The study of metabolism by exercise physiologist began in the 1960’s in laboratories of Bengt Saltin (Stockholm), Eric Hultman (Stockholm), Lars Hermansen (Oslo) and in the US in the laboratories of John Holloszy (St. Louis, Missouri), Charles Tipton (Iowa City, Iowa) and Phil Gollnick (Pullman, Washington). Interest in metabolism, in skeletal muscle was facilitated in humans by the re-introduction of the needle biopsy technique in Scandinavia and by animal studies in the US. Quite quickly, both types of approaches were employed in Europe and North America, and by the end of the 1970s a considerable knowledge-base had developed with respect to the development of exercise-induced muscle (and blood) lactate accumulation and glycogen utilization, and metabolic adaptation to exercise training. Knowledge of the utilization of other substrates by exercising muscle, namely glucose and fatty acids, began in the late 1970s and early 1980s. Studies of exercise-induced alterations in metabolism in other key metabolic organs (liver and adipose tissue) began somewhat later.
In the first decade (approximately 1965-1975) of experimental work in metabolic exercise physiology, the studies were necessarily largely descriptive, as it was completely unknown as to what metabolic effects were provoked by exercise. This “descriptive knowledge” was then used to design experimental work in later years to uncover the mechanisms involved in altering substrate metabolism in selected organs and tissues, using biochemical approaches in the 1980s and 1990s, as well molecular approaches (1990s onward).
The purpose of the historical overview in metabolism is to draw attention to key ideas and methodologies that have occurred over the past 50 years, and the contributions by Canadian scientists, as well as colleagues elsewhere, to some of these areas. Due to space limitations this overview will be confined to broad understandings of the regulatory aspects of substrate metabolism in skeletal muscle.
Glycogen utilization and replenishment
Work in Scandinavia in the 1960s and early 1970s showed that in humans muscle energy provision during exercise was provided by the breakdown of glycogen to form ATP for muscle contraction (Bengt Saltin, Eric Hultman, Lars Hermansen), and that replenishment of muscle glycogen was facilitated by consuming diets high in carbohydrates (Hultman). Studies of glycogen metabolism during aerobic exercise were also undertaken by groups at Waterloo (largely by Howie Green, in collaboration with Mike Houston and Jay Thompson) and Guelph (Lawrence Spriet and Terry Graham). The latter group also examined how caffeine influenced muscle glycogen use, an effect that had implications for high performance athletes. Studies on high intensity exercise and glycogen use were conducted in the UK (Clyde Williams, Loughborough), Denmark (Jens Bangsbo and Bengt Saltin) and Hamilton (Duncan McDougall, John Sutton).
Collectively, these studies by Scandinavian, US and Canadian scientists led to an understanding of when muscle fibers were recruited during exercise at different intensities, and optimal means to replenish muscle glycogen after exercise. Work in the last decade, by Terry Graham (Guelph), Mark Tarnopolsky (Hamilton), and Niels Ortenblad (Denmark) has highlighted that subcellular pools of glycogen exist and appear to serve different metabolic processes. Furthermore, Clara Prats (Denmark) demonstrated that key regulatory enzymes associated with the glycogen granules can translocate to different locales within muscle due to altered metabolic demands.
In the mid-1960s, John Holloszy (St. Louis) showed that muscle contraction facilitated glucose uptake by skeletal muscle, a process that had until then only been thought to be caused solely by insulin. Somewhat later a number of studies showed that 80% of glucose taken into the circulation was destined to be taken up into skeletal muscle, Holloszy’s pivotal studies were a key starting point for examining the mechanism regulating glucose uptake by muscle. However, many laboratories worldwide focused on this process in other tissues, primarily adipocytes. To this end, in the 1970s a massive amount of work was directed at examining altered binding of insulin to its receptor, as this was thought to be key to altering insulin-mediated glucose uptake. How exercise affected insulin binding in muscle was marred by studies using non-specific targets (monocytes) as well as scandal (i.e. data manipulation in Phil Felig’s laboratory).
