Reactions of cyst(e)ine concentration in plasma of Merino sheep from two genetic groups to fasting and ACTH injection

Abstract

Two experiments were conducted with two groups of Merino sheep which, as a result of selective breeding, differed in average wool production and average concentration of cyst(e)ine in plasma. In the first experiment, 10 ewes from each flock were fasted for 49 h. Seven blood samples were collected from each ewe and the plasma assayed for total cyst(e)ine (free plus disulfide bound). After an initial decrease in concentration of 2.65 8mol l-1 between 0 and 4 h, the concentration of cyst(e)ine increased linearly throughout the period of fasting (0.22 8mol l-1 h-1). These responses were similar in sheep from the two flocks so that sheep from the high wool producing flock maintained a lower concentration of cyst(e)ine throughout the period of fasting (50.4 v. 60.9 8mol l-1, P < 0.05). In the second experiment, with sheep from the same two genotypes, both cortisol and cyst(e)ine levels in plasma were monitored for 8 h after an injection of synthetic ACTH: 0, 0.2 or 0.4 i.u. kg-I liveweight. Twelve sheep per flock were studied in each of two replicates. Cortisol levels increased following the ACTH injections with the higher dose level giving higher levels of cortisol and more sustained responses for 7 h. The raised cortisol levels were not associated with any changes in the concentration of cyst(e)ine in plasma, and a 17% lower concentration of cyst(e)ine in the sheep of the high producing flock was maintained throughout the experiment. As the raised cortisol levels did not influence the concentrations of cyst(e)ine in plasma of these laboratory sheep, a shed sampling procedure involving venepuncture should be satisfactory for comparisons of the concentration of cyst(e)ine between groups of animals. If the sampling involves extended periods of fasting, some corrections may be necessary to account for the changes in concentration associated with period of fasting.