Mechanisms of Na+-K+ pump regulation in cardiac myocytes during hyposmolar swelling

Regulation of muscle potassium: exercise performance, fatigue and health implications

This review integrates from the single muscle fibre to exercising human the current understanding of the role of skeletal muscle for whole-body potassium (K⁺) regulation, and specifically the regulation of skeletal muscle [K⁺]. We describe the K⁺ transport proteins in skeletal muscle and how they contribute to, or modulate, K⁺ disturbances during exercise. Muscle and plasma K⁺ balance are markedly altered during and after high-intensity dynamic exercise (including sports), static contractions and ischaemia, which have implications for skeletal and cardiac muscle contractile performance. Moderate elevations of plasma and interstitial [K⁺] during exercise have beneficial effects on multiple physiological systems. Severe reductions of the trans-sarcolemmal K⁺ gradient likely contributes to muscle and whole-body fatigue, i.e. impaired exercise performance. Chronic or acute changes of arterial plasma [K⁺] (hyperkalaemia or hypokalaemia) have dangerous health implications for cardiac function. The current mechanisms to explain how raised extracellular [K⁺] impairs cardiac and skeletal muscle function are discussed, along with the latest cell physiology research explaining how calcium, β-adrenergic agonists, insulin or glucose act as clinical treatments for hyperkalaemia to protect the heart and skeletal muscle in vivo. Finally, whether these agents can also modulate K⁺-induced muscle fatigue are evaluated.

Targeting Cardiac Myocyte Na+-K+ Pump Function With β3 Adrenergic Agonist in Rabbit Model of Severe Congestive Heart Failure

BACKGROUND

Abnormally high cytosolic Na⁺ concentrations in advanced heart failure impair myocardial contractility. Stimulation of the membrane Na⁺-K⁺ pump should lower Na⁺ concentrations, and the β3 adrenoceptor (β3 AR) mediates pump stimulation in myocytes. We examined if β3 AR-selective agonists given in vivo increase myocyte Na⁺-K⁺ pump activity and reverse organ congestion in severe heart failure (HF).

METHODS

Indices for HF were lung-, heart-, and liver: body weight ratios and ascites after circumflex coronary artery ligation in rabbits. Na⁺-K⁺ pump current, I_p, was measured in voltage-clamped myocytes from noninfarct myocardium. Rabbits were treated with the β3 AR agonists CL316,243 or ASP9531, starting 2 weeks after coronary ligation.

RESULTS

Coronary ligation caused ascites in most rabbits, significantly increased lung-, heart-, and liver: body weight ratios, and decreased I_p relative to that for 10 sham-operated rabbits. Treatment with CL316,243 for 3 days significantly reduced lung-, heart-, and liver: body weight ratios and prevalence of ascites in 8 rabbits with HF relative to indices for 13 untreated rabbits with HF. It also increased I_p significantly to levels of myocytes from sham-operated rabbits. Treatment with ASP9531 for 14 days significantly reduced indices of organ congestion in 6 rabbits with HF relative to indices of 6 untreated rabbits, and it eliminated ascites. β3 AR agonists did not significantly change heart rates or blood pressures.

CONCLUSIONS

Parallel β3 AR agonists-induced reversal of Na⁺-K⁺ pump inhibition and indices of congestion suggest pump inhibition is a useful target for treatment with β3 AR agonists in congestive HF.

Thymidine 5'-O-monophosphorothioate induces HeLa cell migration by activation of the P2Y6 receptor

ATP, ADP, UTP, and UDP acting as ligands of specific P2Y receptors activate intracellular signaling cascades to regulate a variety of cellular processes, including proliferation, migration, differentiation, and cell death. Contrary to a widely held opinion, we show here that nucleoside 5'-O-monophosphorothioate analogs, containing a sulfur atom in a place of one nonbridging oxygen atom in a phosphate group, act as ligands for selected P2Y subtypes. We pay particular attention to the unique activity of thymidine 5'-O-monophosphorothioate (TMPS) which acts as a specific partial agonist of the P2Y6 receptor (P2Y6R). We also collected evidence for the involvement of the P2Y6 receptor in human epithelial adenocarcinoma cell line (HeLa) cell migration induced by thymidine 5'-O-monophosphorothioate analog. The stimulatory effect of TMPS was abolished by siRNA-mediated P2Y6 knockdown and diisothiocyanate derivative MRS 2578, a selective antagonist of the P2Y6R. Our results indicate for the first time that increased stability of thymidine 5'-O-monophosphorothioate as well as its affinity toward the P2Y6R may be responsible for some long-term effects mediated by this receptor.

