Combined effect of propofol and GSNO on oxidative phosphorylation of isolated rat liver mitochondria

[Mitochondria in anaesthesia and intensive care]

OBJECTIVE

Mitochondria play a key role in energy metabolism within the cell through the oxidative phosphorylation. They are also involved in many cellular processes like apoptosis, calcium signaling or reactive oxygen species production. The objectives of this review are to understand the interactions between mitochondrial metabolism and anaesthetics or different stress situations observed in ICU and to know the clinical implications.

DATA SOURCES

References were obtained from PubMed data bank (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi) using the following keywords: mitochondria, anaesthesia, anaesthetics, sepsis, preconditioning, ischaemia, hypoxia.

DATA SYNTHESIS

Mitochondria act as a pharmacological target for the anaesthetic agents. The effects can be toxic like in the case of the local anaesthetics-induced myotoxicity. On the other hand, beneficial effects are observed in the anaesthetic-induced myocardial preconditioning. Mitochondrial metabolism could be disturbed in many critical situations (sepsis, chronic hypoxia, ischaemia-reperfusion injury). The study of the underlying mechanisms should allow to propose in the future new specific therapeutics.

Metabolic effects of propofol in the isolated perfused rat liver

Inhibitory effects of the intravenous anaesthetic propofol on mitochondrial energy metabolism have been reported by several authors. Impairment of energy metabolism is usually coupled to reduction in ATP production, which in turn is expected to lead to several alterations in cell metabolism such as stimulation of glycolysis and inhibition of gluconeogenesis. The present work aimed at finding an answer to the question of how propofol affects energy metabolism-linked parameters in the isolated perfused rat liver. In the fed state, propofol increased glycogenolysis (glucose release), glycolysis (lactate and pyruvate production) and oxygen uptake in the range between 10 and 500 microM. In the liver of fasted rats, propofol up to 100 microM increased oxygen uptake but decreased gluconeogenesis from three different substrates: lactate, alanine and glycerol. When lactate was the substrate 50% inhibition occurred at a propofol concentration of 50 microM. Propofol (100 microM) decreased the ATP content of the liver (-33.3%), increased the AMP content (+25%) and decreased the ATP/ADP and ATP/AMP ratios (49 and 45%, respectively). Most effects of propofol are probably due to impairment of oxidative phosphorylation. Particularly, the combined differential action on oxygen uptake (stimulation) and gluconeogenesis (inhibition) is strongly suggestive of an uncoupling action also under the conditions of the intact cell. This effect, in turn, is consistent with the reported high affinity of the cellular hepatic structure, especially membranes, for propofol.

Correlation between fluidising effects on phospholipid membranes and mitochondrial respiration of propofol and p-nitrosophenol homologues

Nitrosopropofol (2-6-diisopropyl-4-nitrosophenol) has dramatic consequences for respiration, ATP synthesis and the transmembrane potential of isolated rat liver mitochondria at concentrations at which propofol (2-6-diisopropylphenol) does not cause any apparent effects. These results correlate well with the observation that nitrosopropofol is also a stronger perturbing agent of phospholipid membranes. In this paper we verify the possible biological activity of different phenols and nitrosophenols on mitochondrial respiration. We then discuss their interactions with phospholipid liposomes, studied with differential scanning calorimetry, spin labelling techniques and UV-Vis spectrophotometry, in order to obtain information on drug distribution and the modifications they impose on lipid bilayer. The results of the experiments performed on mitochondria and model membranes prove an interesting correlation between the effects of the molecules on both systems.

Effects of nitrosopropofol on mitochondrial energy-converting system

Nitrosopropofol (NOPR) is a relatively stable compound obtained from the reaction between the general anesthetic 2,6 diisopropylphenol (propofol) and nitrosoglutathione (GSNO) and bearing a more acidic phenol group than propofol. It interfered with mitochondrial energetic metabolism in a concentration-dependent manner. Concentrations as high as 100 or 200 microM disrupted both oxidative phosphorylation and electron transport. Low concentrations of NOPR (50 microM) markedly slowed down the electron transport rate which was insensitive both to ADP and uncoupler stimulation and spontaneously gradually stopped. Consequently, both the transmembrane potential production and the ATP synthesis system were affected. In the presence of 10 or 20 microM NOPR, mitochondria respired but showed a worsening of the respiratory control and produced a transmembrane potential useful to respond to a phosphorylation pulse, but were not able to restore it. These results were consistent with ATP synthesis and swelling experiments. NOPR was effective at concentrations lower than those required by the combination of propofol and GSNO, suggesting that mitochondria might be able to catalyze the reaction between GSNO and propofol and that the resulting metabolite was more active on mitochondrial membrane structure than the parent compounds. Although the details of the process are yet unknown, the mechanism presented may be of potential relevance to rationalize the pathophysiological effects of propofol.

Different effects of propofol and nitrosopropofol on DMPC multilamellar liposomes

The mechanisms of reaction of propofol with nitrosoglutathione lead to the formation of an active species which was identified, and then synthesised, as 2,6-diisopropyl-4-nitrosophenol. In the present work, we demonstrate the in vitro formation of 2,6-diisopropyl-4-nitrosophenol, then we discuss the interaction of propofol and 2,6-diisopropyl-4-nitrosophenol with dimyristoylphosphatidylcholine and egg yolk phosphatidylcholine multilamellar liposomes using differential scanning calorimetry and spin labelling techniques. It was demonstrated that both molecules are highly lipophylic and absorb almost entirely in the lipid phase. The thermotropic profiles showed that these molecules affect the temperature and the co-operativity of the gel-to-fluid state transition of the liposomes differently: the effects of 2,6-diisopropylphenol on the lipid organisation are quite similar to phenol and coherently interpretable in terms of the disorder produced in the membrane by a bulky group; 2,6-diisopropyl-4-nitrosophenol is a stronger perturbing agent, and ESR spectra suggest that this is due to a relative accumulation of the molecule into the interfacial region of the bilayer.