Formation of functional heterodimers between the TASK-1 and TASK-3 two-pore domain potassium channel subunits.

The potassium channels in the two-pore domain family are widely expressed and regulate the excitability of neurons and other excitable cells. These channels have been shown to function as dimers, but heteromerization between the various channel subunits has not yet been reported. Here we demonstrate that two members of the TASK subfamily of potassium channels, TASK-1 and TASK-3, can form functional heterodimers when expressed in Xenopus laevis oocytes. To recognize the two TASK channel types, we took advantage of the higher sensitivity of TASK-1 over TASK-3 to physiological pH changes and the discriminating sensitivity of TASK-3 to the cationic dye ruthenium red. These features were clearly observed when the channels were expressed individually. However, when TASK-1 and TASK-3 were expressed together, the resulting current showed intermediate pH sensitivity and ruthenium red insensitivity (characteristic of TASK-1), indicating the formation of TASK-1/TASK-3 heterodimers. Expression of a tandem construct in which TASK-3 and TASK-1 were linked together yielded currents with features very similar to those observed when coexpressing the two channels. The tandem construct also responded to AT(1a) angiotensin II receptor stimulation with an inhibition that was weaker than the inhibition of homodimeric TASK-1 and greater than that shown by TASK-3. Expression of epitope-tagged channels in mammalian cells showed their primary presence in the plasma membrane consistent with their function in this location. Heteromerization of two-pore domain potassium channels may provide a greater functional diversity and additional means by which they can be regulated in their native tissues.     

Immunocytochemical Localization of TASK-3 (K<Subscript>2P</Subscript>9.1) Channels in Monoaminergic and Cholinergic Neurons

Monoaminergic and cholinergic systems are important regulators of cortical and subcortical systems, and a variety of vegetative functions are controlled by the respective neurotransmitters. Neuronal excitability and transmitter release of these neurons are strongly regulated by their potassium conductances carried by Kir and K_2P channels. Here we describe the generation and characterization of a polyclonal monospecific antibody against rat TASK-3, a major brain K_2P channel. After removal of cross-reactivities and affinity purification the antibody was characterized by ELISA, immunocytochemistry of TASK-3 transfected cells, and Western blots indicating that the antibody only detects TASK-3 protein, but not its paralogs TASK-1 and TASK-5. Western blot analysis of brain membrane fractions showed a single band around 45 kD, close to the predicted molecular weight of the TASK-3 protein. In addition, specific immunolabeling using the anti-TASK-3 antibody in Western blot analysis and immunocytochemistry was blocked in a concentration dependent manner by its cognate antigen only. Immunocytochemical analysis of rat brain revealed strong expression of TASK-3 channels in serotoninergic neurons of the dorsal and median raphe, noradrenergic neurons of the locus coeruleus, histaminergic neurons of the tuberomammillary nucleus and in the cholinergic neurons of the basal nucleus of Meynert. Immunofluorescence double-labeling experiments with appropriate marker enzymes confirmed the expression of TASK-3 in cholinergic, serotoninergic, and noradrenergic neurons. In the dopaminergic system strong TASK-3 expression was found in the ventral tegmental area, whereas TASK-3 immunoreactivity in the substantia nigra compacta was only weak. All immunocytochemical results were supported by in situ hybridization using TASK-3 specific riboprobes.     

Immunocytochemical Localization of TASK-3 Protein (K2P9.1) in the Rat Brain

Among all K2P channels, TASK-3 shows the most widespread expression in rat brain, regulating neuronal excitability and transmitter release. Using a recently purified and characterized polyclonal monospecific antibody against TASK-3, the entire rat brain was immunocytochemically analyzed for expression of TASK-3 protein. Besides its well-known strong expression in motoneurons and monoaminergic and cholinergic neurons, TASK-3 expression was found in most neurons throughout the brain. However, it was not detected in certain neuronal populations, and neuropil staining was restricted to few areas. Also, it was absent in adult glial cells. In hypothalamic areas, TASK-3 was particularly strongly expressed in the supraoptic and suprachiasmatic nuclei, whereas other hypothalamic nuclei showed lower protein levels. Immunostaining of hippocampal CA1 and CA3 pyramidal neurons showed strongest expression, together with clear staining of CA3 mossy fibers and marked staining also in the dentate gyrus granule cells. In neocortical areas, most neurons expressed TASK-3 with a somatodendritic localization, most obvious in layer V pyramidal neurons. In the cerebellum, TASK-3 protein was found mainly in neurons and neuropil of the granular cell layer, whereas Purkinje cells were only faintly positive. Particularly weak expression was demonstrated in the forebrain. This report provides a comprehensive overview of TASK-3 protein expression in the rat brain.     

