Toxic injury from mercuric chloride in rat hepatocytes

The relationship between cytosolic free Ca²⁺, mitochondrial membrane potential, ATP depletion, pyridine nucleotide fluorescence, cell surface blebbing, and cell death was evaluated in rat hepatocytes exposed to HgCl₂. In cell suspensions, 50 μM HgCl₂ oxidized pyridine nucleotides between 1/2 and 2 min, caused ATP depletion between 2 and 5 min, and produced an 89% loss of cell viability after 20 min. Rates of cell killing were identical in high (1.2 mM) and low (2.6 μM) Ca²⁺ buffers. Cytosolic free Ca²⁺ was determined in 1-day cultured hepatocytes by ratio imaging of Fura-2 employing multiparameter digitized video microscopy. In high Ca²⁺ medium, HgCl₂ caused a 3-4-fold increase of free Ca²⁺ beginning after 6-7 min, but free Ca²⁺ did not change in low Ca²⁺ medium. Bleb formation occurred after about 4-5 min in both buffers prior to any increase of free Ca²⁺. Subsequently, in high Ca²⁺ medium, blebs became hot spots of free Ca²⁺ (> 600 nM). After about 2 min of exposure to HgCl₂, rhodamine 123 fluorescence redistributed from mitochondrial to cytosolic compartments signifying collapse of the mitochondrial membrane potential. The results taken together demonstrate that bleb formation, ATP depletion, and the onset of cell death are not dependent on an increase of cytosolic free Ca²⁺. HgCl₂ toxicity appears to be a consequence of inhibition of oxidative phosphorylation leading to ATP depletion and cell death.