NHE- and diffusion-dependent proton fluxes across the tubular system membranes of fast-twitch muscle fibers of the rat

Lactic acidosis: implications for human exercise performance

During high-intensity exercise a lactic-acidosis occurs with raised myoplasmic and plasma concentrations of lactate- and protons ([lactate-], [H+] or pH). We critically evaluate whether this causes/contributes to fatigue during human exercise. Increases of [lactate-] per se (to 25 mM in plasma, 50 mM intracellularly) exert little detrimental effect on muscle performance while ingestion/infusion of lactate- can be ergogenic. An exercise-induced intracellular acidosis at the whole-muscle level (pHi falls from 7.1-7.0 to 6.9-6.3), incorporates small changes in slow-twitch fibres (pHi ~ 6.9) and large changes in fast-twitch fibres (pHi ~ 6.2). The relationship between peak force/power and acidosis during fatiguing contractions varies across exercise regimes implying that acidosis is not the sole cause of fatigue. Concomitant changes of other putative fatigue factors include phosphate metabolites, glycogen, ions and reactive oxygen species. Acidosis to pHi 6.7-6.6 at physiological temperatures (during recovery from exercise or induced in non-fatigued muscle), has minimal effect on force/power. Acidosis to pHi ~ 6.5-6.2 per se reduces maximum force (~12%), slows shortening velocity (~5%), and lowers peak power (~22%) in non-fatigued muscles/individuals. A pre-exercise induced-acidosis with ammonium chloride impairs exercise performance in humans and accelerates the decline of force/power (15-40% initial) in animal muscles stimulated repeatedly in situ. Raised [H+]i and diprotonated inorganic phosphate ([H2PO4-]i) act on myofilament proteins to reduce maximum cross-bridge activity, Ca2+-sensitivity, and myosin ATPase activity. Acidosis/[lactate-]o attenuates detrimental effects of large K+-disturbances on action potentials and force in non-fatigued muscle. We propose that depressive effects of acidosis and [H2PO4-]i on myofilament function dominate over the protective effects of acidosis/lactate- on action potentials during fatigue. Raised extracellular [H+]/[lactate-] do not usually cause central fatigue but do contribute to elevated perceived exertion and fatigue sensations by activating group III/IV muscle afferents. Modulation of H+/lactate- regulation (via extracellular H+-buffers, monocarboxylate transporters, carbonic anhydrase, carnosine) supports a role for intracellular acidosis in fatigue. In conclusion, current evidence advocates that severe acidosis in fast-twitch fibres can contribute to force/power fatigue during intense human exercise.

Short-Term Mild Hypoxia Modulates Na,K-ATPase to Maintain Membrane Electrogenesis in Rat Skeletal Muscle

The Na,K-ATPase plays an important role in adaptation to hypoxia. Prolonged hypoxia results in loss of skeletal muscle mass, structure, and performance. However, hypoxic preconditioning is known to protect against a variety of functional impairments. In this study, we tested the possibility of mild hypoxia to modulate the Na,K-ATPase and to improve skeletal muscle electrogenesis. The rats were subjected to simulated high-altitude (3000 m above sea level) hypobaric hypoxia (HH) for 3 h using a hypobaric chamber. Isolated diaphragm and soleus muscles were tested. In the diaphragm muscle, HH increased the α2 Na,K-ATPase isozyme electrogenic activity and stably hyperpolarized the extrajunctional membrane for 24 h. These changes were accompanied by a steady increase in the production of thiobarbituric acid reactive substances as well as a decrease in the serum level of endogenous ouabain, a specific ligand of the Na,K-ATPase. HH also increased the α2 Na,K-ATPase membrane abundance without changing its total protein content; the plasma membrane lipid-ordered phase did not change. In the soleus muscle, HH protected against disuse (hindlimb suspension) induced sarcolemmal depolarization. Considering that the Na,K-ATPase is critical for maintaining skeletal muscle electrogenesis and performance, these findings may have implications for countermeasures in disuse-induced pathology and hypoxic therapy.

