Receptor kinase-dependent desensitization of the muscarinic K+ current in rat atrial cells

The Role of KACh Channels in Atrial Fibrillation

This manuscript explores the intricate role of acetylcholine-activated inward rectifier potassium (KACh) channels in the pathogenesis of atrial fibrillation (AF), a common cardiac arrhythmia. It delves into the molecular and cellular mechanisms that underpin AF, emphasizing the vital function of KACh channels in modulating the atrial action potential and facilitating arrhythmogenic conditions. This study underscores the dual nature of KACh activation and its genetic regulation, revealing that specific variations in potassium channel genes, such as Kir3.4 and K2P3.1, significantly influence the electrophysiological remodeling associated with AF. Furthermore, this manuscript identifies the crucial role of the KACh-mediated current, IKACh, in sustaining arrhythmia through facilitating shorter re-entry circuits and stabilizing the re-entrant circuits, particularly in response to vagal nerve stimulation. Experimental findings from animal models, which could not induce AF in the absence of muscarinic activation, highlight the dependency of AF induction on KACh channel activity. This is complemented by discussions on therapeutic interventions, where KACh channel blockers have shown promise in AF management. Additionally, this study discusses the broader implications of KACh channel behavior, including its ubiquitous presence across different cardiac regions and species, contributing to a comprehensive understanding of AF dynamics. The implications of these findings are profound, suggesting that targeting KACh channels might offer new therapeutic avenues for AF treatment, particularly in cases resistant to conventional approaches. By integrating genetic, cellular, and pharmacological perspectives, this manuscript offers a holistic view of the potential mechanisms and therapeutic targets in AF, making a significant contribution to the field of cardiac arrhythmia research.

Structural and Electrical Remodeling of the Sinoatrial Node in Diabetes: New Dimensions and Perspectives

The sinoatrial node (SAN) is composed of highly specialized cells that mandate the spontaneous beating of the heart through self-generation of an action potential (AP). Despite this automaticity, the SAN is under the modulation of the autonomic nervous system (ANS). In diabetes mellitus (DM), heart rate variability (HRV) manifests as a hallmark of diabetic cardiomyopathy. This is paralleled by an impaired regulation of the ANS, and by a pathological remodeling of the pacemaker structure and function. The direct effect of diabetes on the molecular signatures underscoring this pathology remains ill-defined. The recent focus on the electrical currents of the SAN in diabetes revealed a repressed firing rate of the AP and an elongation of its tracing, along with conduction abnormalities and contractile failure. These changes are blamed on the decreased expression of ion transporters and cell-cell communication ports at the SAN (i.e., HCN4, calcium and potassium channels, connexins 40, 45, and 46) which further promotes arrhythmias. Molecular analysis crystallized the RGS4 (regulator of potassium currents), mitochondrial thioredoxin-2 (reactive oxygen species; ROS scavenger), and the calcium-dependent calmodulin kinase II (CaMKII) as metabolic culprits of relaying the pathological remodeling of the SAN cells (SANCs) structure and function. A special attention is given to the oxidation of CaMKII and the generation of ROS that induce cell damage and apoptosis of diabetic SANCs. Consequently, the diabetic SAN contains a reduced number of cells with significant infiltration of fibrotic tissues that further delay the conduction of the AP between the SANCs. Failure of a genuine generation of AP and conduction of their derivative waves to the neighboring atrial myocardium may also occur as a result of the anti-diabetic regiment (both acute and/or chronic treatments). All together, these changes pose a challenge in the field of cardiology and call for further investigations to understand the etiology of the structural/functional remodeling of the SANCs in diabetes. Such an understanding may lead to more adequate therapies that can optimize glycemic control and improve health-related outcomes in patients with diabetes.

