Familial Sinus Node Disease Caused by a Gain of GIRK (G-Protein Activated Inwardly Rectifying K+ Channel) Channel Function

BACKGROUND

Inherited forms of sinus node dysfunction (SND) clinically include bradycardia, sinus arrest, and chronotropic incompetence and may serve as disease models to understand sinus node physiology and impulse generation. Recently, a gain-of-function mutation in the G-protein gene GNB2 led to enhanced activation of the GIRK (G-protein activated inwardly rectifying K⁺ channel). Thus, human cardiac GIRK channels are important for heart rate regulation and subsequently, genes encoding their subunits Kir3.1 and Kir3.4 ( KCNJ3 and KCNJ5) are potential candidates for inherited SND in human.

METHODS

We performed a combined approach of targeted sequencing of KCNJ3 and KCNJ5 in 52 patients with idiopathic SND and subsequent whole exome sequencing of additional family members in a genetically affected patient. A putative novel disease-associated gene variant was functionally analyzed by voltage-clamp experiments using various heterologous cell expression systems (Xenopus oocytes, CHO cells, and rat atrial cardiomyocytes).

RESULTS

In a 3-generation family with SND we identified a novel variant in KCNJ5 which leads to an amino acid substitution (p.Trp101Cys) in the first transmembrane domain of the Kir3.4 subunit of the cardiac GIRK channel. The identified variant cosegregated with the disease in the family and was absent in the Exome Variant Server and Exome Aggregation Consortium databases. Expression of mutant Kir3.4 (±native Kir3.1) in different heterologous cell expression systems resulted in increased GIRK currents ( I_K,ACh) and a reduced inward rectification which was not compensated by intracellular spermidine. Moreover, in silico modeling of heterotetrameric mutant GIRK channels indicates a structurally altered binding site for spermine.

CONCLUSIONS

For the first time, an inherited gain-of-function mutation in the human GIRK3.4 causes familial human SND. The increased activity of GIRK channels is likely to lead to a sustained hyperpolarization of pacemaker cells and thereby reduces heart rate. Modulation of human GIRK channels may pave a way for further treatment of cardiac pacemaking.

Fluorescent Biosensors for Multiplexed Imaging of Phosphoinositide Dynamics

Phosphoinositides constitute a critical family of lipids that regulate numerous cellular processes. Phosphatidylinositol 4,5-bisphosphate (PIP₂) is arguably the most important plasma membrane phosphoinositide and is involved in regulating diverse processes. It is also the precursor of phosphatidylinositol 3,4,5-trisphosphate (PIP₃), which is critical for growth factor signaling, as well as membrane polarization and dynamics. Studying these lipids remains challenging, because of their compartmentalized activities and location-dependent signaling profiles. Here, we introduce several new genetically encoded fluorescent biosensors, including FRET-based and dimerization-dependent fluorescent protein (ddFP)-based biosensors, that enable real-time monitoring of PIP₂ levels in live cells. In addition, we developed a red fluorescent biosensor for 3-phosphoinositides that can be co-imaged with the green PIP₂ indicator. Simultaneous visualization of the dynamics of PIP₂ and 3-phosphoinositides in the same cell shows that plasma membrane PIP₃ formation upon EGF stimulation is coupled to a decrease in the local pool of PIP₂.

Biased signaling of Ca2+-sensing receptors in cardiac myocytes regulates GIRK channel activity

Ca²⁺-sensing receptors (CaSRs) belong to the class C of G protein-coupled receptors and are activated by extracellular Ca²⁺. CaSRs display biased G protein signaling by coupling to different classes of heterotrimeric G proteins depending on agonist and cell type. In this study we used fluorescent biosensors to directly analyze G protein coupling to CaSRs and downstream signaling in living cells. In HEK 293 cells, CaSRs displayed biased signaling: elevation of extracellular Ca²⁺ or application of the alternative agonist spermine caused activation of G_i- and G_q-proteins. Adult cardiac myocytes express endogenous CaSRs, which have been implicated in regulating Ca²⁺ signaling and contractility. Biased signaling of CaSRs has not been investigated in these cells. To evaluate efficiencies of G_i- and G_q-signaling via CaSRs in rat atrial myocytes, we measured G protein-activated K⁺ (GIRK) channels. Activation of GIRK requires binding of Gβγ subunits released from G_i proteins, whereas G_q-signaling results in inhibition of GIRK channel activity. Stimulation of CaSRs by Ca²⁺ or spermine failed to directly activate G_i and GIRK channels. When GIRK channels were pre-activated via endogenous M₂ receptors, stimulation of CaSRs caused pronounced inhibition of GIRK currents. This effect was specific to CaSR activation: GIRK current inhibition was sensitive to NPS-2143, a negative allosteric modulator of CaSRs, and abrogated by FR900359, a direct inhibitor of G_q. GIRK current inhibition was also sensitive to the PKC inhibitor chelerythrine, suggesting that following activation of CaSR and G_q, GIRK currents are modulated by PKC phosphorylation. We conclude from this data that cardiac CaSRs do not activate G_i and affect GIRK currents preferentially via the G_q/PKC pathway.

Receptor Species-dependent Desensitization Controls KCNQ1/KCNE1 K+ Channels as Downstream Effectors of Gq Protein-coupled Receptors

Activation of G_q protein-coupled receptors (G_qPCRs) might induce divergent cellular responses, related to receptor-specific activation of different branches of the G_q signaling pathway. Receptor-specific desensitization provides a mechanism of effector modulation by restricting the spatiotemporal activation of signaling components downstream of G_q We quantified signaling events downstream of G_qPCR activation with FRET-based biosensors in CHO and HEK 293 cells. KCNQ1/KCNE1 channels (I_Ks) were measured as a functional readout of receptor-specific activation. Activation of muscarinic M₁ receptors (M₁-Rs) caused robust and reversible inhibition of I_Ks. In contrast, activation of α_1B-adrenergic receptors (α_1B-ARs) induced transient inhibition of I_Ks, which turned into delayed facilitation after agonist withdrawal. As a novel finding, we demonstrate that G_qPCR-specific kinetics of I_Ks modulation are determined by receptor-specific desensitization, evident at the level of Gα_q activation, phosphatidylinositol 4,5-bisphosphate (PIP₂) depletion, and diacylglycerol production. Sustained I_Ks inhibition during M₁-R stimulation is attributed to robust membrane PIP₂ depletion, whereas the rapid desensitization of α_1B-AR delimits PIP₂ reduction and augments current activation by protein kinase C (PKC). Overexpression of Ca²⁺-independent PKCδ did not affect the time course of α_1B-AR-induced diacylglycerol formation, excluding a contribution of PKCδ to α_1B-AR desensitization. Pharmacological inhibition of Ca²⁺-dependent PKC isoforms abolished fast α_1B receptor desensitization and augmented I_Ks reduction, but did not affect I_Ks facilitation. These data indicate a contribution of Ca²⁺-dependent PKCs to α_1B-AR desensitization, whereas I_Ks facilitation is induced by Ca²⁺-independent PKC isoforms. In contrast, neither inhibition of Ca²⁺-dependent/Ca²⁺-independent isoforms nor overexpression of PKCδ induced M₁ receptor desensitization, excluding a contribution of PKC to M₁-R-induced I_Ks modulation.

