Control of muscle protein breakdown: effects of activity and nutritional states.

We propose that there is a link between muscle protein synthesis and breakdown that is regulated, in part, through maintenance of the free intracellular pool of essential amino acids. For example, we propose that muscle protein breakdown is paradoxically elevated in the anabolic state following resistance exercise in part because the even greater stimulation of synthesis would otherwise deplete this pool. Thus, factors regulating muscle protein breakdown must be evaluated in the context of the prevailing rate of muscle protein synthesis. Further, the direct effect of factors on breakdown may depend on the physiological state. For example, local hyperinsulinemia suppresses accelerated muscle protein breakdown after exercise, but not normal resting breakdown. Thus, factors regulating muscle protein breakdown in human subjects are complex and interactive.