Ac. Mollerloswick et al., INSULIN SELECTIVELY ATTENUATES BREAKDOWN OF NONMYOFIBRILLAR PROTEINS IN PERIPHERAL-TISSUES OF NORMAL MEN, The American journal of physiology, 266(4), 1994, pp. 50000645-50000652
The role of insulin to control protein synthesis and degradation in th
e human leg and forearm was investigated in eight healthy individuals.
The glucose clamp technique with simultaneous infusion of crystalline
amino acids were used to create hyperinsulinemia (100-120 mU/l) in co
mbination with euglycemia and elevated plasma concentrations of amino
acids ( > 4 mmol/ l). A primed constant infusion with L-[U-C-14]tyrosi
ne and L-[phenyl-H-2(5)]phenylalanine was used for simultaneous measur
ements of the disposal (protein synthesis) and the release (protein de
gradation) of tyrosine and phenylalanine, respectively, across the leg
and forearm before and during hyperinsulinemia. The balance of 3-meth
ylhistidine was also determined as a measure of muscle breakdown. Insu
lin stimulated tissue glucose and net amino acid uptake across the arm
and leg tissues, whereas the disposal of both tyrosine and phenylalan
ine (protein synthesis) was not stimulated across the arm and the leg
during hyperinsulinemia. The release of tyrosine and phenylalanine was
significantly decreased from both leg and arm tissues (protein degrad
ation) in response to insulin. However, the release of 3-methylhistidi
ne from skeletal muscles was totally unaffected by hyperinsulinemia. W
e conclude that it is unlikely that insulin contributes to the normal
stimulation of protein synthesis during feeding in humans and that ins
ulin has no effect on breakdown of the large myofibrillar protein pool
in skeletal muscles in unstressed individuals.