Expression of major gap junction connexin types in the working myocardium of eight chordates.

The alpha1 connexin (connexin43) is regarded as the major gap junction protein of the myocardium because it predominates there in mammals. Here, we show that it is not the major connexin of the working myocardium in non-mammalian vertebrates, which instead express beta1-like connexins homologous to mammalian connexin32. A phylogenetic series of hearts was immunostained with seven antibodies raised against peptide sequences specific for three distinct members of the gap junction connexin family: alpha1, beta1 and alpha5 (mammalian connexin40/avian connexin42). Working myocardium from two ascidian chordates (Ciona and Mogula), a teleost (Carassius), a frog (Xenopus) and two reptiles (Anolis and Alligator) was found to express a beta1-like connexin, rather than an alpha1-like connexin. An alpha1-like connexin was nevertheless often detected in other cardiac tissues. In the chicken (by ancestry a reptile), the developing myocardium expressed a beta1-like connexin strongly on embryonic day 6 but less strongly at hatching, and minimally in the adult. Myocardial expression of alpha5 connexin increased during development, but remained strongest in the coronary vascular endothelial and cardiac conduction tissues. The arteriolar smooth muscle of the chicken expressed alpha1 connexin throughout development, but its myocardium did not. In contrast, the working myocardium of a marsupial mammal (the opossum Trichosurus) strongly expressed an alpha1 connexin just like placental mammals. These results imply that a shift from beta1 to alpha1 connexin expression in the heart occurred prior to the evolution of the opossums. The beta and alpha connexin subfamilies have different permeabilities and gating properties, and we discuss factors that might have made this shift beneficial. Copyright 1998 Academic Press.