Differential effects of volatile and intravenous anesthetics on the activity of human TASK-1

Volatile anesthetics have been shown to activate various two-pore (2P) domain K⁺ (K_2P) channels such as TASK-1 and TREK-1 (TWIK-related acid-sensitive K⁺ channel), and mice deficient in these channels are resistant to halothane-induced anesthesia. Here, we investigated whether K_2P channels were also potentially important targets of intravenous anesthetics. Whole cell patch-clamp techniques were used to determine the effects of the commonly used intravenous anesthetics etomidate and propofol on the acid-sensitive K⁺ current in rat ventricular myocytes (which strongly express TASK-1) and selected human K_2P channels expressed in Xenopus laevis oocytes. In myocytes, etomidate decreased both inward rectifier K⁺ (K_ir) current (I_K1) and acid-sensitive outward K⁺ current at positive potentials, suggesting that this drug may inhibit TASK channels. Indeed, in addition to inhibiting guinea pig Kir2.1 expressed in oocytes, etomidate inhibited human TASK-1 (and TASK-3) in a concentration-dependent fashion. Propofol had no effect on human TASK-1 (or TASK-3) expressed in oocytes. Moreover, we showed that, similar to the known effect of halothane, sevoflurane and the purified R-(–)- and S-(+)-enantiomers of isoflurane, without stereoselectivity, activated human TASK-1. We conclude that intravenous and volatile anesthetics have dissimilar effects on K_2P channels. Human TASK-1 (and TASK-3) are insensitive to propofol but are inhibited by supraclinical concentrations of etomidate. In contrast, stimulatory effects of sevoflurane and enantiomeric isoflurane on human TASK-1 can be observed at clinically relevant concentrations. volatile anesthetics; etomidate; propofol; ion channels     

Immunolocalization of TASK-3 (KCNK9) to a subset of cortical neurons in the rat CNS

Tandem pore domain (2P) K channels constitute the most diverse family of K channels and are responsible for background (leak or baseline) K currents. Of the 15 human 2P K channels, TASK-1, TASK-2, and TASK-3 are uniquely sensitive to physiologic pH changes as well as being inhibited by local anesthetics and activated by volatile anesthetics. In this study polyclonal antibodies selective for TASK-3 have been used to localize its expression in the rat central nervous system (CNS). TASK-3 immunostaining was found in rat cortex, hypothalamus, and hippocampus. Double immunofluorescent studies identified a discrete population of TASK-3 expressing neurons scattered throughout cortex. Using immunogold electron microscopy TASK-3 was identified at the cell surface associated with synapses and within the intracellular synthetic compartments. These results provide a more finely detailed picture of TASK-3 expression and indicate a role for TASK-3 in modulating cerebral synaptic transmission and responses to CNS active drugs.     

A di-acidic sequence motif enhances the surface expression of the potassium channel TASK-3

We have characterized a sequence motif, EDE, in the proximal C-terminus of the acid-sensitive potassium channel TASK-3. Human TASK-3 channels were expressed in Xenopus oocytes, and the density of the channels at the surface membrane was studied with two complementary techniques: a luminometric surface expression assay of hemagglutinin epitope-tagged TASK-3 channels and voltage-clamp measurements of the acid-sensitive potassium current. Both approaches showed that mutation of the two glutamate residues of the EDE motif to alanine (ADA mutant) markedly reduced the transport of TASK-3 channels to the cell surface. Mutation of the central aspartate of the EDE motif had no effect on surface expression. The functional role of the EDE motif was further characterized in chimaeric constructs consisting of truncated Kir2.1 channels to which the C-terminus of TASK-3 was attached. In these constructs, too, replacement of the EDE motif by ADA strongly reduced surface expression. Live-cell imaging of enhanced green fluorescent protein-tagged channels expressed in COS-7 cells showed that 24 h after transfection wild-type TASK-3 was mainly localized to the cell surface whereas the ADA mutant was largely retained in the endoplasmic reticulum (ER). Mutation of a second di-acidic motif in the C-terminus of TASK-3 (DAE) had no effect on surface expression. Coexpression of TASK-3 with a GTP-restricted mutant of the coat recruitment GTPase Sar1 (Sar1H79G) resulted in ER retention of the channel. Our data suggest that the di-acidic motif, EDE, in human TASK-3 is a major determinant of the rate of ER export and is required for efficient surface expression of the channel.     

