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1 Department of Exercise and Sport Sciences, CMRC, Section of Human Physiology, Copenhagen, Denmark
* To whom correspondence should be addressed. E-mail: erichter{at}ifi.ku.dk.
Previous studies have proposed that caffeine-induced activation of glucose transport in skeletal muscle is independent of AMP activated protein kinase (AMPK) because 
-AMPK Thr172 phosphorylation was not increased by caffeine. However, our previous studies, as well as the present, show that AMPK phosphorylation measured in whole-muscle lysate is not a good indicator of AMPK activation in rodent skeletal muscle. In lysates from incubated rat soleus muscle, a predominant model in previous caffeine-studies, both ACC
Ser221 and immunoprecipitated 1 AMPK activity increased with caffeine-incubation, without changes in AMPK phosphorylation or immunoprecipitated 2 AMPK activity. This pattern was also observed in mouse soleus muscle, where only ACC and 1 AMPK phosphorylation were increased following caffeine-treatment. Preincubation with the selective CaMKK inhibitor, STO-609 (5 µM), the Ca2+/CaM-competitive inhibitor, KN93 (10 µM), or the SR Ca2+_release blocking agent, dantrolene (10 µM), all inhibited ACC phosphorylation and 1 AMPK phosphorylation, suggesting that SR Ca2+-release may work through a CaMKK-AMPK pathway. Caffeine-stimulated 2-deoxyglucose (2DG) uptake reflected the AMPK activation pattern, being increased with caffeine and inhibited by STO-609, KN93 or dantrolene. The inhibition of 2DG uptake is likely causally linked to AMPK activation, since muscle-specific expression of a kinase-dead AMPK construct greatly reduced caffeine-stimulated 2DG uptake in mouse soleus. We conclude that a SR-Ca2+ activated CaMKK may control 1 AMPK activation and be necessary for caffeine-stimulated glucose uptake in mouse soleus muscle.
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