Conditioning-induced expression of novel glucose transporters in canine skeletal muscle homogenate

Athletic conditioning can increase the capacity for insulin-stimulated skeletal muscle glucose uptake through increased sarcolemmal expression of GLUT4 and potentially additional novel glucose transporters. We used a canine model that has previously demonstrated conditioning-induced increases in basal, insulin- and contraction-stimulated glucose uptake to identify whether expression of glucose transporters other than GLUT4 was upregulated by athletic conditioning. Skeletal muscle biopsies were obtained from 12 adult Alaskan Husky racing sled dogs before and after a full season of conditioning and racing, and homogenates from those biopsies were assayed for expression of GLUT1, GLUT3, GLUT4, GLUT6, GLUT8, and GLUT12 using western blots. Athletic conditioning resulted in a 1.31 ± 0.70 fold increase in GLUT1 (p <0.0001), 1.80 ± 1.99 fold increase in GLUT4 (p = 0.005), and 2.46 ± 2.39 fold increase in GLUT12 (p = 0.002). The increased expression of GLUT1 helps explain the previous findings of conditioning-induced increases in basal glucose clearance in this model, and the increase in GLUT12 provides an alternative mechanism for insulin- and contraction-mediated glucose uptake and likely contributes to the substantial conditioning-induced increases in insulin sensitivity in highly trained athletic dogs. Furthermore, these results suggest that athletic dogs can serve as a valuable resource for the study of alternative glucose transport mechanisms in higher mammals.

Acylcarnitine profile in Alaskan sled dogs during submaximal multiday exercise points out metabolic flexibility and liver role in energy metabolism

Alaskan sled dogs develop a particular metabolic strategy during multiday submaximal exercise, allowing them to switch from intra-muscular to extra-muscular energy substrates thus postponing fatigue. Specifically, a progressively increasing stimulus for hepatic glycogenolysis and gluconeogenesis provides glucose for both fueling exercise and replenishing the depleted muscle glycogen. Moreover, recent studies have shown that with continuation of exercise sled dogs increase their insulin-sensitivity and their capacity to transport and oxidize glucose and carbohydrates rather than oxidizing fatty acids. Carnitine and acylcarnitines (AC) play an essential role as metabolic regulators in both fat and glucose metabolism; they serve as biomarkers in different species in both physiologic and pathologic conditions. We assessed the effect of multiday exercise in conditioned sled dogs on plasma short (SC), medium (MC) and long (LC) chain AC by tandem mass spectrometry (MS/MS). Our results show chain-specific modification of AC profiles during the exercise challenge: LCACs maintained a steady increase throughout exercise, some SCACs increased during the last phase of exercise and acetylcarnitine (C2) initially increased before decreasing during the later phase of exercise. We speculated that SCACs kinetics could reflect an increased protein catabolism and C2 pattern could reflect its hepatic uptake for energy-generating purposes to sustain gluconeogenesis. LCACs may be exported by muscle to avoid their accumulation to preserve glucose oxidation and insulin-sensitivity or they could be distributed by liver as energy substrates. These findings, although representing a “snapshot” of blood as a crossing point between different organs, shed further light on sled dogs metabolism that is liver-centric and more carbohydrate-dependent than fat-dependent and during prolonged submaximal exercise.

Effect of conditioning and physiological hyperthermia on canine skeletal muscle mitochondrial oxygen consumption

Exercise often causes skeletal muscle hyperthermia, likely resulting in decreased efficiency of mitochondrial respiration. We hypothesized that athletic conditioning would improve mitochondrial tolerance to hyperthermia. Skeletal muscle biopsies were obtained from six Alaskan sled dogs under light general anesthesia before and after a full season of conditioning and racing, and respiration of permeabilized muscle fibers was measured at 38, 40, 42, and 44°C. There was no effect of temperature on phosphorylating respiration, and athletic conditioning increased maximal phosphorylating respiration by 19%. Leak respiration increased and calculated efficiency of oxidative phosphorylation decreased with increasing incubation temperature, and athletic conditioning resulted in higher leak respiration and lower calculated oxidative phosphorylation efficiency at all temperatures. Conditioning increased skeletal muscle expression of putative mitochondrial leak pathways adenine nucleotide transporter 1 and uncoupling protein 3, both of which were correlated with the magnitude of leak respiration. We conclude that athletic conditioning in elite canine endurance athletes results in increased capacity for mitochondrial proton leak that potentially reduces maximal mitochondrial membrane potential during periods of high oxidative phosphorylation. This effect may provide a mechanistic explanation for previously reported decreases in exercise-induced muscle damage in well-conditioned subjects.NEW & NOTEWORTHY Athletic conditioning is expected to increase exercise capacity through improved function of cardiopulmonary and musculoskeletal tissues. Our finding of decreased calculated efficiency of skeletal muscle mitochondria in one of the premier mammalian athletes suggests that this mandate for improved function may take the form of sacrificing capacity for maximal oxidative phosphorylation to minimize exercise-induced muscle damage caused by mitochondrial oxidative stress.

