Colonic smooth muscle: Contractile proteins, shortening velocity, and regulation

Smooth muscle in megacolon was studied in the lethal spotted mouse and its normal sibling. In megacolon, structural remodeling and a very large increase in total protein content are associated with some changes in the contractile protein isoform composition. 1) There is a higher actin concentration in megacolon, primarily caused by a larger proportion of gamma-isoforms. 2) There was no difference in myosin concentration or in SM1/SM2 heavy chains in megacolon and normal muscle contractile proteins. 3) Only LC17a essential Light chain is present in both normal and megacolon. 4) The 25- to 50-kDa 5'-insert occurs in 15-20% of the myosin in normal colon, compared with 5- to 10-fold lower amounts in megacolon. In permeabilized muscles there was no significant difference in unloaded shortening velocity (V-o) with maximal thiophosphorylation of the 20-kDa Light chains, nor was there significant difference in the force vs. Ca2+ and force vs. myosin light chain phosphorylation relationships. At approximate to 60% myosin Light chain phosphorylation after Ca2+ activation, V-o of megacolon was approximately two times faster than V-o of normal muscle. These cellular changes largely account for the higher propulsive velocity of the colon in situ. The distribution of myosin and actin isoforms and the lack of a simple relationship between myosin light chain phosphorylation and V-o point to the operation of additional regulatory processes.