NCI-H295R cell line as in vitro model of hyperaldosteronism lacks functional KCNJ5 (GIRK4; Kir3.4) channels

As a major cause of aldosterone producing adenomas, numerous gain-of-function mutations in the KCNJ5 gene (encoding the K(+) channel subunit GIRK4) have been identified. The human adrenocortical carcinoma cell line NCI-H295R is the most frequently used cellular model for in vitro studies related to regulation of aldosterone-synthesis. Because of the undefined role of KCNJ5 (GIRK4) in regulating synthesis of aldosterone, we aimed at identifying basal and G protein-activated GIRK4 currents in this paradigmatic cell line. The GIRK-specific blocker Tertiapin-Q did not affect basal current. Neither loading of the cells with GTP-γ-S via the patch-clamp pipette nor agonist stimulation of an infected A1-adenosine receptor resulted in activation of GIRK current. In cells co-infected with KCNJ5, robust activation of basal and adenosine-activated inward-rectifying current was observed. Although GIRK4 protein can be detected in Western blots of H295R homogenates, we suggest that GIRK4 in aldosterone-producing cells does not form functional G(βγ)-activated channels.

Novel Kv7.1-phosphatidylinositol 4,5-bisphosphate interaction sites uncovered by charge neutralization scanning

Kv7.1 to Kv7.5 α-subunits belong to the family of voltage-gated potassium channels (Kv). Assembled with the β-subunit KCNE1, Kv7.1 conducts the slowly activating potassium current IKs, which is one of the major currents underlying repolarization of the cardiac action potential. A known regulator of Kv7 channels is the lipid phosphatidylinositol 4,5-bisphosphate (PIP2). PIP2 increases the macroscopic current amplitude by stabilizing the open conformation of 7.1/KCNE1 channels. However, knowledge about the exact nature of the interaction is incomplete. The aim of this study was the identification of the amino acids responsible for the interaction between Kv7.1 and PIP2. We generated 13 charge neutralizing point mutations at the intracellular membrane border and characterized them electrophysiologically in complex with KCNE1 under the influence of diC8-PIP2. Electrophysiological analysis of corresponding long QT syndrome mutants suggested impaired PIP2 regulation as the cause for channel dysfunction. To clarify the underlying structural mechanism of PIP2 binding, molecular dynamics simulations of Kv7.1/KCNE1 complexes containing two PIP2 molecules in each subunit at specific sites were performed. Here, we identified a subset of nine residues participating in the interaction of PIP2 and Kv7.1/KCNE1. These residues may form at least two binding pockets per subunit, leading to the stabilization of channel conformations upon PIP2 binding.

Differential effects of genetically-encoded Gβγ scavengers on receptor-activated and basal Kir3.1/Kir3.4 channel current in rat atrial myocytes

Opening of G-protein-activated inward-rectifying K(+) (GIRK, Kir3) channels is regulated by interaction with βγ-subunits of Pertussis-toxin-sensitive G proteins upon activation of appropriate GPCRs. In atrial and neuronal cells agonist-independent activity (I(basal)) contributes to the background K(+) conductance, important for stabilizing resting potential. Data obtained from the Kir3 signaling pathway reconstituted in Xenopus oocytes suggest that I(basal) requires free G(βγ). In cells with intrinsic expression of Kir3 channels this issue has been scarcely addressed experimentally. Two G(βγ)-binding proteins (myristoylated phosducin - mPhos - and G(αi1)) were expressed in atrial myocytes using adenoviral gene transfer, to interrupt G(βγ)-signaling. Agonist-induced and basal currents were recorded using whole cell voltage-clamp. Expression of mPhos and G(αi1) reduced activation of Kir3 current via muscarinic M(2) receptors (IK(ACh)). Inhibition of IK(ACh) by mPhos consisted of an irreversible component and an agonist-dependent reversible component. Reduction in density of IK(ACh) by overexpressed Gαi1, in contrast to mPhos, was paralleled by substantial slowing of activation, suggesting a reduction in density of functional M2 receptors, rather than G(βγ)-scavenging as underlying mechanism. In line with this notion, current density and activation kinetics were rescued by fusing the αi1-subunit to an Adenosine A(1) receptor. Neither mPhos nor G(αi1) had a significant effect on I(basal), defined by the inhibitory peptide tertiapin-Q. These data demonstrate that basal Kir3 current in a native environment is unrelated to G-protein signaling or agonist-independent free G(βγ). Moreover, our results illustrate the importance of physiological expression levels of the signaling components in shaping key parameters of the response to an agonist.

Impaired Na⁺-dependent regulation of acetylcholine-activated inward-rectifier K⁺ current modulates action potential rate dependence in patients with chronic atrial fibrillation

Shortened action-potential duration (APD) and blunted APD rate adaptation are hallmarks of chronic atrial fibrillation (cAF). Basal and muscarinic (M)-receptor-activated inward-rectifier K(+) currents (IK1 and IK,ACh, respectively) contribute to regulation of human atrial APD and are subject to cAF-dependent remodeling. Intracellular Na(+) ([Na(+)]i) enhances IK,ACh in experimental models but the effect of [Na(+)]i-dependent regulation of inward-rectifier K(+) currents on APD in human atrial myocytes is currently unknown. Here, we report a [Na(+)]i-dependent inhibition of outward IK1 in atrial myocytes from sinus rhythm (SR) or cAF patients. In contrast, IK,ACh activated by carbachol, a non-selective M-receptor agonist, increased with elevation of [Na(+)]i in SR. This [Na(+)]i-dependent IK,ACh regulation was absent in cAF. Including [Na(+)]i dependence of IK1 and IK,ACh in a recent computational model of the human atrial myocyte revealed that [Na(+)]i accumulation at fast rates inhibits IK1 and blunts physiological APD rate dependence in both groups. [Na(+)]i-dependent IK,ACh augmentation at fast rates increased APD rate dependence in SR, but not in cAF. These results identify impaired Na(+)-sensitivity of IK,ACh as one potential mechanism contributing to the blunted APD rate dependence in patients with cAF. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Voltage-dependent open-channel block of G protein-gated inward-rectifying K(+) (GIRK) current in rat atrial myocytes by tamoxifen

Tamoxifen (Tmx) is a nonsteroidal selective estrogen receptor antagonist and is frequently used in the treatment and prevention of breast cancer. The compound and its metabolites have been reported to inhibit functions of different classes of membrane proteins, including various ion channels. For members of the inward-rectifying K(+) (Kir) channel family, interference of Tmx with binding of phosphatidylinositol 4,5-bisphosphate (PIP(2)) has been suggested as the mechanism underlying such inhibition. We have studied the inhibition of G protein-activated K(+) (GIRK) current by Tmx in isolated myocytes from hearts of adult rats using whole-cell voltage clamp and experimental conditions for measuring K(+) currents as inward currents (E (K) -50 mV; holding potential -90 mV). Extracellular Tmx reversibly inhibited GIRK current activated by acetylcholine (I (K(ACh))) with an EC(50) of 7.4 × 10(-7) M. This inhibition was composed of two components, a basal reduction in peak current and a block that required opening of channels by ACh. The open-channel block was partially relieved by depolarizing voltage steps in a voltage- and time-dependent fashion. A voltage-dependent open-channel block was not observed when I (K(ACh)) was measured as outward current (E (K) -90 mV; holding potential -40 mV). Intracellular application of Tmx via the patch clamp pipette at a concentration (7 × 10(-6) M) that caused a rapid inhibition of I (K(ACh)) upon extracellular application did not affect the current. Intracellular application of the H(2)O-soluble PIP(2) analog diC(8)-PIP(2) reduced the voltage-independent component of inhibition but had no effect on voltage-dependent open-channel block. The effects of 4-hydroxy-Tmx, a major active metabolite, tested at 2 × 10(-6) M, had effects on I (K(ACh)) analogous to those of Tmx. Inhibition of constitutive inward-rectifying K(+) current (I (K1)) in ventricular myocytes, carried by Kir2 complexes, by Tmx was devoid of a voltage-dependent component. This study suggests the voltage-dependent open-channel block of GIRK inward current as a novel mechanism of Tmx action.

