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

Abstract

Non-technical summary 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. Here, we consider a novel explanation for I KACh relaxation based upon the recent finding that G-protein-coupled receptors are intrinsically voltage sensitive and that the muscarinic agonists acetylcholine and pilocarpine manifest opposite voltage-dependent I KACh modulation. Based on experimental and computational findings, we propose that I KACh relaxation represents a voltage-dependent change in agonist affinity as a consequence of a voltage-dependent conformational change in the muscarinic receptor.Abstract 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.

Voltage regulates adrenergic receptor function

“…4). This finding is in agreement with previous studies that demonstrated voltage-dependent modulation of G-protein mediated signaling pathways, such as the regulation of G-protein regulated K + -channels or Ca 2+ -activated Cl − channels and the liberation of Ca 2+ from intracellular Ca 2+ stores (4,7,10,26). A unique aspect from this study is the finding that the recruitment of β-arrestin to active receptors, which induces G protein-independent receptor signaling pathways, also exhibits voltage-dependence.…”

“…Indeed, voltage-dependent binding of neurotransmitters to their receptors, or at least voltage-dependent regulation of their downstream signal, has been described for a few other members of the GPCR family, specifically for M 1 and M 2 muscarinic receptors (4), as well as P 2 Y 1 purinergic receptors (5,6) and metabotropic glutamate receptors (7). Voltage-dependence of GPCRs has been implicated in fine-tuning of neurotransmitter release (8,9), in regulation of cardiac excitability (10), and in potentiation of IP 3 dependent intracellular Ca 2+ signals (6). So far the mechanism underlying this voltage sensitivity of GPCRs is not very well understood; however, the current view is that allosteric regulation of receptor conformations by docking of the G protein seems to be required for voltage sensitivity.…”

Muscarinic-activated potassium current mediates the negative chronotropic effect of pilocarpine on the rabbit sinoatrial node

“…Despite their frequent use as pharmacological tools, the selectivity of 4-DAMP, p-F-HHSiD, tropicamide, and methoctramine for muscarinic receptor subtypes is poor and insufficient to discriminate between M 2 , M 3 , and M 4 muscarinic receptors [3,8,15,22]. Our data and the previous work in feline atrial myocytes (see [16,17]) suggest that the muscarinic receptor subtype and the current activated by pilocarpine are M 2 and I KACh . However, our electrophysiological approach did not permit us to define the presence or absence of M 3 muscarinic receptors in the rabbit SA node.…”

“…The intrinsic capacity of muscarinic receptors to sense transmembrane potential was confirmed by the recording of "gating" currents, which reflect the reorientation of charges within the receptor in response to changes in voltage [5]. NavarroPolanco et al [17] and Moreno-Galindo et al [16] corroborated that depolarization decreased the affinity of M 2 muscarinic receptors for ACh, but depolarization increased the apparent affinity and efficacy of pilocarpine, as measured by I KACh activation. In addition, they found that voltage-induced conformational changes in M 2 muscarinic receptors (as measured by gating currents) were modified in a ligand-selective manner.…”

“…NavarroPolanco et al [17] and Moreno-Galindo et al [16] studied the effects of ACh and pilocarpine on voltage-dependent M 2 activation of I KACh and on M 2 gating currents. They found that ACh and pilocarpine manifested opposite voltage-dependent I KACh modulation.…”