Protein metabolism in skin and muscle of sheep selected for or against staple strength

Abstract

Two experiments were carried out to determine the mechanisms underlying the reduced effect of nutritional status on wool growth rate in Merino sheep that have been selected for high staple strength (SS). In Expt 1, each group of 6 young sheep of SS+ and SS– genotypes were fed at 0.4 or 1.1 times maintenance, and in Expt 2, groups of 8 sheep of each genotype were fed at 1.1 and 1.8 times maintenance. In both experiments, rates of protein synthesis in skin, muscle, gut, rumen, and liver were determined using a flooding dose of labelled phenylalanine. Feed intake and the digestibility of feed were not affected by genotype. Neither dissection of the carcasses at slaughter, nor deuterated water analysis in Expt 1, detected any differences between the genotypes in body composition. The feeding level affected the total daily amount of protein synthesised in all the organs examined, and the fractional rate of protein synthesis was affected by feeding level in all organs except the liver. The fractional synthesis rate of protein was less responsive to feeding level in the SS+ sheep in both skin and muscle (P < 0.05), but not in the liver, jejunum, or rumen. Total protein synthesis in muscle, and the estimated rate of protein degradation, were also less responsive to feeding level in the SS+ sheep (P < 0.05). We conclude that sheep genetically selected for high or low SS have altered local mechanisms in both skin and muscle that control the way they respond to nutrition. These findings provide a mechanism by which selection for wool growth rate also affects body metabolism.