Inward flux of lactate⁻ through monocarboxylate transporters contributes to regulatory volume increase in mouse muscle fibres

A century of exercise physiology: key concepts in muscle cell volume regulation

Skeletal muscle cells can both gain and lose volume during periods of exercise and rest. Muscle cells do not behave as perfect osmometers because the cell volume changes are less than predicted from the change in extracellular osmolality. Therefore, there are mechanisms involved in regulating cell volume, and they are different for regulatory volume decreases and regulatory volume increases. Also, after an initial rapid change in cell volume, there is a gradual and partial recovery of cell volume that is effected by ion and water transport mechanisms. The mechanisms have been studied in non-contracting muscle cells, but remain to be fully elucidated in contracting muscle. Changes in muscle cell volume are known to affect the strength of contractile activity as well as anabolic/catabolic signaling, perhaps indicating that cell volume should be a regulated variable in skeletal muscle cells. Muscles contracting at moderate to high intensity gain intracellular volume because of increased intracellular osmolality. Concurrent increases in interstitial (extracellular) muscle volume occur from an increase in osmotically active molecules and increased vascular filtration pressure. At the same time, non-contracting muscles lose cell volume because of increased extracellular (blood) osmolality. This review provides the physiological foundations and highlights key concepts that underpin our current understanding of volume regulatory processes in skeletal muscle, beginning with consideration of osmosis more than 200 years ago and continuing through to the process of regulatory volume decrease and regulatory volume increase.

Comparison of Isotonic Activation of Cell Volume Regulation in Rat Peritoneal Mesothelial Cells and in Kidney Outer Medullary Collecting Duct Principal Cells

In disease states, mesothelial cells are exposed to variable osmotic conditions, with high osmotic stress exerted by peritoneal dialysis (PD) fluids. They contain unphysiologically high concentrations of glucose and result in major peritoneal membrane transformation and PD function loss. The effects of isotonic entry of urea and myo-inositol in hypertonic (380 mOsm/kg) medium on the cell volume of primary cultures of rat peritoneal mesothelial cells and rat kidney outer medullary collecting duct (OMCD) principal cells were studied. In hypertonic medium, rat peritoneal mesothelial cells activated a different mechanism of cell volume regulation in the presence of isotonic urea (100 mM) in comparison to rat kidney OMCD principal cells. In kidney OMCD cells inflow of urea into the shrunken cell results in restoration of cell volume. In the shrunken peritoneal mesothelial cells, isotonic urea inflow caused a small volume increase and activated regulatory volume decrease (RVD). Isotonic myo-inositol activated RVD in hypertonic medium in both cell types. Isotonic application of both osmolytes caused a sharp increase of intracellular calcium both in peritoneal mesothelial cells and in kidney OMCD principal cells. In conclusion, peritoneal mesothelial cells exhibit RVD mechanisms when challenged with myo-inositol and urea under hyperosmolar isotonic switch from mannitol through involvement of calcium-dependent control. Myo-inositol effects were identical with the ones in OMCD principal cells whereas urea effects in OMCD principal cells led to no RVD induction.

Lactate metabolism: historical context, prior misinterpretations, and current understanding

Lactate (La^-) has long been at the center of controversy in research, clinical, and athletic settings. Since its discovery in 1780, La^- has often been erroneously viewed as simply a hypoxic waste product with multiple deleterious effects. Not until the 1980s, with the introduction of the cell-to-cell lactate shuttle did a paradigm shift in our understanding of the role of La^- in metabolism begin. The evidence for La^- as a major player in the coordination of whole-body metabolism has since grown rapidly. La^- is a readily combusted fuel that is shuttled throughout the body, and it is a potent signal for angiogenesis irrespective of oxygen tension. Despite this, many fundamental discoveries about La^- are still working their way into mainstream research, clinical care, and practice. The purpose of this review is to synthesize current understanding of La^- metabolism via an appraisal of its robust experimental history, particularly in exercise physiology. That La^- production increases during dysoxia is beyond debate, but this condition is the exception rather than the rule. Fluctuations in blood [La^-] in health and disease are not typically due to low oxygen tension, a principle first demonstrated with exercise and now understood to varying degrees across disciplines. From its role in coordinating whole-body metabolism as a fuel to its role as a signaling molecule in tumors, the study of La^- metabolism continues to expand and holds potential for multiple clinical applications. This review highlights La^-'s central role in metabolism and amplifies our understanding of past research.

