Endothelins in the cat petrosal ganglion and carotid body: effects and immunolocalization

Growth Factors in the Carotid Body-An Update

The carotid body may undergo plasticity changes during development/ageing and in response to environmental (hypoxia and hyperoxia), metabolic, and inflammatory stimuli. The different cell types of the carotid body express a wide series of growth factors and corresponding receptors, which play a role in the modulation of carotid body function and plasticity. In particular, type I cells express nerve growth factor, brain-derived neurotrophic factor, neurotrophin 3, glial cell line-derived neurotrophic factor, ciliary neurotrophic factor, insulin-like-growth factor-I and -II, basic fibroblast growth factor, epidermal growth factor, transforming growth factor-α and -β, interleukin-1β and -6, tumor necrosis factor-α, vascular endothelial growth factor, and endothelin-1. Many specific growth factor receptors have been identified in type I cells, indicating autocrine/paracrine effects. Type II cells may also produce growth factors and express corresponding receptors. Future research will have to consider growth factors in further experimental models of cardiovascular, metabolic, and inflammatory diseases and in human (normal and pathologic) samples. From a methodological point of view, microarray and/or proteomic approaches would permit contemporary analyses of large groups of growth factors. The eventual identification of physical interactions between receptors of different growth factors and/or neuromodulators could also add insights regarding functional interactions between different trophic mechanisms.

Role of glial-like type II cells as paracrine modulators of carotid body chemoreception

Mammalian carotid bodies (CB) are chemosensory organs that mediate compensatory cardiorespiratory reflexes in response to low blood PO₂ (hypoxemia) and elevated CO₂/H⁺ (acid hypercapnia). The chemoreceptors are glomus or type I cells that occur in clusters enveloped by neighboring glial-like type II cells. During chemoexcitation type I cells depolarize, leading to Ca²⁺-dependent release of several neurotransmitters, some excitatory and others inhibitory, that help shape the afferent carotid sinus nerve (CSN) discharge. Among the predominantly excitatory neurotransmitters are the purines ATP and adenosine, whereas dopamine (DA) is inhibitory in most species. There is a consensus that ATP and adenosine, acting via postsynaptic ionotropic P2X2/3 receptors and pre- and/or postsynaptic A2 receptors respectively, are major contributors to the increased CSN discharge during chemoexcitation. However, it has been proposed that the CB sensory output is also tuned by paracrine signaling pathways, involving glial-like type II cells. Indeed, type II cells express functional receptors for several excitatory neurochemicals released by type I cells including ATP, 5-HT, ACh, angiotensin II, and endothelin-1. Stimulation of the corresponding G protein-coupled receptors increases intracellular Ca²⁺, leading to the further release of ATP through pannexin-1 channels. Recent evidence suggests that other CB neurochemicals, e.g., histamine and DA, may actually inhibit Ca²⁺ signaling in subpopulations of type II cells. Here, we review evidence supporting neurotransmitter-mediated crosstalk between type I and type II cells of the rat CB. We also consider the potential contribution of paracrine signaling and purinergic catabolic pathways to the integrated sensory output of the CB during chemotransduction.

Sympathoexcitation and arterial hypertension associated with obstructive sleep apnea and cyclic intermittent hypoxia

Obstructive sleep apnea (OSA) is characterized by repetitive episodes of upper airway obstruction during sleep. These obstructive episodes are characterized by cyclic intermittent hypoxia (CIH), by sleep fragmentation, and by hemodynamic instability, and they result in sustained sympathoexcitation and elevated arterial pressure that persist during waking, after restoration of normoxia. Early studies established that 1) CIH, rather than sleep disruption, accounts for the increase in arterial pressure; 2) the increase in arterial pressure is a consequence of the sympathoactivation; and 3) arterial hypertension after CIH exposure requires an intact peripheral chemoreflex. More recently, however, evidence has accumulated that sympathoactivation and hypertension after CIH are also dependent on altered central sympathoregulation. Furthermore, although many molecular pathways are activated in both the carotid chemoreceptor and in the central nervous system by CIH exposure, two specific neuromodulators-endothelin-1 and angiotensin II-appear to play crucial roles in mediating the sympathetic and hemodynamic response to intermittent hypoxia.

Trophic factors in the carotid body

The aim of the present study is to provide a review of the expression and action of trophic factors in the carotid body. In glomic type I cells, the following factors have been identified: brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor, artemin, ciliary neurotrophic factor, insulin-like growth factors-I and -II, basic fibroblast growth factor, epidermal growth factor, transforming growth factor-alpha and -beta1, interleukin-1beta and -6, tumour necrosis factor-alpha, vascular endothelial growth factor, and endothelin-1 (ET-1). Growth factor receptors in the above cells include p75LNGFR, TrkA, TrkB, RET, GDNF family receptors alpha1-3, gp130, IL-6Ralpha, EGFR, FGFR1, IL1-RI, TNF-RI, VEGFR-1 and -2, ETA and ETB receptors, and PDGFR-alpha. Differential local expression of growth factors and corresponding receptors plays a role in pre- and postnatal development of the carotid body. Their local actions contribute toward producing the morphologic and molecular changes associated with chronic hypoxia and/or hypertension, such as cellular hyperplasia, extracellular matrix expansion, changes in channel densities, and neurotransmitter patterns. Neurotrophic factor production is also considered to play a key role in the therapeutic effects of intracerebral carotid body grafts in Parkinson's disease. Future research should also focus on trophic actions on carotid body type I cells by peptide neuromodulators, which are known to be present in the carotid body and to show trophic effects on other cell populations, that is, angiotensin II, adrenomedullin, bombesin, calcitonin, calcitonin gene-related peptide, cholecystokinin, erythropoietin, galanin, opioids, pituitary adenylate cyclase-activating polypeptide, atrial natriuretic peptide, somatostatin, tachykinins, neuropeptide Y, neurotensin, and vasoactive intestinal peptide.

