Histamine Activates the Chloride Current in Cardiac Ventricular Myocytes

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.

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.

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.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Nitric oxide synthase activity in guinea pig ventricular myocytes is not involved in muscarinic inhibition of cAMP-regulated ion channels

It has recently been demonstrated that NO plays an obligatory role in muscarinic inhibition of beta-adrenergically stimulated ion channels in cardiac sinoatrial node cells (J Gen Physiol. 1995;106:45-65). We looked for evidence that NO might play a similar role in ventricular cells by using histochemical staining for NO synthase (NOS) activity and whole-cell patch-clamp recording of cAMP-regulated Cl- currents. Myocytes isolated from guinea pig hearts stained positively for NADPH-diaphorase activity, suggesting that these cells do express NOS. Acetylcholine (ACh) inhibition of the R(-)-isoproterenol bitartrate (Iso)-activated Cl- current was also reversed by the cGMP-lowering agents LY-83583 and methylene blue, consistent with idea that NO activation of guanylate cyclase may contribute to muscarinic responses. However, LY-83583 and methylene blue activated the Cl- current in the presence of subthreshold concentrations of Iso alone, suggesting that their effects may not be due to antagonism of an NO/cGMP-dependent response. Furthermore, ACh inhibition of Iso-activated Cl- currents could not be mimicked by the NO donors sodium nitroprusside,3-morpholinosydnonimine, and spermine-NO. Similarly, ACh inhibition of the Iso-activated Cl- current could not be blocked by the NOS inhibitor NG-monomethyl-L-arginine. These results indicate that even though ventricular myocytes possess NOS activity, NO production does not play an important role in muscarinic inhibition of beta-adrenergically regulated Cl- channels in these cells.

Properties of single outwardly rectifying Cl- channels in heart

A variety of potentially important macroscopic Cl- currents have been described in the heart. Although the single-channel properties of the cAMP-dependent current (ICl.cAMP) have been well described, the single-channel equivalents of the other forms of cardiac Cl- current remain unknown. Unlike ICl.cAMP, many of these currents show prominent outward rectification in the presence of symmetrical transmembrane Cl- gradients and sensitivity to disulfonic stilbene Cl- transport blockers. We used the patch-clamp technique to search for single Cl- channels in inside-out patches from rabbit atrial cell membranes, under conditions minimizing the chances of observing channels carrying Na+, Ca2+, or K+. Under symmetrical Cl- conditions, single-channel activity was seen in 14 (9%) of 155 patches. Channels showed strong outward rectification and a unitary conductance of 60 +/- 3 picosiemens (mean +/- SEM) at positive voltages. The current-voltage relation was not altered by replacement of cations by the impermeant cation N'-methyl-D-glucamine (NMDG) and shifted as expected for a Cl(-)-selective channel when methanesulfonate was substituted for Cl-. The Cl- transport blockers DIDS (diisothiocyanatostilbene-2,2'-disulfonic acid, 100 mumol/L) and SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid, 1 mmol/L) strongly and reversibly inhibited channel activity when added to the bath and caused channel flickering suggesting open-channel block. Ensemble-average currents showed no time dependence, and the form of the ensemble-average current-voltage relation was similar to that of macroscopic background Cl- current.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of the cardiac protein kinase A-dependent chloride conductance by endothelin-1

Endothelin-1 is a peptide hormone constitutively secreted by vascular and endocardial endothelial cells. Secretion of endothelin-1 is increased under certain pathophysiological conditions, including coronary vasospasm, cardiac ischaemia and myocardial infarction. We have examined the effect of endothelin-1 on the protein kinase A (PKA)-dependent chloride current in voltage-clamped guinea pig ventricular myocytes. This conductance, induced by catecholamines through beta-adrenergic receptors, counteracts the simultaneously increased L-type calcium current by shortening the action potential duration. We report here that endothelin-1, acting through ETA (endothelin-1-selective) receptors, inhibited the current through a pertussis toxin-sensitive mechanism, analogous to muscarinic receptors, by reducing the intracellular cyclic AMP concentration. This effect of endothelin-1 should help protect the ventricle against potentially arrhythmogenic shortening of the action potential during ischaemia when the circulating levels of catecholamines are increased.

