Ultrastructural Evidence for Exocytotic Release of Atrial Natriuretic Polypeptide in the Rat Atrial Muscle With the Use of the Tannic Acid Method

The atrial muscle of the rat was observed by electron microscopy in order to reveal how the atrial granules are released. For provocation of granule release, the atrial muscle was stimulated either by injection ofadrenaline or by expansion of the atrial cavity. The atrial muscle was then fixed with tannic acid in combination with glutaraldehyde and osmium tetroxide. Tannic acid bound to exposed secretory products, and therefore facilitated visualization of the release process in the atrial muscle cell under the electron microscope. With this method we could clearly demonstrate exocytotic granule release in the atrial muscle cell. The ultrastructural images obtained in this study indicate the process of the granule release as follows. The granules move to the periphery of the cytoplasm, then the granule membrane fuses with the plasma membrane and an omega-shaped membrane indentation appears. Successively the granule contents are diffused into the extracellular space, while coated pits adhere to the opening membrane. Multivesicular bodies may be found in the cytoplasm, presumably representing an autolytic or crinophagic process. This sequence of exocytosis in the atrial muscle cell is identical with that in peptideproducing endocrine cells, supporting the view that the atrial muscle cell functions as an endocrine cell.