Intracellular calcium concentration impairment in hepatocytes from thioacetamide-treated rats. Implications for the activity of Ca2+-dependent enzymes

Thioacetamide induced a severe perivenous necrosis followed by a hepatocellular regenerative response, when administered in a single dose of 6.6 mmol/kg to rats. As (Ca2+)i plays an important role in both toxic cell killing and cell proliferation, the disturbances in the basal cytosolic calcium as well as the levels of Ca2+ sequestered in the endoplasmic reticulum were determined in hepatocytes isolated at 0, 12, 24, 48 and 72 h after thioacetamide administration. The basal Ca2+ increased progressively, reaching a maximum at 24 h of the intoxication (205%, p<0.001), while the microsomal sequestered Ca2+ decreased at 24 h to 16% (p<0.001) when compared with untreated controls. Changes in the activity of glycogen phosphorylase a paralleled those of basal free calcium and showed the maximum value also at 24 h (291%; p<0.001). Moreover; there was a close association in time between the basal concentration of Ca2+ and the inhibition of microsomal Ca2+-dependent ATPase activity. Conclusions: The significant decrease in the levels of GSH and protein thiols indicates that oxidative stress is involved in thioacetamide-induced cell injury, but these decreases did not precede changes in cytosolic Ca2+ level. In the sequence of events leading to hepatic cell injury and regeneration, thioacetamide mobilized hepatic (Ca2+)i via inhibition of microsomal Ca2+-ATPase which may have activated Ca2+-dependent mechanisms involved both in cell death and in acute mitogen response.