Abstract
The acetylcholine (ACh)-gated inwardly rectifying K+ current (IKACh) plays a vital role in cardiac excitability by regulating heart rate variability and vulnerability to atrial arrhythmias. These crucial physiological contributions are determined principally by the inwardly rectifying nature of IKACh. Here, we investigated the relative contribution of two distinct mechanisms of IKACh inward rectification measured in atrial myocytes: a rapid component due to KACh channel block by intracellular Mg2+ and polyamines; and a time- and concentration-dependent mechanism. The time- and ACh concentration-dependent inward rectification component was eliminated when IKACh was activated by GTPγS, a compound that bypasses the muscarinic-2 receptor (M2R) and directly stimulates trimeric G proteins to open KACh channels. Moreover, the time-dependent component of IKACh inward rectification was also eliminated at ACh concentrations that saturate the receptor. These observations indicate that the time- and concentration-dependent rectification mechanism is an intrinsic property of the receptor, M2R; consistent with our previous work demonstrating that voltage-dependent conformational changes in the M2R alter the receptor affinity for ACh. Our analysis of the initial and time-dependent components of IKACh indicate that rapid Mg2+-polyamine block accounts for 60–70% of inward rectification, with M2R voltage sensitivity contributing 30–40% at sub-saturating ACh concentrations. Thus, while both inward rectification mechanisms are extrinsic to the KACh channel, to our knowledge, this is the first description of extrinsic inward rectification of ionic current attributable to an intrinsic voltage-sensitive property of a G protein-coupled receptor.
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Abbreviations
- ACh:
-
acetylcholine
- CCh:
-
carbachol
- C-R:
-
concentration-response
- I KACh :
-
acetylcholine-gated K+ current
- M2R:
-
muscarinic type-2 receptor
- Pilo:
-
pilocarpine
- TMA:
-
tetramethylammonium
- V h :
-
holding potential
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Acknowledgements
The authors wish to thank Miguel Angel Flores-Virgen for technical assistance.
Funding
This work was supported by SEP-CONACYT, México. Grant No. CB-2013-01-220742 (R. A. N-P.) and CB-2011-01-167109 (E.G.M-G).
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All animal studies were performed in accordance with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No. 85-23, revised 1996) and after securing approval by the Institutional Animal Care and Use Committee of the University of Colima. This article does not contain any studies with human participation performed by any of the authors.
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Salazar-Fajardo, P.D., Aréchiga-Figueroa, I.A., López-Serrano, A.L. et al. The voltage-sensitive cardiac M2 muscarinic receptor modulates the inward rectification of the G protein-coupled, ACh-gated K+ current. Pflugers Arch - Eur J Physiol 470, 1765–1776 (2018). https://doi.org/10.1007/s00424-018-2196-y
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DOI: https://doi.org/10.1007/s00424-018-2196-y