Effects of competitive and noncompetitive showjumping on total and free iodothyronines, β-endorphin, ACTH and cortisol levels of horses

REASONS FOR PERFORMING STUDY: Limited knowledge exists about the differentiated effects of competitive and noncompetitive showjumping on thyroid function and relationships with hypothalamic-hypophysis-corticoadrenal hormones. OBJECTIVES: To obtain preliminary data about differentiated effects of competitive and noncompetitive showjumping on total and free iodothyronines, β-endorphin, ACTH and cortisol of horses. MATERIAL AND METHODS: Five trained healthy jumper horses were studied during competitive and noncompetitive showjumping, performed in the same circuit design over 10 fences of 1.10 m. Hormone levels before, 5 and 30 min post exercise were recorded. Serum iodothyronines and cortisol concentrations were measured in duplicate utilising EIA kits. Serum ACTH and plasma β-endorphin concentrations were analysed in duplicate utilising RIA kits. Two-way RM ANOVA was applied to test for effects of interaction between different type of session and time. Significant differences between post exercise and basal values were established using Bonferroni's multiple comparison test. A linear correlation analysis (Pearson's method) was performed to analyse the relationships between total and free iodothyronines and between iodothyronines and β-endorphin, ACTH and cortisol. RESULTS: In sampling times adopted no statistical different effects of type of session were recorded on hormone variables. Sampling time affected ACTH (F = 4.25; P < 0.02) and T(4) (F = 4.43; P < 0.02) post exercise changes. During the noncompetitive session, significant correlations existed between T(4) and β-endorphin (r = -0.56), ACTH (r = -0.65), between β-endorphin and ACTH (r = 0.52) and between T(3) and fT(3) (r = 0.72); during competition between β-endorphin and T(3) (r = -0.67), fT(3) (r = -0.59). CONCLUSIONS: These preliminary results could demonstrate correlations between thyroid hormones and β-endorphin response to showjumping, although no definitive conclusion can be produced concerning the relationships between HPA and thyroid function during exercise.