Metabolism of cystine by Merino sheep genetically different in wool production IV. Rates of entry of cystine into plasma, measured with a single intravenous injection of L-[35S]cystine, and the subsequent incorporation of 35S into wool fibres.

Ten, 2-year-old Merino ewes from a flock selectively bred for high clean fleece weight (Fleece Plus) and ten from a flock bred for low clean fleece weight (Fleece Minus) were randomly divided between two dietary treatments: 600 or 1100 g/day of pelleted lucerne hay. After 14 weeks, each ewe received an intravenous injection of L-[35S]cystine (66-4 muCi). Venous blood samples were collected at 15 specified times until 8 h after the injections, and wool fibres were plucked until 65-75 days after the injections. Protein-free filtrates prepared from blood plasma were bulked within sample times for ewes from the same flock and dietary treatment. Equations relating the specific radioactivity of free cystine isolated from the bulked filtrates to time after injection contained three exponential terms. The entry rate and pool size of cystine estimated from these equations were greater in Fleece Minus than in Fleece Plus ewes (by 25 and 44% respectively for entry rate and pool size). Both traits were also higher in ewes offered 1100 g lucerne/day than in those offered 600 g/day (58-7 v. 33-9 mg/h for entry rate and 19-2 v 11-8 mg for pool size). The concentration of free cystine in plasma was greater in ewes offered 1100 g lucerne/day (3-0 v 2-1 mg/1; P less than 0-05), and greater in Fleece Minus ewes (3-0 v. 2-1 mg/l; P less than 0-05). The percentage of the injected radioactivity recovered in the wool clipped to day 70 post-injection differed between genotypes and between dietary treatments (P less than 0.05), being greater in Fleece Plus than in Fleece Minus ewes, and greater in those offered 1100 g lucerne/day than in those offered 600 g/day. The relationships between 35S incorporated per 1000 fibres (R) and time after injection (t) were best fitted by equations of the form (formula: see text). For all sheep, n = 3. The coefficient of the second term was significantly greater (P less than 0-05) in ewes offered 1100 g lucerne/day, whilst the constant of this term was significantly greater in Fleece Minus ewes. The specific radioactivities of cystine incorporated into wool fibres (SRf) during various intervals of time after injection were derived from these equations and from the measured rates of output of cystine in wool. The equations computed to relate SRf to time after injection (t) were of the form (formula: see text). Again there were three components. The coefficient of the third component was significantly greater (P less than 0-05) in ewes offered 1100 g lucerne/day, whilst the constant of the second term was significantly greater in Fleece Minus ewes.