Growth hormone acutely stimulates skeletal muscle but not whole-body protein synthesis in humans.

Abstract

In a previous study, a 6-hour local infusion of growth hormone (GH) into the brachial artery of normal subjects stimulated net muscle protein anabolism by augmenting skeletal muscle protein synthesis. In the present study, we examined whether systemically infused GH affects forearm and whole-body protein metabolism. Normal volunteers aged 18 to 24 years (n = 8) were given an 8-hour systemic infusion of 3H-phenylalanine and 14C-leucine. Between 90 and 120 minutes of tracer infusion, basal samples for determination of forearm and whole-body amino acid kinetics were taken. GH was then infused at 0.06 micrograms/kg/min, increasing GH concentration from 2.4 +/- 0.3 to 32 +/- 3 ng/mL. Systemic insulin-like growth factor 1 (IGF-1) level increased from 224 +/- 20 to 262 +/- 21 ng/mL (basal v 6-hour, P < .01). By 6 hours, GH suppressed forearm phenylalanine and leucine net release (each P < .05) by increasing 3H-phenylalanine (66%, P < .05) and 14C-leucine (13%, P < .05) extraction or disposal (Rd). Whole-body leucine rate of appearance ([Ra] an index of whole-body proteolysis) and nonoxidative leucine Rd (whole-body protein synthesis) did not change over the course of the GH infusion, whereas oxidative leucine Rd decreased (20%, P < .03). Acute stimulation of muscle but not whole-body protein synthesis by systemically infused GH suggests that muscle protein is acutely and specifically regulated by GH.