Effect of muscle length on isometric stress and myosin light chain phosphorylation in gallbladder smooth muscle

These experiments were designed to characterize the effect of muscle length on isometric stress, sensitivity to stimulation, and phosphorylation of the 20,000-Da myosin light chains in guinea pig gallbladder smooth muscle. Basal, active, and total isometric stress were determined in acetylcholine- or K⁺-treated (10^-4 M ACh, 80 mM KCl) muscle strips at 0.6-1.3 times the optimal muscle length (L(o)) for isometric stress development. The effect of muscle length on the sensitivity to ACh and K⁺ was determined in cumulative dose-response experiments (10^-8 to 10^-4 M ACh, 10-80 mM KCl) at 0.7, 1.0, and 1.3 L(o). The effect of muscle length on myosin light chain phosphorylation was determined in ACh- or K⁺-treated (10^-4 M ACh, 80 mM KCl) muscle strips at 0.7, 1.0, and 1.3 L(o). In gallbladder smooth muscle, 1) active isometric stresses at 0.7 and 1.3 L(o) were less than active isometric stress at 1.0 L(o); 2) the sensitivity of developed stress was similar at 1.0 and 1.3 L(o) but decreased at 0.7 L(o); 3) the decline in isometric stress and sensitivity at 0.7 L(o) was associated with reduced levels of phosphorylated myosin light chain; and 4) the decline in isometric stress at 1.3 L(o) was not associated with reduced amounts of phosphorylated myosin light chain. These results suggest that the decline in active stress and sensitivity at short muscle lengths (L < L(o)) in gallbladder smooth muscle is due, at least in part, to decreases in the activation of the myofilaments. The decline in active isometric stress at long muscle lengths (L > L(o)) is not due to changes in myofilament activation.