GH and IGF-I differentially increase protein synthesis in skeletal muscle and jejunum of parenterally fed rats.

Growth hormone (GH) and insulin-like growth factor I (IGF-I) selectively increase tissue mass. We compared the fractional rate of protein synthesis (Ks in skeletal muscle, jejunal mucosa and muscularis, and liver to investigate the differential effects of GH and IGF-I on tissue protein synthesis. Surgically stressed rats were maintained with hypocaloric total parenteral nutrition (TPN) and given recombinant human (rh) GH (rhGH), rhIGF-I, rhGH + rhIGF-I (800 or 800 + 800 micrograms/day, respectively), or TPN alone. After 3 days, a flooding dose of valine (800 mumol with 5.56 MBq L-[3,4-3H]valine) was administered, and rats were killed 20 min later. Body weight gain, nitrogen retention, and serum IGF-I concentrations confirmed that GH plus IGF-I additively increased anabolism. Serum insulin concentrations were significantly increased by GH and decreased by IGF-I. GH significantly increased Ks in skeletal muscle and jejunal muscularis, IGF-I significantly increased Ks in jejunal mucosa and muscularis, and neither GH nor IGF-I altered Ks in liver. GH and IGF-I differentially increase tissue protein synthesis in vivo.