CHARACTERIZATION OF GAP JUNCTION CHANNELS IN ADULT-RABBIT ATRIAL AND VENTRICULAR MYOCARDIUM

For effective cardiac output, it is essential that electrical excitati on spread rapidly throughout the atria and ventricles. This is effecte d by electrical coupling through gap junction channels at contact site s between myocytes. These channels form a low-resistance pathway betwe en adjacent myocytes and consist of connexin proteins. The connexin fa mily is a large multigene family, and the channels formed by different members of this family have distinct electrical and regulatory proper ties. We have studied gap junction channels between adult rabbit atria l and ventricular myocytes using immunocytochemical and electrophysiol ogical methods. Gap junctions of ventricular myocytes were immunoreact ive to antibodies directed against connexin43 (Cx43) and Cx45, but not to antibodies against Cx37 or Cx40. Gap junctions between atrial myoc ytes showed immunostaining with anti-Cx40, -Cx43, and -Cx45 antibodies , but not with anti-Cx37 antibody. Endocardial and endothelial tissue were labeled with both Cx37 and Cx40 antibodies. The conductance of ra bbit myocardial gap junctions was measured using the double whole-cell voltage-clamp method. The average macroscopic junctional conductance, corrected for series resistance, of atrial and ventricular cell pairs did not differ significantly (169+/-146 and 175+/-147 nS, respectivel y), and both were at most only slightly sensitive to the applied trans junctional potential difference. The difference in connexin expression between atrial and ventricular myocytes was reflected in the distribu tion of single gap junction channel conductances. A single population of unitary channel conductances with an average of 100 pS was observed between ventricular myocyte pairs. In addition to this population, a population with an average conductance of 185 pS was present between a trial myocyte pairs. The observed difference in connexin expression be tween atrial and ventricular myocardium may enable differential regula tion of conduction in these tissues.