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.