Antioxidant mechanisms of isoflavones in lipid systems: paradoxical effects of peroxyl radical scavenging.

Oxidation of lipids has been implicated in the pathophysiology of atherosclerosis. It has been suggested that scavenging of lipid peroxyl radicals contribute to the antiatherosclerotic effects of naturally occurring compounds such as the isoflavones. This group of polyphenolics includes genistein and is present in relatively high concentrations in food products containing soy. Soy isoflavones are capable of inhibiting lipoprotein oxidation in vitro and suppressing formation of plasma lipid oxidation products in vivo. However, key aspects of the antioxidant mechanisms remain unknown. In this study the antioxidant effects of genistein and other soy isoflavones on lipid peroxidation initiated by mechanistically diverse oxidants was investigated. Although isoflavones inhibited lipid peroxidation stimulated by both metal-dependent and independent processes, the concentration required for these effects were relatively high compared to those found in vivo. Interestingly, however, isoflavones were not consumed and remained in the native state over the time during which inhibition of lipid peroxidation was observed. This was also the case under conditions where synergistic inhibition of LDL oxidation was observed with ascorbate. Furthermore, in an oxidation system driven solely by peroxyl radicals, isoflavones were found to be relatively poor peroxyl radical scavengers. Consistent with the apparent lack of reactivity with lipid-derived oxidants, isoflavones were also relatively resistant to oxidation mediated by the potent oxidant peroxynitrite. The potential antioxidant mechanisms of isoflavones are discussed in the context of possible reactivities of isoflavone-derived phenoxyl radicals.

Mechanisms of the pro- and anti-oxidant actions of nitric oxide in atherosclerosis.

The association of nitric oxide (NO) with cardiovascular disease has long been recognized and the extensive research on this topic has revealed both pro- and anti-atherosclerotic effects. While these contradictory findings were initially perplexing recent studies offer molecular mechanisms for the integration of these data in the context of our current understanding of the biochemistry of NO. The essential findings are that the biochemical properties of NO allow its exploitation as both a cell signaling molecule, through its interaction with redox centers in heme proteins, and an extremely rapid reaction with other biologically relevant free radicals. The direct reaction of NO with free radicals can have either pro- or antioxidant effects. In the cell, antioxidant properties of NO can be greatly amplified by the activation of signal transduction pathways that lead to the increased synthesis of endogenous antioxidants or down regulate responses to pro-inflammatory stimuli. These findings will be discussed in the context of atherosclerosis.

Metabotropic glutamate receptor activation and intracellular cyclic ADP-ribose release Ca2+ from the same store in cultured DRG neurones.

The whole cell patch clamp technique has been used to record Ca(2+)-activated cation and chloride conductances evoked by release of Ca2+ from intracellular stores of cultured neonatal dorsal root ganglion neurones. The aim of this study was to investigate metabotropic glutamate receptor (mGluR) mechanisms and evaluate a possible role for cyclic ADP-ribose as an intracellular signalling molecule. Glutamate and the metabotropic glutamate receptor agonist (1S, 3R)-ACPD-evoked transient depolarizations, Ca(2+)-activated inward currents and rises in intracellular Ca2+. The (1S, 3R)-ACPD-activated currents were insensitive to InsP3 signalling inhibitors, heparin and pentosan polysulphate. Intracellular application of ryanodine alone activated currents in this study and proved a difficult tool to use as a potential inhibitor of cyclic ADP-ribose-mediated responses. However, intracellular dantrolene did attenuate both (1S, 3R)-ACPD and cyclic ADP-ribose responses. Intracellular photo-release of cGMP and cyclic ADP-ribose mimicked the responses to mGluR receptor activation. Intracellular application of nicotinamide and W7 inhibited the responses to photo-released cGMP but did not prevent responses to mGluR activation. The cyclic ADP-ribose receptor antagonist 8-amino cyclic ADP-ribose attenuated responses to (1S, 3R)-ACPD, cGMP and cyclic ADP-ribose, but some Ca(2+)-activated inward currents were still observed in the presence of this antagonist. In conclusion, mGluR receptor activation, cGMP and cyclic ADP-ribose release Ca2+ from intracellular stores. Some evidence suggests that pharmacologically related pathways are involved.

Activation of Ca2+-dependent currents in dorsal root ganglion neurons by metabotropic glutamate receptors and cyclic ADP-ribose precursors.

Cultured dorsal root ganglion neurons were voltage clamped at -90 mV to study the effects of intracellular application of nicotinamide adenine dinucleotide (betaNAD+), intracellular flash photolysis of caged 3',5'-cyclic guanosine monophosphate (cGMP), and metabotropic glutamate receptor activation. The activation of metabotropic glutamate receptors evoked inward Ca2+-dependent currents in most cells. This was mimicked both by intracellular flash photolysis of the caged axial isomer of cGMP [P-1-(2-nitrophenyl)ethyl cGMP] and intracellular application of betaNAD+. Whole cell Ca2+-activated inward currents were used as a physiological index of raised intracellular Ca2+ levels. Extracellular application of 10 microM glutamate evoked the activation of Ca2+-dependent inward currents, thus reflecting a rise in intracellular Ca2+ levels. Similar inward currents were also activated after isolation of metabotropic glutamate receptor activation by application of 10 microM glutamate in the presence of 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione and 20 microM dizocilpine maleate (MK 801), or by extracellular application of 10 microM trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid. Intracellular photorelease of cGMP, from its caged axial isomer, in the presence of betaNAD+ was also able to evoke similar Ca2+-dependent inward currents. Intracellular application of betaNAD+ alone produced a concentration-dependent effect on inward current activity. Responses to both metabotropic glutamate receptor activation and cGMP were suppressed by intracellular ryanodine, chelation of intracellular Ca2+ by bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid, and depletion of intracellular Ca2+ stores, but were insensitive to the removal of extracellular Ca2+. Therefore both cGMP, possibly via a mechanism that involves betaNAD+ and/or cyclic ADP-ribose, and glutamate can mobilize intracellular Ca2+ from ryanodine-sensitive stores in sensory neurons.

An electrophysiological investigation of the effects of cisplatin and the protective actions of dexamethasone on cultured dorsal root ganglion neurones from neonatal rats.

In this study we have investigated the acute and chronic effects of cisplatin on whole cell currents in cultured dorsal root ganglion neurones. Consistent with effects on action potentials measured under current clamp, acute (5 min) application of cisplatin (5 microM) attenuated voltage-activated potassium, and mixed cation currents by approximately 50% in both cases. Chronic treatment (5-7 days) of cultured neurones with 5 microM cisplatin also resulted in greatly reduced voltage-activated potassium currents (by 50%) and calcium currents (by 60%) compared to events recorded from neurones not treated with cisplatin. In contrast, the amplitude of inward cation current activated by hyperpolarization was doubled by 5-12 days treatment with cisplatin. Studies on action potential after-depolarizations and calcium-activated chloride currents suggest that cisplatin disturbs calcium homeostatic mechanisms. These observations may account for anode break spike excitation and the low efficiency with which cells buffer intracellular calcium following cisplatin treatment. Dexamethasone has been found to enhance the anti-emetic effects of 5-HT3 receptor antagonists in patients treated with cisplatin. For this reason the actions of dexamethasone were studied in combination with cisplatin treatment. Although acute application of dexamethasone (1-10 microM) produced transient depolarizations and bursts of action potentials, after 5 minutes application it had no effect on membrane potential, input resistance, or the properties of action potentials evoked by depolarizing current commands.(ABSTRACT TRUNCATED AT 250 WORDS)