Activation properties of myosin light chain kinase during contraction/relaxation cycles of tonic and phasic smooth muscles

In intact smooth muscle, myosin light chain kinase (MLCK) is phosphorylated at its regulatory site by Ca²⁺/calmodulin-dependent protein kinase II resulting in an increase in the concentration of Ca²⁺/calmodulin required for half-maximal activation of the enzyme (K(CaM)). We investigated the physiological significance of MLCK phosphorylation during cycles of contraction and relaxation in tonic (tracheal) and phasic (uterine) smooth muscles. MLCK phosphorylation and dephosphorylation occurred at rates sufficient to modulate the Ca²⁺ sensitivity of light chain phosphorylation. In contractions of both smooth muscles (though using different sources of activating Ca²⁺), increases in [Ca²⁺](i) preceded light chain phosphorylation; but, the rate of increase in light chain phosphorylation was significantly greater than the rate of increase in [Ca²⁺](i). The onset of MLCK phosphorylation with the resultant increase in K(CaM) coincided with the diminished rate of light chain phosphorylation. During spontaneous contractions of uterine smooth muscle, the Ca²⁺ transient was characterized by an initial rapid increase, a sustained plateau, and rapid decline. During the sustained phase of the Ca²⁺ transient, MLCK phosphorylation increased and coincided with dephosphorylation of light chain and relaxation. These results indicate that MLCK is sensitive to small increases in intracellular Ca²⁺ during the initiation of contraction and that the enzyme subsequently becomes desensitized to Ca²⁺/calmodulin, thereby limiting the extent of light chain phosphorylation.