INTRAPLANTAR INJECTION OF HYALURONIC-ACID AT LOW PH INTO THE RAT HINDPAW PRODUCES TISSUE ACIDOSIS AND ENHANCES WITHDRAWAL RESPONSES TO MECHANICAL STIMULI

Application of buffers covering a range of acidic pH values activates and sensitizes nociceptors and produces pain. The purpose of this stud y was to determine whether a range of acidic pH in tissue produces mec hanical hyperalgesia. Tissue acidosis was produced in the hindpaw of t he rat by intraplantar injections of hyaluronic acid (HA) adjusted to pH 7.4, 6.0, 5.0, 4.0 or 3.0. Mechanical hyperalgesia was assessed by evaluating responses to application of a von Frey monofilament to the plantar surface before and after injection of HA. In separate experime nts, magnitude of tissue acidosis produced by injection of HA was dete rmined by measuring pH of intraplantar tissue using a pH microelectrod e. Although needle stick alone produced mechanical hyperalgesia, intra plantar injections of HA at pH 6.0 or 5.0 produced significantly great er mechanical hyperalgesia. In contrast, mechanical hyperalgesia produ ced by injection of HA at pH 7.4, 4.0 or 3.0 was not different from th at produced by needle stick. Although injection of HA at low pH produc ed tissue acidosis in a pH dependent manner, only a narrow range of ti ssue acidosis (pH = 6.38-6.00) produced mechanical hyperalgesia. Our d ata suggest that tissue acidosis induces mechanical hyperalgesia; howe ver, the range of tissue pH that produces this effect is limited. (C) 1998 International Association for the Study of Pain. Published by Els evier Science B.V.