Increasing phosphate concentration from 0 to 16 mM quadrupled the stretch activation to calcium-activated force ratio from 11% to 43% in mouse soleus slow-twitch fibers.
Stretch activation is a significant force modulator in slow-twitch mammalian skeletal muscle fibers, particularly under conditions of increased phosphate concentration such as prolonged muscle use.
Stretch activation (SA) is a delayed increase in force following a rapid muscle length increase. SA is best known for its role in asynchronous insect flight muscle, where it has replaced calcium’s typical role of modulating muscle force levels during a contraction cycle. SA also occurs in mammalian skeletal muscle but has previously been thought to be too low in magnitude, relative to calcium-activated (CA) force, to be a significant contributor to force generation during locomotion. To test this supposition, we compared SA and CA force at different P i concentrations (0–16 mM) in skinned mouse soleus (slow-twitch) and extensor digitorum longus (EDL; fast-twitch) muscle fibers. CA isometric force decreased similarly in both muscles with increasing P i , as expected. SA force decreased with P i in EDL (40%), leaving the SA to CA force ratio relatively constant across P i concentrations (17–25%). In contrast, SA force increased in soleus (42%), causing a quadrupling of the SA to CA force ratio, from 11% at 0 mM P i to 43% at 16 mM P i , showing that SA is a significant force modulator in slow-twitch mammalian fibers. This modulation would be most prominent during prolonged muscle use, which increases P i concentration and impairs calcium cycling. Based upon our previous Drosophila myosin isoform studies and this work, we propose that in slow-twitch fibers a rapid stretch in the presence of P i reverses myosin’s power stroke, enabling quick rebinding to actin and enhanced force production, while in fast-twitch fibers, stretch and P i cause myosin to detach from actin.
Straight et al. (Wed,) conducted a other in Muscle physiology. Varying phosphate (Pi) concentrations was evaluated on Stretch activation (SA) and calcium-activated (CA) force. Increasing phosphate concentration from 0 to 16 mM quadrupled the stretch activation to calcium-activated force ratio from 11% to 43% in mouse soleus slow-twitch fibers.