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Results for “Chloride Channel Agonists”

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PubMed1998

Opposing effects of different steroid sulfates on GABAA receptor-mediated chloride uptake.

Steroids with the 3alpha-hydroxy-5alpha- or 5beta-reduced configurations of the A ring interact with the gamma-aminobutyric acid (GABA) type A receptor chloride channel complex and potentiate the stimulation of Cl- uptake by GABA agonists. Conversely, the sulfate esters of 3beta-hydroxy-5-ene neurosteroids pregnenolone and dehydroepiandrosterone behave as inhibitory modulators. In the present work, steroid sulfates were tested for their ability to modulate muscimol-induced chloride ion uptake into cortical synaptoneurosomes. 3alpha-Hydroxy-5alpha-pregnan-20-one sulfate and several other 3alpha-hydroxy-steroid sulfates potentiated, whereas 3beta-hydroxy-steroid sulfates inhibited muscimol effect. It is concluded that GABA-agonistic or antagonistic properties of steroid sulfates depend on the alpha or beta orientation of the sulfate moiety linked to the A ring.

Animals↗

PubMed2001

Possible involvement of cyclic adenosine monophosphate-independent mechanism in the positive chronotropic effect of norepinephrine in the isolated guinea pig right atrium.

BACKGROUND: Although both positive chronotropic and inotropic effects of beta-adrenergic stimulation are thought to be mediated by cyclic adenosine 3'5'-monophosphate, phosphodiesterase III inhibitors such as amrinone and milrinone potentiate the positive inotropic effect of catecholamines with minimum influence on the heart rate in clinical setting. The aim of the current study was to compare the positive chronotropic effect of norepinephrine with that of forskolin to elucidate whether cyclic adenosine monophosphate is relevant to the chronotropic effect of norepinephrine. METHODS: Concentration-response curves for the positive chronotropic effects of norepinephrine and forskolin on the spontaneously beating right atria of guinea pigs were determined in the absence and presence of phosphodiesterase inhibitors or ion channel inhibitors. In some experiments, the left atria driven electrically were used to determine the positive inotropic effect of norepinephrine. RESULTS: Norepinephrine and forskolin increased the beating rate in a concentration-dependent manner. The positive chronotropic effect of forskolin was potentiated by amrinone and 3-isobutyl-1-methylxanthine, whereas the positive chronotropic effect of norepinephrine was not potentiated by the phosphodiesterase inhibitors. In contrast, the positive inotropic effect of norepinephrine was potentiated by amrinone. The hyperpolarization-activated inward current inhibitor cesium chloride and L-type voltage-dependent Ca2+ current inhibitor verapamil suppressed the chronotropic effect of norepinephrine, whereas these inhibitors did not affect the chronotropic effect of forskolin. CONCLUSION: Norepinephrine increases the spontaneously beating rate by a different mechanism from that of forskolin, suggesting that cyclic adenosine monophosphate is causally unrelated to the positive chronotropic effect of norepinephrine in the guinea pig heart.

Adrenergic alpha-Agonists↗

PubMed2000

The interrelationship between chloride ions and endothelium on alpha(1)-adrenoceptor-mediated contractions in aortic rings from Dahl normotensive and hypertensive rats.

OBJECTIVES: The effects of chloride-free buffer in the absence or presence of the nitric oxide synthase inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME), or chloride channel antagonist, indanyloxyacetic acid 94 (IAA-94) on alpha(1)-adrenoceptor mediated contractions were investigated in aortic rings from Dahl salt-resistant normotensive (SRN) and salt-sensitive hypertensive (SSH) rats on a 4% salt diet. METHODS: Systolic and diastolic blood pressure were measured via intra-arterial catheters under halothane anesthesia. Subsequently, the aorta was removed and cirazoline-induced contractions were recorded in normal Krebs and chloride-free buffer using ring preparation. Guanosine 3',5'-cyclic monophosphate (cyclic GMP) content of aortic rings was also determined by scintillation proximity assay kits. RESULTS: Systolic and diastolic blood pressure of SSH (180/130+/-1/1, mean+/-S.E.; n=14) were significantly higher than those of SRN (101/76+/-1/1, mean+/-S.E.; n=14) 7 weeks after they were placed on a salt diet. While the presence of L-NAME failed to have any impact on cirazoline-induced contractions in aortic rings from SSH rats, it significantly accentuated the effects of cirazoline in tissues from SRN rats. On the other hand, IAA-94 was able to inhibit cirazoline-mediated contractions in aortic rings from both SRN and SSH rats. The removal of chloride ions potentiated contractions produced by cirazoline in tissues from SRN but not SSH rats. Moreover, cirazoline-evoked responses in tissues from SRN were not further accentuated by the inclusion of L-NAME in chloride-free buffer. Cirazoline-mediated contractions in tissues from SSH rats were not influenced by absence of chloride ions, and the presence of L-NAME. It was also apparent that the inclusion of IAA-94 in absence of chloride ions did not prevent the potentiation of responses to cirazoline. Removal of chloride ions did not significantly decrease basal cyclic GMP levels in aortic rings from either strain. CONCLUSIONS: Basal release of nitric oxide seems to make a greater contribution in the suppression of cirazoline-evoked contractions in vessels from SRN as opposed to SSH rats. Chloride channels appear to contribute to cirazoline-evoked contractions in normal Krebs but not in chloride-free buffer.

