Downregulation of muscarinic M2 receptors linked to K+ current in cultured guinea-pig atrial myocytes

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.

Dynamics of G protein effector interactions and their impact on timing and sensitivity of G protein-mediated signal transduction

G protein coupled receptors regulate numerous cellular functions primarily via coupling to heterotrimeric G proteins and subsequent regulation of effector proteins such as ion channels, enzymes or GTP exchange factors for small G proteins. The dynamics of interactions between G protein subunits and effectors have been difficult to study particularly in a cellular context. The introduction of Förster resonance energy transfer methods into the field of GPCR research led to interesting insights into the temporal patterns of interactions between G protein subunits and their effectors. In this review we specifically focus on the interaction of Gαi subunits with adenylyl cyclases and of Gαq subunits with p63RhoGEF or G protein coupled receptor kinases type 2. Comparing the dynamics of these interactions revealed remarkable differences between different G protein effectors regarding the ability to be modulated by members of the regulator of G protein signalling protein family as well as the sensitivity towards receptor activation.

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.

Autocrine signaling via A(1) adenosine receptors causes downregulation of M(2) receptors in adult rat atrial myocytes in vitro

G protein-activated K(+) channels composed of Kir3 (GIRK) subunits contribute to regulation of heart rate and excitability. Opening of these channels in myocytes is increased by binding of G(βγ) upon activation of muscarinic M(2) receptors (M(2)-R) or A(1) adenosine receptors (A(1)-R). It has been shown that saturating activation of A(1)-R resulted in a smaller GIRK current than activation of M(2)-R. Adenovirus-driven overexpression of the A(1)-R caused an increase in current induced by adenosine (I(K(Ado))), whereas the M(2)-R-activated current (I(K(ACh))) was reduced. Here, we sought to get deeper insight into the mechanism causing this negative crosstalk. GIRK current in cultured rat atrial myocytes was recorded in whole cell mode. Adenovirus-driven RNA interference targeting the M(2)-R resulted in a reduction in I(K(ACh)) without affecting I(K(Ado)), arguing against a competition of the two receptors for common signaling complexes. The negative effect of A(1)-R overexpression on I(K(ACh)) was reduced by the A(1)-R antagonist DPCPX and augmented by the agonist chloro-N6-cyclopentyladenosin (CCPA). In native myocytes incubation with either CCPA or the muscarinic agonist carbachol resulted in reduction in I(K(ACh)) and I(K(Ado)), suggesting common pathways of A(1)-R and M(2)-R downregulation. In the absence of agonist, inhibition of adenosine deaminase by EHNA or exposure to AMP, less to ADP, but not ATP resulted in reduction of I(K(ACh)) and I(K(Ado)). Our data indicate that atrial myocytes generate adenosine from extracellular AMP, which activates A(1)-R in an autocrine fashion. Chronic activation of A(1)-R causes parallel downregulation of both A(1)-R and M(2)-R.

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.

Relaxation gating of the acetylcholine-activated inward rectifier K+ current is mediated by intrinsic voltage sensitivity of the muscarinic receptor

Normal heart rate variability is critically dependent upon the G-protein-coupled, acetylcholine (ACh)-activated inward rectifier K+ current, I(KACh). A unique feature of I(KACh) is the so-called ‘relaxation' gating property that contributes to increased current at hyperpolarized membrane potentials. I(KACh) relaxation refers to a slow decrease or increase in current magnitude with depolarization or hyperpolarization, respectively. The molecular mechanism underlying this perplexing gating behaviour remains unclear. Here, we consider a novel explanation for I(KACh) relaxation based upon the recent finding that G-protein-coupled receptors (GPCRs) are intrinsically voltage sensitive and that the muscarinic agonists acetylcholine (ACh) and pilocarpine (Pilo) manifest opposite voltage-dependent I(KACh) modulation. We show that Pilo activation of I(KACh) displays relaxation characteristics opposite to that of ACh. We explain the opposite effects of ACh and Pilo using Markov models of I(KACh) that incorporate ligand-specific, voltage-dependent parameters. Based on experimental and computational findings, we propose a novel molecular mechanism to describe the enigmatic relaxation gating process: I(KACh) relaxation represents a voltage-dependent change in agonist affinity as a consequence of a voltage-dependent conformational change in the muscarinic receptor.