Involvement of the phosphatidylinositol kinase pathway in augmentation of ATP-sensitive K+ channel currents by hypo-osmotic stress in rat ventricular myocytes

The objective of this study was to investigate the mechanisms of increase in the efficacy of ATP-sensitive K+ channel (KATP) openings by hypo-osmotic stress. The whole-cell KATP currents (IK,ATP) stimulated by 100 μmol/L pinacidil, a K+ channel opening drug, were significantly augmented during hypo-osmotic stress (189 mOsmol/L) compared with normal conditions (303 mOsmol/L). The EC50 and Emax value for pinacidil-activated IK,ATP (measured at 0 mV) was 154 μmol/L and 844 pA, respectively, in normal solution and 16.6 μmol/L and 1266 pA, respectively, in hypo-osmotic solution. Augmentation of IK,ATP during hypo-osmotic stress was attenuated by wortmannin (50 μmol/L), an inhibitor of phosphatidylinositol 3- and 4-kinases, but not by (i) phalloidin (30 μmol/L), an actin filament stabilizer, (ii) the absence of Ca2+ from the internal and external solutions, and (iii) the presence of creatine phosphate (3 mmol/L), which affects creatine kinase regulation of the KATP channels. In the single-channel recordings, an inside-out patch was made after approximately 5 min exposure of the myocyte to hypo-osmotic solution. However, the IC50 value for ATP under such conditions was not different from that obtained in normal osmotic solution. In conclusion, hypo-osmotic stress could augment cardiac IK,ATP through intracellular mechanisms involving the phosphatidylinositol kinase pathway.

P2Y(4) nucleotide receptor: a novel actor in post-natal cardiac development

Communication between endothelial cells and cardiomyocytes is critical for cardiac development and regeneration. However the mechanisms involved in these endothelial-cardiomyocyte interactions remain poorly understood. Nucleotides are released within the heart, especially under ischemia or pressure overload. The function of P2Y nucleotide receptors in cardiac development has never been investigated. Here we show that adult P2Y(4)-null mice display microcardia. P2Y(4) nucleotide receptor is expressed in cardiac endothelial cells but not in cardiomyocytes. Loss of P2Y(4) in cardiac endothelial cells strongly inhibits their growth, migration and PDGF-B secretion in response to UTP. Proliferation of microvessels and cardiomyocytes is reduced in P2Y(4)-null hearts early after birth, resulting in reduced heart growth. Our study uncovers mouse P2Y(4) receptor as an essential regulator of cardiac endothelial cell function, and illustrates the involvement of endothelial-cardiomyocyte interactions in post-natal heart development. We also detected P2Y(4) expression in human cardiac microvessels. P2Y(4) receptor could constitute a therapeutic target to regulate cardiac remodelling and post-ischemic revascularisation.

Nucleotide receptors as targets in the pharmacological enhancement of dermal wound healing

With a growing interest of the involvement of extracellular nucleotides in both normal physiology and pathology, it has become evident that P2 receptor agonists and antagonists may have therapeutic potential. The P2Y2 receptor agonists (diquafosol tetrasodium and denufosol tetrasodium) are in the phase 3 of clinical trials for dry eye and cystic fibrosis, respectively. The thienopyridine derivatives clopidogrel and ticlopidine (antagonists of the platelet P2Y12 receptor) have been used in cardiovascular medicine for nearly a decade. Purines and pyrimidines may be of therapeutic potential also in wound healing since ATP and UTP have been shown to have many hallmarks of wound healing factors. Recent studies have demonstrated that extracellular nucleotides take part in all phases of wound repair: hemostasis, inflammation, tissue formation, and tissue remodeling. This review is focused on the potent purines and pyrimidines which regulate many physiological processes important for wound healing.

Resting and action potentials under hypotonic conditions, unlike Na+ pump activity, depend only on the alteration of intracellular [Na+] and [K+] in frog skeletal muscle

It is well established that hypotonicity generates a marked and unexpected increase in active Na(+) efflux in frog muscle fibers as well as in other cells like cardiac myocytes, astrocytes, brain synaptosomes and renal cells. The effect of hypotonicity on the electrical activity of skeletal muscle related to Na(+) and K(+) voltage-gated channels, however, has not been specifically addressed. The results of the present investigation show that the changes in resting and action potentials produced by hypotonicity can be fully explained by the reduction of intracellular [Na(+)] and [K(+)] due to the increase in cellular water content.

alpha-Adrenoceptor-mediated depletion of phosphatidylinositol 4, 5-bisphosphate inhibits activation of volume-regulated anion channels in mouse ventricular myocytes

BACKGROUND AND PURPOSE

Volume-regulated anion channels (VRACs) play an important role in cell-volume regulation. alpha(1)-Adrenoceptor stimulation by phenylephrine (PE) suppressed the hypotonic activation of VRAC current in mouse ventricular cells and regulatory volume decrease (RVD) was also absent in PE-treated cells. We examined whether the effects of alpha(1)-adrenoceptor stimuli on VRAC current were modulated by phosphatidylinositol signalling.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp method was used to record the hypotonicity-induced VRAC current in mouse ventricular cells. RVD was analyzed by videomicroscopic measurement of cell images.

KEY RESULTS

The attenuation of VRAC current by PE was suppressed by alpha(1A)-adrenoceptor antagonists (prazosin and WB-4101), anti-G(q) protein antibody and a specific phosphoinositide-specific phospholipase C (PLC) inhibitor (U-73122), but not by antagonists for alpha(1B)-, alpha(1D)- or beta-adrenoceptor, or protein kinase C inhibitors. The inhibition of VRAC by PE was antagonized by intracellular excess phosphatidylinositol 4,5-bisphosphate (PIP(2)), while intracellular anti-PIP(2) antibody (PIP(2) Ab) inhibited the activation of VRAC currents. When cells were loaded with phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) with or without PIP(2) Ab, PE little affected the VRAC current. Extracellular m-3M3FBS (an activator of PLC) suppressed VRAC in the absence of PE, and this effect was reversed by intracellular excess PIP(2).

