RESPONSES OF THE SPLANCHNIC TISSUES OF RUMINANTS TO CHANGES IN INTAKE- ABSORPTION OF DIGESTION END-PRODUCTS, TISSUE MASS, METABOLIC-ACTIVITY AND IMPLICATIONS TO WHOLE ANIMAL ENERGY-METABOLISM

This review focuses on the influence of intake on the availability of digestion end products, on splanchnic tissue weights and metabolic act ivity, and on the consequences of these changes on whale animal energy metabolism in ruminants. An increase in intake reduces diet digestibi lity and modifies the relative importance of the sites of digestion. P assive absorption of volatile fatty acids depends on changes in blood flow with intake, whereas active absorption capacity of nutrients such as glucose does not seem to be greatly modified by intake. Consequent ly, with most diets, excluding the maize-based diets, the amount but n ot the balance of digestion end products is altered. Weights of the ga strointestinal tract and of the liver are subsequently increased with intake, due to the effects of both bulk and nutrient supply. Response time is very rapid and probably results from changes in tissue protein degradation rates and, to a lesser extent, from changes in tissue pro tein fractional synthesis rates. Metabolic rate of gut tissue is lower than that of liver but much higher than that of hind limbs. Intake ma y alter splanchnic tissue metabolic rates over a very short time perio d; however, no effects have been noted in the longer term. The combina tion of splanchnic tissue weight changes with intake and of high metab olic rates has important implications on whole animal energy metabolis m. The increment in whole animal energy expenditure with intake origin ating from the portal drained viscera is 17-61%; from the liver, 16-44 %; and from the carcass, 5-7%.