Photolytic release of MgADP reduces rigor force in smooth muscle.
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.
Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville 22906, USA.
A S
AS
Khromov
A P
AP
Somlyo
A V
AV
Somlyo
D000244:Adenosine Diphosphate / Q000737:chemistry* / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D004594:Electrophysiology; D007700:Kinetics; D008027:Light; D008274:Magnesium / Q000378:metabolism; D009130:Muscle, Smooth / Q000737:chemistry*; D010766:Phosphorylation; D010782:Photolysis; D011817:Rabbits; D013314:Stress, Mechanical; D013997:Time Factors
D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D008274:Magnesium