Ultrastructural differences between pulmonary arteriolar muscularization induced by hypoxia and monocrotaline.

A group of Wistar rats was treated with two subcutaneous injections of monocrotaline, at 4 and 6 weeks of age. They were then killed 1 month after the initial injection. A second group of rats was born and reared in hypobaric hypoxia for 1 month before being killed. Both groups had age-matched controls. The ultrastructure of pulmonary arterioles from all groups was studied, and quantitative measurements were made of the volume densities of organelles within the cytoplasm of arteriolar smooth muscle cells. The pulmonary arterioles of rats treated with monocrotaline contained immature smooth muscle cells with coarse peripheral myofilaments and were bounded by thin indistinct elastic laminae. In contrast, the arteriolar smooth muscle cells of hypoxic rats were mature with fine myofilaments and bounded by electron dense laminae. When compared with both their respective controls and the alternative test group, the muscle cells from rats treated with monocrotaline had significantly lower volume densities of dense bodies, and the hypoxic muscle cells had significantly higher densities of mitochondria. The pulmonary arteriolar muscularization in rats would appear to be a nonuniform process producing smooth muscle cells with differing cytoplasmic features that suggest differing cellular functions.