Melatonin attenuates rat carotid chemoreceptor response to hypercapnic acidosis

Respiratory activity is under circadian modulation and the physiological mechanisms may involve the pineal secretory product, melatonin, and the carotid chemoreceptor. We hypothesized that melatonin modulates the carotid chemoreceptor response to hypercapnic acidosis. To determine whether the effect of melatonin on the chemoreceptor response to hypercapnic acidosis is mediated by melatonin receptors in the chemosensitive cells, cytosolic calcium ([Ca ²⁺]_i) was measured by spectrofluorometry in fura-2-loaded glomus cells dissociated from rat carotid bodies. Melatonin (0.01-10 nM) per se did not change the [Ca²⁺]_i levels of the glomus cells but it concentration-dependently attenuated the peak [Ca²⁺]_i response to hypercapnic acidosis in the glomus cells. In addition, the [Ca ²⁺]_i response was attenuated by 2-iodomelatonin, an agonist of melatonin receptors. The melatonin-induced attenuation of the [Ca²⁺]_i response to hypercapnic acidosis was abolished by pretreatment with an non-selective mt₁/MT₂ antagonist, luzindole, and by MT₂ antagonists, 4-phenyl-2-propionamidotetraline or DH97. In situ hybridization study with antisense mt₁ and MT ₂ receptor mRNA oligonucleotide probes showed an expression of mt₁ and MT₂ receptors in the rat carotid body. Also, melatonin attenuated the carotid afferent response to hypercapnic acidosis in single- or pauci-fibers recorded from the sinus nerve in isolated carotid bodies superfused with bicarbonate-buffer saline. Results suggest that an activation of the melatonin receptors expressed in the glomus cells of the rat carotid body reduces the chemoreceptor response to hypercapnic acidosis. This modulation may play a physiological role in the influence of the circadian rhythms on the chemoreflex.