Structural and biochemical analysis of skinned smooth muscle preparations

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Abstract
This paper describes a biochemical and immunocytochemical analysis of smooth muscle strips that were chemically skinned and subjected to contraction and relaxation cycles according to procedures commonly employed in current skinned smooth muscle work. The fate of four major proteins, myosin, filamin, caldesmon and actin, was followed with respect to the proportionate loss of these proteins to the bathing medium as well as to their structural redistribution within the cells in the muscle strips. Large losses (of the order of 50%) of both myosin and filamin occurred at the skinning step, using either Triton X-100 or Saponin as the detergent; losses of actin were up to 30% with Triton X-100 and around 15% with Saponin. Losses of caldesmon were difficult to assess due to the rapid degradation of this protein in the bathing medium. Subsequent cycles of contraction and relaxation resulted in accumulated loss, notably of myosin and filamin, so that after the third contraction as little as 20% and 40% respectively of the original complement of these proteins remained in the muscle strips. These changes in protein composition were accompanied by a drastic redistribution of the proteins in the muscle cells. Most marked were the changes seen with myosin, significant amounts of this protein being already found in the connective tissue space after the first relaxation. These findings point to the need for a careful reappraisal of the conditions currently used in skinned smooth muscle research.