A recent study in Sweden explored the possible role of gastrin as a stress hormone and how it might relate to ulcers and colic in horses. Plasma gastrin and cortisol concentrations were determined during fasting and compared with concentrations during hay feeding.
In addition, gastrin and cortisol concentrations were determined before, during and after two kinds of physical exercise on a treadmill.
Meal stimulation significantly increased the plasma gastrin concentration, irrespective of the meal composition. An immediate and large increase in plasma gastrin concentration was found when voluminous meals were given, whereas a small meal evoked a later onset of gastrin release, suggesting that gastric distention plays an important role in inducing gastrin release during a meal.
Meals consisting of grain seem to evoke a slower onset and then a more prolonged gastrin response than a hay meal, possibly due to different emptying rates of the stomach.
Nervous excitation may play a minor role in the activation of gastrin release in horses. No experimental support was obtained for the idea that gastrin acts as a stress hormone in the horse.
In this study, horses that ate only hay or hay and grain showed a peak in plasma gastrin level about 30 minutes after eating. Horses eating unlimited hay showed the most prolonged gastrin response. When only the small grain meal was given, gastrin response was delayed compared to response time when horses were fed hay, reaching a peak about 2.5 hours after eating.
Differences in gastrin level were seen after horses ate a large hay meal (highest and most prolonged response), small hay meal (rapid response but fairly quick return to base level), or small grain meal (delayed response and fairly quick return to base level).
An immediate and large increase in plasma gastrin concentration was found when high-volume meals (large amount of hay) were given, whereas a small meal (concentrate/grain) evoked a later onset of gastrin release, suggesting that gastric distention plays an important role in inducing gastrin release during a meal.
The researchers suggested that the stomach distention that follows a large hay meal may be a mechanical trigger for the continuing release of gastrin, while a smaller meal does not distend the stomach enough to cause such a release. This explanation is backed up by studies of colicky horses in which there was a correlation between degree of stomach distention and level of gastrin in the blood.
Gastrin levels after feeding were higher than during fasting, while cortisol levels tended to be higher while horses were fasting. Treadmill exercise had little effect on gastrin concentrations, while cortisol levels were found to increase with exercise. No changes in plasma gastrin or plasma cortisol were noted in response to venipuncture for installation of the jugular catheters, a procedure that is associated with sharp increases in gastrin production in human subjects.
Because rises in cortisol did not show consistent correlation with gastrin levels in this experiment, it is suggested that gastrin secretion is not a useful measure of stress in fasting or exercised horses.
Although evidence suggests that gastric ulcers can be caused or aggravated by production of gastric acid, the saliva stimulated by hay consumption is effective in buffering the acid. High levels of gastrin are therefore not harmful to the horse as long as food is present in the stomach.
Unlimited hay may not be an option for feeding certain performance horses, but managers should consider allowing horses access to grass, hay, or another fiber source as much as possible.
If access to fiber is not possible and horses do not eat for long periods of time, gastric acid may lead to abdominal discomfort from ulcers or other factors. As a result of this study, some veterinarians recommend that these horses can be kept more comfortable by the use of supplements such as equine antacids, hindgut buffers, and digestive tract.