Submitted on September 5, 2008
Revised on October 24, 2008
Accepted on October 25, 2008
The fed-state clamp stimulates cellular mechanisms of muscle protein anabolism and modulates glucose disposal in normal men
Olasunkanmi A.J. Adegoke1, Stephanie Chevalier2, Jose A. Morais3, Rejeanne Gougeon2, Scot R. Kimball4, Leonard S. Jefferson5, Simon S. Wing2, and Errol B. Marliss3*
1 York University
2 McGill University
3 Royal Victoria Hospital
4 Pennsylvania State College of Medicine
5 Pennsylvania State University College of Medicine
* To whom correspondence should be addressed. E-mail: marliss.errol{at}muhc.mcgill.ca.
Since maximum anabolism occurs postprandially, we developed a simulated fed state with clamped hyperinsulinemia, physiological hyperglycemia and hyperaminoacidemia (Hyper-3), and explored muscle cellular mechanisms. Whole-body 1-[13C]leucine and 3-[3H]glucose kinetics in healthy men were compared between hyperinsulinemic, euglycemic, isoaminoacidemic (Hyper-1, n=10) vs. Hyper-3 (n=9) clamps. In Hyper-3, vs Hyper-1, nonoxidative leucine Rd (synthesis) was stimulated more (45±4 vs. 24±4, P<0.01) and endogenous Ra (breakdown) was inhibited similarly, hence net balance increased more (86±6 vs. 49±2, P<0.001, µmol/min). Glucose Rd was similar, thus Hyper-3 metabolic clearance rate (331±23 vs. 557±41 mL/min, P<0.0005) and Rd/insulin (M, 0.65±0.10 vs. 1.25±0.10 mg•min-1/pmol•L-1, P<0.001) were less, despite higher insulin (798±74 vs. 450±24 pmol/L, P<0.005). In vastus lateralis muscle biopsies, phosphorylation of Akt (P=0.025), mTOR, p70S6K1 (P=0.008), S6 (P=0.049) and 4E-BP1 (P=0.001) increased. With decreased eIF4E *4E-BP1 complex (P=0.01), these are consistent with increased mTORC1 signaling and translation initiation of protein synthesis. Although mRNA expression of ubiquitin, MAFbx 1 and MuRF-1 was unchanged, total ubiquitinated proteins decreased 20% (P<0.01), consistent with proteolysis suppression. The Hyper-3 clamp increases whole-body protein synthesis, net anabolism, muscle protein translation initiation pathways and decreases protein ubiquitination. The main contribution of hyperaminoacidemia is stimulation of synthesis rather than inhibition of proteolysis, and it attenuates the expected increment of glucose disposal.
