Starvation and experimental diabetes induce a stable increase in
pyruvate dehydrogenase kinase (PDK) activity in skeletal muscle, which is largely due to a selective upregulation of PDK-4 expression. Increased
free fatty acid (FFA) level has been suggested to be responsible for the upregulation. Because these metabolic states are also characterized by
insulin deficiency, the present study was designed to examine whether
insulin has a significant effect to regulate PDK
mRNA expression in rat skeletal muscle. In study 1, overnight-fasted rats received an infusion of saline or
insulin for 5 h (n = 6 each). During the
insulin infusion, plasma
glucose was clamped at basal levels (euglycemic hyperinsulinemic clamp). A third group (n = 6) received
Intralipid infusion during the clamp to prevent a fall in plasma FFA. PDK-2
mRNA level in gastrocnemius muscle was not altered by
insulin or FFA (i.e.,
Intralipid infusion). In contrast, PDK-4
mRNA level was decreased 72% by
insulin (P < 0.05), and
Intralipid infusion prevented only 20% of the decrease. PDK-4
protein level was decreased approximately 20% by
insulin (P < 0.05), but this effect was not altered by
Intralipid infusion. In study 2, overnight-fasted rats were refed or received an infusion of saline or
nicotinic acid (NA, 30 micromol/h) for 5 h (n = 5 each). During the refeeding and NA infusion, plasma FFA levels were similarly (i.e., 60-70% vs. saline control) lowered. Muscle PDK-4
mRNA level decreased 77% after the refeeding (P < 0.05) but not after the NA infusion. In conclusion, the present data indicate that
insulin had a profound effect to suppress PDK-4 expression in skeletal muscle and that, contrary to previous suggestions, circulating FFA had little impact on PDK-4
mRNA expression, at least within 5 h.