Gastrin, cortisol and CRP are involved in different physiological mechanisms including secretion of gastric acid, stress response and inflammatory reactions. The main findings in this study were that dogs participating in this long distance race had an increase in serum concentration of gastrin, cortisol and CRP. It can be hypothesized that an increase in these parameters to some extent may indicate the general level of stress and inflammation arising during extreme physical exercise, but they may also be markers of gastrointestinal dysfunction.
Gastrin
Sled dogs that participated in a long distance race experienced a significant increase in serum gastrin. Previous human studies, which focused on gastrointestinal dysfunction during long distance races, also concluded that there is an exercise-induced increase in serum concentration of gastrin [8–10, 17].
Exercise-induced elevation in serum gastrin can be a result of several factors or mechanisms. Repetitive blows against visceral organs during running are in bipeds thought to trigger the release of gastrointestinal hormones, including gastrin [8], although this has not been documented in quadrupeds. In addition, renal inactivation of gastrin might be reduced during physical exercise, due to redistribution of blood supply from visceral organs to skeletal muscles [8]. However, a study concluded that renal hypoperfusion was less likely in exercising sled dogs [18]. Release of catecholamines due to physical activity [17] may stimulate the release of gastrin, although a study regarding exercising sled dogs showed minimal systemic release of catecholamines, making this mechanism of gastrin increase less likely [12].
Food intake, along with the expectation and smell of food, stimulates secretion of gastrin [19]. In humans and horses, food intake after physical exercise gives a significantly higher increase in serum gastrin compared to what is registered in fasted individuals [17, 20]. Sled dogs are snacked during racing, and this may give a higher exercise-induced increase of gastrin, similar to the rise seen in humans and horses. In this study, all dogs were supposed to be fasted prior to blood sampling, however the competing dogs might have received food along the trail, and seven non-completing dogs were fed prior to blood sampling. It is also possible that competing dogs were expecting food when approaching the checkpoints and finish line. These factors may contribute to the increase in serum gastrin detected in the competing dogs. Serum gastrin was significantly elevated in non-completing dogs when compared to completing dogs. If this is a result of different feeding patterns or may be associated with illness in non-completing dogs is not known and merits further investigation.
The major pathophysiologic factor causing gastritis and gastric ulceration is currently considered to be breakdown of the mucosal barrier, and not hyperacidity as earlier proposed [21]. Some studies have concluded that there is no decrease in gastric pH in humans or dogs during physical activity [8, 9, 22–24], which emphasizes that hyperacidity is not the causation of exercise-related gastritis and gastric ulceration. Release of various inhibitors, such as somatostatin, has been assumed to limit the release of gastric acid during physical exercise [8, 9].
On the other hand, several studies regarding sled dogs demonstrate a significant reduction in the severity of exercise-induced gastritis and gastric ulceration by preventive use of gastric acid reducing agents, like omeprazole and famotidine [25–27]. This would suggest that gastric acid is a contributing factor to exercise-induced gastritis and gastric ulceration in dogs. Further investigation regarding exercise-induced increase in gastrin related to gastric pH should be done. Use of antacid medication, like omeprazole and famotidine, leads to an increase in serum gastrin concentration in dogs [28]. However, at the time of this study, use of such medications prior to and during race was not allowed in Norway. According to the mushers, none of the dogs were medicated prior to blood sampling during training. Therefore, use of antacids did not influence the gastrin results in this study.
Cortisol
There was a significant increase in serum cortisol in the competing dogs after racing. These findings are in accordance with previous human, horse and canine studies [1, 8, 9, 13, 17, 29]. Also, anticipation of exercise before racing can cause increase in cortisol in sled dogs [30].
A negative energy balance during physical activity may trigger the release of cortisol [1, 12]. Racing sled dogs have high-energy requirements, and may not be capable of sufficiently increasing their food intake to meet these requirements. Racing sled dogs are also exposed to cold, which further increases energy demands [31].
Dogs that completed the race had significantly higher serum cortisol levels compared to dogs that did not complete. This may reflect the sustained strain that the dogs experience with greater distance, and the prolonged requirement for energy in completing dogs.
It has been proposed that exercise-induced increase in endogenous cortisol may contribute to development of gastritis and gastric ulceration in sled dogs. A study regarding sled dogs competing in the Iditarod race showed that dogs with abnormal gastroscopic findings after racing also had a significantly higher serum cortisol level, compared to dogs with normal gastroscopic findings. However, the association between endogenous cortisol and gastrointestinal ulceration was not possible to establish [1].
Crp
There was a significant increase in serum CRP in competing dogs. An increase in serum CRP after long distance racing has previously been reported in humans and canines [14–16, 32]. Muscle degradation with subsequent inflammation is assumed to be a contributing factor in humans and dogs [16, 33].
