Regulation of oxidative phosphorylation is different in electrically- and cortically-stimulated skeletal muscle

Training-Induced Increase in V·O2max and Critical Power, and Acceleration of V·O2 on-Kinetics Result from Attenuated Pi Increase Caused by Elevated OXPHOS Activity

Computer simulations using a dynamic model of the skeletal muscle bioenergetic system, involving the Pi-double-threshold mechanism of muscle fatigue, demonstrate that the training-induced increase in V·O2max, increase in critical power (CP) and acceleration of primary phase II of the V·O2 on kinetics (decrease in t0.63) is caused by elevated OXPHOS activity acting through a decrease in and slowing of the Pi (inorganic phosphate) rise during the rest-to-work transition. This change leads to attenuation of the reaching by Pi of Pipeak, peak Pi at which exercise is terminated because of fatigue. The delayed (in time and in relation to V·O2 increase) Pi rise for a given power output (PO) in trained muscle causes Pi to reach Pipeak (in very heavy exercise) after a longer time and at a higher V·O2; thus, exercise duration is lengthened, and V·O2max is elevated compared to untrained muscle. The diminished Pi increase during exercise with a given PO can cause Pi to stabilize at a steady state less than Pipeak, and exercise can continue potentially ad infinitum (heavy exercise), instead of rising unceasingly and ultimately reaching Pipeak and causing exercise termination (very heavy exercise). This outcome means that CP rises, as the given PO is now less than, and not greater than CP. Finally, the diminished Pi increase (and other metabolite changes) results in, at a given PO (moderate exercise), the steady state of fluxes (including V·O2) and metabolites being reached faster; thus, t0.63 is shortened. This effect of elevated OXPHOS activity is possibly somewhat diminished by the training-induced decrease in Pipeak.

Mechanisms of slowed V̇O2 on-kinetics in second step of two-step-incremental exercise in skeletal muscle

Simulations using a computer model of the skeletal muscle bioenergetic system demonstrate that the slowed V̇O₂ on-kinetics of the second step in two-step incremental exercise (exercise initiated from elevated baseline metabolic rate) can be accounted for by a decrease in the stimulation of oxidative phosphorylation (OXPHOS) and/or increase in the stimulation of glycolysis through each-step activation (ESA) in working skeletal muscle. This effect can be caused by either a recruitment of more glycolytic type IIa, IIx and IIb fibers or metabolic regulation in already recruited fibers, or both. The elevated-glycolysis-stimulation mechanism predicts that the end-second-step pH in two-step-incremental exercise is lower than the end-exercise pH in constant-power exercise with the same work intensity (power output). The lowered-OXPHOS-stimulation mechanism predicts higher end-exercise ADP and P_i, and lower PCr in the second step of two-step-incremental than in constant-power exercise. These predictions/mechanisms can be verified or falsified in the experimental way. DATA AVAILABILITY STATEMENT: There are no additional data available.

V̇O2 (non-)linear increase in ramp-incremental exercise vs. V̇O2 slow component in constant-power exercise: Underlying mechanisms

A computer model of the skeletal muscle bioenergetic system involving the P_i double-threshold mechanism of muscle fatigue was used to study the V̇O₂ (non-)linear increase in time in ramp-incremental exercise as compared to the V̇O₂ slow component in constant-power exercise. The P_i double-threshold mechanism applies to both constant-power and ramp-incremental exercise. The additional ATP usage is initiated at a significantly higher ATP usage activity (power output), determining the moderate/heavy exercise border, in ramp-incremental, than in constant-power exercise. A significantly lowered additional ATP usage activity or elevated glycolysis stimulation at the highest power outputs in ramp-incremental exercise in relation to constant-power exercise can additionally explain the much smaller (or zero) V̇O₂ non-linearity in ramp-incremental exercise, than V̇O₂ slow component in constant-power exercise. The V̇O₂ (non-)linearity in ramp-incremental exercise and V̇O₂ slow component in constant-power exercise is a derivative of a balance between the additional ATP usage and ATP production by anaerobic glycolysis.

