CARDIAC MUSCULATURE OF THE CRANIAL VENA-CAVA IN THE RAT

The distribution and morphological features of cardiac musculature in rat cranial venae cavae were examined by light and transmission electr on microscopy. Cardiac myocytes are encountered from the right atrium to the root of the subclavian vein. The musculature consists of severa l well-developed circular or spiral myocyte layers. The ultrastructure of myocytes in the cranial venae cavae exhibits a similar structure t o that of atrial myocytes. Abundant myofibrils and mitochondria are de tected within the cytoplasm of these myocytes, suggesting an active co ntraction of the musculature. Characteristic caveolae are accumulated near the sarcolemma of cardiac myocytes in the cranial venae cavae sho wing their high pinocytotic activity. Immunohistochemical analysis rev eals the presence of an atrial natriuretic polypeptide-like substance in the cranial vena cava and the proximal portion of the subclavian ve in. Ultrastructural studies also demonstrate the distribution of atria l granules within the musculature. This musculature in the vena cava m ay be considered part of the endocrine atrium.