Single-channel study of the cyclic AMP-regulated chloride current in guinea-pig ventricular myocytes

Regulation of CFTR chloride channel macroscopic conductance by extracellular bicarbonate

The CFTR contributes to Cl⁻ and HCO₃⁻ transport across epithelial cell apical membranes. The extracellular face of CFTR is exposed to varying concentrations of Cl⁻ and HCO₃⁻ in epithelial tissues, and there is evidence that CFTR is sensitive to changes in extracellular anion concentrations. Here we present functional evidence that extracellular Cl⁻ and HCO₃⁻ regulate anion conduction in open CFTR channels. Using cell-attached and inside-out patch-clamp recordings from constitutively active mutant E1371Q-CFTR channels, we show that voltage-dependent inhibition of CFTR currents in intact cells is significantly stronger when the extracellular solution contains HCO₃⁻ than when it contains Cl⁻. This difference appears to reflect differences in the ability of extracellular HCO₃⁻ and Cl⁻ to interact with and repel intracellular blocking anions from the pore. Strong block by endogenous cytosolic anions leading to reduced CFTR channel currents in intact cells occurs at physiologically relevant HCO₃⁻ concentrations and membrane potentials and can result in up to ∼50% inhibition of current amplitude. We propose that channel block by cytosolic anions is a previously unrecognized, physiologically relevant mechanism of channel regulation that confers on CFTR channels sensitivity to different anions in the extracellular fluid. We further suggest that this anion sensitivity represents a feedback mechanism by which CFTR-dependent anion secretion could be regulated by the composition of the secretions themselves. Implications for the mechanism and regulation of CFTR-dependent secretion in epithelial tissues are discussed.

An outwardly rectifying chloride channel in human atrial cardiomyocytes

INTRODUCTION

Among a range of chloride channels, outwardly rectifying Cl- channels have been reported in the heart of various species. Although the anionic current carried by this channel has been subjected to intense electrophysiological investigations, paradoxically no examination of single-channel currents has been reported for human cardiomyocytes.

METHODS AND RESULTS

Using the cell-attached and cell-free configurations of the patch-clamp technique, we have characterized the properties of an outwardly rectifying chloride current (ORCC) at the unitary level in freshly isolated human atrial cardiomyocytes. In excised inside-out patches, the channel presented a nonlinear I/V relationship with a conductance of 76.5 +/- 14.7 pS in the positive voltage range and 8.1 +/- 2 pS in the negative voltage range, indicating an outward rectification. Preincubation with the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) significantly increased the number of spontaneously active channels observed. The channel was Cl- selective (Cl- to Na+ permeability ratio, PCl/PNa= 18) with the permeability sequence I- > Br- > Cl- > F- > gluconate. It was blocked by the classical Cl- channels blockers glibenclamide, NPPB, SITS, and DIDS. Channel activity was not dependent upon internal calcium concentration. In the cell-attached configuration, ORCC channel activation was observed under perfusion of a hypotonic solution.

CONCLUSION

Human atrial myocytes express an outwardly rectifying Cl- channel that is sensitive to PKC activation. This channel shares biophysical and pharmacological properties with the swelling-activated chloride current implicated in cardiac pathologies such as myocardial ischemia and dilated cardiopathies.

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.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Control of CFTR channel gating by phosphorylation and nucleotide hydrolysis

Control of CTFR Channel Gating by Phosphorylation and Nucleotide Hydrolysis. Physiol. Rev. 79, Suppl.: S77-S107, 1999. - The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is the protein product of the gene defective in cystic fibrosis, the most common lethal genetic disease among Caucasians. Unlike any other known ion channel, CFTR belongs to the ATP-binding cassette superfamily of transporters and, like all other family members, CFTR includes two cytoplasmic nucleotide-binding domains (NBDs), both of which bind and hydrolyze ATP. It appears that in a single open-close gating cycle, an individual CFTR channel hydrolyzes one ATP molecule at the NH2-terminal NBD to open the channel, and then binds and hydrolyzes a second ATP molecule at the COOH-terminal NBD to close the channel. This complex coordinated behavior of the two NBDs is orchestrated by multiple protein kinase A-dependent phosphorylation events, at least some of which occur within the third large cytoplasmic domain, called the regulatory domain. Two or more kinds of protein phosphatases selectively dephosphorylate distinct sites. Under appropriately controlled conditions of progressive phosphorylation or dephosphorylation, three functionally different phosphoforms of a single CFTR channel can be distinguished on the basis of channel opening and closing kinetics. Recording single CFTR channel currents affords an unprecedented opportunity to reproducibly examine, and manipulate, individual ATP hydrolysis cycles in a single molecule, in its natural environment, in real time.

