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.

The effects of large amounts of volatile fatty acids (VFA) on hepatic metabolism have been investigated in vivo, with rats adapted to a high fiber (HF) diet, or in vitro with isolated hepatocytes. Net absorption of glucose was negligible and lactate production was lower in rats fed the HF diet than in those fed a fiber-free basal diet. VFA (particularly acetate and propionate) were absorbed in very large amounts in rats fed the HF diet. Propionate and butyrate were completely removed by the liver in both groups of rats; the efficiency of acetate uptake was higher in rats fed the HF diet than in those fed the basal diet. Gluconeogenesis was active in rats fed the HF diet, but lactate uptake was very limited in spite of high portal concentrations. Hepatocytes from rats fed the HF diet utilized VFA at different rates: acetate was poorly utilized, propionate utilization plateaued at about 1 mM external propionate, whereas butyrate utilization was utilized about twice as efficiently. Propionate and butyrate displayed opposite effects on lactate utilization. The stimulating effect of butyrate prevailed over the inhibitory effects of propionate, at high concentrations (2 mM). However, the results at lower concentrations (less than or equal to 0.5 mM) suggest that, owing to its higher portal concentrations, the effects of propionate on lactate uptake might prevail in vivo. The effects of acetate in vivo might be greater than on isolated hepatocytes.

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Summary. Factors involved in metabolic adjustments in ruminants.In ruminants, the digestive tract and liver metabolism represents a substantial part of the energy requirements.

The purpose of this study was to characterize the glycogenolytic response to catecholamines and glucagon in isolated sheep hepatocytes. In this species, epinephrine appeared to exert its action on hepatic glycogenolysis by altering the cytosolic concentrations of both adenosine 3',5'-cyclic monophosphate (cAMP) and Ca2+. In contrast to results obtained in rat hepatocytes, glucagon failed to induce a rise in free cytosolic Ca2+ in sheep liver. Experiments on isolated hepatocytes or on liver plasma membranes showed that in sheep, glucagon was more efficient than epinephrine in promoting the production of cAMP. In the presence of glucagon or epinephrine, the activation of the glycogen phosphorylase a always appeared greater in sheep than in rat liver cells, whereas the variations in cellular cAMP were quite limited in sheep. The alpha 1- and beta-agonists (phenylephrine and isoproterenol) were alone as efficient as epinephrine in promoting phosphorylase a activation in sheep hepatocytes. All these results indicate the existence in sheep liver of a glycogen phosphorylase highly responsive to hormones.