In Canada in the early 1980s Arend Bonen’s group (Halifax) was the first to show that insulin receptor quantities differed in muscle (red >>white) and that in exercised skeletal muscle insulin binding to its receptor was not altered. Therefore, during exercise altered insulin binding was not central for increasing glucose uptake. At about the same time, the so-called “local factor” identified by Erik Richter (Copenhagen) that stimulated glucose uptake in exercising muscle was shown in Holloszy’s laboratory to be the glucose transport system. In the 1980s Sam Cushman’s group at the NIH (Washington DC), Laurie Goodyear (Burlington, VT) and Amira Klip’s laboratory (Toronto) showed that this system involved a transport protein that was translocated from an intracellular storage compartment to the cell surface, either by insulin or by muscle contraction. Shortly thereafter (late 1980s and early 1990s), this protein was identified by David James (Boston) as the insulin-sensitive glucose transporter 4 (GLUT4), which importantly was also exercise-sensitive.
In the 1990s understanding the regulation of this GLUT4 protein in muscle became important. Groups in the US (Laurie Goodyear) and Canada (Amira Klip, Toronto; Andre Marette, Quebec City; Arend Bonen, Waterloo) showed that GLUT4 expression was dependent on muscle fiber composition (red >> white), and that muscle inactivity or increased activity reduced or increased the protein level of GLUT4 in muscle with concomitant changes in insulin-stimulated glucose transport (Arend Bonen, Waterloo). A key study in Holloszy’s group (John Ivy) showed that in insulin resistant muscle of obese animals insulin stimulation of GLUT4 was impaired, but that exercise-induced GLUT4 translocation was completely normal.
In more recent years (mid-1990s onward) considerable attention has been devoted to signaling pathways involved in the recruitment of GLUT4 to cell surface. This is an extremely complex process (beyond the scope of the present overview), which for insulin-signaling has been largely deciphered from studies in selected cell lines by groups world-wide and Canada (Amira Klip, Toronto). Observations from these studies have been extrapolated to, and tested in muscle of genetically altered animals. In contrast to the well-known insulin signaling pathways involved in recruiting GLUT4 to the cell surface, the contraction-signaling pathway(s) for GLUT4 remain obscure.
The many years of basic research in this area have brought us to understanding that exercise can be used as a prophylactic and therapeutic instrument to combat systemic insulin resistance, which is largely due to insulin resistance in skeletal muscle due to its large mass (~40% of body weight). Indeed, a large prospective study has shown that mild exercise is twice as effective as a course of drug treatment in preventing the onset of type 2 diabetes.
Fatty acids are another key substrate that provides energy for exercising muscle. Fatty acids for muscle energy provision are largely derived from circulating fatty acids released from their storage in adipose tissue. However intramuscular triacylglycerol can also be broken down to provide for oxidation fatty acids within muscle. In the 1990s this was clearly illustrated via the hindlimb perfusion studies (Lorraine Turcotte, Los Angeles; Arend Bonen Waterloo) and the introduction of “pulse-chase” studies using 14C-labeled and 3H-labeled fatty acids in isolated contracting muscle (David Dyck and Arend Bonen at Waterloo and Guelph). This latter experimental approach provided the means to examine in muscle fatty acid oxidation dynamically and simultaneously from exogenous fatty acids and triacylglycerol breakdown, rather than relying on changes in muscle enzyme activities and/or circulating levels of fatty acids, as had been the case for many years. With this model it was shown that intramuscular triacylglycerol is simultaneously synthesized and hydrolyzed during contractions and that insulin stimulates triacylglycerol synthesis (David Dyck and Arend Bonen, Waterloo), and the key role for leptin-stimulation of AMPK activation to increase fatty acid oxidation (David Dyck, Guelph). a process that is FAT/CD36-dependent (see next paragraph) (Arend Bonen, Guelph).
