Femoral artery occlusion increases expression of ASIC3 in dorsal root ganglion neurons

Acid-sensing ion channels (ASICs) in sensory nerves are responsive to increases in the levels of protons in the extracellular medium. Prior studies suggest that the muscle metabolite, lactic acid, plays a role in reflex sympathetic and cardiovascular responses via stimulation of thin muscle afferent nerves. Also, femoral artery occlusion augments the reflex sympathetic nerve response in rats. ASIC3 is a main subtype to appear in sensory nerves in mediating the response induced by increases in protons in the interstitial space of contracting muscles. Thus, in this article, we hypothesized that femoral occlusion increases the expression of ASIC3 in primary afferent neurons innervating muscles, and this contributes to the exaggerated reflex sympathetic responses. Femoral occlusion/vascular insufficiency of the hindlimb muscles was induced by the femoral artery ligation in rats. First, Western blot analysis shows that 24-72 h of femoral artery ligation significantly increased the expression of ASIC3 protein in dorsal root ganglion (optical density, 1.0 ± 0.07 in control vs. 1.65 ± 0.1 after 24 h of occlusion, P < 0.05; n = 6 in each group). There were no significant differences for increases in ASIC3 24 and 72 h postocclusion. Second, experiments using fluorescent immunohistochemistry and retrograde-labeling technique show that a greater percentage of ASIC3 staining neurons are localized in muscle-innervating dorsal root ganglion neurons after the arterial occlusion (78 ± 3% in 24 h post occlusion vs. 59 ± 5% in control, P < 0.05; n = 6 in each group). Third, the reflex responses in renal sympathetic nerve and arterial blood pressure induced by the stimulation of ASIC were examined after an injection of lactic acid into the arterial blood supply of hindlimb muscles of control rats and ligated rats. The results demonstrate that the sympathetic and pressor responses to lactic acid were significantly augmented after femoral occlusion compared with those in the control group. The data of this study suggest that enhanced ASIC3 expression in muscle afferent nerves contributes to the exaggerated reflex sympathetic and pressor responses to lactic acid as seen in arterial occlusion.