Time-Resolved Measurements of Phosphate Release by Cycling Cross-Bridges in Portal Vein Smooth Muscle

Abstract

The rate of release of inorganic phosphate (P i) from cycling cross-bridges in rabbit portal-anterior mesenteric vein smooth muscle was determined by following the fluorescence of the P i-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 (MLC 20) had been thiophosphorylated, the rate of P i release was biphasic with an initial rate of 80 μM s −1 and amplitude 108 μM, decreasing to 13.7 μM 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 μM myosin heads. Activation by Ca 2+-dependent phosphorylation subsequent to ATP release resulted in slower P i release, paralleling the rate of contraction that was also slower than after thiophosphorylation, and was also biphasic: 51 μM s −1 and 13.2 μM 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 (MLC 20 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.