P2Y purinergic receptor regulation of CFTR chloride channels in mouse cardiac myocytes

Arrhythmogenic Remodeling in the Failing Heart

Chronic heart failure is a clinical syndrome with multiple etiologies, associated with significant morbidity and mortality. Cardiac arrhythmias, including ventricular tachyarrhythmias and atrial fibrillation, are common in heart failure. A number of cardiac diseases including heart failure alter the expression and regulation of ion channels and transporters leading to arrhythmogenic electrical remodeling. Myocardial hypertrophy, fibrosis and scar formation are key elements of arrhythmogenic structural remodeling in heart failure. In this article, the mechanisms responsible for increased arrhythmia susceptibility as well as the underlying changes in ion channel, transporter expression and function as well as alterations in calcium handling in heart failure are discussed. Understanding the mechanisms of arrhythmogenic remodeling is key to improving arrhythmia management and the prevention of sudden cardiac death in patients with heart failure.

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

Potential role of cardiac chloride channels and transporters as novel therapeutic targets

The heart and blood vessels express a range of anion currents (e.g. ICl.PKA) and symporter/antiporters (e.g. Cl(-)/HCO3(-) exchanger) that translocate chloride (Cl(-)). They have been proposed to contribute to a variety of physiological processes including cellular excitability, cell volume homeostasis and apoptosis. Additionally there is evidence that Cl(-) currents or transporters may play a role in cardiac pathophysiology. Arrhythmogenesis, the process of cardiac ischaemic preconditioning, and the adaptive remodelling process in myocardial hypertrophy and heart failure have all been linked to such channels or transporters. We have explored the possibility that selective targeting of one or more of these may provide benefit in cardiovascular disease. Existing evidence points to an emerging role of cardiac cell anion channels as potential therapeutic targets, the 'disease-specificity' of which may represent a substantial improvement on current targets. However, the limitations of current techniques hitherto applied (such as developmental compensation in gene-modified animals) and pharmacological agents (which do not at present possess sufficient selectivity for the adequate probing of function) have thus far hindered translation to the introduction of new therapy.

In vivo characterization of the role of tissue-specific translation elongation factor 1A2 in protein synthesis reveals insights into muscle atrophy

Translation elongation factor 1A2 (eEF1A2), uniquely among translation factors, is expressed specifically in neurons and muscle. eEF1A2‐null mutant wasted mice develop an aggressive, early‐onset form of neurodegeneration, but it is unknown whether the wasting results from denervation of the muscles, or whether the mice have a primary myopathy resulting from loss of translation activity in muscle. We set out to establish the relative contributions of loss of eEF1A2 in the different tissues to this postnatal lethal phenotype. We used tissue‐specific transgenesis to show that correction of eEF1A2 levels in muscle fails to ameliorate the overt phenotypic abnormalities or time of death of wasted mice. Molecular markers of muscle atrophy such as Fbxo32 were dramatically upregulated at the RNA level in wasted mice, both in the presence and in the absence of muscle‐specific expression of eEF1A2, but the degree of upregulation at the protein level was significantly lower in those wasted mice without transgene‐derived expression of eEF1A2 in muscle. This provides the first in vivo confirmation that eEF1A2 plays an important role in translation. In spite of the inability of the nontransgenic wasted mice to upregulate key atrogenes at the protein level in response to denervation to the same degree as their transgenic counterparts, there were no measurable differences between transgenic and nontransgenic wasted mice in terms of weight loss, grip strength, or muscle pathology. This suggests that a compromised ability fully to execute the atrogene pathway in denervated muscle does not affect the process of muscle atrophy in the short term.