Type 10 adenylyl cyclase mediates mitochondrial Bax translocation and apoptosis of adult rat cardiomyocytes under simulated ischaemia/reperfusion

AIMS

Apoptosis of cardiomyocytes significantly contributes to the development of post-ischaemic cardiomyopathy. Although mitochondria have been suggested to play a crucial role in this process, the precise mechanisms controlling the mitochondria-dependent apoptosis in cardiomyocytes under ischaemia/reperfusion are still poorly understood. Here we aimed to analyse the role of the soluble adenylyl cyclase (sAC).

METHODS AND RESULTS

Adult rat cardiomyocytes were subjected to simulated in vitro ischaemia (SI) consisting of glucose-free anoxia at pH 6.4. Apoptosis was detected by DNA laddering, chromatin condensation, and caspases cleavage. SI led to the translocation of sAC to the mitochondria and mitochondrial depolarization followed by cytochrome c release, caspase-9/-3 cleavage and apoptosis during simulated reperfusion (SR). Pharmacological inhibition of sAC during SI, but not during SR, significantly reduced the SI/SR-induced mitochondrial injury and apoptosis. Similarly, sAC knock-down mediated by an adenovirus coding for shRNA targeting sAC prevented the activation of the mitochondrial pathway of apoptosis. Analysis of the link between sAC and apoptosis revealed a sAC and protein kinase A-dependent Bax phosphorylation at Thr(167) and its translocation to mitochondria during SI, which subsequently caused mitochondrial oxygen radical formation followed by cytochrome c release and caspase-9 cleavage during SR.

CONCLUSION

These results suggest a key role of sAC in SI-induced mitochondrial Bax translocation and activation of the mitochondrial pathway of apoptosis in adult cardiomyocytes.

Altered stress stimulation of inward rectifier potassium channels in Andersen-Tawil syndrome

Inward rectifier potassium channels of the Kir2 subfamily are important determinants of the electrical activity of brain and muscle cells. Genetic mutations in Kir2.1 associate with Andersen-Tawil syndrome (ATS), a familial disorder leading to stress-triggered periodic paralysis and ventricular arrhythmia. To identify the molecular mechanisms of this stress trigger, we analyze Kir channel function and localization electrophysiologically and by time-resolved confocal microscopy. Furthermore, we employ a mathematical model of muscular membrane potential. We identify a novel corticoid signaling pathway that, when activated by glucocorticoids, leads to enrichment of Kir2 channels in the plasma membranes of mammalian cell lines and isolated cardiac and skeletal muscle cells. We further demonstrate that activation of this pathway can either partly restore (40% of cases) or further impair (20% of cases) the function of mutant ATS channels, depending on the particular Kir2.1 mutation. This means that glucocorticoid treatment might either alleviate or deteriorate symptoms of ATS depending on the patient's individual Kir2.1 genotype. Thus, our findings provide a possible explanation for the contradictory effects of glucocorticoid treatment on symptoms in patients with ATS and may open new pathways for the design of personalized medicines in ATS therapy.

A genetically encoded tool kit for manipulating and monitoring membrane phosphatidylinositol 4,5-bisphosphate in intact cells

Background

Most ion channels are regulated by phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂) in the cell membrane by diverse mechanisms. Important molecular tools to study ion channel regulation by PtdIns(4,5)P₂ in living cells have been developed in the past. These include fluorescent PH-domains as sensors for Förster resonance energy transfer (FRET), to monitor changes in plasma membrane_. For controlled and reversible depletion of PtdIns(4,5)P₂, voltage-sensing phosphoinositide phosphatases (VSD) have been demonstrated as a superior tool, since they are independent of cellular signaling pathways. Combining these methods in intact cells requires multiple transfections. We used self-cleaving viral 2A-peptide sequences for adenovirus driven expression of the PH-domain of phospholipase-Cδ1 (PLCδ1) fused to ECFP and EYFP respectively and Ciona intestinalis VSP (Ci-VSP), from a single open reading frame (ORF) in adult rat cardiac myocytes.

Methods and Results

Expression and correct targeting of ECFP-PH-PLCδ1_, EYFP-PH-PLCδ1, and Ci-VSP from a single tricistronic vector containing 2A-peptide sequences first was demonstrated in HEK293 cells by voltage-controlled FRET measurements and Western blotting. Adult rat cardiac myocytes expressed Ci-VSP and the two fluorescent PH-domains within 4 days after gene transfer using the vector integrated into an adenoviral construct. Activation of Ci-VSP by depolarization resulted in rapid changes in FRET ratio indicating depletion of PtdIns(4,5)P₂ in the plasma membrane. This was paralleled by inhibition of endogenous G protein activated K⁺ (GIRK) current. By comparing changes in FRET and current, a component of GIRK inhibition by adrenergic receptors unrelated to depletion of PtdIns(4,5)P₂ was identified.

Conclusions

Expression of a FRET sensor pair and Ci-VSP from a single ORF provides a useful approach to study regulation of ion channels by phosphoinositides in cell lines and transfection-resistant postmitotic cells. Generally, adenoviral constructs containing self-cleaving 2A-peptide sequences are highly suited for simultaneous transfer of multiple genes in adult cardiac myocytes.

Pannexin 1 constitutes the large conductance cation channel of cardiac myocytes

A large conductance (∼300 picosiemens) channel (LCC) of unknown molecular identity, activated by Ca(2+) release from the sarcoplasmic reticulum, particularly when augmented by caffeine, has been described previously in isolated cardiac myocytes. A potential candidate for this channel is pannexin 1 (Panx1), which has been shown to form large ion channels when expressed in Xenopus oocytes and mammalian cells. Panx1 function is implicated in ATP-mediated auto-/paracrine signaling, and a crucial role in several cell death pathways has been suggested. Here, we demonstrate that after culturing for 4 days LCC activity is no longer detected in myocytes but can be rescued by adenoviral gene transfer of Panx1. Endogenous LCCs and those related to expression of Panx1 share key pharmacological properties previously used for identifying and characterizing Panx1 channels. These data demonstrate that Panx1 constitutes the LCC of cardiac myocytes. Sporadic openings of single Panx1 channels in the absence of Ca(2+) release can trigger action potentials, suggesting that Panx1 channels potentially promote arrhythmogenic activities.

Adenovirus-mediated delivery of short hairpin RNA (shRNA) mediates efficient gene silencing in terminally differentiated cardiac myocytes

RNA interference (RNAi) represents the most frequently utilized technique to analyze proteins by loss of function assays. Protein synthesis is impaired by sequence-specific degradation of mRNA, which is triggered by short (19-28 nt) silencing RNAs (siRNA). Efficient gene silencing using RNAi has been demonstrated in numerous cell lines and primary cultured cells. Incorporation of siRNA into terminally differentiated mammalian cells, such as adult cardiac myocytes is limited by their resistance to standard transfection protocols. Viral delivery of short-hairpin RNA (shRNA) overcomes these limitations and allows efficient gene silencing in these cells. This chapter describes the generation and characterization of recombinant siRNA-encoding adenoviruses and their application to adult cardiac myocytes, which represent a standard experimental model in research related to cardiac physiology and pathophysiology. Feasibility of this approach is demonstrated by effective ablation (>80%) of both, a transgene encoding for eGFP and the endogenous muscarinic M(2) acetylcholine receptor.

