/2. DISCUSSION: Findings highlight PAR2 and PAR2-driven PKA/MEK1/2 in control of VSM cell growth and provide evidence for continued investigation of PAR2 in VSM pathology.

Lymphatic Vessel Network Structure and Physiology.

The lymphatic system is comprised of a network of vessels interrelated with lymphoid tissue, which has the holistic function to maintain the local physiologic environment for every cell in all tissues of the body. The lymphatic system maintains extracellular fluid homeostasis favorable for optimal tissue function, removing substances that arise due to metabolism or cell death, and optimizing immunity against bacteria, viruses, parasites, and other antigens. This article provides a comprehensive review of important findings over the past century along with recent advances in the understanding of the anatomy and physiology of lymphatic vessels, including tissue/organ specificity, development, mechanisms of lymph formation and transport, lymphangiogenesis, and the roles of lymphatics in disease. © 2019 American Physiological Society. Compr Physiol 9:207-299, 2019.

Soluble guanylyl cyclase-activated cyclic GMP-dependent protein kinase inhibits arterial smooth muscle cell migration independent of VASP-serine 239 phosphorylation.

Coronary artery disease (CAD) accounts for over half of all cardiovascular disease-related deaths. Uncontrolled arterial smooth muscle (ASM) cell migration is a major component of CAD pathogenesis and efforts aimed at attenuating its progression are clinically essential. Cyclic nucleotide signaling has long been studied for its growth-mitigating properties in the setting of CAD and other vascular disorders. Heme-containing soluble guanylyl cyclase (sGC) synthesizes cyclic guanosine monophosphate (cGMP) and maintains vascular homeostasis predominantly through cGMP-dependent protein kinase (PKG) signaling. Considering that reactive oxygen species (ROS) can interfere with appropriate sGC signaling by oxidizing the cyclase heme moiety and so are associated with several CVD pathologies, the current study was designed to test the hypothesis that heme-independent sGC activation by BAY 60-2770 (BAY60) maintains cGMP levels despite heme oxidation and inhibits ASM cell migration through phosphorylation of the PKG target and actin-binding vasodilator-stimulated phosphoprotein (VASP). First, using the heme oxidant ODQ, cGMP content was potentiated in the presence of BAY60. Using a rat model of arterial growth, BAY60 significantly reduced neointima formation and luminal narrowing compared to vehicle (VEH)-treated controls. In rat ASM cells BAY60 significantly attenuated cell migration, reduced G:F actin, and increased PKG activity and VASP Ser239 phosphorylation (pVASP·S239) compared to VEH controls. Site-directed mutagenesis was then used to generate overexpressing full-length wild type VASP (FL-VASP/WT), VASP Ser239 phosphorylation-mimetic (FL-VASP/239D) and VASP Ser239 phosphorylation-resistant (FL-VASP/239A) ASM cell mutants. Surprisingly, FL-VASP/239D negated the inhibitory effects of FL-VASP/WT and FL-VASP/239A cells on migration. Furthermore, when FL-VASP mutants were treated with BAY60, only the FL-VASP/239D group showed reduced migration compared to its VEH controls. Intriguingly, FL-VASP/239D abrogated the stimulatory effects of FL-VASP/WT and FL-VASP/239A cells on PKG activity. In turn, pharmacologic blockade of PKG in the presence of BAY60 reversed the inhibitory effect of BAY60 on naïve ASM cell migration. Taken together, we demonstrate for the first time that BAY60 inhibits ASM cell migration through cGMP/PKG/VASP signaling yet through mechanisms independent of pVASP·S239 and that FL-VASP overexpression regulates PKG activity in rat ASM cells. These findings implicate BAY60 as a potential pharmacotherapeutic agent against aberrant ASM growth disorders such as CAD and also establish a unique mechanism through which VASP controls PKG activity.

Activation of RhoA, but Not Rac1, Mediates Early Stages of S1P-Induced Endothelial Barrier Enhancement.

