RESUMO
Early weaning of ewe lambs strongly stimulates the hypothalamic-pituitary-adrenal axis and is associated with suppressed growth rate despite the increased food intake. At the same time, plasma leptin concentration increases only slightly or undetectably. To better understand this atypical interdependence among somatic stress, leptin, and lamb growth rate, we analyzed impact of leptin and/or adrenocorticotropic hormone (ACTH) on growth hormone (GH) secretion as well as the effect of ACTH on mRNA expression of two splice variants of leptin receptor (LEPRa, LEPRb) in pituitary cells isolated from early weaned ewe lambs. The GH secretion under the influence of leptin and/or ACTH depended on the timing of exposure and hormone concentration. After 6 - 30 h, GH secretion increased under 10-11 - 10-8 M leptin (P ≤ 0.05). However, after 24 - 30 h, GH secretion significantly increased only in cells exposed to both leptin and ACTH compared to culture with leptin only. Simultaneously, there was a significant (P ≤ 0.05) decrease in leptin receptor mRNA expression under the influence of ACTH at 10-8 - 10-6 M after 12 - 30 and 24 - 30 h for LEPRa and LEPRb, respectively. ACTH-related downregulation of LEPR mRNA was associated with a significant (P ≤ 0.05) reduction in leptin-stimulated GH secretion, also after 24 - 30 hours. Thus, the timing of ACTH exposure, followed by decreased leptin receptor mRNA, converged with the timing of decreased GH secretion under the influence of leptin with ACTH. The ACTH-induced downregulation of LEPR mRNA therefore may underlie the decrease in GH. These results show a direct role for leptin, ACTH, and leptin receptor expression in modulation of pituitary GH secretion in early weaned ewe lambs. During the early weaning-induced stress response, the ACTH-mediated decrease in sensitivity of pituitary cells to leptin may abolish a stimulatory effect of leptin on GH secretion and explain in part, the reduction in lamb growth rate.