To address this question, we looked at changes in circulating levels of cortisol, a hormone indicative of stress. Using catheterized fish, Antonella Pagnotta (M.Sc., 1993; Pagnotta et al., 1994) found that circulating levels of cortisol were elevated after exercise for a period of up to 6 hours. She also found that if we experimentally prevent the rise in plasma cortisol (by injection fish with metyrapone, which blocks cortisol synthesis), the fish recover much more quickly (within 2 hours). See Fig 1.

More recently, we have shown that in fish allowed to swim at low velocity after a bout of exhaustive exercise there is no elevation in plasma cortisol and that recovery, as indicated by restoration of muscle glycogen, is complete within 2 hours (Milligan et al., 2000).

It looks like cortisol is affecting the activity of the two key enzymes regulating glycogen metabolism. Glycogen phosphorylase is responsible for catalyzing the breakdown of glycogen and its activity is highest, not surprisingly, right after the period of exercise (indicated by hatched bar). In fish in which cortisol levels are kept low, phosphorylase activity is turned off much more quickly than in controls. This allows glycogen synthase (which catalyzes glycogen synthesis) activity to increase more quickly, resulting in faster glycogen synthesis. See Fig 2.

We now know that cortisol is acting directly at the level of the muscle. Jason Frolow, in his M.Sc. research developed an in vitro muscle slice preparation which is capable of synthesizing glycogen. The results from his work indicate that glycogen levels themselves determine whether the muscle will be glycogenic or glycogenolytic. If glycogen levels are > 5 umol/g, then dexamethasone (a cortisol analog) will stimulate glycogenolysis, but if levels are <5 umol/g, then glycogenesis is the predominant response.

There are several issues still unresolved regarding muscle glycogen metabolism, such as substrate use, metabolic pathway, hormonal regulation, that can be addressed now that we have a viable, glycogen-synthesizing, in vitro muscle preparation.