Effect of intraruminal urea administration on glucose metabolism in dairy steers.

Isotope dilution techniques were used to study steady-state glucose kinetics in four rumen-fistulated Holstein steers and to study the effect of rapid absorption of ammonia from the rumen on glucose metabolism. Steers were fed a high-concentrate diet at hourly intervals from automatic feeders. Plasma glucose specific activity curves following single intravenous injection of [2-3H]glucose were used to construct a two-compartment model of the glucose pool with inflow and outflow from compartment one. Primed continuous infusion of [2-3H]glucose was used to determine the steady-state turnover rate of glucose and to monitor changes in the rates of inflow and outflow of glucose from the glucose pool following a single dosage of urea (0.4 g/kg body weight) into the rumen. Compartment sizes of the glucose pool were 65.6 and 33.5 g for compartments 1 and 2, respectively. Glucose turnover rate during steady-state was 15.4 mg/minute/kg body weight 0.75 and transfer rate of glucose between compartments was 17.9 mg/minute/kg body weight 0.75. Concentrations of rumen ammonia-nitrogen, plasma ammonia-nitrogen and plasma urea-nitrogen were 6.1, 0.5 and 4.0 mg/100 ml, respectively, before urea dosage. Rumen ammonia-nitrogen, plasma ammonia-nitrogen increased after urea dosage and reached peak concentrations, 170.0 and 1.2 mg/100 ml, respectively, approximately 120 minutes after urea dosage. Plasma urea-nitrogen increased linearly throughout the 4-hour sampling period and reached 12.0 mg/100 ml at end of the experiment. Concentration of glucose in plasma increased from 98.2 mg/100 ml before urea dosage to 114.6 mg/100 ml at 100 minutes after urea dosage. Estimates of glucose production and utilization indicated that the increased concentration of glucose in plasma in all steers was due, at least partially, to a decrease in the rate of glucose utilization. A rapid rate of glycogenolysis which resulted in a marked increase in the plasma glucose concentration also was evident in one steer.