Muscle Ionic Shifts During Exercise: Implications for Fatigue and Exercise Performance

Exercise causes major shifts in multiple ions (e.g., K⁺ , Na⁺ , H⁺ , lactate^- , Ca²⁺ , and Cl^- ) during muscle activity that contributes to development of muscle fatigue. Sarcolemmal processes can be impaired by the trans-sarcolemmal rundown of ion gradients for K⁺ , Na⁺ , and Ca²⁺ during fatiguing exercise, while changes in gradients for Cl^- and Cl^- conductance may exert either protective or detrimental effects on fatigue. Myocellular H⁺ accumulation may also contribute to fatigue development by lowering glycolytic rate and has been shown to act synergistically with inorganic phosphate (Pi) to compromise cross-bridge function. In addition, sarcoplasmic reticulum Ca²⁺ release function is severely affected by fatiguing exercise. Skeletal muscle has a multitude of ion transport systems that counter exercise-related ionic shifts of which the Na⁺ /K⁺ -ATPase is of major importance. Metabolic perturbations occurring during exercise can exacerbate trans-sarcolemmal ionic shifts, in particular for K⁺ and Cl^- , respectively via metabolic regulation of the ATP-sensitive K⁺ channel (K_ATP ) and the chloride channel isoform 1 (ClC-1). Ion transport systems are highly adaptable to exercise training resulting in an enhanced ability to counter ionic disturbances to delay fatigue and improve exercise performance. In this article, we discuss (i) the ionic shifts occurring during exercise, (ii) the role of ion transport systems in skeletal muscle for ionic regulation, (iii) how ionic disturbances affect sarcolemmal processes and muscle fatigue, (iv) how metabolic perturbations exacerbate ionic shifts during exercise, and (v) how pharmacological manipulation and exercise training regulate ion transport systems to influence exercise performance in humans. © 2021 American Physiological Society. Compr Physiol 11:1895-1959, 2021.

Wheat germ agglutinin-conjugated fluorescent pH sensors for visualizing proton fluxes

Small-molecule fluorescent wheat germ agglutinin (WGA) conjugates are routinely used to demarcate mammalian plasma membranes, because they bind to the cell’s glycocalyx. Here, we describe the derivatization of WGA with a pH-sensitive rhodamine fluorophore (pHRho; pKa = 7) to detect proton channel fluxes and extracellular proton accumulation and depletion from primary cells. We found that WGA-pHRho labeling was uniform and did not appreciably alter the voltage gating of glycosylated ion channels, and the extracellular changes in pH correlated with proton channel activity. Using single-plane illumination techniques, WGA-pHRho was used to detect spatiotemporal differences in proton accumulation and depletion over the extracellular surface of cardiomyocytes, astrocytes, and neurons. Because WGA can be derivatized with any small-molecule fluorescent ion sensor, WGA conjugates should prove useful to visualize most electrogenic and nonelectrogenic events on the extracellular side of the plasma membrane.

Nox4 - RyR1 - Nox2: Regulators of micro-domain signaling in skeletal muscle

The ability for skeletal muscle to perform optimally can be affected by the regulation of Ca²⁺ within the triadic junctional space at rest. Reactive oxygen species impact muscle performance due to changes in oxidative stress, damage and redox regulation of signaling cascades. The interplay between ROS and Ca²⁺ signaling at the triad of skeletal muscle is therefore important to understand as it can impact the performance of healthy and diseased muscle. Here, we aimed to examine how changes in Ca²⁺ and redox signaling within the junctional space micro-domain of the mouse skeletal muscle fibre alters the homeostasis of these complexes. The dystrophic mdx mouse model displays increased RyR1 Ca²⁺ leak and increased NAD(P)H Oxidase 2 ROS. These alterations make the mdx mouse an ideal model for understanding how ROS and Ca²⁺ handling impact each other. We hypothesised that elevated t-tubular Nox2 ROS increases RyR1 Ca²⁺ leak contributing to an increase in cytoplasmic Ca²⁺, which could then initiate protein degradation and impaired cellular functions such as autophagy and ER stress. We found that inhibiting Nox2 ROS did not decrease RyR1 Ca²⁺ leak observed in dystrophin-deficient skeletal muscle. Intriguingly, another NAD(P)H isoform, Nox4, is upregulated in mice unable to produce Nox2 ROS and when inhibited reduced RyR1 Ca²⁺ leak. Our findings support a model in which Nox4 ROS induces RyR1 Ca²⁺ leak and the increased junctional space [Ca²⁺] exacerbates Nox2 ROS; with the cumulative effect of disruption of downstream cellular processes that would ultimately contribute to reduced muscle or cellular performance.