Phenomics of cardiac chloride channels

Forward genetic studies have identified several chloride (Cl-) channel genes, including CFTR, ClC-2, ClC-3, CLCA, Bestrophin, and Ano1, in the heart. Recent reverse genetic studies using gene targeting and transgenic techniques to delineate the functional role of cardiac Cl- channels have shown that Cl- channels may contribute to cardiac arrhythmogenesis, myocardial hypertrophy and heart failure, and cardioprotection against ischemia reperfusion. The study of physiological or pathophysiological phenotypes of cardiac Cl- channels, however, is complicated by the compensatory changes in the animals in response to the targeted genetic manipulation. Alternatively, tissue-specific conditional or inducible knockout or knockin animal models may be more valuable in the phenotypic studies of specific Cl- channels by limiting the effect of compensation on the phenotype. The integrated function of Cl- channels may involve multiprotein complexes of the Cl- channel subproteome. Similar phenotypes can be attained from alternative protein pathways within cellular networks, which are influenced by genetic and environmental factors. The phenomics approach, which characterizes phenotypes as a whole phenome and systematically studies the molecular changes that give rise to particular phenotypes achieved by modifying the genotype under the scope of genome/proteome/phenome, may provide more complete understanding of the integrated function of each cardiac Cl- channel in the context of health and disease.

Transient receptor potential melastatin 4 inhibitor 9-phenanthrol abolishes arrhythmias induced by hypoxia and re-oxygenation in mouse ventricle

BACKGROUND AND PURPOSE

Hypoxia and subsequent re-oxygenation are associated with cardiac arrhythmias such as early afterdepolarizations (EADs), which may be partly explained by perturbations in cytosolic calcium concentration. Transient receptor potential melastatin 4 (TRPM4), a calcium-activated non-selective cation channel, is functionally expressed in the heart. Based on its biophysical properties, it is likely to participate in EADs. Hence, modulators of TRPM4 activity may influence arrhythmias. The aim of this study was to investigate the possible anti-arrhythmic effect of 9-phenanthrol, a TRPM4 inhibitor in a murine heart model of hypoxia and re-oxygenation-induced EADs.

EXPERIMENTAL APPROACH

Mouse heart was removed, and the right ventricle was pinned in a superfusion chamber. After a period of normoxia, the preparation was superfused for 2 h with a hypoxic solution and then re-oxygenated. Spontaneous electrical activity was investigated by intracellular microelectrode recordings.

KEY RESULTS

In normoxic conditions, the ventricle exhibited spontaneous action potentials. Application of the hypoxia and re-oxygenation protocol unmasked hypoxia-induced EADs, the occurrence of which increased under re-oxygenation. The frequency of these EADs was reduced by superfusion with either flufenamic acid, a blocker of Ca(2+) -dependent cation channels or with 9-phenanthrol. Superfusion with 9-phenanthrol (10(-5) or 10(-4)  mol·L(-1) ) caused a dramatic dose-dependent abolition of EADs.

CONCLUSIONS AND IMPLICATIONS

Hypoxia and re-oxygenation-induced EADs can be generated in the mouse heart model. 9-Phenanthrol abolished EADs, which strongly suggests the involvement of TRPM4 in the generation of EAD. This identifies non-selective cation channels inhibitors as new pharmacological candidates in the treatment of arrhythmias.

Arrhythmia and sudden death associated with elevated cardiac chloride channel activity

The identification and analysis of several cationic ion channels and their associated genes have greatly improved our understanding of the molecular and cellular mechanisms of cardiac arrhythmia. Our objective in this study was to examine the involvement of anionic ion channels in cardiac arrhythmia. We used a transgenic mouse model to overexpress the human cystic fibrosis transmembrane conductance regulator (CFTR) gene, which encodes a cAMP-regulated chloride channel. We used RNase protection and in situ hybridization assays to determine the level of CFTR expression, and radiotelemetry and in vivo electrophysiological study in combination with pharmacological intervention to analyse the cardiac function. Cardiac CFTR overexpression leads to stress-related sudden death in this model. In vivo intracardiac electrophysiological studies performed in anaesthetized mice showed no significant differences in baseline conduction parameters including atrial-His bundle (AH) or His bundle-ventricular (HV) conduction intervals, atrioventricular (AV) Wenckebach or 2:1 AV block cycle length and AV nodal functional refractory period. However, following isoproterenol administration, there was marked slowing of conduction parameters, including high-grade AV block in transgenic mice, with non-sustained ventricular tachycardia easily inducible using programmed stimulation or burst pacing. Our sudden death mouse model can be a valuable tool for investigation of the role of chloride channels in arrhythmogenesis and, potentially, for future evaluation of novel anti-arrhythmic therapeutic strategies and pharmacological agents.

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.

