Biophys J, September 2000, p. 1511-1523, Vol. 79, No. 3

The Smooth Muscle Cross-bridge Cycle Studied Using Sinusoidal Length Perturbations

Albert Y. Rhee^* and Frank V. Brozovich^*

 ^*Department of Physiology and Biophysics and  Department of Medicine, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106-4970 USA

The mechanical characteristics of smooth muscle can be broadly defined as either phasic, or fast contracting, and tonic, or slow contracting (Somlyo and Somlyo, 1968, Pharmacol. Rev. 20:197-272). To determine if differences in the cross-bridge cycle and/or distribution of the cross-bridge states could contribute to differences in the mechanical properties of smooth muscle, we determined force and stiffness as a function of frequency in Triton-permeabilized strips of rabbit portal vein (phasic) and aorta (tonic). Permeabilized muscle strips were mounted between a piezoelectric length driver and a piezoresistive force transducer. Muscle length was oscillated from 1 to 100 Hz, and the stiffness was determined as a function of frequency from the resulting force response. During calcium activation (pCa 4, 5 mM MgATP), force and stiffness increased to steady-state levels consistent with the attachment of actively cycling cross-bridges. In smooth muscle, because the cross-bridge states involved in force production have yet to be elucidated, the effects of elevation of inorganic phosphate (P_i) and MgADP on steady-state force and stiffness were examined. When portal vein strips were transferred from activating solution (pCa 4, 5 mM MgATP) to activating solution with 12 mM P_i, force and stiffness decreased proportionally, suggesting that cross-bridge attachment is associated with P_i release. For the aorta, elevating P_i decreased force more than stiffness, suggesting the existence of an attached, low-force actin-myosin-ADP- P_i state. When portal vein strips were transferred from activating solution (pCa 4, 5 mM MgATP) to activating solution with 5 mM MgADP, force remained relatively constant, while stiffness decreased ~50%. For the aorta, elevating MgADP decreased force and stiffness proportionally, suggesting for tonic smooth muscle that a significant portion of force production is associated with ADP release. These data suggest that in the portal vein, force is produced either concurrently with or after P_i release but before MgADP release, whereas in aorta, MgADP release is associated with a portion of the cross-bridge powerstroke. These differences in cross-bridge properties could contribute to the mechanical differences in properties of phasic and tonic smooth muscle.

Biophys J, September 2000, p. 1511-1523, Vol. 79, No. 3
© 2000 by the Biophysical Society   0006-3495/00/09/1511/13  $2.00

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