Substrate utilization for hepatic gluconeogenesis is altered by increased glucose demand in ruminants.

Abstract

Hepatocytes isolated from 10 Dorset wethers that were treated with excipient or 1.0 g/d of phlorizin for 72 h were used to determine the effects of increased glucose demand on utilization of [1-(14)C]propionate and [1-(14)C] alanine for oxidative metabolism and gluconeogenesis. Control and phlorizin-treated wethers excreted 0 and 62.8 g/d of glucose into the urine, respectively. Phlorizin treatment tended to increase conversion of propionate and alanine to CO2. A phlorizin x substrate interaction for conversion to glucose indicated that conversion of alanine to glucose was increased more by phlorizin treatment than was conversion of propionate (285 vs 166% of controls). Phlorizin treatment did not affect estimated Ks for conversion of substrates to either CO2 or glucose; however, phlorizin increased estimated Vmax for conversion of substrates to CO2 and tended to increase estimated Vmax for conversion of substrates to glucose. Phlorizin treatment slightly increased the ratio of conversion of propionate to glucose compared with CO2 and slightly decreased the ratio of conversion of alanine to glucose compared with CO2. In vitro addition of 2.5 mM NH4Cl decreased conversion of propionate to CO2 and glucose but had little effect on conversion of alanine to CO2 and glucose. Estimated Ks and Vmax for conversion of substrates to CO2, Ks for conversion of substrates to glucose, and Vmax for conversion of alanine to glucose were not affected by NH4Cl; however, Vmax for conversion of propionate to glucose was decreased by NH4Cl. These data indicate that although utilization of propionate for gluconeogenesis is extensive, amino acids have the potential to increase in importance as gluconeogenic substrates when glucose demand is increased substantially. Furthermore, excess ammonia decreases the capacity of hepatocytes to utilize propionate for oxidation and gluconeogenesis.

Propionate supplementation did not increase whole body glucose turnover in growing lambs fed rye grass

“…The ammonia concentrations as well as the isoacid concentrations decreased and suggested a decrease in ruminal proteolysis [25,26]. The increase in propionate concentrations in the rumen was also associated with a decrease in the rumen L-lactic acid concentrations, which disagreed with data reported by Overton et al [27]. All these effects were similar to those observed with an addition of monensin which favours propionate producing bacteria [28,29].…”

“…On the basis of the results obtained by [8] and [27] when glucose requirements are experimentally increased by enhancing urinary glucose excretion, it may be hypothesised that the effect of propionate supplementation on glucose turnover is influenced by the balance between the basal glucose turnover of the animals and their glucose requirements. A key notion is that of glucose requirements which are probably determined by the growth rate of the animals relative to their growth potential and by their nitrogen (N) intake [40].…”

Whole body glucose turnover in growing and non-productive adult ruminants: meta-analysis and review

“…In sheep [9] and steers [46], injection of phlorizin significantly increases glucose turnover rate at fixed intakes. This is accompanied by a reduction in glucose oxidation and an increased efficiency of precursor use.…”

“…Data obtained in lactating ruminants also show that the glucose requirements of the animals and their hormonal status have a clear impact on glucose turnover [7][8][9]. No literature review specifically focused on growing ruminants, which present lower and a smaller range of nutritional requirements than lactating ones, exists.…”

Pre-breeding blood urea nitrogen concentration and reproductive performance of Bonsmara heifers within different management systems

“…It is thought that in dairy cattle, the negative effect of high BUN concentration might be exerted through the exacerbation of an underlying negative energy balance (NEB) and the energy costs of detoxifying large quantities of ammonia in post-partal cattle (Staples et al, 1990;Garcia-Bojalil et al, 1998;Overton et al, 1999). However, it can be assumed that beef heifers will not suffer from NEB as is the case in lactating dairy cattle.…”