Immunocytochemical localization of the G-protein sub-unit, G(o) alpha, in rat heart. Implications for a role of G(o) 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 immunoelectron microscopy. Using antibodies directed against purified G(o) alpha and an antiserum raised against the C-terminal decapeptide of G(o) alpha, strong immunoreactivity was found in the conducting system of the heart, neurons, and atrial cardiomyocytes. Labeling of ventricles was weak compared to that of atria. In neurons, immunoelectron microscopy revealed G(o) alpha was localized along the inner surface of axolemma and on axoplasmal vesicles. G(o) alpha immunoreactivity in atrial and ventricular myocytes was not restricted to sarcolemma, but was also found on sub-sarcolemmal vesicles with characteristic caveolar morphology. At the level of intercalated discs, labeling was spread over the periphery of intercalated discs avoiding its membrane structures. Additionally, in atrial endocrine cardiomyocytes, approximately 60% of secretory granules revealed G(o) alpha-labeling on the cytoplasmic surface. 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 granules undergoing exocytosis. Therefore, G(o) alpha found on secretory granules of endocrine cardiomyocytes may have a function in regulated exocytosis of cardiac hormones. Sarcolemmal localization of G(o) alpha in atrial and ventricular cardiomyocytes supports the role of G(o) alpha in transmembrane signal transduction. Furthermore, caveolar localization of G(o) alpha may provide a compartmental basis for integrating G(o)-mediated signaling events.