Phenomics of cardiac chloride channels: the systematic study of chloride channel function in the heart

Recent studies have identified several chloride (Cl-) channel genes in the heart, including CFTR, ClC-2, ClC-3, CLCA, Bestrophin, and TMEM16A. Gene targeting and transgenic techniques have been used to delineate the functional role of cardiac Cl- channels in the context of health and disease. It has been shown that Cl- channels may contribute to cardiac arrhythmogenesis, myocardial hypertrophy and heart failure, and cardioprotection against ischaemia-reperfusion. The study of physiological or pathophysiological phenotypes of cardiac Cl- channels, however, may be complicated by the compensatory changes in the animals in response to the targeted genetic manipulation. Alternatively, tissue-specific conditional or inducible knockout or knockin animal models may be more valuable in the phenotypic studies of specific Cl- channels by limiting the effect of compensation on the phenotype. The integrated function of Cl- channels may involve multi-protein complexes of the Cl- channel subproteome and similar phenotypes can be attained from alternative protein pathways within cellular networks, which are influenced by genetic and environmental factors. Therefore, the phenomics approach, which characterizes phenotypes as a whole phenome and systematically studies the molecular changes that give rise to particular phenotypes achieved by modifying the genotype (such as gene knockouts or knockins) under the scope of genome/proteome/phenome, may provide a more complete understanding of the integrated function of each cardiac Cl- channel in the context of health and disease.

Involvement of apical P2Y2 receptor-regulated CFTR activity in muscarinic stimulation of Cl(-) reabsorption in rat submandibular gland

Previously, we presented in vivo evidence for a physiological significance of cAMP-regulated CFTR Cl(-) channels in Ca(2+)-activated Cl(-) reabsorption in the ductal system of the rat submandibular gland. Here, we address the mechanism by which basal CFTR activation contributes to the transepithelial Cl(-) movement evoked by muscarinic stimulation. The Cl(-) concentration ([Cl(-)]) was increased in the final saliva from rat submandibular gland during pilocarpine stimulation when a small interfering RNA for CFTR or a specific CFTR inhibitor, CFTR(inh)-172, was injected retrogradely into the gland's own duct, indicating that basal CFTR activation is involved in Cl(-) reabsorption. Systemically administered propranolol failed to alter the [Cl(-)], suggesting little involvement of a beta-adrenergic pathway in the Cl(-) movement that occurs through basal CFTR activation. Intraductal injection of suramin (a nonspecific P2-receptor antagonist) increased the salivary [Cl(-)], indicating the existence of endogenous purinergic activation. Upon separate intraductal injection, ATP and a P2Y(2)-receptor agonist, UTP, decreased the salivary [Cl(-)] almost equipotently. CFTR(inh)-172 and suramin each prevented these effects, whereas 2',3'-O-(4-benzoylbenzoyl)-ATP (Bz-ATP), a P2X(7) agonist, had no specific effect. Pilocarpine stimulation evoked ATP secretion into the salivary fluid. Immunohistochemistry revealed the partial coexistence of CFTR and P2Y(2) receptors on the luminal surface of epithelial cells in the striated ducts of this gland. These results raise the possibility that muscarinic stimulation-induced Cl(-) reabsorption occurs through basal CFTR activity and that this is regulated by P2Y(2) receptors in the ductal epithelium via stimulation by ATP secreted into the salivary fluid.

Cyclic AMP regulates bicarbonate secretion in cholangiocytes through release of ATP into bile

BACKGROUND & AIMS

Bicarbonate secretion is a primary function of cholangiocytes. Either adenosine 3',5'-cyclic monophosphate (cAMP) or cytosolic Ca(2+) can mediate bicarbonate secretion, but these are thought to act through separate pathways. We examined the role of the inositol 1,4,5-trisphosphate receptor (InsP3R) in mediating bicarbonate secretion because this is the only intracellular Ca(2+) release channel in cholangiocytes.

METHODS

Intrahepatic bile duct units (IBDUs) were microdissected from rat liver then luminal pH was examined by confocal microscopy during IBDU microperfusion. Cyclic AMP was increased using forskolin or secretin, and Ca(2+) was increased using acetylcholine (ACh) or adenosine triphosphate (ATP). Apyrase was used to hydrolyze extracellular ATP, and suramin was used to block apical P2Y ATP receptors. In selected experiments, IBDUs were pretreated with short interfering RNA (siRNA) to silence expression of specific InsP3R isoforms.

