A calponin peptide enhances Ca2+ sensitivity of smooth muscle contraction without affecting myosin light chain phosphorylation.

In permeabilized smooth muscle, exogenously applied calponin binds to myofibrils and reduces Ca(2+)-activated tension (Itoh, T., Suzuki, S., Suzuki, A., Nakamura, F., Naka, M., and Tanaka, T. (1994) Pflügers Arch. Eur. J. Physiol. 427, 301-308). A calponin peptide (calponin Phe173-Arg185), which inhibits the binding of calponin to actin, blocks the action of calponin and enhances the contraction induced by submaximal Ca2+ in permeabilized vascular smooth muscle. Unlike calmodulin, this peptide enhances the Ca(2+)-induced contraction without a corresponding increase in the level of myosin light chain phosphorylation. These results suggest that calponin decreases the sensitivity of smooth muscle to Ca2+ at a given level of myosin light chain phosphorylation.