The present study evaluated the substrate competition between
fatty acids (FA) and
glucose in the kidney in vivo in relation to the operation of the "
glucose-FA" and "reverse
glucose-FA" cycles. In fed rats, neither inhibition of adipocyte lipolysis by
5-methylpyrazole-3-carboxylic acid (MPCA) nor inhibition of mitochondrial long-chain FA oxidation by 2-tetradecylglycidate (TDG) influenced the renal ratio of free/acylated
carnitine or the percentage of total renal
pyruvate dehydrogenase complex (PDHC) in the active (dephosphorylated) form (PDHa). The additional provision of
glucose, a precursor for the synthesis of
malonyl-coenzyme A (
coA), did not influence renal PDHa activity or the renal ratio of free to acylated
carnitine, implying that FA oxidation is maximally suppressed in the fed state. A reverse
glucose-FA cycle may therefore be important in suppressing renal FA oxidation in the fed state. After 48 hours of
starvation, MPCA and TDG decreased short- and long-chain
acylcarnitine concentrations (40% to 50%, P < .01) and elevated the renal ratio of free/acylated
carnitine (2.5-fold, P < .001, and 3.3-fold, P < .001, respectively), indicating that FA oxidation is increased after
starvation. Despite suppression of renal FA oxidation, renal PDHa activity in 48-hour starved rats was only partially restored by treatment with MPCA or TDG. The additional administration of
glucose did not remedy this. The failure to reverse completely the effects of prolonged
starvation in suppressing PDHC activity by acute inhibition of FA oxidation suggests additional regulatory mechanisms that dampen the PDHC response to acute changes in substrate supply. Estimations of
PDH kinase (PDK) activity in renal mitochondria showed a significant 1.7-fold stable increase (P < .01) after 48 hours of
starvation. Analysis of PDK
pyruvate sensitivity in renal mitochondria incubated with respiratory substrate (5 mmol/L
2-oxoglutarate/0.5 mmol/L L-
malate) showed that the
pyruvate concentration required for 50% activation was substantially decreased by
starvation.
Enzyme-linked
immunosorbent assay (ELISA) analysis over a range of PDHC activities demonstrated that increased PDK activity was concomitant with a significant (at least P < .01) 1.8-fold increase in the
protein expression of the ubiquitously expressed PDK
isoform, PDK2. We hypothesize that changes in
protein expression and activity of individual PDK
isoforms may dictate the renal response to incoming FA lesterification v oxidation) through modulation of the relationship between glycolytic flux and PDHC activity, and thus the provision of precursor for
malonyl-coA production.