Hypoxia In Carotid Body Research Articles

An important functional role of persistent Na+current in carotid body hypoxia transduction

Systemic hypoxia in mammals is sensed and transduced by the carotid body into increased action potential (AP) frequency on the sinus nerve, resulting in increased ventilation. The mechanism of hypoxia transduction is not resolved, but previous work suggested that fast Na(+) channels play an important role in determining the rate and timing of APs (Donnelly, DF, Panisello JM, and Boggs D. J Physiol. 511: 301-311, 1998). We speculated that Na(+) channel activity between APs, termed persistent Na(+) current (I(NaP)), is responsible for AP generation that and riluzole and phenytoin, which inhibit this current, would impair organ function. Using whole cell patch clamp recording of intact petrosal neurons with projections to the carotid body, we demonstrated that I(NaP) is present in chemoreceptor afferent neurons and is inhibited by riluzole. Furthermore, discharge frequencies of single-unit, chemoreceptor activity, in vitro, during normoxia (Po(2) 150 Torr) and during acute hypoxia (Po(2) 90 Torr) were significantly reduced by riluzole concentrations at or above 5 microM, and by phenytoin at 100 microM, without significant affect on nerve conduction time, AP magnitude (inferred from extracellular field), and AP duration. The effect of both drugs appeared solely postsynaptic because hypoxia-induced catecholamine release in the carotid body was not altered by either drug. The respiratory response of unanesthetized, unrestrained 2-wk-old rats to acute hypoxia (12% inspired O(2) fraction), which was measured with whole body plethysmography, was significantly reduced after treatment with riluzole (2 mg/kg ip) and phenytoin (20 mg/kg ip). We conclude that I(NaP) is present in chemoreceptor afferent neurons and serves an important role in peripheral chemoreceptor function and, hence, in the ventilatory response to hypoxia.

Comparative gene expression profile of mouse carotid body and adrenal medulla under physiological hypoxia

The carotid body (CB) is an arterial chemoreceptor, bearing specialized type I cells that respond to hypoxia by closing specific K+ channels and releasing neurotransmitters to activate sensory axons. Despite having detailed information on the electrical and neurochemical changes triggered by hypoxia in CB, the knowledge of the molecular components involved in the signalling cascade of the hypoxic response is fragmentary. This study analyses the mouse CB transcriptional changes in response to low PO2 by hybridization to oligonucleotide microarrays. The transcripts were obtained from whole CBs after mice were exposed to either normoxia (21% O2), or physiological hypoxia (10% O2) for 24 h. The CB transcriptional profiles obtained under these environmental conditions were subtracted from the profile of control non-chemoreceptor adrenal medulla extracted from the same animals. Given the common developmental origin of these two organs, they share many properties but differ specifically in their response to O2. Our analysis revealed 751 probe sets regulated specifically in CB under hypoxia (388 up-regulated and 363 down-regulated). These results were corroborated by assessing the transcriptional changes of selected genes under physiological hypoxia with quantitative RT-PCR. Our microarray experiments revealed a number of CB-expressed genes (e.g. TH, ferritin and triosephosphate isomerase) that were known to change their expression under hypoxia. However, we also found novel genes that consistently changed their expression under physiological hypoxia. Among them, a group of ion channels show specific regulation in CB: the potassium channels Kir6.1 and Kcnn4 are up-regulated, while the modulatory subunit Kcnab1 is down-regulated by low PO2 levels.

