A compelling mystery in the study of exercise is mechanisms of skeletal muscle fatigue. Broadly described, muscle fatigue is the uncomfortable sensation that particular muscle groups are shutting down and muscle force production cannot continue. More specifically, muscle fatigue is defined as an activity-induced inability to continue to produce a desired level of force. Several groups suggest that a major cause of force loss during fatigue is reductions in the rates of sarcoplasmic reticulum (SR) calcium (Ca2+) release and uptake. These changes result in diminished contractile machinery activation, reduced force production and slowed relaxation.

During exercise, adenosine 5'-triphosphate (ATP) is the energy currency that is used to support force production. As a result of ATP hydrolysis and re-synthesis, adenosine diphosphate (ADP) and adenosine monophosphate (AMP) levels rise. AMP kinase (AMPK) is an enzyme that becomes activated as a result of increased AMP levels. It is thought to function as a metabolic "master switch" within the muscle and plays a major role in carbohydrate and fat metabolism. Once AMPK is activated it regulates several ATP consuming and producing pathways. The overall objective of this project was to determine if increased metabolism during exercise contributes to SR Ca2+ dysfunction during fatigue. If this is true, artificial activation of AMPK via 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) should induce changes in SR function that are qualitatively similar to those caused by fatigue.

In study 1, mice were injected with 0.85 mg/kg AICAR (or saline solution) and both gastrocnemius muscles were removed one hour later. In study 2, EDL muscles were placed in a muscle bath and incubated in AICAR (4mM) or stimulated to fatigue. Glycogen, glucose-6-phosphate (G-6-P), ATP, ADP, and phosphocreatine (PCr) were examined in all groups of muscles. Alterations in SR calcium uptake and release rates due to the presence of AICAR were also studied as a likely cause of muscle fatigue.

AICAR treatment in vivo did not alter muscle glycogen, glucose, ATP, ADP or PCr concentrations. However, G-6-P levels were increased by 137%. This was accompanied by a 36% reduction in the SR Ca2+ uptake rate and a 42% reduction in Ca2+-stimulated Ca2+ ATPase activity as well as 13-15% reductions in the rates of Ca2+ release. These changes were not associated with SR Ca2+ pump content. Administration of AICAR in vitro also increased G-6-P content (200%) without altering the concentrations of glycogen, glucose, G-6-P, ATP, ADP or PCr. AICAR decreased SR Ca2+ uptake rate by 28% and the rate of Ca2+ release by 16%. For comparison, fatiguing stimulation reduced the rates of SR Ca2+ uptake and release by 31 and 41%, respectively.

Taken together, these results indicate that when administered to skeletal muscle both in vivo and in vitro, AICAR evokes metabolic stress. More importantly, activation of AMPK alters skeletal muscle SR function to an extent that is qualitatively similar to that caused by fatiguing activity. At present, it is not clear how AMPK activation causes changes in SR function. However, the present finding is consistent with the notion that metabolic stress caused by exercise, affects SR function. This, in turn, leads to force loss but reduces energy demand and protects the cell from ATP depletion during maximal contractile activity.