RESULTS

Both cAMP and Ca(2+) agonists increased luminal pH. The effect of ACh on luminal pH was reduced by siRNA for basolateral (types I and II) but not apical (type III) InsP3R isoforms. The effect of forskolin on luminal pH was reduced by a cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor and by siRNA for the type III InsP3R. Luminal apyrase or suramin blocked the effects of forskolin but not ACh on luminal pH.

CONCLUSIONS

Cyclic AMP-induced ductular bicarbonate secretion depends on an autocrine signaling pathway that involves CFTR, apical release of ATP, stimulation of apical nucleotide receptors, and then activation of apical, type III InsP3Rs. The primary role of CFTR in bile duct secretion may be to regulate secretion of ATP rather than to secrete chloride and/or bicarbonate.

Regulation of extracellular UTP-activated Cl- current by P2Y-PLC-PKC signaling and ATP hydrolysis in mouse ventricular myocytes

The intracellular signaling pathways responsible for extracellualr uridine-5'-triphosphate (UTPo)-induced chloride (Cl-) currents (I(Cl.UTP)) were studied in mouse ventricular myocytes with the whole-cell clamp technique. UTPo (0.1 to 100 microM) activated a whole-cell current that showed a time-independent activation, a linear current-voltage relationship in symmetrical Cl- solutions, an anion selectivity of Cl- > iodide > aspartate, and an inhibition by a thiazolidinone-derived specific inhibitor (CFTR(inh)-172, 10 microM) of cystic fibrosis transmembrane conductance regulator (CFTR), but not by a disulfonic stilbene derivative (DIDS, 100 microM), these properties matching those of CFTR Cl- channels. The potency order of nucleotides for an activation of the Cl- current was UTP = ATP > uridine-5'-diphosphate (UDP) = ADP. Suramin (100 microM), a P2Y receptor antagonist, strongly inhibited the UTPo -activation of the Cl- current, whereas pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 100 microM), another P2Y receptor antagonist, induced little inhibition of I(Cl.UTP). The activation of I(Cl.UTP) was sensitive to protein kinase C (PKC) inhibitor, phospholipase C (PLC) inhibitor, intracellular GDPbetaS (nonhydrolyzable GDP analogue) or anti-Gq/11 antibody. UTPo failed to activate the Cl- current when the cells were dialyzed with nonhydrolyzable ATP analogues (ATPS or AMP-PNP) without ATP, suggesting that ATP hydrolysis is a prerequisite for the current activation. I(Cl.UTP) was persistently activated with a mixture of ATPgammaS + ATP in the pipette, suggesting the involvement of phosphorylation reaction in the current activation process. Our results strongly suggest that I(Cl.UTP) is due to the activation of CFTR Cl- channels through Gq/11-coupled P2Y2 receptor-PLC-PKC signaling and ATP hydrolysis in mouse heart.

P2X purinergic receptor-mediated ionic current in cardiac myocytes of calsequestrin model of cardiomyopathy: implications for the treatment of heart failure

P2X purinergic receptors, activated by extracellular ATP, mediate a number of cardiac cellular effects and may be important under pathophysiological conditions. The objective of the present study was to characterize the P2X receptor-mediated ionic current and determine its role in heart failure using the calsequestrin (CSQ) model of cardiomyopathy. Membrane currents under voltage clamp were determined in myocytes from both wild-type (WT) and CSQ mice. The P2X agonist 2-methylthio-ATP (2-meSATP) induced an inward current that was greater in magnitude in CSQ than in WT ventricular cells. The novel agonist, MRS-2339, an N-methanocarba derivative of 2-chloro-AMP relatively resistant to nucleotidase, induced a current in the CSQ myocyte similar to that by 2-meSATP. When administered via a miniosmotic pump (Alzet), it significantly increased longevity compared with vehicle-injected mice (log rank test, P = 0.02). The improvement in survival was associated with decreases in the heart weight-to-body weight ratio and in cardiac myocyte cross-sectional area [MRS-2339-treated mice: 281 +/- 15.4 (SE) mum(2), n = 6 mice vs. vehicle-treated mice: 358 +/- 27.8 mum(2), n = 6 mice, P < 0.05]. MRS-2339 had no vasodilator effect in mouse aorta ring preparations, indicating that its salutary effect in heart failure is not because of any vascular unloading. The cardiac P2X current is upregulated in the CSQ heart failure myocytes. Chronic administration of a nucleotidase-resistant agonist confers a beneficial effect in the CSQ model of heart failure, apparently via an activation of the cardiac P2X receptor. Cardiac P2X receptors represent a novel and potentially important therapeutic target for the treatment of heart failure.

