Contribution of propionate to glucose synthesis in sheep

Abstract

The production rate of propionate in the rumen and the entry rate of glucose into the body pool of glucose in sheep were measured by isotope-dilution methods. Propionate production rates were measured by using a continuous infusion of specifically labelled [14c]propionate. Glucose entry rates were estimated by using either a primed infusion or a continuous infusion of [U-14C]glucose. The specific radioactivity of plasma glucose was constant between 4 and 9 hr. after the commencement of intravenous infusion of [U-14C]glucose and between 1 and 3 hr. when a primed infusion was used. Infustion of [14C]propionate intraruminally resulted in a fairly constant specific radioactivity of rumen propionate between about 4 and 9 hr. and of plasma glucose between 6 and 9 hr. after the commencement of the infusion. Comparison of the mean specific radioactivities of glucose and propionate during these periods allowed estimates to be made of the contribution of propionate to glucose synthesis. Comparisons of the specific radioactivities of plasma glucose and rumen propionate during intraruminal infusions of one of [1- C]-, [2-14C]-, [3-14C- and [U-14C]-propionate indicated considerable exchange of C-1 of propionate on conversion into glucose. The incorporation of C-2 and C-3 of propionate into glucose and lactate indicated that 54 percent of both the glucose and lactate synthesized arose from propionate carbon. No differences were found for glucose entry rates measured either by a primed infusion or by a continuous infusion. The mean entry rate ([plus or minus]S. E.M.) of glucose estimated by using a continuous infusion into sheep was 0.33[plus or minus]0.03 (4) mM/min. and by using a primed infusion was 0.32[plus or minus]0.01 (4) mM/min. The mean propionate production rate was 1.24[plus or minus]0.03 (8) mM/min. The conversion of propionate into glucose was 0.36 mM/min., indicating that 32 percent of the propionate produced in the rumen is used for glucose synthesis. It was indicated that a considerable amount of the propionate converted into glucose was first converted into lactate.