Lithocholic acid induces two different calcium-dependent inner membrane permeability systems in liver mitochondria

Abstract

The effect of the most hydrophobic bile acid–lithocholic–as an inducer of two different Ca2+-dependent inner membrane permeability systems was studied on isolated rat liver mitochondria. It is shown that the addition of lithocholic acid at a concentration of 20 μM to the Ca2+-loaded mitochondria leads to swelling of the organelles, rapid release of Ca2+ from the matrix and almost complete collapse of Δψ. Mitochondrial pore blocker cyclosporin A (CsA) eliminates mitochondrial swelling but has no effect on the process of Ca2+ release and Δψ collapse. In the absence of Ca2+ lithocholic acid causes only a transient decrease of Δψ with subsequent complete recovery. Ruthenium red, inhibitor of mitochondrial Ca2+ uniporter, which blocks Ca2+ influx into the matrix, prevents mitochondrial swelling induced by lithocholic acid. At the same time, ruthenium red, which is added before lithocholic acid to the Ca2+-preloaded mitochondria, does not affect the swelling of the organelles but reduces the CsA-insensitive drop in Δψ. It is concluded that lithocholic acid is able to induce two Ca2+-dependent energy dissipation systems in the inner membrane of liver mitochondria: CsA-sensitive mitochondrial pore and CsA-insensitive permeability, which exhibits sensitivity to ruthenium red. It is found that the effect of this bile acid as an inductor of CsA-sensitive mitochondrial pore is not associated with the modulation of Pi effects. It is assumed that CsA-insensitive action of lithocholic acid is associated with the induction of Ca2+ efflux from the matrix in exchange for protons. In this case, the energy-dependent Ca2+ transport in the opposite direction with the participation of mitochondrial calcium uniporter sensitive to ruthenium red leads to the formation of calcium cycle and thereby to energy dissipation.