Adrenergic beta-Agonists↗

PubMed1998

Chloride transport in a human submandibular gland cell line studied by X-ray microanalysis.

Ion transport properties of the human submandibular gland cell line A253 were investigated by X-ray microanalysis. The cells were grown on ultrathin plastic films on titanium grids. After the experiment, where the cells were exposed to various agonists and inhibitors, the specimens were briefly rinsed in distilled water, frozen, and freeze-dried. The elemental composition of the cultured cells most closely resembled that of striated duct cells. Beta-adrenergic stimulation caused a small decrease in the cellular Cl- and K+ concentration, but cholinergic stimulation had no effect. Also stimulation with cAMP caused a decrease in Cl- and K+; this decrease could be inhibited by the chloride channel blocker NPPB. Loss of Cl- and K+ could also be elicited by exposure to a hypotonic solution. The results suggest that there may be several types of chloride channel present in A253 cells.

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed1991

Chloride ion transport in transformed normal and cystic fibrosis epithelial cells.

The inability of beta-adrenergic agonists (eg. isoproterenol) to activate a specific apical membrane chloride channel in epithelial cells is characteristic of cystic fibrosis (CF). The study of these channels has been facilitated by the transformation of human airway epithelial cells. Clonal populations of SV40 large T-antigen transformed airway epithelial cells from both normal and cystic fibrosis individuals have been established. A limitation in the use of these cell lines has been the loss of the ability to form tight junctions after multiple subcultures. In particular, this loss appears to be associated with cell "crisis". A selection protocol that involves growing cells in medium that is high in Ca2+ and supplemented with fetal bovine serum (FBS) has been employed to facilitate progression through crisis. One cell line (1HAEo-) that has a normal phenotype and retains tight junctions post-crisis has been produced using this protocol. Immunofluorescent staining with a monoclonal antibody to the E-cadherin adhesion molecule shows a characteristic pericellular localization, indicating the presence of the junctional complex. The presence of tight junctions has been confirmed by electron microscopy. These cells produce elevated (greater than 30 fold) levels of cAMP in response to exposure to isoproterenol or forskolin. Chloride ion transport, as measured by 36Cl- efflux, is stimulated greater than 2 fold by these agents. Three post-crisis CF cell lines which grow in serum-containing medium have been established. These do not exhibit tight junctions. Elevated (greater than 25 fold) levels of cAMP are detected in these cells after addition of isoproterenol or forskolin, but this increase in cAMP is not accompanied by an increase in 36Cl-efflux. Both normal (1HAEo-) and CF cells show increased 36Cl-efflux following addition of the calcium ionophore A23187. (ABSTRACT TRUNCATED AT 250 WORDS)

Biological Transport, Active↗

PubMed1990

GABAA-receptors: structural requirements and sites of gene expression in mammalian brain.