G protein-activated (GIRK) current in rat ventricular myocytes is masked by constitutive inward rectifier current (I(K1))

Inwardly-rectifying K+ channel subunits are not homogenously expressed in different cardiac tissues. In ventricular myocytes (VM) the background current-voltage relation is dominated by I(K1), carried by channels composed of Kir2.x subunits, which is less important in atrial myocytes (AM). On the other hand in AM a large G protein gated current carried by Kir3.1/3.4 complexes can be activated by stimulation of muscarinic M(2) receptors (I(K(ACh))), which is assumed to be marginal in VM. Recent evidence suggests that total current carried by cardiac inward-rectifiers (I(K(ATP)), I(K(ACh)), I(K1)) in both, AM and VM is limited, due to K+ accumulation/depletion. This lead us to hypothesize that in conventional whole celI recordings I(K(ACh)) in VM is underestimated as a consequence of constitutive I(K1). In that case a reduction in density of I(K1) should be paralleled by an increase in density of I(K(ACh)). Three different experimental strategies have been used to test for this hypothesis: (i) Adenovirus-driven expression of a dominant-negative mutant of Kir2.1, one of the subunits supposed to form I(K1) channels, in VM caused a reduction in I(K1)-density by about 80 %. In those cells I(K(ACh)) was increased about 4 fold. (ii) A comparable increase in I(K(ACh)) was observed upon reduction of I(K1) caused by adenovirus-mediated RNA interference.(iii) Ba2+ in a concentration of 2 microM blocks I(K1) in VM by about 60 % without affecting atrial I(K(ACh)). The reduction in I(K1) by 2 microM Ba2+ is paralleled by a reversible increase in I(K(ACh)) by about 100%. These data demonstrate that the increase in K+ conductance underlying ventricular I(K(ACh)) is largely underestimated, suggesting that it might be of greater physiological relevance than previously thought.

A role for RGS10 in beta-adrenergic modulation of G-protein-activated K+ (GIRK) channel current in rat atrial myocytes

The effect of beta-adrenergic stimulation on endogenous G-protein-activated K(+) (GIRK) current has been investigated in atrial myocytes from hearts of adult rats. Beta-adrenergic stimulation (10 microm isoprenaline, Iso) had no effect on activation kinetics, peak current or steady-state current but resulted in slowing of deactivation upon washout of acetylcholine (ACh), the time constant (tau(d)) being increased by a factor of about 2.5. The effect of Iso could be mimicked by inclusion of cAMP (500 microm) in the filling solution of the patch clamp pipette. The Iso-induced increase in tau(d) was blocked by the selective beta(1) receptor antagonist CGP-20112A (2 microm) and by the PKA inhibitor H9 (100 microm included in the pipette solution). A candidate for mediating these effects is RGS10, one of the regulators of G-protein signalling (RGS) species expressed in cardiac myocytes. Overexpression of RGS10 by adenoviral gene transfer resulted in a reduction in tau(d) of 60%. Sensitivity of tau(d) to Iso remained in cells overexpressing RGS10. Overexpression of RGS4 caused a comparable reduction in tau(d), which became insensitive to Iso. Expression of an RGS10 carrying a mutation (RGS10-S168A), which deletes a PKA phosphorylation site, caused a decrease in tau(d) comparable to overexpression of wild-type RGS10. Sensitivity of tau(d) to Iso was lost in RGS10-S168A-expressing myocytes. Silencing of RGS10 by means of adenovirus-mediated transcription of a short hairpin RNA did not affect basal tau(d) but removed sensitivity to Iso. These data suggest that endogenous RGS10 has GTPase-activating protein (GAP) activity on the G-protein species that mediates activation of atrial GIRK channels. Moreover, RGS10, via PKA-dependent phosphorylation, enables a crosstalk between beta-adrenergic and muscarinic cholinergic signalling.

Generation of a constitutive Na+-dependent inward-rectifier current in rat adult atrial myocytes by overexpression of Kir3.4

Apart from gating by interaction with betagamma subunits from heterotrimeric G proteins upon stimulation of appropriate receptors, Kir.3 channels have been shown to be gated by intracellular Na+. However, no information is available on how Na+-dependent gating affects endogenous Kir3.1/Kir3.4 channels in mammalian atrial myocytes. We therefore studied how loading of adult atrial myocytes from rat hearts via the patch pipette filling solution with different concentrations of Na+ ([Na+]pip) affects Kir3 current. Surprisingly, in a range between 0 and 60 mm, Na+ neither had an effect on basal inward-rectifier current nor on the current activated by acetylcholine. Overexpression of Kir3.4 in adult atrial myocytes forced by adenoviral gene transfer results in formation of functional homomeric channels that interact with betagamma subunits upon activation of endogenous muscarinic receptors. These channels are activated at [Na+]pip >or= 15 mm, resulting in a receptor-independent basal inward rectifier current (I bir). I bir was neither affected by pertussis toxin nor by GDP-beta-S, suggesting G-protein-independent activation. PIP(2) depletion via endogenous PLC-coupled alpha1 adrenergic receptors causes inhibition of endogenous Kir3.1/3.4 channel currents by about 75%. In contrast, inhibition of Na+-activated I bir amounts to < 20%. The effect of the Kir3 channel blocker tertiapin-Q can be described using an IC50 of 12 nm (endogenous I K(ACh)) and 0.61 nm (I bir). These data clearly identify I bir as a homotetrameric Kir3.4 channel current with novel properties of regulation and pharmacology. Ibir shares some properties with a basal current recently described in atrial myocytes from an animal model of atrial fibrillation (AF) and AF patients.

Gene silencing in adult rat cardiac myocytes in vitro by adenovirus-mediated RNA interference

RNA interference (RNAi) by short double stranded RNA (siRNA) represents an efficient and frequently used tool for gene silencing to study gene function. Whereas efficient ablation of genes has been demonstrated in neonatal cardiac myocytes, thus far information on successful application of this technique in adult cardiac myocytes (ACM), a standard experimental model in cardiac physiology and pathophysiology, is sparse. Here we demonstrate efficient ablation of a transgene encoding for enhanced green fluorescent protein (EGFP) and a cell specific endogenous gene encoding for an inward-rectifier channel subunit (Kir2.1) in ACM in vitro using adenovirus driven transcription of siRNA hairpins. EGFP fluorescence and density of background inward rectifier current (IK1) were reduced by > 90% within about 6-8 days after transformation with the corresponding virus. In Kir2.1-silenced myocytes resting membrane potential was significantly reduced. Survival of these cells in culture was compromised, presumably due to Ca2+ -overload caused by the depolarization. The sequence-specific knockdowns of EGFP and Kir2.1 were confirmed on the RNA level using real-time RT-PCR. In Kir2.1-silenced myocytes density of transient outward current, carried predominantly by Kv4.x subunits remained unaffected. This communication for the first time demonstrates proof of principle of efficient RNA interference using adenovirus-based vectors and demonstrates its large potential in phenotyping of ACM.

Inhibition des muskarinergen Kaliumionenstroms durch das neue Klasse-III-Antiarrhythmikum KB130015 im Vergleich zu Ibutilide

BACKGROUND AND PURPOSE

Vagus-induced atrial fibrillation is of particular clinical interest. The muscarinic potassium current I(K(ACh)) mediates the induction of vagus-induced atrial fibrillation. Selective inhibition of I(K(ACh)) seems to be an option to treat atrial fibrillation. The application of amiodarone, presently one of the most important antiarrhythmic agents in the parmacological treatment of atrial fibrillation, is limited by its adverse effects. KB130015, a new amiodarone derivative, and ibutilide are new class III antiarrhythmic agents.

METHODS

In guinea-pig atrial myocytes the muscarinic potassium current (I(K(ACh))) was activated by acetylcholine and adenosine. The effect of KB130015 on I(K(ACh)) was measured using the whole-cell voltage-clamp method.

RESULTS

KB130015 and ibutilide in a concentration of 50 microM effectively inhibited the muscarinic potassium current. The effect was concentrationdependent and reversible. The half-maximum effective concentration was 0.8 microM (KB130015) and 2.8 microM (ibutilide). The inhibition of I(K(ACh)) was independent of the mode of its activation. The adenosine-induced ion current was as well inhibited by both drugs as the acetylcholine-induced ion current. Via GTP-gamma-S irreversibly activated I(K(ACh)) was also inhibited by KB130015 and ibutilide, whereas intracellular application showed no effect on I(K(ACh)).

