Role of AMP-activated protein kinase in exercise capacity, whole body glucose homeostasis, and glucose transport in skeletal muscle -insight from analysis of a transgenic mouse model-.

To examine the role of muscle AMP-activated protein kinase (AMPK) in maximal exercise capacity, whole body glucose homeostasis, and glucose transport in skeletal muscle, we generated muscle-specific transgenic mice carrying cDNAs of inactive AMPK alpha2 (alpha2i TG). Fed blood glucose was slightly higher in alpha2i TG mice compared to wild type littermates, however, the difference was not statistically significant. In alpha2i TG mice, glucose tolerance was slightly impaired in male, but not in female mice, compared to wild type littermates. Maximal exercise capacity was dramatically reduced in alpha2i TG mice, suggesting that AMPK alpha2 has a critical role in skeletal muscle during exercise. We confirmed that known insulin-independent stimuli of glucose transport including mitochondrial respiration inhibition, hyperosmolarity, and muscle contraction increased both AMPK alpha1 and alpha2 activities in isolated EDL muscle in wild type mice. While, alpha2 activation was severely blunted and alpha1 activation was only slightly reduced in alpha2i TG mice by these insulin independent stimuli compared to wild type mice. Mitochondrial respiration inhibition-induced glucose transport was fully inhibited in isolated EDL muscles in alpha2i TG mice. However, contraction- or hyperosmolarity-induced glucose transport was nearly normal. These results suggest that AMPK alpha2 activation is essential for some, but not all insulin-independent glucose transport.

AMP-activated protein kinase alpha2 activity is not essential for contraction- and hyperosmolarity-induced glucose transport in skeletal muscle.

To examine the role of AMP-activated protein kinase (AMPK) in muscle glucose transport, we generated muscle-specific transgenic mice (TG) carrying cDNAs of inactive alpha2 (alpha2i TG) and alpha1 (alpha1i TG) catalytic subunits. Extensor digitorum longus (EDL) muscles from wild type and TG mice were isolated and subjected to a series of in vitro incubation experiments. In alpha2i TG mice basal alpha2 activity was barely detectable, whereas basal alpha1 activity was only partially reduced. Known AMPK stimuli including 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR), rotenone (a Complex I inhibitor), dinitrophenol (a mitochondrial uncoupler), muscle contraction, and sorbitol (producing hyperosmolar shock) did not increase AMPK alpha2 activity in alpha2i TG mice, whereas alpha1 activation was attenuated by only 30-50%. Glucose transport was measured in vitro using isolated EDL muscles from alpha2i TG mice. AICAR- and rotenone-stimulated glucose transport was fully inhibited in alpha2i TG mice; however, the lack of AMPK alpha2 activity had no effect on contraction- or sorbitol-induced glucose transport. Similar to these observations in vitro, contraction-stimulated glucose transport, assessed in vivo by 2-deoxy-d-[(3)H]glucose incorporation into EDL, tibialis anterior, and gastrocnemius muscles, was normal in alpha2i TG mice. Thus, AMPK alpha2 activation is essential for some, but not all, insulin-independent glucose transport. Muscle contraction- and hyperosmolarity-induced glucose transport may be regulated by a redundant mechanism in which AMPK alpha2 is one of multiple signaling pathways.