Specific carotid body chemostimulation is sufficient to elicit phrenic poststimulus frequency decline in a novel in situ dual-perfused rat preparation

Time-dependent ventilatory responses to hypoxic and hypercapnic challenges, such as posthypoxic frequency decline (PHxFD) and posthypercapnic frequency decline (PHcFD), could profoundly affect breathing stability. However, little is known about the mechanisms that mediate these phenomena. To determine the contribution of specific carotid body chemostimuli to PHxFD and PHcFD, we developed a novel in situ arterially perfused, vagotomized, decerebrate rat preparation in which central and peripheral chemoreceptors are perfused separately (i.e., a nonanesthetized in situ dual perfused preparation). We confirmed that 1) the perfusion of central and peripheral chemoreceptor compartments was independent by applying specific carotid body hypoxia and hypercapnia before and after carotid sinus nerve transection, 2) the PCO(2) chemoresponse of the dual perfused preparation was similar to other decerebrate preparations, and 3) the phrenic output was stable enough to allow investigation of time-dependent phenomena. We then applied four 5-min bouts (separated by 5 min) of specific carotid body hypoxia (40 Torr PO(2) and 40 Torr PCO(2)) or hypercapnia (100 Torr PO(2) and 60 Torr PCO(2)) while holding the brain stem PO(2) and PCO(2) constant. We report the novel finding that specific carotid body chemostimuli were sufficient to elicit several phrenic time-dependent phenomena in the rat. Hypoxic challenges elicited PHxFD that increased with bout, leading to progressive augmentation of the phrenic response. Conversely, hypercapnia elicited short-term depression and PHcFD, neither of which was bout dependent. These results, placed in the context of previous findings, suggest multiple physiological mechanisms are responsible for PHxFD and PHcFD, a redundancy that may illustrate that these phenomena have significant adaptive advantages.

Regulation of catecholamines by sustained and intermittent hypoxia in neuroendocrine cells and sympathetic neurons.

Chronic intermittent hypoxia, a characteristic feature of sleep-disordered breathing, induces hypertension through augmented sympathetic nerve activity and requires the presence of functional carotid body arterial chemoreceptors. In contrast, chronic sustained hypoxia does not alter blood pressure. We therefore analyzed the biosynthetic pathways of catecholamines in peripheral nervous system structures involved in the pathogenesis of intermittent hypoxia-induced hypertension, namely, carotid bodies, superior cervical ganglia, and adrenal glands. Rats were exposed to either intermittent hypoxia (90 seconds of room air alternating with 90 seconds of 10% O2) or to sustained hypoxia (10% O2) for 1 to 30 days. Dopamine, norepinephrine, epinephrine, dihydroxyphenylacetic acid, and 5-hydroxytyptamine contents were measured by high-performance liquid chromatography. Expression of tyrosine hydroxylase and its phosphorylated forms, dopamine beta-hydroxylase, phenylethanolamine N-methyltransferase, and GTP cyclohydrolase-1 were determined by Western blot analyses. Both sustained and intermittent hypoxia significantly increased dopamine and norepinephrine content in carotid bodies but not in sympathetic ganglia or adrenal glands. In carotid bodies, both types of hypoxia augmented total levels of tyrosine hydroxylase protein and its phosphorylation on serines 19, 31, 40, as well as levels of GTP cyclohydrolase-1. However, the effects of intermittent hypoxia on catecholaminergic pathways were significantly smaller and delayed than those induced by sustained hypoxia. Thus, attenuated induction of catecholaminergic phenotype by intermittent hypoxia in carotid body may play a role in development of hypertension associated with sleep-disordered breathing. The effects of both types of hypoxia on expression of catecholaminergic enzymes in superior cervical neurons and adrenal glands were transient and small.

Nitric oxide and calcium binding protein in hypoxic rat carotid body.

The occurence and distribution of nitric oxide synthase (NOS)-containing nerve fibers have been demonstrated by means of NOS immunohistochemistry and NADPH-diaphorase (NADPH-d) histochemistry (Wang, et al., 1993; Grimes, et al., 1994; Hohler, et al., 1994; Tanaka, et al., 1994). Recently, we reported the distribution of NOS immunoreactive fibers in the chronically hypoxic rat carotid body (Kusakabe, et al., 1998). The density of NOS fibers in the chronically hypoxic carotid body was significantly reduced. This finding suggests that altered nitrergic innervation of the chronically hypoxic carotid body is a feature of hypoxic adaptation. On the other hand, the first evidence for the presence of the calcium binding protein (spot 35-protein) in afferent nerve fibers in the rat carotid body has been shown by Kondo (1990). Recently, in the same hypoxic carotid body, we reported a similar reduction in the density per unit area of calbindin D-28k immunoreactive nerve fibers (Kusakabe et al., 2000). Calbindin D-28k belongs to a family of calcium binding proteins, and plays important roles in the storage and transport of intracellular Ca2+ (Baimbridge and Miller, 1982).

