Somatotropin and adipose tissue metabolism: Substrate and temporal effects

The purpose of these studies was to determine the time course for changes i n feed intake, blood metabolites, and lipogenic activity in adipose tissue in response to the initiation of porcine somatotropin (pST) treatment and f ollowing withdrawal from treatment in barrows. An initial study was conduct ed to determine the impact of chronic pST treatment (4 wk of daily injectio n; 0 vs 4 mg/d) on adipose tissue lipid metabolism in barrows (initial weig ht 67 kg). Feed efficiency was improved 27%, backfat thickness was decrease d 43%, and glucose and lactate oxidation and incorporation into lipid in ad ipose tissue was reduced 70 to 86% in pST-treated pigs. Palmitate esterific ation was decreased 44%, whereas palmitate oxidation was unaffected. In vit ro metabolism of lactate, glucose, and palmitate in liver slices was not af fected by pST treatment. The time-course for changes in intake and adipose tissue metabolism in response to 7 d of pST (0 vs 4 mg/d) treatment and 7 d of withdrawal was examined in subsequent studies in barrows (initial weigh t 75 kg). Feed intake during pST treatment was significantly (P < .05) less than in control pigs within 24 h of the initiation of treatment and remain ed low through 3 d after withdrawal. Adipose tissue biopsies were obtained on d 0, 1, 2, 4, and 7 of the treatment phase and on d 2, 4, and 7 after wi thdrawal from 7 d of treatment. Maximal inhibition of lipogenesis by pST tr eatment in adipose tissue in vitro was observed on d 4 (-68%) and d 7 (-69% ). Similarly, fatty acid synthase activity declined during the treatment pe riod, with the greatest change noted on d 7 (-26%). After withdrawal from t reatment, lipogenesis gradually increased, returning to control values 7 d after withdrawal. Levels of IGF-I began to increase from d 1 to d 7 of trea tment, continually decreased during withdrawal, and were normalized by the end of the withdrawal period. Plasma urea nitrogen concentrations decreased during treatment, increased during the withdrawal phase, and were normaliz ed 4 d after the last pST treatment. Overall results indicate that most of the metabolic changes in response to pST occur within 1 wk of treatment and return to pretreatment values after 7 d of withdrawal from treatment.