Effects of Chronic Hypoxia on Opioid Peptide and Catecholamine Levels and on the Release of Dopamine in the Rabbit Carotid Body

Abstract: Carotid body catecholamine and opioid levels were measured in rabbits exposed for 8 days to an atmosphere of 11% O₂ in N₂ (Po₂ of ～ 80 mm Hg) and during an identical period of recovery, i.e., after 8 days of returning to the control normoxic atmosphere. Carotid bodies show a decrease in dopamine content at day 2. Thereafter, the levels of this biogenic amine increase progressively to peak at day 10, that is, 2 days after returning to a normoxic atmosphere. Finally, dopamine levels start to decrease and reach prehypoxic control levels at day 16, that is, after 8 days of recovery. In contrast, levels of native opioid peptides remain unchanged during the whole duration of the experiment, except for a decrease at day 2 of the hypoxic exposure. Levels of total opioid peptides are also below control values at day 2 of hypoxia, increase above control values on returning to a normoxic atmosphere (maximal levels at days 10-12), and later decrease to reach prehypoxic levels at day 16. As a result of these changes the ratios of dopamine to opioid levels show a progressive increase from day 0 to day 10 of the experiment and then return to control prehypoxic values. Carotid bodies isolated from animals that have been exposed to hypoxia for 8 days synthesize [³H]dopamine from its natural precursor [³H]tyrosine at a rate of 175 pmol/mg of protein/h, which is about double the rate of synthesis found in the carotid bodies of control animals and those allowed to recover for 8 days. The release of [³H]-dopamine induced by mild hypoxic stimuli and by a high external K⁺ concentration is greater in the carotid bodies isolated from animals hypoxic for 8 days than in those of control animals (catecholamine deposits were labeled by prior incubation with [³H]tyrosine); in contrast, the carotid bodies from chronically hypoxic animals exhibit an attenuated release response to intense hypoxic stimuli and to dinitrophenol. Stimulus-induced release of [³H]dopamine by carotid bodies isolated from animals allowed to recover for 8 days is not different from that of control animals. Our results suggest that modifications in the proportions of neurotransmitters, as well as changes in the stimulus-secretion coupling machinery in chemoreceptor cells, contribute to the adaptative responses seen in the carotid body during high altitude acclimatization.     

Influence of substance P on carotid sinus nerve baro- and chemoreceptor activity in rabbits.

Substance P (SP) is abundant in the carotid sinus nerve (CSN) and has been implicated in baro- and chemoreceptor reflexes. We examined the effect of SP on blood pressure, heart rate, phrenic nerve activity, hindlimb perfusion pressure, and cardiac contractile strength in urethane-anesthetized rabbits with bilaterally cut cervical sympathetic, vagus, and aortic depressor nerves. Retrograde simultaneous injection of SP (0.5-2.7 micrograms/kg in 0.2-0.3 ml saline) into both carotid sinus areas via the internal carotid arteries decreased blood pressure (by 56%), heart rate (by 13%), cardiac contractility (by 25%) and phrenic nerve activity (by 77%). The effect on hindlimb perfusion pressure was variable. There was both a reflex effect and direct hindlimb vasodilation. In another group of rabbits, the carotid sinus areas were vascularly isolated and perfused with SP (0.19 micrograms/min dissolved in Locke's solution) or Locke's solution alone for 5 min. While carotid sinus perfusion pressure was maintained in the range of 80-120 mmHg, mean arterial blood pressure, heart rate, and unit activity from the CSN were recorded. SP increased the activity of 11 of 18 baroreceptor fibers and inhibited all of 20 chemoreceptor fibers. SP decreased mean arterial blood pressure and heart rate, but the changes were less than those obtained with injection of SP into nonisolated carotid sinus arteries because systemic effects of SP, which in some cases counteracted the reflex effects, were eliminated.     