CONCLUSIONS AND IMPLICATIONS

Our results indicate that the stimulation of alpha(1A)-adrenoceptors by PE inhibited the activation of cardiac VRAC current via PIP(3) depletion brought about by PLC-dependent reduction of membrane PIP(2) level.

Hypotonic swelling-induced activation of PKN1 mediates cell survival in cardiac myocytes

Hypotonic cell swelling in the myocardium is induced by pathological conditions, including ischemia-reperfusion, and affects the activities of ion transporters/channels and gene expression. However, the signaling mechanism activated by hypotonic stress (HS) is not fully understood in cardiac myocytes. A specialized protein kinase cascade, consisting of Pkc1 and MAPKs, is activated by HS in yeast. Here, we demonstrate that protein kinase N1 (PKN1), a serine/threonine protein kinase and a homolog of Pkc1, is activated by HS (67% osmolarity) within 5 min and reaches peak activity at 60 min in cardiac myocytes. Activation of PKN1 by HS was accompanied by Thr(774) phosphorylation and concomitant activation of PDK1, a potential upstream regulator of PKN1. HS also activated RhoA, thereby increasing interactions between PKN1 and RhoA. PP1 (10(-5) M), a selective Src family tyrosine kinase inhibitor, significantly suppressed HS-induced activation of RhoA and PKN1. Constitutively active PKN1 significantly increased the transcriptional activity of Elk1-GAL4, an effect that was inhibited by dominant negative MEK. Overexpression of PKN1 significantly increased ERK phosphorylation, whereas downregulation of PKN1 inhibited HS-induced ERK phosphorylation. Downregulation of PKN1 and inhibition of ERK by U-0126 both significantly inhibited the survival of cardiac myocytes in the presence of HS. These results suggest that a signaling cascade, consisting of Src, RhoA, PKN1, and ERK, is activated by HS, thereby promoting cardiac myocyte survival.

P2Y2 receptor desensitization on single endothelial cells

Receptor desensitization, or decreased responsiveness of a receptor to agonist stimulation, represents a regulatory process with the potential to have a significant impact on cell behavior. P2Y(2), a G-protein-coupled receptor activated by extracellular nucleotides, undergoes desensitization at many tissues, including the vascular endothelium. Endothelial cells from a variety of vascular beds are normally exposed to extracellular nucleotides released from damaged cells and activated platelets. The purpose of the present study was to compare P2Y(2) receptor desensitization observed in endothelial cells derived from bovine retina, a model of microvascular endothelium, and human umbilical vein endothelial cells (HUVECs), a model of a large blood vessel endothelium. P2Y(2) receptor desensitization was monitored by following changes in UTP-stimulated intracellular free Ca(2 +) in single cells using fura-2 microfluorometry. Both endothelial cell models exhibited desensitization of the P2Y(2) receptor after stimulation with UTP. However, the cells differed in the rate, dependence on agonist concentration, and percentage of maximal desensitization. These results suggest differential mechanisms of P2Y(2) receptor desensitization and favors heterogeneity in extracellular nucleotide activity in endothelial cells according to its vascular bed origin.

Natriuretic peptides stimulate the cardiac sodium pump via NPR-C-coupled NOS activation

Natriuretic peptides (NPs) and their receptors (NPRs) are expressed in the heart, but their effects on myocyte function are poorly understood. Because NPRs are coupled to synthesis of cGMP, an activator of the sarcolemmal Na+-K+ pump, we examined whether atrial natriuretic peptide (ANP) regulates the pump. We voltage clamped rabbit ventricular myocytes and identified electrogenic Na+-K+ pump current (arising from the 3:2 Na+:K+ exchange and normalized for membrane capacitance) as the shift in membrane current induced by 100 μmol/l ouabain. Ten nanomoles per liter ANP stimulated the Na+-K+ pump when the intracellular compartment was perfused with pipette solutions containing 10 mmol/l Na+ but had no effect when the pump was at near maximal activation with 80 mmol/l Na+ in the pipette solution. Stimulation was abolished by inhibition of cGMP-activated protein kinase with KT-5823, nitric oxide (NO)-activated guanylyl cyclase with 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ), or NO synthase with NG-nitro-l-arginine methyl ester (l-NAME). Since synthesis of cGMP by NPR-A and NPR-B is not NO dependent or ODQ sensitive, we exposed myocytes to AP-811, a highly selective ligand for the NPR-C “clearance” receptor. It abolished ANP-induced pump stimulation. Conversely, the selective NPR-C agonist ANP(4-23) reproduced stimulation. The stimulation was blocked by l-NAME. To examine NO production in response to ANP(4-23), we loaded myocytes with the NO-sensitive fluorescent dye diacetylated diaminofluorescein-2 and examined them by confocal microscopy. ANP(4-23) induced a significant increase in fluorescence, which was abolished by l-NAME. We conclude that NPs stimulate the Na+-K+ pump via an NPR-C and NO-dependent pathway.

