A2A Adenosine Receptors Inhibit ATP-Induced Ca2+ Influx in PC12 Cells by Involving Protein Kinase A

Abstract: The regulatory role of A_2A adenosine receptors in P₂ purinoceptor-mediated calcium signaling was investigated in rat pheochromocytoma (PC12) cells. When PC12 cells were treated with 2- p -(2-carboxyethyl)-phenethylamino-5'- N -ethylcarboxamidoadenosine (CGS-21680), a specific agonist of the A_2A adenosine receptor, the extracellular ATP-evoked rise in cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) was inhibited by 20%. Both intracellular calcium release and inositol 1,4,5-trisphosphate production evoked by ATP were not affected by CGS-21680 treatment. However, ATP-evoked Ca²⁺ influx was inhibited following CGS-21680 stimulation. The CGS-21680-mediated inhibition occurred independently of nifedipine-induced inhibition of the [Ca²⁺]_i rise. The CGS-21680-induced inhibition was completely blocked by reactive blue 2. The CGS-21680 effect was mimicked by forskolin and dibutyryl-cyclic AMP and blocked by Rp -adenosine 3',5'-cyclic monophosphothioate, a protein kinase A inhibitor, or by staurosporine, a general kinase inhibitor. The data suggest that in PC12 cells activation of A_2A adenosine receptors leads to inhibition of P₂ purinoceptor-mediated Ca²⁺ influx through ATP-gated cation channels and involves protein kinase A.     

Tachykinins Potentiate N-Methyl-D-Aspartate Responses in Acutely Isolated Neurons from the Dorsal Horn

Abstract: Substance P and neurokinin A both potentiated N -methyl- d -aspartate (NMDA)-induced currents recorded in acutely isolated neurons from the dorsal horn of the rat. To elucidate the mechanism underlying this phenomenon, we measured the effects of tachykinins and glutamate receptor agonists on [Ca²⁺]_i in these cells. Substance P, but not neurokinin A, increased [Ca²⁺]_i in a subpopulation of neurons. The increase in [Ca²⁺]_i was found to be due to Ca²⁺ influx through voltage-sensitive Ca²⁺ channels. Substance P and neurokinin A also potentiated the increase in [Ca²⁺]_i produced by NMDA, but not by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, kainate, or 50 m M K⁺. Phorbol esters enhanced the effects of NMDA and staurosporine inhibited the potentiation of NMDA effects by tachykinins. It is concluded that activation of protein kinase C may mediate the enhancement of NMDA effects by tachykinins in these cells. However, the effects of tachykinins on [Ca²⁺]_i can be dissociated from their effects on NMDA receptors.     

Differential Regulation of Intracellular Signaling Systems by Sodium Fluoride in Rat Glioma Cells

Abstract: We investigated the rapid and slow effects of NaF on intracellular signaling systems such as Ca²⁺ homeostasis and cyclic GMP (cGMP) generation in rat glioma C6 cells, using the Ca²⁺-sensitive dye fura-2 and cGMP enzyme immunoassay. We found that the following: (a) NaF enhanced cGMP generation in a concentration-dependent manner. This enhancement was abolished by pretreatment with 100 µ M BAPTA tetraacetoxymethyl ester or in the presence of W-7 in a concentration-dependent manner. N ^G-Monomethyl- l -arginine (NMMA), a competitive inhibitor of nitric oxide synthase (NOS), also inhibited the NaF-induced generation of cGMP. These results suggest that NaF-induced cGMP generation occurs via a calcium/calmodulin- and NOS-dependent pathway. (b) The basal intracellular Ca²⁺ concentration ([Ca²⁺]_i) was transiently greater at 1 and 3 h after pretreatment with NaF. W-7 and W-13 antagonized the increase in [Ca²⁺]_i, whereas NMMA had little effect. This suggests that the NaF-induced change in basal [Ca²⁺]_i was mediated by a calmodulin-dependent pathway but was independent of a NOS-sensitive pathway. (c) The serotonin (5-HT)-induced intracellular mobilization of Ca²⁺ was reduced by pretreating the cells with NaF. The reduction in Ca²⁺ mobilization was antagonized by genistein, a tyrosine kinase inhibitor. W-7, W-5, and H-8 had no effect. Results suggest that NaF differentially regulates the cGMP generation, basal [Ca²⁺]_i, and 5-HT_2A receptor function in C6 glioma cells.     