Stable knockdown of CFTR establishes a role for the channel in P2Y receptor-stimulated anion secretion

P2Y receptor regulation of anion secretion was investigated in porcine endometrial gland (PEG) epithelial cells. P2Y2, P2Y4, and P2Y6 receptors were detected in monolayers of PEG cells and immunocytochemistry indicated that P2Y4 receptors were located in the apical membrane. Apical membrane current measurements showed that Ca2+-dependent and PKC-dependent Cl- channels were activated following treatment with uridine triphosphate (UTP) (5 microM). Current-voltage relationships comparing calcium-dependent and PKC-dependent UTP responses under biionic conditions showed significant differences in selectivity between Cl-)and I- for the PKC-dependent conductance (P(I)/P(Cl) = 0.76), but not for Ca2+-dependent conductance (PI/P(Cl) = 1.02). The I-/Cl- permeability ratio for the PKC-dependent conductance was identical to that measured for 8-cpt cAMP. Furthermore, PKC stimulation using phorbol 12-myristate 13-acetate (PMA) activated an apical membrane Cl- conductance that was blocked by the CFTR selective inhibitor, CFTRinh-172. CFTR silencing, accomplished by stable expression of small hairpin RNAs (shRNA), blocked the PKC-activated conductance associated with UTP stimulation and provided definitive evidence of a role for CFTR in anion secretion. CFTR activation increased the initial magnitude of Cl- secretion, and provided a more sustained secretory response compared to conditions where only Ca2+-activated Cl- channels were activated by UTP. Measurements of [cAMP]i following UTP and PMA stimulation were not significantly different than untreated controls. Thus, these results demonstrate that UTP and PMA activation of CFTR occurs independently of increases in intracellular cAMP and extend the findings of earlier studies of CFTR regulation by PKC in Xenopus oocytes to a mammalian anion secreting epithelium.

Beta-adrenergic stimulation does not activate Na+/Ca2+ exchange current in guinea pig, mouse, and rat ventricular myocytes

The effect of beta-adrenergic stimulation on cardiac Na(+)/Ca(2+) exchange has been controversial. To clarify the effect, we measured Na(+)/Ca(2+) exchange current (I(NCX)) in voltage-clamped guinea pig, mouse, and rat ventricular cells. When I(NCX) was defined as a 5 mM Ni(2+)-sensitive current in guinea pig ventricular myocytes, 1 microM isoproterenol apparently augmented I(NCX) by approximately 32%. However, this increase was probably due to contamination of the cAMP-dependent Cl(-) current (CFTR-Cl(-) current, I(CFTR-Cl)), because Ni(2+) inhibited the activation of I(CFTR-Cl) by 1 microM isoproterenol with a half-maximum concentration of 0.5 mM under conditions where I(NCX) was suppressed. Five or ten millimolar Ni(2+) did not inhibit I(CFTR-Cl) activated by 10 microM forskolin, an activator of adenylate cyclase, suggesting that Ni(2+) acted upstream of adenylate cyclase in the beta-adrenergic signaling pathway. Furthermore, in a low-extracellular Cl(-) bath solution, 1 microM isoproterenol did not significantly alter the amplitude of Ni(2+)-sensitive I(NCX) at +50 mV, which is close to the reversal potential of I(CFTR-Cl). No change in I(NCX) amplitude was induced by 10 microM forskolin. When I(NCX) was activated by extracellular Ca(2+), it was not significantly affected by 1 microM isoproterenol in guinea pig, mouse, or rat ventricular cells. We concluded that beta-adrenergic stimulation does not have significant effects on I(NCX) in guinea pig, mouse, or rat ventricular myocytes.

