Series-to-parallel transition in the filament lattice of airway smooth muscle

Force-velocity curves measured at different times during tetani of sheep tr achealis muscle were analyzed to assess whether velocity slowing could be e xplained by thick-filament lengthening. Such lengthening increases force by placing more cross bridges in parallel on longer filaments and decreases v elocity by reducing the number of filaments spanning muscle length. From 2 s after the onset of stimulation, when force had achieved 42% of it final v alue, to 28 s, when force had been at its tetanic plateau for similar to 15 s, velocity decreases were exactly matched by force increases when force w as adjusted for changes in activation, as assessed from the maximum power v alue in the force-velocity curves. A twofold change in velocity could be qu antitatively explained by a series-to-parallel change in the filament latti ce without any need to postulate a change in cross-bridge cycling rate.