After a Century, it's time to turn the page on understanding of lactate metabolism and appreciate that lactate shuttling is an important component of intermediary metabolism in vivo. Cell-Cell and intracellular Lactate Shuttles fulfill purposes of energy substrate production and distribution as well as cell signaling under fully aerobic conditions. Recognition of lactate shuttling came first in studies of physical exercise where the roles of driver (producer) and recipient (consumer) cells and tissues were obvious. Moreover, the presence of lactate shuttling as part of postprandial glucose disposal and satiety signaling has been recognized. Mitochondrial respiration creates the physiological sink for lactate disposal in vivo. Repeated lactate exposure from regular exercise results in adaptive processes such as mitochondrial biogenesis and other healthful circulatory and neurological characteristic such as improved physical work capacity, metabolic flexibility, learning, and memory. The importance of lactate and lactate shuttling in healthful living is further emphasized when lactate signaling and shuttling are dysregulated as occur in particular illnesses and injuries. Like a Phoenix, lactate has risen to major importance in 21 st Century Biology.

No longer viewed as a metabolic waste product and cause of muscle fatigue, a contemporary view incorporates the roles of lactate in metabolism, sensing and signaling in normal as well as pathophysiological conditions. Lactate exists in millimolar concentrations in muscle, blood and other tissues and can rise more than an order of magnitude as the result of increased production and clearance limitations. Lactate exerts its powerful driver-like influence by mass action, redox change, allosteric binding, and other mechanisms described in this article. Depending on the condition, such as during rest and exercise, following injury, or pathology, lactate can serve as a myokine or exerkine with autocrine-, paracrine-, and endocrine-like functions that have important basic and translational implications. For instance, lactate signaling is: involved in reproductive biology, fueling the heart, muscle and brain, controlling cardiac output and breathing, growth and development, and a treatment for inflammatory conditions. Ironically, lactate can be disruptive of normal processes such as insulin secretion when insertion of lactate transporters into pancreatic Beta-cell membranes is not suppressed and in carcinogenesis. Lactate signaling is important in areas of intermediary metabolism, redox biology, mitochondrial biogenesis, cardiovascular and pulmonary regulation, genomics, neurobiology, gut physiology, appetite regulation, nutrition and overall health and vigor. The various roles of lactate as a myokine and exerkine are reviewed.

Isotope tracer infusion studies employing lactate, glucose, glycerol, and fatty acid isotope tracers were central to the deduction and demonstration of the Lactate Shuttle at the whole-body level. In concert with the ability to perform tissue metabolite concentration measurements, as well as determinations of unidirectional and net metabolite exchanges by means of arterial–venous difference (a-v) and blood flow measurements across tissue beds including skeletal muscle, the heart and the brain, lactate shuttling within organs and tissues was made evident. From an extensive body of work on men and women, resting or exercising, before or after endurance training, at sea level or high altitude, we now know that Organ–Organ, Cell–Cell, and Intracellular Lactate Shuttles operate continuously. By means of lactate shuttling, fuel-energy substrates can be exchanged between producer (driver) cells, such as those in skeletal muscle, and consumer (recipient) cells, such as those in the brain, heart, muscle, liver and kidneys. Within tissues, lactate can be exchanged between white and red fibers within a muscle bed and between astrocytes and neurons in the brain. Within cells, lactate can be exchanged between the cytosol and mitochondria and between the cytosol and peroxisomes. Lactate shuttling between driver and recipient cells depends on concentration gradients created by the mitochondrial respiratory apparatus in recipient cells for oxidative disposal of lactate.

Verification tests to confirm graded exercise test (GXT) V˙O2max are growing in popularity, but the validity and reliability of such testing in the heat remains unknown. Purpose This study aimed to evaluate the validity and reliability of a verification test to confirm GXT V˙O2max in a hot environment. Methods Twelve recreationally trained cyclists completed a two-test protocol that included a GXT progressing 20 W·min−1 followed by a biphasic supramaximal-load verification test (1 min at 60% increasing to 110% maximal GXT wattage until failure) in a hot environment (39°C, 32% relative humidity). Rest between tests occurred in a thermoneutral room and was anchored to the duration required for gastrointestinal temperature to return to baseline. Results Mean verification test V˙O2max (51.3 ± 8.8 mL·kg−1·min−1) was lower than GXT (55.9 ± 7.6 mL·kg−1·min−1, P = 0.02). Verification tests confirmed GXT V˙O2max in 92% of participants using individual analysis thresholds. Bland–Altman analysis revealed a sizable mean bias (−4.6 ± 4.9 mL·kg−1·min−1) with wide 95% limits of agreement (−14.0 to 5.0 mL·kg−1·min−1) across a range of V˙O2max values. The high coefficient of variation (9.6%) and typical error (±3.48 mL·kg−1·min−1) indicate potential issues of test–retest reliability in the heat. Conclusions Verification testing in a hot condition confirmed GXT V˙O2max in virtually all participants, indicating robust utility. To enhance test–retest reliability in this environment, protocol recommendations for work rate and recovery between tests are provided.

The roles of exerkines, factors released from tissues during exercise, in promoting health and longevity were recently addressed in the Review by Chow and colleagues (Chow, L. S. et al. Exerkines in health, resilience and disease. Nat. Rev. Endocrinol. 18, 273-289 (2022) 1 ). However, their timely Review did not adequately describe the major and diverse roles of lactate in regulating metabolism and physiology. Here, we highlight important functions of lactate as an exerkine.

