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.
