Interactions between propionate and amino acid metabolism in isolated sheep hepatocytes

Abstract

The purpose of the present study was to evaluate the contribution of various substrates to glucose synthesis in isolated sheep hepatocytes, and more specifically to quantify the contribution of propionate to gluconeogenesis. Liver cells from fed sheep have a very high capacity for propionate utilization and conversion into glucose. The glucogenicity of lactate or amino acids was very low in hepatocytes from fed sheep, but was significantly increased in hepatocytes from starved animals. Amino acids such as alanine or glutamine were characterized by a substantial utilization towards ureogenesis, whereas their conversion to glucose was very low. Propionate utilization and conversion into glucose was inhibited by butyrate, ammonia and especially ethanol (by up to 80%). Ethanol promoted a striking accumulation of intracellular malate in hepatocytes incubated with propionate (reaching 14.9 μmol/g cell) and led to a depletion of phosphoenolpyruvate; ethanol inhibition could be counteracted by pyruvate. Propionate and butyrate enhanced ureogenesis from ammonia in ruminant liver cells but their effects were not additive. Propionate also elicited a marked increase in cellular concentrations of phosphoserine and serine, particularly in the presence of ammonia; such effects could influence phospholipid metabolism in the liver. These findings emphasize the contribution of propionate, compared with the other glucogenic substrates, to glucose synthesis in ruminants and point to the possibilities of modulation of the glucogenicity of propionate by various substrates which may be present in portal blood.

Effects of in vitro addition of ammonia on the metabolism of 15N-labelled amino acids in isolated sheep hepatocytes

“…Oxygen consumption rates (6.15 to 6.77 µL O 2 mg cell DM -1 h -1 ) were similar to those previously reported for adult sheep hepatocytes (McBride and Milligan 1985). Linear rates of gluconeogenesis (1.51 to 2.13 µmol g cell DM -1 min -1 ) and ureagenesis (0.19 to 0.25 µmol g cell DM -1 min -1 ) in the presence of propionate and NH 4 Cl as substrates, respectively, are similar to those previously reported for ruminant hepatocytes isolated using similar techniques (Looney et al 1987;Demigné et al 1991). Hepatocyte dry weight of the original cell preparation was determined in duplicate by drying cells suspended in KrebsHenseleit incubation buffer (1 mL) at 100°C with correction for the residual dry weight of an equal volume of incubation buffer without hepatocytes.…”

Interaction between protein and energy supply on protein utilization in growing cattle: A review

“…Moreover, infusion of either glucose into the duodenum or propionate into the rumen increased milk protein output, reducing oxidation of AA and improving the overall efficiency of AA use (Hurtaud et al, 1998;Raggio et al, 2006). In vitro studies with isolated sheep hepatocytes (Demigne et al, 1991), suggested inhibitory effect of glucogenic energy sources on some other gluconeogenic pathways, sparing AA for increased lean tissue growth or milk protein production. Conversely, glucogenic (glucose and propionate) and lipogenic (fat and acetate) energy sources produced similar increases in N retention in growing steers provided with supplemental energy and protein infusates (see later: Schroeder et al, 2006b).…”

Metabolic implications of ammonia production in the ruminant

“…Krebs et al (1979) suggested that increased amino acid utilization with increased ureagenesis may be the result of competition between gluconeogenic and ureagenic pathways for cytoplasmic oxaloacetate. Gluconeogenesis is a major synthetic pathway in fed ruminant liver and NH3 has been demonstrated to inhibit glucose synthesis by ovine hepatocytes (Weekes et al 1978;Aiello & Armentano, 1987;DemignC et al 1991) and urea feeding to calves has been shown to decrease glucose disposal rates (Spires & Clark, 1979). The experiments of Wilton et al (1988) and Maltby et al (l991,1993a), however, have failed to confirm this effect in vivo since net hepatic glucose release is not altered either by mesenteric vein NH3 infusion or by feeding urea to calves.…”