Mechanism of insulin's anabolic effect on muscle: measurements of muscle protein synthesis and breakdown using aminoacyl-tRNA and other surrogate measures.
Level 3 - non-randomized controlled study
Controlled human physiological trial (randomization not specified in abstract)
PubMed 16705065 · doi:10.1152/ajpendo.00003.2006
What was done
Eighteen healthy subjects in the postabsorptive state received continuous infusions of leucine and phenylalanine tracers. After a 3-hour baseline, 10 participants received a 4-hour femoral arterial infusion of insulin (0.125 mU·kg⁻¹·min⁻¹) while 8 participants received saline. Tracer-to-tracee ratios of leucine, phenylalanine, and ketoisocaproate were measured across arterial and venous plasma, muscle tissue fluid, and aminoacyl-tRNA to calculate rates of muscle protein synthesis and breakdown.
What was found
Insulin infusion significantly decreased leucine and phenylalanine efflux from the muscle bed compared to saline. Muscle protein breakdown based on leucyl-tRNA decreased by 28% with insulin (P < 0.02). Insulin infusion had no significant effect on muscle protein synthesis.
Why it matters
Using direct precursor aminoacyl-tRNA measurements alongside traditional surrogate pools, this study demonstrates that insulin's anabolic effect on skeletal muscle in fasting humans is mediated by suppressing protein breakdown rather than stimulating protein synthesis.
Limits
Small total sample size of 18 subjects. Randomization is not explicitly reported in the abstract. Evaluates only local arterial infusion in the postabsorptive state; effects under systemic hyperinsulinemia or combined with amino acid feeding were not assessed.
Cited by
- supports A published study by K. Sreekumaran Nair's group found that insulin is not required for muscle protein synthesis, but serves primarily to inhibit muscle protein breakdown.