Vanilloid type 1 receptor and the acid-sensing ion channel mediate acid phosphate activation of muscle afferent nerves in rats

Reflex cardiovascular responses to contracting skeletal muscle are mediated by mechanical and metabolic stimulation of thin-fiber muscle afferents. Diprotonated phosphate (H₂PO₄^-) excites those thin-fiber nerves and evokes the muscle pressor reflex. The receptors mediating this response are unknown. Thus we examined the role played by purinergic receptors, vanilloid type 1 receptors (VR1), and acid-sensing ion channels (ASIC) in mediating H₂PO₄^--evoked pressor responses. Phosphate and blocking agents were injected into the arterial blood supply of the hindlimb muscles of 53 decerebrated rats. H₂PO₄^- (86 mM, pH 6.0) increased mean arterial pressure by 25 ± 2 mmHg, whereas monoprotonated phosphate (HPO₄², pH 7.5) had no effect. Pyridoxalphosphate-6-azophenyl-2′,4′-disulfonic acid (a purinergic receptor antagonist, 2 mM) did not block the response. However, capsazepine (a VR1 antagonist, 1 mg/kg) attenuated the reflex by 60% and amiloride (an ASIC blocker, 6 μg/kg) by 52%. Of note, the H₂PO₄^--induced pressor response was attenuated by 87% when both capsazepine and amiloride were injected before the H₂PO₄^-. In conclusion, VR1 and ASIC mediate the pressor response due to H₂PO₄^-. The H₂PO₄^--evoked response was greater when VR1 and ASIC blockers were given simultaneously than when the respective blockers were given separately. Our laboratory's previous study has shown that H⁺ stimulates ASIC (but not VR1) on thin-fiber afferent nerves in evoking the reflex response. Thus VR1 and ASIC are likely to play a coordinated and interactive role in processing the muscle afferent response to H₂PO₄^-. Furthermore, the physiological mechanisms mediating the response to H⁺ and H₂PO₄^- are likely to be different.