Smooth muscle heterogeneity: Does the striated muscle model apply?

Smooth muscle cells are the major component of most smooth muscle tissue. Although the function of these tissues is known to differ, it has been generally assumed that the smooth muscle cells that make up these tissues are all the same. In striated muscle, individual fibers composed of unique contractile and regulatory proteins have unique mechanical properties. Smooth muscles also contain unique proteins. The hexamer myosin has multiple isoforms of its heavy and light chain subunits. Some of these differences result from alternate splicing of a single gene, while others result from multiple genes. At the tissue level, the presence of these isoforms is developmentally regulated and varies between tissues and species. There are numerous contradictory reports in the literature regarding possible unique regulation and function for the multiple myosin heavy and light chain isoforms. By extending molecular and mechanical methods beyond their normally used ranges, we have begun a series of experiments at the single smooth muscle cell level. Preliminary results show extreme heterogeneity of myosin isoforms between smooth muscle cells. Zn addition, preliminary results suggest that the ratio of the SM1 and SM2 MHC tail isoforms do not correlate with unloaded shortening velocity. However, there is a good correlation between the amount of SM2 MHC isoform present in the cell and the minimum length to which the cell can shorten. Thus, cells with a greater SM2 content appear to shorten more than cells with a lesser SM2 content. As with striated muscle, single-cell experiments may be required to determine the unique function of protein isoforms in smooth muscle tissues.