The sodium-calcium exchanger is a mechanosensitive transporter

This report describes the influence of fluid flow and osmotically induced volume changes on Na(+)-Ca(2+) exchange (NCX) activity in transfected CHO cells. Exchange activity was measured as Na(+)-dependent Ca(2+) or Ba(2+) fluxes using the fluorescent probe fura-2. When exchange activity was initiated by superfusing Ba(2+)-containing solutions over the cells for a 20 s interval, a high rate of Ba(2+) uptake was observed while the solution was being applied but the rate of Ba(2+) uptake declined > 10-fold when the solution flow ceased. Ba(2+) efflux in exchange for extracellular Na(+) or Ca(2+) (Ba(2+)-Ca(2+) exchange) was similarly biphasic. During NCX-mediated Ca(2+) uptake, a rapid increase in cytosolic [Ca(2+)] to a peak value occurred, followed by a decline in [Ca(2+)](i) to a lower steady-state value after solution flow ceased. When NCX activity was initiated by an alternate procedure that minimized the duration of solution flow, the rapid phase of Ba(2+) influx was greatly reduced in magnitude and Ca(2+) uptake became nearly monophasic. Solution superfusion did not produce any obvious changes in cell shape or volume. NCX-mediated Ba(2+) and Ca(2+) influx were also sensitive to osmotically induced changes in cell volume. NCX activity was stimulated in hypotonic media and inhibited in hypertonic media; the osmotically induced changes in activity occurred within seconds and were rapidly reversible. We conclude that NCX activity is modulated by both solution flow and osmotically induced volume changes.

Extracellular ATP is a pro-angiogenic factor for pulmonary artery vasa vasorum endothelial cells

Expansion of the vasa vasorum network has been observed in a variety of systemic and pulmonary vascular diseases. We recently reported that a marked expansion of the vasa vasorum network occurs in the pulmonary artery adventitia of chronically hypoxic calves. Since hypoxia has been shown to stimulate ATP release from both vascular resident as well as circulatory blood cells, these studies were undertaken to determine if extracellular ATP exerts angiogenic effects on isolated vasa vasorum endothelial cells (VVEC) and/or if it augments the effects of other angiogenic factors (VEGF and basic FGF) known to be present in the hypoxic microenvironment. We found that extracellular ATP dramatically increases DNA synthesis, migration, and rearrangement into tube-like networks on Matrigel in VVEC, but not in pulmonary artery (MPAEC) or aortic (AOEC) endothelial cells obtained from the same animals. Extracellular ATP potentiated the effects of both VEGF and bFGF to stimulate DNA synthesis in VVEC but not in MPAEC and AOEC. Analysis of purine and pyrimidine nucleotides revealed that ATP, ADP and MeSADP were the most potent in stimulating mitogenic responses in VVEC, indicating the involvement of the family of P2Y1-like purinergic receptors. Using pharmacological inhibitors, Western blot analysis, and Phosphatidylinositol-3 kinase (PI3K) in vitro kinase assays, we found that PI3K/Akt/mTOR and ERK1/2 play a critical role in mediating the extracellular ATP-induced mitogenic and migratory responses in VVEC. However, PI3K/Akt and mTOR/p70S6K do not significantly contribute to extracellular ATP-induced tube formation on Matrigel. Our studies indicate that VVEC, isolated from the sites of active angiogenesis, exhibit distinct functional responses to ATP, compared to endothelial cells derived from large pulmonary or systemic vessels. Collectively, our data support the idea that extracellular ATP participates in the expansion of the vasa vasorum that can be observed in hypoxic conditions.

Regulation of swelling-activated Cl(-) current by angiotensin II signalling and NADPH oxidase in rabbit ventricle

AIMS

We assessed whether hypoosmotic swelling of cardiac myocytes activates volume-sensitive Cl(-) current (I Cl,swell) via the angiotensin II (AngII)-reactive oxygen species (ROS) signalling cascade. The AngII-ROS pathway previously was shown to elicit I(Cl,swell) upon mechanical stretch of beta(1D) integrin. Integrin stretch and osmotic swelling are, however, distinct stimuli. For example, blocking Src kinases stimulates swelling-induced but inhibits stretch-induced I Cl,swell.

METHODS AND RESULTS

I Cl,swell was measured in rabbit ventricular myocytes by whole-cell voltage clamp. Swelling-induced I Cl,swell was completely blocked by losartan and eprosartan, AngII type I receptor (AT1) antagonists. AT1 stimulation transactivates epidermal growth factor receptor (EGFR) kinase. Blockade of EGFR kinase with AG1478 abolished both I Cl,swell and AngII-induced Cl(-) current, whereas exogenous EGF evoked a Cl(-) current that was suppressed by osmotic shrinkage. Phosphatidylinositol 3-kinase (PI-3K) is downstream of EGFR kinase, and PI-3K inhibitors LY294002 and wortmannin blocked I Cl,swell. Ultimately, AngII signals via NADPH oxidase (NOX) and superoxide anion, O2*. NOX inhibitors, diphenyleneiodonium, apocynin and gp91ds-tat, eliminated I Cl,swell, whereas scramb-tat, an inactive gp91ds-tat analogue, was ineffective. O2* rapidly dismutates to H2O2. Consistent with H2O2 being a downstream effector, catalase inhibited I Cl,swell, and exogenous H2O2 overcame suppression of I Cl,swell by AT1 receptor, EGFR kinase, and PI-3K blockers. H2O2-induced current was not blocked by osmotic shrinkage, however.

CONCLUSION

Activation of I Cl,swell by osmotic swelling is controlled by the AngII-ROS cascade, the same pathway previously implicated in I Cl,swell activation by integrin stretch. This in part explains why I Cl,swell is persistently activated in several models of cardiac disease.

