ROLE OF MYOSIN LIGHT-CHAIN PHOSPHORYLATION IN GUINEA-PIG GALLBLADDER SMOOTH-MUSCLE CONTRACTION

In acetylcholine (ACh)-stimulated gallbladder smooth muscle, we have p reviously shown that phosphorylation of the 20,000-Da myosin light cha ins is necessary for the initiation of contraction, that myosin is sta bly phosphorylated at steady state, and that dephosphorylation of cros s bridges is not necessary for the slowing of cross-bridge cycling rat es during the period of steady-state isometric stress. The present stu dies were undertaken to determine whether 1) K+ (60 or 80 mM) or chole cystokinin (CCK, 10(-8) M) stimulation is accompanied by changes in my osin light-chain phosphorylation in gallbladder smooth muscle and 2) d ephosphorylated noncycling cross bridges exist in K+- or CCK-stimulate d gallbladder smooth muscle. Isometric stress, isotonic shortening vel ocity, and myosin light-chain phosphorylation were determined during c ontraction with K+ or CCK. Steady-state isometric stress was reached w ithin 2.5 min of stimulation with K+ or CCK and was maintained for the duration of the stimulation. Stimulation with K+ or CCK was associate d with rapid increases in myosin light-chain phosphorylation and maint enance of myosin light-chain phosphorylation during the stimulation. I n contrast, isotonic shortening velocity was maximal at 1 min of stimu lation with either K+ or CCK and then declined significantly to values that were only 26-32% of the peak velocity. These data, along with da ta from previous experiments with ACh, suggest that myosin light-chain phosphorylation is essential in the initiation of contraction in gall bladder smooth muscle, regardless of the source of Ca2+ or of the cont ractile agonist. Because myosin is stably phosphorylated during the pe riod of steady-state isometric stress, these data also suggest that cr oss-bridge dephosphorylation is not necessary for the slowing of cross bridge cycling rates in gallbladder smooth muscle. Thus, although myos in light-chain phosphorylation is essential in the initiation of contr action of gallbladder smooth muscle, factors other than myosin light-c hain phosphorylation are involved in the regulation of cross-bridge cy cling rates at steady state.