Role of MCT1 and CAII in skeletal muscle pH homeostasis, energetics, and function: in vivo insights from MCT1 haploinsufficient mice

The purpose of this study was to investigate the effects of a partial suppression of monocarboxylate transporter (MCT)-1 on skeletal muscle pH, energetics, and function (MCT1^+/- mice). Twenty-four MCT1^+/- and 13 wild-type (WT) mice were subjected to a rest-exercise-recovery protocol, allowing assessment of muscle energetics (by magnetic resonance spectroscopy) and function. The study included analysis of enzyme activities and content of protein involved in pH regulation. Skeletal muscle of MCT1^+/- mice had lower MCT1 (-61%; P < 0.05) and carbonic anhydrase (CA)-II (-54%; P < 0.05) contents. Although intramuscular pH was higher in MCT1^+/- mice at rest (P < 0.001), the mice showed higher acidosis during the first minute of exercise (P < 0.01). Then, the pH time course was similar among groups until exercise completion. MCT1^+/- mice had higher specific peak (P < 0.05) and maximum tetanic (P < 0.01) forces and lower fatigability (P < 0.001) when compared to WT mice. We conclude that both MCT1 and CAII are involved in the homeostatic control of pH in skeletal muscle, both at rest and at the onset of exercise. The improved muscle function and resistance to fatigue in MCT1^+/- mice remain unexplained.-Chatel, B., Bendahan, D., Hourdé, C., Pellerin, L., Lengacher, S., Magistretti, P., Fur, Y. L., Vilmen, C., Bernard, M., Messonnier, L. A. Role of MCT1 and CAII in skeletal muscle pH homeostasis, energetics, and function: in vivo insights from MCT1 haploinsufficient mice.

Limitations in intense exercise performance of athletes - effect of speed endurance training on ion handling and fatigue development

Mechanisms underlying fatigue development and limitations for performance during intense exercise have been intensively studied during the past couple of decades. Fatigue development may involve several interacting factors and depends on type of exercise undertaken and training level of the individual. Intense exercise (½-6 min) causes major ionic perturbations (Ca²⁺ , Cl^- , H⁺ , K⁺ , lactate^- and Na⁺ ) that may reduce sarcolemmal excitability, Ca²⁺ release and force production of skeletal muscle. Maintenance of ion homeostasis is thus essential to sustain force production and power output during intense exercise. Regular speed endurance training (SET), i.e. exercise performed at intensities above that corresponding to maximum oxygen consumption (V̇O2, max ), enhances intense exercise performance. However, most of the studies that have provided mechanistic insight into the beneficial effects of SET have been conducted in untrained and recreationally active individuals, making extrapolation towards athletes' performance difficult. Nevertheless, recent studies indicate that only a few weeks of SET enhances intense exercise performance in highly trained individuals. In these studies, the enhanced performance was not associated with changes in V̇O2, max and muscle oxidative capacity, but rather with adaptations in muscle ion handling, including lowered interstitial concentrations of K⁺ during and in recovery from intense exercise, improved lactate^- -H⁺ transport and H⁺ regulation, and enhanced Ca²⁺ release function. The purpose of this Topical Review is to provide an overview of the effect of SET and to discuss potential mechanisms underlying enhancements in performance induced by SET in already well-trained individuals with special emphasis on ion handling in skeletal muscle.

