Using immunoblot analysis with
antibodies raised against recombinant
pyruvate dehydrogenase kinase (PDK)
isoenzymes PDK2 and PDK4, we demonstrate selective changes in PDK
isoenzyme expression in slow-twitch versus fast-twitch skeletal muscle types in response to prolonged (48 h)
starvation and refeeding after
starvation.
Starvation increased PDK activity in both slow-twitch (soleus) and fast-twitch (anterior tibialis) skeletal muscle and was associated with loss of sensitivity of PDK to inhibition by
pyruvate, with a greater effect in anterior tibialis.
Starvation significantly increased PDK4
protein expression in both soleus and anterior tibialis, with a greater response in anterior tibialis.
Starvation did not effect PDK2
protein expression in soleus, but modestly increased PDK2 expression in anterior tibialis. Refeeding for 4 h partially reversed the effect of 48-h
starvation on PDK activity and PDK4 expression in both soleus and anterior tibialis, but the response was more marked in soleus than in anterior tibialis.
Pyruvate sensitivity of PDK activity was also partially restored by refeeding, again with the greater response in soleus. It is concluded that targeted regulation of PDK4
isoenzyme expression in skeletal muscle in response to
starvation and refeeding underlies the modulation of the regulatory characteristics of PDK in vivo. We propose that switching from a
pyruvate-sensitive to a
pyruvate-insensitive PDK
isoenzyme in
starvation (a) maintains a sufficiently high
pyruvate concentration to ensure that the
glucose-->
alanine-->
glucose cycle is not impaired, and (b) may 'spare'
pyruvate for anaplerotic entry into the tricarboxylic acid cycle to support the entry of
acetyl-CoA derived from
fatty acid (FA) oxidation into the tricarboxylic acid cycle. We further speculate that FA oxidation by skeletal muscle is both forced and facilitated by upregulation of PDK4, which is perceived as an essential component of the operation of the
glucose-FA cycle in
starvation.