Quantifying Ca2+ current and permeability in ATP-gated P2X7 receptors.

ATP-gated P2X7 receptors are prominently expressed in inflammatory cells and play a key role in the immune response. A major consequence of receptor activation is the regulated influx of Ca(2+) through the self-contained cation non-selective channel. Although the physiological importance of the resulting rise in intracellular Ca(2+) is universally acknowledged, the biophysics of the Ca(2+) flux responsible for the effects are poorly understood, largely because traditional methods of measuring Ca(2+) permeability are difficult to apply to P2X7 receptors. Here we use an alternative approach, called dye-overload patch-clamp photometry, to quantify the agonist-gated Ca(2+) flux of recombinant P2X7 receptors of dog, guinea pig, human, monkey, mouse, rat, and zebrafish. We find that the magnitude of the Ca(2+) component of the ATP-gated current depends on the species of origin, the splice variant, and the concentration of the purinergic agonist. We also measured a significant contribution of Ca(2+) to the agonist-gated current of the native P2X7Rs of mouse and human immune cells. Our results provide cross-species quantitative measures of the Ca(2+) current of the P2X7 receptor for the first time, and suggest that the cytoplasmic N terminus plays a meaningful role in regulating the flow of Ca(2+) through the channel.

Mechanisms of C-peptide-mediated rescue of low O2-induced ATP release from erythrocytes of humans with type 2 diabetes.

The circulating erythrocyte, by virtue of the regulated release of ATP in response to reduced oxygen (O2) tension, plays a key role in maintaining appropriate perfusion distribution to meet tissue needs. Erythrocytes from individuals with Type 2 diabetes (DM2) fail to release ATP in response to this stimulus. However, the administration of C-peptide and insulin at a 1:1 ratio was shown to restore this important physiological response in humans with DM2. To begin to investigate the mechanisms by which C-peptide influences low O2-induced ATP release, erythrocytes from healthy humans and humans with DM2 were exposed to reduced O2 in a thin-film tonometer, and ATP release under these conditions was compared with release during normoxia. We determined that 1) low O2-induced ATP release from DM2 erythrocytes is rescued by C-peptide in the presence and absence of insulin, 2) the signaling pathway activated by C-peptide in human erythrocytes involves PKC, as well as soluble guanylyl cyclase (sGC) and 3) inhibitors of cGMP degradation rescue low O2-induced ATP release from DM2 erythrocytes. These results provide support for the hypothesis that both PKC and sGC are components of a signaling pathway activated by C-peptide in human erythrocytes. In addition, since both C-peptide and phosphodiesterase 5 inhibitors rescue low O2-induced ATP release from erythrocytes of humans with DM2, their administration to humans with DM2 could aid in the treatment and/or prevention of the vascular disease associated with this condition.

Prostacyclin receptor-mediated ATP release from erythrocytes requires the voltage-dependent anion channel.

Erythrocytes have been implicated as controllers of vascular caliber by virtue of their ability to release the vasodilator ATP in response to local physiological and pharmacological stimuli. The regulated release of ATP from erythrocytes requires activation of a signaling pathway involving G proteins (G(i) or G(s)), adenylyl cyclase, protein kinase A, and the cystic fibrosis transmembrane conductance regulator as well as a final conduit through which this highly charged anion exits the cell. Although pannexin 1 has been shown to be the final conduit for ATP release from human erythrocytes in response to reduced oxygen tension, it does not participate in transport of ATP following stimulation of the prostacyclin (IP) receptor in these cells, which suggests that an additional protein must be involved. Using antibodies directed against voltage-dependent anion channel (VDAC)1, we confirm that this protein is present in human erythrocyte membranes. To address the role of VDAC in ATP release, two structurally dissimilar VDAC inhibitors, Bcl-x(L) BH4(4-23) and TRO19622, were used. In response to the IP receptor agonists, iloprost and UT-15C, ATP release was inhibited by both VDAC inhibitors although neither iloprost-induced cAMP accumulation nor total intracellular ATP concentration were altered. Together, these findings support the hypothesis that VDAC is the ATP conduit in the IP receptor-mediated signaling pathway in human erythrocytes. In addition, neither the pannexin inhibitor carbenoxolone nor Bcl-x(L) BH4(4-23) attenuated ATP release in response to incubation of erythrocytes with the ß-adrenergic receptor agonist isoproterenol, suggesting the presence of yet another channel for ATP release from human erythrocytes.

