CALPONIN AND SMOOTH-MUSCLE REGULATION

Calponin has been implicated in the regulation of smooth muscle contra ction as a result of its ability to inhibit the actin-activated Mg ATP ase of smooth muscle myosin. This inhibitory effect is abolished by ph osphorylation of calponin by Ca2+/calmodulin-dependent protein kinase II or protein kinase C, and restored following dephosphorylation by a type 2A protein phosphatase. Confocal immunofluorescent images of isol ated smooth muscle cells colabeled with antibodies to calponin and act in or to calponin and tropomyosin indicate that calponin is present on thin filaments throughout the cell cytoplasm. Both calponin phosphory lation and myosin light chain phosphorylation increased in intact smoo th muscle tissue strips when they contracted in response to carbachol or the phosphatase inhibitor okadaic acid. These results support the h ypothesis that calponin phosphorylation - dephosphorylation plays a ro le in regulating smooth muscle contraction.