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 act n-myos n-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.
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