CONCLUSION

KB130015 and ibutilide are potent inhibitors of IK(ACh). Their effect is most likely mediated by direct interaction with the extracellular part of the ion channel. Acute effects of KB130015 on ventricular myocardium are not known so far. Ibutilide on the other hand is known to inhibit I(kr). KB130015 is a promising antiarrhythmic agent for the pharmacotherapy of vagus-induced atrial fibrillation.

[PMMA pulmonary embolism and post interventional associated fractures after percutaneous vertebroplasty]

A 78-year old woman with osteoporotic collapse of the Th12 and L4 vertebrae was treated by percutaneous vertebroplasty (pVp) with PMMA (polymethylmethacrylate). Postoperatively, the Th11 and L1 vertebrae collapsed so that a second vertebroplasty was performed. Postoperatively, the patient developed a severe pulmonary embolism which was treated conservatively. In this report, the complications of pulmonary embolism, perivertebral leakage of PMMA and of additional vertebral collapses after pVp are discussed.

Differential subunit composition of the G protein-activated inward-rectifier potassium channel during cardiac development

Parasympathetic slowing of the heart rate is predominantly mediated by acetylcholine-dependent activation of the G protein-gated potassium (K+) channel (IK,ACh). This channel is composed of 2 inward-rectifier K+ (Kir) channel subunits, Kir3.1 and Kir3.4, that display distinct functional properties. Here we show that subunit composition of IK,ACh changes during embryonic development. At early stages, IK,ACh is primarily formed by Kir3.1, while in late embryonic and adult cells, Kir3.4 is the predominant subunit. This change in subunit composition results in reduced rectification of IK,ACh, allowing for marked K+ currents over the whole physiological voltage range. As a consequence, IK,ACh is able to generate the membrane hyperpolarization that underlies the strong negative chronotropy occurring in late- but not early-stage atrial cardiomyocytes upon application of muscarinic agonists. Both strong negative chronotropy and membrane hyperpolarization can be induced in early-stage cardiomyocytes by viral overexpression of the mildly rectifying Kir3.4 subunit. Thus, a switch in subunit composition is used to adopt IK,ACh to its functional role in adult cardiomyocytes.

Acute desensitization of GIRK current in rat atrial myocytes is related to K+ current flow

We have investigated the acute desensitization of acetylcholine-activated GIRK current (I(K(ACh))) in cultured adult rat atrial myocytes. Acute desensitization of I(K(ACh)) is observed as a partial relaxation of current with a half-time of < 5 s when muscarinic M2 receptors are stimulated by a high concentration (> 2 micromol l(-1)) of ACh. Under this condition experimental manoeuvres that cause a decrease in the amplitude of I(K(ACh)), such as partial block of M2 receptors by atropine, intracellular loading with GDP-beta-S, or exposure to Ba2+, caused a reduction in desensitization. Acute desensitization was also identified as a decrease in current amplitude and a blunting of the response to saturating [ACh] (20 micromol l(-1)) when the current had been partially activated by a low concentration of ACh or by stimulation of adenosine A1 receptors. A reduction in current analogous to acute desensitization was observed when ATP-dependent K+ current (I(K(ATP))) was activated either by mitochondrial uncoupling using 2,4-dinitrophenole (DNP) or by the channel opener rilmakalim. Adenovirus-driven overexpression of Kir2.1, a subunit of constitutively active inwardly rectifying K+ channels, resulted in a large Ba2+-sensitive background K+ current and a dramatic reduction of ACh-activated current. Adenovirus-driven overexpression of GIRK4 (Kir3.4) subunits resulted in an increased agonist-independent GIRK current paralleled by a reduction in I(K(ACh)) and removal of the desensitizing component. These data indicate that acute desensitization depends on K+ current flow, independent of the K+ channel species, suggesting that it reflects a reduction in electrochemical driving force rather than a bona fide signalling mechanism. This is supported by the observation that desensitization is paralleled by a significant negative shift in reversal potential of I(K(ACh)). Since the ACh-induced hyperpolarization shows comparable desensitization properties as I(K(ACh)), this novel current-dependent desensitization is a physiologically relevant process, shaping the time course of parasympathetic bradycardia.

Voltage dependence of ATP-dependent K+ current in rat cardiac myocytes is affected by IK1 and IK(ACh)

In this study we have investigated the voltage dependence of ATP-dependent K+ current (I(K(ATP))) in atrial and ventricular myocytes from hearts of adult rats and in CHO cells expressing Kir6.2 and SUR2A. The current-voltage relation of 2,4-dinitrophenole (DNP) -induced I(K(ATP)) in atrial myocytes and expressed current in CHO cells was linear in a voltage range between 0 and -100 mV. In ventricular myocytes, the background current-voltage relation of which is dominated by a large constitutive inward rectifier (I(K1)), the slope conductance of I(K(ATP)) was reduced at membrane potentials negative to E(K) (around -50 mV), resulting in an outwardly rectifying I-V relation. Overexpression of Kir2.1 by adenoviral gene transfer, a subunit contributing to I(K1) channels, in atrial myocytes resulted in a large I(K1)-like background current. The I-V relation of I(K(ATP)) in these cells showed a reduced slope conductance negative to E(K) similar to ventricular myocytes. In atrial myocytes with an increased background inward-rectifier current through Kir3.1/Kir3.4 channels (I(K(ACh))), irreversibly activated by intracellular loading with GTP-gamma-S, the I-V relation of I(K(ATP)) showed a reduced slope negative to E(K), as in ventricular myocytes and atrial myocytes overexpressing Kir2.1. It is concluded that the voltage dependencies of membrane currents are not only dependent on the molecular composition of the charge-carrying channel complexes but can be affected by the activity of other ion channel species. We suggest that the interference between inward I(K(ATP)) and other inward rectifier currents in cardiac myocytes reflects steady-state changes in K+ driving force due to inward K+ current.

Coupling to Gs and G(q/11) of histamine H2 receptors heterologously expressed in adult rat atrial myocytes

The predominant histamine receptor subtype in the supraventricular and ventricular tissue of various mammalian species is the H2 receptor (H2-R) subtype, which is known to couple to stimulatory G proteins (Gs), i.e. the major effects of this autacoid are an increase in sinus rate and in force of contraction. To investigate histamine effects in H2-R-transfected rat atrial myocytes, endogenous GIRK currents and L-type Ca2+ currents were used as functional assays. In H2-R-transfected myocytes, exposure to His resulted in a reversible augmentation of L-type Ca2+ currents, consistent with the established coupling of this receptor to the Gs-cAMP-PKA signalling pathway. Mammalian K+ channels composed of GIRK (Kir3.x) subunits are directly controlled by interaction with betagamma subunits released from G proteins, which couple to seven-helix receptors. In mock-transfected atrial cardiomyocytes, activation of muscarinic K+ channels (IK(ACh)) was limited to Gi-coupled receptors (M2R, A1R). In H2-R-overexpressing cells, histamine activated IK(ACh) via Gs-derived betagamma subunits since the histamine-induced current was insensitive to pertussis toxin. These data indicate that overexpression of Gs-coupled H2-R results in a loss of target specificity due to an increased agonist-induced release of Gs-derived betagamma subunits. When IK(ACh) was maximally activated by GTP-gamma-S, histamine induced an irreversible inhibition of the inward current in a fraction of H2-R-transfected cells. This inhibition is supposed to be mediated via a G(q/11)-PLC-mediated depletion of PIP2, suggesting a partial coupling of overexpressed H2-R to G(q/11). Dual coupling of H2-Rs to Gs and Gq is demonstrated for the first time in cardiac myocytes. It represents a novel mechanism to augment positive inotropic effects by activating two different signalling pathways via one type of histamine receptor. Activation of the Gs-cAMP-PKA pathway promotes Ca2+ influx through phosphorylation of L-type Ca2+ channels. Simultaneous activation of Gq-signalling pathways might result in phosphoinositide turnover and Ca2+ release from intracellular stores, thereby augmenting H2-induced increases in [Ca2+]i.

