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CYTOKINES BLOCK THE EFFECTS OF INSULIN‐LIKE GROWTH FACTOR‐I (IGF‐I) ON GLUCOSE UPTAKE AND LACTATE PRODUCTION IN SKELETAL MUSCLE BUT DO NOT INFLUENCE IGF‐I-INDUCED CHANGES IN PROTEIN TURNOVER

There is evidence that proinflammatory cytokines are involved in the regulation of muscle protein breakdown in various catabolic conditions but the mechanisms are not fully understood. Previous studies suggest that cytokines reduce circulating and tissue levels of insulin-like growth factor-I (IGF-I) and may block the anabolic effects of the hormone in certain cell types and tissues. We tested the hypothesis that a mixture of tumor necrosis factor α, interleukin-1α, and interferon-γ block the anabolic effects of IGF-I in skeletal muscle. Muscles from burned or unburned rats were incubated in the absence or presence of 1 μg/mL of IGF-I with or without the addition of the cytokines. As expected, IGF-I stimulated protein synthesis and inhibited protein breakdown in incubated muscles. The cytokines did not influence protein turnover rates in muscles incubated with or without IGF-I. In additional experiments, the effects of IGF-I on glucose uptake and lactate production were tested. IGF-I increased glucose uptake ∼2.5-fold and stimulated lactate production ∼5-fold. These effects of the hormone were significantly inhibited by the cytokine mixture. The results suggest that cytokines do not induce protein catabolism by directly inhibiting the anabolic effects of IGF-I in muscle tissue. The inhibitory effects of the cytokines on IGF-I-stimulated glucose transport and lactate production suggest that the lack of effect of cytokines on protein metabolism was not due to a metabolic unresponsiveness of the incubated muscles to the cytokines.