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We have investigated the importance of covalent binding and lipid peroxidation on the depression of microsomal calcium sequestration associated with in vitro metabolism of 14CCl4. Studies with CBrCl3 are also reported. In aerobic systems, promethazine was used to block lipid peroxidation, measured as malondialdehyde (MDA) generation. Effects of low levels of lipid peroxidation were tested in Fe2+-supplemented systems free of halogenated hydrocarbons. The results indicate that microsomal calcium sequestration can be depressed significantly by metabolism of either CCl4 or CBrCl3 in the absence of MDA generation, or by lipid peroxidation occurring in the absence of halogenated hydrocarbons.

The calcium fluorescent probe fura2 was used to measure concentration of free calcium in the cytosol of isolated rat hepatocytes in suspension. The resting level in untreated hepatocytes was 121 nM. On addition of CCl4 at a concentration of 0.5 mM, cytosolic free calcium rose sharply and reached a statistically significant (P less than 0.05) steady plateau level of about 190 nM within five minutes. With a concentration of 1.0 mM CCl4, cytosolic free calcium rose within ten minutes to a plateau level of about 200 nM. Use of fura2, along with the capacity of Mn2+ ions to effectively quench fura2 fluorescence, provided the basis for a simple and decisive method to determine whether the added CCl4 was permeabilizing the hepatocyte plasma membrane by direct solvent action. It was found that up to a concentration of 1.0 mM, CCl4 did not permeabilize the plasma membrane, but direct attack on the plasma membrane was unequivocally demonstrated for concentrations of 2 mM CCl4 and above. Finally, an hypothesis is presented for resolution of the puzzling dilemma that emerged from the observation, reported from two laboratories, that CCl4 can rapidly mobilize liver mitochondrial calcium despite the well-known relative resistance of these organelles to the damaging effects of this toxic agent.