Skeletal Muscle Biochemical Origin of Exercise Intensity Domains and their Relation to Whole-Body V̇O2 Kinetics

This article presents the biochemical intra-skeletal-muscle basis of exercise intensity domains: moderate (M), heavy (H), very heavy (VH) and severe (S). Threshold origins are mediated by a "Pi double-threshold" mechanism of muscle fatigue, which assumes: (1) additional ATP usage, underlying muscle V̇O2 and metabolite slow components, is initiated when inorganic phosphate (Pi) exceeds a critical value (Picrit); (2) exercise is terminated because of fatigue, when Pi reaches a peak value (Pipeak); (3) the Pi increase and additional ATP usage increase mutually stimulate each other forming a positive feedback. M/H and H/VH borders are defined by Pi on-kinetics in relation to Picrit and Pipeak. The values of the ATP usage activity, proportional to power output (PO), for the M/H, H/VH and VH/S borders are lowest in untrained muscle and highest in well-trained muscle. The metabolic range between the M/H and H/VH border (or "H space") decreases with muscle training, while the difference between the H/VH and VH/S border (or "VH space") is only weakly dependent on training status. The absolute magnitude of the muscle V̇O2 slow-component, absent in M exercise, rises gradually with PO to a maximal value in H exercise, and then decreases with PO in VH and S exercise. Simulations of untrained, physically-active and well-trained muscle demonstrate that the muscle M/H border need not be identical to the whole-body M/H border determined from pulmonary V̇O2 on-kinetics and blood lactate, while suggesting that the biochemical origins of the H/VH border reside within skeletal muscle and correspond to whole-body critical power.

Energy metabolism design of the striated muscle cell

The design of the energy metabolism system in striated muscle remains a major area of investigation. Here, we review our current understanding and emerging hypotheses regarding the metabolic support of muscle contraction. Maintenance of ATP free energy, so called energy homeostasis, via mitochondrial oxidative phosphorylation is critical to sustained contractile activity, and this major design criterion is the focus of this review. Cell volume invested in mitochondria reduces the space available for generating contractile force, and this spatial balance between mitochondria acontractile elements to meet the varying sustained power demands across muscle types is another important design criterion. This is accomplished with remarkably similar mass-specific mitochondrial protein composition across muscle types, implying that it is the organization of mitochondria within the muscle cell that is critical to supporting sustained muscle function. Beyond the production of ATP, ubiquitous distribution of ATPases throughout the muscle requires rapid distribution of potential energy across these large cells. Distribution of potential energy has long been thought to occur primarily through facilitated metabolite diffusion, but recent analysis has questioned the importance of this process under normal physiological conditions. Recent structural and functional studies have supported the hypothesis that the mitochondrial reticulum provides a rapid energy distribution system via the conduction of the mitochondrial membrane potential to maintain metabolic homeostasis during contractile activity. We extensively review this aspect of the energy metabolism design contrasting it with metabolite diffusion models and how mitochondrial structure can play a role in the delivery of energy in the striated muscle.

Factors determining training-induced changes in V̇O2max, critical power, and V̇O2 on-kinetics in skeletal muscle

Computer simulations, using the "P_i double-threshold" mechanism of muscle fatigue postulated previously (the first threshold initiating progressive reduction in work efficiency and the second threshold resulting in exercise intolerance), demonstrated that several parameters of the skeletal muscle bioenergetic system can affect maximum oxygen consumption (V̇O_2max), critical power (CP), and oxygen consumption (V̇O₂) on-kinetics in skeletal muscle. Simulations and experimental observations together demonstrate that endurance exercise training increases oxidative phosphorylation (OXPHOS) activity and/or each-step activation (ESA) intensity, the latter, especially in the early stages of training. Here, new computer simulations demonstrate that an endurance training-induced increase in OXPHOS activity and decrease in peak P_i (Pi_peak), at which exercise is terminated because of exercise intolerance, result in increased V̇O_2max and CP, speeding of the primary phase II of V̇O₂ on-kinetics, and decreases V̇O₂ slow component magnitude, consistent with their observed behavior in vivo. It is possible, but remains unknown, whether there is a contribution to this behavior of an increase in the critical P_i (Pi_crit), above which the additional ATP usage underlying the slow component begins, and a decrease in the activity of the additional ATP usage (k_add). Thus, we offer a mechanism, involving P_i accumulation, Pi_crit and Pi_peak, of the training-induced adaptations in V̇O_2max, CP, and the primary and slow component phases of V̇O₂ on-kinetics that was absent in the literature.NEW & NOTEWORTHY A mechanism of the training-induced changes in V̇O_2max, critical power, and V̇O₂ on-kinetics in skeletal muscle reported in the literature is postulated. It involves the self-driving "P_i double-threshold" mechanism of muscle fatigue underlying exercise inefficiency, the slow component of the V̇O₂ on-kinetics, and termination of exercise. It is proposed that an increase in OXPHOS activity and decrease in peak P_i at which exercise terminates are responsible for the training-induced changes in the muscle bioenergetic system.