Impaired regulation of heart rate and sinoatrial node function by the parasympathetic nervous system in type 2 diabetic mice

Heart rate (HR) and sinoatrial node (SAN) function are modulated by the autonomic nervous system. HR regulation by the parasympathetic nervous system (PNS) is impaired in diabetes mellitus (DM), which is denoted cardiovascular autonomic neuropathy. Whether blunted PNS effects on HR in type 2 DM are related to impaired responsiveness of the SAN to PNS agonists is unknown. This was investigated in type 2 diabetic db/db mice in vivo and in isolated SAN myocytes. The PNS agonist carbachol (CCh) had a smaller inhibitory effect on HR, while HR recovery time after CCh removal was accelerated in db/db mice. In isolated SAN myocytes CCh reduced spontaneous action potential firing frequency but this effect was reduced in db/db mice due to blunted effects on diastolic depolarization slope and maximum diastolic potential. Impaired effects of CCh occurred due to enhanced desensitization of the acetylcholine-activated K⁺ current (I_KACh) and faster I_KACh deactivation. I_KACh alterations were reversed by inhibition of regulator of G-protein signaling 4 (RGS4) and by the phospholipid PIP₃. SAN expression of RGS4 was increased in db/db mice. Impaired PNS regulation of HR in db/db mice occurs due to reduced responsiveness of SAN myocytes to PNS agonists in association with enhanced RGS4 activity.

Serine/Threonine Phosphatases in Atrial Fibrillation

Serine/threonine protein phosphatases control dephosphorylation of numerous cardiac proteins, including a variety of ion channels and calcium-handling proteins, thereby providing precise post-translational regulation of cardiac electrophysiology and function. Accordingly, dysfunction of this regulation can contribute to the initiation, maintenance and progression of cardiac arrhythmias. Atrial fibrillation (AF) is the most common heart rhythm disorder and is characterized by electrical, autonomic, calcium-handling, contractile, and structural remodeling, which include, among other things, changes in the phosphorylation status of a wide range of proteins. Here, we review AF-associated alterations in the phosphorylation of atrial ion channels, calcium-handling and contractile proteins, and their role in AF-pathophysiology. We highlight the mechanisms controlling the phosphorylation of these proteins and focus on the role of altered dephosphorylation via local type-1, type-2A and type-2B phosphatases (PP1, PP2A, and PP2B, also known as calcineurin, respectively). Finally, we discuss the challenges for phosphatase research, potential therapeutic significance of altered phosphatase-mediated protein dephosphorylation in AF, as well as future directions.

Acute desensitization of acetylcholine and endothelin-1 activated inward rectifier K+ current in myocytes from the cardiac atrioventricular node

The atrioventricular node (AVN) is a vital component of the pacemaker-conduction system of the heart, co-ordinating conduction of electrical excitation from cardiac atria to ventricles and acting as a secondary pacemaker. The electrical behaviour of the AVN is modulated by vagal activity via activation of muscarinic potassium current, I_KACh. However, it is not yet known if this response exhibits ‘fade’ or desensitization in the AVN, as established for the heart’s primary pacemaker – the sinoatrial node. In this study, acute activation of I_KACh in rabbit single AVN cells was investigated using whole-cell patch clamp at 37 °C. 0.1–1 μM acetylcholine (ACh) rapidly activated a robust I_KACh in AVN myocytes during a descending voltage-ramp protocol. This response was inhibited by tertiapin-Q (TQ; 300 nM) and by the M2 muscarinic ACh receptor antagonist AFDX-116 (1 μM). During sustained ACh exposure the elicited I_KACh exhibited bi-exponential fade (τ_f of 2.0 s and τ_s 76.9 s at −120 mV; 1 μM ACh). 10 nM ET-1 elicited a current similar to I_KACh, which faded with a mono-exponential time-course (τ of 52.6 s at −120 mV). When ET-1 was applied following ACh, the ET-1 activated response was greatly attenuated, demonstrating that ACh could desensitize the response to ET-1. For neither ACh nor ET-1 was the rate of current fade dependent upon the initial response magnitude, which is inconsistent with K⁺ flux mediated changes in electrochemical driving force as the underlying mechanism. Collectively, these findings demonstrate that TQ sensitive inwardly rectifying K⁺ current in cardiac AVN cells, elicited by M2 muscarinic receptor or ET-1 receptor activation, exhibits fade due to rapid desensitization.

