Aim: 

Heatstroke is a life-threatening illness that shows similarities to malignant hyperthe rmia (MH), a disorder typically caused by mutations in the major Ca²⁺ release channel in skeletal muscle (RyR1). MH predisposes affected individuals to potentially fatal hypermetabolic reactions triggered by halogenetic anesthetics, exertion, heat challenge, and febrile illness. There is no approved drug treatment for heatstroke and the only pharmacotherapy for MH is dantrolene, which has adverse side effects. Here we treat MH susceptible mice with 5-aminoimidazole-4carboxamide ribonucleoside (AICAR) to see if this can prevent heatstroke.

Methods/Results: 

Mice with knock-in mutation (Y524S) in RyR1 undergo an MH response when exposed to elevations (37°C) in ambient temperature and less than 5% of untreated Y524S mice survived a heat-challenge (37°C, 15 min). Injection with AICAR 10 min prior heat-challenge resulted in 100% survival. AICAR is commonly used to activate the energy sensing AMP-activated kinase (AMPK). However, our results show: no increase in AMPK phosphorylation after 25 min of AICAR injection; Y524S mice crossed with constitutively active AMPK mice still suffer heat-induced death; AICAR prevents heat-induced death in Y524S mice crossed with dominant negative AMPK expressing mice. ³H-ryanodine binding experiments show that AICAR is a partial agonist of RyR1 and prevents full activation of RyR1 in the presence of cellular concentrations of ATP both in Y524S and WT mice. Ca²⁺ imaging shows that AICAR inhibits the SR Ca²⁺ leak in Y524S muscle fibers at 37°C.

Conclusion: 

We show a novel, AMPK-independent function of AICAR: it acts as a partial RyR1 agonist that dampens RyR1 Ca²⁺ leak at high temperature, and thus, prevents the activation of RyR1 that underlies heatstroke.