Fibronectin and laminin in transverse tubules of cardiac myocytes studied by laser confocal microscopy and immunocytochemistry.

The distribution of two non-collagenous glycoproteins of high molecular weight, fibronectin (FN) and laminin (LMN), was investigated in myocardial cells from the ventricle of rats, and from biopsies collected from the auricle of patients undergoing a coronary bypass operation. In order to elucidate the expression of FN and LMN across cells, non-invasive serial sectioning has been carried out by laser scanning confocal microscopy of frozen, immunostained tissue sections. In addition, immunoelectron microscopy was used to study the distribution of these antigens at higher magnifications. These studies show that FN is part of the basement membrane of the surface sarcolemma of both ventricular and atrial cells, in addition to being an abundant protein of the extracellular matrix (ECM). Along transverse tubular(TT)-membranes, FN was only detected in tubules exceeding 200 nm in diameter. Even here, the intensity of labelling varied greatly and was generally low. By contrast, a heavy investment of LMN was organized in the basal lamina along the surface sarcolemma and along ramifications of the entire TT-system in ventricular heart muscle cells. In this way, the network of TT-membrane systems of working heart muscle cells provides a supply of LMN to all depths of the myocardial fibre. In human atrial muscle cells, a regular TT-system appears to be absent. Instead occasional, deep sarcolemmal invaginations occur with diameters of 300-500 nm, the surfaces of which are also invested with LMN. The significance of the present findings has been discussed, with special reference to LMN as a possible component of a series of proteins involved in transmembrane communication between the ECM and the sarcoplasm.