Effect of a synthetic progestin on ventilatory response to hypoxia in anesthetized rats

Effects on ventilatory responses to progressive isocapnic hypoxia of a synthetic potent progestin, chlormadinone acetate (CMA), were determined in the halothane-anesthetized male rat. Ventilation during the breathing of hyperoxic gas was largely unaffected by treatment with CMA when carotid chemoreceptor afferents were kept intact. The sensitivity to hypoxia evaluated by hyperbolic regression analysis of the response curve did not differ between the control and CMA groups. The reduction of ventilation after bilateral section of the carotid sinus nerve (CSN) in hyperoxia was less severe in CMA-treated than in untreated animals. Furthermore, the CMA-treated rats showed a larger increase in ventilation during the hypoxia test and a lower PO2 break point for ventilatory depression. Inhibition of hypoxic ventilatory depression by CMA persisted even after the denervation of CSN. We conclude that exogenous progestin likely protects regulatory mechanism(s) for respiration against hypoxic depression through a stimulating action independent of carotid chemoreceptor afferents and without a change in the sensitivity of the ventilatory response to hypoxia.     

Adult carotid chemoafferent responses to hypoxia after 1, 2, and 4 wk of postnatal hyperoxia.

Exposing newborn rats to postnatal hyperoxia (60% O2) for 1-4 wk attenuates the ventilatory and phrenic nerve responses to acute hypoxia in adult rats. The goal of this research was to increase our understanding of the carotid chemoreceptor afferent neural input in this depressed response with different durations of postnatal hyperoxic exposure. Rats were exposed from a few days before birth to 1, 2, or 4 wk of 60% O2 and studied after 3-5 mo in normoxia. The rats were anesthetized with urethane. Whole carotid sinus nerve (CSN) responses to NaCN (40 microg/kg iv), 10 s of asphyxia and acute isocapnic hypoxia (arterial Po2 45 Torr) were determined. Mean CSN responses to stimuli after postnatal hyperoxia were reduced compared with controls. Responses in rats exposed to 1 wk of postnatal hyperoxia were less affected than those exposed to 2 and 4 wk of hyperoxia, which were equivalent to each other. These studies illustrate the importance of normoxia during the first 2 wk of life in development of carotid chemoreceptor afferent function.     

Role of substance P in hypercapnic excitation of carotid chemoreceptors

Experiments were performed on 17 anesthetized, paralyzed, and artificially ventilated cats to evaluate the importance of substance P-like peptide (SP) on the carotid body responses to CO2. Single or paucifiber carotid chemoreceptor activity was recorded from the peripheral end of the cut carotid sinus nerve. In eight of the cats the influence of SP on hyperoxic hypercapnic responses was studied. While the animals breathed 100% O2, intracarotid infusion of SP (1 microgram.kg-1.min-1, 3 min) increased chemoreceptor activity by +4.8 +/- 0.3 impulses/s. After SP infusion, inhalation of CO2 in O2 caused a rapid increase in activity that reached a peak and then adapted to a lower level, whereas similar levels of CO2 before SP caused only a gradual increase in carotid body discharge rate without any overshoot in response. Furthermore SP significantly increased the magnitude and slope of the CO2 response. In the other nine cats the effect of intracarotid infusion of an SP antagonist, [D-Pro2,D-Trp7,9] SP (10-15 micrograms.kg-1.min-1), on carotid body responses to 1) hyperoxic hypercapnia (7% CO2-93% O2), 2) isocapnic hypoxia (11% O2-89% N2), and 3) hypoxic hypercapnia (11% O2-7% CO2-82% N2) was examined. SP antagonist had no effect on carotid body response to hyperoxic hypercapnia but significantly attenuated the chemoreceptor excitation caused by isocapnic hypoxia and hypoxic hypercapnia. These results suggest that 1) SP may play an important role in carotid body responses to hypoxia but not to CO2, and 2) the mechanisms of stimulation of the carotid body by hypercapnia and by hypoxia differ.     

