Some comparative studies of sulfate metabolism in Merino sheep genetically different in wool production

Abstract

Several aspects of sulfate metabolism were compared in sheep from two selection flocks, selectively bred for either high (Fl+) or low (FI-) clean fleece weight per head. These were studied within a 23 factorial design: 2 flocksx2 levels of formaldehyde treated casein in diet (0 or 80 g day-1)x2 levels of subcutaneous L-thyroxine (0 or 1 mg day-1) with 4 sheep per cell. The treatments influenced the rates of wool growth and the output of sulfur in wool (P < 0.05), but with interactive effects (P < 0.05). The F1+ sheep produced 24 and 32 8g sulfur in wool cm-2 day-1at the midside when consuming the control and casein supplemented diet. The outputs of sulfur in wool by F1- sheep consuming these two diets were 16 and 17 8g cm-2 day-1. The thyroxine treatment increased the rate of growth of wool by 11% (P < 0.05), but did not increase average fibre diameter or the sulfur content of wool. Sulfate was cleared more rapidly (P < 0.05) from the plasma of sheep consuming the casein supplemented diet, with a rate constant of 0.134 h-1, compared to 0.099 h-1for the control sheep. The rate constants for Fl+ and F1- sheep were 0.102 and 0.131 h-1 respectively (P < 0.05). The concentration of sulfate in plasma was similar in all treatment groups: 1.3 mmol L-1The differences in the clearance of sulfate from plasma were reflected in similar differences in the proportion of the injected 35S-sulfate excreted in urine during the following 24 h. The sheep of the F1- flock and those consuming the casein-supplemented diet excreted significantly greater proportions of the injected 35S in urine (P < 0.05). In a second experiment, adult sheep (n = 24), the progeny of F1+ or F1- rams mated with a flock of medium wool Merino ewes, were randomly allocated to a low (c. 0.8 maintenance, 1 g sulfur day-1) or high (c. 2xM, 3 g sulfur day-1) dietary regime. [35S] sodium sulfate was again injected intravenously. There were no significant differences between the genetic groups for (i) sulfate concentration in plasma, (ii) total and net rates of irreversible loss of sulfate from plasma, (iii) the proportion of the injected [35S] sodium sulfate excreted in urine, (iv) the proportion of the excreted 35S present in urine as sulfate, and (v) the quantity of sulfate excreted in urine daily. However, the dietary treatments significantly influenced all these traits, except the concentration of sulfate in plasma. The direction of these effects was consistent with the proposition that sheep regulate excretion of sulfate from kidneys to maintain homeostasis of sulfate within the body. The smaller phenotypic difference in wool production between the two half-bred flocks in the second experiment, probably limited the chances of detecting any differences in the metabolism of sulfate.