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 8-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 mns. 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 (similar to 3 times) apparent second-order rate constant of MgATP association with the similar to 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.