Caffeine-induced Ca2+ release increases AMPK-dependent glucose uptake in rodent soleus muscle

Previous studies have proposed that caffeine-induced activation of glucose transport in skeletal muscle is independent of AMP-activated protein kinase (AMPK) because α-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 acetyl-CoA carboxylase-β (ACCβ) Ser221 and immunoprecipitated α₁-AMPK activity increased with caffeine incubation, without changes in AMPK phosphorylation or immunoprecipitated α₂-AMPK activity. This pattern was also observed in mouse soleus muscle, where only ACCβ and α₁-AMPK phosphorylation were increased following caffeine treatment. Preincubation with the selective CaMKK inhibitor STO-609 (5 μM), the CaM-competitive inhibitor KN-93 (10 μM), or the SR Ca ²⁺ release blocking agent dantrolene (10 μM) all inhibited ACCβ phosphorylation and α₁-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, KN-93, 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 α₁-AMPK activation and be necessary for caffeine-stimulated glucose uptake in mouse soleus muscle.