Novel, albeit initially very controversial work, from the mid-1990s onward by Arend Bonen (at Waterloo and thereafter at Guelph) convincingly established that fatty acid entry into muscle is not simply a diffusive process, as had been believed for most of the 20th century. This Canadian research was the quintessential work, worldwide, for a major paradigm shift in the understanding of fatty acid utilization by skeletal muscle, namely that fatty acid entry into muscle is a highly regulated, protein-mediated process involving at least one fatty acid binding protein (FAT/CD36), and possibly as yet unknown other proteins. Essentially, acute metabolic perturbations (muscle contraction, insulin, leptin) activate various signaling mechanisms induce FAT/CD36 translocation to the sarcolemma. This transport system is central to the well-known increases in fatty acid oxidation that occur with exercise training. Moreover, this transport system, via a permanent relocation of FAT/CD36 to the sarcolemma, also correlates highly with the increased intramuscular lipid deposition, observed in animal models of obesity, and in human obesity and type 2 diabetes, which has been associated with the observed insulin resistance in muscle of these individuals.
Since about 2010 work on fatty acid breakdown in adipose tissue during exercise has begun by David Wright (Guelph). At the same time, Graham Holloway (Guelph), Greg Steinburg and Mark Tarnopolsky (Hamilton) have provided approaches to study the regulation of mitochondrial fatty acid oxidation. Concurrently, work in Copenhagen (Jorgen Wojtaszewski) and Hamilton (Greg Steinberg) is furthering understanding of fatty acid oxidation by AMPK.
Taken collectively, understandings of muscle lipid metabolism has progressed primarily in the past 20 years due the introduction of new approaches and questioning of extant dogma.
Production of lactate by exercising muscle has fascinated physiologists since the early 1800s. The popular notion that lactate “causes” muscle fatigue has never been scientifically demonstrated.
Many studies have shown that lactate production increases directly with increasing exercise intensity, particularly when exercise is highly anaerobic. However, work in the 1970s (Terry Graham, Kingston and Guelph; Graham Mainwood, Ottawa; Bruce Gladden, Knoxville TN; Jan Karlsson Stockholm) showed that blood lactate did not mirror the dynamic lactate metabolism observed in muscle, that lactate formation was required to regenerate very modest concentrations of cytosolic NAD that is vital for maintaining glycolysis, and that muscle can also oxidize lactate from intramuscular accumulation as well as from the circulation. However, reconverting lactate to glucose and/or glycogen, as occurs in liver, does not occur to a meaningful extent in muscle (John Holloszy, St. Louis; Arend Bonen, Halifax). The controversial notion by George Brooks (Berkely, CA) that lactate was converted to pyruvate within mitochondria, rather than in the cytosol where 99% of the LDH is located, has been proven to be untenable (Arend Bonen, Guelph).
Molecular work in the mid-1990s by Andrew Halestrap (Bristol, UK) revealed the existence of transport proteins for lactate. Subsequently, Arend Bonen (Waterloo and Guelph) found that muscle co-expressed monocarboxylate transporters (MCT) that were associated with their oxidative (MCT1) or glycolytic capacity (MCT4), suggesting that MCT1 functions to take up lactate from the circulation and MCT4 extrudes it into the circulation. Definitive proof for these suggestions remains to be established.
Amino acids are not normally viewed as a significant energy source, and yet Mark Tarnopolsky (Hamilton) reported that the protein requirements for endurance athletes were greater than that for resistance athletes and sedentary adults. However, during exercise blood ammonia increased during exercise (Ron Terjung, Syracuse NY). In exercising human muscle it was shown by Terry Graham (Guelph) in collaboration with Bengt Saltin and Erik Richter (Copenhagen) that this was due to the release of both essential and nonessential amino acids. Subsequently, Martin Gibala (Hamilton) showed that there was a large anaplerosis during exercise, and while some tricarboxylic acid (TCA) intermediates increased far more than others, there was no evidence that this was a regulator of metabolism but rather the result of increased oxidation.
Future work in the understandings of muscle metabolism will increasingly rely on combining modern molecular approaches with classic, functional dynamic assessments of substrate utilization and/or storage in skeletal muscle.