The relationship between ATP depletion and the loss of cell integrity was examined in the killing of hepatocytes by anoxia and P388D1 macrophages by silica. ATP depletion is a feature of the reaction to either hazard. Treatment of hepatocytes, however, with antimycin, oligomycin, sodium azide, or N,N'-dicyclohexylcarbodiimide produced a rate and extent of ATP depletion comparable with anoxia without significant loss of viability. Treatment of P388D1 cells with 2-deoxyglucose plus antimycin, oligomycin, or sodium azide reproduced the loss of ATP accompanying silica particle intoxication. Again, there was no loss of viability. These data dissociate the loss of cellular ATP from the genesis of lethal injury in both cell types. ATP depletion was, however, associated with a loss of lysosomal integrity. With the metabolic inhibitors, loss of lysosomal integrity occurred in the absence of irreversible cell injury over the time course that anoxia and silica intoxication significantly damaged the cells. This implies that neither hazard produces lethal damage through mechanisms dependent on intracellular lysosomal enzyme release. While ATP depletion can cause lysosomal rupture in P388D1 macrophages, phagocytosis of silica particles in the absence of extracellular Ca2+ ions is associated with release of lysosomal contents without depletion of ATP or loss of cell integrity. Silica particles are concluded to interact directly with both the plasma and lysosomal membranes. The former leads to Ca2+ influx with resultant cell death and ATP depletion. The latter leads to release of lysosomal contents that is not followed by irreversible cell injury.

The relationship between intracellular lysosomal rupture and cell death caused by silica was studied in P388D, macrophages. After 3 h of exposure to 150 Jig silica in medium containing 1 .8 mM Ca", 60% of the cells were unable to exclude trypan blue . In the absence of extracellular Ca", however, all of the cells remained viable . Phagocytosis of silica particles occurred to the same extent in the presence or absence of Ca" . The percentage of P388D, cells killed by silica depended on the dose and the concentration of Ca" in the medium . Intracellular lysosomal rupture after exposure to silica was measured by acridine orange fluorescence or histochemical assay of horseradish peroxidase . With either assay, 60% of the cells exposed to 150 Ftg silica for 3 h in the presence or absence of Ca" showed intracellular lysosomal rupture, whereas cell death occurred only in the presence of Ca" . Intracellular lysosomal rupture was not associated with measurable degradation of total DNA, RNA, protein, or phospholipid or accelerated turnover of exogenous horseradish peroxidase . Pretreatment with promethazine (20 pg/ml) protected 80% of P388D, macrophages against silica toxicity although lysosomal rupture occurred in 60-70% of the cells. Intracellular lysosomal rupture was prevented in 80% of the cells by pretreatment with indomethacin (5 x 10 -5 M), yet 40-50% of the cells died after 3 h of exposure to 150 p,g silica in 1 .8 mM extracellular Ca" . The calcium ionophore A23187 also caused intracellular lysosomal rupture in 90-98% of the cells treated for 1 h in either the presence or absence of extracellular Ca t+ . With the addition of 1 .8 mM Ca 2+, 80% of the cells was killed after 3 h, whereas all of the cells remained viable in the absence ofCa". These experiments suggest that intracellular lysosomal rupture is not causally related to the cell death caused by silica or A23187 . Cell death is dependent on extracellular Ca 2+ and may be mediated by an influx of these ions across the plasma membrane permeability barrier damaged directly by exposure to these toxins .Lysosomes participate in the physiological turnover of cellular macromolecules (15), limited autophagy of cellular organelles, and storage of undegradable materials (13). At least 60 different enzymes capable of digesting nucleic acids, proteins, lipids, and carbohydrates (6) are contained in these membrane-bound organelles, separated from their potential substrates. Endogenous or exogenous substances enter the lysosome by fusion of autophagic (13) or endocytic vacuoles (34) with lysosomes. This confines hydrolytic processes to the lysosome and presumably prevents unregulated intracellular digestion (13) .In various pathological states, cell injury may be produced by either extracellular or intracellular release of lysosomal enzymes (16). Extracellular release of lysosomal enzymes ac-

Rat peritoneal macrophages were studied for their ability to undergo metabolic cooperation with rat fibroblasts or with other macrophages. In contrast, rat reticular cells, mesothelial cells, and fibroblasts were able to cooperate with human fibroblasts.

Genetic determinants of metabolic cooperation were studied by fusing chick erythrocytes to HGPRT- mammalian cells. Heterokaryons were then tested for their ability to incorporate [3H]hypoxanthine and to transfer radioactive material to HGPRT- recipient cells. Chick erythrocytes (CE) have nuclei which are inactive but contain the HGPRT gene and some cytoplasmic HGPRT enzyme activity. They are unable, however, to cooperate with HGPRT- cells. Of the two mammalian cell lines used, the human GM29 line is HGPRT- and capable of functioning as a receptor cell in cooperation experiments with HGPRT+ cells. The HGPRT- mouse A9 line on the other hand is unable to cooperate. Immediately after fusion, both types of heterokaryons incorporated [3H]hypoxanthine, indicating the presence of some chick HGPRT enzyme contributed by the erythrocyte partner at the time of fusion. While the CE-GM29 heterokaryons participated in metabolic cooperation shortly after fusion, the CE-A9 heterokaryons did not. However, four days after fusion, i.e., at a time when the erythrocyte nucleus had been reactivated, the CE-A9 heterokaryons did cooperate. This suggests that in CE-A9 heterokaryons the genes required for metabolic cooperation are expressed by the previously dormant chick erythrocyte nucleus.