Effects of acetylcholine on electrical remodeling of human atrial fibers

Autonomic nervous system activation can result in significant changes of atrial electrophysiology and facilitate induction of atrial fibrillation. By recording influence of different concentrations of acetylcholine (ACh) on atrial fibers (AF), we investigated the role of the increased vagal tone in electrical remodeling in atrial fibrillation. Parameters of action potentials and force contraction (Fc) in atrial fibers were recorded by using standard intracellular microelectrode technique and force transducer. It was found that: (1) ACh at 0.1 μmol/L had no significant influence on spontaneous action potentials (SAPs) and Fc (n=6, P>0.05); ACh at both 1.0 and 10.0 μmol/L shortened action potential duration (APD) and Fc of human AF from right atrium (n=6, P<0.05); there was no significant difference in shortening APD between 10.0 and 1.0 μmol/L of ACh; (2) ACh at 0.1 μmol/L had no significant desensitization (n=6, P>0.05), but ACh at 1.0 and 10.0 μmol/L had desensitization (n=6, P<0.05) to SAPs and Fc. The desensitization of ACh on APD in AF was concentration- and time-dependent. It was shown that APD was longer than the control along with extending time of continuous Tyrode's solution perfusion after desensitization. It is concluded that ACh changes the electrophysiological characteristics of human AF, indicating that increased vagal tone plays a role in the development of a vulnerable substrate for atrial electrical remodeling in atrial fibrillation.

RGS4 regulates parasympathetic signaling and heart rate control in the sinoatrial node

Heart rate is controlled by the opposing activities of sympathetic and parasympathetic inputs to pacemaker myocytes in the sinoatrial node (SAN). Parasympathetic activity on nodal myocytes is mediated by acetylcholine-dependent stimulation of M(2) muscarinic receptors and activation of Galpha(i/o) signaling. Although regulators of G protein signaling (RGS) proteins are potent inhibitors of Galpha(i/o) signaling in many tissues, the RGS protein(s) that regulate parasympathetic tone in the SAN are unknown. Our results demonstrate that RGS4 mRNA levels are higher in the SAN compared to right atrium. Conscious freely moving RGS4-null mice showed increased bradycardic responses to parasympathetic agonists compared to wild-type animals. Moreover, anesthetized RGS4-null mice had lower baseline heart rates and greater heart rate increases following atropine administration. Retrograde-perfused hearts from RGS4-null mice showed enhanced negative chronotropic responses to carbachol, whereas SAN myocytes showed greater sensitivity to carbachol-mediated reduction in the action potential firing rate. Finally, RGS4-null SAN cells showed decreased levels of G protein-coupled inward rectifying potassium (GIRK) channel desensitization and altered modulation of acetylcholine-sensitive potassium current (I(KACh)) kinetics following carbachol stimulation. Taken together, our studies establish that RGS4 plays an important role in regulating sinus rhythm by inhibiting parasympathetic signaling and I(KACh) activity.

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.

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.

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.

Inhibition of acetylcholine-activated K(+) current by chelerythrine and bisindolylmaleimide I in atrial myocytes from mice

The effects of the protein kinase C inhibitors chelerythrine and bisindolylmaleimide I on acetylcholine-activated K+ currents (I(KACh)) were examined in atrial myocytes of mice, using the patch clamp technique. Chelerythrine and bisindolylmaleimide I inhibited I(KACh) in a reversible and dose-dependent manner. Half-maximal effective concentrations were 0.49+/-0.01 microM for chelerythrine and 98.69+/-12.68 nM for bisindolylmaleimide I. However, I(KACh) was not affected either by calphostin C, which is also known as a protein kinase C inhibitor, or by a protein kinase C activator, phorbol 12,13-dibutyrate. When K(ACh) channels were activated directly by adding 1 mM GTPgammaS to the bath solution in inside-out patches, chelerythrine (10 microM) decreased the open probability from 0.043+/-0.01 to 0.014+/-0.007 (n=5), but bisindolylmaleimide I did not affect the channel activity. From these results, it is concluded that both chelerythrine and bisindolylmaleimide I inhibit K(ACh) channels independently of protein kinase C inhibition, but the level of inhibition is different.

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.

