Inhibition of prostanoid formation in intact cells by 2,5-di-(tert-butyl)-1,4-benzohydroquinone, a blocker of Ca(2+)-ATPases

Calcium signalling in the guidance of nerve growth by netrin-1

Pathfinding by growing axons in the developing nervous system is guided by diffusible or bound factors that attract or repel the axonal growth cone. The cytoplasmic signalling mechanisms that trigger the responses of the growth cone to guidance factors are mostly unknown. Previous studies have shown that the level and temporal patterns of cytoplasmic Ca2+ can regulate the rate of growth-cone extension in vitro and in vivo. Here we report that Ca2+ also mediates the turning behaviour of the growth cones of cultured Xenopus neurons that are induced by an extracellular gradient of netrin-1, an established diffusible guidance factor in vivo. The netrin-1-induced turning response depends on Ca2+ influx through plasma membrane Ca2+ channels, as well as Ca2+-induced Ca2+ release from cytoplasmic stores. Reduction of Ca2+ signals by blocking either of these two Ca2+ sources converted the netrin-1-induced response from attraction to repulsion. Activation of Ca2+-induced Ca2+ release from internal stores with a gradient of ryanodine in the absence of netrin-1 was sufficient to trigger either attractive or repulsive responses, depending on the ryanodine concentration used. These results support the model that cytoplasmic Ca2+ signals mediate growth-cone guidance by netrin-1, and different patterns of Ca2+ elevation trigger attractive and repulsive turning responses.

2,5-Di-t-butyl-1,4-benzohydroquinone induces endothelium-dependent relaxation of rat thoracic aorta

The aim of this work was to clarify the mechanism by which 2,5-di-t-butyl-1,4-benzohydroquinone (BHQ) induces relaxation of rat thoracic aorta. In particular, the role of endothelium-derived nitric oxide (NO) was investigated. BHQ concentration dependently (0.1-10 microM) relaxed rat aorta rings precontracted with phenylephrine. This effect was dependent on the intactness of the endothelium, suppressed by preincubation with 100 microM N(omega)-nitro-L-arginine methyl ester and antagonised by 3-30 microM methylene blue. The 10 microM BHQ-induced relaxation, however, was followed by the gradual and slow return to phenylephrine-induced tone. Superoxide dismutase (250 U/ml) increased the BHQ-induced relaxation, while preincubation with 3 mM diethyldithiocarbamate inhibited it in a time-dependent fashion. BHQ gave rise to superoxide anion formation which was markedly inhibited by the addition of superoxide dismutase (250 U/ml), either in the presence or in the absence of aorta rings. The non-specific blocker of Ca2+ channels, Ni2+, concentration dependently attenuated the BHQ relaxing effect. BHQ did not modify the relaxation induced by the NO donor 3-morpholino-sydnonimine in endothelium-deprived rings. In conclusion, BHQ induces endothelium-dependent relaxation and gives rise, by auto-oxidation, to the formation of superoxide anion. The former effect results from the enhanced synthesis of NO rather than from its enhanced biological activity; NO synthase is presumed to be stimulated by BHQ-induced activation of Ca2+ influx through Ni2+-sensitive Ca2+ channels.

Mechanism of rise and decay of 2,5-di-tert-butylhydroquinone-induced Ca2+ signals in Madin Darby canine kidney cells

We examined the effect of 2,5-di-tert-butylhydroquinone (BHQ) on intracellular Ca2+ concentrations ([Ca2+]i) measured by fura-2 fluorimetry in Madin Darby canine kidney (MDCK) cells. BHQ increased [Ca2+]i in a dose-dependent manner with an EC50 of 40 microM. The Ca2+ signal showed a slow onset, a gradual decay and a sustained plateau in normal Ca2+ medium. Depletion of the endoplasmic reticulum Ca2+ store by incubation with 0.1 mM BHQ for 6 min abolished the [Ca2+]i increase evoked by bradykinin or ATP, suggesting that BHQ depleted the inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store. Removal of extracellular Ca2+ reduced the BHQ response by 50%. The Ca2+ signal was initiated by Ca2+ release from the internal store, followed by capacitative Ca2+ entry which was abolished by 100 microM La3+ or 50 microM Gd3+ and was partly inhibited by 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride (SKF 96365). After depletion of the endoplasmic reticulum Ca2+ store, by incubation with another inhibitor of the endoplasmic reticulum Ca2+ pump, thapsigargin for 30 min, BHQ did not elevate [Ca2+]i, suggesting that the BHQ-induced Ca2+ influx was largely due to capacitative Ca2+ entry, and that BHQ released Ca2+ from the thapsigargin-sensitive endoplasmic reticulum store. We investigated the mechanism of decay of the BHQ response. Pretreatemt with La3+ (or Gd3+) or alkalization of the extracellular medium to pH 8 significantly potentiated the Ca2+ signal, whereas pretreatment with carbonylcyanide m-chlorophenylhydrazone (CCCP) or oligomycin, or removal of extracellular Na+, had no effect. Collectively, our results suggest that BHQ increased [Ca2+]i in MDCK cells by depleting the endoplasmic reticulum Ca2+ store followed by capacitative Ca2+ entry, with both pathways contributing equally. The decay of the BHQ response is effected by Ca2+ efflux via the plasma membrane Ca2+ pump, but not by efflux via Na+/Ca2+ exchange or sequestration by the mitochondria.