Interindividual variation in hypoxic ventilatory response: potential role of carotid body

There is considerable interindividual variation in ventilatory response to hypoxia in humans but the mechanism remains unknown. To examine the potential contribution of variable peripheral chemorecptor function to variation in hypoxic ventilatory response (HVR), we compared the peripheral chemoreceptor and ventilatory response to hypoxia in 51 anesthetized cats. We found large interindividual differences in HVR spanning a sevenfold range. In 23 cats studied on two separate days, ventilatory measurements were correlated (r = 0.54, P less than 0.01), suggesting stable interindividual differences. Measurements during wakefulness and in anesthesia in nine cats showed that although anesthesia lowered the absolute HVR it had no influence on the range or the rank of the magnitude of the response of individuals in the group. We observed a positive correlation between ventilatory and carotid sinus nerve (CSN) responses to hypoxia measured during anesthesia in 51 cats (r = 0.63, P less than 0.001). To assess the translation of peripheral chemoreceptor activity into expiratory minute ventilation (VE) we used an index relating the increase of VE to the increase of CSN activity for a given hypoxic stimulus (delta VE/delta CSN). Comparison of this index for cats with lowest (n = 5, HVR A = 7.0 +/- 0.8) and cats with highest (n = 5, HVR A = 53.2 +/- 4.9) ventilatory responses showed similar efficiency of central translation (0.72 +/- 0.06 and 0.70 +/- 0.08, respectively). These results indicate that interindividual variation in HVR is associated with comparable variation in hypoxic sensitivity of carotid bodies. Thus differences in peripheral chemoreceptor sensitivity may contribute to interindividual variability of HVR.     

Effect of chronic hypoxia on cholinergic chemotransmission in rat carotid body.

Current views suggest that oxygen sensing in the carotid body occurs in chemosensory type I cells, which excite synaptically apposed chemoafferent nerve terminals in the carotid sinus nerve (CSN). Prolonged exposure in a low-oxygen environment [i.e., chronic hypoxia (CH)] elicits an elevated stimulus-evoked discharge in chemoreceptor CSN fibers (i.e., increased chemosensitivity). In the present study, we evaluated cholinergic chemotransmission in the rat carotid body in an effort to test the hypothesis that CH enhances ACh-mediated synaptic activity between type I cells and chemoafferent nerve terminals. Animals were exposed in a hypobaric chamber (barometric pressure = 380 Torr) for 9-22 days before evaluation of chemoreceptor activity using an in vitro carotid body/CSN preparation. Nerve activity evoked by ACh was significantly larger (P < 0.01) after CH, suggesting increased expression of cholinergic receptors. Approximately 80% of the CSN impulse activity elicited by ACh (100- or 1,000-microg bolus) in both normal and CH preparations was blocked by the specific nicotinic receptor antagonist mecamylamine (100 microM). CSN activity elicited by acute hypoxia or hypercapnia in normal preparations was likewise blocked (> or =80%) in the presence of 100 muM mecamylamine, but after CH the enhanced CSN activity elicited by acute hypoxia or hypercapnia was not reduced in the presence of 100 or 500 microM mecamylamine. A muscarinic receptor antagonist, atropine (10 microM), and a specific nicotinic receptor alpha7 subunit antagonist, methyllycaconatine (50 nM), blocked approximately 50% of the hypoxia-evoked activity in normal preparations but were ineffective after CH. Prolonged exposure to hypoxia appears to dramatically alter chemotransmission in the carotid body, and may induce alternative neurotransmitter mechanisms and/or electrical coupling between type I cells and chemoafferent nerve terminals.     

Peripheral chemoreceptors in health and disease.

Peripheral chemoreceptors (carotid and aortic bodies) detect changes in arterial blood oxygen and initiate reflexes that are important for maintaining homeostasis during hypoxemia. This mini-review summarizes the importance of peripheral chemoreceptor reflexes in various physiological and pathophysiological conditions. Carotid bodies are important for eliciting hypoxic ventilatory stimulation in humans and in experimental animals. In the absence of carotid bodies, compensatory upregulation of aortic bodies as well as other chemoreceptors contributes to the hypoxic ventilatory response. Peripheral chemoreceptors are critical for ventilatory acclimatization at high altitude. They also contribute in part to the exercise-induced hyperventilation, especially with submaximal and heavy exercise. During pregnancy, hypoxic ventilatory sensitivity increases, perhaps due to the actions of estrogen and progesterone on chemoreceptors. Augmented peripheral chemoreceptors have been implicated in early stages of recurrent apneas, congestive heart failure, and certain forms of hypertension. It is likely that chemoreceptors tend to maintain oxygen homeostasis and act as a defense mechanism to prevent the progression of the morbidity associated with these diseases. Experimental models of recurrent apneas, congestive heart failure, and hypertension offer excellent opportunities to unravel the cellular mechanisms associated with altered chemoreceptor function.     

