Nucleotide binding by actomyosin as a determinant of relaxation kinetics of rabbit phasic and tonic smooth muscle.

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Abstract
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.