Abstract
M2 receptors have long been believed to be the only functional subtype of muscarinic acetylcholine receptor (mAChR) in the heart, although recent studies have provided evidence for the presence of other subtypes. We performed a detailed study to clarify this issue. In the presence of tetramethylammonium (1 μM to 10 mM), a novel K+ current with both delayed rectifying and inward rectifying properties (IKTMA) was activated in single canine atrial myocytes. 4-Aminopyridine (0.05–2 mM) also induced a K+ current (IK4AP) with characteristics similar to but distinct from those of IKTMA. Both IKTMAand IK4AP were abolished by 1 μM atropine. IK4AP, but not IKTMA, was minimized by treatment with pertussis toxin. IKTMA was markedly decreased by 4-diphenylacetoxy-N-methylpiperidine methiodide (a selective antagonist for M3 subtype) but was not altered by pirenzepine (for M1), methoctramine (for M2), and tropicamide (for M4). Tropicamide substantially reduced IK4AP, but the antagonists for other mAChR subtypes had no effects on IK4AP. By comparison, IKACh (ACh-induced K+ current) was significantly depressed by methoctramine but was unaltered by other antagonists. Results from displacement binding of [methyl-3H]N-scopolamine methyl chloride with pirenzepine, methoctramine, 4-diphenylacetoxy-N-methylpiperidine methiodide, or tropicamide revealed the coexistence of multiple mAChR subtypes in canine atrium. Cloning of cDNA fragments and detection of mRNAs coding for M2, M3, and M4 provided further supporting evidence. Our results suggest that 1) multiple subtypes of mAChRs (M2/M3/M4) coexist in the dog heart and 2) different subtypes of mAChRs are coupled to different K+ channels. Our findings represent the first functional evidence for the physiological role of cardiac M3 and M4 receptors.
Footnotes
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Send reprint requests to: Zhiguo Wang, Ph.D, Research Center, Montreal Heart Institute, 5000 Bélanger Street East, Montreal, Quebec, Canada H1T 1C8. E-mail:wangz{at}icm.umontreal.ca
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1 This work was supported in part by the Medical Research Council of Canada, the Heart and Stroke Foundation of Quebec, an Establishment Grant for young investigators from the Fonds de Recherche en Santé du Québec (Z.W.), and the Fonds de Recherche de l’Institut de Cardiologie de Montréal. Z.W. is a Heart and Stroke Foundation of Canada Research Scholar.
- Abbreviations:
- mAChR
- muscarinic acetylcholine receptor
- ACh
- acetylcholine
- TMA
- tetramethylammonium
- IKTMA
- tetramethylammonium-induced K+ currents
- PTX
- pertussis toxin
- 4AP
- 4-aminopyridine
- IK4AP
- 4-aminopyridine-induced K+ currents
- PZ
- pirenzepine
- Meth
- methoctramine
- 4-DAMP
- 4-diphenylacetoxy-N-methylpiperidine methiodide
- p-F-HHSiD
- hexahydro-sila-difenidol hydrochloride,p-fluoro analog
- Trop
- tropicamide
- [3H]NMS
- [methyl-3H]N-scopolamine methyl chloride
- pKi
- equilibrium dissociation constant
- RT
- reverse transcription
- PCR
- polymerase chain reaction
- IKr
- rapid component of delayed rectifier K+ current
- IKs slow component of delayed rectifier K+ current.
- Received June 1, 1998.
- Accepted December 7, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
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