Energetics of Muscle Contraction

White epaxial muscle aerobic and anaerobic potential and muscle fiber structure in surface and cave morphotypes of the Mexican cavefish (Astyanax mexicanus)

Proper muscle function and muscle fiber structures that match the environmental demands of organisms are imperative to their success in any ecosystem. The Mexican cavefish, Astyanax mexicanus, has two morphotypes: an obligate cave-dwelling form that lives in thermally insulated caves and an O2 poor environment, and a surface form that lives in a more thermally variable, but O2 rich river environment. As environment can determine physiological adaptations, it is of interest to compare the aerobic and anaerobic metabolic profiles of white muscle metabolism in both morphotypes of this species, as well as their muscle structures. Here, we used white muscle of both morphotypes of the Mexican cavefish to determine citrate synthase (CS) activity as a measure of aerobic potential, and lactate concentration as a measure of anaerobic potential at three different chronic acclimation temperatures (14°C, 25°C, and 31°C). By examining aerobic and anaerobic potential in both morphs, we sought to link environmental thermal flexibility to muscle metabolism. We found that the surface morphotype had higher CS activity and lower lactate concentration, suggesting an overall more efficient usage of aerobic metabolism; whereas the cave morphotype showed lower CS activity and higher lactate concentration, suggesting a stronger reliance on anaerobic pathways. We also measured white muscle histological variables that have been previously linked to whole-animal metabolism: fiber diameter, number of nuclei per mm of fiber and myonuclear domain (MND) of both morphotypes at 25°C to examine cell-level differences in muscle morphology. However, we found no differences in fiber diameter, number of nuclei per mm of fiber or MND between the two morphotypes. Thus, although the cellular morphology is similar in these species, the environmental differences in the evolution of the two morphs has led to differences in their metabolic profiles.

The legacy of A. V. Hill's Nobel Prize winning work on muscle energetics

A. V. Hill was awarded the 1922 Nobel Prize, jointly with Otto Meyerhof, for Physiology or Medicine for his work on energetic aspects of muscle contraction. Hill used his considerable mathematical and experimental skills to investigate the relationships among muscle mechanics, biochemistry and heat production. The main ideas of the work for which the Nobel Prize was awarded were superseded within a decade and the legacy of Hill and Meyerhof's Nobel work was not a set of persistent, influential ideas but rather was a prolonged period of extraordinary activity that advanced the understanding of how muscles work far beyond the concepts that led to the Nobel Prize. Hill pioneered the integration of mathematics into the study of physiology and pharmacology. Particular aspects of Hill's own work that remain in common use in muscle physiology include mathematical descriptions of the relationships between muscle force output and shortening velocity and between force output and calcium concentration and the model of muscle as a contractile element in series with an elastic element. We describe some of the characteristics of Hill's broader scientific activities and then outline how Hill's work on muscle energetics was extended after 1922, as a result of Hill's own work and that of others, to the present day. Abstract figure legend. A. V. Hill's scientific legacy. Vertical from the base. A. V. Hill (pictured) and his colleague Otto Meyerhof shared the 1922 Nobel Prize for Physiology or Medicine. Seven enduring elements of Hill's legacy to muscle physiology are shown. Arrows link concepts and approaches to related disciplines where they have been foundational and highly influential. This article is protected by copyright. All rights reserved.

Meijer and Vloedman's histochemical demonstration of mitochondrial coupling obeys Lambert-Beer's law in the myocardium

Uncoupling of mitochondrial proton pumping and adenosine triphosphate (ATP) production lowers mitochondrial efficiency. Current methods to determine mitochondrial efficiency require substantial amounts of tissue and permeabilization or isolation procedures. A simple histochemical method has been described by Meijer and Vloedman (Histochemistry 69:217–232, 1980, 10.1007/BF00489769), but this was not quantitative. We found linear correlations between (1) absorbance and sections thickness and (2) absorbance and incubation time. Because the method obeys Lambert–Beer’s law, we can estimate ATP/O₂ ratios for healthy and overloaded right-sided rat myocardium. We related mitochondrial efficiency to the ratio between cardiolipin and its precursor phosphatidylglycerol. We found a non-linear relationship between mitochondrial efficiency and this ratio, indicating that lower mitochondrial efficiency as found in experimental pulmonary hypertension may be due to altered composition of the mitochondrial inner membrane. We conclude that the histochemical method of Meijer and Vloedman can be applied to quantify mitochondrial efficiency.

