The mechanism of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity: Role of intracellular calcium

Abstract

The mechanism of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced toxicity to isolated hepatocytes was studied. MPTP was more toxic to hepatocytes than its major metabolite, 1-methyl-4-phenylpyridine (MPP +); this may, in part, be explained by the lesser permeability of the hepatocyte plasma membrane to the cation compared to its parent compound, MPTP. Loss of cell viability was preceded by plasma membrane bleb formation and disturbance of intracellular Ca 2+ homeostasis. MPTP caused a rapid depletion of the mitochondrial Ca 2+ pool which was followed by a marked and sustained elevation of cytosolic free Ca 2+ concentration. This increase of cytosolic Ca 2+ level appeared to be associated with the impairment of the cell's Ca 2+ extrusion system since the plasma membrane Ca 2+-ATPase was markedly inhibited in MPTP-treated hepatocytes. Preincubation of hepatocytes with inhibitors of monoamine oxidase type B, but not A, protected the cells from MPTP-induced cytotoxicity. Moreover, the monoamine oxidase B inhibitor, pargyline, prevented the rise in cytosolic free Ca 2+ concentration and partially protected the plasma membrane Ca 2+-ATPase from inhibition by MPTP. As observed with MPTP, MPP + caused an extensive loss of mitochondrial Ca 2+ and significantly decreased the rate of Ca 2+ efflux from hepatocytes. However, MPP + was without effect on the plasma membrane Ca 2+-ATPase. In conclusion, our studies demonstrate that MPTP caused a substantial elevation of cytosolic Ca 2+ which preceded loss of cell viability and we propose that calcium ions are of major importance in the mechanism of MPTP- and MPP +-induced toxicity in hepatocytes.