Compromised endothelial barrier function is a hallmark of inflammation. Rho family GTPases are critical in regulating endothelial barrier function, yet their precise roles, particularly in sphingosine-1-phosphate (S1P)-induced endothelial barrier enhancement, remain elusive. Confluent cultures of human umbilical vein endothelial cells (HUVEC) or human dermal microvascular endothelial cells (HDMEC) were used to model the endothelial barrier. Barrier function was assessed by determining the transendothelial electrical resistance (TER) using an electrical cell-substrate impedance sensor (ECIS). The roles of Rac1 and RhoA were tested in S1P-induced barrier enhancement. The results show that pharmacologic inhibition of Rac1 with Z62954982 failed to block S1P-induced barrier enhancement. Likewise, expression of a dominant negative form of Rac1, or knockdown of native Rac1 with siRNA, failed to block S1P-induced elevations in TER. In contrast, blockade of RhoA with the combination of the inhibitors Rhosin and Y16 significantly reduced S1P-induced increases in TER. Assessment of RhoA activation in real time using a fluorescence resonance energy transfer (FRET) biosensor showed that S1P increased RhoA activation primarily at the edges of cells, near junctions. This was complemented by myosin light chain-2 phosphorylation at cell edges, and increased F-actin and vinculin near intercellular junctions, which could all be blocked with pharmacologic inhibition of RhoA. The results suggest that S1P causes activation of RhoA at the cell periphery, stimulating local activation of the actin cytoskeleton and focal adhesions, and resulting in endothelial barrier enhancement. S1P-induced Rac1 activation, however, does not appear to have a significant role in this process.

Histamine activates p38 MAP kinase and alters local lamellipodia dynamics, reducing endothelial barrier integrity and eliciting central movement of actin fibers.

The role of the actin cytoskeleton in endothelial barrier function has been debated for nearly four decades. Our previous investigation revealed spontaneous local lamellipodia in confluent endothelial monolayers that appear to increase overlap at intercellular junctions. We tested the hypothesis that the barrier-disrupting agent histamine would reduce local lamellipodia protrusions and investigated the potential involvement of p38 mitogen-activated protein (MAP) kinase activation and actin stress fiber formation. Confluent monolayers of human umbilical vein endothelial cells (HUVEC) expressing green fluorescent protein-actin were studied using time-lapse fluorescence microscopy. The protrusion and withdrawal characteristics of local lamellipodia were assessed before and after addition of histamine. Changes in barrier function were determined using electrical cell-substrate impedance sensing. Histamine initially decreased barrier function, lamellipodia protrusion frequency, and lamellipodia protrusion distance. A longer time for lamellipodia withdrawal and reduced withdrawal distance and velocity accompanied barrier recovery. After barrier recovery, a significant number of cortical fibers migrated centrally, eventually resembling actin stress fibers. The p38 MAP kinase inhibitor SB203580 attenuated the histamine-induced decreases in barrier function and lamellipodia protrusion frequency. SB203580 also inhibited the histamine-induced decreases in withdrawal distance and velocity, and the subsequent actin fiber migration. These data suggest that histamine can reduce local lamellipodia protrusion activity through activation of p38 MAP kinase. The findings also suggest that local lamellipodia have a role in maintaining endothelial barrier integrity. Furthermore, we provide evidence that actin stress fiber formation may be a reaction to, rather than a cause of, reduced endothelial barrier integrity.

Involvement of the H1 Histamine Receptor, p38 MAP Kinase, Myosin Light Chains Kinase, and Rho/ROCK in Histamine-Induced Endothelial Barrier Dysfunction.

