Abstract
Elevated levels of the metabolic by-products, including acidosis (i.e., high [H+]) and phosphate (Pi) are putative agents of muscle fatigue; however, the mechanism through which they affect myosin’s function remain unclear. To elucidate these mechanisms, we directly examined the effect of acidosis (pH 6.5 vs. 7.4), alone and in combination with elevated levels of Pi on the force-generating capacity of a mini-ensemble of myosin using a laser trap assay. Acidosis decreased myosin’s average force-generating capacity by 20% (p < 0.05). The reduction was due to both a decrease in the force generated during each actomyosin interaction, as well as an increase in the number of binding events generating negative forces. Adding Pi to the acidic condition resulted in a quantitatively similar decrease in force but was solely due to an elimination of all high force-generating events (>2 pN), resulting from an acceleration of the myosin’s rate of detachment from actin. Acidosis and Pi also had distinct effects on myosin’s steady state ATPase rate with acidosis slowing it by ∼90% (p > 0.05), while the addition of Pi under acidic conditions caused a significant recovery in the ATPase rate. These data suggest that these two fatigue agents have distinct effects on myosin’s cross-bridge cycle that may underlie the synergistic effect that they have muscle force. Thus these data provide novel molecular insight into the mechanisms underlying the depressive effects of Pi and H+ on muscle contraction during fatigue.
Highlights
Muscle fatigue from intense contractile activity is due, in large part, to the accumulation metabolic by-products, primarily hydrogen ions (H+) and phosphate (Pi), inhibiting myosin’s ability to generate force and motion (Allen et al, 2008; Fitts, 2008; Debold et al, 2016)
The mini-ensemble of myosin molecules stochastically interacted with the actin filament to produce a range of low and high force-generating events (Figure 1)
Examination of the distribution of events revealed this was caused by a decrease in the frequency of high force-generating events, as well as an increase in the frequency of negative force-generating events
Summary
Muscle fatigue from intense contractile activity is due, in large part, to the accumulation metabolic by-products, primarily hydrogen ions (H+) and phosphate (Pi), inhibiting myosin’s ability to generate force and motion (Allen et al, 2008; Fitts, 2008; Debold et al, 2016). Pi and H+ Inhibit Myosin state (AM.ADP), reverses myosin’s powerstroke, and induces detachment, restoring myosin to the pre-powerstroke state (Pate and Cooke, 1988; Takagi et al, 2004) This model is based, primarily, on the effects of Pi single fiber contractile properties (Hibberd et al, 1985a,b; Pate and Cooke, 1988, 1989; Dantzig et al, 1992; Debold et al, 2004; Caremani et al, 2008), which represent the average behavior of billions of myosin molecules, making it difficult to determine how Pi affects a single actomyosin cross-bridge. The Pi-induced increase in velocity at low pH led us to propose that the rebinding of Pi to actomyosin does not reverse the powerstroke, but instead induces myosin’s detachment in a post-powerstroke state (Debold et al, 2011, 2013)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