A specific two-pore domain potassium channel blocker defines the structure of the TASK-1 open pore

Two-pore domain potassium (K(2P)) channels play a key role in setting the membrane potential of excitable cells. Despite their role as putative targets for drugs and general anesthetics, little is known about the structure and the drug binding site of K(2P) channels. We describe A1899 as a potent and highly selective blocker of the K(2P) channel TASK-1. As A1899 acts as an open-channel blocker and binds to residues forming the wall of the central cavity, the drug was used to further our understanding of the channel pore. Using alanine mutagenesis screens, we have identified residues in both pore loops, the M2 and M4 segments, and the halothane response element to form the drug binding site of TASK-1. Our experimental data were used to validate a K(2P) open-pore homology model of TASK-1, providing structural insights for future rational design of drugs targeting K(2P) channels.     

The sensitivity of G protein-activated K+ channels toward halothane is essentially determined by the C terminus

G protein-activated K(+) channels (GIRKs or Kir3.x) are targets for the volatile anesthetic, halothane. When coexpressed with the m(2) acetylcholine (ACh) receptor in Xenopus oocytes, agonist-activated GIRK1(F137S)- and GIRK2-mediated currents are inhibited by halothane, whereas in the absence of ACh, high concentrations of halothane induce GIRK1(F137S)-mediated currents. To elucidate the molecular mechanism of halothane action on GIRK currents of different subunit compositions, we constructed deletion mutants of GIRK1(F137S) (GIRK1(Delta363*)) and GIRK2 (GIRK2(Delta356)) lacking the C-terminal ends, as well as chimeric GIRK channels. Mutated GIRK channels showed normal currents when activated by ACh but exhibited different pharmacological properties toward halothane. GIRK2(Delta356) showed no sensitivity against the inhibitory action of halothane but was activated by halothane in the absence of an agonist. GIRK1(Delta363*) was activated by halothane more efficiently. Currents mediated by chimeric channels were inhibited by anesthetic concentrations that were at least 30-fold lower than those necessary to decrease GIRK2 wild type currents. Glutathione S-transferase pulldown experiments did not show displacement of bound Gbetagamma by halothane, indicating that halothane does not interfere with Gbetagamma binding. Single channel experiments revealed an influence of halothane on the gating of the channels: The agonist-induced currents of GIRK1 and GIRK2, carried mainly by brief openings, were inhibited, whereas higher concentrations of the anesthetic promoted long openings of GIRK1 channels. Because the C terminus is crucial for these effects, an interaction of halothane with the channel seems to be involved in the mechanism of current modulation.     

Both TASK-3 and TREK-1 two-pore loop K channels are expressed in H295R cells and modulate their membrane potential and aldosterone secretion

The rate of aldosterone synthesis by adrenal glomerulosa cells relies on the selective permeability of the glomerulosa cell to K(+) ions. In rodent and bovine adrenal glomerulosa cells, this background potassium current is provided by a two-pore loop potassium (K2P) channel: largely TASK-3 in the rat and TREK-1 in the cow. The nature of the K2P channel in the human adrenal cortex is not known, and we have addressed this issue here using the H295R human adrenal cell line. We show that these cells express mRNA and protein for both TASK-3 and TREK-1 K2P channels. Using a potentiometric dye (FMP), we also show that TASK-3 and TREK-1 channel modulators can affect the membrane potential of H295R cells. Transfecting H295R cells with TASK-3 or TREK-1 dominant-negative mutants (TASK-3 G95E or TREK-1 G144E) produced depolarization of H295R cells and altered K-stimulated aldosterone secretion. Finally, transfection of a constitutively active mutant of Galpha(q) into H295R cells (GTPase-deficient Galpha(q)-QL) depolarized them and increased basal aldosterone secretion. Taken together, our data support both TASK-3 and TREK-1 as being functionally operational in the H295R cell line. This suggests that human adrenal glomerulosa cells may utilize both of these K2P channels for their background potassium current.     