OBJECTIVE: The mechanisms by which histamine increases microvascular permeability remain poorly understood. We tested the hypothesis that H1 receptor activation disrupts the endothelial barrier and investigated potential downstream signals. METHODS: We used confluent EC monolayers, assessing TER as an index of barrier function. HUVEC, HCMEC, and HDMEC were compared. Receptor expression was investigated using Western blotting, IF confocal microscopy and RT-PCR. Receptor function and downstream signaling pathways were tested using pharmacologic antagonists and inhibitors, respectively. RESULTS: We identified H1-H4 receptors on all three EC types. H1 antagonists did not affect basal TER but prevented the histamine-induced decrease in TER. Blockade of H2 or H3 attenuated the histamine response only in HDMEC, while inhibition of H4 attenuated the response only in HUVEC. Combined inhibition of both PKC and PI3K caused exaggerated histamine-induced barrier dysfunction in HDMEC, whereas inhibition of p38 MAP kinase attenuated the histamine response in all three EC types. Inhibition of RhoA, ROCK, or MLCK also prevented the histamine-induced decrease in TER in HDMEC. CONCLUSION: The data suggest that multiple signaling pathways contribute to histamine-induced endothelial barrier dysfunction via the H1 receptor.

Phosphodiesterases Regulate BAY 41-2272-Induced VASP Phosphorylation in Vascular Smooth Muscle Cells.

BAY 41-2272 (BAY), a stimulator of soluble guanylyl cyclase, increases cyclic nucleotides and inhibits proliferation of vascular smooth muscle cells (VSMCs). In this study, we elucidated mechanisms of action of BAY in its regulation of vasodilator-stimulated phosphoprotein (VASP) with an emphasis on VSMC phosphodiesterases (PDEs). BAY alone increased phosphorylation of VASP(Ser239) and VASP(Ser157), respective indicators of PKG and PKA signaling. IBMX, a non-selective inhibitor of PDEs, had no effect on BAY-induced phosphorylation at VASP(Ser239) but inhibited phosphorylation at VASP(Ser157). Selective inhibitors of PDE3 or PDE4 attenuated BAY-mediated increases at VASP(Ser239) and VASP(Ser157), whereas PDE5 inhibition potentiated BAY-mediated increases only at VASP(Ser157). In comparison, 8Br-cGMP increased phosphorylation at VASP(Ser239) and VASP(Ser157) which were not affected by selective PDE inhibitors. In the presence of 8Br-cAMP, inhibition of either PDE4 or PDE5 decreased VASP(Ser239) phosphorylation and inhibition of PDE3 increased phosphorylation at VASP(Ser239), while inhibition of PDE3 or PDE4 increased and PDE5 inhibition had no effect on VASP(Ser157) phosphorylation. These findings demonstrate that BAY operates via cAMP and cGMP along with regulation by PDEs to phosphorylate VASP in VSMCs and that the mechanism of action of BAY in VSMCs is different from that of direct cyclic nucleotide analogs with respect to VASP phosphorylation and the involvement of PDEs. Given a role for VASP as a critical cytoskeletal protein, these findings provide evidence for BAY as a regulator of VSMC growth and a potential therapeutic agent against vasculoproliferative disorders.

Identification of cytosolic phosphodiesterases in the erythrocyte: a possible role for PDE5.

BACKGROUND: Within erythrocytes (RBCs), cAMP levels are regulated by phosphodiesterases (PDEs). Increases in cAMP and ATP release associated with activation of ß-adrenergic receptors (ßARs) and prostacyclin receptors (IPRs) are regulated by PDEs 2, 4 and PDE 3, respectively. Here we establish the presence of cytosolic PDEs in RBCs and determine a role for PDE5 in regulating levels of cGMP. MATERIAL/METHODS: Purified cytosolic proteins were obtained from isolated human RBCs and western analysis was performed using antibodies against PDEs 3A, 4 and 5. Rabbit RBCs were incubated with dbcGMP, a cGMP analog, to determine the effect of cGMP on cAMP levels. To determine if cGMP affects receptor-mediated increases in cAMP, rabbit RBCs were incubated with dbcGMP prior to addition of isoproterenol (ISO), a ßAR receptor agonist. To demonstrate that endogenous cGMP produces the same effect, rabbit and human RBCs were incubated with SpNONOate (SpNO), a nitric oxide donor, and YC1, a direct activator of soluble guanylyl cyclase (sGC), in the absence and presence of a selective PDE5 inhibitor, zaprinast (ZAP). RESULTS: Western analysis identified PDEs 3A, 4D and 5A. dbcGMP produced a concentration dependent increase in cAMP and ISO-induced increases in cAMP were potentiated by dbcGMP. In addition, incubation with YC1 and SpNO in the presence of ZAP potentiated ßAR-induced increases in cAMP. CONCLUSIONS: PDEs 2, 3A and 5 are present in the cytosol of human RBCs. PDE5 activity in RBCs regulates cGMP levels. Increases in intracellular cGMP augment cAMP levels. These studies suggest a novel role for PDE5 in erythrocytes.