P2Y2 receptors induced cell surface redistribution of alpha(v) integrin is required for activation of ERK 1/2 in U937 cells

Nucleotides released from cells due to stress, injury or inflammation, induce mitogenic effects in monocytes via activation of P2Y(2) nucleotide receptors (P2Y(2)Rs). Here we show that P2Y(2) nucleotide receptors in U937 monocytic cells regulate the activation of extracellular signal-regulated kinases 1 and 2 (ERK 1/2) by inducing the clustering of alpha(v) integrins. The activation of phosphatidylinositol 3-kinase by P2Y(2)R ligands was required for alpha(v) clustering, suggesting a means whereby two different classes of receptors communicate to induce mitogenic responses in monocytic cells. P2Y(2)R-induced alpha(v) clustering was also associated with a flattened phenotype of the U937 cells, consistent with the role of the P2Y(2)R in regulating early events in cell migration.

EGFR kinase regulates volume-sensitive chloride current elicited by integrin stretch via PI-3K and NADPH oxidase in ventricular myocytes

Stretch of beta1 integrins activates an outwardly rectifying, tamoxifen-sensitive Cl(-) current (Cl(-) SAC) via AT1 receptors, NADPH oxidase, and reactive oxygen species, and Cl(-) SAC resembles the volume-sensitive Cl(-) current (I(Cl,swell)). Epidermal growth factor receptor (EGFR) kinase undergoes transactivation upon stretch, integrin engagement, and AT1 receptor activation and, in turn, stimulates NADPH oxidase. Therefore, we tested whether Cl(-) SAC is regulated by EGFR kinase signaling and is volume sensitive. Paramagnetic beads coated with mAb for beta1 integrin were attached to myocytes and pulled with an electromagnet. Stretch activated a Cl(-) SAC that was 1.13 +/- 0.10 pA/pF at +40 mV. AG1478 (10 muM), an EGFR kinase blocker, inhibited 93 +/- 13% of Cl(-) SAC, and intracellular pretreatment with 1 muM AG1478 markedly suppressed Cl(-) SAC activation. EGF (3.3 nM) directly activated an outwardly rectifying Cl(-) current (0.81 +/- 0.05 pA/pF at +40 mV) that was fully blocked by 10 muM tamoxifen, an I(Cl,swell) blocker. Phosphatidylinositol 3-kinase (PI-3K) is downstream of EGFR kinase. Wortmannin (500 nM) and LY294002 (100 microM), blockers of PI-3K, inhibited Cl(-) SAC by 67 +/- 6% and 91 +/- 25% respectively, and the EGF-induced Cl(-) current also was fully blocked by LY294002. Furthermore, gp91ds-tat (500 nM), a cell-permeable, chimeric peptide that specifically blocks NADPH oxidase assembly, profoundly inhibited the EGF-induced Cl(-) current. Inactive permeant and active impermeant control peptides had no effect. Myocyte shrinkage with hyperosmotic bathing media inhibited the Cl(-) SAC and EGF-induced Cl(-) current by 88 +/- 9% and 127 +/- 11%, respectively. These results suggest that beta1 integrin stretch activates Cl(-) SAC via EGFR, PI-3K, and NADPH oxidase, and that both the Cl(-) SAC and the EGF-induced Cl(-) currents are likely to be the volume-sensitive Cl(-) current, I(Cl,swell).

The P2Y2 nucleotide receptor interacts with alphav integrins to activate Go and induce cell migration

Extracellular ATP and UTP induce chemotaxis, or directed cell migration, by stimulating the G protein-coupled P2Y(2) nucleotide receptor (P2Y(2)R). Previously, we found that an arginine-glycine-aspartic acid (RGD) integrin binding domain in the P2Y(2)R enables this receptor to interact selectively with alpha(v)beta(3) and alpha(V)beta(5) integrins, an interaction that is prevented by mutation of the RGD sequence to arginine-glycine-glutamic acid (RGE) (Erb, L., Liu, J., Ockerhausen, J., Kong, Q., Garrad, R. C., Griffin, K., Neal, C., Krugh, B., Santiago-Perez, L. I., Gonzalez, F. A., Gresham, H. D., Turner, J. T., and Weisman, G. A. (2001) J. Cell Biol. 153, 491-501). This RGD domain also was found to be necessary for coupling the P2Y(2)R to G(o)- but not G(q)-mediated intracellular calcium mobilization, leading us to investigate the role of P2Y(2)R interaction with integrins in nucleotide-induced chemotaxis. Here we show that mutation of the RGD sequence to RGE in the human P2Y(2)R expressed in 1321N1 astrocytoma cells completely prevented UTP-induced chemotaxis as well as activation of G(o), Rac, and Vav2, a guanine nucleotide exchange factor for Rac. UTP also increased expression of vitronectin, an extracellular matrix protein that is a ligand for alpha(v)beta(3)/beta(5) integrins, in cells expressing the wild-type but not the RGE mutant P2Y(2)R. P2Y(2)R-mediated chemotaxis, Rac and Vav2 activation, and vitronectin up-regulation were inhibited by pretreatment of the cells with anti-alpha(v)beta(5) integrin antibodies, alpha(v) integrin antisense oligonucleotides, or the G(i/o) inhibitor, pertussis toxin. Thus, the RGD-dependent interaction between the P2Y(2)R and alpha(v) integrins is necessary for the P2Y(2)R to activate G(o) and to initiate G(o)-mediated signaling events leading to chemotaxis.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Src homology 3 binding sites in the P2Y2 nucleotide receptor interact with Src and regulate activities of Src, proline-rich tyrosine kinase 2, and growth factor receptors

