The atrioventricular junction of the fish heart, namely the segment interposed between the single atrium and the single ventricle, has been studied anatomically and histologically in several chondrichthyan and teleost species. However, knowledge about Myosin heavy chain (MyHC) in the atrioventricular myocardium remains scarce. The present report is the first one to provide data on the MyHC isoform distribution in the myocardium of the atrioventricular junction in chondrichthyans, specifically in the lesser spotted dogfish, Scyliorhinus canicula, a shark species whose heart reflects the primitive cardiac anatomical design in gnathostomes. Hearts from five dogfish were examined using histochemical and immunohistochemical techniques. The anti-MyHC A4.1025 antibody was used to detect differences in the occurrence of MyHC isoforms in the dogfish, as the fast-twitch isoforms MYH2 and MYH6 have a higher affinity for this antibody than the slow-twitch isoforms MYH7 and MYH7B. The histochemical findings show that myocardium of the atrioventricular junction connects the trabeculated myocardium of the atrium with the trabeculated layer of the ventricular myocardium. The immunohistochemical results indicate that the distribution of MyHC isoforms in the atrioventricular junction is not homogeneous. The atrial portion of the atrioventricular myocardium shows a positive reactivity against the A4.1025 antibody, similar to that of the atrial myocardium. In contrast, the ventricular portion of the atrioventricular junction is not labeled, as is the case with the ventricular myocardium. This dual condition suggests that the myocardium of the atrioventricular junction has two contraction patterns: the myocardium of the atrial portion contracts in line with the atrial myocardium, while that of the ventricular portion follows the contraction pattern of the ventricular myocardium. Thus, the transition of the contraction wave from the atrium to the ventricle may be established in the atrioventricular segment due to its heterogenous MyHC isoform distribution. Our findings support the hypothesis that a distinct Myosin heavy chain isoform distribution in the atrioventricular myocardium enables synchronous contraction of inflow and outflow cardiac segments in vertebrates lacking a specialized cardiac conduction system. This article is protected by copyright. All rights reserved.

中文翻译：

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Scyliorhinus canicula (Chondrichthyes, Carcharhiniformes) 房室交界处心肌中的肌球蛋白重链亚型

鱼心脏的房室交界处，即插入单心房和单心室之间的部分，已经在几种软骨鱼类和硬骨鱼类中进行了解剖学和组织学研究。然而，关于房室心肌中肌球蛋白重链 (MyHC) 的知识仍然很少。本报告是第一个提供有关软骨鱼类房室交界处心肌中 MyHC 同种型分布的数据的报告，特别是在较小斑点的角鲨中，Scyliorhinus canicula，一种鲨鱼物种，其心脏反映了有颚类动物的原始心脏解剖设计。使用组织化学和免疫组织化学技术检查了五条角鲨的心脏。抗 MyHC A4.1025 抗体用于检测角鲨中 MyHC 亚型出现的差异，因为快缩肌同种型 MYH2 和 MYH6 对这种抗体的亲和力高于慢缩肌同种型 MYH7 和 MYH7B。组织化学结果表明，房室交界处的心肌将心房的小梁心肌与心室心肌的小梁层连接起来。免疫组织化学结果表明 MyHC 亚型在房室交界处的分布不均匀。房室心肌的心房部分对 A4.1025 抗体呈阳性反应，与心房心肌相似。相比之下，房室交界处的心室部分没有标记，就像心室心肌的情况一样。这种双重条件表明房室交界处的心肌有两种收缩模式：心房部分的心肌收缩与心房心肌一致，而心室部分的心肌遵循心室心肌的收缩模式。因此，收缩波从心房到心室的过渡可能在房室段中建立，因为其 MyHC 异构体​​分布不均匀。我们的研究结果支持这样的假设，即房室心肌中独特的肌球蛋白重链异构体分布能够使缺乏专门心脏传导系统的脊椎动物的流入和流出心脏节段同步收缩。本文受版权保护。版权所有。而心室部分的收缩遵循心室心肌的收缩模式。因此，由于其不均匀的 MyHC 异构体​​分布，收缩波从心房到心室的过渡可能在房室段中建立。我们的研究结果支持这样的假设，即房室心肌中独特的肌球蛋白重链异构体分布能够使缺乏专门心脏传导系统的脊椎动物的流入和流出心脏节段同步收缩。本文受版权保护。版权所有。而心室部分的收缩遵循心室心肌的收缩模式。因此，由于其不均匀的 MyHC 异构体​​分布，收缩波从心房到心室的过渡可能在房室段中建立。我们的研究结果支持这样的假设，即房室心肌中独特的肌球蛋白重链异构体分布能够使缺乏专门心脏传导系统的脊椎动物的流入和流出心脏节段同步收缩。本文受版权保护。版权所有。我们的研究结果支持这样的假设，即房室心肌中独特的肌球蛋白重链异构体分布能够使缺乏专门心脏传导系统的脊椎动物的流入和流出心脏节段同步收缩。本文受版权保护。版权所有。我们的研究结果支持这样的假设，即房室心肌中独特的肌球蛋白重链异构体分布能够使缺乏专门心脏传导系统的脊椎动物的流入和流出心脏节段同步收缩。本文受版权保护。版权所有。