The effects of fructose on adenosine triphosphate depletion following mitochondrial dysfunction and lethal cell injury in isolated rat hepatocytes

Abstract

Mitochondrial injury in aerobic mammalian cells is associated with a rapid depletion of adenosine triphosphate (ATP) which occurs prior to the onset of lethal cell injury. In this report, the relationships between ATP depletion and lethal cell injury were examined in rat hepatocytes using oligomycin as a model mitochondrial toxicant and fructose as an alternative carbohydrate source for glycolysis. Oligomycin was more potent in causing lethal cell injury in hepatocytes isolated from fasted animals than cells from fed animals. The onset of cell injury (leakage of lactate dehydrogenase) in cells from fed animals correlated with the depletion of stored glycogen and ATP. The degree and time course profile of oligomycin-induced ATP depletion could be duplicated with 50 mM fructose alone in hepatocytes from fasted animals; however, fructose did not cause lethal cell injury. Oligomycin caused marked accumulation of adenosine monophosphate (AMP) and inorganic phosphate (Pi) and a conservation of adenine nucleotides. In contrast, fructose (50 mM) caused a decrease in Pi, no persistent change in AMP, and a depletion of the adenine nucleotide pool. Fructose, at concentrations greater than 1.0 mM, protected hepatocytes from oligomycin-induced toxicity. Blockade of mitochondrial ATP synthesis with oligomycin resulted in massive ATP depletion. In the presence of oligomycin, 5.0 mM fructose maintained cellular ATP content similar to that of control cells, whereas 50 mM fructose did not, demonstrating the biphasic effect of increasing fructose concentrations on cellular ATP content. Fructose-induced protection of hepatocytes from oligomycin toxicity was due to glycolytic fructose metabolism as hepatocytes incubated with iodoacetate (30 microM), fructose, and oligomycin had reduced viability and ATP content. In conclusion, interruption of mitochondrial ATP synthesis leads to marked ATP depletion and lethal cell injury. Cell injury is clearly not due to ATP depletion alone since increased glycolytic ATP production from either glycogen or fructose can maintain cell integrity in the absence of mitochondrial ATP synthesis and at low cellular ATP levels.