Energetics of muscle contraction: further trials

Knowledge accumulated in the field of energetics of muscle contraction has been reviewed in this article. Active muscle converts chemical energy into heat and work. Therefore, measurements of heat production and mechanical work provide the framework for understanding the process of energy conversion in contraction. In the 1970s, precise comparison between energy output and the associated chemical reactions was performed. It has been found that the two do not match in several situations, resulting in an energy balance discrepancy. More recently, efforts in resolving these discrepancies in the energy balance have been made involving chemical analysis, phosphorus nuclear magnetic resonance spectroscopy, and microcalorimetry. Through reviewing the evidence from these studies, the energy balance discrepancy developed early during isometric contraction has become well understood on a quantitative basis. In this situation energy balance is established when we take into account the binding of Ca to sarcoplasmic proteins such as troponin and parvalbumin, and also the shift of cross-bridge states. On the other hand, the energy balance discrepancy observed during rapid shortening still remains to be clarified. The problem may be related to the essential mechanism of cross-bridge action.

Muscle mechanical advantage of human walking and running: implications for energy cost

Muscular forces generated during locomotion depend on an animal's speed, gait, and size and underlie the energy demand to power locomotion. Changes in limb posture affect muscle forces by altering the mechanical advantage of the ground reaction force ( R) and therefore the effective mechanical advantage (EMA = r/ R, where r is the muscle mechanical advantage) for muscle force production. We used inverse dynamics based on force plate and kinematic recordings of humans as they walked and ran at steady speeds to examine how changes in muscle EMA affect muscle force-generating requirements at these gaits. We found a 68% decrease in knee extensor EMA when humans changed gait from a walk to a run compared with an 18% increase in hip extensor EMA and a 23% increase in ankle extensor EMA. Whereas the knee joint was extended (154–176°) during much of the support phase of walking, its flexed position (134–164°) during running resulted in a 5.2-fold increase in quadriceps impulse (time-integrated force during stance) needed to support body weight on the ground. This increase was associated with a 4.9-fold increase in the ground reaction force moment about the knee. In contrast, extensor impulse decreased 37% ( P < 0.05) at the hip and did not change at the ankle when subjects switched from a walk to a run. We conclude that the decrease in limb mechanical advantage (mean limb extensor EMA) and increase in knee extensor impulse during running likely contribute to the higher metabolic cost of transport in running than in walking. The low mechanical advantage in running humans may also explain previous observations of a greater metabolic cost of transport for running humans compared with trotting and galloping quadrupeds of similar size.

Theoretical simulation of K+-based mechanisms for regulation of capillary perfusion in skeletal muscle

Muscle fibers release K(+) into the interstitial space upon recruitment. Increased local interstitial K(+) concentration ([K(+)]) can cause dilation of terminal arterioles, leading to perfusion of downstream capillaries. The possibility that capillary perfusion can be regulated by vascular responses to [K(+)] was examined using a theoretical model. The model takes into account the spatial relationship between functional units of muscle fiber recruitment and capillary perfusion. Diffusion of K(+) in the interstitial space was simulated. Two hypothetical mechanisms for vascular sensing of interstitial [K(+)] were considered: direct sensing by arterioles and sensing by capillaries with stimulation of feeding arterioles via conducted responses. Control by arteriolar sensing led to poor tissue oxygenation at high levels of muscle activation. With control by capillary sensing, increases in perfusion matched increases in oxygen demand. The time course of perfusion after sudden muscle activation was considered. Predicted capillary perfusion increased rapidly within the first 5 s of muscle fiber activation. The reuptake of K(+) by muscle fibers had a minor effect on the increase of interstitial [K(+)]. Uptake by perfused capillaries was primarily responsible for limiting the increase in [K(+)] in the interstitial space at the onset of fiber activation. Vascular responses to increasing interstitial [K(+)] may contribute to the rapid increase in blood flow that is observed to occur after the onset of muscle contraction.

