The skill levels of the dogs reported in this study was determined by the rules set out by the USDAA and may have seemed arbitrary. The groupings proved accurate in these dogs, however, since each successive skill group was significantly faster than the novice group and the group level below it. The labeling of these groups as novice, intermediate and masters could have been labeled instead by their time through the simulated exercise test, however, this would have been less applicable to dogs in actual agility competition.
Agility exercise, as performed by dogs in this study, is of sufficient intensity to induce mild oxidant stress and lipid peroxidation based upon the slightly increased urinary excretion of ISO following the exercise test. The intensity of muscle contraction associated with agility exercise increases production of reactive oxygen species due to increased oxygen consumption, resulting in elevated superoxide anion production in skeletal muscle mitochondria
. All dogs in this study had similar increases in ISO regardless of skill level and speed through the course, indicating that agility exercise routinely stimulates intense muscle contraction and results in mild lipid peroxidation. It is unlikely, given the results reported here, that the mild oxidant stress encountered in a single agility exercise session has a significant effect on physiologic function and performance. Previous reports have documented oxidative stress in dogs exercising for 20 minutes (hunting dogs) and many hours (sled dogs), however, this is the first report, to the authors’ knowledge, of the effect of very short duration, intense exercise on lipid peroxidation and oxidation in the dog
[13, 20, 21]. Many agility dogs participate in more than one trial in a single day or weekend. In humans, intense exercise elevates urinary ISO excretion for 24 hours, and following a soccer game, humans experience oxidant stress and reduced performance for up to 72 hours
[12, 22]. Muscle fatigue due to oxidant stress may be a contributing factor to joint injuries, at least in humans
[23, 24]. Whether oxidant stress plays a role in injury to dogs during agility competition with repeated runs through a course in a single day remains unknown, however determining the level of oxidant stress that develops over time and the length of time before isoprostane production returns to pre-exercise levels in actual agility competition (rather than simulated as reported here) may be warranted based upon the results of this study and the high incidence of injury following participation in this activity
. Collection of urine at more precise time points following the agility exercise may have altered the results in our study by reducing variability between the subjects. Future research determining other markers of oxidant stress at exact time points prior to and following agility exercise is indicated by the results of this study.
A significant limitation of this study is the use of an EIA for determination of F2 isoprostanes in the urine of dogs. Analysis of the methods used to determine urinary F2 isoprostane excretion have found that EIA results do not correlate with gas chromatography negative ion chemical ionization-mass spectroscopy (GC/NICI-MS, considered the gold standard) in dogs
. The results of this study cannot be compared to other studies, however, mildly increased lipid peroxidation following the intense, short-duration exercise of agility competition occurred in the dogs reported here and was not affected by the speed or training of the dogs. Further research, preferably using GC/NICI-MS for urinary F2 isoprostane determination, is required to determine the extent of oxidant stress induced by this type of exercise in relation to long duration exercise in sled dogs and to disease states including lung injury
, neurologic disease
 and heart disease
. Future research is warranted to determine the effect of agility exercise on other parameters of oxidant stress including glutathione, glutathione-peroxidase, catalase, superoxide dismutase.
Another limitation of this study is the lack of data regarding antioxidant status in these dogs. Antioxidants are either supplements or endogenous mechanisms of consumption or removal of reactive oxygen species and include superoxide dismutase, catalase, glutathioperoxidase, glutathione reductase, vitamin E, vitamin C, and ubiquinone
. Hunting dogs exercised for 20 minutes to 4 hours duration decrease their biological antioxidant potential following the exercise but the return of this potential is within one hour
. Determination of antioxidant status may be warranted in future studies of dogs participating in agility exercise.
The plasma lactate increased markedly in the dogs following the exercise test. The changes in plasma lactate were inversely proportional to the speed through the course (and proportional to increasing skill level). Plasma lactate concentration at a specific speed or level of intensity is strongly correlated to physical fitness in horses as well as human athletes
[30, 31]. In the current study, all three skill groups had normal plasma lactate concentrations by 4 hours after exercise, indicating that recovery from lactic acidosis in agility dogs occurs quickly, similar to other athletic events, and before recovery from oxidative stress
In Greyhounds following high intensity sprint exercise, plasma lactate increased to 29.3 meq/L or more, and similar to the dogs reported here, returns to baseline in less than 4 hours following the exercise
[32–35]. During field trial competition, Labrador retrievers also increase plasma lactate concentrations similar to the masters level dogs in this study
[36–38]. Increased plasma lactate was most likely the result of anaerobic metabolism in contracting muscles during the sprint exercise
. Studies of the Greyhound have indicated that this breed has higher concentrations of type II fast twitch muscle fibers
. These fibers have the greatest glycolytic/anaerobic capacity and therefore are capable of producing large amounts of lactate during high intensity exercise
. The relative amount of muscle fiber types present in the dogs in this study is unknown, however, a higher percentage of type II fibers may have resulted in greater increases in plasma lactate following the exercise test and significantly shorter times through the agility course. Elite sprinters in men and horses have higher proportions of type II fibers than their slower counterparts and plasma lactate has been shown to correlate with racing speed in thoroughbred horses and female cyclists
Lactic acidosis and oxidant stress may be causes of muscle fatigue
. The mechanism by which reactive oxygen species cause muscle fatigue during intense exercise is not fully understood, however, supplementation with the antioxidant, N-acetylcysteine alleviates muscle fatigue in humans and animals
[46–49]. Muscle fatigue due to oxidant stress may be a contributing factor to joint injuries, at least in humans
[23, 24]. Increasing lactate production by high intensity-contracting muscle will result in decreased ionized calcium release from the sarcolemma and contribute to muscle fatigue
. Muscle fatigue has been linked to increased bone strain in dogs experimentally, and may contribute to the development of stress fractures
[51, 52]. Further research is warranted to determine the muscle fiber types of elite agility competitors and whether, in competition, plasma lactate correlates with injury rates in these dogs.