Production of clover broomrape seed germination stimulants by red clover root requires nitrate but is inhibited by phosphate and ammonium

The effect of nutrients (nitrate, ammonium, urea, phosphate and potassium) on the production and/or exudation of germination stimulants for clover broomrape ( Orobanche minor Sm.), a root holoparasite, by its host red clover ( Trifolium pratense L.) was examined using hydroponically grown material. Potassium (K₂SO₄) concentrations up to 100 mg l^–1 (based on K) did not affect the production of germination stimulants by red clover while, in contrast, phosphate (NaH₂PO₄) was highly inhibitory even at concentrations as low as 1 mg l^–1 (based on P). Nitrate (NaNO₃) markedly promoted stimulant production in a dose-dependent manner from 2 to 50 mg l^–1 (based on N). Ammonium [(NH₄)₂SO₄] had no effect at 2 mg l^–1 (based on N) but was inhibitory at higher concentrations. Ammonium is known to be a seed germination inhibitor of root parasites, indicating that ammonium has a dual inhibitory action. Urea had no effect at 2 mg l^–1 (based on N) but was promotive at higher concentrations. These results provide a basis for the inhibitory effects of nitrogen fertilizer on infection by root parasitic weeds, broomrapes and witchweeds, and explain why these parasites prevail in areas where there is lower phosphorus availability in soils.     

Ecology of Peridinium willei and P. volzii (Dinophyceae) in Danish lakes

The distribution of Peridinium willei and P. volzii was studied in Danish lakes. Both species were confined to lakes with concentrations of Total P < 0.15 mg 1^-1, with the majority of occurrences at Total P concentration between 0.020–0.040 mg 1^-1 and concentrations of PO₄ P between detection limit and 0.040 mg 1^-1. The occurrence of the species in relation to inorganic N compounds (NH₄ N and NO₂+ NO₃ N) was significantly broader for P. willei than for P. volzii: P. willei had an almost even distribution within a wide range of NH₄ N, whereas P. volzii mainly occurred between 0.001 and 0.10 NH₄ N 1^-1. P. willei had an almost even distribution at values beween 0.005 and 0.42 mg NO₂+ NO₃ N 1^-1, whereas P. volzii mainly occurred below 0.050 mg NO₂+ NO₃ N 1¹. P. willei was found at pH values between 4.2 and 8.5, whereas P. volzii was confined to lakes with a slightly basic pH. The study confirmed the broad limits of P. willei and the much more narrow limits of P. volzii in relation to seasonal occurrence and pH, as well as an affinity of the former to ponds and lakes with a rich bottom vegetation. The study also showed, however, that the species were not as widespread and common in recent Danish lake phytoplankton as generally stated by previous authors. The use of different ecological factors to give weight to species separation is discussed. The inclusion of P. volzii in P. willei proposed by Popovsky & Phiester is not supported by the present study, as the two taxa appear to have different ecological tolerances.     

The Effect of Divalent Cations on Aggregation of Dictyostelium discoideum

Following the initiation of development, amoebae of Dictyostelium discoideum aggregate chemotactically toward cyclic AMP (cAMP). Adenyl cyclase, cAMP phosphodiesterase, and cAMP binding sites all increase 20–40 fold during the first few hours of development. It has been shown that addition of 1 mM EDTA and 5 mM MgCl₂ accelerates the aggregation process. Likewise, the calcium ionophore, A23187, leads to precocious aggregation while 4 × 10⁻⁵ M progesterone considerably delays it These treatments have now been shown to result in increased accumulation of adenyl cyclase in the case of EDTA and Mg²⁺ or the ionophore and greatly decreased accumulation in the case of the steroid.
Treatment with EDTA and Mg²⁺ or the ionophore has been shown not only to accelerate aggregation in wild-type amoebae but to overcome complete blocks to aggregation in certain mutant strains. We have found that addition of Mn²⁺ will also permit aggregation of mutant cells otherwise unable to aggregate. This divalent ion, unlike EDTA and Mg²⁺ or the ionophore, was shown to directly stimulate adenyl cyclase. Calcium ions were also found to affect the enzyme such that at Ca²⁺ concentrations found within the cells the great majority of the activity is inhibited. Manganese ions can overcome the inhibition by Ca²⁺.
These findings show that conditions which stimulate aggregation result in increased activity of adenyl cyclase either by increased accumulation of the enzyme or by increased activity of the available enzyme, and support the proposed central role of adenyl cyclase in aggregation.     

