THE EFFECTS OF ACUTE AND CHRONIC PROTEIN DEPLETION AND ACCRETION ON PLASMA-CONCENTRATIONS OF INSULIN-LIKE GROWTH-FACTOR-I, FIBRONECTIN AND TOTAL PROTEIN FOR RUMINANTS NOURISHED BY INTRAGASTRIC INFUSION OF NUTRIENTS

The relationships of N input or protein status and the concentrations of serum insulin-like growth factor-1 (IGF-1), plasma fibronectin (FN) and total protein (TP) were examined in three experiments with steers and sheep nourished by intragastric infusion of nutrients. In Expt 1, three steers (340 kg live weight) were infused with three levels of v olatile fatty acids (0, 300 and 600 kJ/kg metabolic weight (W-0.75) pe r d) and six levels of casein (0, 200, 400, 650, 1500 and 2500 mg N/kg W-0.75 per d). Each N treatment was imposed for 5 d. In Expts 2 and 3 , five groups of sheep (about 35 kg live weight) were infused with cas ein at 500 mg N/kg W-0.75 per d for 2 weeks followed by 1500, 500 or 5 0 mg N/kg W-0.75 per d in Expt 2, and in Expt 3, with 100 mg N/kg W-0. 75 per d for 6 weeks or 10 mg N/kg W-0.75 per d for 4 weeks. Non-prote in energy was maintained constant at 500 kJ/kg W-0.75 per d throughout . Daily N balance and total body N content at the end were measured, a nd protein status was defined as a percentage of cumulative N accretio n or depletion in relation to the total body N content at maintenance. It was found that IGF-1 and FN responded rapidly and substantially to altered N input, and that when daily N input was maintained constantl y at sub-maintenance, their continuous declines were related closely t o progressive protein depletion in the sheep. Plasma TP concentration was independent of N input when N input was altered acutely in the ste ers, but declined significantly and gradually with severe, chronic bod y protein depletion in the sheep. Plasma content of TP in the sheep ho wever reduced acutely with a reduction in N input. Plasma volume fell substantially over the first 2 weeks of protein depletion, compensatin g for the declines in TP content and maintaining TP concentration plat eau. The possible implications of the changes in TP concentration and content (concentration x volume) to body protein Loss in sheep are dis cussed.