Acid-evoked Ca(2+) signalling in rat sensory neurones: effects of anoxia and aglycaemia

Ischaemia excites sensory neurones (generating pain) and promotes calcitonin gene-related peptide release from nerve endings. Acidosis is thought to play a key role in mediating excitation via the activation of proton-sensitive cation channels. In this study, we investigated the effects of acidosis upon Ca(2+) signalling in sensory neurones from rat dorsal root ganglia. Both hypercapnic (pH(o) 6.8) and metabolic–hypercapnic (pH(o) 6.2) acidosis caused a biphasic increase in cytosolic calcium concentration ([Ca(2+)](i)). This comprised a brief Ca(2+) transient (half-time approximately 30 s) caused by Ca(2+) influx followed by a sustained rise in [Ca(2+)](i) due to Ca(2+) release from caffeine and cyclopiazonic acid-sensitive internal stores. Acid-evoked Ca(2+) influx was unaffected by voltage-gated Ca(2+)-channel inhibition with nickel and acid sensing ion channel (ASIC) inhibition with amiloride but was blocked by inhibition of transient receptor potential vanilloid receptors (TRPV1) with (E)-3-(4-t-butylphenyl)-N-(2,3-dihydrobenzo[b][1,4] dioxin-6-yl)acrylamide (AMG 9810; 1 μM) and N-(4-tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl) tetrahydropryazine-1(2H)-carbox-amide (BCTC; 1 μM). Combining acidosis with anoxia and aglycaemia increased the amplitude of both phases of Ca(2+) elevation and prolonged the Ca(2+) transient. The Ca(2+) transient evoked by combined acidosis, aglycaemia and anoxia was also substantially blocked by AMG 9810 and BCTC and, to a lesser extent, by amiloride. In summary, the principle mechanisms mediating increase in [Ca(2+)](i) in response to acidosis are a brief Ca(2+) influx through TRPV1 followed by sustained Ca(2+) release from internal stores. These effects are potentiated by anoxia and aglycaemia, conditions also prevalent in ischaemia. The effects of anoxia and aglycaemia are suggested to be largely due to the inhibition of Ca(2+)-clearance mechanisms and possible increase in the role of ASICs. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00424-009-0715-6) contains supplementary material, which is available to authorized users.