Ascorbate in the carotid body.

In previous work, we found that ascorbic acid (AA) or ascorbate is present in the carotid body of the cat (Pokorski and Gonet, 1997), the animal that, as opposed to man and other primates, is capable of synthesizing ascorbate (Chatterjee, 1973). The content of ascorbate is decreased in the hypoxic carotid body, which implies the compound might be involved in the chemosensing process, although the determinants of such an involvement remain unraveled.

Altered expression of vascular endothelial growth factor and FLK-1 receptor in chronically hypoxic rat carotid body.

The mammalian carotid body maintains a high level of chemoreceptor output and provides support for increased ventilatory drive over an extended period in animals chronically exposed to a low O2 environment (i.e., chronic hypoxia, CH; Bisgard, 2000). Important features of carotid body adaptation to CH include hypertrophy and hyperplasia of (O2-sensitive type I cells, and marked dilation of surrounding microvascular elements (McGregor et al 1984; Laidler & Kay, 1975). Although little is known about the cellular mechanisms which mediate these remarkable morphological changes (Pequignot et al 1987; Bisgard, 2000), similar (O2-sensitive changes occur in the lung and heart where CH elicits constriction and thickening of vessel walls in the pulmonary circuit and hypertrophy of the right ventricle, resulting in pulmonary hypertension (Rabinovitch et al., 1979).

Importance of glomus cell plasma membrane and mitochondrial membrane potentials during acute hypoxia signaling in the rat carotid body.

The mammalian carotid body (CB) is an oxygen addicted chemoreceptor organ, which consists of type I glomus, cells; the seat of oxygen sensing. There are two consensus hypotheses regarding the CB hypoxia signaling pathway; (a) non-respiratory/ plasma membrane and (b) respiratory/ metabolic.

Postnatal hypoxemia increases angiotensin II sensitivity and up-regulates AT1a angiotensin receptors in rat carotid body chemoreceptors.

In the present study, the effects of postnatal hypoxemia on the AT1 angiotensin receptor-mediated activities in the rat carotid body were studied. Angiotensin II (Ang II) concentration-dependently increased the chemoreceptor afferent activity in the isolated carotid body. Single- or pauci-fiber recording of the sinus nerve revealed that the afferent response to Ang II was enhanced in the postnatally hypoxic carotid body. To determine whether the increased sensitivity to Ang II is mediated by changes in the functional expression of Ang II receptors in the carotid body chemoreceptors, cytosolic calcium ([Ca2+]i) was measured by spectrofluorimetry in fura-2 acetoxymethyl ester-loaded type I cells dissociated from carotid bodies. Ang II (25-100 nM) concentration-dependently increased [Ca2+]i in the type I cells. The proportion of clusters of type I cells responsive to Ang II was higher in the postnatally hypoxic group than in the normoxic control (89 vs 66%). In addition, the peak [Ca2+]i response to Ang II was enhanced 2- to 3-fold in the postnatally hypoxic group. The [Ca2+]i response to Ang II was abolished by pretreatment with losartan (1 microM), an AT1 receptor antagonist, but not by PD-123177 (1 microM), an AT(2) antagonist. Double-labeling immunohistochemistry confirmed that an enhanced immunoreactivity for AT1 receptor was co-localized to the lobules of type I cells in the hypoxic group. In addition, RT-PCR analysis of subtypes of AT1 receptors showed an up-regulation of AT1a but a down-regulation of AT1b receptors, indicating a differential regulation of the expression of AT1 receptor subtypes by postnatal hypoxia in the carotid body. These data suggest that postnatal hypoxemia is associated with an increased sensitivity of peripheral chemoreceptors in response to Ang II and an up-regulation of AT1a receptor-mediated [Ca2+]i activity of the chemoreceptors. This modulation may be important for adaptation of carotid body functions in the hypoxic ventilatory response and in electrolyte and water homeostasis during perinatal and postnatal hypoxia.