Second-order simultaneous components model for the overshoot and "slow component" in V̇O2 kinetics

The human oxygen uptake responses to exercise step on-transients present different shapes depending on the overshoot and/or the "slow component" manifestations. The conventional First-Order Multi-Exponential (FOME) model incorporates delayed add-on terms to comprise these phenomena, increasing parameter quantity, requiring a delayed recruitment of type II fibers to explain the "slow component," and not offering a unified structure for different individuals and intensity domains. We hypothesized that a model composed of two Second-Order Simultaneous Components (SOSC) would present a better overall fitting performance than the FOME. Fourteen well-trained male cyclists performed repeated step on-transitions to moderate, heavy, and severe cycling intensities, whose responses were fitted with FOME and SOSC models. The SOSC presented significantly smaller (p < 0.05) root mean squared errors for moderate, supra-moderate, and all intensities combined. Along with conceptual analyses, these findings suggest the SOSC as a comprehensive alternative to the FOME model, explaining all oxygen uptake step responses with as many parameters and without delayed add-on components.

Exceeding a "critical" muscle Pi: implications for [Formula: see text] and metabolite slow components, muscle fatigue and the power-duration relationship

PURPOSE

The consequences of the assumption that the additional ATP usage, underlying the slow component of oxygen consumption ([Formula: see text]) and metabolite on-kinetics, starts when cytosolic inorganic phosphate (P_i) exceeds a certain "critical" P_i concentration, and muscle work terminates because of fatigue when P_i exceeds a certain, higher, "peak" P_i concentration are investigated.

METHODS

A previously developed computer model of the myocyte bioenergetic system is used.

RESULTS

Simulated time courses of muscle [Formula: see text], cytosolic ADP, pH, PCr and P_i at various ATP usage activities agreed well with experimental data. Computer simulations resulted in a hyperbolic power-duration relationship, with critical power (CP) as an asymptote. CP was increased, and phase II [Formula: see text] on-kinetics was accelerated, by progressive increase in oxygen tension (hyperoxia).

CONCLUSIONS

P_i is a major factor responsible for the slow component of the [Formula: see text] and metabolite on-kinetics, fatigue-related muscle work termination and hyperbolic power-duration relationship. The successful generation of experimental system properties suggests that the additional ATP usage, underlying the slow component, indeed starts when cytosolic P_i exceeds a "critical" P_i concentration, and muscle work terminates when P_i exceeds a "peak" P_i concentration. The contribution of other factors, such as cytosolic acidification, or glycogen depletion and central fatigue should not be excluded. Thus, a detailed quantitative unifying mechanism underlying various phenomena related to skeletal muscle fatigue and exercise tolerance is offered that was absent in the literature. This mechanism is driven by reciprocal stimulation of P_i increase and additional ATP usage when "critical" P_i is exceeded.

Pi-induced muscle fatigue leads to near-hyperbolic power-duration dependence

PURPOSE

Consequences of combining three ideas proposed previously by other authors: (1) that there exists a critical power (CP), above which no steady state in [Formula: see text]O₂ (oxygen consumption) and metabolites can be achieved in voluntary constant-power exercise; (2) that muscle fatigue is related to decreased exercise efficiency (increased [Formula: see text]O₂/power output ratio); and (3) that P_i (inorganic phosphate) is the main fatigue-related metabolite are investigated.

METHODS

A previously-developed computer model of the skeletal muscle bioenergetic system is used. It was assumed in computer simulations that skeletal muscle work terminates when cytosolic P_i (inorganic phosphate) exceeds a certain critical level.

