Distribution of light chains and ATPase activity of myosin in the atrioventricular conducting tissue of bovine heart

Summary The relations of the light chains of myosins of the atria, ventricles, and atrioventricular conducting tissue (specialized myocardial tissue) and the distribution of the light chains of myosin in different regions of the atrioventricular conducting tissue in bovine heart were examined. Two-dimensional gel electrophoresis showed that the atrial and ventricular myosins each had two light chains (LC₁ and LC₂). Ventricular LC₁ differed from atrial LC₁, but ventricular LC₂ corresponded to atrial LC₂. The specialized myocardial tissue myosin had three light chains (named here SL₁, SL₂, and SL₃). SL₁ comigrated with ventricular LC₁, SL₂ with atrial LC₁, and SL₃ with ventricular LC₂ and atrial LC₂. The compositions of the three light chains of myosins in various regions of the atrioventricular conducting tissue were determined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The percentage proportion of SL₁ decreased in the order—atrioventricular node (AVN), right and left bundle branches (RLBB), His bundle (HIS), false tendon (FT) myosin; while the percentage proportion of SL₂ decreased in the order—FT and HIS, RLBB, AVN myosin. The percentages of SL₃ in these four regions were similar. The Ca²⁺-activated ATPase activity of myosin was highest in the AVN and lowest in the FT. The activities in the HIS and RLBB were intermediate between those in the AVN and FT. Thus, the composition of the light chains and the Ca²⁺-activated ATPase activity were different in various regions of the atrioventricular conducting tissue.