Growth hormone stimulates skeletal muscle protein synthesis and antagonizes insulin's antiproteolytic action in humans.
Level 4 - case-series / case-control
Non-randomized, uncontrolled human physiological interventional study with sequential infusions
PubMed 1607069 · doi:10.2337/diab.41.4.424
What was done
Seven healthy adults received a 6-hour brachial artery infusion of growth hormone (GH) at a dose that did not alter systemic hormone concentrations. For the final 3 hours, insulin was coinfused locally at a dose known to suppress proteolysis by 30-40% without altering systemic levels. Forearm glucose balance and amino acid kinetics were measured at 3 hours (GH alone) and 6 hours (GH plus insulin) using [3H]phenylalanine and [14C]leucine tracer infusions.
What was found
Forearm glucose uptake did not change significantly between 3 and 6 hours. By 6 hours, combined GH and insulin infusion promoted a significantly more positive net balance for phenylalanine, leucine, isoleucine, and valine (all P < 0.05). Net balance improvements were driven by an increase in the rate of disappearance (Rd) for phenylalanine (+51%, P < 0.05) and leucine (+50%, P < 0.05), while the rate of appearance (Ra, reflecting proteolysis) showed no significant reduction.
Why it matters
This study demonstrates that while growth hormone acts alongside insulin to stimulate muscle protein synthesis, it directly antagonizes insulin's normal ability to inhibit muscle breakdown.
Limits
The sample size was very small (n = 7), and the study lacked a randomized parallel control group receiving insulin alone or vehicle under identical conditions. Effects were measured locally in forearm skeletal muscle over hours, so systemic and longer-term adaptations were not assessed.
Cited by
- supports Only a low concentration of insulin is required to inhibit muscle proteolysis.