Time-Dependent Uncoupling between Myosin Phosphorylation and Contractile Force Induced by Ca²⁺-Depletion in Smooth Muscle

Phosphorylation of the 20,000-Da myosin light chain is an important regulatory mechanism of smooth muscle contraction. In this study, we investigated the uncoupling between carbachol-activated myosin phosphorylation and isometric stress in Ca²⁺-depleted bovine tracheal smooth muscle at low [Ca²⁺]. In control tissues, lowering extracellular [CaCl₂] from 1.6 to 0.1 mM had insignificant effects on carbachol-activated steady-state isometric stress and myosin phosphorylation. In contrast, in Ca²⁺-depleted tissues, lowering [CaCl₂]₀ from 1.6 to 0.1 mM significantly reduced steady-state isometric stress without significantly changing steady-state myosin phosphorylation, thus uncoupling contractile force from myosin phosphorylation. Time-course data of myosin phosphorylation and isometric stress revealed that isometric stress and myosin phosphorylation were coupled at the beginning of contractions, but then gradually became uncoupled at steady state. We attempted to stabilize the cell membrane and contractile filaments using high [Mg²⁺]. However, 25 mM [MgSO₄] further reduced steady-state isometric stress development at 0.1 mM [CaCL₂]₀ without significantly changing steady-state myosin phosphorylation. These results indicated that Ca²⁺-depletion induced a time-dependent cellular process which gradually uncouples contractile force from myosin phosphorylation. Furthermore, steady-state isometric stress appeared to be not limited by myosin phosphorylation, but sensitive to the [Ca²⁺]/[Mg²⁺] ratio near the cell membrane.