Platelets increased immediately following the agility exercise but by 4 hours later had returned to values similar to pre-exercise; and at all time-points, values were within reference range for the laboratory (200-900 × 1000/μl). In humans, increased platelet count occurs with intense exercise and are thought to be mobilized from the spleen, bone marrow and lungs, but they may also increase due to hemoconcentration from respiratory water loss as a result of panting
[53–55]. The small reduction in platelet counts following exercise may be due to plasma volume expansion that occurs during recovery or reduced catecholamine-induced platelet release
Similar to previous reports in agility dogs, increases in red blood cell count, hemoglobin and hematocrit occurred in all groups immediately following exercise
[3, 4]. Increases in these variables without a rise in total plasma protein is consistent with splenic contraction with similar findings reported in Greyhounds after racing
. RBC count, HCT, and hemoglobin concentrations were decreased at 4-hours following the agility exercise test. In humans, during the hours following exercise, there is an increase in plasma volume termed “autohemodilution” resulting in decreased concentrations of RBC’s, HCT and hemoglobin
[57–59]. This phenomenon may explain the decreased red blood cell parameters in the dogs reported here at 4-hours following exercise without alteration in protein or albumin concentrations as fluid from the interstitial space moves into the vascular space. To the authors’ knowledge, this is the first report indicating “autohemodilution” in dogs and further research is needed. Plasma albumin concentration increased immediately after exercise (P<0.01) but remained within reference range and returned to pre-exercise values within 4 hours, Table
1. No other alterations in hematological or biochemical parameters were found.
Increased TXB2 production without concomitant increase in prostaglandin E2 or PGF1α as reported here in agility dogs has also been reported in horses following treadmill exercise
. In endurance exercise, horses may also increase 6-keto-prostaglandin F1α production, however this did not occur in the dogs of this study where the duration of exercise lasted less than 20 minutes
. The dogs with the greatest skill level (and fastest run time) had a significantly greater increase in TXB2 and this association has also been reported in horses
. High intensity exercise reportedly stimulates increased TXB2 production whereas submaximal exercise induces increases in PGF1α rather than TXB2 in humans
. Urinary 11-TXB2 has been determined to be a reflection of in vivo platelet activation in humans
. TXB2 in urine includes renal production of thromboxane A2 and may not reflect systemic circulation or platelet production of the unstable prostaglandin, thromboxane A2. In this study both TXB2 and 11-TXB2 increased following exercise and remained increased 4 hours post exercise indicating systemic increases in thromboxane A2 occurred in the dogs. Interestingly, only TXB2 was significantly increased when skill levels were examined and not 11-TXB2 which may indicate greater renal production of thromboxane A2 only in the fastest dogs. Potential sources of TXB2 may include endothelial cells activated by shear stress or catecholamine activated platelets, however the source was not determined in this study. Future research to determine why improved performance is associated with increased TXB2 in the urine is warranted.
The dogs participating in the exercise test were not controlled for breed, age, sex, or nutritional status. Restriction of these variables was not enforced since a very wide variety of dogs participate in agility sports and many of them go on to injure themselves in this exercise; therefore, we sought to ascertain the degree of physiological alteration that occurs in a variety of agility dogs. Future research may limit the participants by breed etc. and investigate dogs during actual agility competition rather than a simulated test.
Agility competition in dogs has become a popular sport with a high incidence of injury
. Development of methods to reduce increases in urinary TXB2 and plasma lactate in elite canine athletes warrants investigation and determination of their effects on injury rates in dogs is a possible next step in this research. Possible methods to reduce these alterations might include improved maintenance of hydration with readily accessible water during agility exercise, adequate warm-up and cool-down exercise prior to participation in any agility exercise to improve musculoskeletal blood flow, and training methods that increase aerobic muscle capacity. Further understanding of the physiological responses to this type of intense, short-duration exercise may improve performance and reduce injury in dogs participating in this sport.