Does force depression resulting from shortening against series elasticity contribute to the activation dependence of optimum length?

Abstract

ABSTRACTThe optimum length for force generation (L0) increases as activation is reduced, challenging classic theories of muscle contraction. Although the activation dependence ofL0is seemingly consistent with length-dependent Ca2+sensitivity, this mechanism can’t explain the apparent force dependence ofL0, or the effect of series compliance on activation-related shifts inL0. We have tested a theory proposing that the activation dependence ofL0relates to force depression resulting from shortening against series elasticity. This theory predicts that significant series compliance would cause tetanicL0to be shorter than the length corresponding to optimal filament overlap, thereby increasing the activation dependence ofL0. We tested this prediction by determiningL0and maximum tetanic force (P0) with (L0_spring,P0_spring) and without added compliance in bullfrog semitendinosus muscles. The activation dependence ofL0was characterised with the addition of twitch and doublet contractions. Springs attached to muscles gave added fixed-end compliances of 11-39%, and this added compliance induced force depression for tetanic fixed-end contractions (P0_spring/P0< 1). We found strong, negative correlations between spring compliance and bothP0_spring(r2= 0.89-91) andL0_spring(r2= 0.60-63;P< 0.001), while the activation dependence ofL0was positively correlated to added compliance (r2= 0.45,P= 0.011). However, since the compliance-mediated reduction inL0was modest relative to the activation-related shift reported for the bullfrog plantaris muscle, additional factors must be considered. Our demonstration of force depression under novel conditions adds support to the involvement of a stress-induced inhibition of cross-bridge binding.