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

Vagal Nerve Stimulation Evoked Heart Rate Changes and Protection from Cardiac Remodeling

This study investigated whether vagal nerve stimulation (VNS) leads to improvements in ischemic heart failure via heart rate modulation. At 7 ± 1 days post left anterior descending artery (LAD) ligation, 63 rats with myocardial infarctions (MI) were implanted with ECG transmitters and VNS devices (MI + VNS, N = 44) or just ECG transmitters (MI, N = 17). VNS stimulation was active from 14 ± 1 days to 8 ± 1 weeks post MI. The average left ventricular (LV) end diastolic volumes at 8 ± 1 weeks were MI = 672.40 μl and MI + VNS = 519.35 μl, p = 0.03. The average heart weights, normalized to body weight (± std) at 14 ± 1 weeks were MI = 3.2 ± 0.6 g*kg(-1) and MI + VNS = 2.9 ± 0.3 g*kg(-1), p = 0.03. The degree of cardiac remodeling was correlated with the magnitude of acute VNS-evoked heart rate (HR) changes. Further research is required to determine if the acute heart rate response to VNS activation is useful as a heart failure biomarker or as a tool for VNS therapy characterization.

Short-term desensitization of muscarinic K+ current in the heart

Acetylcholine (ACh) rapidly increases cardiac K(+) currents (IKACh) by activating muscarinic K(+) (KACh) channels followed by a gradual amplitude decrease within seconds. This phenomenon is called short-term desensitization and its precise mechanism and physiological role are still unclear. We constructed a mathematical model for IKACh to examine the conditions required to reconstitute short-term desensitization. Two conditions were crucial: two distinct muscarinic receptors (m2Rs) with different affinities for ACh, which conferred an IKACh response over a wide range of ACh concentrations, and two distinct KACh channels with different affinities for the G-protein βγ subunits, which contributed to reconstitution of the temporal behavior of IKACh. Under these conditions, the model quantitatively reproduced several unique properties of short-term desensitization observed in myocytes: 1), the peak and quasi-steady states with 0.01-100 μM [ACh]; 2), effects of ACh preperfusion; and 3), recovery from short-term desensitization. In the presence of 10 μM ACh, the IKACh model conferred recurring spontaneous firing after asystole of 8.9 s and 10.7 s for the Demir and Kurata sinoatrial node models, respectively. Therefore, two different populations of KACh channels and m2Rs may participate in short-term desensitization of IKACh in native myocytes, and may be responsible for vagal escape at nodal cells.

Cardiac inotropic rebound effect after washout of acetylcholine is associated with electrophysiological heterogeneity in Langendorff-perfused rabbit heart

Cardiac electrophysiological heterogeneity related to the washout of acetylcholine (ACh) remains incompletely characterized. The aim of this study was to examine whether positive cardiac inotropic action is associated with electrophysiological heterogeneity between the atrium and the ventricle after ACh perfusion and washout. Epicardial monophasic action potentials (MAPs) from the right ventricle and right atrium, as well as cardiac contractility, were recorded from isolated Langendorff-perfused rabbit hearts using MAP electrodes and a force transducer. The results indicated that rebound positive inotropic actions were induced by ACh washout with adrenaline preconditioning. This effect was accompanied by an increase in MAP amplitude (MAPA) in the right ventricle but not the right atrium. These findings indicate that cholinergic muscarinic stimulation may lead to positive cardiac inotropic action followed by changes in regional electrophysiological heterogeneity between the atrial and ventricular myocardium. Therefore, we hypothesize that electrophysiological heterogeneity is an underlying cause of arrhythmogenesis as well as hemodynamic disturbance elicited by sudden termination of vagus stimulation.

Muscarinic receptor trafficking

Knowledge of the mechanisms responsible for the trafficking of neurotransmitter receptors away from the cell surface is of obvious importance in understanding what regulates their expression and function. This chapter will focus on the mechanisms responsible for the internalization and degradation of muscarinic receptors. There are both receptor subtype-specific and cell-type specific differences in muscarinic receptor trafficking. Studies on muscarinic receptor trafficking both in cells in culture and in vivo will be described, and the potential physiological consequences of this trafficking will be discussed.

GABAB receptor coupling to G-proteins and ion channels

GABA(B) receptors have been found to play a key role in regulating membrane excitability and synaptic transmission in the brain. The GABA(B) receptor is a G-protein coupled receptor (GPCR) that associates with a subset of G-proteins (pertussis toxin sensitive Gi/o family), that in turn regulate specific ion channels and trigger cAMP cascades. In this review, we describe the relationships between the GABA(B) receptor, its effectors and associated proteins that mediate GABA(B) receptor function within the brain. We discuss a unique feature of the GABA(B) receptor, the requirement for heterodimerization to produce functional receptors, as well as an increasing body of evidence that suggests GABA(B) receptors comprise a macromolecular signaling heterocomplex, critical for efficient targeting and function of the receptors. Within this complex, GABA(B) receptors associate specifically with Gi/o G-proteins that regulate voltage-gated Ca(2+) (Ca(V)) channels, G-protein activated inwardly rectifying K(+) (GIRK) channels, and adenylyl cyclase. Numerous studies have revealed that lipid rafts, scaffold proteins, targeting motifs in the receptor, and regulators of G-protein signaling (RGS) proteins also contribute to the function of GABA(B) receptors and affect cellular processes such as receptor trafficking and activity-dependent desensitization. This complex regulation of GABA(B) receptors in the brain may provide opportunities for new ways to regulate GABA-dependent inhibition in normal and diseased states of the nervous system.