Control of the cardiac muscarinic K+ channel by beta-arrestin 2

Control of the cardiac muscarinic K(+) current (i(K,ACh)) by beta-arrestin 2 has been studied. In Chinese hamster ovary cells transfected with m2 muscarinic receptor, muscarinic K(+) channel, receptor kinase (GRK2), and beta-arrestin 2, desensitization of i(K,ACh) during a 3-min application of 10 micrometer ACh was significantly increased as compared with that in cells transfected with receptor, channel, and GRK2 only (fade in current increased from 45 to 78%). The effect of beta-arrestin 2 was lost if cells were not co-transfected with GRK2. Resensitization (recovery from desensitization) of i(K,ACh) in cells transfected with beta-arrestin 2 was significantly slowed (time constant increased from 34 to 232 s). Activation and deactivation of i(K,ACh) on application and wash-off of ACh in cells transfected with beta-arrestin 2 were significantly slowed from 0.9 to 3.1 s (time to half peak i(K,ACh)) and from 6.2 to 13.8 s (time to half-deactivation), respectively. In cells transfected with a constitutively active beta-arrestin 2 mutant, desensitization occurred in the absence of agonist (peak current significantly decreased from 0.4 +/- 0.05 to 0.1 +/- 0.01 nA). We conclude that beta-arrestin 2 has the potential to play a major role in desensitization and other aspects of the functioning of the muscarinic K(+) channel.

Phosphatidylinositol 4,5-bisphosphate is acting as a signal molecule in alpha(1)-adrenergic pathway via the modulation of acetylcholine-activated K(+) channels in mouse atrial myocytes

We have investigated the effect of alpha(1)-adrenergic agonist phenylephrine (PE) on acetylcholine-activated K(+) currents (I(KACh)). I(KACh) was recorded in mouse atrial myocytes using the patch clamp technique. I(KACh) was activated by 10 microm ACh and the current decreased by 44.27 +/- 2.38% (n = 12) during 4 min due to ACh-induced desensitization. When PE was applied with ACh, the extent of desensitization was markedly increased to 69.34 +/- 2.22% (n = 9), indicating the presence of PE-induced desensitization. I(KACh) was fully recovered from desensitization after a 6-min washout. PE-induced desensitization of I(KACh) was not affected by protein kinase C inhibitor, calphostin C, but abolished by phospholipase C (PLC) inhibitor, neomycin. When phophatidylinositol 4,5-bisphosphate (PIP(2)) replenishment was blocked by wortmannin (an inhibitor of phophatidylinositol 3-kinase and phophatidylinositol 4-kinase), desensitization of I(KACh) in the presence of PE was further increased (97.25 +/- 7.63%, n = 6). Furthermore, the recovery from PE-induced desensitization was inhibited, and the amplitude of I(KACh) at the second exposure after washout was reduced to 19.65 +/- 2.61% (n = 6) of the preceding level. These data suggest that the K(ACh) channel is modulated by PE through PLC stimulation and depletion of PIP(2).

Endogenous regulator of G-protein signaling proteins modify N-type calcium channel modulation in rat sympathetic neurons

Experiments using heterologous overexpression indicate that regulator of G-protein signaling (RGS) proteins play important roles in Gbetagamma-mediated ion channel modulation. However, the roles subserved by endogenous RGS proteins have not been extensively examined because tools for functionally inhibiting natively expressed RGS proteins are lacking. To address this void, we used a strategy in which Galpha(oA) was rendered insensitive to pertussis toxin (PTX) and RGS proteins by site-directed mutagenesis. Either PTX-insensitive (PTX-i) or both PTX- and RGS-insensitive (PTX/RGS-i) mutants of Galpha(oA) were expressed along with Gbeta(1) and Ggamma(2) subunits in rat sympathetic neurons. After overnight treatment with PTX to suppress natively expressed Galpha subunits, voltage-dependent Ca(2+) current inhibition by norepinephrine (NE) (10 microm) was reconstituted in neurons expressing either PTX-i or PTX/RGS-i Galpha(oA). When compared with neurons expressing PTX-i Galpha(oA), the steady-state concentration-response relationships for NE-induced Ca(2+) current inhibition were shifted to lower concentrations in neurons expressing PTX/RGS-i Galpha(oA). In addition to an increase in agonist potency, the expression of PTX/RGS-i Galpha(oA) dramatically retarded the current recovery after agonist removal. Interestingly, the alteration in current recovery was accompanied by a slowing in the onset of current inhibition. Together, our data suggest that endogenous RGS proteins contribute to membrane-delimited Ca(2+) channel modulation by regulating agonist potency and kinetics of G-protein-mediated signaling in neuronal cells.

