Microvessel precursor smooth muscle cells express head-inserted smooth muscle myosin heavy chain (SM-B) isoform in hyperoxic pulmonary hypertension

The present study analyzes smooth muscle myosin heavy chain (SMMHC) expression as lung microvascular precursor smooth muscle cells (PSMCs), cells derived from fibroblasts and intermediate cells (immature SMCs), acquire a smooth muscle phenotype in an in vivo model of pulmonary hypertension (PH). Because of the unique contractile properties of the SMMHC isoform SM-B, we analyzed its expression in the microvessels (<100 mu m diameter) and in larger vessels (100-700 mu m) quantitatualy by the labeled [strept]avidin-biotin technique (day 1-28), and related this to cell phenotype by transmission microscopy and protein A-gold labeling (at day 28). Airway SMCs of the normal and hypertensive lung uniformly expressed SM-B whereas vascular SMC expression was heterogeneous. Thus, in some large arteries (and veins) SMCs contained cells expressing SM-B while in others all the cells were immunonegative. Microvascular cells expressing SM-B (arteries and veins) were rare in normal lung and numerous in PH, increasing as wall muscle developed in smaller segments with time. As in large vessels, some microvessels had immunopositive cells and others only negative ones. Ultrastructural analysis confirmed that the SMCs of bronchial vessels, and the septal SMCs adjoining alveolar ducts, contained dense filament arrays decorated with SM-B. While the PSMC processes of the normal lung contained sparse filaments decorated with SM-B, these cells expressed dense filament arrays in PH. Fibroblasts migrating to align around the microvessels also expressed SM-B but in the absence of a filament network. For the first time, we demonstrate in vivo that newly developed microvascular PSMCs express the SMMHC SM-B isoform in PH.