Coregulation of natively expressed pertussis toxin-sensitive muscarinic receptors with G-protein-activated potassium channels

Many inhibitory neurotransmitters in the brain activate Kir3 channels by stimulating pertussis toxin (PTX)-sensitive G-protein-coupled receptors. Here, we investigated the regulation of native muscarinic receptors and Kir3 channels expressed in NGF-differentiated PC12 cells, which are similar to sympathetic neurons. Quantitative reverse transcription-PCR and immunocytochemistry revealed that NGF treatment significantly upregulated mRNA and protein for m2 muscarinic receptors, PTX-sensitive G alpha(o) G-proteins, and Kir3.2c channels. Surprisingly, these upregulated muscarinic receptor/Kir3 signaling complexes were functionally silent. Ectopic expression of m2 muscarinic receptors or Kir3.2c channels was unable to produce muscarinic receptor-activated Kir3 currents with oxotremorine. Remarkably, pretreatment with muscarinic (m2/m4) receptor antagonists resulted in robust oxotremorine-activated Kir3 currents. Thus, sustained cholinergic stimulation of natively expressed m2/m4 muscarinic receptors controlled cell surface expression and functional coupling of both receptors and Kir3 channels. This new pathway for controlling Kir3 signaling could help limit the potential harmful effects of excessive Kir3 activity in the brain.

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.

A difference in inward rectification and polyamine block and permeation between the Kir2.1 and Kir3.1/Kir3.4 K+ channels

Inward rectification is caused by voltage-dependent block of the channel pore by intracellular Mg2+ and polyamines such as spermine. In the present study, we compared inward rectification in the Kir3.1/Kir3.4 channel, which underlies the cardiac current I(K,ACh), and the Kir2.1 channel, which underlies the cardiac current I(K,1). Sustained outward current at potentials positive to the K+ reversal potential was observed through Kir3.1/Kir3.4, but not Kir2.1, demonstrating that Kir3.1/Kir3.4 exhibits weaker inward rectification than Kir2.1. We show that Kir3.1/Kir3.4 is more sensitive to extracellular spermine block than Kir2.1, and that intracellular and extracellular polyamines can permeate Kir3.1/Kir3.4, but not Kir2.1, to a limited extent. We describe a simple kinetic model in which polyamines act as permeant blockers of Kir3.1/Kir3.4, but as relatively impermeant blockers of Kir2.1. The model shows the difference in sensitivity to extracellular spermine block, as well as the difference in the extent of inward rectification between the two channels. This suggests that Kir3.1/Kir3.4 exhibits weaker inward rectification than Kir2.1 because of the difference in the balance of polyamine block and permeation of the two channels.

Endogenous RGS proteins enhance acute desensitization of GABA(B) receptor-activated GIRK currents in HEK-293T cells

The coupling of GABA(B) receptors to G-protein-gated inwardly rectifying potassium (GIRK) channels constitutes an important inhibitory pathway in the brain. Here, we examined the mechanism underlying desensitization of agonist-evoked currents carried by homomeric GIRK2 channels expressed in HEK-293T cells. The canonical GABA(B) receptor agonist baclofen produced GIRK2 currents that decayed by 57.3+/-1.4% after 60 s of stimulation, and then deactivated rapidly (time constant of 3.90+/-0.21 s) upon removal of agonist. Surface labeling studies revealed that GABA(B) receptors, in contrast to micro opioid receptors (MOR), did not internalize with a sustained stimulation for 10 min, excluding receptor redistribution as the primary mechanism for desensitization. Furthermore, heterologous desensitization was observed between GABA(B) receptors and MOR, implicating downstream proteins, such G-proteins or the GIRK channel. To investigate the G-protein turnover cycle, the non-hydrolyzable GTP analogue (GTPgammaS) was included in the intracellular solution and found to attenuate desensitization to 38.3+/-2.0%. The extent of desensitization was also reduced (45.3+/-1.3%) by coexpressing a mutant form of the Galphaq G-protein subunit that has been designed to sequester endogenous RGS proteins. Finally, reconstitution of GABA(B) receptors with Galphao G-proteins rendered insensitive to RGS resulted in significantly less desensitization (28.5+/-3.2%). Taken together, our results demonstrate that endogenous levels of RGS proteins effectively enhance GABA(B) receptor-dependent desensitization of GIRK currents.

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.