Endogenous regulator of G-protein signaling proteins modify N-type calcium channel modulation in rat sympathetic neurons

Experiments using heterologous overexpression indicate that regulator of G-protein signaling (RGS) proteins play important roles in Gbetagamma-mediated ion channel modulation. However, the roles subserved by endogenous RGS proteins have not been extensively examined because tools for functionally inhibiting natively expressed RGS proteins are lacking. To address this void, we used a strategy in which Galpha(oA) was rendered insensitive to pertussis toxin (PTX) and RGS proteins by site-directed mutagenesis. Either PTX-insensitive (PTX-i) or both PTX- and RGS-insensitive (PTX/RGS-i) mutants of Galpha(oA) were expressed along with Gbeta(1) and Ggamma(2) subunits in rat sympathetic neurons. After overnight treatment with PTX to suppress natively expressed Galpha subunits, voltage-dependent Ca(2+) current inhibition by norepinephrine (NE) (10 microm) was reconstituted in neurons expressing either PTX-i or PTX/RGS-i Galpha(oA). When compared with neurons expressing PTX-i Galpha(oA), the steady-state concentration-response relationships for NE-induced Ca(2+) current inhibition were shifted to lower concentrations in neurons expressing PTX/RGS-i Galpha(oA). In addition to an increase in agonist potency, the expression of PTX/RGS-i Galpha(oA) dramatically retarded the current recovery after agonist removal. Interestingly, the alteration in current recovery was accompanied by a slowing in the onset of current inhibition. Together, our data suggest that endogenous RGS proteins contribute to membrane-delimited Ca(2+) channel modulation by regulating agonist potency and kinetics of G-protein-mediated signaling in neuronal cells.

Muscarinic regulation of Ca2+ signaling in mammalian atrial and ventricular myocardium

The differential regulation of the contractility of mammalian atrial and ventricular myocardium upon activation of muscarinic receptors can be ascribed, for the most part, to alterations in intracellular Ca2+ transients. However, alterations in myofibrillar sensitivity to Ca2+ ions also contribute to such regulation. In atrial muscle, the following actions are all associated with the corresponding alterations in the amplitude of Ca2+ transients in the same direction as those in the strength of the contractile force: (1) the direct inhibitory action on the basal force of contraction; (2) the increase (recovery) in force that is induced during the prolonged stimulation of muscarinic receptors; and (3) the rebound increase in force induced by washout of muscarinic receptor agonists. In addition, for a given decrease in force induced by muscarinic receptor stimulation in atrial muscle, the amplitude of Ca2+ transients is decreased to a smaller extent than the decrease in amplitude induced by reduction of extracellular Ca2+ concentration ([Ca2+]o), an indication that muscarinic receptor stimulation might increase myofibrillar sensitivity to Ca2+ ions simultaneously with the reduction in the amplitude of Ca2+ transients during induction of the direct inhibitory action. In mammalian ventricular myocardium, the direct inhibitory action of muscarinic receptor stimulation exhibits a wide range of species-dependent variation. A pronounced direct inhibitory action is induced in ferret papillary muscle, which is also associated with a definite increase in myofibrillar sensitivity to Ca2+ ions. By contrast, in the ventricular myocardium of other species including the rabbit and the dog, muscarinic receptor stimulation scarcely affects the baseline Ca2+ transients and the force, but it results in a pronounced decrease in Ca2+ transients and force when applied in the presence of beta-adrenoceptor stimulation, a phenomenon known as 'accentuated antagonism' or the 'indirect inhibitory action' of muscarinic receptor stimulation in mammalian ventricular myocardium. During induction of the indirect inhibitory action in mammalian ventricular myocardium, muscarinic receptor stimulation reverses all the effects induced by beta-adrenoceptor stimulation, including the increase in Ca2+ transients, the positive inotropic and lusitropic effects, and the decrease in myofibrillar sensitivity to Ca2+ ions. The relationship between the amplitude of Ca2+ transients and force is unaffected during induction of the indirect inhibitory action in rabbit and dog ventricular myocardium. The direct and indirect inhibitory actions of muscarinic receptor stimulation on Ca2+ transients have clearly different dependences on frequency: the former is more pronounced at a higher rate of stimulation, while the latter is more pronounced at a lower rate. The more complex interaction of muscarinic receptor and beta-adrenoceptor stimulation in mammalian atrial muscle and ferret ventricular muscle might be explained by the contribution of both the direct and the indirect regulatory mechanisms to the interaction.

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.

