Influence of Homeorhetic State on Ruminant Metabolism and Substrate Selection for Hepatic Gluconeogenesis

Abstract

We hypothesized that the contribution of amino acids to hepatic gluconeogenesis would increase when glucose demand was increased in ruminants. Our first approach was to establish a sheep model for increased glucose demand. Four wethers injected with excipient or.5, 1.0, or 2.0 g/d of phlorizin excreted an average of 0,72.7, 97.9, and 98.5 g/d of glucose into the urine. Wethers injected with 1.0 g/d of phlorizin excreted approximately 50% of their probable daily glucose production into their urine and adapted by 24 h after the first injection. Phlorizin-induced changes in plasma variables suggested increased gluconeogenesis from amino acids. In experiment 2, isolated hepatocytes from 10 Dorset wethers injected with excipient or 1.0 g/d of phlorizin for 72 h were used to determine the effects of increased glucose demand on conversion of (1-$\sp{14}$C) propionate and (1-$\sp{14}$C) alanine to CO$\sb2$ and glucose. Phlorizin tended to increase conversion of substrates to CO$\sb2.$ Conversion of (1-$\sp{14}$C) alanine to glucose was increased more by phlorizin treatment than that of (1-$\sp{14}$C) propionate (285 vs. 166% of controls). Phlorizin increased estimated V$\sb{\rm max}$ for conversion of substrates to CO$\sb2$ and tended to increase estimated V$\sb{\rm max}$ for conversion of substrates to glucose. Addition of 2.5 mM NH$\sb4$Cl in vitro decreased conversion of (1-$\sp{14}$C) propionate to glucose and CO$\sb2$ but had little effect on conversion of (1-$\sp{14}$C) alanine. These data suggest that potential utilization of amino acids for gluconeogenesis increases when glucose demand is increased. In experiment 3, thirty-six Holsteins were fed high grain or high fat diets for restricted or ad libitum intake prepartum, and then all fed a lactation diet postpartum. Liver slices determined in vitro conversion of (1-$\sp{14}$C) propionate and (1-$\sp{14}$C) alanine to CO$\sb2$ and glucose. Conversion of substrates to products was increased at 1 and 21 d postpartum compared with 21 d prepartum and 65 d postpartum. Conversion of (1-$\sp{14}$C) propionate to products was greater than conversion of (1-$\sp{14}$C) alanine; however, conversion of (1-$\sp{14}$C) alanine to glucose at 1 and 21 d postpartum was 198% and 150%, respectively, of that at 21 d prepartum and conversion of (1-$\sp{14}$C) propionate to glucose at 1 and 21 d postpartum was 119% and 129%, respectively, of that at 21 d prepartum. The ratio of 3-methyl histidine to creatinine in urine was increased markedly postpartum until 21 d postpartum. Results indicate that amino acids are more important as gluconeogenic substrates during the first 3 wk of lactation than at other stages of the lactation cycle. Furthermore, prepartum manipulation of dietary energy has more influence on hepatic propionate metabolism than amino acid metabolism.