Summary. The effect of a single subcutaneous injection of monocrotaline on the right atrial muscle cells of rats was examined by electron microscopy. Particular attention was paid to changes in the number of specific atrial granules containing atrial natriuretic polypeptide, and to the degree of development of the Golgi apparatus in the cells induced by the drug.The effect of the drug was maximum at 5 weeks after injection of 60 mg/kg monocrotaline. The Golgi apparatuses were markedly developed, and an increase in the number of specific atrial granules was observed in the central sarcoplasmic core, in the sarcoplasmic layer and under the plasma membrane of the atrial muscle cells. Swelling and increased numbers of mitochondria and swelling of the sarcoplasmic reticulum were also noted. The number of granules and development of the Golgi apparatuses were decreased at 7 weeks after the injection when cell degeneration became apparent.These findings support the idea that the synthesis of atrial natriuretic polypeptide is accelerated by an increase in the atrial pressure.

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