GABAA-receptors, the major synaptic targets for the neurotransmitter GABA, are gated chloride channels. By their allosteric drug-induced modulation they serve as molecular control elements through which the levels of anxiety, vigilance, muscle tension and epileptiform activity can be regulated. Despite their functional prominence, the structural requirements of fully functional GABAA-receptors are still elusive. Expression of cDNAs coding for the alpha 1- beta 1-subunits of rat brain yielded GABA-gated chloride channels which were modulated by barbiturates but displayed only agonistic responses to ligands of the benzodiazepine receptor. GABAA-receptors with fully functional benzodiazepine receptor sites were formed when the alpha 1- and beta 1-subunits were co-expressed with the gamma 2-subunit of rat brain. These receptors, however, failed to show cooperativity of GABA in gating the channel. In order to determine the subunit repertoire available for receptor assembly in different neuronal populations in vivo, the sites of subunit gene expression were (alpha 1, alpha 2, alpha 3, alpha 5, alpha 6, beta 1, beta 2, beta 3, gamma 2) mapped by in situ hybridization histochemistry in brain sections. The mRNAs of the alpha 1-, beta 1- and gamma 2-subunits were co-localized e.g. in mitral cells of olfactory bulb, pyramidal cells of hippocampus as well as granule cells of dentate gyrus and cerebellum. The lack of colocalization in various other brain areas points to an extensive receptor heterogeneity. The presence of multiple GABAA-receptors in brain may contribute to synaptic plasticity, differential responsiveness of neurons to GABA and to variations in drug profiles.

Animals↗

PubMedDate not supplied

Chloride channel blockers 5-nitro-2-(3-phenylpropylamino) benzoic acid and anthracene-9-carboxylic acid inhibit contractions of pregnant rat myometrium in vitro.

OBJECTIVE: We compared in vitro relaxant effect of chloride channel modulators, such as 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and anthracene-9-carboxylate (9-AC), and beta(2)-adrenergic agonists, such as ritodrine, in pregnant rat myometrium. METHODS: Isolated myometrial strips were obtained from eight pregnant rats, and the strips were mounted in organ baths for recording isometric tension. The effects of 10(-8)-10(-4) M ritodrine, 10(-6)-3 x 10(-4) M NPPB, and 10(-6)-10(-3) M 9-AC on spontaneous contractions were recorded. RESULTS: Ritodrine (10(-8)-10(-5) M) completely inhibited the amplitude and frequency of spontaneous contractions in myometrial strips isolated from pregnant rats in a concentration-dependent manner, but the relaxant effect of ritodrine at 10(-4) M concentration resulted in tachyphylaxis. The chloride channel blocker NPPB (10(-6)-3 x 10(-4) M) and the chloride transport inhibitor 9-AC (10(-5)-10(-3) M) decreased the amplitude of spontaneous myometrial contractions in a concentration-dependent manner; the maximum inhibition produced by the highest tested concentration of each drug was 43.8% and 42.1% of the original degree of spontaneous contractions, respectively. The frequency of myometrial contractions was significantly inhibited by NPPB and 9-AC beginning with the concentration of 10(-4) M. CONCLUSION: NPPB and 9-AC appear to be effective relaxants of pregnant rat myometrium. These effects of NPPB and 9-AC might be therapeutically advantageous in clinical management of preterm labor.

Adrenergic beta-Agonists↗

PubMed2001

Regulation of a human chloride channel. a paradigm for integrating input from calcium, type ii calmodulin-dependent protein kinase, and inositol 3,4,5,6-tetrakisphosphate.

We have studied the regulation of Ca(2+)-dependent chloride (Cl(Ca)) channels in a human pancreatoma epithelial cell line (CFPAC-1), which does not express functional cAMP-dependent cystic fibrosis transmembrane conductance regulator chloride channels. In cell-free patches from these cells, physiological Ca(2+) concentrations activated a single class of 1-picosiemens Cl(-)-selective channels. The same channels were also stimulated by a purified type II calmodulin-dependent protein kinase (CaMKII), and in cell-attached patches by purinergic agonists. In whole-cell recordings, both Ca(2+)- and CaMKII-dependent mechanisms contributed to chloride channel stimulation by Ca(2+), but the CaMKII-dependent pathway was selectively inhibited by inositol 3,4,5,6-tetrakisphosphate (Ins(3,4,5,6)P(4)). This inhibitory effect of Ins(3,4,5,6)P(4) on Cl(Ca) channel stimulation by CaMKII was reduced by raising [Ca(2+)] and prevented by inhibition of protein phosphatase activity with 100 nm okadaic acid. These data provide a new context for understanding the physiological relevance of Ins(3,4,5,6)P(4) in the longer term regulation of Ca(2+)-dependent Cl(-) fluxes in epithelial cells.

