Myosin isoforms and functional diversity in vertebrate smooth muscle.

The expression of fast and slow myosin isoforms in individual cells is associated with differences in shortening velocities and power output in fully differentiated vertebrate striated muscle. This paradigm in which shortening velocity is determined by the myosin isoform (and load) is inappropriate for smooth muscle. Smooth muscle tissues express multiple myosin heavy and light chain isoforms, and it is not currently possible to separate and identify chemically distinct native myosin hexamers (i.e., isoforms). It is not known if different isoforms are localized in subpopulations of cells or in specific cellular domains nor whether they combine preferentially to form a small number of native myosin hexamer isoforms. Potentially, thick filaments are aggregates of many different combinations of heavy and light chain isoforms that may or may not exhibit different kinetics. Shortening velocities in smooth muscle are regulated by Ca(2+)-dependent crossbridge phosphorylation of the myosin regulatory light chains. Much of the observed diversity in power output in smooth muscle may be attributed to regulatory mechanisms modulating crossbridge cycling rates rather than contractile protein isoform expression.