Lupins and energy-yielding nutrients in ewes. 1. Glucose and acetate biokinetics and metabolic hormones in sheep fed a supplement of lupin grain

Abstract

Measurements were made of metabolism of glucose and acetate and changes in plasma hormones in sheep fed a basal ration to meet maintenance requirements for metabolizable energy (ME) or fed the same level of basal ration and given either a supplement of 750 g day-1 lupin grain (Experiment I) or intravenous glucose infusions over periods of 17 h (Experiment 2).In ewes given supplementary lupins, concentrations of volatile fatty acids (VFAs) in rumen liquor increased but molar proportions of VFAs were unchanged. Plasma concentrations of glucose, acetate and propionate increased; entry rates of glucose, acetate and CO2 increased and glucose oxidation (but not acetate oxidation) increased during supplementation with lupins. Other changes measured during lupin supplementation included increased plasma urea, a transient and marked increase in plasma insulin and a concomitant decrease in plasma growth hormone during the first three days, and an increase in plasma tri-iodothyronine (T3) with no changes in plasma thyroxine (T4).When glucose was infused intravenously at rates ranging from 0 to 46.8 mmoles h-1 (cf. 31 mmoles h-1 due to lupins) endogenous entry rate of glucose only increased at the highest infusion rate. There was a negative linear relationship between infusion rate of glucosc and plasma urea, whereas positive linear relationships were found between glucose infusion rate and both plasma insulin and T3.It was concluded that ewes on a maintenance basal diet and fed a supplement of 750 g day-1 lupin grain would rapidly switch to the anabolic mode concomitant with increases in glucose entry rate by more than 100% and acetate entry rate by more than 50%. Long-term intravenous infusion of the amount of glucose equivalent to that promoted by the lupin grain supplement would probably not suppress endogenous glucose production.