The role of calcium in liver cell damage. Comparative studies with carbon tetrachloride and D-galactosamine

Pathobiochemical mechanisms of hepatocellular damage following lipid peroxidation

In hepatocytes, cytotoxic events induced by haloalkanes or acute iron-overload exhibit neither a quantitative nor a temporal correlation to lipid peroxidation or covalent binding. Thus, secondary pathological mechanisms have been postulated linking initial focal reactions of free radicals and end stage pathological consequences. Due to the crucial role of plasma-membrane integrity in the cytotoxic process it has to be supposed that relevant secondary pathological mechanisms finally impair the physico-chemical and functional properties of this membrane. Based on recent developments a chain of causality is proposed as a two-step activation of phospholipase A2 producing cytolytic amounts of lysophosphatides. In this cascade, the initial activating step is a decrease of membrane lipid fluidity induced by lipid peroxidation and/or by calcium binding and intramembranous formation of 4-hydroxynonenal. This enzyme activation is further amplified by the early rise of cytosolic calcium. Consequently, increasing amounts of lysophosphatides progressively impair membrane configuration thus improving the substrate accessibility for phospholipase A2 in a second activation step. Finally, the lysophosphatides reach plasma membrane-lytic concentrations by this autocatalytic enzyme activation.

The intracellular distribution of liver cell calcium in normal rats and one hour after administration of carbon tetrachloride

In order to recognize the significance of elevated liver calcium level in the early phase of carbon tetrachloride intoxication, the subcellular distribution of calcium in liver was investigated one hour after administration of CCl4 intraperitoneally. During preparation of subcellular fractions attempts were made to prevent redistribution of calcium by adding Ruthenium Red and EGTA or LaCl3 to homogenization medium, and by shortening of differential centrifugation to a minimum. The latter caused a loss of purity in sub cellular fractions which was overcome by correction of calcium values from atom absorption spectrometry by means of marker enzyme activity. The calcium levels in normal rat liver were found to be 70 nmoles/g liver wet wt in cytosol, 310 nmoles/g liver in microsomes and about 500 nmoles/g liver in mitochondria. A minor part of the latter fraction may belong to nuclei and plasma membranes. One hour after CCl4 administration calcium levels in cytoplasma were not altered, in microsomes were decreased to 200 nmoles/g liver and in mitochondria were elevated to 2.5 mumoles/g liver. In rats pretreated with vitamin D the whole additional calcium, after carbon tetrachloride application in the range of 9 mumoles/g liver, was sequestrated in mitochondria. In the early phase of carbon tetrachloride intoxication all border membranes of liver cells have to participate in bringing about the reversible increase of liver cell calcium.