Peptidergic innervation in the rat carotid body after 2, 4, and 8 weeks of hypocapnic hypoxic exposure

The distribution and abundance of neuropeptide-containing nerve fibers were examined in the carotid bodies of rats exposed to hypocapnic hypoxia (10% O2 in N2) for 2, 4, and 8 weeks. The carotid bodies after 2, 4, and 8 weeks of hypoxic exposure were enlarged by 1.2-1.5 times in the short axis, and 1.3-1.7 times in the long axis in comparison with the normoxic control ones. The enlarged carotid bodies contained a number of expanded blood vessels. Mean density per unit area (10(4) microm2) of substance P (SP) and calcitonin gene-related peptide (CGRP) immunoreactive fibers was transiently high in the carotid bodies after 4 weeks of hypoxic exposure, and decreased significantly to nearly or under 50% after 8 weeks of hypoxic exposure. Density of vasoactive intestinal polypeptide (VIP) immunoreactive fibers increased significantly in all periods of hypoxic exposure observed, and was especially high in the carotid bodies after 4 weeks of hypoxic exposure. Density of neuropeptide Y immunoreactive fibers was unchanged in the carotid bodies during hypoxic exposure. These characteristic changes in the density of SP, CGRP, and VIP fibers in the carotid bodies after 4 weeks of hypoxic exposure suggest that the role of these neuropeptide-containing fibers may be different in the carotid bodies after each of three periods of hypoxic exposure, and that the peptidergic innervation after 8 weeks of hypoxic exposure may show an acclimatizing state.     

Acute lung injury augments hypoxic ventilatory response in the absence of systemic hypoxemia.

The objective of the present study was to examine the impact of early stages of lung injury on ventilatory control by hypoxia and hypercapnia. Lung injury was induced with intratracheal instillation of bleomycin (BM; 1 unit) in adult, male Sprague-Dawley rats. Control animals underwent sham surgery with saline instillation. Five days after the injections, lung injury was present in BM-treated animals as evidenced by increased neutrophils and protein levels in bronchoalveolar lavage fluid, as well as by changes in lung histology and computed tomography images. There was no evidence of pulmonary fibrosis, as indicated by lung collagen content. Basal core body temperature, arterial Po(2), and arterial Pco(2) were comparable between both groups of animals. Ventilatory responses to hypoxia (12% O(2)) and hypercapnia (7% CO(2)) were measured by whole body plethysmography in unanesthetized animals. Baseline respiratory rate and the hypoxic ventilatory response were significantly higher in BM-injected compared with control animals (P = 0.003), whereas hypercapnic ventilatory response was not statistically different. In anesthetized, spontaneously breathing animals, response to brief hyperoxia (Dejours' test, an index of peripheral chemoreceptor sensitivity) and neural hypoxic ventilatory response were augmented in BM-exposed relative to control animals, as measured by diaphragmatic electromyelograms. The enhanced hypoxic sensitivity persisted following bilateral vagotomy, but was abolished by bilateral carotid sinus nerve transection. These data demonstrate that afferent sensory input from the carotid body contributes to a selective enhancement of hypoxic ventilatory drive in early lung injury in the absence of pulmonary fibrosis and arterial hypoxemia.     

Role of components of the phagocytic NADPH oxidase in oxygen sensing.