Changes in K+, Na+ and calcium contents during in vivo stimulation of rat skeletal muscle

The effects of in vivo stimulation via the sciatic nerve on Na+, K+ and calcium contents in slow-twitch and fast-twitch muscles were compared. Whereas intermittent stimulation for 24 h at 20 Hz caused only minor changes in soleus (SOL), a considerable loss of K+ (around 24%) and gain of Na+ (around 84%) was observed in extensor digitorum longus (EDL) and tibialis anterior (TA) muscles. These changes could be detected within 0.5 h and a plateau was maintained from 2 to 24 h. Total calcium content increased progressively, reaching values 245 and 382% above the control level in EDL and TA muscle, respectively, after 24 h of 20 Hz stimulation. Whereas the Na+ and K+ content recovered within a few hours, calcium content did not return towards control level until after 48 h of rest. In a pilot study performed with continuous stimulation at 10 Hz, the changes in Na+ and K+ contents in SOL, EDL and TA muscle were comparable to those at 20 Hz. The concentration of the Na(+)-K+ pumps was highest in the fast-twitch EDL and TA muscles and was unaffected by 10 Hz stimulation. It is concluded that a stimulation pattern leading to a rise in intracellular Na+ and a loss of K+ may cause a marked accumulation of calcium. These events seem to be related to insufficient activation of the Na-K+ pump rather than to variations in the total Na(+)-K+ pump capacity.

Presence of NAD pyrophosphorylase in skeletal muscle in dystrophic mice

NAD pyrophosphorylase (ATP:NMN adenylyltransferase) activity has been measured in the skeletal muscle of dystrophic mice. The amount of this enzyme in the dystrophic mice, as determined by three different methods, was about one half of that in the controls. In addition, the concentration of ATP was too low to be detected in crude extracts of dystrophic mouse skeletal muscle, which were prepared using Tris buffer alone or Tris buffer containing either 3 M KCl, or 1 mM PMSF.

Phosphorus NMR spectroscopy of cat biceps and soleus muscles

It is clear that the combination of 31P NMR studies of muscle and of other organs with biochemical and physiological measurements is an extremely powerful method for studying cellular energetics. We have presented examples of our on-going experiments. Well defined and stable in vitro preparations of mammalian fast-twitch (cat biceps) and slow-twitch (cat soleus) muscles are developed. Erythrocyte-containing perfusate is needed for adequate oxygenation during stimulation. It is not clear why fluorocarbon suspensions are inadequate. Inorganic phosphate levels in the fast-twitch biceps as determined from NRM spectra are close to lmM, that is very much lower than the value (6 mM) derived from chemical measurements of muscle extracts. A similar discrepancy in the slow-twitch soleus cannot yet be established; if it exists, the discrepancy is small. From the metabolite contents, the free energy available from ATP splitting is similar in both muscle types: -63 kJ/mole in biceps and -57 kJ/mole in soleus. Kinetic NMR measurements indicate the creatine phosphokinase reaction is at equilibrium, and in biceps the unidirectional fluxes are 5 mumole per gram per sec. Transient alkalinization of intracellular pH during stimulation periods and acidification during recovery periods are clearly seen. In the biceps the alkalinization is much smaller than in soleus, and suggests a greater buffer capacity in fast-twitch muscles. Both muscles recovery from prolonged ischemic periods upon reperfusion, although the recovery rate is very much slower than that from 10-20 min periods of stimulation in which similar decreases of PCr and increases of Pi occur. The pattern of recovery is however quite similar. Perfused muscles may be a useful model for studying ischemia, anoxia and hypoxia and are useful preparations for a wide variety of metabolic and energetic studies of fast-twitch and slow-twitch mammalian skeletal muscles.