Changes in the peptidergic innervation of the rat carotid body a month after the termination of chronic hypoxia.

The carotid bodies are enlarged by several fold in rats exposed to chronic hypoxia; their volume increases and endothelial cells multiply (Barer et al. 1976; Heath et al. 1973; Laider and Kay 1975a, b). Recently, we examined the peptidergic innervation in the carotid body of the rats exposed to chronically isocapnic hypoxia (Kusakabe et al. 1998). The density of substance P (SP) and calcitonin gene-related peptide (CGRP) immunoreactive fibers in the chronically hypoxic carotid body decreased significantly to under 50%, the density of vasoactive intestinal polypeptide (VIP) immunoreactive fibers increased significantly 1.80 times, and the density of neuropeptide Y (NPY) immunoreactive fibers was unchanged in comparison with the controls. These findings suggest that altered peptidergic innervation of the chronically hypoxic carotid body is one feature of hypoxic adaptation, which may involve modulation of chemosensory mechanisms by modification of carotid body circulation. As far as we are aware, however, there are no immunohistochemical studies on the peptidergic innervation in the carotid body during deacclimatization after chronic hypoxia is terminated. In the present study, the distribution and abundance of neuropeptide-

Age-Dependent Effect of Domperidone on Dopamine Release by the Hypoxic Carotid Body in the Rabbit

The aim of this study was to identify the mechanisms that regulate dopamine release (DA_r) by the hypoxic carotid body (CB) during development. CBs sampled from adult (n = 58) and 10-day-old (n = 53) rabbits were incubated for 1 h in a medium equilibrated with 8% O₂ in N₂ without or in the presence of the specific DA D₂ receptor antagonist domperidone, 0.01, 0.1 and 1 µM. DA and its major metabolite DOPAC were measured in the CB (DA_CB) and in the medium (DA_r) by HPLC+ED. In adults, each concentration of domperidone significantly decreased DA_CB and increased DA_r, compared with control (p < 0.01). In contrast, in 10-day old, only the 1 µM domperidone concentration decreased DA_CB and increased DA_r compared with control (p < 0.001). The data show that domperidone increases CB DA_r in response to hypoxia in a concentration- and age-dependent manner and suggest this response depends, in part, on the functional maturation of CB DA D₂ receptors.

Ventilatory Response to Sustained Hypoxia in Carotid Body Denervated Rats

Hypoxia stimulates ventilation, but when it is sustained, a decline in the ventilatory response is seen. The mechanism responsible for this decline lies within the CNS, but still remains unknown. In this study, we attempted to elucidate the possible role of hypoxia-induced depression of respiratory neurons by comparing the ventilatory response to hypoxia in intact rats and those with denervated carotid bodies. A whole-body plethysmograph was used to measure tidal volume, frequency of breathing and minute ventilation (VE) in awake and anesthetized intact rats and rats after carotid body denervation during exposure to hypoxia (FIO2 0.1). Fifteen-minute hypoxia induced an initial increase of VE in intact rats (to 248 % of control ventilation in awake and to 227 % in anesthetized rats) followed by a consistent decline (to 207 % and 196 % of control VE, respectively). Rats with denervated carotid bodies responded with a smaller increase in VE (to 134 % in awake and 114 % in anesthetized animals), but without a secondary decline (145 % and 129 % of control VE in the 15th min of hypoxia). These results suggest that afferentation from the carotid bodies and/or the substantial increase in ventilation are crucial for the biphasicity of the ventilatory response to sustained hypoxia and that a central hypoxic depression cannot fully explain the secondary decline in VE.

