LEPTIN mRNA EXPRESSION AND SERUM LEPTIN CONCENTRATIONS AS INFLUENCED BY AGE, WEIGHT, AND ESTRADIOL IN PIGS H. Qian,* ,2 C.R. Barb,*** M.M. Compton,** G.J. Hausman,*** M.J. Azain,* R.R. Kraeling,*** and C.A. Baile* ,1 Departments of *Animal and Dairy Science and **Poultry Science, University of Georgia, Athens, GA 30602 ***Animal Physiology Research Unit, USDA-ARS, Russell Agriculture Research Center, Athens, GA 30604 Received October 5, 1998 Accepted October 21, 1998 Two experiments (EXP) were conducted to determine the roles of age, weight and estradiol (E) treatment on serum leptin concentrations and leptin gene expression. In EXP I, jugular blood samples were collected from gilts at 42 to 49 (n = 8), 105 to 112 (n = 8) and 140 to 154 (n = 8) d of age. Serum leptin concentrations increased (P  0.05) with age and averaged 0.66, 2.7, and 3.0 ng/ml (pooled SE 0.21) for the 42- to 49-, 105- to 112-, and 140- to 154-d-old gilts, respectively. In EXP II, RNase protection assays were used to assess leptin mRNA in adipose tissue of ovariectomized gilts at 90 (n = 12), 150 (n = 11) or 210 (n = 12) d of age. Six pigs from each age group received estradiol (E) osmotic pump implants and the remaining animals received vehicle control implants (C; Day 0). On Day 7, back fat and blood samples were collected. Estradiol treatment resulted in greater (P  0.05) serum E levels in E (9  1 pg/ml) than C (3  1 pg/ml) pigs. Serum leptin concentrations were not affected by age, nor E treatment. Leptin mRNA expression was not increased by age in C pigs nor by E in 90- and 150-d-old pigs. However, by 210 d of age, leptin mRNA expression was 2.5-fold greater (P  0.01) in E-treated pigs compared to C animals. Serum insulin concentrations were similar between treatments for 210-d-old pigs. However, insulin concentrations were greater (P  0.05) in E than C pigs at 90 d and greater in C than E animals at 150 d. Plasma glucose and serum insulin-like growth factor-I concentrations were not influenced by treatment. These results demonstrate that serum leptin concentrations increased with age and E-induced leptin mRNA expression is age- and weight-dependent. © Elsevier Science Inc. 1999 INTRODUCTION The recently discovered protein leptin is a 16-kDa protein consisting of 146 amino acids, which is synthesized by adipose tissue and secreted into the blood after cleavage of the 21 amino acid signal peptide (1). Leptin impacts feed intake, the neuroendocrine axis, and immunologic processes. This protein was first identified as the ob/ob gene product, which is absent in the obese ob/ob mouse. Leptin serves as a circulating signal of nutritional status and plays a pivotal role in the regulation of body weight, energy expenditure, growth, and reproduction. Leptin treatment increased gonadal steroid secre- tion, restored fertility and influenced reproductive function in the mouse (2– 4). Moreover, central administration of leptin increased growth hormone (GH) secretion in the pig (5) and prevented the fasting-induced decrease in GH concentrations in the rat (6). Serum leptin concentrations increased during puberty in mice (3) and boys (7) and in the human DOMESTIC ANIMAL ENDOCRINOLOGY Vol. 16(2):135–143, 1999 © Elsevier Science Inc. 1999 0739-7240/99/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S0739 –7240(99)00004 –1