Coupling mode of receptors and G proteins

Signaling via G-protein-coupled receptors (GPCRs) is crucial to many physiological and pathophysiological processes in multicellular organisms, and GPCRs themselves are targets for important drugs. Classical cell supplementation experiments suggest a collision coupling model, in which receptors and G proteins diffuse randomly within the cell membrane and interact only if receptors are activated. This model is also backed by kinetic and live cell imaging data. According to the challenging theory, receptors and G proteins are precoupled--meaning they are forming stable complexes in the absence of agonist, which prevail during signaling. This model has been favored on the basis of copurification and coimmunoprecipitation of inactive receptors with G proteins and more recently by some approaches measuring energy transfer between labeled receptors and G proteins. This article reviews key findings regarding the receptor/G protein coupling mode, including most recent findings obtained by optical techniques.

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.

Role of internalization of M2muscarinic receptor via clathrin-coated vesicles in desensitization of the muscarinic K+current in heart

In the heart, ACh activates the ACh-activated K+current ( IK,ACh) via the M2muscarinic receptor. The relationship between desensitization of IK,AChand internalization of the M2receptor has been studied in rat atrial cells. On application of the stable muscarinic agonist carbachol for 2 h, IK,AChdeclined by ∼62% with time constants of 1.5 and 26.9 min, whereas ∼83% of the M2receptor was internalized from the cell membrane with time constants of 2.9 and 51.6 min. Transfection of the cells with β-adrenergic receptor kinase 1 (G protein-receptor kinase 2) and β-arrestin 2 significantly increased IK,AChdesensitization and M2receptor internalization during a 3-min application of agonist. Internalized M2receptor in cells exposed to carbachol for 2 h was colocalized with clathrin and not caveolin. It is concluded that a G protein-receptor kinase 2- and β-arrestin 2-dependent internalization of the M2receptor into clathrin-coated vesicles could play a major role in IK,AChdesensitization.

Modulation of membrane potential by an acetylcholine-activated potassium current in trout atrial myocytes

Application of the current-clamp technique in rainbow trout atrial myocytes has yielded resting membrane potentials that are incompatible with normal atrial function. To investigate this paradox, we recorded the whole membrane current ( Im) and compared membrane potentials recorded in isolated cardiac myocytes and multicellular preparations. Atrial tissue and ventricular myocytes had stable resting potentials of −87 ± 2 mV and −83.9 ± 0.4 mV, respectively. In contrast, 50 out of 59 atrial myocytes had unstable depolarized membrane potentials that were sensitive to the holding current. We hypothesized that this is at least partly due to a small slope conductance of Imaround the resting membrane potential in atrial myocytes. In accordance with this hypothesis, the slope conductance of Imwas about sevenfold smaller in atrial than in ventricular myocytes. Interestingly, ACh increased Imat −120 mV from 4.3 pA/pF to 27 pA/pF with an EC50of 45 nM in atrial myocytes. Moreover, 3 nM ACh increased the slope conductance of Imfourfold, shifted its reversal potential from −78 ± 3 to −84 ± 3 mV, and stabilized the resting membrane potential at −92 ± 4 mV. ACh also shortened the action potential in both atrial myocytes and tissue, and this effect was antagonized by atropine. When applied alone, atropine prolonged the action potential in atrial tissue but had no effect on membrane potential, action potential, or Imin isolated atrial myocytes. This suggests that ACh-mediated activation of an inwardly rectifying K+current can modulate the membrane potential in the trout atrial myocytes and stabilize the resting membrane potential.

Kinetic Analysis of G Protein–Coupled Receptor Signaling Using Fluorescence Resonance Energy Transfer in Living Cells

We describe and review methods for the kinetic analysis of G protein-coupled receptor (GPCR) activation and signaling that are based on optical methods. In particular, we describe the use of fluorescence resonance energy transfer (FRET) as a means of analyzing conformational changes within a single protein (for example a receptor) or between subunits of a protein complex (such as a G protein heterotrimer) and finally between distinct proteins (such as a receptor and a G protein). These methods allow the analysis of signaling kinetics in intact cells with proteins that retain their essential functional properties. They have produced a number of unexpected results: fast receptor activation kinetics in the millisecond range, similarly fast kinetics for receptor-G protein interactions, but much slower activation kinetics for G protein activation.

