The relationship between plasma leptin and ghrelin concentrations and the presence of the ghrelin receptor in beef cattle exhibiting differences in composition of gain

Beef steers (n = 72) of similar age, weight (292 +/- 1.4 kg), and genetic background (Angus crossbred) were used to determine the effects of growing period diet on the relationship of plasma ghrelin and leptin concentrations with growth performance and carcass composition, and on the expression and abundance of growth hormone secretagogue receptor (GHS-R) and peroxisome proliferator-activated receptor gamma 2 (PPARgamma2) in metabolically important tissues of beef cattle. At trial initiation (d 0), 8 steers were harvested for initial carcass composition. The remaining 64 steers were allotted, by weight, to pen and treatment was assigned randomly. Treatments were (1) a high-forage diet fed during the growing period (112 d) followed by a high-concentrate during the finishing period (113--209 d; GRW-FNSH) or (2) a high-concentrate fed for the duration of the trial (0--209 d; FNSH-FNSH). Steers were allowed ad libitum consumption regardless of dietary treatment. Eight steers per treatment were harvested on d 88, 116, 165, and 209, 9-10-11th rib sections were dissected for chemical composition, and carcass characteristics were recorded. Replicate blood samples were collected from every steer prior to each harvest, and assayed for ghrelin, leptin, GH, insulin, and NEFA concentrations. Immediately following harvest, samples were collected for compositional analyses and liver, muscle and subcutaneous adipose tissue samples were collected from each steer and immersed in liquid nitrogen. Extraction of RNA occurred using TRI-Reagent and a SYBR GreenRTM amplification kit was used for semi-quantitative RT-PCR. Protein separation and quantification was determined using SDS-PAGE and Western blotting techniques. Protein abundance of GHS-R and PPARgamma2 were detected using the LI-CORRTM system and standardized to beta-Actin. Hormone and growth performance were statistically analyzed using the GLM procedure of SAS comparing dietary treatment at each serial harvest point. Carcass composition, protein abundance and expression were statistically analyzed using the GLM procedure of SAS to evaluate diet, harvest date, and their interaction. Linear, quadratic, and cubic contrasts also were performed. Plasma ghrelin concentrations for FNSH-FNSH increased quadraticly (P < 0.001) over time, whereas plasma ghrelin concentrations were not different over time for GRW-FNSH. Plasma leptin concentrations for FNSH-FNSH increased (P < 0.001) from d 0 to 88 and then plateaued, whereas plasma leptin concentrations increased linearly ( P < 0.001) for the GRW-FNSH. Percent carcass protein decreased linearly (P < 0.001) and percent carcass fat increased linearly (P < 0.001) in both treatments overtime. At each harvest day, FNSH-FNSH steers had greater carcass fat (P < 0.01) compared with GRW-FNSH steers. Plasma ghrelin and leptin concentration fluctuated relative to nutritional status, however plasma ghrelin concentrations were increased in excessively fat cattle.;Abundance and expression of PPARgamma2 in subcutaneous adipose tissue fluctuated (P < 0.05) throughout the feeding period. In both dietary treatments, protein abundance of PPARgamma2 was highest on d 0 with a secondary increase in abundance on d 165, whereas expression of PPARgamma2 was highest on d 0 with a secondary increase in expression on d 116. A dietary treatment by serial harvest day interaction (P < 0.05) occurred for protein abundance of GHS-R in subcutaneous adipose tissue. Abundance of GHS-R in the GRW-FNSH was greatest on d 88, whereas abundance for the FNSH-FNSH treatment was greatest at the end of the finishing period (d 209). Protein abundance of GHS-R in longissimus dorsi tissue fluctuated relative to serial harvest date (P < 0.001), and was highest on d 88 in both treatment groups. The FNSH-FNSH steers had increased abundance of GHS-R in longissimus dorsi on d 88 and 116 compared with the GRW-FNSH steers. An interaction of dietary treatment and serial harvest day resulted (P < 0.05) for GHS-R abundance in liver tissue. Protein abundance for GHS-R in both dietary treatments increased quadrically (P < 0.001) throughout the feeding period. These data are not consistent with the hypothesis that GHS-R abundance decreases in beef cattle with greater fat deposition. Additionally, increased PPARgamma2 abundance in growing cattle consuming a high concentrate diet was not observed. However, increased PPARgamma2 abundance in younger cattle warrants further investigation.;Key Words: ghrelin, leptin, composition of gain, beef cattle...