Calcium↗

PubMed1997

Effects of 5-(4-piperidyl) isoxazol-3-ol (4-PIOL), a GABA(A) receptor partial agonist, on recombinant human GABA(A) receptors.

gamma-Aminobutyric acidA (GABA(A)) gated chloride ion channels were expressed from human recombinant cDNA using the baculovirus/Sf-9 insect cell expression system. The electrophysiological effects in whole-cell currents of 5-(4-piperidyl) isoxazol-3-ol (4-PIOL), a GABA(A) receptor partial agonist, were investigated on GABA(A) receptor complexes of alpha1beta2gamma2S subunits as well as a slightly modified construct of alpha1(valine 121)beta2gamma2S subunits. Here we report that (1)4-PIOL induces an inward whole-cell current in a concentration-dependent manner in both alpha1(val 121)beta2gamma2S and alpha1(ile 121)beta2gamma2S receptor subunit combinations. (2) The 4-PIOL induced whole-cell currents were more pronounced in alpha1(val 121)beta2gamma2S than in alpha1(ile 121)beta2gamma2S receptor subunit combinations. (3) 4-PIOL inhibited GABA-induced responses on alpha1(ile 121)beta2gamma2S and alpha1(val 121)beta2gamma2S receptor combinations with similar potency.

Animals↗

PubMed1993

GABA receptor molecules of insects.

Receptors for 4-aminobutyric acid (GABA) have been identified in both central and peripheral nervous systems of several invertebrate phyla. To date, much of the information derived from physiological and biochemical studies on insect GABA receptors relates to GABA-gated chloride channels that show some similarities with vertebrate GABAA receptors. Like their vertebrate central nervous system (CNS) counterparts, agonist activation of such insect GABA receptors leads to a rapid, picrotoxin-sensitive increase in chloride ion conductance across the cell membrane. In insects, responses to GABA can be modulated by certain benzodiazepines and barbiturates. However, recent studies have detected a number of striking pharmacological differences between GABA-gated chloride channels of insects and vertebrates. Receptor binding, electrophysiological and 36Cl- flux assays have indicated that many insect receptors of this type are insensitive to the vertebrate GABAA antagonists bicuculline and pitrazepin. Benzodiazepine binding sites coupled to insect GABA receptors display a pharmacological profile distinct from that of corresponding sites in vertebrate CNS. Receptor binding studies have also demonstrated differences between convulsant binding sites of insect and vertebrate receptors. Insect GABA receptor molecules are important target sites for several chemically-distinct classes of insecticidally-active molecules. By characterizing these pharmacological properties in detail, it may prove possible to exploit differences between vertebrate and insect GABA receptors in the rational design of novel, more selective pest control agents. The recent application of the powerful techniques of molecular biology has revealed a diversity of vertebrate GABAA receptor subunits and their respective isoforms that can assemble in vivo to form a multiplicity of receptor subtypes. Molecular cloning of insect GABA receptor subunits will not only enhance our understanding of invertebrate neurotransmitter receptor diversity but will also permit the precise identification of the sites of action of pest control agents.

Amino Acid Sequence↗

PubMed2005

Gaboxadol--a new awakening in sleep.

Drugs that enhance synaptic gamma-aminobutyric acid (GABA)ergic neurotransmission are widely utilized in the clinical setting. Barbiturates and benzodiazepine receptor agonists, for example, both potentiate an inhibitory chloride conductance through GABA-gated channels, and thereby achieve their sedative-hypnotic effects. The primary locus of action of these agents, and indeed most neuroactive drugs, is the postsynaptic junction. By contrast, gaboxadol, a selective extrasynaptic GABA receptor agonist and late-stage investigational treatment for insomnia, acts on a unique delta-containing GABAA receptor subtype found exclusively outside of the synapse. Although the mechanistic details of extrasynaptic neurotransmission remain to be fully established, it is now clear that these receptors demonstrate unique pharmacological, biophysical and electrophysiological properties. Importantly, the delta-containing GABAA receptor subtype activated by gaboxadol is highly expressed in the thalamus, where it might behave as a 'gain control' (independently controlling the strength of signals) in the corticothalamic pathways that govern sleep-relevant neuronal oscillations. This unique mechanism has contributed to our increased understanding of sleep mechanisms, and targeting of this system offers potential advantages over existing insomnia treatments.

Animals↗

PubMed1994

Decreased agonist affinity and chloride conductance of mutant glycine receptors associated with human hereditary hyperekplexia.

