IMMUNOELECTRON MICROSCOPIC VISUALIZATION OF THE GAP JUNCTION PROTEIN CONNEXIN40 IN THE MAMMALIAN HEART

In mammalian myocardium, myocytes are electrically coupled by gap junc tions, which are collections of membrane channels spanning the interce llular gap. Each channel is composed of twelve protein molecules - six in each membrane - enclosing a pore of 2 nm diameter. In addition to the main cardiac gap junction protein connexin43 (cx43), two other con stituents, connexin40 (cx40) and connexin 45 (cx45), recently have bee n identified. In this study, the ultrastructural distribution of cx40 has been.traced in several mammalian cardiac tissues, using a specific antibody and immunogold labeling. In rabbit, cx40 was found in gap ju nctions between myocytes of the right atrium as well as in the false t endons of the left ventricle. Labeling within the free ventricular wal l could be obtained only with higher primary antibody concentrations. In rat atrial and ventricular working myocardium a similar distributio n of label was observed. Double labeling of cx40 and cx43 in rat atriu m, with 5 nm and 15 nm gold particles respectively, demonstrates limit ed patches of cx40 in gap junctions otherwise homogeneously labeled fo r cx43. Since cx40 has a much higher single channel conductance than c x43, topographical differences in their relative distribution might co ntribute to regional differences in conduction velocity of the cardiac impulse.