This study was designed to investigate gastrointestinal dysfunctions, especially the effects of gastrointestinal prokinetic agents on digestive disorders involving lowered motility. The effects of α2-adrenergic agonists for treating acute abdominal disorders involving intestinal torsion in horses were also investigated. The results are as follows: (1) It appeared that the functions of migrating contractions (MC) in horses were to transfer the contents of the digestive tract as soon as possible to the cecum and colon, in addition to maintaining homeostatis in the digestive tract. (2) Motilin was seen to be involved in the regulation mechanism of intestinal motility in the horse as well as in humans and dogs, and brought about MC. (3) The intravenous injection of cisapride brought about MC in the small intestine. With administration of a higher dose of 0.75 or 1.0 mg/kg, a significant increase (P<0.05) in MC frequency was observed. (4) It was clear that the inhibitory effects of the α2-adrenergic agonists persisted for a longer time with a more remarkable reduction of contractions in the cecum and colon than in the jejunum. When α2-adrenergic agonists are used for relieving intestinal tension, it is recommended that medetomidine is more effective than xylazine.
Seven horses were exercised five days a week for 12 months at different gaits on a treadmill, and eight horses were pastured freely in a paddock without any training programs. They were 60 days of age at the beginning of the experiment. We compared the number of C cells in the thyroid, ultrastructure of chief cells in the parathyroid and radiography of the third metatarsal bone of the training group with the control group. In the training group, the number of C cells was not increased and chief cells were not in a phase of secretion. In addition, the bone mineral content and the cortical area did not show high values. We considered that an additional exercise used in this study had suppressive effects on the thyroid and parathyroid. Our results also suggest an additional and/or intense exercise in early stage of life (foal to weanling) may have an adverse effect on bone growth and metabolism.