1992 Volume 4 Issue 2 Pages 189-201
Ultrastructural distribution and nature of atrial granules (AGs) in canine atrial muscle cells were examined by Immunoelectron microscopy. Changes in subcellular distribution of AGs after atrial pacing were evaluated relative to the serum level of atrial natriuretic polypeptide (ANP) and atrial pressure. Thirty mongrel dogs were divided into three groups: group A, nonpaced group; group B, paced for 20 min; and group C, paced for 60 min. Atrial pacing was performed at a rate of 300 per minute, and serum ANP levels and hemodynamics were measured before and after the pacing for 10, 20, 40 and 60 min, in groups B and C. Specimens were excised from both atrial appendages and atria at the end of each experiment. Some were prepared for electron microscopy by either the usual transmission method or an immunocytochemical method, and the others were prepared for measurement of tissue concentration of ANP by radioimmunoassay (RIA) . In groups B and C, mean right atrial pressure and pulmonary capillary wedge pressure were both significantly higher throughout the pacing. In groups B and C, serum levels of ANP increased rapidly from 24±11 to 75±39 pg/ml within the first 10 min of pacing and remained higher, accompanied by higher atrial pressures, during atrial pacing. However, tissue concentration of ANP was not changed significantly by pacing in any part of the atrial tissue. Immunoelectron microscopy revealed that all AGs contained immuno-positive α-hANP. In group B, AGs were predominant in the subsarcolemmal area. A number of caveolae, reflecting exocytosis of AGs, were also found along the sarcolemma. In group C, AGs and coated vesicles were increased especially in the Golgi area, suggesting accelerated synthesis of AGs. These results indicate that α-ANP containing in AGs are released by exocytosis, and the secretion is promoted by atrial pacing for 20 min, which also results in higher atrial pressures. Synthesis of α-ANP is concluded to be stimulated by further pacing for 60 min.
