Dipalmitoylphosphatidylcholine liposomes inhibit calcium-dependent [3H]acetylcholine release

Effect of inhibition of cyclooxygenase on pre- and postjunctional actions of peroxides in the iris-ciliary body

In the present study, we investigated the effect of inhibition of cyclooxygenase (COX) with flurbiprofen (FBF) on peroxide-induced enhancement of field-stimulated [3H]norepinephrine ([3H]NE) release from bovine isolated irides. Furthermore, the effect of FBF was examined on peroxide-induced attenuation of contractions evoked by carbachol on this tissue. Irides were prepared for studies of neurotransmitter release and for measurement of contractile tension in vitro. Pretreatment of tissues with FBF (10 microM) caused significant (P < 0.001) rightward shifts of concentration-response curves to H2O2 and also decreased cumene hydroperoxide (cuOOH)-induced enhancement of evoked [3H]NE release. FBF (10 microM) partially prevented the attenuation of carbachol-induced contractions induced by H2O2 (300 microM) and cuOOH (300 microM). We conclude that inhibition of the biosynthesis of prostanoids reduced both the prejunctional stimulatory effects of H2O2 and cuOOH on sympathetic neurotransmission and inhibitory effects of peroxides on carbachol-induced contractions the in the bovine isolated iris.

Role of intracellular calcium in peroxide-induced potentiation of norepinephrine release from bovine isolated irides

1. BAPTA AM (10 microM), thapsigargin (10 microM), ruthenium red (30 microM) and oligomycin (30 microM) inhibited field-stimulated [3H]NE release from bovine isolated irides by 54%, 30%, 30% and 26%, respectively. 2. Both BAPTA AM and thapsigargin had no significant effect (P>0.05) on H2O2-induced potentiation of evoked [3H]NE release. 3. Ruthenium red prevented (but oligomycin enhanced) H2O2-induced enhancement of evoked [3H]NE release. 4. We conclude that, although intracellular calcium participates in field-stimulation evoked [3H]NE release from bovine isolated irides, only the mitochondrial pool of calcium may be involved in peroxide-induced enhancement of sympathetic neurotransmission.

Prejunctional alpha2-adrenoceptors and peroxide-induced potentiation of norepinephrine release from the bovine iris

Peroxides can enhance field-stimulated [3H]norepinephrine ([3H]NE) release in isolated irides from several mammalian species. In the present study, we investigated the role of prejunctional alpha2-adrenoceptors in peroxide-induced potentiation of sympathetic neurotransmission in bovine isolated irides. Isolated hemi-irides were incubated in a Krebs buffered-solution containing [3H]NE and prepared for studies of neurotransmitter release using the superfusion method. Alpha2-adrenoceptor agonists, oxymetazoline, UK-14304 and clonidine inhibited field-stimulated [3H]NE overflow without affecting basal tritium efflux. Pretreatment of tissues with H2O2 (300 microM) had no effect on inhibition of evoked [3H]NE release caused by the alpha2-adrenergic agonists. However, H2O2 (300 microM) caused significant (P < 0.01) leftward shifts of excitatory concentration-response curves to yohimbine (10 nM-1 microM). In contrast, yohimbine (1 microM) did not prevent the enhancement of evoked [3H]NE overflow induced by H2O2 (300 microM). In conclusion, excitatory effects of peroxides on sympathetic neurotransmission in bovine irides are not mediated by prejunctional alpha2-adrenoceptors.

Effects of peroxidation and aging on rat neocortical ACh-release and protein kinase C

Cerebral cortical synaptosomes were prepared from 2- or 24-month-old Fischer 344 rats and exposed to a peroxidizing condition (50 microM ferrous ions and 2 microM ascorbate ions) before measuring either the release of newly synthesized [3H]acetylcholine (ACh) or protein kinase C activity (PKC). Several secretagogues with different mechanisms of action and different responses to aging were used to trigger release: K+ depolarization (5 mM-60 mM), calcium ionophore A23187 (1-10 micrograms/ml), and 4-aminopyridine (0.1-10 mM). Aging reduced K+ depolarization-induced release at every K+ concentration studied, reduced A23187-induced release at low but not high concentrations and did not affect 4-aminopyridine-induced release. Membrane peroxidation of synaptosomes from 2-month-old rats altered the response to secretagogues to match that seen in 24-month-old rats. Membrane peroxidation also attenuated the A23187-stimulated translocation of free to bound synaptosomal PKC activity in 2-month-old but not 24-month-old animals. These results suggest that membrane peroxidation may mimic some age-related deficits in secretagogue-induced [3H]ACh release.

Effects of membrane peroxidation on [3H]acetylcholine release in rat cerebral cortical synaptosomes

[3H]Acetylcholine (ACh) release, malonaldehyde formation and 45calcium-uptake were measured in rat cerebral cortical nerve terminal that were exposed to various concentrations of ferrous and ascorbate ions. At a constant molar ratio of 25:1, ferrous:ascorbate, these ions increased malonaldehyde (MA) synthesis in a concentration-dependent manner. Treatment with these ions in the same ratio also induced a dose-related inhibition of the K(+)-depolarization-induced release of newly synthesized [3H]ACh. Combined exposure to Fe2+/ascorbate also reduced calcium ionophore A23187-induced [3H]ACh release. Neither ferrous nor ascorbate ions alone altered depolarization- or ionophore-induced [3H]ACh release over this concentration range. Depolarization- and A23187-induced 45calcium uptake were not affected by peroxidation, suggesting that membrane peroxidation influenced some process in the release-process subsequent to calcium influx in a manner similar to what is observed during aging.

Dipalmitoylphosphatidylcholine liposomes reduce [3H]acetylcholine levels in an eserine-sensitive manner in rat cerebral cortical synaptosomes

We investigated the effects of various phospholipids on the presynaptic levels of newly synthesized [3H]acetylcholine (ACh) in rat cerebral cortical synaptosomes. When administered as small unilamellar vesicles (200-500 A diameters) dipalmitoylphosphatidylcholine (DPPC) reduced [3H]ACh levels in concentration and time-related manners, while increasing the efflux of labelled choline to a similar extent. The reductions in synaptosomal [3H]-ACh levels induced by DPPC (3 mg/ml) were found in the cytosolic S3 but not microsomal P3 fraction, arguing for a cytoplasmic, non-vesicular site of action. DPPC-induced reductions in [3H]ACh levels were blocked by 100 microM eserine, a tertiary amine cholinesterase inhibitor, but not with 100 microM neostigmine, a quaternary ammonium inhibitor. Large unilamellar vesicles (2000-5000 A diameters) consisting of soybean-phosphatidylcholine reduced [3H]ACh levels to the same extent that small vesicles did at the same concentration (3 mg/ml). Taken together, these results suggest that DPPC can fuse with membranes to increase the hydrolysis of cytoplasmic ACh via a small intra-terminal subpopulation of cholinesterases.