Effect of glucose or amino acid infusion during pregnancy in ewes with different wool growth responsiveness to nutrition

Abstract

The reason why wool growth is suppressed in pregnancy is unclear. The present study examined the response to the nutrient drains imposed by pregnancy in 3 genotypes that had been selected for low (SS–) or high (SS+) staple strength, and a random control line. These flocks differ in the responsiveness of their protein metabolism to nutrient supply. Three groups of 20 pregnant ewes were fed to maintain conceptus-free liveweight, and were challenged with an infusion of glucose or amino acids in mid (Day 104) or late (Day 138) pregnancy. Infusion of glucose in late pregnancy caused a lower increment in plasma glucose concentration (P < 0.001) compared with mid-pregnancy, whereas plasma insulin responses to glucose infusion were not significantly different across time. In contrast, infusion of amino acids caused a greater increase in plasma concentrations of α-amino nitrogen in all genotypes in late pregnancy (P < 0.01), whereas the insulin response to amino acids was reduced (P < 0.001). Despite slower clearance of amino acids and decreased wool protein synthesis, the concentrations of blood urea nitrogen (BUN) were elevated in late pregnancy. The genotypes did not differ in their response to infusion with amino acids, but the SS– genotype had higher basal glucose and insulin concentrations. We conclude that the suppression in wool growth as pregnancy progresses was accompanied by greater oxidation but slower clearance of amino acids from plasma, associated with a lower insulin response. This defect in insulin response was selective for amino acids, because it was not observed in response to glucose infusion.