Somatotropin increases protein balance by lowering body protein degradation in fed, growing pigs

Abstract

Somatotropin (ST) administration enhances protein deposition in well-nourished, growing animals. To determine whether the anabolic effect is due to an increase in protein synthesis or a decrease in proteolysis, pair-fed, weight-matched (∼20 kg) growing swine were treated with porcine ST (150 μg ⋅ kg− 1 ⋅ day− 1, n = 6) or diluent ( n = 6) for 7 days. Whole body leucine appearance (Ra), nonoxidative leucine disposal (NOLD), urea production, and leucine oxidation, as well as tissue protein synthesis (Ks), were determined in the fed steady state using primed continuous infusions of [13C]leucine, [13C]bicarbonate, and [15N2]urea. ST treatment increased the efficiency with which the diet was used for growth. ST treatment also increased plasma insulin-like growth factor I (+100%) and insulin (+125%) concentrations and decreased plasma urea nitrogen concentrations (−53%). ST-treated pigs had lower leucine Ra (−33%), leucine oxidation (−63%), and urea production (−70%). However, ST treatment altered neither NOLD nor Ks in the longissimus dorsi, semitendinosus, or gastrocnemius muscles, liver, or jejunum. The results suggest that in the fed state, ST treatment of growing swine increases protein deposition primarily through a suppression of protein degradation and amino acid catabolism rather than a stimulation of protein synthesis.