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 research suggests that reduced activity of the GH/insulin-like growth factor-I system could be a determinant of this slowing of protein synthesis. To test the hypothesis that GH could rejuvenate the rate of myofibrillar 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 months 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-[1-13C]leucine. GH reduced whole-body leucine oxidation by 36% (P < 0.01) in the single injection study. There was no effect of GH on whole-body protein breakdown or synthesis, or on myofibrillar protein synthesis in the quadriceps, 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 evaluated by 40K counting), muscle mass (3.3 +/- 1.1 kg, P < 0.02, as evaluated by urinary creatinine excretion), and thigh strength (14 +/- 5%, P < 0.05, as evaluated by isokinetic dynamometry). We conclude that GH can increase muscle mass and strength in healthy men over 60 yr old, but does not restore a youthful rate of myofibrillar protein synthesis.

Postprandial myofibrillar and whole body protein synthesis in young and old human subjects.

Rates of incorporation of leucine (using L-[1-13C]leucine as a tracer) into myofibrillar and whole body proteins were determined in healthy old (> 60 yr old, n = 7) and young (< 30 yr old, n = 9) men and women who were fed small meals (4% of daily energy) every 30 min. There was no difference in whole body incorporation of leucine into proteins in the young (148 +/- 5 mumol.h-1.kg lean body mass-1, means +/- SE) and old groups (150 +/- 3 mumol.h-1.kg lean body mass-1). However, the fractional myofibrillar protein synthesis in the vastus lateralis muscle was 28% slower in the older group (0.063 +/- 0.004 vs. 0.088 +/- 0.003 %/h, P < 0.001). Extrapolation of these results to whole body myofibrillar synthesis (fractional rate x myofibrillar mass estimated by creatinine excretion) indicated that, in the older group, total myofibrillar synthesis was 43% slower (1.8 +/- 0.2 vs. 3.1 +/- 0.2 g/h, P < 0.01) and that their myofibrillar synthesis was a smaller portion of whole body protein synthesis (15 +/- 1 vs. 23 +/- 1%, P < 0.001). Compared with age-matched postabsorptive subjects, whole body protein synthesis was approximately 25% faster, and fractional myofibrillar synthesis was approximately 50% faster in these fed subjects, both young and old. We conclude that myofibrillar synthesis is slower in older subjects during both postabsorptive and postprandial conditions but that aging does not impair the stimulatory effect of feeding on protein synthesis.