Fluid transport by the cornea endothelium is dependent on buffering lactic acid efflux

Maintenance of corneal hydration is dependent on the active transport properties of the corneal endothelium. We tested the hypothesis that lactic acid efflux, facilitated by buffering, is a component of the endothelial fluid pump. Rabbit corneas were perfused with bicarbonate-rich (BR) or bicarbonate-free (BF) Ringer of varying buffering power, while corneal thickness was measured. Perfusate was collected and analyzed for lactate efflux. In BF with no added HEPES, the maximal corneal swelling rate was 30.0 ± 4.1 μm/h compared with 5.2 ± 0.9 μm/h in BR. Corneal swelling decreased directly with [HEPES], such that with 60 mM HEPES corneas swelled at 7.5 ± 1.6 μm/h. Perfusate [lactate] increased directly with [HEPES]. Similarly, reducing the [HCO3 (-)] increased corneal swelling and decreased lactate efflux. Corneal swelling was inversely related to Ringer buffering power (β), whereas lactate efflux was directly related to β. Ouabain (100 μM) produced maximal swelling and reduction in lactate efflux, whereas carbonic anhydrase inhibition and an monocarboxylic acid transporter 1 inhibitor produced intermediate swelling and decreases in lactate efflux. Conversely, 10 μM adenosine reduced the swelling rate to 4.2 ± 0.8 μm/h and increased lactate efflux by 25%. We found a strong inverse relation between corneal swelling and lactate efflux (r = 0.98, P < 0.0001). Introducing lactate in the Ringer transiently increased corneal thickness, reaching a steady state (0 ± 0.6 μm/h) within 90 min. We conclude that corneal endothelial function does not have an absolute requirement for bicarbonate; rather it requires a perfusing solution with high buffering power. This facilitates lactic acid efflux, which is directly linked to water efflux, indicating that lactate flux is a component of the corneal endothelial pump.

The Role of Monocarboxylate Transporters and Their Chaperone CD147 in Lactate Efflux Inhibition and the Anticancer Effects of Terminalia chebula in Neuroblastoma Cell Line N2-A

AIMS

In the presence of oxygen, most of the synthesized pyruvate during glycolysis in the cancer cell of solid tumors is released away from the mitochondria to form lactate (Warburg Effect). To maintain cell homeostasis, lactate is transported across the cell membrane by monocarboxylate transporters (MCTs). The major aim of the current investigation is to identify novel compounds that inhibit lactate efflux that may lead to identifying effective targets for cancer treatment.

STUDY DESIGN

In this study, 900 ethanol plant extracts were screened for their lactate efflux inhibition using neuroblastoma (N2-A) cell line. Additionally, we investigated the mechanism of inhibition for the most potent plant extract regarding monocarboxylate transporters expression, and consequences effects on viability, growth, and apoptosis.

METHODOLOGY

The potency of lactate efflux inhibition of ethanol plant extracts was evaluated in N2-A cells by measuring extracellular lactate levels. Caspase 3- activity and acridine orange/ethidium bromide staining were performed to assess the apoptotic effect. The antiproliferative effect was measured using WST assay. Western blotting was performed to quantify protein expression of MCTs and their chaperone CD147 in treated cells lysates.

RESULTS

Terminalia chebula plant extract was the most potent lactate efflux inhibitor in N2-A cells among the 900 - tested plant extracts. The results obtained show that extract of Terminalia chebula fruits (TCE) significantly (P = 0.05) reduced the expression of the MCT1, MCT3, MCT4 and the chaperone CD147. The plant extract was more potent (IC₅₀ of 3.59 ± 0.26 μg/ml) than the MCT standard inhibitor phloretin (IC₅₀ 76.54 ± 3.19 μg/ml). The extract also showed more potency and selective cytotoxicity in cancer cells than DI-TNC1 primary cell line (IC₅₀ 7.37 ± 0.28 vs. 17.35 ± 0.19 μg/ml). Moreover, TCE Inhibited N2-A cell growth (IG₅₀ = 5.20 ± 0.30 μg/ml) and induced apoptosis at the 7.5 μg/ml concentration.

CONCLUSION

Out of the 900 plant extracts screened, Terminalia chebula ethanol extract was found to be the most potent lactate efflux inhibitor with the ability to inhibit chaperone CD147 expression and impact the function of monocarboxylate transporters. Furthermore, TCE was found to have growth inhibition and apoptotic effects. The results obtained indicate that Terminalia chebula constituent(s) may contain promising compounds that can be useful in the management of neuroblastoma cancer.