It has been hypothesized that O(2) sensing in type I cells of the carotid body and erythropoietin (EPO)-producing cells of the kidney involves protein components identical to the NADPH oxidase system responsible for the respiratory burst of phagocytes. In the present study, we evaluated O(2) sensing in mice with null mutant genotypes for two components of the phagocytic oxidase. Whole body plethysmography was used to study unanesthetized, unrestrained mice. When exposed to an acute hypoxic stimulus, gp91(phox)-null mutant and wild-type mice increased their minute ventilation by similar amounts. In contrast, p47(phox)-null mutant mice demonstrated increases in minute ventilation in response to hypoxia that exceeded that of their wild-type counterparts: 98.0 +/- 18.0 vs. 20.0 +/- 13.0% (n = 11, P = 0.003). In vitro recordings of carotid sinus nerve (CSN) activity demonstrated that resting (basal) neural activity was marginally elevated in p47(phox)-null mutant mice. With hypoxic challenge, mean CSN discharge was 1.5-fold greater in p47(phox)-null mutant than in wild-type mice: 109.61 +/- 13.29 vs. 72.54 +/- 7.65 impulses/s (n = 8 and 7, respectively, P = 0.026). Consequently, the hypoxia-evoked CSN discharge (stimulus-basal) was approximately 58% larger in p47(phox)-null mutant mice. Quantities of EPO mRNA in kidney were similar in gp91(phox)- and p47(phox)-null mutant mice and their respective wild-type controls exposed to hypobaric hypoxia for 72 h. These findings confirm the previous observation that absence of the gp91(phox) component of the phagocytic NADPH oxidase does not alter the O(2)-sensing mechanism of the carotid body. However, absence of the p47(phox) component significantly potentiates ventilatory and chemoreceptor responses to hypoxia. O(2) sensing in EPO-producing cells of the kidney appears to be independent of the gp91(phox) and p47(phox) components of the phagocytic NADPH oxidase.     

Changes in the peptidergic innervation in the carotid body of rats chronically exposed to hypercapnic hypoxia: an effect of arterial CO2 tension

The abundance of neuropeptide Y (NPY)-, vasoactive intestinal polypeptide (VIP)-, substance P (SP)-, and calcitonin gene-related peptide (CGRP)-immunoreactive nerve fibers in the carotid body was examined in chronically hypercapnic hypoxic rats (10% O2 and 6-7% CO2 for 3 months), and the distribution and abundance of these four peptidergic fibers were compared with those of previously reported hypocapnic- and isocapnic hypoxic carotid bodies to evaluate the effect of arterial CO2 tension. The vasculature in the carotid body of chronically hypercapnic hypoxic rats was found to be enlarged in comparison with that of normoxic control rats, but the rate of vascular enlargement was smaller than that in the previously reported hypocapnic- and isocapnic hypoxic carotid bodies. In the chronically hypercapnic hypoxic carotid body, the density per unit area of parenchymal NPY fibers was significantly increased, and that of VIP fibers was unchanged, although the density of NPY and VIP fibers in the previously reportetd chronically hypocapnic and isocapnic hypoxic carotid bodies was opposite to that in hypercapnic hypoxia as observed in this study. The density of SP and CGRP fibers was decreased. These results along with previous reports suggest that different levels of arterial CO2 tension change the peptidergic innervation in the carotid body during chronically hypoxic exposure, and altered peptidergic innervation of the chronically hypercapnic hypoxic carotid body is one feature of hypoxic adaptation.     

Opioid Peptides in the Rabbit Carotid Body: Identification and Evidence for Co-Utilization and Interactions with Dopamine

Abstract: The rabbit carotid body is a catecholaminergic organ that contains dopamine and norepinephrine in a proportion of nearly 5:1. Chronic (15 days) carotid sinus nerve denervation or superior cervical ganglionectomy did not modify the carotid body dopamine content (5–6 nmol/mg of protein, equivalent to 250 pmol per carotid body), but sympathectomy reduced by ～ 50% the norepinephrine content. The carotid body has also a very high content of opioid activity (250 equivalent pmol of Leu-enkephalin/mg of protein) as measured by a radioreceptor assay that detects preferentially δ-opioid activity. In the carotid body the degree of opioid posttranslational processing to low-molecular-weight peptides (mostly Leu- and Met-enkephalin) is nearly 80%. HPLC identification of opioid peptides revealed that the sequences of Met- and Leu-enkephalin were in a proportion of nearly 6:1, indicating that the main opioid precursor in the carotid body is proenkephalin A. Chronic denervations of the carotid body did not modify the levels or the degree of opioid precursor processing. Acute hypoxic exposure of the animals (8% 0₂ in N₂; 3 h) resulted in a parallel decrease of dopamine and opioid activity, without any change in the degree of opioid processing. Norepinephrine levels were not affected by hypoxia. These findings suggest corelease of dopamine and opioids during natural hypoxic stimulation. In agreement with the analytical data. [d -Ala², d -Leu⁵]enkephalin, but not [d -Ala²,N-Me-Phe⁴ Gly⁵-ol]-enkephalin, reduced the in vitro release of dopamine induced by low Po₂, a high external K⁺ concentration, and dinitrophenol. Naloxone augmented the release response elicited by low Po₂ stimulation. These findings indicate that the previously described inhibitory actions of opioids are mediated, at least in part, by receptors located in chemoreceptor cells. Additional targets for opioid peptides, e.g., sensory nerve endings or blood vessels, and additional actions of opioids on chemoreceptor cells, e.g., long-term trophic actions, are not excluded.     