Phosphatase-mediated enhancement of cardiac cAMP-activated Cl-conductance by a Cl- channel blocker, anthracene-9-carboxylate

An aromatic carboxylate, anthracene-9-carboxylic acid (9-AC), is known as a Cl- channel blocker. However, variable 9-AC effects have hitherto been reported on the cardiac cAMP-activated Cl- conductance, when applied extracellularly. We have reexamined the 9-AC effect on the Cl-conductance activated by isoproterenol or forskolin in guinea pig ventricular myocytes under whole-cell patch-clamp conditions. The inward current was blocked by 9-AC at > or = 0.5 mmol/L, but in contrast, the outward current was enhanced at much lower concentrations (ED50, approximately 13 mumol/L). 9-AC applied by the intracellular perfusion technique increased both the inward and outward currents. In the presence of intracellular 9-AC, deactivation of the conductance after washout of isoproterenol or forskolin was largely prevented. 9-AC produced an enhancing effect, even after inhibiting the deactivation process by okadaic acid (OA), whereas it failed to produce additional-effects in the presence of orthovanadate. Intracellular application of 9-AC together with OA virtually abolished the current deactivation. The 9-AC effects on the Cl-conductance were not dependent on intracellular Ca2+ or pH. Putative inhibitors of alkaline (bromotetramisole) and acid phosphatases (tartrate) were without effect. 9-AC failed to inhibit the activities of purified protein phosphatase (PP)-1, -2A, and -2C. In the extract of guinea pig ventricle, 9-AC (> or = 10 mumol/L for full action) significantly inhibited a fraction of endogenous phosphatase activity that was sensitive to orthovanadate but not to OA, bromotetramisole, and tartrate. It is concluded that 9-AC blocks cardiac cAMP-activated (cystic fibrosis transmembrane conductance regulator) Cl- conductance from the extracellular side but enhances the conductance from the intracellular side by inhibiting an orthovanadate-sensitive phosphatase distinct from PP-1, -2A, -2B, or -2C and alkaline or acid phosphatase.

Purification of the CIC-0 chloride channel from Torpedo california electroplax identification of a phosphorylation site for cAMP-dependent protein kinase

The voltage-gated chloride channel (CIC-0) from the electric organ of Torpedo californica was purified by immunoaffinity chromatography. A polyclonal antibody was shown to specifically recognize the CIC-0 channel (M(r) 85,000-90,000) in a Western blot of total membrane proteins. As monitored by immunoprecipitation, the formation of antibody-antigen complexes in solution strongly depends on the detergent composition. The highest yield of precipitated CIC-0 was obtained from an incubation mixture containing both an anionic detergent, cholate or lauryl sulfate, and the zwitterionic detergent CHAPS. In contrast, immuno-precipitation of CIC-0 was largely reduced when cholate was exchanged for the nonionic detergent Triton x-100, suggesting that the efficient formation of immuno-complexes is favored by negatively charged detergent. In initial immunopurification experiments, in addition to CIC-0 a major contaminating polypeptide of M(r) approximately 115,000 was copurified, which represents the SITS-binding protein [Jentsch et al. (1989) Biochem. J. 261, 155]. Purification of CIC-0 could be increased from about 35% up to 60% homogeneity when immunoaffinity chromatography was performed in the presence of N-acetylglucosamine. Therefore the highly glycosylated SITS-binding protein most likely interacts with the CIC-0 protein via its carbohydrate parts. The purified CIC-0 channel was found to be phosphorylated by PKA in vitro to a level of 0.35-0.4 mol of phosphate incorporated per mol of CIC-0. Proteolytic digestion with endoproteinase GluC and HPLC-separation revealed two major phosphopeptides, which could be identified by amino acid sequence analysis as different size fragments of the same consensus phosphorylation site. Comparison of the peptide sequences with the deduced protein sequence of CIC-0 [Jentsch et al. (1990) Nature 348, 510; O'Neill et al. (1991) Biochem. Biophys. Acta 1129, 131] indicates serine 600 as the phosphorylated residue. Therefore, our results provide strong evidence that CIC-0 is phosphorylated at this single site by PKA in vitro.

