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The myocardial layer of the pulmonary vein of adult rats was examined by electron microscopy. Among ordinary myocardial cells resembling those of the atrial myocardium, clear cells with structural features similar to those of sinus node cells were identified. They were distributed in the intrapulmonary, preterminal portion of the pulmonary vein. They appeared singly or in small groups among the ordinary myocardial cells. Their cytoplasm was characterised by a paucity of myofilaments, irregular disposition of myofilament bundles, small and oval mitochondria, absence of atrial specific granules and a wide cytoplasmic matrix between intracellular organelles. The intercalated discs of node-like cells were composed of small junctional specialisations. Nerve fibres containing small and large vesicles with and without dense cores were juxtaposed to the node-like cells over an intercellular space of more than 200 nm. Taking into consideration physiological data, the possibility is discussed that the node-like cells may have a potential pacemaking activity and represent an ectopic pacemaker centre in the pulmonary vein.

The distribution of atrial natriuretic polypeptide (ANP) was immunohistochemically surveyed in the rat heart and lung using an antiserum raised against alpha-human ANP. The ANP-immunoreactive cells were seen to be distributed in the atrial walls and proximal portions of the pulmonary vein and venae cavae, but were absent from the aorta, pulmonary arteries, trachea, bronchus, and alveolar cells. The immunoreactive cells were present in a narrow region just beneath the endothelium of the pulmonary vein and vena cavae, and, ultrastructurally and immunocytochemically, were seen to be striated muscle cells with ANP-containing specific granules similar to those seen in atrial cardiocytes. A radioimmunoassay for ANP revealed a content of 604 +/- 51 pg/mg wet weight in the pulmonary vein, and 3343 +/- 1620 pg/mg wet weight in the venae cavae. In addition to the atrial wall, the proximal portion of both the pulmonary vein and venae cavae are suggested to be constituents of an ANP-producing organ.

Endothelial cells in culture have emerged as a model system for evaluation of endothelial properties. Culture conditions may affect expression of these endothelial properties or growth characteristics. Therefore, we evaluated cell cycle kinetics, prostacyclin release, and angiotensin-converting enzyme activity in primary and subcultured cells from bovine pulmonary and systemic vessels. Prostacyclin release was highly variable during passage in culture. However, release following stimulation with ionophore A23187 declined with serial passage. Angiotensin-converting enzyme activity also declined with repeated subculturing. Greater than 10% of primary confluent bovine endothelial cells were cycling compared to 2 to 8% of human umbilical vein endothelial cells. With passage in culture, 25% of bovine endothelial cells were cycling at confluence. Occasional strains of subcultured bovine endothelium arrested in G2 but would reenter G1 upon subculturing. Exposure to trypsin did not appear to contribute to this phenomenon. These studies have demonstrated variability in endothelial cell properties and growth characteristics during in vitro propagation. The existence of this variability must be considered when studying properties or responses of subcultured endothelium.