1. Circulating concentrations of glucose, propionate, lactate and pyruvate, and net exchange of these compounds across the liver and gut, were measured in lactating and non-lactating dairy cows (a) in the normal fed state, (b) before, during and after intravenous infusion of an aqueous solution of glucose, propionate or lactate (lactating cows only) in fed animals, and (c) before and during 6 days of food deprivation. 2. In the normal fed state, gut output of propionate, hepatic output of glucose and hepatic uptake of lactate were all higher in the lactating group. There was a net uptake of pyruvate across the liver in the lactating cows and a net output in the non-lactating cows. In the lactating cows there was a net uptake of lactate and pyruvate by the splanchnic bed (i.e. gut and liver combined). 3. In the lactating cows, the glucose and propionate infusions had the following effects: decrease in net hepatic uptake of lactate; a switch in pyruvate exchange across the liver from uptake to output; suppression of uptake of lactate and pyruvate by the splanchnic bed; increase in the magnitude of the liver (propionate uptake)/(glucose output) ratio. Lactate infusion did not affect hepatic propionate uptake. 4. Food deprivation increased hepatic extraction of lactate and pyruvate and decreased the liver (propionate uptake)/(glucose output) ratio in both groups. 5. It is concluded that mechanisms exist to ensure an inverse relationship between the availability to the cow of glucose or propionate and utilization by the splanchnic bed of endogenously derived lactate and pyruvate.

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