Phorbol ester activation of chloride current in guinea-pig ventricular myocytes

1. Although earlier studies with phorbol esters indicate that protein kinase C (PKC) may be an important regulator of Cl- current (Icl) in cardiac cells, there is a need for additional quantitative data and investigation of conflicting findings. Our objectives were to measure the magnitude, time course, and concentration-dependence of Icl activated in guinea-pig ventricular myocytes by phorbol 12-myristate 13-acetate (PMA), evaluate its PKC dependence, and examine its modification by external and internal ions. 2. The whole-cell patch clamp technique was used to apply short depolarizing and hyperpolarizing pulses to myocytes superfused with Na(+)-, K(+)-, Ca(2+)-free solution (36 degrees C) and dialysed with Cs+ solution. Stimulation of membrane currents by PMA (threshold < or = 1nM, EC50 approximately equal to 14 nM, maximal 40% increase with > or = 100 nM) plateaued within 6-10 min. 3. PMA-activated current was time-independent, and suppressed by l mM 9-anthracenecarboxylic acid (9-AC). Its reversal potential (Erev) was sensitive to changes in the Cl- gradient, and outward rectification of the current-voltage (I-V) relationship was more pronounced with 30 mM than 140 mM Cl- dialysate. 4. The relative permeability of PMA-activated channels estimated from Erev measurements was I- > Cl- > > aspartate. Channel activation was independent of external Na+. 5. PMA failed to activate Icl in myocytes pretreated with 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) or dialysed with pCa 10.5 solution. Lack of response to 4 alpha-phorbol 12, 13-didecanoate (alpha PDD) was a further indication of mediation by PKC. 6. Icl induced by 2 microM forskolin was far larger than that induced by PMA, suggesting that endogenous protein kinase A is a much stronger Cl- channel activator than endogenous PKC in these myocytes. 7. The macroscopic properties of PMA-induced Icl appear to be indistinguishable from those of PKA-activated Icl. We discount stimulation of PKA by PMA as an explanation, and conclude that endogenous PKC may activate PKA-regulated Cl- channels in these myocytes.

alpha-Adrenergic inhibition of the beta-adrenoceptor-dependent chloride current in guinea-pig ventricular myocytes

1. alpha 1-Adrenoceptor-mediated inhibition of the beta-adrenoceptor-dependent Cl- current was investigated in guinea-pig ventricular myocytes using the patch clamp technique. The Cl- conductance activated by noradrenaline (0.1-10 microM) with an alpha 1-blocker (prazosin, 5 microM) was significantly greater than that activated by noradrenaline alone. Phenylephrine and methoxamine, alpha 1-agonists, exerted an inhibitory effect on the Cl- conductance activated by isoprenaline. The dose-response relationship for isoprenaline and the Cl- current activation was shifted to higher doses in the presence of phenylephrine (30 microM). 2. The interaction of alpha 1- and beta-agonists on Cl- current was also observed on the single channel level; in some of the outside-out membrane patches, phenylephrine (50 microM) depressed the activity of the single Cl- channel which was induced by 5 microM adrenaline. 3. Phenylephrine had no effect on the Cl- conductance induced by forskolin (0.5-5 microM), an activator of adenylate cyclase. The Cl- conductance activated persistently by isoprenaline in GTP gamma S-loaded cells was also insensitive to phenylephrine. The results suggest that the observed alpha 1-adrenergic attenuation of the beta-adrenergic response is not primarily due to inhibition of adenylate cyclase activity. The alpha 1-adrenergic action may interfere with the processes leading to enzyme activation in the beta-adrenergic pathway.

Activation of the plasma membrane chloride channel by protein kinase C in isolated guinea-pig hepatocytes

1. To assess the nature of the underlying mechanism of noradrenaline-induced increase of Cl- conductances in hepatocytes, macroscopic and unitary currents through noradrenaline-induced Cl- channels were examined in enzymatically isolated guinea-pig hepatocytes using whole-cell, cell-attached and excised inside-out configurations of the patch-clamp technique. 2. When K+ conductances were blocked and the intracellular Ca2+ concentration ([Ca2+]i) was set at 0.1 microM, bath application of noradrenaline activated the time-independent membrane currents under whole-cell voltage-clamp conditions. The current was similarly activated by phorbol ester (PMA), an activator of protein kinase C (PKC), while a specific protein kinase C inhibitor, H-9, reversed PMA activation of the current. The inactive phorbol ester, 4 alpha-phorbol 12-myristate, 13-acetate (alpha PMA), failed to activate the channel. 3. The reversal potential of the PMA-activated current shifted by approximately 60 mV per 10-fold change in the external Cl- concentration, indicating that the current was Cl- selective. Bath application of 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) partially inhibited both the noradrenaline- and PMA-induced currents. 4. In single channel recordings from cell-attached patches, bath application of noradrenaline or PMA induced unitary current activity, the averaged slope conductance of which was 10.1 +/- 1.5 pS (mean +/- S.D.; n = 12) in the noradrenaline-induced current and 9.7 +/- 1.3 pS (n = 7) in the PMA-induced current. The open time distribution was moderately well fitted by a single exponential function with mean open lifetime of 88.5 +/- 10.6 ms (n = 10), while at least two exponentials were required to fit the closed time distributions with a time constant for the fast component of 24.4 +/- 5.8 ms (n = 10) and for the slow component of 316.9 +/- 49.2 ms (n = 10). 5. Bath application of purified PKC to excised inside-out patches activated the channel. The PKC selective inhibitor, PKC(19-36), and DIDS inhibited the PKC-activated channel. 6. These results suggest that PKC can phosphorylate the channel protein or a related structure leading to the activation of Cl- channels in guinea-pig hepatocytes.

