Nucleotide binding by actomyosin as a determinant of relaxation kinetics of rabbit phasic and tonic smooth muscle.
1. The apparent second-order rate constants (k+T) of ATP-induced cross-bridge detachment from rigor in the absence of Ca2+ were determined with laser flash photolysis of caged ATP (cATP) in alpha-toxin-permeabilized tonic, rabbit femoral artery and phasic, rabbit bladder smooth muscles. The potential effect of cATP binding to actomyosin (AM) on cross-bridge kinetics was examined by varying the initial concentration of cATP 2-fold. For a given [ATP] released from either 10 or 5 mM cATP, the kinetics of relaxation were not significantly different; the estimated dissociation constant for cATP binding to smooth muscle AM was 1-3 mM. 2. k+T was significantly higher ((9.5 +/- 1.3) x 10(4) M-1 s-1) in the phasic than in the tonic ((3.0 +/- 1.0) x 10(4) M-1 s-1) smooth muscle. 3. We conclude that the combination of the significantly lower (approximately 3 times) apparent second-order rate constant of MgATP association with the approximately 5 times higher affinity of cross-bridges for MgADP in tonic, than in phasic, smooth muscle is a major determinant of the slower kinetics of relaxation and, probably, shortening velocity of tonic smooth muscle.
Khromov
A S
AS
Department of Molecular Physiology, University of Virginia Health Sciences Center, Charlottesville 22908, USA.
Somlyo
A V
AV
Somlyo
A P
AP
D000205:Actomyosin / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D066298:In Vitro Techniques; D007700:Kinetics; D007834:Lasers; D008297:Male; D009126:Muscle Relaxation / Q000502:physiology*; D009130:Muscle, Smooth / Q000378:metabolism*; D009711:Nucleotides / Q000378:metabolism*; D010539:Permeability; D010766:Phosphorylation; D010782:Photolysis; D011485:Protein Binding / Q000502:physiology; D011817:Rabbits; D010738:Type C Phospholipases / Q000494:pharmacology
D009711:Nucleotides; D000255:Adenosine Triphosphate; D000205:Actomyosin; D010738:Type C Phospholipases