Hereditary hyperekplexia is a dominant neurological disorder associated with point mutations at the channel-forming segment M2 of the glycine receptor alpha 1 subunit. Voltage-clamp recordings from the heterologously expressed mutants (alpha 1R271L or alpha 1R271Q) revealed 146- to 183-fold decreased potencies of glycine to activate the chloride channel, and significantly reduced maximal whole-cell currents as compared with wild-type receptors. In contrast, the ability of the competitive antagonist strychnine to block glycine-induced currents was similar in all cases. Radioligand binding assays showed a 90- to 1365-fold reduction in the ability of glycine to displace [3H]strychnine from its binding site on the mutant receptors. Paralleling the reductions in whole-cell current, the elementary main-state conductances of the mutants (alpha 1R271L, 64 pS; alpha 1R271Q, 14 pS) were lower than that of the wild-type receptor (86 pS). The decreased agonist affinities and chloride conductances of the mutants are likely to cause neural hyperexcitability of affected patients by impairing glycinergic inhibition. In addition, our data reveal that structural modifications of the ion-channel region can affect agonist binding to the glycine receptor.

Animals↗

PubMed1998

Adrenergic stimulation of Na+ transport across alveolar epithelial cells involves activation of apical Cl- channels.

Alveolar epithelial cells were isolated from adult Sprague-Dawley rats and grown to confluence on membrane filters. Most of the basal short-circuit current (Isc; 60%) was inhibited by amiloride (IC50 0. 96 microM) or benzamil (IC50 0.5 microM). Basolateral addition of terbutaline (2 microM) produced a rapid decrease in Isc, followed by a slow recovery back to its initial amplitude. When Cl- was replaced with methanesulfonic acid, the basal Isc was reduced and the response to terbutaline was inhibited. In permeabilized monolayer experiments, both terbutaline and amiloride produced sustained decreases in current. The current-voltage relationship of the terbutaline-sensitive current had a reversal potential of -28 mV. Increasing Cl- concentration in the basolateral solution shifted the reversal potential to more depolarized voltages. These results were consistent with the existence of a terbutaline-activated Cl- conductance in the apical membrane. Terbutaline did not increase the amiloride-sensitive Na+ conductance. We conclude that beta-adrenergic stimulation of adult alveolar epithelial cells results in an increase in apical Cl- permeability and that amiloride-sensitive Na+ channels are not directly affected by this stimulation.

Adrenergic beta-Agonists↗

PubMed2002

6,7-Dihydro-2-benzothiophen-4(5H)-ones: a novel class of GABA-A alpha5 receptor inverse agonists.

Nonselective inverse agonists at the benzodiazepine binding site on the GABA-A chloride ion channel enhance cognitive performance in animals but cannot be used in the treatment of cognitive disorders because of anxiogenic and convulsant side effects. We have identified a novel series of GABA-A alpha5 receptor ligands during our search for alpha5 receptor inverse agonists as potential cognition enhancers. In particular, 6,6-dimethyl-3-(2-hydroxyethyl)thio-1-(thiazol-2-yl)-6,7-dihydro-2-benzothiophen-4(5H)-one (26) has been identified as a functionally selective GABA-A alpha5 inverse agonist.

Animals↗

PubMed1986

Possible involvement of low and high affinity GABA receptors in the chloride influx into synaptosomes.

Experiments carried out in the absence or presence of GABA using a synaptosomal fraction from which endogenous GABA was as far as possibly eliminated, seem to indicate that both GABA receptors are involved in the chloride channel opening. This hypothesis is supported by results obtained in the presence of GABA agonist (muscimol) or drugs which are related to the complex GABA receptor-ionophore (diazepam and phenobarbital).

4-Aminobutyrate Transaminase↗

PubMed2000

Bidirectional transepithelial water transport: chloride-dependent mechanisms.