RESULTS

Simulated changes in muscle [Formula: see text]O₂, cytosolic ADP, pH, PCr and P_i as a function of time at various ATP usage activities (corresponding to power outputs) agreed well with experimental data. Computer simulations resulted in a fourth previously-published idea: (4) that the power-duration relationship describing the dependence of power output (PO) on the time to exhaustion of voluntary constant-power exercise at a given PO has a (near-)hyperbolic shape.

CONCLUSIONS

P_i is a major factor contributing to muscle fatigue, as such an assumption leads to a (near-)hyperbolic shape of the power-duration relationship, at least for exercise duration of ~ 1-10 min. Thus, a potential mechanism underlying the power-duration relationship shape is offered that was absent in the literature. Other factors/mechanisms, such as cytosol acidification, glycogen stores depletion and central fatigue can contribute to this relationship, especially in longer exercises.

Muscle V˙O2-power output nonlinearity in constant-power, step-incremental, and ramp-incremental exercise: magnitude and underlying mechanisms

A computer model of the skeletal muscle bioenergetic system was used to simulate time courses of muscle oxygen consumption (V˙O2), cytosolic metabolite (ADP, PCr, P_i, and ATP) concentrations, and pH during whole‐body constant‐power exercise (CPE) (6 min), step‐incremental exercise (SIE) (30 W/3 min), and slow (10 W/min), medium (30 W/min), and fast (50 W/min) ramp‐incremental exercise (RIE). Different ESA (each‐step activation) of oxidative phosphorylation (OXPHOS) intensity‐ATP usage activity relationships, representing different muscle fibers recruitment patterns, gave best agreement with experimental data for CPE, and for SIE and RIE. It was assumed that the muscle V˙O2‐power output (PO) nonlinearity is related to a time‐ and PO‐dependent increase in the additional ATP usage underlying the slow component of the V˙O2 on‐kinetics minus the increase in ATP supply by anaerobic glycolysis leading to a decrease in V˙O2. The muscle V˙O2‐PO relationship deviated upward (+) or downward (−) from linearity above critical power (CP), and the nonlinearity equaled +16% (CPE),+12% (SIE), +8% (slow RIE), +1% (moderate RIE), and −2% (fast RIE) at the end of exercise, in agreement with experimental data. During SIE and RIE, changes in PCr and P_i accelerated moderately above CP, while changes in ADP and pH accelerated significantly with time and PO. It is postulated that the intensity of the additional ATP usage minus ATP supply by anaerobic glycolysis determines the size of the muscle V˙O2‐PO nonlinearity. It is proposed that the extent of the additional ATP usage is proportional to the time integral of PO ‐ CP above CP.

Mechanisms underlying extremely fast muscle V˙O2 on-kinetics in humans

The time constant of the primary phase of pulmonary V˙O2 on-kinetics (τp ), which reflects muscle V˙O2 kinetics during moderate-intensity exercise, is about 30 s in young healthy untrained individuals, while it can be as low as 8 s in endurance-trained athletes. We aimed to determine the intramuscular factors that enable very low values of t0.63 to be achieved (analogous to τp , t0.63 is the time to reach 63% of the V˙O2 amplitude). A computer model of oxidative phosphorylation (OXPHOS) in skeletal muscle was used. Muscle t0.63 was near-linearly proportional to the difference in phosphocreatine (PCr) concentration between rest and work (ΔPCr). Of the two main factors that determine t0.63 , a huge increase in either OXPHOS activity (six- to eightfold) or each-step activation (ESA) of OXPHOS intensity (>3-fold) was needed to reduce muscle t0.63 from the reference value of 29 s (selected to represent young untrained subjects) to below 10 s (observed in athletes) when altered separately. On the other hand, the effect of a simultaneous increase of both OXPHOS activity and ESA intensity required only a twofold elevation of each to decrease t0.63 below 10 s. Of note, the dependence of t0.63 on OXPHOS activity and ESA intensity is hyperbolic, meaning that in trained individuals a large increase in OXPHOS activity and ESA intensity are required to elicit a small reduction in τp . In summary, we postulate that the synergistic action of elevated OXPHOS activity and ESA intensity is responsible for extremely low τp (t0.63 ) observed in highly endurance-trained athletes.