Studies on the influence of diet on the metabolism of propionate in ovine liver and adipose tissue in vitro
The effects of an increased production of rumen propionate, achieved by manipulation of the diet, on the metabolism of propionate in ovine liver were investigated. In experiment 1 two groups of six sheep were fed on diets of either 100% cubed dried grass or a pelleted diet containing 25% dried grass, 70% ground barley. It was shown that the rate of production of [14C02] from [1-14] or [2-14C] propionate by liver slices prepared from sheep receiving the barley diet increased nearly up to a propionate concentration of 2M. The rate of [14CO2] production by liver slices prepared from sheep given the diet of dried grass showed no further increase above a propionate concentration of 500 mM. The proportion of odd-numbered fatty acids (ONFA) in the subcutaneous adipose tissue of sheep receiving the barley diet was significantly higher than that in the adipose tissue of sheep given the dried grass diet. In experiment two three groups of four sheep were fed diets of 100% dried grass (Diet G) or 10% dried grass, 90% pelleted ground barley (Diet B) or 10% dried grass, 90% pelleted ground barley containing 33% mg/kg monensin (Diet M). Diet M was not readily accepted, one animal on this diet died. The specific activities of acetyl-CoA synthetase, NADP -dependent malate dehydrogenase in liver and adipose tissue, of phosphoenolpyruvate carboxykinase in liver cytoplasm and of propionyl-CoA carboxylase and NADP+-dependent malate dehydrogenase in adipose tissue were found to be significantly higher in sheep receiving diet B when compared to those in sheep given diet G. No increase in the proportion of ONFA was found. An HPLC method was developed to determine the specific activities of certain TCA cycle intermediates and glucose. Over a range of concentrations of propionate from 0-20 mM it was found that in hepatocytes prepared from sheep receiving diet B, the flux of carbon from malate to citrate was maintained at the higher concentrations of propionate. In hepatocytes prepared from sheep fed on diet G the flux of carbon through this reaction was lower at high propionate concentrations than it was at low propionate concentrations. When hepatocytes prepared from sheep fed of diet G were incubated with 2 mM propionate, 40% of the propionate carbon converted to malate was further metabolised to glucose whereas 60% was converted to citrate. At a propionate concentration of 20 mM the proportion of propionate-derived malate that was converted to glucose increased to 62% whereas that converted to citrate decreased to 35%. Much smaller changes were observed when similar incubations were carried out using hepatocytes prepared from sheep fed in diet B. A comparison of the results obtained from incubations of hepatocytes and liver slices showed the advantages offered by the former tissue preparation.