Inhibition of ATP release from erythrocytes: a role for EPACs and PKC.

OBJECTIVE: Here we demonstrate that, in human erythrocytes, increases in cAMP that are not localized to a specific receptor-mediated signaling pathway for ATP release can activate effector proteins resulting in inhibition of ATP release. Specifically we sought to establish that exchange proteins activated by cAMP (EPACs) inhibit ATP release via activation of protein kinase C (PKC). METHODS: ATP release stimulated by iloprost (ILO), or isoproterenol (ISO), was determined in the absence and presence of selective phosphodiesterase inhibitors and/or the EPAC activator, 8CPT2OMecAMP (8CPT). To determine whether EPACs inhibit ATP release via activation of PKC, erythrocytes were incubated with phorbol 12-myristate 13-acetate (PMA) prior to either forskolin or ILO in the absence and presence of a PKC inhibitor, calphostin C (CALC). RESULTS: Selective inhibition of PDEs in one pathway inhibited ATP release in response to activation of the other cAMP-dependent pathway. 8CPT and PMA inhibited both ILO- and ISO-induced ATP release. Inhibition of ATP release with 8CPT was rescued by CALC. CONCLUSION: These results support the hypothesis that cAMP not localized to a specific signaling pathway can activate EPACs which inhibit ATP release via activation of PKC and suggest a novel role for EPACs in erythrocytes.

Diamide decreases deformability of rabbit erythrocytes and attenuates low oxygen tension-induced ATP release.

Exposure of erythrocytes to reduced oxygen (O(2)) tension activates the heterotrimeric G-protein Gi, resulting in the accumulation of cyclic AMP (cAMP) and release of ATP. The mechanism by which exposure of erythrocytes to reduced O(2) tension activates Gi is not known. Here we investigate the hypothesis that, in rabbit erythrocytes, ATP release in response to exposure to reduced O(2) tension is linked to erythrocyte membrane deformability. If this hypothesis is correct, then decreasing the deformability of the erythrocyte membrane should decrease the release of ATP in response to reduced O(2) tension. We report that treating erythrocytes with diamide, a compound that decreases erythrocyte deformability, inhibits low O(2) tension-induced ATP release. Treating erythrocytes with diamide does not, however, interfere with cAMP accumulation or ATP release in response to a direct activator of Gi (mastoparan 7) or in response to receptor-mediated activation of Gs (the prostacyclin analog, iloprost). These results demonstrate that diamide (100 micromol/L) does not directly inhibit the signaling pathways for ATP release from rabbit erythrocytes and support the hypothesis that low O(2) tension-induced ATP release from these cells is linked to membrane deformability.

Regulation of cAMP by phosphodiesterases in erythrocytes.

