Mechanisms of fatigue-induced activation of group IV muscle afferents: theroles played by lactic acid and inflammatory mediators

We already showed that group IV muscle afferents are activated during elect rically-induced fatigue. The purpose of this study is to identify the mecha nisms of stimulation of these muscle afferents by electrically-induced fati gue at a high (100 Hz; high frequency fatigue, HFF) or a low rate (10 Hz; l ow frequency fatigue, LFF) of stimulation. In 23 paralyzed and anaesthetize d rabbits, group IV afferent activity from the tibialis anterior muscle was recorded before and after 3-min HFF or 5-min LFF runs eliciting the same f orce failure. Plasma lactic acid concentration (LA) was also measured in le g venous blood. We tested the effects of dichloroacetate (DCA), which reduc es lactic acid production, and of acetylsalicylic acid (ASA), a blocker of cyclooxygenase, on fatigue-induced activation of group IV muscle afferents after HFF and LFF trials. Pretreatment by ASA or DCA did not modify HFF-ind uced activation of muscle afferents. On the other hand, LFF-induced respons e was markedly depressed by each pharmacological agent (-44% after ASA and -75% after DCA). We verified that DCA markedly lowered LA production after LFF. The present results show that the activation of group IV muscle affere nts by LFF or HFF results from different mechanisms and also demonstrate th e major role played by lactic acid production and, to a lesser extent, the release of inflammatory mediators in LFF-induced activation of group IV mus cle afferents. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.