The basis of tonic vs. phasic contractile phenotypes of visceral smooth muscles is poorly understood. We used gel electrophoresis and quantitative scanning densitometry to measure the content and isoform composition of contractile proteins in opossum lower esophageal sphincter (LES), to represent tonic muscle, and circular muscle of the esophageal body (EB), to represent phasic smooth muscle. The amount of protein in these two types of muscles is similar: ∼27 mg/g of frozen tissue. There is no difference in the relative proportion of myosin, actin, calponin, and tropomyosin in the two muscle types. However, the EB contains ∼2.4-times more caldesmon than the LES. The relative ratios of α- to γ-contractile isoforms of actin are 0.9 in the LES and 0.3 in EB. The ratio between acidic (LC17a) and basic (LC17b) isoforms of the 17-kDa essential light chain of myosin is 0.7:1 in the LES, compared with 2.7:1 in the EB. There is no significant difference in the ratios of smooth muscle myosin SM1 and SM2 isoforms in the two muscle types. The level of the myosin heavy chain isoform, which contains the seven-amino acid insert in the myosin head, is about threefold higher in the EB compared with LES. In conclusion, the esophageal phasic muscle in contrast to the tonic LES contains proportionally more caldesmon, LC17a, and seven-amino acid-inserted myosin and proportionally less α-actin. These differences may provide a basis for functional differences between tonic and phasic smooth muscles.

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The contribution of electrostatic interactions to the effects of chicken gizzard calponin on the kinetics of actin polymerization and the bundling of F-actin were characterized by a combination of fluorescence, light-scattering, co-sedimentation, and electron-microscopic methods. Stoichiometric amounts of calponin accelerate actin polymerization in low-ionic-strength solutions, but this effect is diminished at [KCI] = 150 mM. At low ionic strengths, micromolar concentrations of calponin induce the formation of large bundles of actin filaments, and lower concentrations of calponin quench the fluorescence of pyrene-labeled F-actin. The latter effect is related to binding of calponin to F-actin rather than to bundling of the filaments. The concentration of calponin required to bundle a fixed concentration of actin filaments increases with increasing ionic strength, as the average diameter of the bundles decreases. Millimolar concentrations of ATP, GTP or ITP are equally efficient at dispersing actin bundles to single filaments or smaller aggregates, even though a significant fraction of calponin remains bound to F-actin. Our findings show that the binding of calponin to actin is determined at least in part by electrostatic interactions, and that the polycationic nature of calponin is primarily responsible for the formation of F-actin bundles via its ability to reduce the electrostatic repulsion between the negatively charged actin filaments.