The erythrocyte, a cell responsible for carrying and delivering oxygen in the body, has often been regarded as simply a vehicle for the circulation of hemoglobin. However, it has become evident that this cell also participates in the regulation of vascular caliber in the microcirculation via release of the potent vasodilator, adenosine triphosphate (ATP). The regulated release of ATP from erythrocytes occurs via a defined signaling pathway and requires increases in cyclic 3',5'- adenosine monophosphate (cAMP). It is well recognized that cAMP is a critical second messenger in diverse signaling pathways. In all cells increases in cAMP are localized and regulated by the activity of phosphodiesterases (PDEs). In erythrocytes activation of either beta adrenergic receptors (beta(2)AR) or the prostacyclin receptor (IPR) results in increases in cAMP and ATP release. Receptor-mediated increases in cAMP are tightly regulated by distinct PDEs associated with each signaling pathway as shown by the finding that selective inhibitors of the PDEs localized to each pathway potentiate both increases in cAMP and ATP release. Here we review the profile of PDEs identified in erythrocytes, their association with specific signaling pathways and their role in the regulation of ATP release from these cells. Understanding the contribution of PDEs to the control of ATP release from erythrocytes identifies this cell as a potential target for the development of drugs for the treatment of vascular disease.

Pannexin 1 is the conduit for low oxygen tension-induced ATP release from human erythrocytes.

Erythrocytes release ATP in response to exposure to the physiological stimulus of lowered oxygen (O(2)) tension as well as pharmacological activation of the prostacyclin receptor (IPR). ATP release in response to these stimuli requires activation of adenylyl cyclase, accumulation of cAMP, and activation of protein kinase A. The mechanism by which ATP, a highly charged anion, exits the erythrocyte in response to lowered O(2) tension or receptor-mediated IPR activation by iloprost is unknown. It was demonstrated previously that inhibiting pannexin 1 with carbenoxolone inhibits hypotonically induced ATP release from human erythrocytes. Here we demonstrate that three structurally dissimilar compounds known to inhibit pannexin 1 prevent ATP release in response to lowered O(2) tension but not to iloprost-induced ATP release. These results suggest that pannexin 1 is the conduit for ATP release from erythrocytes in response to lowered O(2) tension. However, the identity of the conduit for iloprost-induced ATP release remains unknown.

Protein kinases A and C regulate receptor-mediated increases in cAMP in rabbit erythrocytes.

Activation of the beta-adrenergic receptor (beta-AR) or the prostacyclin receptor (IPR) results in increases in cAMP and ATP release from erythrocytes. cAMP levels depend on a balance between synthesis via adenylyl cyclase and hydrolysis by phosphodiesterases (PDEs). Previously, we reported that cAMP increases associated with activation of the beta-AR and IPR in rabbit and human erythrocytes are tightly regulated by distinct PDEs. Importantly, inhibitors of these PDEs potentiated both increases in cAMP and ATP release. It has been shown that increases in protein kinase (PK) activity can activate PDE3 and PDE4. Both PKA and PKC are present in the erythrocyte and can phosphorylate and activate these PDEs. Here we investigate the hypothesis that PKA regulates PDE activity associated with the beta-AR and both PKA and PKC regulate the PDE activity associated with the IPR in rabbit erythrocytes. Pretreatment of erythrocytes with the PKA inhibitor, H89 (10 microM), in the presence of the PDE4 inhibitor, rolipram (10 microM), augmented isoproterenol (1 microM)-induced cAMP increases. In contrast, in the presence of the PDE3 inhibitor, cilostazol (10 microM), pretreatment of erythrocytes with either H89 (1 microM) or two chemically dissimilar inhibitors of PKC, calphostin C (1 microM) or GFX109203X (1 microM), potentiated iloprost (1 microM)-induced cAMP increases. Furthermore, pretreatment of erythrocytes with both H89 and GFX109203X in the presence of cilostazol augmented the iloprost-induced increases in cAMP to a greater extent than either PK inhibitor individually. These results support the hypothesis that PDEs associated with receptor-mediated increases in cAMP in rabbit erythrocytes are regulated by kinases specific to the receptor's signaling pathway.

Insulin inhibits low oxygen-induced ATP release from human erythrocytes: implication for vascular control.