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Nox2 ROS does not influence RyR1 Ca²⁺ leak in skeletal muscle.

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Lack of Nox2 ROS increases Nox4 expression.

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Nox4 ROS induces RyR1 Ca²⁺ leak via S-nitrosylation.

The SLC9A-C Mammalian Na+/H+ Exchanger Family: Molecules, Mechanisms, and Physiology

Na⁺/H⁺ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na⁺ and H⁺ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na⁺/H⁺ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.

Measurement of force and calcium release using mechanically skinned fibers from mammalian skeletal muscle

The mechanically skinned (or “peeled”) skeletal muscle fiber technique is a highly versatile procedure that allows controlled examination of each of the steps in the excitation-contraction (EC)-coupling sequence in skeletal muscle fibers, starting with excitation/depolarization of the transverse tubular (T)-system through to Ca2+ release from sarcoplasmic reticulum (SR) and finally force development by the contractile apparatus. It can also show the overall response of the whole EC-coupling sequence together, such as in twitch and tetanic force responses. A major advantage over intact muscle fiber preparations is that it is possible to set and rapidly manipulate the “intracellular” conditions, allowing examination of the effects of key variables (e.g., intracellular pH, ATP levels, redox state, etc.) on each individual step in EC coupling. This Cores of Reproducibility in Physiology (CORP) article describes the rationale, procedures, and experimental details of the various ways in which the mechanically skinned fiber technique is used in our laboratory to examine the physiological mechanisms controlling Ca2+ release and contraction in skeletal muscle fibers and the aberrations and dysfunction occurring with exercise and disease.

Carriers, exchangers, and cotransporters in the first 100 years of the Journal of General Physiology

Jennings reviews the many contributions of JGP articles to our current understanding of solute transporter mechanisms.

Transporters, pumps, and channels are proteins that catalyze the movement of solutes across membranes. The single-solute carriers, coupled exchangers, and coupled cotransporters that are collectively known as transporters are distinct from conductive ion channels, water channels, and ATP-hydrolyzing pumps. The main conceptual framework for studying transporter mechanisms is the alternating access model, which comprises substrate binding and release events on each side of the permeability barrier and translocation events involving conformational changes between inward-facing and outward-facing conformational states. In 1948, the Journal of General Physiology began to publish work that focused on the erythrocyte glucose transporter—the first transporter to be characterized kinetically—followed by articles on the rates, stoichiometries, asymmetries, voltage dependences, and regulation of coupled exchangers and cotransporters beginning in the 1960s. After the dawn of cDNA cloning and sequencing in the 1980s, heterologous expression systems and site-directed mutagenesis allowed identification of the functional roles of specific amino acid residues. In the past two decades, structures of transport proteins have made it possible to propose specific models for transporter function at the molecular level. Here, we review the contribution of JGP articles to our current understanding of solute transporter mechanisms. Whether the topic has been kinetics, energetics, regulation, mutagenesis, or structure-based modeling, a common feature of these articles has been a quantitative, mechanistic approach, leading to lasting insights into the functions of transporters.

The ins and outs of acid-base transport in skeletal muscle

Aalkjær and Nielsen discuss new data revealing the basis of acid–base transport in t-tubules of skeletal muscle.