Smooth Muscle Myosin in Precursor and Mature Smooth Muscle Cells in Normal Pulmonary Arteries and the Effect of Hypoxia

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Abstract
Exposure to hypoxia increases pulmonary arterial muscularity. In the intra-acinar arteries new muscle appears in the normally nonmuscular regions and in the preacinar arteries, medial thickness increases. By immunofluorescence techniques the myosin content of the pulmonary arterial walls at 2 levels of the circulation (intra-acinar and preacinar) were studied in control rats and those exposed to hypobaric hypoxia of 380 torr for 3, 7, 10 or 14 days. The precursor smooth muscle cells, pericytes and intermediate cells normally present in the nonmuscular regions of the intra-acinar arteries, contain smooth muscle myosin. With exposure to hypoxia, smooth muscle myosin in the intra-acinar arteries increases to day 10, both in area of staining and fluorescent intensity. This is in contrast to the preacinar arteries where only the area of myosin increases. Antihuman platelet (nonmuscle) myosin shows a little faint staining in both control and hypoxic animals. Adaptation to hypoxia by the intra-acinar precursor and preacinar mature smooth muscle cells is different and suggests that the functions subserved by the myosin filaments at each of the 2 levels differs.