Glucocentric Metabolism in Ultra-Endurance Sled Dogs

The total energy expenditure of a racing sled dog can exceed 500 kcal/kg bodyweight during typical events spanning a week or more. Based on early research, as well as practical considerations, modern commercial diets are formulated to meet these energy requirements using fat and protein. However, more recent research provides a clear picture of a canine athlete with glucocentric metabolism, including many conditioning-induced adaptations oriented toward sustaining submaximal exercise with glucose as the primary metabolic fuel despite the consumption of fat as the primary dietary energy source. The specific strategies used by racing sled dogs to maintain a robust supply of glucose during exercise, as well as the possible role of fat in facilitating that metabolic priority, is the subject of ongoing research.

Effect of Endurance Conditioning on Insulin-mediated Glucose Clearance in Dogs

INTRODUCTION

Physical activity has been shown to improve insulin sensitivity in subjects with insulin resistance, but the effect of athletic conditioning on subjects with normal insulin sensitivity has received less scrutiny. Because strenuous exercise can be limited by the availability of substrates, it is reasonable to hypothesize that conditioning would increase the capacity for muscle uptake of substrates like glucose and to the extent that improvement in this process would include upregulation of the portions of the glucose uptake pathway in muscle, this increased capacity would also be reflected in insulin sensitivity. Therefore, we tested the hypothesis that conditioning for endurance exercise would result in increased insulin sensitivity using elite racing sled dogs.

METHODS

A frequent-sampled intravenous glucose tolerance test was performed on these dogs before and after a full 7-month season of conditioning in preparation for a 1600-km race.

RESULTS

Compared with the results in unconditioned dogs, conditioned dogs rapidly cleared the intravenous glucose bolus through increases in both glucose mediated (7.6%·min ± 3.4%·min vs 3.0%·min ± 2.2%·min, P = 0.008) and insulin-mediated (36.3 ± 18.4 × 10 L·min·mU vs 11.5 ± 8.0 × 10 L·min·mU, P = 0.007) mechanisms. The more modest increase in serum insulin after the intravenous glucose bolus in conditioned dogs failed to suppress lipolysis and serum concentrations of nonesterified fatty acids remained constant in the conditioned dogs throughout the 4-h test.

CONCLUSIONS

These results, in particular the increase in insulin-independent peripheral uptake of glucose, describe novel alterations in metabolism induced by athletic conditioning that arguably result in near-continuous provision of oxidizable substrates to peripheral muscle in support of sustained muscular work typical of these dogs.

Effect of conditioning on exercise-induced hyperthermia and post-exercise cooling in dogs

Our objective was to evaluate the effect of conditioning and hypoxia on rectal and gastrointestinal temperature changes in dogs exercising at cold ambient temperature. Six Alaskan Husky sled dogs, each in a physically conditioned and unconditioned state, were used in the prospective study. Dogs in peak physical condition were run untethered on a treadmill under normoxic and hypoxic conditions of 20 and 12.5% environmental oxygen concentration, respectively, on separate days. After undergoing a deconditioning period of four months, the same dogs were run again under the same environmental conditions of 20 and 12.5% O2. Body temperature measurements were obtained via digital rectal thermometer and ingestible gastrointestinal thermistor at baseline, every 5 min for 30 min of exercise, and for 15 min following cessation of exercise. Under hypoxic conditions, peak gastrointestinal temperature was lower in conditioned vs unconditioned dogs. Gastrointestinal cooling was faster in conditioned dogs under normoxic conditions only. There was no difference in the peak rectal temperature, or rate of rectal temperature cooling in either normoxic or hypoxic conditions. 3 of 6 (50%) of the conditioned dogs reached a plateau temperature after approximately 20 to 25 min. This was observed less frequently when the dogs were unconditioned. Gastrointestinal and rectal temperatures reacted differently to exercise depending on conditioning or environmental oxygen concentration and this suggests that they cannot be used interchangeably to assess body temperature. Under these conditions, conditioning had no effect on rectal temperature. Presence of a plateau effect of rectal or gastrointestinal temperature may demonstrate a thermoregulatory balance of heat production and heat loss and may be a helpful measure in assessing individual dogs’ level of conditioning with regard to resistance to thermal stress.