G protein-independent inhibition of GIRK current by adenosine in rat atrial myocytes overexpressing A1 receptors after adenovirus-mediated gene transfer

G protein-activated inwardly rectifying K+ (GIRK) channels, important regulators of membrane excitability in the heart and central nervous system, are activated by interaction with betagamma subunits from heterotrimeric G proteins upon receptor stimulation. In atrial myocytes various endogenous receptors couple to GIRK channels, including the canonical muscarinic M2 receptor (M2AChR) and the A1 adenosine receptor (A1AdoR). Saturating stimulation of A1AdoR in atrial myocytes activates only a fraction of the GIRK current that is activated via M2AChR, which reflects a lower density of A1AdoR. In the present study A1AdoR were overexpressed by means of adenovirus-mediated gene transfer using green fluorescent protein (GFP) as the reporter. Confirmatory to a previous study, this resulted in an increased sensitivity of macroscopic GIRK current (ACh-activated K+ current (IK(ACh))) to stimulation by Ado. However, in the majority of GFP-positive myocytes, exposure to Ado at concentrations > or =1 microM resulted in activation of IK(ACh) followed by a rapid inhibition. In those cells a rebound activation of current was recorded upon washout of Ado. The inhibitory component could be recorded in isolation when IK(ACh) was activated by M2AChR-stimulation and brief pulses of Ado were superimposed. In myocytes loaded with GTP-gamma-S, IK(ACh), irreversibly activated by brief exposure to agonist, was still reversibly inhibited by Ado, suggesting that inhibition is independent of G protein cycling. In myocytes co-transfected with adenoviral vectors encoding A1AdoR and GIRK4 subunit, no inhibition of GIRK current by Ado was observed. As acute desensitization of atrial GIRK current, which is reminiscent of the inhibition described here, has been shown to be absent in myocytes overexpressing GIRK4, this suggests that acute desensitization and the novel inhibition might share a common pathway whose target is the GIRK channel complex or its GIRK1 subunit.

Transfection with 5-HT1A receptor gene and antisense directed against muscarinic M2 receptors reveal a mutual influence between Gi/o-coupled receptors in rat atrial myocytes

A recently described reduction in sensitivity of G protein-activated inward-rectifying K(+) (GIRK) channels to stimulation of muscarinic M(2) receptors (M(2)AChR) in atrial myocytes overexpressing purinergic A(1) receptors (A(1)AdoR) was further investigated by heterologous expression of a 5-HT(1A) receptor (5-HT(1A)R) and by reducing the expression level of endogenous M(2)AChR receptors using antisense. In 5-HT(1A)R-expressing myocytes, in line with previous studies, sizable GIRK currents could be activated by 5-HT. In these cells, the mean current density and activation rate of M(2)AChR-activated current were significantly reduced, supporting the notion that signalling via this receptor is negatively regulated by other G protein-coupled receptors (GPCR) coupling to the same class (G(i/o)) of G proteins. To study if reducing M(2)AChR expression affects sensitivity of GIRK current to stimulation of A(1)AdoR, antisense oligodinucleotides (AsODN) against the M(2)AChR were used. Incubation of myocytes with M(2)AChR-specific AsODN resulted in a significant reduction in mean amplitude and activation rate of ACh-induced currents. This was paralleled by an increase in mean amplitude and activation rate of current activated by stimulation of A(1)AdoR. Plotting amplitudes of 5-HT- or Ado-induced currents from individual manipulated cells against the amplitude of ACh-induced current yielded a positive correlation between these data. Although difficult to interpret in mechanistic terms, this argues against a competition of receptors for a common pool of G(i/o). The mutual interaction between G(i/o)-coupled receptors depends on manipulation of the expression level, since long-term desensitization or down regulation of M(2)AChR by treatment with carbachol did not affect sensitivity of GIRK current to A(1)AdoR stimulation, despite a substantial reduction in amplitude and activation rate of M(2)AChR-activated currents. These data suggest a novel crosstalk between parallel receptors converging on the same class of G proteins.

Transfection of a phosphatidyl-4-phosphate 5-kinase gene into rat atrial myocytes removes inhibition of GIRK current by endothelin and alpha-adrenergic agonists

GIRK (G protein-activated inward-rectifying K(+) channel) channels, important regulators of membrane excitability in the heart and in the central nervous, are activated by interaction with betagamma subunits from heterotrimeric G proteins upon receptor stimulation. For activation interaction of the channel with phosphatidylinositol 4,5-bisphosphate (PtIns(4,5)P(2)) is conditional. Previous studies have provided evidence that in myocytes PtIns(4,5)P(2) levels relevant to GIRK channel regulation are under regulatory control of receptors activating phospholipase C. In the present study a phosphatidyl-4-phosphate 5-kinase was expressed in atrial myocytes by transient transfection. This did not affect basal properties of GIRK current activated by acetylcholine via M(2) receptors but completely abolished inhibition of guanosine triphosphate-gamma-S activated current by endothelin-1 or alpha-adrenergic agonists. We conclude that though PtIns(4,5)P(2) is conditional for channel gating, its normal level in the membrane is not limiting basal function of GIRK channels. Moreover, our data provide further evidence for a regulation of GIRK channels by alpha(1A) receptors and endothelin-A receptors, endogenously expressed in atrial myocytes, via depletion of PtIns(4,5)P(2).

Overexpression of beta 1 and beta 2 adrenergic receptors in rat atrial myocytes. Differential coupling to G protein-gated inward rectifier K(+) channels via G(s) and G(i)/o

G protein-activated inwardly rectifying K(+) (GIRK) channels, expressed in atrial myocytes, various neurons, and endocrine cells, represent the paradigmatic target of beta gamma subunits released from activated heterotrimeric G proteins. These channels contribute to physiological slowing of cardiac frequency and synaptic inhibition. They are activated by beta gamma dimers released upon stimulation of receptors coupled to pertussis toxin-sensitive G proteins (G(i/o)), whereas beta gamma released from G(s) do not converge on the channel subunits. This is in conflict with the finding that dimeric combinations of various beta and gamma subunits can activate GIRK channels with little specificity. In the present study, we have overexpressed the major subtypes of cardiac beta-adrenergic receptors (beta(1)-AR and beta(2)-AR) in atrial myocytes by transient transfection. Whereas in native cells beta-adrenergic stimulation with isoproterenol failed to induce measurable GIRK current, robust currents were recorded from myocytes overexpressing either beta(1)-AR or beta(2)-AR. Whereas the beta(2)-AR-induced current showed the same sensitivity to pertussis toxin as the current evoked by the endogenous G(i/o)-coupled muscarinic M(2) receptor, isoproterenol-activated currents were insensitive to pertussis toxin treatment in beta(1)-AR-overexpressing myocytes. In contrast to a recent publication (Leaney, J. L., Milligan, G., and Tinker, A. (2000) J. Biol. Chem. 275, 921-929), sizable GIRK currents could also be activated by isoproterenol when the signaling pathway was reconstituted by transient transfection in two different standard cell lines (Chinese hamster ovary and HEK293). These results demonstrate that specificity of receptor-G protein signaling can be disrupted by overexpression of receptors. Moreover, the alpha subunit of heterotrimeric G proteins does not confer specificity to G beta gamma-mediated signaling.

