Shi H, Wang H, Wang Z
Research Center, Montreal Heart Institute, Montreal, Quebec, Canada.
Mol Pharmacol. 1999 Mar;55(3):497-507.
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 microM 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 IKTMA and IK4AP were abolished by 1 microM 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.
长期以来，人们一直认为M2受体是心脏中毒蕈碱型乙酰胆碱受体（mAChR）的唯一功能亚型，尽管最近的研究已经为其他亚型的存在提供了证据。我们进行了一项详细研究以阐明这个问题。在存在四甲基铵（1微摩尔至10毫摩尔）的情况下，在单个犬心房肌细胞中激活了一种具有延迟整流和内向整流特性的新型钾电流（IKTMA）。4-氨基吡啶（0.05 - 2毫摩尔）也诱导出一种钾电流（IK4AP），其特性与IKTMA相似但又不同。IKTMA和IK4AP都被1微摩尔阿托品消除。IK4AP在用百日咳毒素处理后最小化，但IKTMA不受影响。4-二苯基乙酰氧基-N-甲基哌啶甲碘化物（M3亚型的选择性拮抗剂）使IKTMA显著降低，但哌仑西平（针对M1）、甲氧基氨甲酰胆碱（针对M2）和托吡卡胺（针对M4）对其无影响。托吡卡胺显著降低IK4AP，但其他mAChR亚型的拮抗剂对IK4AP无作用。相比之下，IKACh（乙酰胆碱诱导的钾电流）被甲氧基氨甲酰胆碱显著抑制，但其他拮抗剂对其无影响。用[甲基-3H]N-东莨菪碱甲基氯与哌仑西平、甲氧基氨甲酰胆碱、4-二苯基乙酰氧基-N-甲基哌啶甲碘化物或托吡卡胺进行置换结合的结果揭示了犬心房中多种mAChR亚型的共存。cDNA片段的克隆以及编码M2、M3和M4的mRNA的检测提供了进一步的支持证据。我们的结果表明：1）多种mAChR亚型（M2/M3/M4）共存于犬心脏中；2）不同亚型的mAChR与不同的钾通道偶联。我们的发现代表了心脏M3和M4受体生理作用的首个功能证据。
