Figure 4.

In response to high workload, rates of glucose uptake exceed rates of glucose oxidation, hexose 6‐phosphate accumulates, and mTOR is activated. A, Rates of glucose uptake exceeded rates of glucose oxidation in hearts from rats pretreated with either vehicle or rapamycin and perfused at normal and high workloads. Pretreating rats with rapamycin significantly reduced rates of both glucose uptake and oxidation (right). Data shown are mean±SEM; n=5 to 6 per group. *P=0.07 with Mann–Whitney rank sum test. B, Glucose 6‐phosphate (G6P) levels in freeze‐clamped hearts. Subjecting hearts to high workload ex vivo induced a 4‐fold increase in average G6P levels, which was not observed when rats were pretreated with rapamycin. Dot plots show G6P levels for each heart. Kruskal–Wallis test yielded overall P=0.012. C, To test the hypothesis that hexose‐6‐phosphate (and no other glucose metabolite) activates mTOR, we also perfused hearts with NCS plus 2‐deoxyglucose or 3‐O‐methylglucose (see text for details). Representative Western blots of Akt, TSC2, mTOR and p70S6K phosphorylation from hearts perfused with glucose (5 mmol/L) or NCS plus the glucose analogues 2‐deoxyglucose (2DG; 5 mmol/L), or 3‐O‐methylglucose (3OMG; 5 mmol/L) at either normal or high workload are shown. Glucose or 2DG, but not 3OMG, increased phosphorylation of TSC2, mTOR, and p70S6K at high workload. mTOR indicates mammalian target of rapamycin; TSC2, tuberin; NCS, noncarbohydrate substrate.