Characterization of molecular determinants of smooth muscle cell heterogeneity

Broad diversity in contractile and pharmacological properties of different smooth muscles is well recognized. Differences in proliferative capacity, electrophysiology, phenotypic marker protein content, matrix synthesis, and expression of cell-specific transcription factors between individual smooth muscle cells (SMCs) have also been reported. Precise developmental and molecular mechanisms underlying heterogeneity are not known; however, their elucidation is the thrust of much current research involving vascular smooth muscle. In contrast, limited studies of heterogeneity of subtypes of airway SMCs are available. In this report, we review molecular aspects of differentiation that may determine phenotypic heterogeneity of SMCs and also present data from our own studies characterizing heterogeneity in the proliferative capacity and marker protein content of airway SMCs. Using flow cytometry, cell cycle transit was monitored for cultured canine tracheal SMCs. Only 70% of arrested cells responded and traversed the cell cycle when stimulated with 10% fetal bovine serum. Furthermore, heparin inhibited 40% of serum-responsive cells from entering the cell cycle, suggesting that both serum- and heparin-sensitive and -insensitive airway SMCs exist. Flow cytometric analysis of contractile protein and DNA content in freshly dissociated canine tracheal SMCs revealed that diploid (approximately 87%) and tetraploid (approximately 13%) populations exist. Clusters of SMCs having "high" or "low" smooth muscle myosin or alpha-actin content were also discerned, indicating that distinct subtypes of SMCs exist in mature airways. Diversity of SMCs may be a critical factor determining specific responses of smooth muscles to a number of physiological or pathophysiological stimuli that may include, for example, inflammatory mediators in asthmatic airways.