Ruminants, like other animals, have to meet the nutritional demands of the many organs and cell types of the body. This has to be done against a background of a varying, and not always adequate, supply of nutrients. Thus, once absorbed there are a number of potential fates for a given nutrient and a plethora of mechanisms . | 17 Metabolic Regulation . Vernon Hannah Research Institute Ayr KA6 5HL UK Introduction Ruminants like other animals have to meet the nutritional demands of the many organs and cell types of the body. This has to be done against a background of a varying and not always adequate supply of nutrients. Thus once absorbed there are a number of potential fates for a given nutrient and a plethora of mechanisms and factors which influence the probability of a given fate. Such mechanisms operate within cells between different cells and types within a tissue and between organs. Mechanisms may be brought into play to deal with acute or chronic challenges the former are important for homoeo-stasis while the latter are critical for the homoeorrhetic adaptations needed for different developmental physiological nutritional or pathological states. The nature of these mechanisms and the various types of factors involved are considered in subsequent sections. It will be obvious to those familiar with the previous edition of this book that the flavour of the current chapter is very different from that written by the late Bernard Crabtree. He focused on the important but still rather specialized field of mathematical modelling of metabolic pathways and their regulation for those interested in this aspect I strongly recommend Bernard s chapter Crabtree 1993 and also articles by Brown 1994 Kacser et al. 1995 and Hofmeyr and Cornish-Bowden 2000 . Levels of Metabolic Control Within cells Metabolic pathways Within cells the fate of a nutrient is determined not only by the activity of relevant enzymes but in some cases at least by i translocases reflecting the fact that the CAB International 2005. Quantitative Aspects of Ruminant Digestion and Metabolism 2nd edition eds J. Dijkstra . Forbes and J. France 443 444 . Vernon cell is highly structured and a metabolic pathway may be split between more than one compartment necessitating translocation of metabolites between compartments and
