Myosin heavy chain isoforms in the myocardium of the atrioventricular junction of Scyliorhinus canicula (Chondrichthyes, Carcharhiniformes)

Abstract

The atrioventricular (AV) junction of the fish heart, interposed between the atrium and the ventricle, has been studied anatomically and histologically in several chondrichthyan and teleost species. Nonetheless, knowledge about myosin heavy chain (MyHC) in the AV myocardium remains scarce. This is the first report to provide data on the MyHC isoform distribution in the myocardium of the AV junction in chondrichthyans, specifically in Scyliorhinus canicula, a shark species whose heart reflects the primitive cardiac anatomical design in gnathostomes. The anti-MyHC A4.1025 antibody was used to detect differences regarding MyHC isoforms in the five dogfish examined, 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 the myocardium of the AV junction connects the atrial trabeculated myocardium with the trabeculated layer of the ventricular myocardium. The immunohistochemical results indicate that the distribution of MyHC isoforms in the AV junction is heterogeneous. The atrial portion of the AV myocardium is positive against the A4.1025 antibody, as that of the atrial myocardium. In contrast, the ventricular portion of the AV junction is not labelled, as is the case with the ventricular myocardium. This dual condition suggests that the myocardium of the AV junction has two contraction patterns: its atrial portion contracts in line with the atrial myocardium, whereas its 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 AV segment because of its heterogeneous MyHC isoform distribution. The findings support the hypothesis that a distinct MyHC isoform distribution in the AV myocardium enables a synchronous contraction of inflow and outflow cardiac segments in vertebrates lacking a specialized cardiac conduction system.