Temporal changes in the involvement of pyruvate dehydrogenase complex in muscle lactate accumulation during lipopolysaccharide infusion in rats

Abstract

A characteristic manifestation of sepsis is muscle lactate accumulation. This study examined any putative (causative) association between pyruvate dehydrogenase complex (PDC) inhibition and lactate accumulation in the extensor digitorum longus (EDL) muscle of rats infused with lipopolysaccharide (LPS), and explored the involvement of increased transcription of muscle-specific pyruvate dehydrogenase kinase (PDK) isoenzymes. Conscious, male Sprague-Dawley rats were infused i.v. with saline (0.4 ml h(-1), control) or LPS (150 mug kg(-1) h(-1)) for 2 h, 6 h or 24 h (n = 6-8). Muscle lactate concentration was elevated after 2, 6 and 24 h LPS infusion. Muscle PDC activity was the same at 2 h and 6 h, but was 65% lower after 24 h of LPS infusion (P < 0.01), when there was a 47% decrease in acetylcarnitine concentration (P < 0.05), and a 24-fold increase in PDK4 mRNA expression (P < 0.001). These changes were preceded by marked increases in tumour necrosis factor-alpha and interleukin-6 mRNA expression at 2 h. The findings indicate that the early (2 and 6 h) elevation in muscle lactate concentration during LPS infusion was not attributable to limited muscle oxygen availability or ATP production (evidenced by unchanged ATP and phosphocreatine (PCr) concentrations) or to PDC inhibition, whereas after 24 h, muscle lactate accumulation appears to have resulted from PDC activation status limiting pyruvate flux, most probably due to cytokine-mediated up-regulation of PDK4 transcription.

Figure 1. Extensor digitorum longus muscle lactate concentrations during saline (control) and LPS infusion

Values represent mean ± s.e.m.‡P < 0.05, significantly different from time-matched control. *P < 0.05, significantly different from 2 h LPS-treated muscle. n = 6–8 in each group.

Figure 2. Extensor digitorum longus muscle PDC activity during saline and LPS infusion

Values represent mean ± s.e.m.‡‡‡P < 0.001, significantly different from time-matched control. *P < 0.05, significantly different from 2 h LPS-treated muscle. †P < 0.05, significantly different from 6 h LPS treated muscle. n = 6–8 in each group.

Figure 3. Fold changes in PDK2 (A) and PDK4 (B) mRNA expression from corresponding control value after LPS infusion within extensor digitorum longus

A value > 1 indicates greater than control mRNA expression and < 1 is lower than control mRNA expression. Values represent mean ± s.e.m.‡‡‡P < 0.001, significantly different from time-matched control. †P < 0.05, significantly different from 6 h LPS-treated muscle. n = 6–8 in each group.

Figure 4. Fold changes in TNF-α (A) and IL-6 (B) mRNA expression from corresponding control value after LPS infusion within extensor digitorum longus

A value > 1 indicates greater than control mRNA expression and < 1 is lower than control mRNA expression. Values represent mean ± s.e.m.‡P < 0.05, ‡‡P < 0.01, significantly different from time-matched control. n = 6–8 in each group.

Figure 5. Mechanism of TNF and IL-6 inhibition of PDC activity and lactate accumulation

LDH, lactate dehydrogenase; PDC, pyruvate dehydrogenase complex; PDK4, pyruvate dehydrogenase kinase isoform 4; CAT, carnitine acetyltransferase; TCA, tricarboxylic acid cycle; FOXO, forkhead transcription factor; MURF-1, muscle ring finger 1; MAFbx, muscle atrophy F-box/Atrogin-1. ↑ denotes up-regulation ↓ denotes down-regulation.