Regulation of phospholipase C activity by calcium ions and guanine nucleotide in the normoxic cat carotid body.

The carotid bodies (CB) are a paired chemoreceptor organ located at the bifurcation of the common carotid arteries. High O2 tension suppresses while low tension activates afferent carotid chemoreceptor activity and the chemoreflex ventilatory response in the cat. The intracellular mechanism of chemotransduction is till now unknown. Previously we have shown different activities of phospholipase C (PLC) in normoxic, hypoxic and hyperoxic cat carotid body. Now we have addressed the question whether calcium ions and G-protein could be regulators of the formation of lipid derived messenger molecules in the cat carotid body. To answer this question, the PLC acting against [3H] inositol-phosphatidylinositol (PtdIns) and [3H] inositol-phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] in the cat CB were investigated using labelled phospholipids as a source of the substrate. CB homogenate was used as a source of the enzyme. The results indicate that PLC acting on PtdIns is Ca2+-dependent, in contrary to that acting on PtdIns(4,5)P2 which remains active in the presence of 10 mM EGTA. PtdIns(4,5)P2-PLC is stimulated by GTPgammaS. In the presence of Ca2+, GTPgammaS has a synergistic stimulatory effect. PLC acting on PtdIns is not activated by GTPgammaS. In the presence of calcium ions dopamine and a nonhydrozylable analogue of acetylcholine, carbachol, have a small stimulatory effect of about 30% on PLC acting on PtdIns(4,5)P2. GTPgammaS enhances this effect. These results allow us to suggest that there are two pathways of phosphoinositides degradation in the CB, one of them is regulated by calcium ions/PtdIns-PLC/, the other one by G-protein / PtdIns(4,5)P2-PLC/.

Role of ET-1 in hypoxia-induced mitosis of cultured rat carotid body chemoreceptors.

The mammalian carotid body (CB) contains O2-chemoreceptors, i.e. glomus cells, which display increased mitoses and endothelin-1 (ET-1) expression during chronic hypoxia. To investigate whether endogenous ET-1 might mediate these mitogenic effects, we quantified bromodeoxyuridine (BrdU) uptake by tyrosine hydroxylase (TH)-positive glomus cells in rat CB cultures using double-label immunofluorescence. In normoxia (20% O2), 2-day exposure to ET-1 (10-1000 nM) caused a dose-dependent increase in BrdU uptake which peaked (approximately 55% of TH+ cells) at around 500 nM ET-1. In chronic hypoxia (5% O2) alone, BrdU uptake was stimulated (approximately 46% of TH+ cells) relative to normoxia (approximately 30%), but the effect was abolished in the presence of specific (BQ 123) or non-specific (PD 142893) ETA receptor antagonists (10(-5) M). Thus paracrine/autocrine release of ET-1 in the hypoxic carotid body may promote glomus cell mitosis via ET(A) receptors.

Hypoxia inhibits the synthesis of phosphoinositides in the rabbit carotid body.

Hypoxic transduction in the carotid body (CB) is regulated by several systems of second messengers, but the role of the phospholipase C system has not been studied. The aim of the present study was to characterize the turnover rate of inositol phosphates (InsPs) and phosphoinositides (PIs) and their modifications by hypoxia in the rabbit CB in vitro. In CBs, in which the PIs had been labelled previously with 3H-myo-inositol, hypoxia in the presence of LiCl did not modify the accumulation of 3H-InsPs, whilst exposure to hypoxia during the loading period in the presence of LiCl reduced the accumulation of 3H-InsPs by more than 50%. Endogenous levels of inositol 1,4,5-trisphosphate were unaltered by hypoxia. Synthesis of 3H-PIs from 3H-myo-inositol was markedly inhibited by hypoxia in the CB, but not in the rat superior cervical ganglion used as control tissue. Levels of 3H-phosphatidylinositol (3H-PtdIns), 3H-phosphatidylinositol 4-monophosphate and 3H-phosphatidylinositol 4,5-bisphosphate were similarly decreased, indicating that inhibition occurs at a step prior to PtdIns synthesis. It is concluded that the phospholipase C system of second messengers does not play a significant role in the short-term regulation of hypoxic transduction cascade. It can be speculated that the decrease in PI availability produced by hypoxia might be involved in the functional changes observed in the CB on chronic hypoxic exposure.

