Seasonal Changes in Plasma Adrenocorticotropic Hormone and alpha-Melanocyte-Stimulating Hormone in Response to Thyrotropin-Releasing Hormone in Normal, Aged Horses

作者：Funk R A; Stewart A J^*; Wooldridge A A; Kwessi E; Kemppainen R J; Behrend E N; Zhong Q; Johnson A K

来源：JOURNAL OF VETERINARY INTERNAL MEDICINE, 2011, 25(3): 579-585.

DOI：10.1111/j.1939-1676.2011.0712.x

• Endocrine
• Equine
• Pituitary
• PPID

Background
Results of diagnostic tests for equine pituitary pars intermedia dysPPID), including endogenous ACTH concentration and the overnight dexamethasone suppression test (DST), are affected by season. New and potentially more sensitive diagnostic tests for equine PPID, such as thyrotropin-releasing hormone (TRH)-stimulated ACTH response, have been developed, but have had limited evaluation of seasonality.
Objective
Our purpose was to evaluate seasonal changes in plasma ACTH and alpha-melanocyte-stimulating hormone (alpha-MSH) responses to TRH administration.
Animals
Nine, healthy, aged horses with normal DST results.
Methods
Synthetic TRH (1 mg) was administered IV. Plasma ACTH and alpha-MSH concentrations were measured at 0, 5, 10, 15, 20, 25, 30, 45, 60, and 180 minutes. Testing was performed in February, July, August, September, October, and November. Mean TRH-stimulated ACTH and alpha-MSH concentrations were compared across months and time by repeated measures analysis of variance. Significance was set at the P < .05 level.
Results
Concentrations of ACTH and alpha-MSH significantly increased after TRH administration. Endogenous and TRH-stimulated ACTH and alpha-MSH concentrations were significantly different across months with higher concentrations in the summer and fall compared with February.
Conclusions and Clinical Importance
Plasma ACTH and alpha-MSH responses to TRH administration experience seasonal variation, with TRH-stimulated ACTH and alpha-MSH concentrations increasing from summer through fall. These results support previous evidence of a seasonal influence on the equine pituitary-adrenal axis. More research is warranted with a larger number of horses to determine if seasonal reference ranges for TRH stimulation testing need to be defined.

• 出版日期2011-6