Several factors could explain these findings. First,
several studies have suggested that endotoxin or proinflammatory
mediators may inactivate pyruvate dehydrogenase
or phosphoenolpyruvate carboxykinase enzymes,
thus potentially blocking two metabolic pathways for lactate
metabolism [30,31]. Second, hepatic lactate uptake might
be a saturable process with second order kinetics [18].
Therefore, repeated exogenous lactate loading over a short
period could lead to lactate accumulation by saturating
metabolic clearance. However, the same loading in sham
animals did not produce any accumulation, suggesting that
a metabolic dysfunction should be present for this to occur.
Third, impairment in hepatic microcirculation might be
implicated [32-35]. Doubtlessly, severe liver microcirculatory
abnormalities can be found in the presence of normal
total hepatic flow. Either portal or hepatic arterial LPS
injection leads to a rapid and transient increase in
intrahepatic resistances at the presinusoidal level
[31-35]. Redistribution of intrahepatic blood flow in
concert with a complex interplay between sinusoidal
endothelial cells, liver macrophages, and passing leukocytes,
leads to a decreased perfusion and blood flow
velocity in the liver sinusoids [31-35]. Activation and
dysfunction of the endothelial cell barrier, together with
abnormalities in the nitric oxide pathway, with subsequent
invasion of neutrophils and formation of microthrombi
may further enhance liver tissue ischemia
and damage [31-35]. Unfortunately, we did not specifically
assess hepatic microcirculation. However, preserved liver
VO2 and normality of ex-vivo mitochondrial respiration,
taken together with the absence of positive portohepatic
vein lactate gradients, makes it unlikely that this
phenomena had some impact in lactate clearance.
Fourth, local production of lactate by activated
immune cells in the liver during sepsis is another
biologically plausible cofactor [11,36]. Theoretically, the
impact on local lactate production could be proportional
to the inflammatory stimulus, and thus might
change according to the experimental model selected.
Unfortunately, our study design only allowed us to
address some of these potential mechanisms, but we were
able to rule out liver hypoperfusion or a generalized
metabolic dysfunction.