A selective phosphodiesterase 3 inhibitor rescues low PO2-induced ATP release from erythrocytes of humans with type 2 diabetes: implication for vascular control.

Erythrocytes, via release of ATP in areas of low oxygen (O(2)) tension, are components of a regulatory system for the distribution of perfusion in skeletal muscle ensuring optimal O(2) delivery to meet tissue needs. In type 2 diabetes (DM2), there are defects in O(2) supply to muscle as well as a failure of erythrocytes to release ATP. The goal of this study was to ascertain if a phosphodiesterase 3 (PDE3) inhibitor, cilostazol, would rescue low O(2)-induced ATP release from DM2 erythrocytes and, thereby, enable these cells to dilate isolated erythrocyte-perfused skeletal muscle arterioles exposed to decreased extraluminal O(2). Erythrocytes were obtained from healthy humans (HH; n = 12) and humans with DM2 (n = 17). We determined that 1) PDE3B is similarly expressed in both groups, 2) mastoparan 7 (G(i) activation) stimulates increases in cAMP in HH but not in DM2 erythrocytes, and 3) pretreatment of DM2 erythrocytes with cilostazol resulted in mastoparan 7-induced increases in cAMP not different from those in HH cells. Most importantly, cilostazol restored the ability of DM2 erythrocytes to release ATP in response to low O(2). In contrast with perfusion with HH erythrocytes, isolated hamster retractor muscle arterioles perfused with DM2 erythrocytes constricted in response to low extraluminal PO(2). However, in the presence of cilostazol (100 µM), DM2 erythrocytes induced vessel dilation not different from that seen with HH erythrocytes. Thus rescue of low O(2)-induced ATP release from DM2 erythrocytes by cilostazol restored the ability of erythrocytes to participate in the regulation of perfusion distribution in skeletal muscle.

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.

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.

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.

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.

Divergent effects of low-O(2) tension and iloprost on ATP release from erythrocytes of humans with type 2 diabetes: implications for O(2) supply to skeletal muscle.

Erythrocytes release both O(2) and a vasodilator, ATP, when exposed to reduced O(2) tension. We investigated the hypothesis that ATP release is impaired in erythrocytes of humans with type 2 diabetes (DM2) and that this defect compromises the ability of these cells to stimulate dilation of resistance vessels. We also determined whether a general vasodilator, the prostacyclin analog iloprost (ILO), stimulates ATP release from healthy human (HH) and DM2 erythrocytes. Finally, we used a computational model to compare the effect on tissue O(2) levels of increases in blood flow directed to areas of increased O(2) demand (erythrocyte ATP release) with nondirected increases in flow (ILO). HH erythrocytes, but not DM2 cells, released increased amounts of ATP when exposed to reduced O(2) tension (Po(2) < 30 mmHg). In addition, isolated hamster skeletal muscle arterioles dilated in response to similar decreases in extraluminal O(2) when perfused with HH erythrocytes, but not when perfused with DM2 erythrocytes. In contrast, both HH and DM2 erythrocytes released ATP in response to ILO. In the case of DM2 erythrocytes, amounts of ATP released correlated inversely with glycemic control. Modeling revealed that a functional regulatory system that directs blood flow to areas of need (low O(2)-induced ATP release) provides appropriate levels of tissue oxygenation and that this level of the matching of O(2) delivery with demand in skeletal muscle cannot be achieved with a general vasodilator. These results suggest that the inability of erythrocytes to release ATP in response to exposure to low-O(2) tension could contribute to the peripheral vascular disease of DM2.

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.

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.

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.

Reduced expression of G(i) in erythrocytes of humans with type 2 diabetes is associated with impairment of both cAMP generation and ATP release.

Human erythrocytes, by virtue of their ability to release ATP in response to physiological stimuli, have been proposed to participate in the regulation of local blood flow. A signal transduction pathway that relates these stimuli to ATP release has been described and includes the heterotrimeric G protein G(i) and adenylyl cyclase (AC). In this cell, G(i) activation results in increases in cAMP and, ultimately, ATP release. It has been reported that G(i) expression is decreased in animal models of diabetes and in platelets of humans with type 2 diabetes. Here, we report that G(i2) expression is selectively decreased in erythrocytes of humans with type 2 diabetes and that this defect is associated with reductions in cAMP accumulation and ATP release in response to incubation of erythrocytes with mastoparan 7 (10 micromol/l), an activator of G(i). Importantly, this defect in ATP release correlates inversely with the adequacy of glycemic control as determined by levels of HbA(1c) (A1C). These results demonstrate that in erythrocytes of humans with type 2 diabetes, both G(i) expression and ATP release in response to mastoparan 7 are impaired, which is consistent with the hypothesis that this defect in erythrocyte physiology could contribute to the vascular disease associated with this clinical condition.