Functional role of anion channels in cardiac diseases

In comparison to cation (K+, Na+, and Ca2+) channels, much less is currently known about the functional role of anion (Cl-) channels in cardiovascular physiology and pathophysiology. Over the past 15 years, various types of Cl- currents have been recorded in cardiac cells from different species including humans. All cardiac Cl- channels described to date may be encoded by five different Cl- channel genes: the PKA- and PKC-activated cystic fibrosis tansmembrane conductance regulator (CFTR), the volume-regulated ClC-2 and ClC-3, and the Ca2+-activated CLCA or Bestrophin. Recent studies using multiple approaches to examine the functional role of Cl- channels in the context of health and disease have demonstrated that Cl- channels might contribute to: 1) arrhythmogenesis in myocardial injury; 2) cardiac ischemic preconditioning; and 3) the adaptive remodeling of the heart during myocardial hypertrophy and heart failure. Therefore, anion channels represent very attractive novel targets for therapeutic approaches to the treatment of heart diseases. Recent evidence suggests that Cl- channels, like cation channels, might function as a multiprotein complex or functional module. In the post-genome era, the emergence of functional proteomics has necessitated a new paradigm shift to the structural and functional assessment of integrated Cl- channel multiprotein complexes in the heart, which could provide new insight into our understanding of the underlying mechanisms responsible for heart disease and protection.

S100A9 deficiency alters adenosine-5'-triphosphate induced calcium signalling but does not generally interfere with calcium and zinc homeostasis in murine neutrophils

The two calcium- and zinc-binding proteins, S100A9 and S100 A8, abundant in myeloid cells are considered to play important roles in both calcium signalling and zinc homeostasis. Polymorphonuclear neutrophils from S100A9 ko mice are also devoid of S100A8. Therefore, S100A9-deficient neutrophils were used as a model to study the role of the two S100 proteins in the neutrophils's calcium and zinc metabolism. Analysis of the intracellular zinc level upon pyrithione and (+/-)-(E)-methyl-2-[(E)-hydroxyimino]-5-nitro-6-methoxy-3-hexeneamide (NOR-1) treatment revealed no differences between S100A9-deficient and wildtype neutrophils. Similar, the calcium signals were not distinguishable from S100A9-deficient and wildtype neutrophils upon stimulation with platelet activating factor (PAF), thapsigargin or macrophage inflammatory protein 1 alpha (MIP-1 alpha), indicating despite their massive expression S100A8/A9 do neither serve as calcium nor as zinc buffering proteins in granulocytes. In contrast, stimulation with adenosine-5'-triphosphate (ATP) induces a significant stronger increase of the intracellular free calcium level in S100A9-deficient cells compared to wildtype cells. Moreover, the ATP-induced calcium signal was still different when the cells were incubated in calcium free buffer suggesting that pirinergic receptors of the P(2Y) class could be involved in this signalling pathway.

Inhibition of Na+-K+ pump and L-type Ca2+ channel by glibenclamide in Guinea pig ventricular myocytes

Glibenclamide, a potent cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel blocker, is frequently used to study function and regulation of CFTR Cl(-) channels. In this study, the effects of glibenclamide on intracellular Na(+) concentration ([Na(+)](i)), contraction, Ca(2+) transient, and membrane potential were investigated in isolated guinea pig ventricular myocytes. Glibenclamide increased [Na(+)](i) and decreased contraction and Ca(2+) transient. However, glibenclamide did not change membrane potential. To determine whether inhibition of Na(+)-K(+) pumps and L-type Ca(2+) channels is responsible for the increase of [Na(+)](i) and the decrease of contraction, we tested the effects of glibenclamide on Na(+)-K(+) pump current and L-type Ca(2+) current (I(Ca,L)). Glibenclamide decreased Na(+)-K(+) pump current and I(Ca,L) in a concentration-dependent manner. In the presence of Cl(-) channel inhibitors, glibenclamide depolarized diastolic membrane potential and reduced action potential duration. This result suggests that the reason for lack of effect of glibenclamide on membrane potential might be due to its combined inhibitory effects on the Na(+)-K(+) pump, the L-type Ca(2+) channel, and Cl(-) channels, which may have opposing effects on membrane potential. These results indicate that glibenclamide increases [Na(+)(i)] by inhibiting the Na(+)-K(+) pump and decreases contraction and Ca(2+) transient, in addition, by blocking the L-type Ca(2+) channel.