Nitrogen metabolism and somatotropin secretion in beef heifers receiving abomasal arginine infusions

Twelve Angus x Hereford heifer calves (233 kg) were fitted with abomasal infusion cannulas and used to study N and endocrine responses to abomasally infused arginine (Arg). Heifers were allotted randomly to three treatment groups and received continuous abomasal infusions (2 liters/d) of water (CON) or Arg solutions providing .33 g Arg.HCl/kg BW (LOW) or .50 g Arg.HCl/kg BW (HIGH) each day. A 12-d dietary adjustment period preceded a 7-d infusion and collection period. Each calf received 4,544 g DM/d of a basal diet in equal portions at 0600, 1200, 1800 and 2400. Calves were housed in individual metabolism crates and fitted with urinary bladder catheters for total excreta collection. On d 1 and 5, blood samples were collected at 15-min intervals for 8 h between 1200 and 2000. Single samples were obtained at 1400 on remaining days. The infusion of Arg increased the quantity of N retained by heifers (P less than .01) and the percentage of total N retained (P less than .10); however, no differences were observed between LOW and HIGH heifers. Increased (P less than .01) urinary N excretion by Arg heifers was associated with greater (P less than .05) quantities of urinary urea N and ammonia N. Blood urea N and serum Arg concentrations were highest (P less than .05) in Arg heifers, whereas total serum AA concentrations were lower (P less than .05) in Arg heifers than in CON heifers. Serum glucose and insulin concentrations were not affected (P greater than .10) by treatment. Characterization of somatotropin (STH) profiles revealed that amplitude and frequency of STH pulses were not affected (P greater than .10) by treatment, whereas mean (P less than .10) and basal (P less than .05) STH concentrations were elevated in HIGH compared to LOW heifers on d 1 and 5. The similar N retention responses of LOW and HIGH heifers and similar STH profiles of CON and LOW heifers suggest that the stimulatory effect of the HIGH dose on STH secretion occurred only after tissue N requirements had been satisfied.