Airway smooth muscle contractile, regulatory and cytoskeletal protein expression in health and disease.

The major part of research dealing with the biophysical and biochemical properties of airway smooth muscle is based on the assumption that the cells constituting the tissue are homogenous. For striated muscle this has been shown untenable. In recent years almost every property of vascular smooth muscle has been also demonstrated to be heterogeneous. This realization has been late in arriving on the airway smooth muscle research scene. Our own studies have shown that mechanical properties are, in quantitative terms, heterogeneously distributed down the airways and that contractility, for example, in extrapulmonary and intrapulmonary airways differs markedly. Another indication of heterogeneity is derived from studies of the biochemical properties of airway smooth muscle cells (ASMCs) in culture. Dramatic changes in phenotype expression were found with days in culture. Just after isolation from the tissue, the cells were of contractile type and contained mature isoforms of contractile, regulatory and cytoskeletal proteins. After the fourth day in culture the cellular phenotype changed such that contractile filaments diminished rapidly with smooth muscle isoforms being replaced by non-muscle isoforms. The cell assumed secretory or synthetic properties and commenced proliferating rapidly. It is possible that similar changes in phenotype could occur in vivo in cells undergoing hypertrophy or hyperplasia. Thus, a thickened medial layer of the type seen in the walls of airways from asthmatic airways is not necessarily one endowed with increased contractility and, in fact, the latter may be subnormal. Finally, using the so-called motility assay, we studied the velocity of translation of actin filaments by myosin molecules obtained from antigen-sensitized and control airway smooth muscle. We found no change in maximum velocity of actin translation. This was under conditions where the myosin light chain (MLC) was fully phosphorylated. However, in these tissues we found heterogeneity in myosin light chain kinase (MLCK) content which, we inferred, accounted for the difference in shortening velocity between control and sensitized muscle strips in vitro.