The effect of feed intake on nutrient and hormone levels in jugular and portal blood in goats

SUMMARY Adult goats provided with permanent jugular and portal vein catheters were used to study the effect of feeding hay and concentrates on blood composition in order to determine which blood constituents might act as signals in the control of feed intake. Animals were fed limited amounts of feed, or were allowed to eat unlimited amounts for restricted periods, once or twice daily. Blood samples were collected at hourly intervals up to 9 h after feeding and often more frequently in the initial hour, and were analysed for volatile fatty acids (VFA), lactate, glucose, insulin and glucagon. Feed consumption resulted in rapid and large increments of jugular blood acetate, propionate, n-butyrate and 3-methylbutyrate concentrations whereas isobutyrate and 2-methylbutyrate increased only slightly. Jugular propionate, n–butyrate and 3–methyl–butyrate increased within 10 min and acetate within 20 min after feed was offered, both when hay and when concentrates were eaten. Fasting decreased circulating VFA to very low values. As a rule changes of portal VFA paralleled those in the general circulation although both the concentrations and the changes were much greater. Plasma lactate increased within 30 min on a concentrate diet but not at all on a hay diet. Blood glucose declined in the initial hour after feeding and subsequently increased, resulting in a more or less elevated plateau. In some but not all experiments glucose declined before feed consumption, probably owing to ‘anticipation’. Circulating plasma insulin and glucagon appeared to increase slightly, if at all, after feeding. Increased feed intake was reflected in higher base line glucose and insulin concentrations in the jugular vein but the jugular glucose and insulin response to feed consumption was unaffected. Jugular glucagon was not influenced by the amount of feed. Jugular insulin and glucagon sometimes showed a brief peak during eating. Portal glucose, insulin and glucagon gave essentially the same pattern as seen in the jugular vein. In conclusion, it appears that, of the substances studied, acetate, propionate, nbutyrate and 3-methylbutyrate are most suited for the function of feedback signals in the physiological control of feed intake in goats under the conditions of the present experiment. It seems less probable that glucose, insulin and glucagon can play such a role.