Inflammatory mediators potentiate pain induced by experimental tissue acidosis

Abstract

Electrophysiological evidence from cutaneous nociceptors suggested a synergism between excitatory actions of inflammatory mediators (IM) and low pH. In human skin it is possible to induce constant ongoing pain with continuous infusion of acid buffer. This method was used to study the interaction with mediators of inflammation psychophysiologically. A skin area on the palmar forearm of 6 subjects (either gender, age 22-35 years) was continuously infiltrated with a phosphate buffered electrolyte solution (pH 5.2) using a motorized syringe pump that was adjusted so as to produce constant pain of about 20% on a visual analog scale (VAS; extending from 'no' to 'unbearable pain'). Pain was assessed on the VAS at 10-sec intervals; the rating was called up by means of an acoustic signal. An additional cannula was placed in the skin before the infusion of acidic buffer started. Injections of an acidic combination of IM (BK, 5-HT, HIS, PGE2) 0.2 ml were then given through the cannula at intervals of 10 min in a randomized double blind order of concentrations. The other arm was used for negative control, i.e. IM in neutral solution were injected into normal skin continuously infiltrated with a buffer solution at pH 7.4. The IM induced dose-dependent, transient burning pain on both arms-markedly more intense and prolonged, however, in the acidotic skin (P < 0.004, U-test). The difference corresponded to a 10-fold increase in algogenic potency with 10(-7) M IM, being smaller with 10(-6) and 10(-5) M concentration. The interaction between low pH and IM was mutual: additional injections of plain phosphate buffer (pH 5.2) into the acidotic skin were significantly more painful (20-fold) after application of IM than under control conditions. Thus, we tend to conclude that it is the inflammatory mediators that potentiate the algogenic effect of low pH rather than vice versa. Tissue acidosis appears as a dominant factor in inflammatory pain.

Identification, functional expression and chromosomal localisation of a sustained human proton‐gated cation channel

“…Subcutaneous perfusion of human volunteers with acidic buffer causes pain. At pH 5.2, the pain was rated 20% on a scale ranging from 0 to 100% (unbearable pain) [2]. Furthermore, a subpopulation of polymodal C-fibres in rat nerve-skin preparations can be excited by acidic pH [4].…”

Sensitization of central trigeminovascular neurons: Blockade by intravenous naproxen infusion

“…Activation and sensitization of central trigeminovascular neurons was induced using an 'inflammatory soup' (IS) containing 0.1 mM prostaglandin E 2 , 1 mM histamine, serotonin, bradykinin, pH 5.5 (Steen et al, 1992, Steen et al, 1995, Steen et al, 1996. The exposed area of the cerebral dura was bathed in IS for 5 min as described before (Burstein et al, 1998, Levy andStrassman, 2002), and the establishment of central sensitization was verified 2 h later using any one of the following criteria: (a) increase in ongoing firing rate; (b) increase in responses magnitude to mechanical stimulation of the dura and skin; (c) shift in response threshold to heat stimulation of the skin; (d) expansion of dural or cutaneous receptive fields.…”

pH Dependency and Desensitization Kinetics of Heterologously Expressed Combinations of Acid-sensing Ion Channel Subunits

“…It is well established that tissue acidification, which may be present in inflammatory and ischemic conditions, causes pain (1)(2)(3). In line with this, peripheral sensory neurons exhibit sensitivity toward acid by activating several types of depolarizing currents (4 -6).…”