Evidence in vivo for elevation of intracellular free Ca2+ in the liver after diquat, acetaminophen, and CCl4.

Several hepatotoxic agents with varied chemical mechanisms of toxicity (acetaminophen, diquat, and CCl4) depress membrane calcium pumps and/or enhance the permeability of membranes to calcium. To probe the relevance of these findings to maintenance of calcium homeostasis after toxins in vivo, we measured the activity of glycogen phosphorylase a, as an index of cytosolic free [Ca2+], in freeze-clamped liver samples obtained at several times after the toxin dose. Both acetaminophen and diquat caused significant increases of phosphorylase a activity, and activity remained elevated for several hours after the dose. Significantly, the administration prior to diquat of desferrioxamine, which offers protection against the liver necrosis and depression of microsomal Ca2+ accumulation observed after diquat alone (Tsokos-Kuhn et al., Mol Pharmacol 34: 209-214, 1988), decreased phosphorylase activation. Activation of phosphorylase was observed also after CCl4 administration, as previously reported by Long and Moore (Biochem Pharmacol 35: 4131-4137, 1986). We conclude that perturbations in liver membrane Ca2+ regulation observed after administration of these hepatotoxins in vivo correlate directly with phosphorylase a activity, thereby providing additional in vivo evidence for an alteration of Ca2+ homeostasis early in the development of the liver damage produced by these chemicals.