IMMUNOCYTOCHEMICAL LOCALIZATION OF THE G-PROTEIN SUBUNIT, G(0-ALPHA),IN RAT-HEART - IMPLICATIONS FOR A ROLE OF G(0-ALPHA) IN SECRETION OF CARDIAC HORMONES

The cellular and sub-cellular localization of the G-protein subunit, G (o alpha), in rat heart was determined by immunofluorescence and immun oelectron microscopy. Using antibodies directed against purified G(o a lpha) and an antiserum raised against the C-terminal decapeptide of G( o alpha), strong immunoreactivity was found in the conducting system o f the heart, neurons, and atrial cardiomyocytes. Labeling of ventricle s was weak compared to that of atria. In neurons, immunoelectron micro scopy revealed G(o alpha) was localized along the inner surface of axo lemma and on axoplasmal vesicles. G(o alpha) immunoreactivity in atria l and ventricular myocytes was not restricted to sarcolemma, but was a lso found on sub-sarcolemmal vesicles with characteristic caveolar mor phology. At the level of intercalated discs, labeling was spread over the periphery of intercalated dir;cs avoiding its membrane structures. Additionally in atrial endocrine cardiomyocytes, approximately 60% of secretory granules revealed G(o alpha)-labeling on the cytoplasmic su rface. A small number of these granules stood out due to particularly intense labeling. The observation that these granules were found most- frequently in sub-sarcolemmal areas suggests that they may be mature g ranules undergoing exocytosis. Therefore, G(o alpha) found on secretor y granules of endocrine cardiomyocytes may have a function in regulate d exocytosis of cardiac hormones. Sarcolemmal localization of G(o alph a) in atrial and ventricular cardiomyocytes supports the role of G(o a lpha) in transmembrane signal transduction. Furthermore, caveolar loca lization of G(o alpha) may provide a compartmental basis for integrati ng G(0)-mediated signaling events. (C) 1998 Academic Press.