Effects of modulators of myosin light-chain kinase activity in single smooth muscle cells.

Phosphorylation of myosin light chains by a calmodulin-myosin light-chain kinase (MLCK) pathway is considered to be responsible for coupling increased calcium concentration with contraction in smooth muscle. This simple view has, however, recently been questioned. To test this hypothesis directly, we microinjected individual smooth muscle cells with modulators of the MLCK pathway while measuring contraction and calcium-ion concentration. Injection of a constitutively active proteolyzed form of MLCK causes contraction but no change in calcium concentration. By contrast, injection of peptide inhibitors of MLCK blocks contraction in response to K+ depolarization, despite the fact that the change in calcium concentration in response to stimulation was enhanced over controls. These results provide a direct demonstration at the level of a single cell that activation of the calmodulin-MLCK pathway is both necessary and sufficient to trigger contraction of smooth muscle.