Photolytic release of MgADP reduces rigor force in smooth muscle |
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Authors: | Khromov A S Somlyo A P Somlyo A V |
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Affiliation: | Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville 22906, USA. |
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Abstract: | Photolytic release of MgADP (25-300 microM) from caged ADP in permeabilized tonic (rabbit femoral artery-Rfa) and phasic (rabbit bladder-Rbl) smooth muscle in high-tension rigor state, in the absence of Ca(2+), caused an exponential decline (approximately 1.5% in Rfa and approximately 6% in Rbl) of rigor force, with the rate proportional to the liberated [MgADP]. The apparent second-order rate constant of MgADP binding was estimated as approximately 1.0 x 10(6) M(-1) s(-1) for both smooth muscles. In control experiments, designed to test the specificity of MgADP, photolysis of caged ADP in the absence of Mg(2+) did not decrease rigor force in either smooth muscle, but rigor force decreased after photolytic release of Mg(2+) in the presence of ADP. The effects of photolysis of caged ADP were similar in smooth muscles containing thiophosphorylated or non-phosphorylated regulatory myosin light chains. Stretching or releasing (within range of 0.1-1.2% of initial Ca(2+)-activated force) did not affect the rate or relative amplitude of the force decrease. The effect of additions of MgADP to rigor cross-bridges could result from rotation of the lever arm of smooth muscle myosin, but this need not imply that ADP-release is a significant force-producing step of the physiological cross-bridge cycle. |
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