AbstractThe incubation of isolated rat hepatocytes with extracellular adenosine 5′‐trihosphate (ATP) resulted in an inhibition of Ca2+efflux. The ATP‐induced Ca2+accumulation as determined by the increase in phosphorylaseaactivity and the Ca2+‐sensitive fluorescent indicator (2‐[(2‐bis‐[carboxymethyl]‐amino‐5‐methylphenoxy)‐methyl]‐6‐methoxy‐8‐bis‐[carboxymethyl] aminoquinoline‐tetrakis‐[acetoxymethyl]ester) (Quin 2‐AM) was associated with both the hydrolysis of ATP and the phosphorylation of a 110 kDa protein. No significant alteration in the intracellular ATP level was observed.The appearance of surface blebs and cytotoxicity followed the rise in cytosolic Ca2+, suggesting that the increased free Ca2+may be responsible for the loss of viability. When a calmodulin inhibitor, 1‐[bis(4‐chlorophenyl)methyl]‐3‐[2‐(2,4‐dichlorophenyl)‐2‐[(2,4‐dichlorophenyl)methoxy]ethyl]‐1H‐imidazolium chloride (calmidazolium), was included in the medium prior to ATP addition, bleb formation was reduced and the loss of viability was completely prevented, indicating that a Ca2+‐calmodu