title: Time-resolved measurements of phosphate release by cycling cross-bridges in portal vein smooth muscle.
abstract: The rate of release of inorganic phosphate (Pi) from cycling cross-bridges in rabbit portal-anterior mesenteric vein smooth muscle was determined by following the fluorescence of the Pi-reporter, MDCC-PBP (Brune, M., J. L. Hunter, S. A. Howell, S. R. Martin, T. L. Hazlett, J. E. T. Corrie, and M. R. Webb. 1998. Biochemistry. 37:10370-10380). Cross-bridge cycling was initiated by photolytic release of ATP from caged-ATP in Triton-permeabilized smooth muscles in rigor. When the regulatory myosin light chains (MLC20) had been thiophosphorylated, the rate of Pi release was biphasic with an initial rate of 80 microM s-1 and amplitude 108 microM, decreasing to 13.7 microM s-1. These rates correspond to fast and slow turnovers of 1.8 s-1 and 0.3 s-1, assuming 84% thiophosphorylation of 52 microM myosin heads. Activation by Ca2+-dependent phosphorylation subsequent to ATP release resulted in slower Pi release, paralleling the rate of contraction that was also slower than after thiophosphorylation, and was also biphasic: 51 microM s-1 and 13.2 microM s-1. These rates suggest that the activity of myosin light chain kinase and phosphatase ("pseudo-ATPase") contributes <20% of the ATP usage during cross-bridge cycling. The extracellular "ecto-nucleotidase" activity was reduced eightfold by permeabilization, conditions in which the ecto-ADPase was 17% of the ecto-ATPase. Nevertheless, the remaining ecto-ATPase activity reduced the precision of the estimate of cross-bridge ATPase. We conclude that the transition from fast to slow ATPase rates reflects the properties and forces directly acting on cross-bridges, rather than the result of a time-dependent decrease in activation (MLC20 phosphorylation) occurring in intact smooth muscle. The mechanisms of slowing may include the effect of positive strain on cross-bridges, inhibition of the cycling rate by high affinity Mg-ADP binding, and associated state hydrolysis.
authors.affiliation: National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, United Kingdom.
authors.forename: Z H
authors.initials: ZH
authors.lastname: He
authors.forename: M A
authors.initials: MA
authors.lastname: Ferenczi
authors.forename: M
authors.initials: M
authors.lastname: Brune
authors.forename: D R
authors.initials: DR
authors.lastname: Trentham
authors.forename: M R
authors.initials: MR
authors.lastname: Webb
authors.forename: A P
authors.initials: AP
authors.lastname: Somlyo
authors.forename: A V
authors.initials: AV
authors.lastname: Somlyo
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000031:analogs & derivatives / Q000378:metabolism; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D066298:In Vitro Techniques; D007700:Kinetics; D009119:Muscle Contraction / Q000502:physiology; D009131:Muscle, Smooth, Vascular / Q000378:metabolism* / Q000502:physiology; D009218:Myosins / Q000378:metabolism; D010539:Permeability; D010710:Phosphates / Q000378:metabolism*; D010766:Phosphorylation; D010782:Photolysis; D011169:Portal Vein / Q000378:metabolism / Q000502:physiology; D011817:Rabbits
chemical_list: D010710:Phosphates; C029273:P(3)-1-(2-nitro)phenylethyladenosine 5'-triphosphate; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D009218:Myosins