Rectification of whole cell cystic fibrosis transmembrane conductance regulator chloride current

Whole cell epithelial cystic fibrosis transmembrane conductance regulator (CFTR) Cl- currents exhibited a linear current-voltage (I-V) relationship with high symmetrical transmembrane Cl- concentrations. However, when intracellular Cl- (Cli-) was reduced by replacement with glutamate, I-V relationships were outwardly rectifying. Rectification was not affected by reducing extracellular Cl- to eliminate or reverse the gradient, indicating that rectification is not a function of the Cl- gradient. Rectification was affected by Cli- in a concentration-dependent manner, and it was weaker when Cli- was reduced by replacement with sucrose. These characteristics are identical to those of the cardiac isoform of CFTR, and the experimental data could be simulated by an Eyring rate theory model assuming that permeating anions interact at a single binding site within the channel pore. No evidence was found for multiple binding sites. These results indicate that rectification is a function of the concentration and permeability of the anions inside the cell. It is concluded that rectification of CFTR Cl- current is a property of ion channel permeation that would occur under physiological conditions and that permeation of the epithelial and cardiac isoforms of CFTR is identical.

Unitary chloride channels activated by protein kinase C in guinea pig ventricular myocytes

Recent evidence suggests that protein kinase A (PKA)-activated Cl- channels in heart are encoded by an isoform of the epithelial cystic fibrosis transmembrane conductance regulator gene (CFTR). Macroscopic current measurements indicate that a similar time-independent Cl- conductance can be activated through a protein kinase C (PKC)-dependent pathway in guinea pig and feline ventricle. However, it is presently not clear whether PKC is activating the same population of channels as PKA or a separate class of Cl- channels. even though the regulatory (R) domain of CFTR is known to contain consensus phosphorylation sites for both PKA and PKC. In the present study we directly compare the properties of single Cl- channels activated by PKC and PKA in cell-attached patches of guinea pig ventricular myocytes. Pipette and bath solutions contained N-methyl-D-glucamine and Cs+ or tetraethylammonium as substitutes for Na+ and K+, respectively, and Cl- concentration in the patch pipette was either 150 mmol/L or 40 mmol/L. Bath application of phorbol 12,13-dibutyrate or phorbol 12-myristate 13-acetate (PDBu or PMA; 50 nmol/L), activators of PKC, resulted in the appearance of unitary Cl- channels with a mean conductance of 9.31 +/- 0.94 pS (n = 8) and 8.8 pS (n = 2), respectively, and reversal potentials were close to predicted ECl. Addition of staurosporine (500 nmol/L) reduced open probability (Po) of channels activated by PDBu. Bath application of the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX, 500 mumol/L) resulted in the activation of Cl- channels with a conductance (mean 8.76 +/- 0.67 pS, n = 3) similar to those activated by PDBu.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Regulation of the gating of cystic fibrosis transmembrane conductance regulator C1 channels by phosphorylation and ATP hydrolysis

Opening of cystic fibrosis transmembrane conductance regulator (CFTR) Cl channels requires their phosphorylation by protein kinase A followed by exposure to ATP. We examined the interaction between nucleotides and phosphorylated CFTR channels by recording currents in intact cardiac myocytes and in excised patches. We found that, although the hydrolysis-resistant ATP analogue 5'-adenosine(beta,gamma- imino)triphosphate (AMP-PNP) cannot open phosphorylated CFTR channels, it can cause channels opened by ATP to remain open for many minutes. This suggests that ATP action at one site on CFTR is a prerequisite for AMP-PNP action at a second site. However, this action of AMP-PNP is restricted to highly phosphorylated CFTR channels, which, in the presence of ATP, display a relatively high open probability, but is not seen in partially phosphorylated CFTR channels, which have a low open probability in the presence of ATP. Our findings argue that incremental phosphorylation differentially regulates the interactions between nucleotides and the two nucleotide binding domains of CFTR. The nature of those interactions suggests that ATP hydrolysis at one nucleotide binding domain controls channel opening and ATP hydrolysis at the other regulates channel closing.

Coupling of CFTR Cl- channel gating to an ATP hydrolysis cycle

For cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels to open, they must be phosphorylated by protein kinase A and then exposed to a hydrolyzable nucleoside triphosphate, such as ATP. To test whether channel opening is linked to ATP hydrolysis, we applied VO4 and BeF3 to CFTR channels in inside-out patches excised from cardiac myocytes. These inorganic phosphate analogs interrupt ATP hydrolysis cycles by binding tightly in place of the released hydrolysis product, inorganic phosphate. The analogs acted only on CFTR channels opened by ATP and locked them open, increasing their mean open time by 2-3 orders of magnitude. These findings establish that opening and closing of CFTR channels are coupled to an ATP hydrolysis cycle.