Measuring the modulatory effects of RGS proteins on GIRK channels

Discovery of "regulators of G-protein signaling" (RGS) as GTPase-activating proteins for heterotrimeric G proteins has provided a highly sought "missing link," reconciling past discrepancies between the in vitro GTPase activity of purified G proteins and the kinetics of physiological responses mediated by G-protein signaling in vivo. With the number of RGS genes in the mammalian genome at more than 30, associating specific RGS proteins to specific G-protein-coupled receptor (GPCR) signaling events has become a focus of RGS investigators. The ubiquitous expression of multiple RGS proteins has complicated this effort, yet the outlook has been encouraged with the identification of RGS9 as the determinant mediating rapid recovery of the transducin-dependent photoresponse. G-protein-gated inwardly rectifying potassium (GIRK) channels that mediate inhibitory synaptic transmission via GPCR activation of pertussis toxin-sensitive G proteins are similarly accelerated by RGS proteins when reconstituted in heterologous cell expression systems and fully reproduce the gating properties of native GIRK channels in neurons and cardiomyocytes. The endogenous neuronal and cardiac RGS protein(s) that accelerate GPCR-->GIRK channel-gating kinetics are currently not known. This article describes methods used to measure the receptor-dependent GIRK channel-gating parameters reconstituted in Chinese hamster ovary (CHO-K1) cells and Xenopus oocytes, as well as rat atrial myocytes and rat cerebellar granule neurons as model cells with native GPCR-->GIRK channel signaling. Applications of these methods for structure-function-based studies of RGS proteins, G proteins, and GPCRs are discussed. We also describe single cell reverse transcriptase polymerase chain reaction methods developed to profile atrial myocyte and neuronal RGS expression to identify specific RGS proteins for targeted knockdown or knockout.

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.

Functional M3 muscarinic acetylcholine receptors in mammalian hearts

In contrast to most peripheral tissues where multiple subtypes of muscarinic acetylcholine receptor (mAChR) coexist, with each of them playing its part in the orchestra of parasympathetic innervation, the myocardium has been traditionally considered to possess a single mAChR subtype. Although there is much evidence to support the notion that one receptor subtype (M2) orchestrates myocardial muscarinic transduction, there is emerging evidence that M1 and M3 receptors are also expressed and are of potential physiological, pathophysiological and pharmacological relevance. Clarifying this issue has a profound impact on our thinking about the cholinergic control of the heart function and disease and approaches to new drug development for the treatment of heart disease associated with parasympathetic dysfunction. This review article presents evidence for the presence of the M3 receptor subtype in the heart, and analyzes the controversial data from published pharmacological, functional and molecular studies. The potential roles of the M3 receptors, in parasympathetic control of heart function under normal physiological conditions and in heart failure, myocardial ischemia and arrhythmias, are discussed. On the basis of these considerations, we have made some proposals concerning the future of myocardial M3 receptor research.

Short-Term Desensitization of G-Protein-Activated, Inwardly Rectifying K+ (GIRK) Currents in Pyramidal Neurons of Rat Neocortex

Whole cell recordings from acutely isolated rat neocortical pyramidal cells were performed to study the kinetics and the mechanisms of short-term desensitization of G-protein-activated, inwardly rectifying K+ (GIRK) currents during prolonged application (5 min) of baclofen, adenosine, or serotonin. Most commonly, desensitization of GIRK currents was characterized by a biphasic time course with average time constants for fast and slow desensitization in the range of 8 and 120 s, respectively. The time constants were independent of the agonist used to evoke the current. The biphasic time course was preserved in perforated-patch recordings, indicating that neither component of desensitization is attributable to cell dialysis. Desensitization of GIRK currents displayed a strong heterologous component in that application of a second agonist substantially reduced the responsiveness to a test agonist. Fast desensitization, but not slow desensitization, was lost in cells loaded with GDP, suggesting that the hydrolysis cycle of G proteins might underlie the initial, rapid current decline. Hydrolysis of phosphatidylinositol biphosphate is an unlikely candidate underlying short-term desensitization, because both components of desensitization were preserved in the presence of the phospholipase C inhibitor U73122. We conclude that short-term desensitization does neither result from receptor downregulation nor from altered channel gating but might involve modifications of the G-protein-dependent pathway that serves to translate receptor activation into channel opening.

