Hormonal and nutritional modulation of hepatic arginase activity in growing cattle.

The hormonal and nutritional modulation of hepatic arginase activity (HARG) was characterized in growing cattle in two studies. In the first study, 20 steers (initial weight, 182 +/- 2 kg) were assigned in equal numbers to either Synovex-S (SYN) (ear implant), recombinant bovine somatotropin (Somavubove; SbV; 0.1 mg/kg intramuscularly daily), SYN + SbV, or nothing (control). Steers were individually fed, for 56 d, a concentrate (80% diet dry matter [DM]) and silage (20% diet DM) diet providing 20 g of crude protein (CP) and 252 kcal metabolizable energy (ME) per kg body weight0.75. On Day 57, samples of liver were obtained at slaughter and subsequently assayed for HARG by the incubation of a tissue homogenate for 2 hr with 250 mM arginine, with and without Mn2+ and heat activation, and the measurement of the resulting urea. HARG was uniformly increased by divalent cation (Mn2+) and heating. SYN had no effect on HARG, whereas SbV treatment resulted in an overall 34% decrease in HARG. Plasma urea nitrogen (PUN) was decreased by SbV but not consistently affected by SYN. In the second study, 16 steers (avg. initial weight, 284 +/- 5 kg) were initially fed a concentrate basal diet consisting of 11% CP and 1.96 Mcal ME for 21 d. Steers were then assigned to one of four dietary treatments (6.4 kg DM/hr per day) in a factorial arrangement of high and low CP (8 and 14%) and two levels of energy (1.96 and 2.67 Mcal ME/kg of diet DM) for 210 d and slaughtered. HARG and PUN were higher in steers fed 14% CP but were lower at each level of CP fed at the higher level of ME. The data suggest that hormonal repartitioning compounds and diet composition may modulate nitrogen metabolism by affecting the activity of arginase in the liver as well as by affecting the total content of arginase in association with changes in organ size.