G protein {beta}{gamma} gating confers volatile anesthetic inhibition to Kir3 channels

G protein-activated inwardly rectifying potassium (GIRK or Kir3) channels are directly gated by the βγ subunits of G proteins and contribute to inhibitory neurotransmitter signaling pathways. Paradoxically, volatile anesthetics such as halothane inhibit these channels. We find that neuronal Kir3 currents are highly sensitive to inhibition by halothane. Given that Kir3 currents result from increased Gβγ available to the channels, we asked whether reducing available Gβγ to the channel would adversely affect halothane inhibition. Remarkably, scavenging Gβγ using the C-terminal domain of β-adrenergic receptor kinase (cβARK) resulted in channel activation by halothane. Consistent with this effect, channel mutants that impair Gβγ activation were also activated by halothane. A single residue, phenylalanine 192, occupies the putative Gβγ gate of neuronal Kir3.2 channels. Mutation of Phe-192 at the gate to other residues rendered the channel non-responsive, either activated or inhibited by halothane. These data indicated that halothane predominantly interferes with Gβγ-mediated Kir3 currents, such as those functioning during inhibitory synaptic activity. Our report identifies the molecular correlate for anesthetic inhibition of Kir3 channels and highlights the significance of these effects in modulating neurotransmitter-mediated inhibitory signaling.     

Receptor-mediated inhibition of G protein-coupled inwardly rectifying potassium channels involves G(alpha)q family subunits, phospholipase C, and a readily diffusible messenger

G protein-coupled inwardly rectifying K+ (GIRK) channels can be activated or inhibited by distinct classes of receptor (G(alpha)i/o- and G(alpha)q-coupled), providing dynamic regulation of cellular excitability. Receptor-mediated activation involves direct effects of G(beta)gamma subunits on GIRK channels, but mechanisms involved in GIRK channel inhibition have not been fully elucidated. An HEK293 cell line that stably expresses GIRK1/4 channels was used to test G protein mechanisms that mediate GIRK channel inhibition. In cells transiently or stably cotransfected with 5-HT1A (G(alpha)i/o-coupled) and TRH-R1 (G(alpha)q-coupled) receptors, 5-HT (5-hydroxytryptamine; serotonin) enhanced GIRK channel currents, whereas thyrotropin-releasing hormone (TRH) inhibited both basal and 5-HT-activated GIRK channel currents. Inhibition of GIRK channel currents by TRH primarily involved signaling by G(alpha)q family subunits, rather than G(beta)gamma dimers: GIRK channel current inhibition was diminished by Pasteurella multocida toxin, mimicked by constitutively active members of the G(alpha)q family, and reduced by minigene constructs that disrupt G(alpha)q signaling, but was completely preserved in cells expressing constructs that interfere with signaling by G(beta)gamma subunits. Inhibition of GIRK channel currents by TRH and constitutively active G(alpha)q was reduced by, an inhibitor of phospholipase C (PLC). Moreover, TRH- R1-mediated GIRK channel inhibition was diminished by minigene constructs that reduce membrane levels of the PLC substrate phosphatidylinositol bisphosphate, further implicating PLC. However, we found no evidence for involvement of protein kinase C, inositol trisphosphate, or intracellular calcium. Although these downstream signaling intermediaries did not contribute to receptor-mediated GIRK channel inhibition, bath application of TRH decreased GIRK channel activity in cell-attached patches. Together, these data indicate that receptor-mediated inhibition of GIRK channels involves PLC activation by G(alpha) subunits of the G(alpha)q family and suggest that inhibition may be communicated at a distance to GIRK channels via unbinding and diffusion of phosphatidylinositol bisphosphate away from the channel.     

Immunocytochemical Localization of TASK-3 Channels in Rat Motor Neurons

Motor neurons are large cholinergic neurons located in the brain stem and spinal cord. In recent years, a functional role for TASK channels in cellular excitability and vulnerability to anesthetics of motor neurons has been described. Using a polyclonal monospecific antibody against the tandem pore domain K⁺ channel (K2P channel) TWIK-related acid-sensitive K⁺ channel (TASK-3), we analyzed the expression of the TASK-3 protein in motor systems of the rat CNS. Immunocytochemical staining showed strong TASK-3 expression in motor neurons of the facial, trigeminal, ambiguus, and hypoglossal nuclei. Oculomotor nuclei (including trochlear and abducens nucleus) were also strongly positive for TASK-3. The parasympathetic Edinger-Westphal nucleus and dorsal vagal nucleus showed significant, but weaker expression compared with somato- and branchiomotoric neurons. In addition, motor neurons in the anterior horn of the spinal cord were also strongly labeled for TASK-3 immunoreactivity. Based on morphological criteria, TASK-3 was found in the somatodendritic compartment of motor neurons. Cellular staining using methyl green and immunofluorescence double-labeling with anti-vesicular acetylcholine transporter (anti-vAChT) indicated ubiquitous TASK-3 expression in motor neurons, whereas in other brain regions TASK-3 showed a widespread but not ubiquitous expression. In situ hybridization using a TASK-3 specific riboprobe verified the expression of TASK-3 in motor neurons at the mRNA level.     