Endogenous cannabinoid anandamide inhibits nicotinic acetylcholine receptor function in mouse thalamic synaptosomes

The effects of the endogenous cannabinoid anandamide [arachidonylethanolamide (AEA)] on the function of nicotinic acetylcholine receptor (nAChR) were investigated using the ⁸⁶Rb⁺ efflux assay in thalamic synaptosomes. AEA reversibly inhibited ⁸⁶Rb⁺ efflux induced by 300 μM ACh with an IC₅₀ value of 0.9 ± 2 μM. Pre-treatment with the cannabinoid (CB₁) receptor antagonist SR141716A (1 μM), the CB₂ receptor antagonist SR144528 (1 μM), or pertussis toxin (0.2 mg/mL) did not alter the inhibitory effects of AEA, suggesting that known CB receptors are not involved in AEA inhibition of nAChRs. AEA inhibition of ⁸⁶Rb⁺ efflux was not reversed by increasing acetylcholine (ACh) concentrations. In radioligand binding studies, the specific binding of [³H]-nicotine was not altered in the presence of AEA, indicating that AEA inhibits the function of nAChR in a non-competitive manner. Neither the amidohydrolase inhibitor phenylmethylsulfonyl fluoride (0.2 mM) nor the cyclooxygenase inhibitor, indomethacin, (5 μM) affected AEA inhibition of nAChRs, suggesting that the effect of AEA is not mediated by its metabolic products. Importantly, the extent of AEA inhibition of ⁸⁶Rb⁺ efflux was significantly attenuated by the absence of 1% fatty acid free bovine serum albumin pre-treatment, supporting previous findings that fatty acid-like compounds modulate the activity of nAChRs. Collectively, the results indicate that AEA inhibits the function of nAChRs in thalamic synaptosomes via a CB-independent mechanism and that the background activity of these receptors is affected by fatty acids and AEA.     

Separation of Soluble Amoebicidal and Tumoricidal Activity of Activated Macrophages

ABSTRACT. Macrophage-conditioned medium (MøCM) prepared from mouse peritoneal macrophages activated in vivo with bacillus Calmette-Guérin (BCG) or Propionibacterium acnes and triggered with lipopolysaccharide in vitro contained tumoricidal and amoebicidal activity. The murine fibroblast cell line L929 was used as the indicator of tumoricidal activity and Naegleria fowleri amoeba was used to detect amoebicidal activity in MøCM. The protease inhibitor, soybean trypsin inhibitor, decreased tumoricidal activity but had little effect on amoebicidal activity in MøCM. Anti-TNF _α antiserum inhibited tumoricidal activity in MøCM. The antiserum reduced amoebicidal activity in BCG-activated MøCM but had no effect on amoebicidal activity in P. acnes -activated MøCM. Recombinant TNF _α , rIL-1 _α , or rIL-1 _β independently did not affect cytolysis of amoebae. Also, rTNF _α had no effect on the growth of amoebae. Preparative flat-bed electrofocusing of BCG-activated MøCM yielded fractions that exhibited different amoebicidal and tumoricidal activity profiles. Three domains of activity were analyzed (acidic, neutral, and basic). Anti-TNF _α antiserum eliminated tumoricidal activity, but not amoebicidal activity, in fractions from the acidic domain. A combination of anti-TNF _α and anti-IL-1 _α antisera failed to eliminate amoebicidal activity in fractions from the basic domain. These results indicate that different factors are responsible for macrophage amoebicidal and tumoricidal activity. The amoebicidal factors in MøCM affected cytolysis of several species of amoebae.     

