Atrial Specific Granules in Various Mammals

The osmiophilic specific granules of the atrial muscle cells were electron microscopically examined in the atria of the rat, mouse, guinea pig, dog, cat, rabbit, flying squirrel, pig, ox, monkey and man. In all the animals the granules were round or oval in shape and bounded by a limiting membrane. The content of the granules was moderately electron dense and changed its density with the use of different fixatives. Groups of granules were gathered in the perinuclear sarcoplasm but some were seen between myofibrils and beneath the plasma membrane. The material of the granules is believed to be condensed into visible granules in the Golgi complex and released by eruptocrine type secretion.Certain differences in the distribution, the average diameter and the number per cell of the specific granules were recognized among different species. The atria of the rat and mouse contained a large number of the glanules with a large diameter. The dog and the cat showed a small number of small granules gathered in the perinuclear region. In the monkey and human atria the granules were considerably numerous though small in size. The granules in the atria of the ungulata (pig and ox) were sparce and dispersed all over the muscle cell. Some species differences in the electron density of the granules were also found. They are characteristically dense in the rabbit whereas much less dense in the mouse.Experiments in the mouse confirmed our previous results in the rat in that the administration of reserpine caused a decrease whereas nialamide, L-DOPA and L-methyl-DOPA an increase of the atrial specific granules. Catecholamines were not proved in the granules by fluorescence method. The possible nature and significance of the granules were briefly discussed.