Comparative analysis of neonatal and adult rat carotid body responses to chronic intermittent hypoxia.

Previous studies suggest that carotid body responses to long-term changes in environmental oxygen differ between neonates and adults. In the present study we tested the hypothesis that the effects of chronic intermittent hypoxia (CIH) on the carotid body differ between neonates and adult rats. Experiments were performed on neonatal (1-10 days) and adult (6-8 wk) males exposed either to CIH (9 episodes/h; 8 h/day) or to normoxia. Sensory activity was recorded from ex vivo carotid bodies. CIH augmented the hypoxic sensory response (HSR) in both groups. The magnitude of CIH-evoked hypoxic sensitization was significantly greater in neonates than in adults. Seventy-two episodes of CIH were sufficient to evoke hypoxic sensitization in neonates, whereas as many as 720 CIH episodes were required in adults, suggesting that neonatal carotid bodies are more sensitive to CIH than adult carotid bodies. CIH-induced hypoxic sensitization was reversed in adult rats after reexposure to 10 days of normoxia, whereas the effects of neonatal CIH persisted into adult life (2 mo). Acute intermittent hypoxia (IH) evoked sensory long-term facilitation of the carotid body activity (sensory LTF, i.e., increased baseline neural activity following acute IH) in CIH-exposed adults but not in neonates. The effects of CIH were associated with hyperplasia of glomus cells in neonatal but not in adult carotid bodies. These observations demonstrate that responses to CIH differ between neonates and adults with regard to the magnitude of sensitization of HSR, susceptibility to CIH, induction of sensory LTF, reversibility of the responses, and morphological remodeling of the chemoreceptor tissue.     

Involvement of substance P in neutral endopeptidase modulation of carotid body sensory responses to hypoxia.

Previously, we showed that carotid bodies express neutral endopeptidase (NEP)-like enzyme activity and that phosphoramidon, a potent inhibitor of NEP, potentiates the chemosensory response of the carotid body to hypoxia in vivo. NEP has been shown to hydrolyze methionine enkephalin (Met-Enk) and substance P (SP) in neuronal tissues. The purpose of the present study is to determine whether NEP hydrolyzes Met-Enk and SP in the carotid body and if so whether these peptides contribute to phosphoramidon-induced potentiation of the sensory response to hypoxia. Experiments were performed on carotid bodies excised from anesthetized adult cats (n = 72 carotid bodies). The hydrolysis of Met-Enk and SP was analyzed by HPLC. The results showed that both SP and Met-Enk were hydrolyzed by the carotid body, but the rate of Met-Enk hydrolysis was approximately fourfold higher than that of SP. Phosphoramidon (400 microM) markedly inhibited SP hydrolysis ( approximately 90%) but had only a marginal effect on Met-Enk hydrolysis ( approximately 15% inhibition). Hypoxia (PO(2), 68 +/- 6 Torr) as well as exogenous administration of SP (10 and 20 nmol) increased the sensory discharge of the carotid body in vitro. Sensory responses to hypoxia and SP (10 nmol) were potentiated by approximately 80 and approximately 275%, respectively (P < 0.01), in the presence of phosphoramidon. SP-receptor antagonists Spantide (peptidyl) and CP-96345 (nonpeptidyl) either abolished or markedly attenuated the phosphoramidon-induced potentiation of the sensory response of the carotid body to hypoxia as well as to SP. These results demonstrate that SP is a preferred substrate for NEP in the carotid body and that SP is involved in the potentiation of the hypoxic response of the carotid body by phosphoramidon.     

Ventilatory effects of substance P, vasoactive intestinal peptide, and nitroprusside in humans.