Many G protein-coupled receptors activate growth factor receptors, although the mechanisms controlling this transactivation are unclear. We have identified two proline-rich, SH3 binding sites (PXXP) in the carboxyl-terminal tail of the human P2Y(2) nucleotide receptor that directly associate with the tyrosine kinase Src in protein binding assays. Furthermore, Src co-precipitated with the P2Y(2) receptor in 1321N1 astrocytoma cells stimulated with the P2Y(2) receptor agonist UTP. A mutant P2Y(2) receptor lacking the PXXP motifs was found to stimulate calcium mobilization and serine/threonine phosphorylation of the Erk1/2 mitogen-activated protein kinases, like the wild-type receptor, but was defective in its ability to stimulate tyrosine phosphorylation of Src and Src-dependent tyrosine phosphorylation of the proline-rich tyrosine kinase 2, epidermal growth factor receptor (EGFR), and platelet-derived growth factor receptor. Dual immunofluorescence labeling of the P2Y(2) receptor and the EGFR indicated that UTP caused an increase in the co-localization of these receptors in the plasma membrane that was prevented by the Src inhibitor PP2. Together, these data suggest that agonist-induced binding of Src to the SH3 binding sites in the P2Y(2) receptor facilitates Src activation, which recruits the EGFR into a protein complex with the P2Y(2) receptor and allows Src to efficiently phosphorylate the EGFR.

Dietary cholesterol alters Na+/K+ selectivity at intracellular Na+/K+ pump sites in cardiac myocytes

A modest diet-induced increase in serum cholesterol in rabbits increases the sensitivity of the sarcolemmal Na+/K+ pump to intracellular Na+, whereas a large increase in cholesterol levels decreases the sensitivity to Na+. To examine the mechanisms, we isolated cardiac myocytes from controls and from rabbits with diet-induced increases in serum cholesterol. The myocytes were voltage clamped with the use of patch pipettes that contained osmotically balanced solutions with Na+ in a concentration of 10 mM and K+ in concentrations ([K+]pip) ranging from 0 to 140 mM. There was no effect of dietary cholesterol on electrogenic Na+/K+ current (Ip) when pipette solutions were K+ free. A modest increase in serum cholesterol caused a [K+]pip-dependent increase in Ip, whereas a large increase caused a [K+]pip-dependent decrease in Ip. Modeling suggested that pump stimulation with a modest increase in serum cholesterol can be explained by a decrease in the microscopic association constant KK describing the backward reaction E1 + 2K+ --> E2(K+)2, whereas pump inhibition with a large increase in serum cholesterol can be explained by an increase in KK. Because hypercholesterolemia upregulates angiotensin II receptors and because angiotensin II regulates the Na+/K+ pump in cardiac myocytes in a [K+]pip-dependent manner, we blocked angiotensin synthesis or angiotensin II receptors in vivo in cholesterol-fed rabbits. This abolished cholesterol-induced pump inhibition. Because the epsilon-isoform of protein kinase C (epsilonPKC) mediates effects of angiotensin II on the pump, we included specific epsilonPKC-blocking peptide in patch pipette filling solutions. The peptide reversed cholesterol-induced pump inhibition.

Hypotonic stimulation of the Na+ active transport in frog skeletal muscle: role of the cytoskeleton

Hypotonicity produces a marked activation of the Na+ pump in frog sartorius muscle. The increase in net Na+ efflux under hypotonic conditions occurs despite the reductions in [Na+]i that are due to fibre swelling and Na+ loss. The pump density (ouabain binding) increases not only upon reduction of the medium osmotic pressure (pi) from its normal value (pi = 1) to one-half (pi = 0.5), but also in muscles that are returned to pi = 1 after equilibration in pi = 2 medium. The equilibration in pi = 2 medium does not affect pump density. Ouabain-binding increments cannot be ascribed to a rise in the Na+-K+ exchange rate of a fixed number of pumps: they also occurred in the continued presence of a saturating concentration of ouabain (50 microM). Under those conditions, the pi = 1 pi = 0.5 transfer produced a 43 % increase in pump sites, while the pi = 2 pi = 1 transfer induced a rise of 46 %. Actinomycin D did not alter the stimulation of Na+ extrusion elicited by hypotonicity, suggesting that de novo synthesis of pumps was not involved in the increase of the apparent number of pump sites. Disruption of microtubules by colchicine (100 microM) and intermediate filaments by acrylamide (4 mM) did not alter the hypotonic effect. Likewise, genistein (100 microM), a specific inhibitor of tyrosine kinase, did not affect significantly the hypotonic response. Microfilament-disrupting agents like cytochalasin B (5 microM) and latrunculin B (10 microM) reduced the increase in Na+ efflux induced by pi = 1 pi = 0.5 transfer by about 35 % and 72 %, respectively. Latrunculin B reduced the increases in pump density generated by pi = 1 pi = 0.5 and pi = 2 pi = 1 transfers by about 79 % and 91 %, respectively. The results suggest that the membrane stretch due to hypotonic fibre volume increase would promote a microfilament-mediated insertion of submembranous spare Na+ pumps in the sarcolemma and, consequently, the rise in active Na+ transport.

