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The pyrrolizidine alkaloid senecionine has been shown to be hepatotoxic, genotoxic, and cytotoxic. However, the biochemical mechanism by which senecionine produces hepatocellular toxicity remains to be elucidated. The role of calcium homeostasis in toxic liver injury was examined in isolated rat hepatocytes treated with senecionine and trans-4-OH-2-hexenal (t-4HH), a microsomal metabolite of senecionine, and appropriate cofactors. Hepatocytes treated with senecionine and t-4HH demonstrated greater cytotoxicity (leakage of lactate dehydrogenase) when incubated in the absence of extracellular Ca2+ than in its presence. Both compounds elicited an increase in cytosolic Ca2+ levels of isolated hepatocytes in the presence of extracellular Ca2+. In the following study, senecionine and t-4HH depleted intracellular glutathione levels and induced lipid peroxidation and cytotoxicity in isolated hepatocytes. Pretreatment with the thiol-group reducing agent dithiothreitol prevented depletion of intracellular glutathione and protected hepatocytes against senecionine and t-4HH-induced lipid peroxidation and cytotoxicity. Both compounds also depleted intracellular ATP and NADPH levels. These results suggest that hepatotoxicity induced by senecionine and t-4HH is not dependent on the influx of extracellular Ca2+; however, alterations in intracellular Ca2+, possibly associated with depletion of intracellular glutathione, NADPH, and ATP, may play a critical role.

Lipid peroxidation was examined as a possible mechanism for cell injury by trans-4-OH-2-hexenal, the macrocyclic pyrrolizidine alkaloid senecionine and related alkenals in isolated rat hepatocytes. Each compound elicited a positive dose response for peroxidation of cellular lipids as measured by the formation of thiobarbituric acid-reactive products. The addition of the anti-oxidant N,N'-diphenyl-p-phenylenediamine to the hepatocyte suspensions inhibited the production of thiobarbituric acid-reactants. However, the presence of the anti-oxidant had no protective effects on the cell membrane integrity as evidenced by the leakage of lactate dehydrogenase from the cells into the surrounding media. These results suggest that lipid peroxidation which occurs in the presence of senecionine, trans-4-OH-2-hexenal or related alkenals is not entirely responsible for the cellular damage in isolated rat hepatocytes.

Gas-liquid chromatography with nitrogen-phosphorus detection was used to analyze blood and urine from a volunteer who ingested 50 mg of cyclizine hydrochloride. A peak blood cyclizine concentration of 69 ng/mL was observed 2 hr after drug administration, and the levels declined thereafter in a biphasic manner, with estimated half-lives of 7 and 24 hr for the early and late phases, respectively. The peak urine cyclizine concentration of 12.5 ng/mL was achieved at 4 hr after administration; only 0.01% of the dose was excreted in the 24 hr urine. Norcyclizine was not observed in blood or urine; however, the detectability of the method for this metabolite is poor relative to the parent drug.

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