ten Velde I, de Jonge B, Verheijck E E, van Kempen M J, Analbers L, Gros D, Jongsma H J
Department of Physiology, University of Amsterdam, Netherlands.
Circ Res. 1995 May;76(5):802-11. doi: 10.1161/01.res.76.5.802.
Myocytes are electrically coupled by gap junctions, which are composed of low-resistance intercellular channels. The major cardiac gap junction protein is connexin43 (Cx43). The distribution of Cx43 has been studied by immunofluorescence to visualize the electrical coupling between atrial tissue and sinoatrial node. From modeling studies, this coupling was inferred to be gradual in order to shield the sinoatrial node from the atrial hyperpolarizing influence. The actual Cx43 labeling pattern did not show the expected gradient but instead a rather black and white staining in a striking pattern of strands of cells. We used an immunohistochemical marker (anti-alpha-smooth muscle actin [alpha SMA]) that specifically cross-reacts with guinea pig sinoatrial node cells together with Cx43 antibody to stain previously electrophysiologically mapped sinoatrial nodes. We found that in the guinea pig sinoatrial node the impulse originates in an alpha SMA-positive, virtually Cx43-negative, region (primary pacemaker region). The impulse then travels obliquely upward to the crista terminalis through a region where layers of alpha SMA-positive cells alternate with layers of Cx43-positive SMA-negative cells. The layers of Cx43-positive cells appear to become broader and thicker in the direction of the crista terminalis, whereas the layers of alpha SMA-positive cells become thinner and narrower. Lateral contacts between Cx43- and alpha SMA-positive cells were very sparse and only detected where the Cx43-positive strands ended (the region where alpha SMA-positive cells fill the whole space between endocardium and epicardium, ie, the putative primary pacemaker region). From these results, we conclude that the primary pacemaker is shielded from the hyperpolarizing influence of the atrium by a gradient in coupling brought about by tissue geometric factors rather than by a gradient of gap junction density.
心肌细胞通过间隙连接实现电偶联，间隙连接由低电阻细胞间通道组成。主要的心脏间隙连接蛋白是连接蛋白43（Cx43）。通过免疫荧光研究了Cx43的分布，以观察心房组织与窦房结之间的电偶联。从模型研究推断，这种偶联是渐进的，以便使窦房结免受心房超极化影响。实际的Cx43标记模式并未显示出预期的梯度，而是呈现出一种相当黑白分明的染色，呈明显的细胞条索状。我们使用一种免疫组织化学标记物（抗α-平滑肌肌动蛋白[α-SMA]），它能与豚鼠窦房结细胞特异性交叉反应，同时结合Cx43抗体对先前进行电生理标测的窦房结进行染色。我们发现，在豚鼠窦房结中，冲动起源于一个α-SMA阳性、几乎Cx43阴性的区域（初级起搏区域）。然后，冲动通过一个区域斜向上传播至终嵴，在该区域，α-SMA阳性细胞层与Cx43阳性、SMA阴性细胞层交替排列。Cx43阳性细胞层在朝向终嵴的方向上似乎变得更宽更厚，而α-SMA阳性细胞层则变得更薄更窄。Cx43阳性细胞与α-SMA阳性细胞之间的横向接触非常稀疏，仅在Cx43阳性条索末端（α-SMA阳性细胞充满心内膜和心外膜之间整个空间的区域，即假定的初级起搏区域）被检测到。从这些结果中，我们得出结论，初级起搏点是通过组织几何因素导致的偶联梯度而非间隙连接密度梯度来免受心房超极化影响的。