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.

The M2 muscarinic G-protein-coupled receptor is voltage-sensitive

G-protein coupled receptors are not considered to exhibit voltage sensitivity. Here, using Xenopus oocytes, we show that the M2 muscarinic receptor (m2R) is voltage-sensitive. The m2R-mediated potassium channel (GIRK) currents were used to assay the activity of m2R. We found that the apparent affinity of m2R toward acetylcholine (ACh) was reduced upon depolarization. Binding experiments of [3H]ACh to individual oocytes expressing m2R confirmed the electrophysiological findings. When the GIRK channels were activated either by overexpression of Gbetagamma subunits or by injection of GTPgammaS, the ratio between the currents measured at -60 mV and +40 mV was the same as for the basal activity of the GIRK channel. Thus, the steps downstream to agonist activation of m2R are not voltage-sensitive. We further found that, in contrast to m2R, the apparent affinity of m1R was increased upon depolarization. We also found that the voltage sensitivity of binding of [3H]ACh to oocytes expressing m2R was greatly diminished following pretreatment with pertussis toxin. The cumulative results suggest that m2R is, by itself, voltage-sensitive. Furthermore, the voltage sensitivity does not reside in the ACh binding site, rather, it most likely resides in the receptor region that couples to the G-protein.

Acetylcholine-induced phosphatidylinositol 4,5-bisphosphate depletion does not cause short-term desensitization of G protein-gated inwardly rectifying K+ current in mouse atrial myocytes

Depletion of phosphatidylinositol 4,5-bisphosphate (PIP(2)) induced by phenylephrine or endothelin causes the inhibition of acetylcholine-activated K(+) current (I(KACh)) in atrial myocytes. In the present study, we have investigated the hypothesis that muscarinic receptor induced PIP(2) depletion also causes inhibition of I(KACh), resulting in desensitization. We confirmed the expression of G(q)-coupled muscarinic receptors in mouse atrial myocytes using reverse transcriptase-polymerase chain reaction. The involvement of M(1) and M(3) receptors in desensitization is examined using specific antagonists, 4-DAMP and pirenzepine, but they significantly reduced peak I(KACh), implying nonspecific M(2) blockade. When ACh-induced phosphoinositide depletion was specifically inhibited using PLCbeta1 knock-out mice, the extent of desensitization during 4 min was 47.5 +/- 3.2%, which was not different from that in wild type (46.8 +/- 2.1%). Phenylephrine-induced phosphoinositide hydrolysis and phenylephrine-induced inhibition of I(KACh) were not affected by PLCbeta1 knock-out. To facilitate PIP(2) depletion, replenishment of PIP(2) was blocked by wortmannin. Wortmannin did not affect the desensitization and the recovery from desensitization. These results suggest that PIP(2) depletion by acetylcholine does not contribute to short-term desensitization of I(KACh). The differential regulation of I(KACh) by different phospholipase C-linked receptors may imply that receptor co-localization is required for PIP(2) to act as a signaling molecule.

Role of receptor kinase in long-term desensitization of the cardiac muscarinic receptor-K+ channel system

Desensitization of the cardiac muscarinic K+ channel was studied in cultured neonatal rat atrial cells and in Chinese hamster ovary (CHO) cells transfected with muscarinic receptor (HM(2)), G protein-coupled inward rectifying K+ channels 1 and 4, and G protein-coupled receptor kinase 2. In atrial cells incubated in 10 microM carbachol for 24 h, channel activity in cell-attached patches was substantially reduced as a result of long-term desensitization. The long-term desensitization was also observed in CHO cells transfected with the wild-type receptor and receptor kinase (as well as the channel). However, long-term desensitization was greatly reduced or abolished if the cells were 1) not transfected with the receptor kinase, 2) transfected with a mutant receptor lacking phosphorylation sites (rather than the wild-type receptor), or 3) transfected with a mutant receptor kinase lacking kinase activity (rather than the wild-type receptor kinase). We suggest that long-term desensitization of the cardiac muscarinic receptor-K+ channel system to muscarinic agonist may involve phosphorylation of the receptor by receptor kinase.