Overexpression of monomeric and multimeric GIRK4 subunits in rat atrial myocytes removes fast desensitization and reduces inward rectification of muscarinic K(+) current (I(K(ACh))). Evidence for functional homomeric GIRK4 channels

K(+) channels composed of G-protein-coupled inwardly rectifying K(+) channel (GIRK) (Kir3.0) subunits are expressed in cardiac, neuronal, and various endocrine tissues. They are involved in inhibiting excitability and contribute to regulating important physiological functions such as cardiac frequency and secretion of hormones. The functional cardiac (K((ACh))) channel activated by G(i)/G(o)-coupled receptors such as muscarinic M(2) or purinergic A(1) receptors is supposed to be composed of the subunits GIRK1 and GIRK4 in a heterotetrameric (2:2) fashion. In the present study, we have manipulated the subunit composition of the K((ACh)) channels in cultured atrial myocytes from hearts of adult rats by transient transfection of vectors encoding for GIRK1 or GIRK4 subunits or GIRK4 concatemeric constructs and investigated the effects on properties of macroscopic I(K(ACh)). Transfection with a GIRK1 vector did not cause any measurable effect on properties of I(K(ACh)), whereas transfection with a GIRK4 vector resulted in a complete loss in desensitization, a reduction of inward rectification, and a slowing of activation. Transfection of myocytes with a construct encoding for a concatemeric GIRK4(2) subunit had similar effects on desensitization and inward rectification. Following transfection of a tetrameric construct (GIRK4(4)), these changes in properties of I(K(ACh)) were still observed but were less pronounced. Heterologous expression in Chinese hamster ovary cells and human embryonic kidney 293 cells of monomeric, dimeric, and tetrameric GIRK4 resulted in robust currents activated by co-expressed A(1) and M(2) receptors, respectively. These data provide strong evidence that homomeric GIRK4 complexes form functional G(beta)gamma gated ion channels and that kinetic properties of GIRK channels, such as activation rate, desensitization, and inward rectification, depend on subunit composition.

Sphingosylphosphocholine is a naturally occurring lipid mediator in blood plasma: a possible role in regulating cardiac function via sphingolipid receptors

Blood plasma and serum contain factors that activate inwardly rectifying GIRK1/GIRK4 K+ channels in atrial myocytes via one or more non-atropine-sensitive receptors coupled to pertussis-toxin-sensitive G-proteins. This channel is also the target of muscarinic M(2) receptors activated by the physiological release of acetylcholine from parasympathetic nerve endings. By using a combination of HPLC and TLC techniques with matrix-assisted laser desorption ionization-time-of-flight MS, we purified and identified sphingosine 1-phosphate (SPP) and sphingosylphosphocholine (SPC) as the plasma and serum factors responsible for activating the inwardly rectifying K+ channel (I(K)). With the use of MS the concentration of SPC was estimated at 50 nM in plasma and 130 nM in serum; those concentrations exceeded the 1.5 nM EC(50) measured in guinea-pig atrial myocytes. With the use of reverse-transcriptase-mediated PCR and/or Western blot analysis, we detected Edg1, Edg3, Edg5 and Edg8 as well as OGR1 sphingolipid receptor transcripts and/or proteins. In perfused guinea-pig hearts, SPC exerted a negative chronotropic effect with a threshold concentration of 1 microM. SPC was completely removed after perfusion through the coronary circulation at a concentration of 10 microM. On the basis of their constitutive presence in plasma, the expression of specific receptors, and a mechanism of ligand inactivation, we propose that SPP and SPC might have a physiologically relevant role in the regulation of the heart.

Depletion of phosphatidylinositol 4,5-bisphosphate by activation of phospholipase C-coupled receptors causes slow inhibition but not desensitization of G protein-gated inward rectifier K+ current in atrial myocytes

G protein-gated inwardly rectifier K+ current in atrial myocytes (I(K(ACh))) upon stimulation with acetylcholine (ACh) shows a fast desensitizing component (t(1/2) approximately 5 s). After washout of ACh, I(K(ACh)) recovers from fast desensitization within < 30 s. A recent hypothesis suggests that fast desensitization is caused by depletion of phosphatidylinositol 4,5-bisphosphate (PtIns(4,5)P(2)), resulting from costimulation of phospholipase C (PLC)-coupled M3 receptors (M3AChR). The effects of stimulating two established PLC-coupled receptors, alpha-adrenergic and endothelin (ET(A)), on I(K(ACh)) were studied in rat atrial myocytes. Stimulation of these receptors caused activation of I(K(ACh)) and inhibition of the M2AChR-activated current. In myocytes loaded with GTPgammaS (guanosine 5'-3-O-(thio)triphosphate), causing stable activation of I(K(ACh)), inhibition via alpha-agonists and ET-1 was studied in isolation. Stimulation of either type of receptor under this condition, via G(q/11), caused a slow inhibition (t(1/2) approximately 50 s) by about 70%. No comparable effect on GTPgammaS-activated I(K(ACh)) was induced by ACh, suggesting that PLC-coupled M3AChRs are not functionally expressed in rat myocytes, which was supported by the finding that M3AChR transcripts were not detected by reverse transcriptase-polymerase chain reaction in identified atrial myocytes. Supplementing the pipette solution with PtIns(4,5)P(2) significantly reduced inhibition of I(K(ACh)) but had no effect on fast desensitization. From these data it is concluded that stimulation of PLC-coupled receptors causes slow inhibition of I(K(ACh)) by depletion of PtIns(4,5)P(2), whereas fast desensitization of I(K(ACh)) is not related to PtIns(4,5)P(2) depletion. As muscarinic stimulation by ACh does not exert inhibition of I(K(ACh)) comparable to stimulation of alpha(1)- and ET(A) receptors, expression of functional PLC-coupled muscarinic receptors in rat atrial myocytes is unlikely.

Uncoupling of ATP-sensitive potassium current from cell metabolism due to antisense oligonucleotides against sulfonylurea receptor in guinea-pig atrial myocytes

ATP-sensitive K+ current (IK(ATP)) plays an important role in the regulation of cardiac electrical activity. In the myocardium, IK(ATP) is regulated by the sulfonylurea receptor (SURIIA) which mediates the inhibition of IK(ATP) due to glibenclamide (Gli). The role played by SURIIA in the sensitivity of IK(ATP) to metabolic inhibition is unclear. We studied the effect of SURIIA antisense oligonucleotides (ODNs) on the properties of IK(ATP) in cultured guinea-pig atrial myocytes. IK(ATP) was measured by the whole-cell voltage clamp method and was activated with cromakalim (Cro; 200 microns) and dinitrophenole (DNP; 100 microns). Mean IK(ATP) density activated by DNP and Cro in nonincubated cells was 117 +/- 12 pA/pF (n = 17) and 17 +/- 9 pA/pF (n = 16) respectively. No significant difference was observed after incubation with nODN [DNP: 121 +/- 13 pA/pF (n = 20); Cro:19 +/- 4 pA/pF (n = 8)]. Cells incubated with ODNs showed a significant reduction of IK(ATP) due to DNP (19 +/- 13 pA/pF; P < 0.05, n = 6), whereas Cro-induced IK(ATP) was unaffected (16 +/- 8 pA/pF, n = 8). The effectiveness of DNP-induced metabolic inhibition was apparent in a concomitant reduction of the nucleotide-phosphate dependent muscarinic K+ current (inhibition of IK(ACh) in ODN incubated myocytes without activation of IK(ATP)). The ATP sensitivity of IK(ATP) appears mediated by SURIIA. Activation of this current by Cro seems to be SURIIA-independent. ODN-induced metabolic uncoupling of IK(ATP) may be a useful experimental tool. A reduced sensitivity of IK(ATP) to intracellular ATP concentrations may be of clinical interest.

