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 electron microscopy. Cardiac myocytes are encountered from the right atrium to the root of the subclavian vein. The musculature consists of several well-developed circular or spiral myocyte layers. The ultrastructure of myocytes in the cranial venae cavae exhibits a similar structure to that of atrial myocytes. Abundant myofibrils and mitochondria are detected within the cytoplasm of these myocytes, suggesting an active contraction of the musculature. Characteristic caveolae are accumulated near the sarcolemma of cardiac myocytes in the cranial venae cavae showing their high pinocytotic activity. Immunohistochemical analysis reveals the presence of an atrial natriuretic polypeptide-like substance in the cranial vena cava and the proximal portion of the subclavian vein. Ultrastructural studies also demonstrate the distribution of atrial granules within the musculature. This musculature in the vena cava may be considered part of the endocrine atrium.