Agmatine, a divalent diamine with two positive charges at physiological pH, is transported into the matrix of liver mitochondria by an energy-dependent mechanism the driving force of which is DeltaPsi (electrical membrane potential). Although this process showed strict electrophoretic behaviour, qualitatively similar to that of polyamines, agmatine is most probably transported by a specific uniporter. Shared transport with polyamines by means of their transporter is excluded, as divalent putrescine and cadaverine are ineffective in inhibiting agmatine uptake. Indeed, the use of the electroneutral transporter of basic amino acids can also be discarded as ornithine, arginine and lysine are completely ineffective at inducing the inhibition of agmatine uptake. The involvement of the monoamine transporter or the existence of a leak pathway are also unlikely. Flux-voltage analysis and the determination of activation enthalpy, which is dependent upon the valence of agmatine, are consistent with the hypothesis that the mitochondrial agmatine transporter is a channel or a single-binding centre-gated pore. The transport of agmatine was non-competitively inhibited by propargylamines, in particular clorgilyne, that are known to be inhibitors of MAO (monoamine oxidase). However, agmatine is normally transported in mitoplasts, thus excluding the involvement of MAO in this process. The I2 imidazoline receptor, which binds agmatine to the mitochondrial membrane, can also be excluded as a possible transporter since its inhibitor, idazoxan, was ineffective at inducing the inhibition of agmatine uptake. Scatchard analysis of membrane binding revealed two types of binding site, S1 and S2, both with mono-co-ordination, and exhibiting high-capacity and low-affinity binding for agmatine compared with polyamines. Agmatine transport in liver mitochondria may be of physiological importance as an indirect regulatory system of cytochrome c oxidase activity and as an inducer mechanism of mitochondrial-mediated apoptosis.

Functional alterations in mitochondria such as overproduction of ROS (reactive oxygen species) and overloading of calcium, with subsequent change in the membrane potential, are traditionally regarded as pro-apoptotic conditions. Although such events occur in the early phases of LR (liver regeneration) after two-thirds PH (partial hepatectomy), hepatocytes do not undergo apoptosis but continue to proliferate until the mass of the liver is restored. The aim of the present study was to establish whether tyrosine phosphorylation, an emerging mechanism of regulation of mitochondrial function, participates in the response to liver injury following PH and is involved in contrasting mitochondrial pro-apoptotic signalling. Mitochondrial tyrosine phosphorylation, negligible in the quiescent liver, was detected in the early phases of LR with a trend similar to the events heralding mitochondrial apoptosis and was attributed to the tyrosine kinase Lyn, a member of the Src family. Lyn was shown to accumulate in an active form in the mitochondrial intermembrane space, where it was found to be associated with a multiprotein complex. Our results highlight a role for tyrosine phosphorylation in accompanying, and ultimately counteracting, mitochondrial events otherwise leading to apoptosis, hence conveying information required to preserve the mitochondrial integrity during LR.

Treatment of rat liver mitochondria with aluminum in the presence of Ca2+ results in large amplitude swelling accompanied by loss of endogenous Mg2+ and K+ and oxidation of endogenous pyridine nucleotides. The presence of cyclosporin A, ADP, bongkrekic acid, N-ethylmaleimide and dithioerythritol prevent these effects, indicating that binding of aluminum to the inner mitochondrial membrane, most likely at the level of adenine nucleotide translocase, correlates with the induction of the membrane permeability transition (MPT). Indeed, aluminum binding promotes such a perturbation at the level of ubiquinol-cytochrome c reductase, which favors the production of reactive oxygen species. These metabolites generate an oxidative stress involving two previously defined sites in equilibrium with the glutathione and pyridine nucleotides pools, the levels of which correlate with the increase in MPT induction. Although the above-described phenomena are typical of MPT, they are not paralleled by other events normally observed in response to treatment with inducers of MPT (e.g., phosphate), such as the collapse of the electrochemical gradient and the release of accumulated Ca2+ and oxidized pyridine nucleotides. Biochemical and ultrastructural observations demonstrate that aluminum induces a pore opening having a conformation intermediate between fully open and closed in a subpopulation of mitochondria. While inorganic phosphate enhances the MPT induced by ruthenium red plus a deenergizing agent, aluminum instead inhibits this phenomenon. This finding suggests the presence of a distinct binding site for aluminum differing from that involved in MPT induction.

Treatment of Ca2+-loaded mitochondria with both aluminum and tyramine results in a swelling of higher amplitude than with aluminum alone, while tyramine alone is ineffective. The phenomenon is accompanied by H2O2 production and thiol and pyridine nucleotide oxidation. Cyclosporin A, N-ethylmaleimide or dithioerythritol completely prevent these effects, while catalase exhibits a lower inhibition, pointing to the induction of the permeability transition (MPT) by an oxidative stress. Reactive oxygen species are generated by the interaction of aluminum with the inner membrane and the oxidation of tyramine by monoamine oxidase on the outer membrane. This different localization determines the oxidation of critical thiol groups located on both internal and external sides of pore-forming structures, resulting in MPT induction. The reduced effect by aluminum or the inefficacy by tyramine, when implied alone, can be attributable to the oxidation of thiol groups located only on the internal or external side, respectively. Ultrastructural observations show that aluminum plus tyramine induce the typical configuration of mitochondria that have undergone the MPT. Instead, with aluminum alone, the sensitive subpopulation, although swollen, preserves the outer membrane and shows an apparently orthodox configuration.