Novel inhibition of gbetagamma-activated potassium currents induced by M(2) muscarinic receptors via a pertussis toxin-insensitive pathway

G(i) protein-coupled receptors such as the M(2) muscarinic acetylcholine receptor (mAChR) and A(1) adenosine receptor have been shown to activate G protein-activated inwardly rectifying K(+) channels (GIRKs) via pertussis toxin-sensitive G proteins in atrial myocytes and in many neuronal cells. Here we show that muscarinic M(2) receptors not only activate but also reversibly inhibit these K(+) currents when stimulated with agonist for up to 2 min. The M(2) mAChR-mediated inhibition of the channel was also observed when the channels were first activated by inclusion of guanosine 5'-O-(thiotriphosphate) in the pipette. Under these conditions the M(2) mAChR-induced inhibition was quasi-irreversible, suggesting a role for G proteins in the inhibitory process. In contrast, when GIRK currents were maximally activated by co-expressing exogenous Gbetagamma, the extent of acetylcholine (ACh)-induced inhibition was significantly reduced, suggesting competition between the receptor-mediated inhibition and the large pool of available Gbetagamma subunits. The signaling pathway that led to the ACh-induced inhibition of GIRK channels was unaffected by pertussis toxin pretreatment. Furthermore, the internalization and agonist-induced phosphorylation of M(2) mAChR was not required because a phosphorylation- and internalization-deficient mutant of the M(2) mAChR was as potent as the wild-type counterpart. Pharmacological agents modulating various protein kinases or phosphatidylinositol 3-kinase did not affect the inhibition of GIRK currents. Furthermore, the signaling pathway that mediates GIRK current inhibition was found to be membrane-delimited because bath application of ACh did not inhibit GIRK channel activity in cell-attached patches. Other G protein-coupled receptors including M(4) mAChR and alpha(1A) adrenergic receptors also caused the inhibition, whereas other G protein-coupled receptors including A(1) and A(3) adenosine receptors and alpha(2A) and alpha(2C) adrenergic receptors could not induce the inhibition. The presented results suggest the existence of a novel signaling pathway that can be activated selectively by M(2) and M(4) mAChR but not by adenosine receptors and that involves non-pertussis toxin-sensitive G proteins leading to an inhibition of Gbetagamma-activated GIRK currents in a membrane-delimited fashion.

Overexpressed A(1) adenosine receptors reduce activation of acetylcholine-sensitive K(+) current by native muscarinic M(2) receptors in rat atrial myocytes

In adult rat atrial myocytes, muscarinic acetylcholine (ACh)-sensitive K(+) current activated by a saturating concentration of adenosine (I(K(ACh),(Ado))) via A(1) receptors (A(1)Rs) amounts to only 30% of the current activated by a saturating concentration of ACh (I(K(ACh),(ACh))) via muscarinic M(2) receptors. The half-time of activation of I(K(ACh),(Ado)) on a rapid exposure to agonist was approximately 4-fold longer than that of I(K(ACh),(ACh)). Furthermore, I(K(ACh),(Ado)) never showed fast desensitization. To study the importance of receptor density for A(1)R-I(K(ACh),(Ado)) signaling, adult atrial myocytes in vitro were transfected with cDNA encoding for rat brain A(1)R and enhanced green fluorescent protein (EGFP) as a reporter. Whole-cell current was measured on days 3 and 4 after transfection. Time-matched cells transfected with only the EGFP vector served as controls. In approximately 30% of EGFP-positive cells (group I), the density of I(K(ACh),(Ado)) was increased by 72%, and its half-time of activation was reduced. Density and kinetic properties of I(K(ACh),(ACh)) were not affected in this fraction. In approximately 70% of transfection-positive myocytes (group II), the density of I(K(ACh),(ACh)) was significantly reduced, its activation was slowed, and the fast desensitizing component was lost. Adenosine-induced currents were larger in group II than in group I, their activation rate was further increased, and a fast desensitizing component developed. These data indicate that in native myocytes the amplitude and activation kinetics of I(K(ACh),(Ado)) are limited by the expression of A(1)R. Overexpression of A(1)R negatively interferes with signal transduction via the muscarinic M(2) receptor-linked pathway, which might reflect a competition of receptors with a common pool of G proteins. Negative interference of an overexpressed receptor with physiological regulation of a target protein by a different receptor should be considered in attempts to use receptor overexpression for gene therapy.

Ca2+ entry but not Ca2+ release is necessary for desensitization of ET(A)receptors in airway apithelial cells

Ca2+ transients evoked by endothelin-1 (ET-1) were measured in single cells of a human tracheal epithelial cell line using the fluorescent Ca2+ indicator fura-2. In line with a previous study, a single exposure to ET-1 (10 nM) for 10-20 s resulted in a long-lasting desensitization to a subsequent challenge by the peptide, without affecting sensitivity to agonists for other Ca2+-mobilizing receptors such as P2y or H1, respectively. In the absence of extracellular Ca2+ ET-1 elicited a Ca2+ signal of comparable amplitude as in the presence of extracellular Ca2+ but of shorter duration. Exposure to ET-1 in the absence of Ca2+ caused significantly less desensitization. Inhibition of the Ca2+ entry component of the Ca2+ transient by means of SK&F 96365, an inhibitor of Ca2+ entry, had effects comparable to Ca2+ removal. The Ca2+ transient was shortened but not significantly reduced in amplitude, and desensitization was reduced in the presence of the compound. These data demonstrate that desensitization of ET(A) receptors (ET(A)R) is promoted by transmembrane Ca2+ entry but not by Ca2+ release.

Extracellular links in Kir subunits control the unitary conductance of SUR/Kir6.0 ion channels

Potassium (K+) channels are highly selective for K+ ions but their unitary conductances are quite divergent. Although Kir6.1 and Kir6.2 are highly homologous and both form functional K+ channels with sulfonylurea receptors, their unitary conductances measured with 150 mM extracellular K+ are approximately 35 and 80 pS, respectively. We found that a chain of three amino acid residues N123-V124-R125 of Kir6.1 and S113-I114-H115 of Kir6.2 in the M1-H5 extracellular link and single residues M148 of Kir6.1 and V138 of Kir6.2 in the H5-M2 link accounted for the difference. By using a 3D structure model of Kir6.2, we were able to recognize two independent plausible mechanisms involved in the determination of single channel conductance of the Kir6.0 subunits: (i) steric effects at Kir6.2V138 or Kir6.1M148 in the H5-M2 link influence directly the diffusion of K+ ions; and (ii) structural constraints between Kir6.2S113 or Kir6. 1N123 in the M1-H5 link and Kir6.2R136 or Kir6.1R146 near the H5 region control the conformation of the permeation pathway. These mechanisms represent a novel and possibly general aspect of the control of ion channel permeability.

Antisense oligonucleotides against receptor kinase GRK2 disrupt target selectivity of beta-adrenergic receptors in atrial myocytes

K+ channels composed of GIRK subunits are predominantly expressed in the heart and various regions of the brain. They are activated by betagamma-subunits released from pertussis toxin-sensitive G-proteins coupled to different seven-helix receptors. In rat atrial myocytes, activation of K(ACh) channels is strictly limited to receptors coupled to pertussis toxin-sensitive G-proteins. Upon treatment of myocytes with antisense oligodesoxynucleotides against GRK2, a receptor kinase with Gbetagamma binding sites, in a fraction of cells, K(ACh) channels can be activated by beta-adrenergic receptors. Sensitivity to beta-agonist is insensitive to pertussis toxin treatment. These findings demonstrate a potential role of Gbetagamma binding proteins for target selectivity of G-protein-coupled receptors.