PLASTICITY IN SMOOTH-MUSCLE, A HYPOTHESIS

The controversial finding that the thick filaments of smooth muscle ca n be evanescent leads to the hypothesis that the large functional rang e of this muscle is accommodated by plastic rearrangements that place more thick filaments in series at longer lengths. Our preliminary find ing that the shortening velocity and compliance of dog tracheal muscle were strongly dependent on adapted muscle length, while force was muc h less length dependent, supports this hypothesis (V.R. Pratusevich, C .Y. Seow, and L.E. Ford. Biophys. J. 66: A139, 1994). The hypothesis l eads to two further corollaries. The first is that the lengthening of the thick filaments that must accompany their reformation will cause a series to parallel transition: fewer long filaments span the muscle l ength, but the longer filaments have more cross bridges acting in para llel. The second is that there is more than one activating mechanism i n smooth muscle. It is known that myosin light chain phosphorylation a ctivates the actomyosin ATPase, but this same phosphorylation also cau ses a structural change that facilitates filament formation. The consi deration that the unaggregated, phosphorylated myosin must be prevente d from competing with myosin in thick filaments and hydrolyzing ATP su ggests that there must be a second mechanism that must allow the thin filaments to interact selectively with filamentous myosin. This need f or a second activating mechanism may explain the presence of tropomyos in, calponin, and caldesmon on thin filaments. Although the two coroll aries follow from the initial hypothesis, it should be emphasized that the three are not mutually dependent, and that the proof or disproof of any one of them would not prove or disprove the others.