Aequorin luminescence, myosin phosphorylation, and active stress in tracheal smooth muscle

During muscarinic activation of canine tracheal smooth muscle with carbachol, myosin phosphorylation is significantly more sensitive than stress to the external Ca2+ concentration ([Ca2+]o) [W. T. Gerthoffer. Am. J. Physiol. 250 (Cell Physiol. 19): C597-C604, 1986]. To determine whether the intracellular Ca2+ concentration ([Ca2+]i) correlated more closely with changes in phosphorylation or force, we measured isometric force and light emitted by the luminescent intracellular Ca2+ indicator aequorin as [Ca2+]o was increased in the presence of 1 microM carbachol or 60 mM K+. Myosin phosphorylation was measured using an immunoblot assay in a second set of muscle strips treated identically. Stimulation with carbachol increased aequorin luminescence slightly in strips incubated in Ca2+-free solution. Active stress and aequorin luminescence subsequently increased in parallel as [Ca2+]o was increased. Myosin phosphorylation at 0.05 mM [Ca2+]o (0.30 +/- 0.04 mol Pi/mol light chain) was significantly higher than phosphorylation in Ca2+-free solution with no carbachol (0.12 +/- 0.048 mol Pi/mol light chain) and increased to a maximum of 0.56 +/- 0.03 mol Pi/mol light chain at 1.6 mM [Ca2+]o. In contrast, active stress and aequorin luminescence remained low at low [Ca2+]o and reached a maximum at 2.4 mM [Ca2+]o. Stimulation with carbachol produced greater increases in myosin phosphorylation and active stress for a given change in aequorin luminescence than did K+ depolarization. Stimulation with carbachol also produced a different phosphorylation-stress relationship than did K+ depolarization. These observations are consistent with the possibility that carbachol induces increases in the Ca2+ sensitivity of contractile proteins in tracheal smooth muscle.