Na+-driven ATP synthesis of a psychrophilic marine bacterium, Vibrio sp. strain ABE-1

Abstract: In vivo ATP synthesis of a psychrophilic marine bacterium, Vibrio sp. strain ABE-1, derived from endogenous respiration, was examined. ATP was synthesized at both pH 6.5 and 8.5 after the start of the endogenous respiration by supplying O₂ to the anaerobic cell suspension. The ATP synthesis at pH 6.5, but not at pH 8.5, was completely inhibited by a H⁺ conductor, carbonylcyanide m -chlorophenylhydrazone (CCCP). The CCCP-resistant ATP synthesis at pH 8.5 was strongly inhibited by an inhibitor of the respiration-dependent primary Na⁺ pump, 2- n -heptyl-4-hydroxyquinoline N -oxide, and essentially required Na⁺. These results show that this bacterium synthesizes ATP at pH 6.5 by electrochemical potentials across the membrane Δ ∼ μ _H+, whereas at pH 8.5 by Δ ∼ μ _Na+ but not Δ ∼ μ _H+.     

Decreased endo-lysosomal acidification capacity in methylene diphosphonate-resistant mutants of Dictyostelium discoideum

Abstract The in vivo capacity for endo-lysosomal acidification has been monitored in Dictyostelium discoideum amoebae with acridine orange, a fluorescent weak base dye commonly used to probe transmembrane pH gradients. In the presence of aerobic amoebae, the initial rate of fluorescence quenching was found to be proportional to cell density between 5 × 10⁵ and 2.5 × 10⁶ cells ml⁻¹ and independent of acridine orange concentration in the 1.5 to 7.5 μM range. The dye response was sensitive to agents that perturb endo-lysosomal acidification such as NaN₃, nigericin or imidazole. Several mutant cell lines whose growth was resistant to methylene diphosphonate were found to be partially deficient in the acridine orange quenching test, suggesting that endo-lysosomal acidification was altered in these mutants.     

Specific Receptor-Mediated Inhibition of Cyclic AMP Synthesis by Dopamine in a Neuroblastoma × Brain Hybrid Cell Line NCB-20

Abstract: Dopamine and dopamine receptor agonists were found to inhibit adenylate cyclase activity dose-de-β ndently in a neuroblastoma × Chinese hamster brain explant hybrid cell line NCB-20. Apomorphine (with an IC₅₀ value of 10 n M ) was the most effective inhibitor, followed by 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydro-naphthaline (ADTN), dopamine, and N -dipropyldopa-mine. The inhibition was potently reversed by sulpiride, butaclamol, and flupenthixol in a stereospecific manner, but was unaffected by yohimbine, except at high concentrations. Clonidine also inhibited adenylate cyclase activity in these cells and this was reversed by the α₂-adrenoreceptor antagonist yohimbine, but not by sulpiride. [ d -Ala², d -Leu⁵]Enkephalin inhibited adenylate cyclase activity in NCB-20 cells at nanomolar concentrations; this was reversed by naloxone. All three inhibitory neurotransmitters were able to reverse the stimulation of cyclic AMP synthesis by serotonin or prostaglandin E₁The dopamine receptor that modulates cyclic AMP synthesis in NCB-20 cells is pharmacologically quite distinct from a high-affinity spiperone binding site identified in these cells, but shows the pharmacologic specificity of the D₂ receptor previously described in mammalian brain.     

Characterization of Neuronal Amino Acid Transporters: Uptake of Nitric Oxide Synthase Inhibitors and Implication for Their Biological Effects

Abstract: In the present study we investigated uptake of the nitric oxide (NO) synthase inhibitors N ^G-methyl- l -arginine and N ^G-nitro- l -arginine by the mouse neuroblastoma × rat glioma hybrid cell line NG108-15. Uptake of N ^G-methyl- l -arginine was characterized by biphasic kinetics ( K _m1 = 8 µmol/L, V _max1 = 0.09 nmol × mg⁻¹× min⁻¹; K _m2 = 229 µmol/L, V _max2 = 2.9 nmol × mg⁻¹× min⁻¹) and was inhibited by basic but not by neutral amino acids. Uptake of N ^G-nitro- l -arginine followed Michaelis-Menten kinetics ( K _m = 265 µmol/L, V _max = 12.8 ± 0.86 nmol × mg⁻¹× min⁻¹) and was selectively inhibited by aromatic and branched chain amino acids. Further characterization of the transport systems revealed that uptake of N ^G-methyl- l -arginine is mediated by system y⁺, whereas systems L and T account for the transport of N ^G-nitro- l -arginine. In agreement with these data on uptake of the inhibitors, l -lysine and l -ornithine antagonized the inhibitory effects of N ^G-methyl- l -arginine on bradykinin-induced intracellular cyclic GMP accumulation, whereas l -tryptophan, l -phenylalanine, and l -leucine interfered with the effects of N ^G-nitro- l -arginine. These data suggest that rates of uptake are limiting for the biological effects of NO synthase inhibitors.     

