Extensions of the cardiac plasma membrane from the intercalated disk of one cell into the transverse tubules of the adjacent cell

Abstract

In the rat's left ventricle finger-like processes of cell membrane extend from the intercalated disk of one cell into the transverse tubular system of the adjacent cell. The extensions take place through extracellular channels that are oriented parallel to the longitudinal axis of the cell and connect the T-tubules with the tissue interspaces at the intercalated disk. The extensions have an irregular surface with projections varying in height from knobs to longer structures with stalks. The plasma membranes lining these projections may fuse with each other or with the plasma membrane lining the transverse tubules, forming typical tight junctions. These observations indicate that low electrical resistance contacts between cardiac cells are distributed over a wider area than previously suspected. The findings further suggest that in heart muscle the total area of cell membrane exceeds previous estimates and that there may be T-tubular functions other than excitation-contraction coupling.