OBJECTIVE: ATP released from human erythrocytes in response to reduced oxygen tension (pO(2)) participates in the matching of oxygen (O(2)) supply with need in skeletal muscle by stimulating increases in blood flow to areas with increased O(2) demand. Here, we investigated the hypothesis that hyperinsulinemia inhibits ATP release from erythrocytes and impairs their ability to stimulate dilation of isolated arterioles exposed to decreased extraluminal pO(2). MATERIALS AND METHODS: Erythrocyte ATP release was stimulated pharmacologically (mastoparan 7) and physiologically (reduced pO(2)) in the absence or presence of insulin. We also examined the ability of isolated skeletal muscle arterioles perfused with buffer containing erythrocytes treated with insulin or its vehicle (saline) to dilate in response to decreased extraluminal pO(2). RESULTS: Insulin significantly attenuated mastoparan 7- and reduced pO(2)-induced ATP release. In vessels perfused with untreated erythrocytes, low extraluminal pO(2) resulted in an increase in vessel diameter. In contrast, when erythrocytes were treated with insulin, no vasodilation occurred. CONCLUSIONS: These studies demonstrate that insulin inhibits ATP release from erythrocytes in response to reduced pO(2) and impairs their ability to stimulate dilation of skeletal muscle arterioles. These results suggest that hyperinsulinemia could hinder the matching of O(2) supply with need in skeletal muscle.

Iloprost- and isoproterenol-induced increases in cAMP are regulated by different phosphodiesterases in erythrocytes of both rabbits and humans.

Activation of the G protein G(s) results in increases in cAMP, a necessary step in the pathway for ATP release from rabbit and human erythrocytes. In all cells, the level of cAMP is the product of its synthesis by adenylyl cyclase and its hydrolysis by phosphodiesterases (PDEs). Both iloprost (Ilo), a PGI(2) analog, and isoproterenol (Iso), a beta-agonist, stimulate receptor-mediated increases in cAMP in rabbit and human erythrocytes. However, the specific PDEs associated with each of these signaling pathways in the erythrocyte have not been fully characterized. Previously, we reported that PDE3B is present in rabbit and human erythrocyte membranes and that PDE3 inhibitors potentiate Ilo-induced increases in cAMP. Here we report that inhibitors of either PDE2 or PDE4, erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) and rolipram, respectively, potentiate Iso-induced increases in cAMP in rabbit and human erythrocytes. Importantly, these inhibitors had no effect on cAMP increases associated with the incubation of erythrocytes with Ilo. In addition, we establish, for the first time, the presence of PDE2A protein in rabbit and human erythrocyte membranes. Finally, we determined that preincubation of human erythrocytes with EHNA and rolipram together potentiate Iso-induced ATP release, whereas preincubation with cilostazol enhances Ilo-induced release of ATP. These results are consistent with the hypothesis that, in rabbit and human erythrocytes, Ilo-induced increases in cAMP and ATP release are regulated by PDE3, whereas those associated with Iso are regulated by the activities of both PDE2 and PDE4. These studies demonstrate that PDE activity in these cells is localized to specific signaling pathways.

Rabbit erythrocytes release ATP and dilate skeletal muscle arterioles in the presence of reduced oxygen tension.