Heterogeneous expression of TASK-3 and TRAAK in rat paraganglionic cells

In the present study, we investigated the immunohistochemical localization of the two-pore K⁺ channels, TASK-3 and TRAAK, in paraganglionic cells within the superior cervical ganglion, stellate ganglion, and aortic body in comparison with membrane channels in chief cells of the carotid body. TASK-3 immunoreactivity was observed in the paraganglionic cells in all tissues examined. TRAAK immunoreactivity was observed in the chief cells of the aortic body as well as these of the carotid body, but not in the paraganglionic cells in the sympathetic (superior cervical and stellate) ganglia. Our findings indicate that sympathetic paraganglionic cells and glossopharyngeal/vagal paraganglionic cells were different from each other in the expression patterns of TASK-3 and TRAAK to result in the different chemoreception properties of sympathetic paraganglionic cells from those of chief cells of the aortic and carotid bodies.     

Activation of protein kinase C epsilon inhibits the two-pore domain K+ channel, TASK-1, inducing repolarization abnormalities in cardiac ventricular myocytes

Activation of the platelet-activating factor (PAF) receptor leads to a decrease in outward current in murine ventricular myocytes by inhibiting the TASK-1 channel. TASK-1 carries a background or "leak" current and is a member of the two-pore domain potassium channel family. Its inhibition is sufficient to delay repolarization, causing prolongation of the action potential duration, and in some cases, early after depolarizations. We set out to determine the cellular mechanisms that control regulation of TASK-1 by PAF. Inhibition of TASK-1 via activation of the PAF receptor is protein kinase C (PKC)-dependent. Using isoform-specific PKC inhibitor or activator peptides in patch clamp experiments, we now demonstrate that activation of PKCepsilon is both necessary and sufficient to regulate murine TASK-1 current in a heterologous expression system and to induce repolarization abnormalities in isolated myocytes. Furthermore, site-directed mutagenesis studies have identified threonine 381, in the C-terminal tail of murine TASK-1, as a critical residue in this regulation.     

Inhibition of TASK-1 potassium channel by phospholipase C

Thetwo-pore-domain K⁺ channel, TASK-1, was recently shown tobe a target of receptor-mediated regulation in neurons and in adrenalglomerulosa cells. Here, we demonstrate that TASK-1 expressed inXenopus laevis oocytes is inhibited by differentCa²⁺-mobilizing agonists. Lysophosphatidic acid, via itsendogenous receptor, and ANG II and carbachol, via their heterologouslyexpressed ANG II type 1a and M₁ muscarinic receptors,respectively, inhibit TASK-1. This effect can be mimicked by guanosine5'-O-(3-thiotriphosphate), indicating the involvementof GTP-binding protein(s). The phospholipase C inhibitor U-73122reduced the receptor-mediated inhibition of TASK-1. Downstream signalsof phospholipase C action (inositol 1,4,5-trisphosphate, cytoplasmicCa²⁺ concentration, and diacylglycerol) do not mediate theinhibition. Unlike the G_q-coupled receptors, stimulation ofthe G_i-activating M₂ muscarinic receptorcoexpressed with TASK-1 results in an only minimal decrease of theTASK-1 current. However, additional coexpression of phospholipaseC-₂ (which is responsive also to G_i-subunits) renders M₂ receptor activation effective.This indicates the significance of phospholipase C activity in thereceptor-mediated inhibition of TASK-1.