Tetramethylammonium activation of muscarinic receptors in cardiac ventricular myocytes

Replacement of extracellular Na+ with tetramethylammonium (TMA) reduces the magnitude of the Cl- current activated by beta-adrenergic receptor stimulation in guinea pig ventricular myocytes. However, the effects of replacing Na+ appear to be associated with the presence of TMA, rather than the absence of Na+. Direct addition of TMA to extracellular solutions, without changing the Na+ concentration, was able to inhibit the Cl- current activated by isoproterenol (Iso) in a concentration-dependent manner. The concentration of TMA that caused half-maximal inhibition was 327 microM when the Cl- current was activated by 1 microM Iso and 29 microM when the Cl- current was activated by 0.03 microM Iso. The effect of TMA was also blocked by atropine, suggesting that TMA exerts its effect through stimulation of the muscarinic receptors. Furthermore, TMA inhibited the Iso-activated Ca2+ current, as would be expected for an effect involving muscarinic receptor stimulation. The response to complete Na+ replacement with TMA could not be overcome by increasing the concentration of Iso 1,000-fold, and direct addition of TMA was able to antagonize the Cl- current activated independently of the beta-adrenergic receptor, using forskolin and histamine. These results are consistent with the hypothesis that TMA does not exert its effects through a competitive mechanism at the beta-adrenergic receptor. It is concluded that TMA is able to antagonize adenosine 3',5'-cyclic monophosphate-dependent activation of ion channels in the heart through activation of muscarinic receptors.

Anion and cation modulation of the guinea-pig ventricular action potential during beta-adrenoceptor stimulation

Modulation of the ventricular action potential by beta-adrenergic activation of Ca2+, K+ and cyclic adenosine monophosphate (cAMP)-dependent Cl- channels was assessed in enzymatically isolated guinea-pig ventricular myocytes. The effectiveness and relative selectivity of 9-anthracene carboxylic acid (9-AC), as an antagonist of cAMP-dependent Cl- channels was also tested. Membrane currents and action potentials were recorded using the conventional whole-cell variant of the patch-clamp technique or with the amphotericin B perforated-patch technique. The beta-adrenergic agonist isoproterenol either increased or decreased action potential duration depending on whether the dominant effect was on inward Ca2+ currents or on outward K+ or Cl- currents. When Ca2+ and K+ channel modulation was prevented by nisoldipine and low temperature respectively, beta-adrenergic activation of Cl- channels caused a significant reduction in action potential duration and a slight depolarization of the membrane potential. The beta-adrenergic-mediated effects were reversed by the Cl- channel blocker, 9-AC. In the absence of beta-adrenergic stimulation, 9-AC had no detectable effects on action potentials or Ca2+ currents. These results suggest that beta-adrenergic activation of Cl- channels is a potent mechanism for regulation of action potential duration and that 9-AC may be a useful, relatively specific, pharmacological tool for evaluating the physiological role of cAMP-activated Cl- channels in heart. 9-AC also reversed the ability of isoproterenol to antagonize prolongation of action potential duration by the class III antiarrhythmic agent E-4031.

Modulation by histamine of the delayed outward potassium current in guinea-pig ventricular myocytes

1. Histamine receptor-mediated modulation of the delayed outward potassium current (IK) was investigated in guinea-pig single ventricular cells by the whole-cell voltage clamp. 2. Histamine increased IK in a dose- dependent manner with a half-maximum dose of 3.8 x 10(-8) M. Histamine (10(-6) M) increased IK by a factor of 3.02 without a significant change in the current kinetics. The threshold dose of histamine for increasing IK was 10(-9) M and this value was similar to that for calcium current. 3. Cimetidine decreased IK in the presence of histamine, by shifting the dose-response curve to histamine to the right. The pA2 value of cimetidine against histamine was 6.38. 4. Forskolin did not increase IK after application of 10(-6) M histamine, and histamine scarcely increased IK in the presence of a heat-stable inhibitor of cyclic AMP-dependent protein kinase (PKI). 5. We conclude that stimulation by histamine of IK is mainly by way of the H2-receptor, and is mediated by cyclic AMP-dependent phosphorylation.