Liposomal-delivery of phosphodiesterase 5 inhibitors augments UT-15C-stimulated ATP release from human erythrocytes.

The use of liposomes to affect targeted delivery of pharmaceutical agents to specific sites may result in the reduction of side effects and an increase in drug efficacy. Since liposomes are delivered intravascularly, erythrocytes, which constitute almost half of the volume of blood, are ideal targets for liposomal drug delivery. In vivo, erythrocytes serve not only in the role of oxygen transport but also as participants in the regulation of vascular diameter through the regulated release of the potent vasodilator, adenosine triphosphate (ATP). Unfortunately, erythrocytes of humans with pulmonary arterial hypertension (PAH) do not release ATP in response to the physiological stimulus of exposure to increases in mechanical deformation as would occur when these cells traverse the pulmonary circulation. This defect in erythrocyte physiology has been suggested to contribute to pulmonary hypertension in these individuals. In contrast to deformation, both healthy human and PAH erythrocytes do release ATP in response to incubation with prostacyclin analogs via a well-characterized signaling pathway. Importantly, inhibitors of phosphodiesterase 5 (PDE5) have been shown to significantly increase prostacyclin analog-induced ATP release from human erythrocytes. Here we investigate the hypothesis that targeted delivery of PDE5 inhibitors to human erythrocytes, using a liposomal delivery system, potentiates prostacyclin analog- induced ATP release. The findings are consistent with the hypothesis that directed delivery of this class of drugs to erythrocytes could be a new and important method to augment prostacyclin analog-induced ATP release from these cells. Such an approach could significantly limit side effects of both classes of drugs without compromising their therapeutic effectiveness in diseases such as PAH.

Liposomal delivery of a phosphodiesterase 3 inhibitor rescues low oxygen-induced ATP release from erythrocytes of humans with type 2 diabetes.

ATP release from erythrocytes in response to low oxygen tension requires an increase in cAMP, the level of which is regulated by phosphodiesterase 3 (PDE3). Such release is defective in erythrocytes of humans with type 2 diabetes (DM2). This study tested a hypothesis that direct delivery of the clinically useful PDE3 inhibitor, cilostazol, to erythrocytes of humans with type 2 diabetes using liposomes would restore low-oxygen tension-induced ATP release. Cilostazol was incorporated into liposomes prepared from dimyristoylphosphatidylcholine (DMPC). Liposome-delivery of cilostazol restored ATP release from DM2 erythrocytes to levels which were not different from that released from non-cilostazol treated healthy erythrocytes under the same conditions. There were no observed adverse effects of the liposomes on either healthy or DM2 erythrocytes. The directed liposomal delivery of PDE inhibitors to erythrocytes may help prevent or slow the development of peripheral vascular disease in individuals with DM2 by restoring an important physiological controller of microvascular perfusion while minimizing side effects associated with systemic delivery of some of these inhibitors.

Phosphodiesterase 5 inhibitors augment UT-15C-stimulated ATP release from erythrocytes of humans with pulmonary arterial hypertension.

Both prostacyclin analogs and phosphodiesterase 5 (PDE5) inhibitors are effective treatments for pulmonary arterial hypertension (PAH). In addition to direct effects on vascular smooth muscle, prostacyclin analogs increase cAMP levels and ATP release from healthy human erythrocytes. We hypothesized that UT-15C, an orally available form of the prostacyclin analog, treprostinil, would stimulate ATP release from erythrocytes of humans with PAH and that this release would be augmented by PDE5 inhibitors. Erythrocytes were isolated and the effect of UT-15C on cAMP levels and ATP release were measured in the presence and absence of the PDE5 inhibitors, zaprinast or tadalafil. In addition, the ability of a soluble guanylyl cyclase inhibitor to prevent the effects of tadalafil was determined. Erythrocytes of healthy humans and humans with PAH respond to UT-15C with increases in cAMP levels and ATP release. In both groups, UT-15C-induced ATP release was potentiated by zaprinast and tadalafil. The effect of tadalafil was prevented by pre-treatment with an inhibitor of soluble guanylyl cyclase in healthy human erythrocytes. Importantly, UT-15C-induced ATP release was greater in PAH erythrocytes than in healthy human erythrocytes in both the presence and the absence of PDE5 inhibitors. The finding that prostacyclin analogs and PDE5 inhibitors work synergistically to enhance release of the potent vasodilator ATP from PAH erythrocytes provides a new rationale for the co-administration of these drugs in this disease. Moreover, these results suggest that the erythrocyte is a novel target for future drug development for the treatment of PAH.