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.

Evidence of involvement of GIRK1/GIRK4 in long-term desensitization of cardiac muscarinic K+ channels

The cardiac M2 muscarinic receptor/G protein/K+ channel system was studied in neonatal rat atrial cells cultured with and without 10 microM carbachol (CCh) for 24 h. Channel activity in CCh-pretreated cells was substantially reduced as a result of long-term desensitization regardless of whether the channel was activated by ACh in cell-attached patches or GTP in inside-out patches. Channel activity in CCh-pretreated cells was also low when the receptor was bypassed and the G protein and channel were directly activated by [gamma-S]GTP or both the receptor and G protein were bypassed and the channel was directly activated by trypsin. Finally, in CCh-pretreated cells, the whole cell K+ current was low when the channel was activated via the independent adenosine receptor. This suggests that the channel is involved in long-term desensitization. However, in CCh-pretreated cells, although the receptor was internalized, there was no internalization of the channel. We suggest that the function of the muscarinic K+ channel declines in long-term desensitization of the cardiac M2 muscarinic receptor/G protein/K+ channel system.

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.

Slow modal gating of single G protein-activated K+ channels expressed in Xenopus oocytes

The slow kinetics of G protein-activated K+ (GIRK) channels expressed in Xenopus oocytes were studied in single-channel, inside-out membrane patches. Channels formed by GIRK1 plus GIRK4 subunits, which are known to form the cardiac acetylcholine (ACh)-activated GIRK channel (KACh), were activated by a near-saturating dose of G protein betagamma subunits (Gbetagamma; 20 nM). The kinetic parameters of the expressed GIRK1/4 channels were similar to those of cardiac KACh. GIRK1/4 channels differed significantly from channels formed by GIRK1 with the endogenous oocyte subunit GIRK5 (GIRK1/5) in some of their kinetic parameters and in a 3-fold lower open probability, Po. The unexpectedly low Po (0.025) of GIRK1/4 was due to the presence of closures of hundreds of milliseconds; the channel spent approximately 90 % of the time in the long closed states. GIRK1/4 channels displayed a clear modal behaviour: on a time scale of tens of seconds, the Gbetagamma-activated channels cycled between a low-Po mode (Po of about 0.0034) and a bursting mode characterized by an approximately 30-fold higher Po and a different set of kinetic constants (and, therefore, a different set of channel conformations). The available evidence indicates that the slow modal transitions are not driven by binding and unbinding of Gbetagamma. The GTPgammaS-activated Galphai1 subunit, previously shown to inhibit GIRK channels, substantially increased the time spent in closed states and apparently shifted the channel to a mode similar, but not identical, to the low-Po mode. This is the first demonstration of slow modal transitions in GIRK channels. The detailed description of the slow gating kinetics of GIRK1/4 may help in future analysis of mechanisms of GIRK gating.

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.

G-protein coupled receptor kinases as modulators of G-protein signalling

G-protein coupled receptors (GPCRs) comprise one of the largest classes of signalling molecules. A wide diversity of activating ligands induce the active conformation of GPCRs and lead to signalling via heterotrimeric G-proteins and downstream effectors. In addition, a complex series of reactions participate in the 'turn-off' of GPCRs in both physiological and pharmacological settings. Some key players in the inactivation or 'desensitization' of GPCRs have been identified, whereas others remain the target of ongoing studies. G-protein coupled receptor kinases (GRKs) specifically phosphorylate activated GPCRs and initiate homologous desensitization. Uncoupling proteins, such as members of the arrestin family, bind to the phosphorylated and activated GPCRs and cause desensitization by precluding further interactions of the GPCRs and G-proteins. Adaptor proteins, including arrestins, and endocytic machinery participate in the internalization of GPCRs away from their normal signalling milieu. In this review we discuss the roles of these regulatory molecules as modulators of GPCR signalling.