Hypoxic adaptation of the peptidergic innervation in the rat carotid body

The abundance of substance P (SP)-, calcitonin gene-related peptide (CGRP)-, vasoactive intestinal polypeptide (VIP)-, and neuropeptide Y (NPY)-immunoreactive nerve fibers in the carotid body was compared between normoxic and chronically hypoxic rats (10% O 2 and 3.0–4.0% CO 2 for 3 months). The immunoreactive fibers appeared as thin processes with many varicosities, and were distributed mainly around the vasculatures. In the normoxic control carotid body, NPY fibers were more numerous than VIP, CGRP, and SP fibers. In the chronically hypoxic rats, the carotid body was enlarged several fold, and the mean absolute number of VIP and NPY fibers was 3.88 and 2.22 times higher than in the normoxic carotid body, respectively, although that of SP and CGRP fibers was not changed. When expressed as density per unit area of the parenchyma, the density of SP and CGRP fibers in the chronically hypoxic carotid body decreased significantly to under 50%, the density of VIP fibers increased significantly 1.80 times, and the density of NPY fibers were unchanged. Immunoreactivity for four neuropeptides was not found in the glomus cells of normoxic or chronically hypoxic carotid bodies. These results suggest that altered peptidergic innervation of the chronically hypoxic carotid body is one feature of hypoxic adaptation. Because these neuropeptides are vasoactive in nature, altered carotid body circulation may contribute to modulation of the chemosensory mechanisms by chronic hypoxia.

Changes in the distribution of nitric oxide synthase immunoreactive nerve fibers in the chronically hypoxic rat carotid body

The distribution of nitric oxide synthase (NOS) immunoreactive nerve fibers in the carotid body was compared between normoxic and chronically hypoxic rats (10% O 2 and 3.0–4.0% CO 2 for 3 months). NOS immunoreactive fibers appeared as thin processes with many varicosities. They were distributed predominantly around small arteries and arterioles, and around clusters of glomus cells. When expressed by the density of varicosities per unit area in the parenchyma, the density of NOS fibers associated with the vasculature and with the glomus cells in the chronically hypoxic carotid bodies was significantly decreased. Because nitric oxide (NO) is an inhibitory neuronal messenger in the normoxic carotid body, the present findings suggest that the sensory mechanisms in the hypoxic carotid body may be involved in `disinhibition' resulting from reduced NO synthesis.

Strain-associated differences in hypoxic chemosensitivity of the carotid body in rats.

Studies in humans indicate genetic effects on the ventilatory response to hypoxia, but the site of these effects is unknown. The present study explores the question of whether there are genetically directed effects on the intrinsic hypoxic chemosensitivity of the carotid body. The approach was to study these responses in two inbred rat strains [spontaneously hypertensive rats (SHR) and Fischer 344 (F-344)] and to measure in vivo carotid chemosensitivity as the change in carotid sinus nerve (CSN) activity during progressive, isocapnic hypoxia and the isolated, in vitro responses of excised superfused carotid bodies, loaded with the fluorimetric indicator fura 2, measured as the cytosolic calcium response to moderate hypoxia (PO2 = 55 mmHg). CSN responses in F-344 rats (n = 12) were uniformly low, with a shape parameter A of 13.8 +/- 6.59 (SE), whereas responses in SHR (n = 15) were sevenfold higher (108 +/- 24.1; P < 0.002) and showed greater variation. In vitro, intracellular calcium responses of superfused carotid bodies estimated from the fluorimetric ratio (340/380 nm) showed a greater peak increase during hypoxia in carotid bodies from SHR (140 +/- 4.7%) than from F-344 rats (114 6.0%; P < 0.01). Our results indicate strain-related differences in hypoxic chemosensitivity that are intrinsic to the carotid body and that could mediate genetic effects on ventilatory responsiveness to hypoxia.

