Photolytic Release of MgADP Reduces Rigor Force in Smooth Muscle

AbstractPhotolytic release of MgADP (25–300μM) 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 Ca2+, caused an exponential decline (∼1.5% in Rfa and ∼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 ∼1.0×106M−1s−1 for both smooth muscles. In control experiments, designed to test the specificity of MgADP, photolysis of caged ADP in the absence of Mg2+ did not decrease rigor force in either smooth muscle, but rigor force decreased after photolytic release of Mg2+ 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 Ca2+-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