     

TASK-3 dominates the background potassium conductance in rat adrenal glomerulosa cells

In a preceding study we showed that the highly negative resting membrane potential of rat adrenal glomerulosa cells is related to background potassium channel(s), which belong to the two-pore domain channel family. TWIK-related acid-sensitive K+ channel (TASK-1) expression was found in glomerulosa tissue, and the currents elicited by injection of glomerulosa mRNA (I(glom)) or TASK-1 cRNA (I(TASK-1)) showed remarkable similarity in Xenopus laevis oocytes. However, based on the different sensitivity of these currents to acidification, we concluded that TASK-1 may be responsible for a maximum of 25% of the weakly pH-dependent glomerulosa background K+ current. Here we demonstrate that TASK-3, a close relative of TASK-1, is expressed abundantly in glomerulosa cells. Northern blot detected TASK-3 message in adrenal glomerulosa, but not in other tissues. Quantitative RT-PCR experiments indicated even higher mRNA expression of TASK-3 than TASK-1 in glomerulosa tissue. Similarly to the glomerulosa background current, the current expressed by injection of TASK-3 cRNA (I(TASK-3)) was less acid-sensitive than I(TASK-1). Ruthenium red in the micromolar range inhibited I(glom) and I(TASK-3), but not I(TASK-1). Like I(TASK-1), I(TASK-3) was inhibited by stimulation of AT1a angiotensin II receptor coexpressed with the potassium channel. The high level of expression and its pharmacological properties suggest that TASK-3 dominates the resting potassium conductance of glomerulosa cells.     

The versatile regulation of K2P channels by polyanionic lipids of the phosphoinositide and fatty acid metabolism

Work over the past three decades has greatly advanced our understanding of the regulation of K_ir K⁺ channels by polyanionic lipids of the phosphoinositide (e.g., PIP₂) and fatty acid metabolism (e.g., oleoyl-CoA). However, comparatively little is known regarding the regulation of the K_2P channel family by phosphoinositides and by long-chain fatty acid–CoA esters, such as oleoyl-CoA. We screened 12 mammalian K_2P channels and report effects of polyanionic lipids on all tested channels. We observed activation of members of the TREK, TALK, and THIK subfamilies, with the strongest activation by PIP₂ for TRAAK and the strongest activation by oleoyl-CoA for TALK-2. By contrast, we observed inhibition for members of the TASK and TRESK subfamilies. Our results reveal that TASK-2 channels have both activatory and inhibitory PIP₂ sites with different affinities. Finally, we provided evidence that PIP₂ inhibition of TASK-1 and TASK-3 channels is mediated by closure of the recently identified lower X-gate as critical mutations within the gate (i.e., L244A, R245A) prevent PIP₂-induced inhibition. Our findings establish that K⁺ channels of the K_2P family are highly sensitive to polyanionic lipids, extending our knowledge of the mechanisms of lipid regulation and implicating the metabolism of these lipids as possible effector pathways to regulate K_2P channel activity.     

Developmental expression of a functional TASK-1 2P domain K+ channel in embryonic chick heart

Background

Background K⁺ channels are the principal determinants of the resting membrane potential (RMP) in cardiac myocytes and thus, influence the magnitude and time course of the action potential (AP).

Methods

RT-PCR and in situ hybridization are used to study the distribution of TASK-1 and whole-cell patch clamp technique is employed to determine the functional expression of TASK-1 in embryonic chick heart.

Results

Chicken TASK-1 was expressed in the early tubular heart, then substantially decreased in the ventricles by embryonic day 5 (ED5), but remained relatively high in ED5 and ED11 atria. Unlike TASK-1, TASK-3 was uniformly expressed in heart at all developmental stages. In situ hybridization studies further revealed that TASK-1 was expressed throughout myocardium at Hamilton-Hamburger stages 11 and 18 (S11 &; S18) heart. In ED11 heart, TASK-1 expression was more restricted to atria. Consistent with TASK-1 expression data, patch clamp studies indicated that there was little TASK-1 current, as measured by the difference currents between pH 8.4 and pH 7.4, in ED5 and ED11 ventricular myocytes. However, TASK-1 current was present in the early embryonic heart and ED11 atrial myocytes. TASK-1 currents were also identified as 3 μM anandamide-sensitive currents. 3 μM anandamide reduced TASK-1 currents by about 58% in ED11 atrial myocytes. Zn²⁺ (100 μM) which selectively inhibits TASK-3 channel at this concentration had no effect on TASK currents. In ED11 ventricle where TASK-1 expression was down-regulated, I_K1 was about 5 times greater than in ED11 atrial myocytes.

Conclusion

Functional TASK-1 channels are differentially expressed in the developing chick heart and TASK-1 channels contribute to background K⁺ conductance in the early tubular embryonic heart and in atria. TASK-1 channels act as a contributor to background K⁺ current to modulate the cardiac excitability in the embryonic heart that expresses little I_K1.