Restricted distribution of connexin40, a gap junctional protein, in mammalian heart

Connexin40 (Cx40) is a member of the connexin family of gap junction proteins. Its mRNA, abundant in lung, is also present in mammalian heart, although in lower amount. Rabbit antipeptide antibodies directed to the COOH terminus (residues 335 to 356) of rat Cx40 were characterized to investigate the distribution of Cx40 in rat and guinea pig cardiac tissues. The affinity-purified antibodies detect specifically a major protein (M(r), 40,000) in immunoblots of total extracts from rat lung and rat and guinea pig heart. In sections of guinea pig atrial tissue treated for immunofluorescence, a strong labeling associated with myocytes was seen with a distribution consistent with that of intercalated disks. The results of immunoelectron microscopy carried out with guinea pig atrial tissue showed that epitopes recognized by these antibodies were exclusively associated with gap junctions. These results, added to those of control experiments, demonstrate that antibodies 335-356 are specific for Cx40. Double-labeling experiments carried out with lung sections using anti-factor VIII and anti-Cx40 antibodies suggest that Cx40 is expressed in blood vessel endothelial cells. In guinea pig and rat heart sections, investigated using both immunofluorescence and immunoperoxidase techniques, a signal was also found to be associated with vascular walls. In guinea pig heart, only atrial myocytes are Cx40-positive. No labeling was detected in ventricular myocytes, including those of the His bundle and the bundle branches, which otherwise do express connexin43 (Cx43). In rat heart Cx40-expressing myocytes are localized in branches, and the Purkinje fibers. Cx43 is not detected either in the His bundle or in the proximal parts of the bundle branches, and consequently, Cx40 is the first connexin demonstrated in this region of the rat conduction system. Cx40 was not detected in the working ventricular myocytes. Double-labeling experiments carried out with hen anti-Cx43 antibodies and rabbit anti-Cx40 antibodies demonstrated that, in tissues expressing both Cx43 and Cx40, these two connexins were localized in the same immunoreactive sites. A few sites, however, appear to contain only one or the other of these two connexins.