Arrested exocytosis of atrial secretory granules

Release of atrial natriuretic peptide (ANP) from atrial muscle cells is thought to occur by exocytosis of secretory granules, as in other secretory systems. However, in the atrial myocyte, exocytosis has previously proved difficult to detect ultrastructurally. In order to study the mechanism of ANP release and related events, we have applied two separate techniques--tannic acid perfusion and ultrarapid freezing--specifically designed to arrest exocytosis. An increased number of fusion sites was found after ultrathin sectioning and freeze-fracture of tannic acid-treated atria enabling a hypothetical release sequence to be constructed. Detail of granule substructure was also obtained, suggesting the presence of a coat to the secretory granule core. Ultrarapid freezing, followed by freeze-fracture and freeze substitution, has confirmed aspects of the proposed exocytotic sequence, suggesting that this release is not due to the application of tannic acid, and these techniques also produced further evidence for the existence of the granule-core coat. Coated pits and vesicles were also found in large numbers in tannic acid-treated atria and interactions between coated vesicles and secretory granules were visualized. The possible role of coated vesicles in an exocytotic/endocytotic membrane retrieval pathway is discussed.