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

Abstract

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 ETA 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) concentration-dependently elevated [Ca2+]i in the glomus cells. This response to ET-1 (100 nM) was 49% greater in the chronically hypoxic (CH) group. The ET-1 response was abolished completely by the ETA receptor antagonist BQ610 (1 microM), but not by the ETB antagonist BQ788 (1 microM). 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 ETA receptor mRNA oligonucleotide probe revealed very intense and ubiquitous specific expression of ETA 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 ETA 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 ETA 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 ETA receptor in the CB during chronic hypoxia.

Effect of Endothelin Receptor Antagonist Bosentan on Chronic Hypoxia-Induced Inflammation and Chemoafferent Neuron Adaptation in Rat Carotid Body

“…Moreover, we reported that concurrent treatment of CH rats with an orally-active mixed ET A/B antagonist, bosentan, partially blocked CHinduced mitosis in the carotid body, as well as chemoreceptor adaptation (Chen et al, 2007). Importantly, these findings are consistent with the known CH-induced upregulation of ET-1 and ET A receptor in rat carotid body (Chen et al, 2002a;Chen et al, 2002b). …”

Cellular and Molecular Mechanisms Associated with Carotid Body Adaptations to Chronic Hypoxia

“…ET-1 and its receptors are either undetectable or expressed in low abundance in glomus cells from normal carotid bodies. However, chronic hypoxia leads to robust expression of ET-1 and ET A receptors in glomus cells (Chen et al, 2002a(Chen et al, , 2002bHe et al, 1996). Blockade of ET A receptors prevents chronic hypoxia-induced hypersensitivity of the carotid body to acute low P O 2 .…”

The influence of chronic hypoxia upon chemoreception

“…Chronic hypoxia increases the expression of ET-1 and ET A receptors in glomus cells and blocking ET A receptors reverses the increased hypoxic sensitivity observed in chronic hypoxia, returning it to control levels (Chen et al, 2002b). The effect of ET-1 to enhance hypoxic sensitivity of glomus cells involves increased calcium currents that could result from ion channel modulation by signals from ETA receptors (Chen et al, 2002c).…”