Abstract: 87

DAILY ADMINISTRATION OF CLENBUTEROL DECREASES LEPTIN CONCENTRATIONS BUT DOES NOT INFLUENCE THE TIMING OF ANESTRUS IN MARES.

Christina J. McManus^{1 *}, Barry P. Fitzgerald¹
Dept. Veterinary Sci. Univ. KY, Lexington KY. ¹

Although a seasonal breeder, a small proportion of mares do not exhibit anestrus and display estrous cycles throughout the winter months. In preliminary studies, we have determined that this phenomenon is a characteristic of mares which exhibit high body fat stores (unpublished). Timing and occurrence of anestrus, therefore, may be causally related to body fat stores. Leptin concentrations reflect the level of fat stores and may provide a metabolic signal of nutritional well-being. The aim of this study was to examine the hypothesis that a reduction in circulating leptin would advance the onset of anestrus. This proposal was examined by administration of the B-adrenergic agonist, clenbuterol (CLN), which increases lipolysis and reduces fat stores. Eight mares received CLN (3.2 mg/kg BW daily i.v) from mid-Oct through Jan. Six untreated mares served as controls (CTL). Progesterone was determined in blood samples collected x3/week. Bi-weekly samples were analyzed for leptin concentrations by RIA (Linco, St. Charles, MO). In CLN mares, mean (± SEM) leptin decreased between days 7 to 22 from 5.97 ± 0.90 to 2.37 ± 0.27 ng/ml human equivalent (HE); this low level was sustained for the remainder of the experiment (d 78: 2.29 ± 0.21 ng/ml; p < 0.05). CTL mares also showed a decline in serum leptin, but the timing of the decline was later than CLN mares (d 7: 6.24 ± 0.81; d 22: 5.65 ± 0.50; d 78: 2.58 ± 0.09 ng/ml). Despite a marked decrease in leptin, the timing of anestrus was not advanced in CLN mares compared to controls. Progesterone profiles indicated that 5 of 8 CLN mares and 2 of 6 CTL mares exhibited anestrus. The results of this study suggest that a CLN-induced decrease in leptin is associated with but may not induce the onset of anestrus.

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