GROWTH-HORMONE INCREASES MUSCLE MASS AND STRENGTH BUT DOES NOT REJUVENATE MYOFIBRILLAR PROTEIN-SYNTHESIS IN HEALTHY-SUBJECTS OVER 60 YEARS OLD

The rate of synthesis of myofibrillar proteins is slower in muscle of healthy subjects over 60 yr old than it is in young adults. Previous r esearch suggests that reduced activity of the GH/insulin-like growth f actor-I system could be a determinant of this slowing of protein synth esis. To test the hypothesis that GH could rejuvenate the rate of myof ibrillar protein synthesis, we studied healthy subjects over 60 yr old , after a single injection (0.03 mg/kg, sc) of recombinant human GH (n = 6 males/2 females) or placebo (n = 6 males/2 females), or after 3 m onths of either GH (0.03 mg/kg, sc, 3 x/week, n = 5 males) or placebo (n = 5 males) treatment. Myofibrillar protein synthesis and whole-body protein metabolism were evaluated with the tracer L-[1-C-13]leucine. GH reduced whole-body leucine oxidation by 36% (P < 0.01) in the singl e injection study. There was no effect of GH on whole-body protein bre akdown or synthesis, or on myofibrillar protein synthesis in the quadr iceps, either acutely or after 3 months of treatment. GH treatment for 3 months increased lean body mass (3.3 +/- 0.7 kg, P < 0.01, as evalu ated by K-40 counting), muscle mass (3.3 +/- 1.1 kg, P < 0.02, as eval uated by urinary creatinine excretion), and thigh strength (14 +/- 5%, P < 0.05, as evaluated by isokinetic dynamometry): We conclude that G H can increase muscle mass and strength in healthy men over 60 yr old, but does not restore a youthful rate of myofibrillar protein synthesi s.