High versus low body condition in mares: Interactions with responses to somatotropin, GnRH analog, and dexamethasone1

Abstract

Mares that had previously been fed to attain body condition scores (BCS) of 7.5 to 8.5 (high) or 3.0 to 3.5 (low) were used to determine the interaction of BCS with the responses to 1) administration of equine somatotropin (eST) daily for 14 d beginning January 20 followed by administration of GnRH analog (GnRHa) daily for 21 d and 2) 4-d treatment with dexamethasone later in the spring when mares in low BCS had begun to ovulate. The majority of mares with high BCS continued to cycle throughout the winter, as evidenced by larger ovaries (P < 0.002), more corpora lutea (P < 0.05), greater progesterone concentrations during eST treatment (P < 0.04), and more (P < 0.05) large- and medium-sized follicles. Treatment with eST alone or in combination with GnRHa had no effect (P > 0.05) on ovarian activity or ovulation. Plasma leptin concentrations were greater (P < 0.002) in mares with high BCS; however, there was no effect (P > 0.10) of eST treatment. Plasma IGF-I concentrations were greater (P < 0.0001) in mares treated with eST compared with mares given vehicle, and mares with high BCS had greater IGF-I (P < 0.02) and LH concentrations (P < 0.02) than mares with low BCS. Plasma leptin concentrations in mares with high BCS were increased (P < 0.001) within 12 h of dexamethasone treatment; the leptin response (P < 0.001) in mares with low BCS was greatly reduced (P < 0.001) and transient. Glucose and insulin concentrations also increased (P < 0.0001) after dexamethasone treatment in both groups, and the magnitude of the response was greater (P < 0.0001) in mares with high BCS than in mares with low BCS. In summary,low BCS in mares was associated with a consistent seasonal anovulatory state that was affected little by eST and GnRHa administration. In contrast, all but one mare with high BCS continued to experience estrous cycles and(or) have abundant follicular activity on their ovaries. The IGF-I response to eST treatment was also reduced in mares with low BCS, as was the basal leptin concentration and leptin response to dexamethasone. Although low BCS and leptin concentrations were associated with inactive ovaries during winter and early spring, mares with low BCS eventually ovulated in April and May while leptin concentrations remained low.

Blood Levels of Selected Metabolic Factors, Cytokines, and Lymphocyte Subpopulations in Arabian and Thoroughbred Horses During the Longest and Shortest Days of the Year

“…Plasma glucose levels were analyzed by colorimetric procedures (glucose [HK] assay kit; Sigma-Aldrich, St. Louis, MO) [23]. Leptin and IGF-1 assays were performed using radioimmunoassay (RIA) kits (multispecies leptin RIA kit; Linco Research Inc., St. Charles, MO; IGF-1 RIA kit, Diagnostic Systems Laboratories, Inc., Webster, TX).…”

Effects of nutritional cues on the duration of the winter anovulatory phase and on associated hormone levels in adult female Welsh pony horses (Equus caballus)

“…The main site of action of nutrition on the ovarian cycle in cows [15,16], gilts [17] and mares [10,18] appears to be the hypothalamus, and the mechanism probably has multiple components including insulin, leptin, growth hormone (GH) and insulin-like growth factors (IGFs). It has been shown that induced hypoglycaemia decreases secretion of gonadotrophins and results in anovulation in different species including mares [14,19-22].…”

Serum Leptin Concentrations in Lipizzan Fillies

“…2005). The amount of energy stored as body fat may determine the time when the mares cease reproductive activity during the winter months, and this energy status could be signalled to the brain via leptin (Fitzgerald and McManus 2000; Gentry et al. 2002; McManus and Fitzgerald 2003).…”