Electrophysiological and immunocytological demonstration of cell-type specific responses to hypoxia in the adult cat carotid body

We have recently shown two types of cat carotid body cells based on the oxygen sensitivity of voltage-gated potassium channels. In the present study, we attempted to determine the correlation between cell types (glomus cells, sheath cells, and subtypes of glomus cells) and oxygen sensitivity of potassium channels. Further, changes in membrane potentials in response to hypoxia were also examined. Carotid body cells harvested from adult cats were cultured, and a whole cell patch clamp method was applied to determine the oxygen sensitivity of outward current. The tested cells were identified by Lucifer Yellow in the patch pipette. Glomus cells and sheath cells were immunocytochemically identified using tyrosine hydroxylase (TH) and glial fibrillary acidic protein (GFAP) as markers. The cells whose outward current was inhibited by hypoxia showed TH-immunoreactivity but not GFAP-immunoreactivity. The cells whose outward current was not sensitive to hypoxia were GFAP-positive or TH-negative. One TH-positive cell had oxygen-insensitive outward current. The resting membrane potentials of the cells having oxygen-sensitive outward current were significantly higher (−55±3 mV) than those of the cells having oxygen-insensitive outward current (−35±2 mV). The former type of cells was depolarized during hypoxia, but not the latter type of cells. These results suggest that most glomus cells of the adult cat carotid body possess oxygen-sensitive potassium channels and are depolarized in response to hypoxia. On the other hand, sheath cells and possibly a small fraction of glomus cells possess oxygen-insensitive potassium channels and their membrane potential is not affected by hypoxia.

Changes in Tyrosine Hydroxylase and Substance P Immunoreactivity in the Cat Carotid Body Following Chronic Hypoxia and Denervation

Long-term hypoxia elicits functional changes in the cat carotid body which are manifest as altered chemosensitivity in response to hypoxia. Previous studies have suggested that these functional adjustments may be mediated by changes in neurotransmitter levels in chemosensory type I cells. Neurotransmitter metabolism in the carotid body has also been shown to be regulated by the neural innervation to the organ. The present study using the cat carotid body demonstrates profound changes in the levels of immunoreactivity of the catecholamine-synthesizing enzyme, tyrosine hydroxylase, and the neuropeptide, substance P, in response to a two-week exposure to hypoxia (10% O 2 in 90% N 2). Furthermore, these changes were modulated both by sensory and sympathetic denervation of the organ. For TH, the intensity of immunostaining in type I cells was markedly increased by long-term hypoxia in both normal and chronic carotid sinus nerve-denervated carotid bodies, but this effect was blocked following chronic sympathectomy. Substance P immunoreactivity in type I cells was dramatically attenuated by hypoxia in both intact and chronic carotid sinus nerve-denervated preparations, but this effect was reduced following chronic sympathectomy. Tyrosine hydroxylase- and substance P-positive axon terminals were observed to innervate type I cells. These axons were also present in chronically sympathectomized preparations, but they disappeared following chronic carotid sinus nerve-denervation suggesting that they very likely arise from sensory neurons in the petrosat ganglion. Our data indicate that chronic chemoreceptor stimulation by hypoxia elicits multiple neurochemical adjustments in the cat carotid body. These changes suggest that catecholaminergic enzymes and neuropeptides play a significant role in the adaptive mechanisms of chemoreceptor function which occur in response to chronic physiological stimulation. Furthermore, the data suggest that neurotrophic mechanisms may influence neurotransmitter metabolism in chemosensory type I cells.