In skeletal muscle, oxygen (O(2)) delivery to appropriately meet metabolic need requires mechanisms for detection of the magnitude of O(2) demand and the regulation of O(2) delivery. Erythrocytes, when exposed to a decrease in O(2) tension, release both O(2) and the vasodilator adenosine triphosphate (ATP). The aims of this study were to establish that erythrocytes release ATP in response to reduced O(2) tension and determine if erythrocytes are necessary for the dilation of isolated skeletal muscle arterioles exposed to reduced extraluminal O(2) tension. Rabbit erythrocytes exposed to reduced O(2) tension in a tonometer (n = 5, pO(2) = 27 +/- 3, p < 0.01) released ATP in response to reduced O(2) tension. ATP release increased in proportion to the decrease in O(2) tension. The contribution of erythrocytes to the response of skeletal muscle arterioles to reduced extraluminal O(2) tension was determined using isolated hamster cheek pouch retractor muscle arterioles perfused with buffer (n = 11, mean diameter 52 +/- 3 mum) in the absence and presence of rabbit erythrocytes. Without erythrocytes, arterioles did not dilate when exposed to reduced extraluminal O(2) tension (pO(2) = 32 +/- 4 mmHg). In contrast, when rabbit erythrocytes were present in the perfusate (hematocrit 15%), the same decrease in O(2) tension resulted in a 20 +/- 4% dilation (p < 0.01). These results provide support for the hypothesis that erythrocytes, via their ability to release O(2) along with ATP in response to exposure to reduced O(2) tension, can participate in the matching of O(2) delivery with metabolic need in skeletal muscle.

Phosphodiesterase 3 is present in rabbit and human erythrocytes and its inhibition potentiates iloprost-induced increases in cAMP.

Increases in the second messenger cAMP are associated with receptor-mediated ATP release from erythrocytes. In other signaling pathways, cAMP-specific phosphodiesterases (PDEs) hydrolyze this second messenger and thereby limit its biological actions. Although rabbit and human erythrocytes possess adenylyl cyclase and synthesize cAMP, their PDE activity is poorly characterized. It was reported previously that the prostacyclin analog iloprost stimulated receptor-mediated increases in cAMP in rabbit and human erythrocytes. However, the PDEs that hydrolyze erythrocyte cAMP synthesized in response to iloprost were not identified. PDE3 inhibitors were reported to augment increases in cAMP stimulated by prostacyclin analogs in platelets and pulmonary artery smooth muscle cells. Additionally, PDE3 activity was identified in embryonic avian erythrocytes, but the presence of this PDE in mammalian erythrocytes has not been investigated. Here, using Western blot analysis, we determined that PDE3B is a component of rabbit and human erythrocyte membranes. In addition, we report that the preincubation of rabbit and human erythrocytes with the PDE3 inhibitors milrinone and cilostazol potentiates iloprost-induced increases in cAMP. In addition, cilostamide, the parent compound of cilostazol, potentiated iloprost-induced increases in cAMP in human erythrocytes. These findings demonstrate that PDE3B is present in rabbit and human erythrocytes and are consistent with the hypothesis that PDE3 activity regulates cAMP levels associated with a signaling pathway activated by iloprost in these cells.

Prostacyclin analogs stimulate receptor-mediated cAMP synthesis and ATP release from rabbit and human erythrocytes.

OBJECTIVES: The purpose of this study was to establish that the prostacyclin (PGI(2)) receptor (IP receptor) is present on rabbit and human erythrocytes and that its activation stimulates cyclic adenosine monophosphate (cAMP) synthesis and adenosine triphosphate (ATP) release. METHODS: The effect of incubation of erythrocytes with the active PGI(2) analogs, iloprost or UT-15C, on cAMP levels and ATP release was determined in the absence and presence of the IP receptor antagonist, CAY10441. Western analysis was used to determine the presence of the IP receptor on isolated membranes. To establish that effects of PGI(2) analogs were not due to prostaglandin E(2)(PGE(2)) receptor activation, the effect of PGE(2) on cAMP levels and ATP release was determined. RESULTS: Rabbit and human erythrocytes possess IP receptors. Iloprost and UT-15C stimulated increases in cAMP and ATP release that were prevented by the IP receptor antagonist, CAY10441. PGE(2) did not stimulate cAMP accumulation or ATP release and did not inhibit iloprost-induced increases in cAMP. CONCLUSIONS: This study establishes that the IP receptor is present on rabbit and human erythrocytes and that its activation results in increases in cAMP and ATP release. These results suggest a novel mechanism by which PGI(2) and its active analogs, when administered pharmacologically, could produce vasodilation.