Arrhythmogenic effects of isoproterenol-activated Cl- current in guinea-pig ventricular myocytes

Possible arrhythmogenic effects of the isoproterenol-activated Cl- current were examined in isolated guinea-pig ventricular myocytes under various intra- and extracellular Cl- concentrations. Experiments were carried out with external K+ concentration ([K+]o) decreased to 2 or 3 mM. Under symmetrical concentrations of Cl- in intra- and extra-cellular solutions (ECl = 0 mV), 1 microM isoproterenol (ISP) depolarized resting membrane potential (RMP) by 6.2 +/- 1.1 mV and slowed repolarization with induction of early afterdepolarizations (EADs) in 9 out of 9 cells. EADs appeared at voltages positive to -40 mV, where L-type Ca2+ current is assumed to be activated. When Cl- concentrations were settled near physiological conditions (ECl = -40 - -50 mV), ISP depolarized RMP by 2.8 +/- 0.4 mV and elicited abnormal repolarization with occasional EADs in 6 out of 19 cells. When ECl was set to -80 mV, however, ISP depolarized RMP by only 0.5 +/- 0.5 mV without induction of abnormal activities. Thus, depolarizing effects of ISP and incidence of repolarization abnormalities including EADs were increased as ECl shifted to more positive potential levels. At [K+]o = 4 mM, no abnormal activities were observed when ECl was around -50 mV (0/8), and 6 out of 6 cells showed abnormal activities when ECl was set to 0 mV. ISP-elicited abnormal activities were abolished by 1 mM DNDS (4,4'-dinitrostilbene-2,2'-disulphonic acid), a blocker for Cl- channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of chloride current by purinergic stimulation in guinea pig heart cells

Single atrial cells from guinea pig heart were voltage-clamped using the whole-cell configuration of the patch-clamp technique under conditions in which most of the ionic and exchange currents known in cardiac cells were minimized. Extracellular 5 or 50 microM ATP activated a Cl- current, in addition to a rapidly desensitizing cation-selective current. A nonhydrolyzable ATP analogue, adenosine-5'-O-(3-thiotriphosphate) (50 microM), also evoked these two currents, indicating involvement of purinoceptors rather than ecto-ATPase on the membrane. ADP, AMP, and adenosine were also effective in inducing the Cl- current, showing no clear order of potency for the purinoceptor subtypes involved. The purinoceptor-activated Cl- current, like the beta-catecholamine-cAMP-dependent cardiac Cl- current, showed outward rectification and time independence.

Chloride conductance pathways in heart

Nonelectrogenic movement of Cl- is believed to be responsible for the active accumulation of intracellular Cl- in cardiac muscle. The electro-neutral pathways underlying this nonpassive distribution of Cl- are believed to include Cl(-)-HCO3- exchange, Na(+)-dependent cotransport (operating as Na(+)-Cl- and Na(+)-K(+)-2Cl- cotransport), and K(+)-Cl- cotransport. The electrogenic movement of Cl- in cardiac muscle is particularly interesting from a historical perspective. Until recently, there was some doubt as to whether Cl- carried any current in the heart. Early microelectrode experiments indicated that a Cl- conductance probably played an important role in regulating action potential duration and resting membrane potential. Subsequent voltage-clamp experiments identified a repolarizing, transient outward current that was believed to be conducted by Cl-, yet further investigation suggested that this transient outward current was more likely a K+ current, not a Cl- current. This left some doubt as to whether Cl- played any role in regulating membrane potential in cardiac muscle. More recent studies, however, have identified a highly selective Cl- conductance that is regulated by intracellular adenosine 3',5'-cyclic monophosphate, and it appears that this Cl- current may play an important role in the regulation of action potential duration and resting membrane potential.