Electron-opaque granules in the atrial muscle cells of the rat were studied electron microscopically. The granules accumulated around the Golgi-complex in the cell. Besides, they occurred in less numbers in the sarcoplasmic layers between myofibrils and under the plasma membrane. The Golgi-complex seemed intimately related to the production of the granules.
The granules decreased after treatment of the rats with reserpine, while markedly increased after administration of L-DOPA.
In the histochemical examination of FALCK and HILLARP, however, fluorescence due to catecholamine was found neither in normal rats nor in the animals treated with reserpine or L-DOPA. It was thus concluded that the atrial specific granules do not contain catecholamine.