Does Endothelin Play a Role in Chemoreception During Acute Hypoxia in Normal Men?

BACKGROUND

The peripheral chemoreceptors are the dominant reflex mechanism responsible for the rise in ventilation and muscle sympathetic nerve activity (MSNA) in response to hypoxia. Animal studies have suggested that endothelin (ET) plays an important role in chemosensitivity. Moreover, several human clinical conditions in which circulating ET levels are increased are accompanied by enhanced chemoreflex sensitivity. Whether ET plays a role in normal human chemosensitivity is unknown.

METHODS

We determined whether bosentan, a nonspecific ET receptor antagonist, would decrease chemoreflex sensitivity in 14 healthy subjects. We assessed the effects of bosentan on the response to isocapnic hypoxia, using a randomized, crossover, double-blinded study design.

RESULTS

Bosentan increased mean (+/- SEM) plasma ET levels from 1.97 +/- 0.28 to 2.53 +/- 0.23 pg/mL (p = 0.01). Hypoxia increased mean minute ventilation from 6.7 +/- 0.3 to 8+/0.4 L/min (p < 0.01), mean MSNA from 100 to 111 +/- 5% (p < 0.01), mean heart rate from 67 +/- 3 to 86 +/- 3 beats/min (p < 0.01), and mean systolic BP from 116 +/- 3 to 122 +/- 3 mm Hg (p < 0.01). However, none of these responses differed between therapy with bosentan and therapy with placebo (p = 0.26). Bosentan did not affect the mean MSNA responses to the apneas, during normoxia (change from baseline: placebo, 259 +/- 58%; bosentan, 201 +/- 28%; p = 0.17) or during hypoxia (change from baseline: placebo, 469 +/- 139%; bosentan, 329 +/- 46%; p = 0.24). The durations of the voluntary end-expiratory apneas in normoxia and hypoxia, and the subsequent reductions in oxygen saturation, were also similar with therapy using bosentan and placebo (p = 0.42).

CONCLUSION

In healthy men, ET does not play an important role in peripheral chemoreceptor activation by acute hypoxia.

Expression and immunolocalization of endothelin peptides and its receptors, ETA and ETB, in the carotid body exposed to chronic intermittent hypoxia

Increased levels of endothelin-1 (ET-1) in the carotid body (CB) contribute to the enhancement of chemosensory responses to acute hypoxia in cats exposed to chronic intermittent hypoxia (CIH). However, it is not known if the ET receptor types A (ETA-R) and B (ETB-R) are upregulated. Thus, we studied the expression and localization of ETA-R and ETB-R using Western blot and immunohistochemistry (IHC) in CBs from cats exposed to cyclic hypoxic episodes, repeated during 8 hr for 4 days. In addition, we determined if ET-1 is expressed in the chemoreceptor cells using double immunofluorescence for ET-1 and tyrosine hydroxylase (TH). We found that ET-1 expression was ubiquitous in the blood vessels and CB parenchyma, although double ET-1 and TH-positive chemoreceptor cells were mostly found in the parenchyma. ETAR was expressed in most chemoreceptor cells and blood vessels of the CB vascular pole. ETB-R was expressed in chemoreceptor cells, parenchymal capillaries, and blood vessels of the vascular pole. CIH upregulated ETB-R expression by approximately 2.1 (Western blot) and 1.6-fold (IHC) but did not change ETA-R expression. Present results suggest that ET-1,ETA-R, and ETB-R are involved in the enhanced CB chemosensory responses to acute hypoxia induced by CIH.

Contribution of endothelin-1 to the enhanced carotid body chemosensory responses induced by chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) enhances carotid body (CB) chemosensory responses to acute hypoxia. We tested the hypothesis that endothelin-1 (ET-1), an excitatory modulator of CB chemoreception may contribute to the enhanced CB chemosensory responses in cats exposed to cyclic hypoxic episodes repeated during 8 h for 4 days. Accordingly, we measured the ET-1 immunoreactivity (ET-ir) in the CB and plasma. Using a perfused CB preparation, we studied the effects of exogenous ET-1 and bosentan, a non-selective endothelin receptor type A and B antagonist, on the frequency of chemosensory discharges (f(x)) during normoxia, mild and severe hypoxia. We found that CIH increased ET-ir in the CB by approximately 10-fold leaving ET-1 plasma levels unchanged. Application of ET-1 to control and CIH-treated CBs produced long-lasting dose-dependent increases in f(x), although the dose-response curve showed a rightward-shift in the CIH-treated CBs. CIH increased baseline f(x) and hypoxic chemosensory responses, which were reduced by 50 microM bosentan in CBs from CIH-treated cats. Present results suggest that a local increase of ET-1 in the CB may contribute to the enhanced chemosensory responses induced by CIH predominantly through a vasomotor mechanism.