Desensitization of G-protein-coupled receptors in the cardiovascular system

Multiple mechanisms exist to control the signaling and density of G-protein-coupled receptors (GPRs). Upon agonist binding and receptor activation, a series of reactions participate in the turn off or desensitization of GPRs. Many GPRs are phosphorylated by protein kinases and consequently uncoupled from G proteins. In addition, many GPRs are sequestered from the cell surface and become inaccessible to their activating ligands. Both receptor:G protein uncoupling and receptor sequestration may involve the participation of arrestins or other proteins. A model for receptor regulation has been developed from studies of the beta-adrenergic receptor. However, recent studies suggest that other GPRs important in the cardiovascular system, such as the muscarinic cholinergic receptors that regulate heart rate, might be regulated by mechanisms other than those that regulate the beta-adrenergic receptors. This review summarizes our current understanding of the processes involved in the desensitization of GPRs.

Activation of muscarinic K+ current by beta-adrenergic receptors in cultured atrial myocytes transfected with beta1 subunit of heterotrimeric G proteins

Muscarinic K+ channels (IK(ACh)) in native atrial myocytes are activated by betagamma subunits of pertussis toxin (Ptx)-sensitive heterotrimeric G proteins coupled to different receptors. betagamma subunits of Ptx-insensitive Gs, coupled to beta-adrenergic receptors, do not activate native IK(ACh). In atrial myocytes from adult rats transfected with rat brain beta1 subunit IK(ACh) can be activated by stimulation of beta-adrenergic receptors using isoprenaline. This effect is insensitive to Ptx. These findings demonstrate for the first time promiscuous (Ptx-insensitive) coupling of Gsbetagamma to GIRK channels in their native environment.

Non-receptor-mediated activation of IK(ATP) and inhibition of IK(ACh) by diadenosine polyphosphates in guinea-pig atrial myocytes

1. The effects of diadenosine polyphosphates (APnA, where n = 4-6) were studied on beating frequency of perfused guinea-pig hearts and on muscarinic K+ current (IK(ACh)) and ATP-regulated K+ current (IK(ATP)) in atrial myocytes from guinea-pig hearts using whole-cell voltage clamp. 2. Bradycardia induced by APnA in perfused hearts was completely inhibited by 8-cyclopentyl- 1,3-dipropylxanthine (CPX, 20 microM), a selective antagonist at A1 adenosine receptors, and was augmented by dipyridamole (Dipy), an inhibitor of cellular adenosine (Ado) uptake. 3. Whereas exposure of atrial myocytes to Ado (100 microM) within about 1 s induced a significant whole-cell IK(ACh), APnA up to 1 mM applied for some tens of seconds failed to activate IK(ACh). If present for periods > 2 min, APnA caused inhibition of agonist-evoked IK(ACh) and activation of a weakly inward rectifying K+ current, which was identified as IK(ATP) by its sensitivity to glibenclamide and its current-voltage curve. 4. The actions of extracellular APnA on IK(ACh) and IK(ATP) were mimicked by intracellular loading of compounds via the patch clamp pipette and by intracellular loading of AMP. 5. The results from isolated myocytes exclude APnA acting as A1 agonists. It is suggested that myocytes can take up APnA, which are degraded to AMP. In the presence of ATP, AMP is converted to ADP, a physiological activator of ATP-regulated K+ channels, by adenylate kinase. A similar mechanism resulting in a reduction of the [GTP]/[GDP] ratio might be responsible for inhibition of IK(ACh). 6. In the perfused heart and other multicellular cardiac preparations the actions of APnA are mediated by Ado via A1 receptors. It is suggested that APnA in multicellular cardiac tissue are hydrolysed by an ectohydrolase to yield AMP which is converted to Ado by ectonucleotidases.

Role of receptor kinase in short-term desensitization of cardiac muscarinic K+ channels expressed in Chinese hamster ovary cells

1. The cardiac muscarinic receptor-K+ channel system was reconstructed in Chinese hamster ovary (CHO) cells by transfecting the cells with the various components of the system. The activity of the muscarinic K+ channel was measured with the cell-attached configuration of the patch clamp technique. 2. In CHO cells transfected with the channel (Kir3.1/Kir3.4), receptor (hm2) and receptor kinase (GRK2), on exposure to agonist, there was a decline in channel activity as a result of desensitization, similar to that in atrial cells. 3. Whereas the desensitization was almost abolished by not transfecting with the receptor kinase or by transfecting with a mutant receptor lacking phosphorylation sites, it was only reduced (by approximately 39%) by transfecting with a mutant receptor kinase with little/kinase activity. 4. These results suggest that the receptor kinase is responsible for desensitization of the muscarinic K+ channel and that this involves phosphorylation-dependent and -independent mechanisms.

