Hepatic metabolism of propionate and methylmalonate in growing lambs

The hepatic extraction ratio or propionate and the net hepatic flux of methylmalonate (MMA) were investigated in six Suffolk lambs (32.7 +/- 1.7 kg BW) implanted with catheters in a mesenteric artery and in the portal, hepatic, and ruminal veins and a cannula in the rumen. The lambs were fed a pelleted barley-based diet (1.1 kg DM per day) and subjected to three intravenous infusion protocols separated by at least 7 days: saline (C; control), butyrate (B; 0.2 mmol/h/kg BW) or lactate (L; 0.96 mmol/h/kg BW). The solutions were infused continuously into the ruminal vein for 13 h. At the same time as the intravenous infusion treatments, propionate (1.78 mmol/h/kg BW) was infused intraruminally for 4 h starting 4 h after the initiation of the experimental protocol. Arterial, portal and hepatic blood samples were obtained during the last 10 h of the infusion protocol. Blood flow was measured by down stream dilution of p-aminohippuric acid. The hepatic extraction ratio of propionate decreased from 85 +/- 3 to 75 +/- 1% after intraruminal infusion of propionate and the net splanchnic appearance of propionate increased threefold with the doubling of the net portal appearance of propionate. Intravenous infusion of lactate or butyrate did not affect the hepatic extraction ratio or net splanchnic flux of propionate. The net portal appearance, net hepatic flux and net splanchnic appearance of MMA were not (P > 0.10) different from zero in all situations. The arterial concentration of MMA increased with increasing absorption rate of propionate but remained below 2 mumol/l throughout the experiment. The results of the present study do not support the hypothesis that the MMA possibly incorporated into methyl-branched chain fatty acids in adipose tissue of intensively-reared lambs is of hepatic origin. (C) 2002 Elsevier Science B.V. All rights reserved.