Biphasic effects of cell volume on excitation-contraction coupling in rabbit ventricular myocytes

We studied the effects of osmotic swelling on the components of excitation-contraction coupling in ventricular myocytes. Myocyte volume rapidly increased 30% in hyposmotic (0.6T) solution and was constant thereafter. Cell shortening transiently increased 31% after 4 min in 0.6T but then decreased to 68% of control after 20 min. In parallel, the L-type Ca(2+) current (I(Ca-L)) transiently increased 10% and then declined to 70% of control. Similar biphasic effects on shortening were observed under current clamp. In contrast, action potential duration was unchanged at 4 min but decreased to 72% of control after 20 min. Ca(2+) transients were measured with fura 2-AM. The emission ratio with excitation at 340 and 380 nm (f(340)/f(380)) decreased by 12% after 3 min in 0.6T, whereas shortening and I(Ca-L) increased at the same time. After 8 min, shortening, I(Ca-L), and the f(340)/f(380) ratio decreased 28, 25, and 59%, respectively. The results suggest that osmotic swelling causes biphasic changes in I(Ca-L) that contribute to its biphasic effects on contraction. In addition, swelling initially appears to reduce the Ca(2+) transient initiated by a given I(Ca-L), and later, both I(Ca-L) and the Ca(2+) transient are inhibited.

Purinergic signaling and vascular cell proliferation and death

Evidence for the role of purinergic signaling (via P1 and P2Y receptors) in the proliferation of vascular smooth muscle and endothelial cells is reviewed. The involvement of the mitogen-activated protein kinase second-messenger cascade in this action is clearly implicated, although details of the precise intracellular pathways involved still remain to be determined. Synergistic actions of purines and pyrimidines with growth factors occur in promoting cell proliferation. Interaction between purinergic signaling for vascular cell proliferation and cell death mediated by P2X7 receptors is discussed. There is evidence of the release of ATP from endothelial cells, platelets, and sympathetic nerves as well as from damaged cells in atherosclerosis, hypertension, restenosis, and ischemia; furthermore, there is evidence that vascular smooth muscle and endothelial cells proliferate in these pathological conditions. Thus, the involvement of ATP and its breakdown product, adenosine, is implicated; it is hoped that with the development of selective P1 (A2) and P2Y receptor agonists and antagonists, new therapeutic strategies will be explored.

Activation of background membrane conductance by the tyrosine kinase inhibitor tyrphostin A23 and its inactive analog tyrphostin A1 in guinea pig ventricular myocytes

ABSTRACT-The effects of the tyrosine kinase (TK) inhibitor tyrphostin A23 and its inactive analog tyrphostin Al on background membrane conductance were investigated in guinea pig ventricular myocytes. TK-inhibiting A23 reversibly increased membrane conductance under conditions designed to minimize Na+, Ca2+, K+, and Na+-K+ pump currents. Similar stimulatory action was obtained with TK-inactive Al. The tyrphostin-induced current was inhibited by omitting external Na+ or Ca+, suppressed by chelating internal Ca2+, blocked by external Cd2+ and Ni2+, and insensitive to changes in internal Cl- concentration. We conclude that tyrphostins have a direct, TK-independent action that increases membrane conductance probably by stimulating Na+-Ca2+ exchange.

Electrophysiology of the sodium-potassium-ATPase in cardiac cells

Like several other ion transporters, the Na(+)-K(+) pump of animal cells is electrogenic. The pump generates the pump current I(p). Under physiological conditions, I(p) is an outward current. It can be measured by electrophysiological methods. These methods permit the study of characteristics of the Na(+)-K(+) pump in its physiological environment, i.e., in the cell membrane. The cell membrane, across which a potential gradient exists, separates the cytosol and extracellular medium, which have distinctly different ionic compositions. The introduction of the patch-clamp techniques and the enzymatic isolation of cells have facilitated the investigation of I(p) in single cardiac myocytes. This review summarizes and discusses the results obtained from I(p) measurements in isolated cardiac cells. These results offer new exciting insights into the voltage and ionic dependence of the Na(+)-K(+) pump activity, its effect on membrane potential, and its modulation by hormones, transmitters, and drugs. They are fundamental for our current understanding of Na(+)-K(+) pumping in electrically excitable cells.

Inhibition of Na,K-ATPase activates PI3 kinase and inhibits apoptosis in LLC-PK1 cells

In the present study we used LLC-PK1 cells, a porcine renal proximal tubular cell line, to investigate whether PI3 kinase activation was involved in the anti-apoptotic effect of ouabain, a specific inhibitor of Na,K-ATPase. Apoptosis was induced by actinomycin D (Act D, 5 microM) and assessed by appearance of hypodiploid nuclei and DNA fragmentation. Ouabain attenuated Act D-induced apoptotic response in a dose-dependent manner. Incubation in a low K(+) medium (0.1 mM) which is another way to decrease Na,K-ATPase activity also had anti-apoptotic effect. Both ouabain and low K(+) medium increased the PI3 kinase activity in p85 immunoprecipitates. Ouabain, as well as incubation in the low K(+) medium, also increased the phosphorylation of Akt. Inhibition of PI3 kinase by either wortmannin or LY294002 reversed the cytoprotective effect of ouabain. These data together indicate that inhibition of Na,K-ATPase activates PI3 kinase in LLC-PK1 cells which could then exert the cytoprotective effect.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Biphasic effects of hyposmotic challenge on excitation-contraction coupling in rat ventricular myocytes