A β-N-acetylhexosaminidase from Penicillium oxalicum implicated in its cell-wall degradation

High β- N -acetylhexosaminidase (EC.3.2.1.52) activity was detected during autolysis of Penicillium oxalicum . Purification of the enzyme to homogeneity yielded an enzyme with a molecular weight of 132 000 Da by gel filtration and 71 900 Da by SDS polyacrylamide gel electrophoresis, suggesting a dimeric structure. The enzyme is an acidic protein with a pl of 5.0. Optimal activity was at pH 4.0 and 40°C, with a K _m of 0.80 mmol 1^-1 for p -nitrophenyl-β- N -acetylglucosaminide and 1.03 mmol 1^-1 for p -nitrophenyl-β- N -acetylgalactosaminide. The K _i with the competitive inhibitor O-(2-acetamido-2-deoxy-D-glucopyranosylidene) amino- N -phenylcarbamate was 1 μmol 1^-1. Hg²⁺, Ag⁺ and Fe³⁺ were effective inhibitors. β- N -acetylhexosaminidase hydrolysed chitobiose, chitotriose, chitotetrose and chitopentose to monomer to an extent of 92, 74, 44 and 17% respectively in 40 min. This enzyme, in conjunction with a purified endochitinase from P. oxalicum , hydrolysed a cell-wall chitin fraction isolated from this fungus, with the production of N -acetylglucosamine.     

Lysophosphatidic Acid-Induced Proliferation-Related Signals in Astrocytes

Abstract: Lysophosphatidic acid (LPA) is a potent lipid biomediator that is likely to have diverse roles in the brain. Thus, LPA-induced events in astrocytes were defined. As little as 1 n M LPA induced a rapid increase in the concentration of intracellular free calcium ([Ca²⁺]_i) in astrocytes from neonatal rat brains. This increase was followed by a slow return to the basal level. Intracellular calcium stores were important for the initial rise in [Ca²⁺]_i, whereas the influx of extracellular calcium contributed significantly to the extended elevation of [Ca²⁺]_i. LPA treatment also resulted in increases in lipid peroxidation and DNA synthesis. These increases in [Ca²⁺]_i, lipid peroxidation, and DNA synthesis were inhibited by pretreatment of cells with pertussis toxin or H7, a serine/threonine protein kinase inhibitor. Moreover, the LPA-induced increase in [Ca²⁺]_i was inhibited by a protein kinase C inhibitor, Ro 31-8220, and a calcium-dependent protein kinase C inhibitor, Gö 6976. The increase in [Ca²⁺]_i was important for the LPA-induced increase in lipid peroxidation, whereas the antioxidant, propyl gallate, inhibited the LPA-stimulated increases in lipid peroxidation and DNA synthesis. In contrast, pertussis toxin, H7, and propyl gallate had no effect on LPA-induced inhibition of glutamate uptake. Thus, LPA appears to signal via at least two distinctive mechanisms in astrocytes. One is a novel pathway, namely, activation of a pertussis toxin-sensitive G protein and participation of a protein kinase, leading to sequential increases in [Ca²⁺]_i, lipid peroxidation, and DNA synthesis.     

