Submission Number: GAR-4-27-4

Abstract Number: 209

LEPTIN SYNTHESIS AND SECRETION DURING PUBERTAL DEVELOPMENT IN HEIFERS RE-VISITED: USE OF A RUMINANT SPECIFIC RIA.

MR Garcia* ^1,2, M Amstalden ^1,2, SW Williams ^1,2, RL Stanko ^1,2,3, SE Nizielski ², CD Morrison* ⁴, DH Keisler* ⁴ and GL Williams* ^1,2

Animal Reproduction Laboratory, Texas A&M University Agricultural Research Station, Beeville, TX ¹
Department of Animal Science, Texas A&M University, College Station, TX ²
Deparment of Animal and Wildlife Sciences, Texas A&M University-Kingsville, Kingsville, TX ³
Department of Animal Science, University of Missouri-Columbia, Columbia, MO ⁴

Abstract:
Leptin is an important metabolic hormone implicated in the regulation of caloric intake, thermal regulation, and reproduction in many species, and may be particularly important for sexual maturation. In a previous report [Garcia et al., Biol Reprod 1999; 60 (suppl. 1)263], we characterized circulating concentrations of leptin in 11 beef heifers during pubertal development utilizing the Linco Multispecies leptin RIA . In the current study, circulating concentrations of leptin were reevaluated in 35 heifers using an ovine specific RIA validated for bovine leptin (Delavaud et al., J Endocrinol 2000; in press). Serum concentrations of leptin, IGFI and progesterone were measured once (leptin, IGFI) or twice weekly (progesterone) in blood samples collected beginning an average of 18 weeks before pubertal ovulation. Northern analyses for leptin mRNA were performed on fat samples collected from a subset of heifers (n=11) every 3 weeks. Estimates of proportions of circulating bound and free leptin were made by incubating ¹²⁵Ihleptin in bovine serum (n = 11) at 4 C for 24 h, followed by Sephadex G100 chromatography. Mean body weight (BW) and age at the start of the study was 3025.4 kg and 2994 d, respectively. Heifers gained an average of 118.313.9 kg until puberty was confirmed at 1218 mos. of age. Multivariate stepwise regression analysis revealed, as expected, that BW accounted for a majority of the variation (R² = 0.37, p < 0.0001). In contrast to our previous report involving a hleptin RIA and fewer heifers, circulating concentrations of leptin exhibited a linear increase (p < 0.01) for 1516 weeks before puberty. Serum IGFI increased (p < 0.05) from a mean of 2111.4 ng/ml during weeks -19 to -10 to 2372.2 ng/ml during the 9week period prior to puberty. Circulating leptin was positively correlated with leptin mRNA in adipose tissue (r = 0.28, p < 0.03), BW (r = 0.31, p < 0.0001) and serum IGFI (r = 0.14, p < 0.001), and negatively correlated (r = -0.38, p < 0.001) with the proportion of bound leptin in serum, which declined (p < 0.003) as puberty approached. The greater specificity and sensitivity of the ruminantspecific RIA reported herein allowed more precise determinations of circulating bleptin, and strengthens the possibility that leptin may participate in the pubertal process in heifers. .

Keywords:

This abstract is being presented at: 8:00 AM in session:
Puberty