G protein-coupled receptor kinases

G protein-coupled receptor kinases (GRKs) constitute a family of six mammalian serine/threonine protein kinases that phosphorylate agonist-bound, or activated, G protein-coupled receptors (GPCRs) as their primary substrates. GRK-mediated receptor phosphorylation rapidly initiates profound impairment of receptor signaling, or desensitization. This review focuses on the regulation of GRK activity by a variety of allosteric and other factors: agonist-stimulated GPCRs, beta gamma subunits of heterotrimeric GTP-binding proteins, phospholipid cofactors, the calcium-binding proteins calmodulin and recoverin, posttranslational isoprenylation and palmitoylation, autophosphorylation, and protein kinase C-mediated GRK phosphorylation. Studies employing recombinant, purified proteins, cell culture, and transgenic animal models attest to the general importance of GRKs in regulating a vast array of GPCRs both in vitro and in vivo.

Evidence that the nucleotide exchange and hydrolysis cycle of G proteins causes acute desensitization of G-protein gated inward rectifier K+ channels

The G-protein gated inward rectifier K+ channel (GIRK) is activated in vivo by the Gbeta gamma subunits liberated upon Gi-coupled receptor activation. We have recapitulated the acute desensitization of receptor-activated GIRK currents in heterologous systems and shown that it is a membrane-delimited process. Its kinetics depends on the guanine nucleotide species available and could be accounted for by the nucleotide exchange and hydrolysis cycle of G proteins. Indeed, acute desensitization is abolished by nonhydrolyzable GTP analogues. Whereas regulators of G-protein signaling (RGS) proteins by their GTPase-activating protein activities are regarded as negative regulators, a positive regulatory function of RGS4 is uncovered in our study; the opposing effects allow RGS4 to potentiate acute desensitization without compromising GIRK activation.

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.

Adenovirus-mediated expression of 5-HT1B receptors in cardiac ventricle myocytes; coupling to inwardly rectifying K+ channels

The 5-HT1B receptor is expressed on nerve terminals where it inhibits neurotransmitter release. When expressed ectopically in fibroblasts, the 5-HT1B receptor inhibits adenylyl cyclase. However, in the central nervous system, the effect of this receptor on neurotransmitter release appears to be cAMP-independent. We therefore investigated alternative effector systems that might be activated by the 5-HT1B receptor. We constructed a recombinant adenovirus that allows expression of high levels of the 5-HT1B receptor in a variety of cells. We chose cardiac ventricle myocytes because they express a muscarinic-gated, inwardly rectifying K+ channel (i[KACh]). In infected ventricle cells, both 5-HT and the muscarinic receptor agonist, carbachol, elicited a similar inwardly rectifying K+ current. The currents elicited by these agonists were pertussis-toxin sensitive and were not additive. These results suggest a common signal transduction pathway for 5-HT1B and muscarinic receptors in ventricle cells.

RGS8 accelerates G-protein-mediated modulation of K+ currents

Transmembrane signal transduction via heterotrimeric G proteins is reported to be inhibited by RGS (regulators of G-protein signalling) proteins. These RGS proteins work by increasing the GTPase activity of G protein alpha-subunits (G alpha), thereby driving G proteins into their inactive GDP-bound form. However, it is not known how RGS proteins regulate the kinetics of physiological responses that depend on G proteins. Here we report the isolation of a full-length complementary DNA encoding a neural-tissue-specific RGS protein, RGS8, and the determination of its function. We show that RGS8 binds preferentially to the alpha-subunits G(alpha)o and G(alpha)i3 and that it functions as a GTPase-activating protein (GAP). When co-expressed in Xenopus oocytes with a G-protein-coupled receptor and a G-protein-coupled inwardly rectifying K+ channel (GIRK1/2), RGS8 accelerated not only the turning off but also the turning on of the GIRK1/2 current upon receptor stimulation, without affecting the dose-response relationship. We conclude that RGS8 accelerates the modulation of G-protein-coupled channels and is not just a simple negative regulator. This property of RGS8 may be crucial for the rapid regulation of neuronal excitability upon stimulation of G-protein-coupled receptors.

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.

