Metabolic effects of porcine somatotropin: nitrogen and energy balance and characterization of the temporal pattern of blood metabolites and hormones.

Twelve barrows (61 kg) received daily injections of either pituitary-derived porcine somatotropin (pST; 120 micrograms/kg body weight) or an equivalent volume of excipient. Blood samples were taken 1 d before initiation and on d 9 of treatment to characterize the temporal patterns of blood hormones and metabolites. Nitrogen (N) and energy (E) balances were performed on d 14 through 19 of treatment. Pigs treated with pST retained more N than controls (38.4 vs 23.0 g/d, P less than .01) and had a slightly higher apparent digestibility of N and E (88 vs 84% and 88 vs 85%, respectively, P less than .05). However, this improvement in apparent digestibility was probably a consequence of the lower feed intake (23% reduction) rather than a direct effect of pST. Treatment with pST resulted in chronic elevations in circulating concentrations of insulin-like growth factor I (406 vs 171 ng/ml), glucose (114 vs 86 mg/dl), insulin (3.9 vs 1.6 ng/ml), and nonesterified fatty acids (NEFA; 135 vs 59 mu Eq/liter). Diurnal patterns of glucose, insulin, and NEFA were also altered in pST-treated pigs. Plasma urea N levels were decreased with pST treatment (9.6 vs 24.0 mg/dl), but the diurnal pattern was unaltered. Our results indicate that the effect of pST is primarily on postabsorptive use of nutrients. In growing pigs, pST exerts a profound effect on N metabolism and on circulating concentrations of hormones and metabolites.