Effect of a prolonged low-dose lipopolysaccharide infusion on feed intake and metabolism in heifers

Prolonged infusions of bacterial lipopolysaccharides (LPS) are known to mod el Gramnegative bacterial infections, but the basic mechanisms of the LPS e ffects on feed intake and metabolism and their-potential interdependence ar e largely unknown. The aim of the present study was to distinguish and to b etter characterize the feeding suppressive:and metabolic effects of LPS. Si x heifers were infused intravenously for 100 min with either 1) LPS (2 mu g /kg BW) with free access to feed, 2) saline with free access to feed, or 3) saline with feeding restricted to the amount of feed consumed after LPS in fusion. Feed intake, body temperature, plasma concentrations of various met abolites and hormones, and the respiratory quotient and heat production wer e measured. The LPS reduced feed intake and induced pronounced changes in m etabolic energy turnover and fat and carbohydrate metabolism that were larg ely independent of the concomitant feed intake reduction. Some of the metab olic changes were biphasic; the first phase resembled a stress response wit h increases in plasma glucose and cortisol, and the second phase reflected a beginning energy deficit with low plasma glucose-and enhanced lipolysis. The coincidence of a short-term surge of plasma insulin with marked transie nt decreases in plasma FFA, glycerol, and beta-hydroxybutyrate as well as w ith the transition from hyper- to hypoglycemia indicates that insulin plays a role in some of the metabolic responses to LPS. The failure of LPS to cl early increase energy expenditure despite the increase in body temperature suggests that anaerobic mechanisms of heat production and, perhaps, a reduc ed peripheral blood flow contributed to the fever. Many of the initial meta bolic responses occurred before and, therefore, independent of, an increase in circulating tumor necrosis factor-alpha.