LOCALIZATION OF SPECTRIN ISOFORMS IN THE ADULT-MOUSE HEART

The distribution of two isoforms of spectrin in the adult mouse heart was investigated by Western blotting and immunocytochemistry by use of monospecific antibodies to erythrocyte spectrin and nonerythroid brai n spectrin (240/235). Western blotting revealed proteins analogous to both isoforms of alpha-spectrin in adult heart. Light-microscopic immu nocytochemistry indicated that erythroid spectrin was distributed thro ughout the myocardium, with immunofluorescence localized to plasma mem branes, Z-lines, and intercalated discs. Antibodies to brain spectrin (240/235) exhibited staining throughout the heart, with a generally di ffuse distribution except for the prominent immunoreactivity associate d with the intercalated discs. Nonerythroid spectrin immunofluorescenc e was detected in the endothelial cells of the endocardium and the mes othelial cell lining of the epicardium. Erythrocyte spectrin was not d etected in the endocardium or the epicardium. The identification and l ocalization of spectrin isoforms in the mammalian heart suggest the im portance of spectrin proteins in the structural integrity and proper f unction of cardiac cells and tissues. This is the first demonstration of two different a-spectrin subunits in the mammalian heart.