Chronic hypoxia enhances endothelin-1-induced intracellular calcium elevation in rat carotid body chemoreceptors and up-regulates ET_A receptor expression

Endothelin-1 (ET-1) excites carotid body (CB) chemoreceptors and induces mitosis of the chemoreceptors in chronic hypoxia. The aim of the present study was to examine the hypothesis that up-regulation of both ET_A receptor and endogenous ET-1 expression in CB chemoreceptors enhances the response of intracellular Ca²⁺ to ET-1 following adaptation to chronic hypoxia (10% inspired O₂ for 3-4 weeks). Cytosolic free [Ca²⁺] ([Ca²⁺]_i) in type-I (glomus) cells freshly dissociated from rat CBs was measured by spectrofluorometry. Application of exogenous ET-1 (1-100 nM) pendently elevated [Ca²⁺]_i in the glomus cells. The response to ET-1 (100 nM) was 49% greater in the chronically hypoxic (CH) group. The ET-1 response was abolished completely by the ET_A receptor antagonist BQ610 (1 μM), but not by the ET_B antagonist BQ788 (1 μM). The transient [Ca²⁺]_i elevation induced by caffeine (30 mM) in the normoxic group was similar to that in the CH group, suggesting no differences in the intracellular Ca²⁺ stores. In situ hybridization with a digoxigenin-labelled antisense ET_A receptor mRNA oligonucleotide probe revealed very intense and ubiquitous specific expression of ET_A receptors in the lobules of glomus cells in the CH group, whereas staining in normoxic controls was light. Immunohistochemical studies revealed intense cytoplasmic staining for ET-1-immunoreactivity in most of the cell clusters in glomera in the CBs of CH rats but was faint in normoxic CBs. These findings indicate increased expression of both the ET_A receptor and ET-1 in CB chemoreceptors during chronic hypoxia. Taken together, our results suggest that the [Ca²⁺]_i response to ET-1 in rat CB chemoreceptors is augmented by up-regulation of ET_A receptors and ET-1 expression. The enhancement of the paracrine/autocrine effect of ET-1 on the chemoreceptors is consistent with an excitatory and mitogenic role of the ET-1 and ET_A receptor in the CB during chronic hypoxia.