ULTRASTRUCTURE OF MEMBRANE-ALTERATIONS OF ENZYMATICALLY DISSOCIATED CARDIAC MYOCYTES

Enzymic dissociation of cardiac myocytes results in a homogeneous population of cylindrical cells, if the Ca2+ concentration in the medium is rigorously controlled. In the presence of 0.5 to 1 μm Ca2+ the myocytes undergo cyclic contraction, even though no resting or action potentials can be recorded. The automatic behavior of the myocytes is highly sensitive to the Ca2+ concentration in the medium, but is not affected by tetrodotoxin or changes in K+ and Na+ concentrations. Structural studies by scanning and transmission electron microscopy on whole cells, sections, and freeze-fracture preparations reveal a loss of basement membrane, but a normal continuity of the plasma membrane over the entire surface of cylindrical myocytes, consistent with the exclusion of dyes of approximately 900 MW. The dissociation results in separation of the intercalated discs with either uncoupling or tearing of gap junctions. Tearing results in both membranes of the gap junction remaining with one of the cells and resealing of the torn membranes appears to occur readily. Twenty to 30% of gap junctions, however, are left with one cytoplasmic face exposed to the medium, likely providing channels for entry of electrolytes. This is consistent with the observed electrochemical shunt, the permeability to Ca2+ and the pattern of contractile activation originating at the polar ends of the myocytes where the exposed gap junctions are located. Freeze-fracture studies reveal normal ultrastructure detail, with the exception of lower and altered distribution of protein particles in the plane of nonjunctional myolemma. Sarcoplasmic reticulum and sarcomeres appear intact in all details, consistent with a normal involvement of these structures in contractile activation of dissociated myocytes. © 1979 Academic Press Inc. (London) Limited.