Signalling via the G protein-activated K+ channels

The inwardly rectifying K+ channels of the GIRK (Kir3) family, members of the superfamily of inwardly rectifying K+ channels (Kir), are important physiological tools to regulate excitability in heart and brain by neurotransmitters, and the only ion channels conclusively shown to be activated by a direct interaction with heterotrimeric G protein subunits. During the last decade, especially since their cloning in 1993, remarkable progress has been made in understanding the structure, mechanisms of gating, activation by G proteins, and modulation of these channels. However, much of the molecular details of structure and of gating by G protein subunits and other factors, mechanisms of modulation and desensitization, and determinants of specificity of coupling to G proteins, remain unknown. This review summarizes both the recent advances and the unresolved questions now on the agenda in GIRK studies.

In vivo downregulation of M2 receptors revealed by measurement of muscarinic K+ current in cultured guinea-pig atrial myocytes

1. Muscarinic K+ current (IK(ACh)) elicited by acetylcholine (ACh) was measured in guinea-pig atrial myocytes, which were either freshly isolated or cultured for up to 8 days. 2. The half-time of activation of inward IK(ACh) by a saturating concentration (10 microM) of ACh decreased from approximately 400 ms (in freshly isolated cells) to 250 ms after 6 days in culture. This was paralleled by an increase in the fast desensitizing component of IK(ACh). The density of steady-state currents was not changed. Downregulation of M2 receptors by long-term treatment of isolated myocytes with carbachol in vitro had opposite effects. 3. The EC50 of ACh for the activation of steady-state IK(ACh) was reduced from 5 x 10(-7) M (day 0) to 8 x 10(-8) M (day 6). The shift in EC50 occurred with a half-time of about 2 days, similar to the recovery from downregulation induced by treating atrial myocytes with carbachol in vitro. 4. The increase in sensitivity to ACh can be accounted for by an approximately 6-fold increase in the density of M2 receptors. 5. It is concluded that sensitization in culture reflects recovery from downregulation of M2 receptors due to the tonic vagal input to the heart in the intact animal.

Rapid down regulation of Ca2+ signals induced by endothelin-1 in a human bronchial epithelial cell line

Ca2+ transients evoked by endothelin-1 (ET-1) were measured in single cells of an immortalized human tracheal epithelial cell line using Fura-2. ET-induced Ca2+ transients were compared to signals evoked via established phospholipase-C linked receptors (H1 histamine; P2y purinergic, ATP). Saturating concentrations of histamine (100 microM) and ATP (10 microM) caused Ca2+ transients of identical amplitude, whereas a saturating concentration of ET-1 (10 nM) on average resulted in a slightly smaller change in fluorescence ratio (80 +/- 27%). H1 and P2y induced Ca2+ signals caused by brief (10-30 s) application of the agonists were highly reproducible. No desensitization to these ligands was observed, if between two exposures cells were superfused with agonist-free solution for > or = 200 s. A single exposure to ET-1 (10 nM) for > or = 6 s reduced sensitivity of the cell to a second exposure to ET-1. On average, the signal upon a second application of 10 nM ET-1 had an amplitude of 30% of the first ET-1 induced signal in that cell. After two 10 s exposures to the peptide, less than 10% of the initial amplitude was measured. This desensitization did not affect responsiveness to histamine or ATP. No recovery from desensitization to ET-1 was observed for 12 h after a single brief treatment with the peptide. Thereafter, responsiveness to ET-1 re-appeared with a half time of about 5 h and was complete by about 20 h. Ca2+ signals to all three agonists were absent in thapsigargin treated cells. Their amplitude was not affected by superfusion of the cells with Ca(2+)-free solution. Under conditions of store-depletion either by ET-1 or by thapsigargin, a change from Ca(2+)-free to Ca(2+)-containing solution induced a slow rise in [Ca2+]i, suggesting the existence of a capacitative Ca2+ entry pathway. ET-receptors most likely of the ETA subtype are subject to a novel type of desensitization-at least with regard to Ca2+ signalling-which might reflect a signal transduction mechanism specific to ET receptors.