Can oligomeric myosin participate in smooth muscle contraction?

An experimental model was constructed to examine the possible fact that myosin oligomers take a part in smooth muscle contraction. Single fibres prepared from glycerinated rabbit skeletal muscle were emptied of myosin and then irrigated with purified chicken gizzard myosin. This was a good preparation in which to see the structural change of gizzard myosin and its interaction with actin filaments under the electron microscope. The structure of gizzard myosin was thoroughly changed by varying the concentration of free Mg2+ in the bathing solution. Myosin thick filaments were formed at the position of the A-band longitudinally bridging adjacent I-segments at high concentration of free Mg2+, while most of them disappeared from the A-band and some localized in the I-segment at low concentration of free Mg2+. When this preparation was induced to contract isometrically, it showed quite different sarcomere patterns at high and low concentrations of free Mg2+. Tensions developed by this preparation were of the same magnitude at high and low concentrations of free Mg2+, which were approximately 5% of that developed by glycerinated skeletal muscle fibres. The shortening of the prepared fibre in a contracting medium was almost the same at varying concentrations of free Mg2+. The relation between the structural organization and the contracting nature of the prepared fibre as the concentration of free Mg2+ was varied is discussed in this paper with respect to the significance of oligomeric myosin in smooth muscle contraction.