Arachidonic acid potentiates acid sensing ion channels by a direct mechanism

E. St. J. Smith, H. Cadiou and P. A. McNaughton. Department of Pharmacology, Tennis Court Road, Cambridge, CB2 1PD, United Kingdom.

A reduction of extracellular pH activates an inward current in nociceptors (Krishtal et al. 1980). During the inflammatory process numerous mediators are released, including protons and arachidonic acid (AA). Here we demonstrate that AA potentiates proton-gated currents in both dorsal root ganglia (DRG) neurones and in Chinese hamster ovary (CHO) cells expressing Acid Sensing Ion Channel 1a (ASIC1a).

Neonatal Wistar rats (3 – 10 days old, 10 – 25 g) were killed by cervical dislocation. Proton-gated currents in DRG neurones and CHO cells expressing ASIC1a were examined using the whole-cell patch clamp method. Data are presented as mean ± S.E.M.

Stimulation with a 5 second pulse of pH 6.3 solution evoked three different currents in DRG neurones: a slowly inactivating transient current (-2.57 ± 0.4 nA, τinactivation 2.99 ± 0.25 sec, n = 52), a biphasic current (rapidly inactivating transient phase = 1.48 ± 0.16 nA, τinactivation 0.30 ± 0.02 sec, sustained phase = 0.058 ± 0.01 nA, n = 34) and a sustained current (0.072 ± 0.018 nA, n = 17). The different inactivation times for the slowly inactivating transient current and the rapidly inactivating transient phase of the biphasic current (P < 0.0001, Student’s t test) suggest different proton-gated ion channels may produce them.

Addition of 10 µM AA to the extracellular solution potentiated the maximal response to pH 6.3 solution of each current: slowly inactivating transient = 122.6 ± 4.4 % of control (P < 0.01, paired t test n = 7); biphasic rapidly inactivating transient phase = 171.3 ± 18.5 % of control (P < 0.05, n = 6), sustained phase = 182.7 ± 28.9 % of control (P < 0.05, n = 6); and sustained = 263.6 ± 48.3 % of control (P < 0.05, n = 7). To investigate the contribution of ASIC1a to the potentiating effect of AA in DRGs, experiments were conducted with CHO cells expressing ASIC1a. 1 µM AA was found to significantly potentiate the response to pH 6.9 solution (198.2 ± 32.3 % of control, P < 0.05, n = 9). Inhibiting AA metabolism, protein kinase C and scavenging free radicals did not prevent the effect of AA upon the ASIC1a response to low pH.

This work suggests that enhancement of acid mediated pain in inflammation arises from direct potentiation of ASIC mediated proton-gated currents by AA.

Krishtal O. et al. (1980). Neuroscience 5, 2325-2327.

Work supported by the M. R. C. (E. S.) and the B. B. S. R. C. (H. C.) We thank Prof M. Lazdunski for supply of ASIC1a.