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 phosphory lated at its regulatory site by Ca2+/calmodulin-dependent protein kina se II resulting in an increase in the concentration of Ca2+/calmodulin required for half-maximal activation of the enzyme (K-CaM). We invest igated the physiological significance of MLCK phosphorylation during c ycles of contraction and relaxation in tonic (tracheal) and phasic (ut erine) smooth muscles. MLCK phosphorylation and dephosphorylation occu rred at rates sufficient to modulate the Ca2+ sensitivity of light cha in phosphorylation. In contractions of both smooth muscles (though usi ng different sources of activating Ca2+), increases in [Ca2+], precede d light chain phosphorylation; but, the rate of increase in light chai n phosphorylation was significantly greater than the rate of increase in [Ca2+](i). The onset of MLCK phosphorylation with the resultant inc rease in K-CaM coincided with the diminished rate of light chain phosp horylation. During spontaneous contractions of uterine smooth muscle, the Ca2+ transient was characterized by an initial rapid increase, a s ustained plateau, and rapid decline. During the sustained phase of the Ca2+ transient, MLCK phosphorylation increased and coincided with dep hosphorylation of light chain and relaxation. These results indicate t hat 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.