Serum CRP concentration was slightly higher in non-completing dogs compared to completing dogs, although the difference was not significant. It could be speculated that this may be due to injuries or illness causing an inflammatory response in the majority of the non-completing dogs. Studies with larger case numbers and appropriate statistical power are necessary to reveal a true difference and support the proposed hypothesis.
The control dogs had a significantly higher serum concentration of CRP during training compared to the results post-race. It is likely that they underwent less physical activity in the post-race period compared to the training season. Also, median CRP for Samples A was significantly higher than the upper reference value for CRP, implicating either an inflammatory state of all dogs or a methodical problem in the laboratory analysis of CRP bringing higher results than previously described for healthy dogs when using the same assay. The serum concentration of CRP was significantly higher in Alaska huskies compared to Siberian huskies. The Siberian huskies in the present study represented a single team and one musher, and the finding might be spurious and possibly not reflect true breed differences.
In a study with experimentally induced damage to the gastric mucosa in dogs, it was concluded that CRP could be a useful indicator for gastric lesions [34]. However, as serum CRP also increases as a result of exercise or general illness in sleds dogs, the use of serum CRP as a general diagnostic tool for gastritis or gastric ulceration may be limited in racing sled dogs.
Strengths and limitations
A strength of this study is the prospective design. The use of a control group for the post-race results and a genuine endurance race setting increase the external validity of this study. However, there were also limitations that need to be kept in mind.
Standardization of blood sampling during the race was difficult and not optimal. Appropriate storage of the blood samples was difficult to standardize completely, especially regarding the time passed before freezing at −70 °C. Although temperatures in February in this part of Norway are generally well below 0 °C, storage procedures could have influenced the results. It is known that serum gastrin concentration can decrease significantly if stored in room temperature for a few hours [35]. Likewise, the cortisol concentration can decrease if serum is stored at temperatures above 0 °C over several days [36]. CRP on the other hand is considered to be far more stable in different storage conditions, and it is likely that the serum concentration of CRP were not affected to the same extent as gastrin and cortisol, by transport and temperatures [37, 38]. It is therefore possible that an even greater rise in both serum gastrin and cortisol levels would have been measured, if the samples had been stored in optimal conditions.
Some samples from dogs that did not complete the race were collected after the dogs had been rested for up to 8 h. This could influence the values of the measured parameters. The impact of this resting period is probably a decrease in serum concentrations of gastrin, cortisol and CRP. It is possible that the measured values would have been even higher if the samples had been collected straight after racing.
Because the dogs were from different mushers, confounding factors in the dogs’ home environment might influence the results and interpretation of the statistical analysis.
One musher and his team of dogs withdrew from the race after 500 km. All dogs belonging to this team were recorded as non-completing. Having a whole team of dogs not completing the race was not preplanned during the design of the study, but merely a result of the study being performed in a genuine race setting. Sensitivity-analyses excluding this team had negligible influence on the results and conclusions.
During a race non-completing dogs mostly withdraw due to clinical signs of disease. However, the presumption that they are clinically ill and that completing dogs are healthy, is not always correct. In the authors’ experience, reasons for dropping may vary from observed clinical disease to consideration of a dog’s fitness. Withdrawal of entire teams, including sick and healthy individuals, also happens. There may be different opinions regarding when it is unjustifiable to let a dog continue a race, making the definition sick relative and subjective. Completing dogs have additionally been observed with signs compatible with lethargy and diarrhea. Consequently, the dogs’ health status should not be defined as healthy just because they have completed a race. These challenges need to be addressed in future studies.
No age or sex related differences in the serum concentrations of gastrin, cortisol or CRP could be detected in this study. This might be due to the sample size, and therefore could have appeared if the number of dogs included had been higher. However, it seems unlikely that there should be age or sex differences in either of these parameters. It can be speculated that a rise in CRP may appear with increasing age due to processes linked to ageing, or that a rise in cortisol may be observed in younger, inexperienced dogs due to a higher degree of psychological stress.
In the analysis of gastrin, both inter- and intra assay variation were available (Table 4), which strengthens the results of this study. CV was low and relatively similar for both inter- and intra-assay variation. In the analysis of cortisol and CRP, only inter-assay variation was measured. How this could have affected the results in this study is unknown.
Future research
Further investigation is needed to evaluate if an exercise-induced increase in serum gastrin and cortisol may contribute to gastritis and gastric ulceration in racing sled dogs. In future studies it would be interesting to focus on the association between distinct signs of illness and different biomarkers in completing and non-completing racing dogs. However, the clinical evaluation of the participating sled dogs needs to be done by the same person in a standardized matter, and clinical suspicion of gastrointestinal disease needs to be confirmed by diagnostic procedures. Because sled dogs with gastritis and gastric ulcers often are asymptomatic [7], gastroscopy together with measurement of biomarkers should be performed during training season, and immediately before and after participation in a race. This could increase our knowledge concerning exercise-induced metabolic changes in sled dogs during racing and enlighten possible causalities to gastritis and gastric ulceration in racing sled dogs.