The Lactate Shuttle hypothesis is supported by a variety of techniques including mass spectrometry analytics following infusion of carbon labeled isotopic tracers. However, there has been controversy over whether lactate tracers measure lactate (L) or pyruvate (P) turnover. Here we review the analytical errors, use of inappropriate tissue and animal models, failure to consider L and P pool sizes in modeling results, inappropriate tracer and blood sampling sites, and failure to anticipate roles of heart and lung parenchyma on L:P interactions. With support from magnetic resonance spectroscopy (MRS) and immunocytochemistry we conclude that carbon-labeled lactate tracers can be used to quantitate lactate fluxes.

The verification phase is becoming the norm for confirming VO 2max during a graded exercise test (GXT), but the use of such testing in untrained participants in the heat remains unknown. Purpose: This study aimed to assess the VO 2 uptake obtained during a GXT and subsequent verification phase in untrained participants in a hot environment. Methods: Twelve sedentary males completed a GXT followed by a biphasic supramaximal-load verification phase in a hot environment (39°C, 32% relative humidity). Rest between tests occurred in a temperate chamber and lasted until gastrointestinal temperature returned to baseline. Results: Mean verification phase VO 2max (37.8 ± 4.3 mL•kg −1 •min −1 ) was lower than GXT (39.8 ± 4.1 mL•kg −1 •min −1 ; P = 0.03) and not statistically equivalent. Using an individualized analysis approach, only 17% (2/12) of participants achieved a VO 2 plateau during the GXT. Verification phase confirmed GXT VO 2max in 100% of participants, whereas the traditional and the new age-dependent secondary VO 2max criteria indicated GXT VO 2max achievement at much lower rates (8/12 [67%] vs 7/12 [58%], respectively). Correlational indices between GXT and verification phase VO 2max were strong (intraclass correlation coefficient = 0.95, r = 0.86), and Bland-Altman analysis revealed a low mean bias of −2.1 ± 1.9 mL•kg −1 •min −1 and 95% limits of agreement (−5.8 to 1.7 mL•kg −1 •min −1 ). Conclusions: Very few untrained males achieved a VO 2 plateau during GXT in the heat. When conducting GXT in a hot condition, the verification phase remains a valuable addition to confirm VO 2max in untrained males.

The confirmation of the Intracellular Lactate Shuttle proposed by Brooks and colleagues in 1998 revolutionized the way lactate metabolism was studied in the field of exercise physiology, nutrition, metabolism, and medicine. Previous investigations identified the structure of the mitochondrial Lactate Oxidation Complex (mLOC) which includes MCT1, LDH, COX4, and CD147. While the predominant view in research and textbooks was that pyruvate was imported into the mitochondria to be oxidized it wasn’t until 2009 that Jiang and Colleagues elucidated the mitochondrial pyruvate carrier (mPC). Knowing this, many researchers developed their questions around either pyruvate or lactate metabolism, which may have led to confusion in the field since both substrates play an important role in carbohydrate oxidation. This dilemma led us to the hypothesis that the mPC is in fact an extended member of the mLOC. Mixed skeletal muscle and liver were excised and whole blood was collected post-mortem from (n=5) 4-month-old C57B/l6 mice. Tissues were homogenized in an isotonic solution supplemented with a protease and phosphatase inhibitor. Blood was resuspended in H2O to rupture the cell membranes and the lysates (RBC) were used as negative controls. Mitochondria were isolated by differential centrifugation and resuspended in RIPA buffer. Protein concentrations of tissues were determined using a BCA standard. Western blotting was performed by loading 20 ug of each lysate, separated on an SDS-PAGE gel, and transferred to a PVDF membrane. Membranes were blocked and probed for MCT1(1:1000), mPC1 (1:500), COX4(1:1000), LDHA (1:1000). An SDS-PAGE was repeated and the mPC was excised from the gel. A trypsin digest was performed to isolate the peptides and then analyzed by one-dimensional LCMS/MS at the UC Berkeley QB3 proteomics/mass spectrometry lab. For cDNA preparation, RNA was extracted from 15 mg of skeletal muscle using the Qiagen RNeasy Fibrous tissue kit, reverse transcribed, and concentrated using Quibit ssDNA kit. Quantitative PCR was run on a QuantStudio3 using Power SYBR Green chemistry to confirm the presence of mPC1 in skeletal muscle. Our results revealed that mPC1 was expressed in skeletal muscle along with other genes associated with the mLOC. Immunoblots of mitochondrial lysates contained MCT1, mPC1, LDH, and COX4 in both the skeletal muscle and liver mitochondrial fractions. As expected, MCT1 was also present in RBC lysates as the MCTs are present in both the sarcolemma and mitochondrial membrane. Moreover, LCMS/MS analysis confirmed the presence of mPC1 corroborating our immunoblots. Colocalization using immunocytochemistry confirms the revised mLOC structure. Accordingly, both the pyruvate and lactate transporters assist in maintaining the redox balance and varying rates of carbohydrate oxidation during changing physiological conditions. This promising data has implications for a variety of fields ranging from cancer biology to exercise physiology. NIH Grant #1 R01 AG059715-01 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.