Synergistic effects of prostacyclin analogs and phosphodiesterase inhibitors on cyclic adenosine 3',5' monophosphate accumulation and adenosine 3'5' triphosphate release from human erythrocytes.

Prostacyclin (PGI2) and phosphodiesterase 5 (PDE5) inhibitors are potent vasodilators that are used alone and in combination for the treatment of pulmonary arterial hypertension (PAH). Although these vasodilators are known to stimulate relaxation of vascular smooth muscle directly, other cells in circulation, including erythrocytes, express prostacyclin receptor (IPR) and contain PDE5. The binding of PGI2 analogs to the erythrocyte IPR results in activation of a signaling pathway that increases cyclic adenosine 3',5' monophosphate (cAMP), a requirement for adenosine 3'5' triphosphate (ATP) release. Within this pathway, cAMP levels are regulated by phosphodiesterase 3 (PDE3), a PDE that is inhibited by cGMP, a cyclic nucleotide regulated by the activity of PDE5. Since inhibition of PDE3 enhances ATP release in response to PGI2 analogs, we investigated if the selective PDE5 inhibitors, zaprinast (ZAP) and tadalafil (TAD), would similarly increase cAMP and ATP release from human erythrocytes in response to the same stimulus. We determined that pretreatment of erythrocytes with one of two chemically dissimilar PDE5 inhibitors (ZAP or TAD, 10 µM) potentiated increases in cAMP and ATP release in response to incubation of human erythrocytes with the PGI2 analog, UT-15C (100 nM). These results suggest that a heretofore unrecognized synergism exists between IPR agonists and PDE5 inhibitors that could provide a new rationale for the co-administration of these agents as vasodilators in humans with PAH.

Insulin inhibits human erythrocyte cAMP accumulation and ATP release: role of phosphodiesterase 3 and phosphoinositide 3-kinase.

In non-erythroid cells, insulin stimulates a signal transduction pathway that results in the activation of phosphoinositide 3-kinase (PI3K) and subsequent phosphorylation of phosphodiesterase 3 (PDE3). Erythrocytes possess insulin receptors, PI3K and PDE3B. These cells release adenosine triphosphate (ATP) when exposed to reduced O(2) tension via a signaling pathway that requires activation of the G protein, Gi, as well as increases in cAMP. Although insulin inhibits ATP release from human erythrocytes in response to Gi activation by mastoparan 7 (Mas 7), no effect on cAMP was described. Here, we investigated the hypothesis that insulin activates PDE3 in human erythrocytes via a PI3K-mediated mechanism resulting in cAMP hydrolysis and inhibition of ATP release. Incubation of human erythrocytes with Mas 7 resulted in a 62 +/- 7% increase in cAMP (n = 9, P < 0.05) and a 306 +/- 69% increase in ATP release (n = 9, P < 0.05), both of which were attenuated by pre-treatment with insulin. Selective inhibitors of PDE3 (cilostazol) or PI3K (LY294002) rescued these effects of insulin. These results support the hypothesis that insulin activates PDE3 in erythrocytes via a PI3K-dependent mechanism. Once activated, PDE3 limits Mas 7-induced increases in intracellular cAMP. This effect of insulin leads, ultimately, to decreased ATP release in response to Mas 7. Activation of Gi is required for reduced O(2) tension-induced ATP release from erythrocytes and this ATP release has been shown to participate in the matching of O(2) supply with demand in skeletal muscle. Thus, pathological increases in circulating insulin could, via activation of PDE3 in erythrocytes, inhibit ATP release from these cells, depriving the peripheral circulation of one mechanism that could aid in the regulation of the delivery of O(2) to meet tissue metabolic need.

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.

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.