Characterization of a Human NK1 Tachykinin Receptor in the Astrocytoma Cell Line U 373 MG

Abstract: The human NK₁ tachykinin receptor in the astrocytoma cell line U 373 MG was characterized using selective agonists and antagonists described for this receptor in the rat. Specific [³H]substance P binding sites were present on cell homogenates, whereas [³H]neurokinin A or [³H]-senktide binding sites were absent. The binding was saturable and reversible. The binding of [³H]substance P was inhibited by very low concentrations of [L-Pro⁹]substance P and [Sar⁹,Met(O₂)¹¹]substance P; septide was ∼ 1,000-fold less potent. The most potent peptide antagonist was trans -4-hydroxy-1-(1 H -indol-3-ylcarbonyl)-L-prolyl- N -methyl- N -(phenylmethyl)-L-tyrosineamide. The rank order of potency for the nonpeptide antagonists was ( S , S )-CP 96,345 > (±)-CP 96,345 > (±)-2-chlorobenzylquinuclidinone > ( R , R )-CP 96,345 > RP 67580 > RP 68651. In [³H]-inositol-labeled cells, substance P stimulated phosphatidylinositol turnover. A good correlation was found when the abilities of NK₁ receptor agonists for stimulating inositol phosphate production and for inhibiting [³H]substance P binding were compared. Similarly, the binding and functional assays were well correlated for the antagonists. As a result of its high sensitivity and selectivity, the U 373 MG cell line thus appears an excellent tool for investigating the pharmacology of the human NK₁ receptor.     

Endothelin-1 Increases Arachidonic Acid Release in C6 Glioma Cells Through a Potassium-Modulated Influx of Calcium

Abstract: Endothelin-1 (Et-1) but not a range of other receptor agonists stimulated the release of arachidonic acid (AA) in C6 glioma. Et-1 activation was concentration dependent and was inhibited by chelation of extracellular calcium. The calcium ionophores A23187 and ionomycin could also stimulate release of AA. Et-1 caused an early increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) followed by a sustained but lower plateau level. The sensitivity of the response to quinacrine, its dependence on Ca²⁺, and the demonstration of an increase in phospholipase A₂ (PLA₂) activity that was insensitive to dithiothreitol suggested that the release of AA was due to activation of cytosolic PLA₂ in the cells. Staurosporine, a protein kinase C (PKC) inhibitor, had no effect on Et-1-induced AA release but abolished that by phorbol 12-myristate 13-acetate, demonstrating that the Et-1 response was PKC independent. Raised levels of extracellular KCI inhibited both AA release and the increase in [Ca²⁺]_i triggered by Et-1, whereas valinomycin, which causes K⁺ efflux, not only caused a rapid rise in [Ca²⁺]_i but also caused AA mobilisation. The results therefore suggest that Et-1 activation of PLA₂ in this cell type requires calcium influx dependent on K⁺ efflux.     

Cellular Differentiation in Submerged Monolayers of Dictyostelium discoideum: Possible Functions of Cytoplasmic Ca2+and DIF

Cytoplasmic calcium ion (Ca²⁺) has generally been proposed to be a key factor of numerous cellular processes. Among several agents which might be expected to alter cytoplasmic Ca²⁺-concentration ([Ca²⁺]_i), unexpectedly Ca²⁺-antagonist TMB-8 was found to raise considerably [Ca²⁺]_i, and inhibited not only the formation of prespore cells, but also their maintenance in the monolayer cultures of Dictyostelium discoideum . This seems to indicate that higher [Ca²⁺]_i is unfavorable to the prespore differentiation. In this study, we adopted the monolayer culture technique to monitor cell differentiation. However, in high density monolayers there arised a number of unique cells which was highly vacuolated and morphologically intermediate between the stalk and spore cells. These vacuolated cells having both cellulosic wall and spore coat were also induced by differentiation inducing factor (DIF). Thus the monolayer culture system used might be not necessarily qualified to monitor the terminal differentiation of Dictyostelium cells. Nevertheless, the data presented here have strongly suggested that DIF have two physiologically valued roles: 1) Induction of the membrane fusion of vesicles and/or vacuoles (vacuolization), and 2) Induction of the fusion between the cell membrane and vacuole (or vesicle) membrane (exocytosis).     