ATP-dependent desensitization of the muscarinic K+ channel in rat atrial cells

1. Fast desensitization of the muscarinic K+ channel has been studied in excised patches from rat atrial cells. 2. In inside-out patches, ACh was present in the pipette and GTP was applied via the bath to activate the channel. In outside-out patches, GTP was present in the pipette and ACh was applied via the bath to activate the channel. In both cases, during a 30 s exposure to GTP or ACh there was a decline in channel activity as a result of fast desensitization if ATP was present. 3. In inside-out patches, fast desensitization was still observed if the muscarinic ACh receptor was bypassed and the channel was activated by GTP gamma S. This suggests that fast desensitization is a result of a modification of the channel (or the connecting G protein) and not the receptor. 4. In both inside-out and outside-out patches, channel activity was depressed and fast desensitization was reduced or absent, if ATP was not present. 5. The non-hydrolysable analogue of ATP, AMP-PNP, did not substitute for ATP in its effects on the channel. 6. The results are consistent with the hypothesis that fast desensitization of the muscarinic K+ channel is the result of a dephosphorylation of the channel.

RGS proteins reconstitute the rapid gating kinetics of gbetagamma-activated inwardly rectifying K+ channels

G protein-gated inward rectifier K+ (GIRK) channels mediate hyperpolarizing postsynaptic potentials in the nervous system and in the heart during activation of Galpha(i/o)-coupled receptors. In neurons and cardiac atrial cells the time course for receptor-mediated GIRK current deactivation is 20-40 times faster than that observed in heterologous systems expressing cloned receptors and GIRK channels, suggesting that an additional component(s) is required to confer the rapid kinetic properties of the native transduction pathway. We report here that heterologous expression of "regulators of G protein signaling" (RGS proteins), along with cloned G protein-coupled receptors and GIRK channels, reconstitutes the temporal properties of the native receptor --> GIRK signal transduction pathway. GIRK current waveforms evoked by agonist activation of muscarinic m2 receptors or serotonin 1A receptors were dramatically accelerated by coexpression of either RGS1, RGS3, or RGS4, but not RGS2. For the brain-expressed RGS4 isoform, neither the current amplitude nor the steady-state agonist dose-response relationship was significantly affected by RGS expression, although the agonist-independent "basal" GIRK current was suppressed by approximately 40%. Because GIRK activation and deactivation kinetics are the limiting rates for the onset and termination of "slow" postsynaptic inhibitory currents in neurons and atrial cells, RGS proteins may play crucial roles in the timing of information transfer within the brain and to peripheral tissues.

Inhibition of muscarinic K+ current in guinea-pig atrial myocytes by PD 81,723, an allosteric enhancer of adenosine binding to A1 receptors

1. PD 81,723 has been shown to enhance binding of adenosine to A1 receptors by stabilizing G protein-receptor coupling ('allosteric enhancement'). Evidence has been provided that in the perfused hearts and isolated atria PD 81,723 causes a sensitization to adenosine via this mechanism. 2. We have studied the effect of PD 81,723 in guinea-pig isolated atrial myocytes by use of whole-cell measurement of the muscarinic K+ current (I[K(ACh)]) activated by different Gi-coupled receptors (A1, M2, sphingolipid). PD 81,273 caused inhibition of I[K(ACh)] (IC50 approximately 5 microM) activated by either of the three receptors. Receptor-independent I[K(ACh)] in cells loaded with GTP-gamma-S and background I[K(ACh)], which contributes to the resting conductance of atrial myocytes, were equally sensitive to PD 81,723. At no combination of concentrations of adenosine and PD 81,723 could an enhancing effect be detected. 3. The compound was active from the outside only. Loading of the cells with PD 81,723 (50 microM) via the patch pipette did not affect either I[K(ACh)] or its sensitivity to adenosine. We suggest that PD 81,723 acts as an inhibitor of inward rectifying K+ channels; this is supported by the finding that ventricular I(K1), which shares a large degree of homology with the proteins (GIRK1/GIRK4) forming I[K(ACh)] but is not G protein-gated, was also blocked by this compound. 4. It is concluded that the functional effects of PD 81,723 described in the literature are not mediated by the A1 adenosine receptor-Gi-I[K(ACh)] pathway.

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.