The effects of short (1 min) and long (7-10 min) exposure to hyposmotic solution on excitation-contraction coupling in rat ventricular myocytes were studied. After short exposure, the action potential duration at 90% repolarization (APD(90)), the intracellular Ca(2+) concentration ([Ca(2+)](i)) transient amplitude, and contraction increased, whereas the L-type Ca(2+) current (I(Ca, L)) amplitude decreased. Fractional sarcoplasmic reticulum (SR) Ca(2+) release increased but SR Ca(2+) load did not. After a long exposure, I(Ca,L), APD(90), [Ca(2+)](i) transient amplitude, and contraction decreased. The abbreviation of APD(90) was partially reversed by 50 microM DIDS, which is consistent with the participation of Cl(-) current activated by swelling. After 10-min exposure to hyposmotic solution in cells labeled with di-8-aminonaphthylethenylpyridinium, t-tubule patterning remained intact, suggesting the loss of de-t-tubulation was not responsible for the fall in I(Ca,L). After long exposure, Ca(2+) load of the SR was not increased, and swelling had no effect on the site-specific phosphorylation of phospholamban, but fractional SR Ca(2+) release was depressed. The initial positive inotropic response to hyposmotic challenge may be accounted for by enhanced coupling between Ca(2+) entry and release. The negative inotropic effect of prolonged exposure can be accounted for by shortening of the action potential duration and a fall in the I(Ca,L) amplitude.

P2Y receptor-mediated inhibition of tumor necrosis factor alpha -stimulated stress-activated protein kinase activity in EAhy926 endothelial cells

In the EAhy926 endothelial cell line, UTP, ATP, and forskolin, but not UDP and epidermal growth factor, inhibited tumor necrosis factor alpha (TNFalpha)- and sorbitol stimulation of the stress-activated protein kinases, JNK, and p38 mitogen-activated protein (MAP) kinase, and MAPKAP kinase-2, the downstream target of p38 MAP kinase. In NCT2544 keratinocytes, UTP and a proteinase-activated receptor-2 agonist caused similar inhibition, but in 13121N1 cells, transfected with the human P2Y(2) or P2Y(4) receptor, UTP stimulated JNK and p38 MAP kinase activities. This suggests that the effects mediated by P2Y receptors are cell-specific. The inhibitory effects of UTP were not due to induction of MAP kinase phosphatase-1, but were manifest upstream in the pathway at the level of MEK-4. The inhibitory effect of UTP was insensitive to the MEK-1 inhibitor PD 098059, changes in intracellular Ca(2+) levels, or pertussis toxin. Acute phorbol 12-myristate 13-acetate pretreatment also inhibited TNFalpha-stimulated SAP kinase activity, while chronic pretreatment reversed the effects of UTP. Furthermore, the protein kinase C inhibitors Ro318220 and Go6983 reversed the inhibitory action of UTP, but GF109203X was ineffective. These results indicate a novel mechanism of cross-talk regulation between P2Y receptors and TNFalpha-stimulated SAP kinase pathways in endothelial cells, mediated by Ca(2+)-independent isoforms of protein kinase C.

P2Y receptor regulation of PAI-1 expression in vascular smooth muscle cells

P2Y-type purine and pyrimidine nucleotide receptors play important roles in the regulation of vascular hemostasis. In this article, the regulation of plasminogen activator inhibitor-1 (PAI-1) expression in rat aortic smooth muscle cells (RASMCs) by adenine and uridine nucleotides was examined and compared. Northern analysis revealed that RASMCs express multiple P2Y receptor subtypes, including P2Y(1), P2Y(2), and P2Y(6). Treatment of RASMCs with UTP increased PAI-1 mRNA expression and extracellular PAI-1 protein levels by 21-fold (P<0.001) and 7-fold (P<0.001), respectively. The ED(50) for the effect of UTP on PAI-1 expression was approximately 1 micromol/L, and its maximal effect occurred at 3 hours. UDP stimulated a 5-fold increase (P<0.005) in PAI-1 expression. In contrast to these potent stimulatory effects of uridine nucleotides, ATP and 2-methylthioadenosine triphosphate (2-MeSATP) caused a small and transient increase in PAI-1 mRNA at 1 hour, followed by a rapid decrease to baseline levels. ADP produced only an inhibitory effect, reducing PAI-1 mRNA levels by 63% (P<0.05) at 3 hours. The relative nucleotide potency in stimulating PAI-1 expression is UTP>UDP>ATP=2-MeSATP, consistent with a predominant role of the P2Y(6) receptor. Further studies revealed that exposure of RASMCs to either ATP or ADP for 3 hours inhibited both UTP- and angiotensin II-stimulated PAI-1 expression by up to 90% (P<0.001). Thus, ATP induced a small and transient upregulation of PAI-1 that was followed by a strong inhibition of PAI-1 expression. These results show that extracellular adenine and uridine nucleotides exert potent and opposing effects on vascular PAI-1 expression.