We hypothesized that inhibition and activation of basolateral to luminal chloride transport mechanisms were associated with respective decreases and increases in basolateral to luminal water fluxes. The luminal to basolateral (J(W)(L-->B)) and basolateral to luminal (J(W)(B-->L)) water fluxes across ovine tracheal epithelia were measured simultaneously. The mean J(W)(L-->B) (6.5 microl/min/cm(2)) was larger than J(W)(B-->L) (6.1 microl/min/cm(2)). Furosemide reduced J(W)(B-->L) from 6.0 to 5.6 microl/min/cm(2). Diphenylamine-2-carboxylate (DPC) reduced J(W)(B-->L) from 7.9 to 7. 3 microl/min/cm(2) and reduced the membrane potential difference by 38%. Furosemide together with DPC decreased J(W)(L-->B) by 30% and J(W)(B-->L) by 15%. Norepinephrine increased J(W)(B-->L) from 4.9 to 6.0 microl/min/cm(2). Neuropeptide Y in the presence of norepinephrine decreased J(W)(L-->B) (6.4 to 5.2 microl/min/cm(2)) and returned J(W)(B-->L) to its baseline value. Vasopressin increased J(W)(B-->L) from 4.1 to 5.1 microl/min/cm(2). Endothelin-1 induced a simultaneous increase in J(W)(B-->L) (7.0 to 7.7 microl/min/cm(2)) and decrease in J(W)(L-->B) (7.4 to 6.4 microl/min/cm(2)); and decreased the membrane resistance. These data indicate that in tracheal epithelia under homeostatic conditions J(W)(B-->L) has a approximately 15% actively coupled component. Consistent with our hypothesis, inhibition and receptor-induced stimulation of chloride effluxes were associated with decreases and increases in J(W)(B-->L), respectively. However, as inhibition of transcellular chloride transport always decreased J(W)(L-->B) more than J(W)(B-->L), reducing transepithelial chloride transport did not result in less water being transported into the airway lumen.

Adrenergic alpha-Agonists↗

PubMed1990

Modulation by neuropeptides of bradykinin-stimulated second messenger release in dorsal root ganglion neurons.

Fetal rat dorsal root ganglion neurons (7-8 days in culture) were labeled with [3H]arachidonic acid for 24 h. Stimulation with 10 microM bradykinin (BK) for 30 s resulted in nearly 2-fold increases in levels of radioactive diglyceride and arachidonic acid. A similar result was obtained in the absence of receptor stimulation using the Ca2+ channel agonist BAY K 8644 (10 microM, in the presence of 100 mM potassium chloride) or the Ca2+ ionophore, ionomycin (2.5 microM). If Ca2+ influx was inhibited by adding 3 mM Co2+, a blocker of voltage-sensitive calcium channels, or 2.5 mM EDTA, then BK-stimulated accumulation of both arachidonate and diglyceride was inhibited. These data suggest Ca2+ influx is required for ligand-stimulated accumulation of both arachidonate (a product of diglyceride-lipase or phospholipase A2) and diglyceride (a product of phospholipase C). Two distinct populations of channels may be involved in these reactions since pretreatment with 10 microM nifedipine or 50 microM verapamil (agents which block a subset of voltage-sensitive Ca2+ channels) inhibited BK-stimulated accumulation of arachidonic acid, but did not inhibit diglyceride accumulation. Such functional discrimination appears to have physiological importance; the inhibitory effect of nifedipine and verapamil on BK-stimulated arachidonate release was mimicked by pretreatment with peptides which decrease Ca2+ channel conductance in dorsal root ganglion neurons. The three peptides used were 1 microM neuropeptide Y, 10 microM somatostatin, and 10 microM [N-MePhe3,D-Pro4]-morphiceptin. The effect of neuropeptide Y was blocked by pretreatment with pertussis toxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Analgesics↗

PubMed1991

Phosphorylation of the R domain by cAMP-dependent protein kinase regulates the CFTR chloride channel.

CFTR, the protein associated with cystic fibrosis, is phosphorylated on serine residues in response to cAMP agonists. Serines 660, 737, 795, and 813 were identified as in vivo targets for phosphorylation by protein kinase A. The SPQ fluorescence assay revealed that mutagenesis of any one of these sites did not affect Cl- channel activity. Indeed, concomitant mutagenesis of three of the four sites still resulted in cAMP-responsive Cl- channel activity. However, mutagenesis of all four sites abolished the response. One interpretation of these results is that the CFTR Cl- channel is blocked by the R domain and that phosphorylation on serines by protein kinase A electrostatically repels the domain, allowing passage of Cl-. The four phosphorylation events appear to be degenerate: no one site is essential for channel activity, and, at least in the case of serine 660, phosphorylation at one site alone is sufficient for regulation of Cl- channel activity.

Chloride Channels↗