Inhibition of respiratory oxidation of elemental sulfur (S0) and thiosulfate in Thiobacillus versutus and another sulfur-oxidizing bacterium

Abstract The rates of thiosulfate, elemental sulfur (S⁰) and sulfite oxidation were measured respirometrically with an oxygen electrode using young cells of Thiobacillus versutus growing chemolithoautotrophically on thiosulfate under normal air pressure. Myxothiazol, an inhibitor of the cytochrome b−c₁ segment, and HQNO (2-N-heptyl-4-hydroxyquiniline N-oxide), acting in the quinone-cytochrome b region, both significantly inhibited the thiosulfate oxidation rate. The effect on the oxidation rate of S⁰ was even stronger. The oxidation of sulfite or ascorbate + TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine) (substrates releasing electrons at the level of cytochrome c) was not inhibited by myxothiazol and HQNO. Thiosulfate, S⁰, sulfite and ascorbate + TMPD oxidations were strongly inhibited by KCN. These respiratory activities were almost completely eliminated by cell breakage. The reduction of b-type cytochrome was observed in thiosulfate-reduced minus sulfite-reduced difference spectra. This study confirms that S⁰ is an important intermediate of thiosulfate oxidation in Thiobacillus versutus , and that electrons released by S⁰ oxidation enter the respiratory chain in the quinone-cytochrome b region. This would allow an increased gain of energy, while less energy would probably be required for pyridine-nucleotide reduction.     

Adenosine A2a Receptor-Mediated Modulation of Striatal [3H]GABA and [3H]Acetylcholine Release

Abstract: The ability of adenosine agonists to modulate K⁺-evoked 4D†-[³H]aminobutyric acid ([³H]GABA) and acetylcholine (ACh) release from rat striatal synaptosomes was investigated. The A_2a receptor-selective agonist CGS 21680 inhibited Ca²⁺-dependent [³H]GABA release evoked by 15 m M KCI with a maximal inhibition of 29 ± 4% (IC₅₀ of ∼4 ± 10 ⁻¹² M ). The relative order of potency of three agonists was CGS 21680 ± 5'- N -ethylcarboxamidoadenosine > R-phenylisopropyladenosine (R-PIA), with the inhibition being blocked by A_2a receptor-selective antagonists (CP 66,713 and CGS 15943A) but not by the A₁-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). When release of [³H]GABA was evoked by 30 mM KCI, no significant inhibition was observed. In contrast, CGS 21680 stimulated the release of [³H]ACh evoked by 30 m M KCI, with a maximal stimulation of 26 ± 5% (IC₅₀ of ∼10⁻¹¹ M ). This effect was blocked by CP 66,713 but not by DPCPX. The A₁ agonist R -PIA inhibited [³H]ACh release, an effect blocked by DPCPX. It is concluded that adenosine A_2a receptors are present on both GABAergic and cholinergic striatal nerve terminals where they inhibit and stimulate transmitter release, respectively. Key Words : GABA—Acetylcholine—Adenosine receptors—Striatum.     

Blockade by Neurotransmitter Antagonists of Veratridine-Activated Ion Channels in Neuronal Cell Lines

Abstract: The voltage-dependent Na⁺ ionophore of various neuronal cells is permeable not only to Na⁺ ions but also to guanidinium ions. Therefore, the veratridine-(or aconitine-) stimulated influx of [¹⁴C]guanidinium in neuroblastoma × glioma hybrid cells was measured to characterize the Na⁺ ionophore of these cells. Half-maximal stimulation of guanidinium uptake was seen at 30 μ M veratridine. At 1 m M guanidinium, the veratridine-stimulated uptake of guanidinium was lowered to 50% by approximately 60 m M Li⁺, Na⁺, or K⁺ and by a few millimolar Mn²⁺, Co²⁺, or Ni²⁺. The basal, as well as the veratridine-stimulated, uptake of guanidinium was inhibited by the cholinergic antagonists (+)-tubocurarine ( K_i = 50 to 500 n M ) and atropine ( K_i = 5 to 30 μ M ) and the adrenergic antagonists phentolamine ( K_i = 5 μ M ) and propranolol ( K_i = 60 μ M ). The specificity of the inhibitory effects of these agents is stressed by the ineffectiveness of various other neurotransmitter antagonists. However, the corresponding ionophore in neuroblastoma cells (clone N1E-115) seems to be regulated differently. While phentolamine and propranolol inhibit the veratridine-activated uptake as in the hybrid cells, (+)-tubocurarine and atropine exert only a slight effect.