G-protein-mediated desensitization of metabotropic glutamatergic and muscarinic responses in CA3 cells in rat hippocampus

1. Desensitization of a metabotropic response was investigated in CA3 pyramidal neurons in hippocampal slice cultures using the patch-clamp technique. 2. 1S,3R-1-aminocyclopentane-1,3-dicarboxylate (1S,3R-ACPD), an agonist at metabotropic glutamate receptors (mGluRs), and metacholine (MCh), an agonist at muscarinic receptors, induced a cationic current that appears to be activated through a G-protein-independent transduction process, as previously shown. Prolonged or repetitive bath application of agonists led to rapid desensitization of the cationic current with a time constant of approximately 20 s. 3. Complete recovery from desensitization was observed within 6 min. 4. These responses mediated by mGluRs and muscarinic receptors cross-desensitized. 5. Preventing the activation of G-proteins by loading cells with GDP beta S strongly reduced or suppressed desensitization, and resulted in a sustained inward cationic current. When cells were filled with GTP gamma S to irreversibly activate G-proteins, the desensitization process was enhanced such that a first application of agonist caused a markedly reduced response. 6. These results show that a cationic current induced by metabotropic agonists in hippocampal pyramidal cells undergoes apparent desensitization and suggests that this process occurs through a G-protein-mediated inhibition of the underlying membrane conductance.

RGS family members: GTPase-activating proteins for heterotrimeric G-protein alpha-subunits

Signaling pathways using heterotrimeric guanine-nucleotide-binding-proteins (G proteins) trigger physiological responses elicited by hormones, neurotransmitters and sensory stimuli. GTP binding activates G proteins by dissociating G alpha from G beta gamma subunits, and GTP hydrolysis by G alpha subunits deactivates G proteins by allowing heterotrimers to reform. However, deactivation of G-protein signalling pathways in vivo can occur 10- to 100-fold faster than the rate of GTP hydrolysis of G alpha subunits in vitro, suggesting that GTPase-activating proteins (GAPs) deactivate G alpha subunits. Here we report that RGS (for regulator of G-protein signalling) proteins are GAPs for G alpha subunits. RGS1, RGS4 and GAIP (for G alpha-interacting protein) bind specifically and tightly to G alphai and G alpha0 in cell membranes treated with GDP and AlF4(-), and are GAPs for G alphai, G alpha0 and transducin alpha-subunits, but not for G alphas. Thus, these RGS proteins are likely to regulate a subset of the G-protein signalling pathways in mammalian cells. Our results provide insight into the mechanisms that govern the duration and specificity of physiological responses elicited by G-protein-mediated signalling pathways.

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

1. Desensitization of muscarinic K+ current (IK(ACh)) was studied in cultured atrial myocytes from guinea-pig hearts using whole-cell voltage clamp. 2. Three different types of desensitization could be identified. A fast component which upon rapid superfusion with ACh resulted in a partial relaxation of IK(ACh) within a few seconds to a plateau which was maintained in the presence of ACh. Recovery from this type of desensitization paralleled the decay of IK(ACh) after washout of the agonist. A second type of desensitization was observed within minutes. This was reversed around 10 min after washout of ACh. Both types were heterologous with regard to the A1 receptor and the novel phospholipid (Pl) receptor, both of which activate IK(ACh) via the same signalling pathway. 3. A third type of desensitization (downregulation) occurred upon exposure of the cultures for 24-48 h to the muscarinic agonist carbachol (CCh). The level of downregulation depended on the concentration of CCh (0.1 microM < or = [CCh] < or = 10 microM). No recovery was observed within 5 h after washout of CCh. Thereafter sensitivity to ACh slowly returned (half-time (t1/2), approximately 20 h). 4. Downregulation by CCh (0.1-5 microM) was characterized by an increase in EC50 for ACh with no reduction in maximum IK(ACh). With 5 microM CCh, EC50 was increased from 0.1 to 3.7 microM. At 10 microM CCh EC50 was increased to 15 microM and maximal current that could be evoked by ACh was reduced to 15%. 5. Downregulation by CCh was homologous with regard to A1 and Pl receptors. Maximum IK(ACh), assayed by a saturating concentration of Pl, was not reduced in downregulated cells, suggesting a mechanism localized at the M2 receptor. 6. The changes in the concentration-response curves can be accounted for by assuming an excess of M2 receptors relative to the subsequent component of the signalling pathway. 7. As the intact heart is under tonic vagal control, downregulation is likely to contribute to controlling the sensitivity of the heart to vagal activity in situ.