Sicouri S, Fish J, Antzelevitch C
Masonic Medical Research Laboratory, Utica, New York 13504.
J Cardiovasc Electrophysiol. 1994 Oct;5(10):824-37. doi: 10.1111/j.1540-8167.1994.tb01121.x.
M cells and transitional cells residing in the deep structures of the ventricular free walls are distinguished by the ability of their action potentials to prolong disproportionately to those of other ventricular cells at relatively slow rates. This feature of the M cell due, at least in part, to a smaller contribution of the slowly activating component of the delayed rectifier current (IKs) is thought to contribute to the unique pharmacologic responsiveness of M cells, making them the primary targets in ventricular myocardium for agents that cause action potential prolongation and induce early and delayed afterdepolarizations and triggered activity. Previous studies dealt exclusively with the characteristics and distribution of M cells in the canine right and left ventricular free wall near the base of the ventricles. The present study uses standard microelectrode techniques to define their behavior and distribution in the apical region of the ventricular wall as well as in the endocardial structures of the ventricle, including the interventricular septum, papillary muscles, and trabeculae.
Action potentials recorded from the M region (deep subepicardium) displayed similar characteristics (steep action potential duration [APD]-rate relations) in the base and apex. However, important differences were apparent in the other regions. In epicardium, the spike and dome morphology of the action potential was less accentuated and the rate dependence of APD more pronounced in the apex versus the base. In endocardium, and especially deep subendocardium, rate dependence of APD was considerably more pronounced in the apex. Transmembrane recordings from the subsurface layers of the septum, trabeculae, and papillary muscles revealed M cell behavior (steep APD-rate relations) in the deep subendocardium. Epicardial and transitional behavior were also observed in the deep layers of these endocardial structures.
Our results indicate that M cells reside throughout the deep subepicardial layers of the free wall of the canine left ventricle as well as in the deep subendocardial layers of the septum, papillary muscles, and trabeculae. The data also demonstrate prominent transmural as well as apicobasal gradients of phase 1 and phase 3 repolarization. These findings may have implications relative to our understanding of the electrocardiographic J wave, T wave, U wave, and long QTU intervals.
位于心室游离壁深层结构的M细胞和移行细胞，其动作电位在相对缓慢的频率下延长的程度与其他心室细胞不成比例，以此与其他细胞相区分。M细胞的这一特性至少部分归因于延迟整流电流（IKs）缓慢激活成分的贡献较小，这被认为有助于M细胞具有独特的药理反应性，使其成为心室肌中导致动作电位延长、诱发早期和延迟后去极化以及触发活动的药物的主要作用靶点。以往的研究仅涉及犬心室底部附近右心室和左心室游离壁中M细胞的特征和分布。本研究采用标准微电极技术来确定它们在心室壁心尖区域以及心室内膜结构（包括室间隔、乳头肌和小梁）中的行为和分布。
从M区域（深心外膜下）记录的动作电位在心底和心尖显示出相似的特征（陡峭的动作电位时程[APD] - 频率关系）。然而，在其他区域明显存在重要差异。在心外膜，动作电位的锋和圆顶形态在心底不如在心尖明显，并且APD的频率依赖性在心尖比在心底更显著。在心内膜，尤其是深心内膜下，APD的频率依赖性在心尖更为显著。从室间隔、小梁和乳头肌的表层进行的跨膜记录显示深心内膜下存在M细胞行为（陡峭的APD - 频率关系）。在这些心内膜结构的深层也观察到了心外膜和移行细胞的行为。
我们的结果表明，M细胞存在于犬左心室游离壁的心外膜深层以及室间隔、乳头肌和小梁的心内膜深层。数据还表明在复极化1期和3期存在明显的跨壁以及心尖 - 心底梯度。这些发现可能对我们理解心电图的J波、T波、U波和长QTU间期有影响。
