Acidosis and Phosphate Directly Reduce Myosin’s Force-Generating Capacity Through Distinct Molecular Mechanisms

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.

Bioenergetic basis for the increased fatigability with ageing

“…; Jarvis et al . ; Woodward & Debold, ) that used a fatigue‐mimicking environment of pH 6.2–6.5 and ∼20–30 m m P i are similar to the metabolic environment that can be achieved volitionally in vivo . Interpreted together, the greater accumulation of H + and P i in old compared to young adults (Fig.…”

Monitoring Exercise-Induced Muscle Fatigue and Adaptations: Making Sense of Popular or Emerging Indices and Biomarkers

“…Indeed, a strong connection between pH regulation and work capacity has been shown, suggesting that acidosis strongly contributes to fatigue [54]. More specifically, acidosis may impair function of contractile properties by reducing: (a) sarcoplasmic Ca 2+ release and re-uptake, (b) myofibrillar Ca 2+ sensitivity [55] and (c) activity of ATPase [56] and key enzymes of glycolysis such as phosphofructokinase and phosphorylase [57] (Figure 1). However, skinned fiber experiments in variable pH and temperatures have indicated that acidosis, while still important, is not the only reason behind the slowing of contractile velocity observed during fatigue (e.g., [10]).…”

“…A simplified scheme of the purported effects of acidosis and reactive oxygen species (ROS) based on muscle fatigue literature reviewed in this article. Acidosis has been shown to cause impaired Ca 2+ handling [55] and inhibition of key metabolic enzyme activities [57] as well as to inhibit myosin ATPase [56] affecting force and velocity of contraction [10]. ROS have been shown to cause: impaired sarcolemma ability to depolarize [87], disturbance in calcium release from the sarcoplasmic reticulum and decreased calcium sensitivity of the myofilaments [89], impaired acto-myosin interaction [91], enzyme inhibition [92] and blood flow restriction [93].…”

The time to peak blood bicarbonate (HCO3−), pH, and strong ion difference (SID) following sodium bicarbonate (NaHCO3) ingestion in highly trained adolescent swimmers

“…Increasing blood alkalosis alters the pH gradient between the intracellular and extracellular compartments, subsequently leading to the upregulation of the lactate–hydrogen ion (H + ) co-transporter to efflux acidic H + from the active musculature and into circulation [4,5]. During exercise, accelerating the removal of H + is purported to offset fatigue since intracellular acidosis is associated with debilitating effects on the capacity to sustain muscle force production [6–10]. More specifically, cellular acidosis is suggested to reduce muscle shortening velocity and cross-bridge cycling by inhibiting calcium ion (Ca 2+ ) sensitivity [6,9] and myosin ATPase activity [10,11], whereas additional impairments of key glycolytic enzymes [12,13] and the strong ion difference (SID) [7,14,15] could reduce the available ATP substrates and action potentials necessary to maintain muscular contractions, respectively.…”

“…During exercise, accelerating the removal of H + is purported to offset fatigue since intracellular acidosis is associated with debilitating effects on the capacity to sustain muscle force production [6–10]. More specifically, cellular acidosis is suggested to reduce muscle shortening velocity and cross-bridge cycling by inhibiting calcium ion (Ca 2+ ) sensitivity [6,9] and myosin ATPase activity [10,11], whereas additional impairments of key glycolytic enzymes [12,13] and the strong ion difference (SID) [7,14,15] could reduce the available ATP substrates and action potentials necessary to maintain muscular contractions, respectively. Due to the complexity of fatigue, the role of acidosis in each of these mechanisms is contested as a causative factor [16–18].…”