Animal studies suggest that the neuropeptides, substance P and vasoactive intestinal peptide (VIP), may influence carotid body chemoreceptor activity and that substance P may take part in the carotid body response to hypoxia. The effects of these peptides on resting ventilation and on ventilatory responses to hypoxia and to hypercapnia have been investigated in six normal humans. Infusions of substance P (1 pmol.kg-1.min-1) and of VIP (6 pmol.kg-1.min-1) were compared with placebo and with nitroprusside (5 micrograms.kg-1.min-1) as a control for the hypotensive action of the peptides. Both peptides caused significantly less hypotension than nitroprusside. Substance P and nitroprusside caused significantly greater increases in ventilation and in the hypoxic ventilatory response than VIP. No changes were seen in hypercapnic sensitivity. The stimulation of ventilation and the differential effects on ventilatory chemosensitivity that accompanied hypotension are consistent either with stimulation of carotid body chemoreceptor activity or with an interaction with peripheral chemoreceptor input to the respiratory center, as is seen in animals. The similar cardiovascular but different ventilatory effects of the peptides suggest that substance P may also stimulate the carotid body in a manner independent of the effect of hypotension. This is consistent with a role of substance P in the hypoxic ventilatory response in humans.     

Poly(ADP-ribose) polymerase activity in the cat carotid body in hypoxia and hyperoxia.

Reactive oxygen species (ROS) induce DNA damage with the ensuing activation of the chromosomal repair enzyme poly(ADP-ribose) polymerase (PARP). ROS also interact with the function of carotid body chemoreceptor cells. The possibility arises that PARP is part of the carotid chemosensing process. This study seeks to determine the presence of PARP and its changes in response to contrasting chemical stimuli, hypoxia and hyperoxia, both capable of generating ROS, in cat carotid bodies. The organs were dissected from anesthetized cats exposed in vivo to acute normoxic (PaO2 approximately 90 mmHg), hypoxic (PaO2 approximately 25 mmHg), and hyperoxic (PaO2 > 400 mmHg) conditions. Carotid body homogenate was the source of PARP and [adenine 14C] NAD was the substrate in the assay. Specimens of the superior cervical ganglion and brainstem were used as reference tissues. We found that PARP activity amounted to 27 pmol/mg protein/min in the normoxic carotid body. The activity level more than doubled in both hypoxic and hyperoxic carotid bodies. Changes of PARP in the reference tissues were qualitatively similar. We conclude that PARP is present in the carotid body but the augmentation of the enzyme activity in both hypoxia and hyperoxia reflects DNA damage, induced likely by ROS and being universal for neural tissues, rather than a specific involvement of PARP in the chemosensing process.     

Substance P as a baro- and chemoreceptor afferent neurotransmitter: immunocytochemical and neurochemical evidence in the rat

The possibility that substances P (SP) is a neurotransmitter of baro- and chemoreceptor afferents in the rat was investigated. SP-like immunoreactivity (SP-I) was analyzed quantitatively by radioimmunoassay in various levels of the nucleus tractus solitarius (NTS), the site of termination of these afferents while SP-containing afferent neurons were studied in various portions of the peripheral pathways by immunocytochemistry. It was found that the NTS contained significant amounts of SP-I and that unilateral removal of the nodose ganglia reduces the SP-I content of those portions of the NTS known to receive vagal afferents. In addition, SP-I was visualized in discrete fibers in the tunica adventitia of the aortic arch and carotid sinus regions, the vagus nerve and nodose ganglia. These results in the rat are consistent with our previous studies in the cat and provide further evidence that SP is contained within baro- and chemoreceptor afferent nerves.     

Carotid chemoreceptor activity during acute and sustained hypoxia in goats

The role of carotid body chemoreceptors in ventilatory acclimatization to hypoxia, i.e., the progressive, time-dependent increase in ventilation during the first several hours or days of hypoxic exposure, is not well understood. The purpose of this investigation was to characterize the effects of acute and prolonged (up to 4 h) hypoxia on carotid body chemoreceptor discharge frequency in anesthetized goats. The goat was chosen for study because of its well-documented and rapid acclimatization to hypoxia. The response of the goat carotid body to acute progressive isocapnic hypoxia was similar to other species, i.e., a hyperbolic increase in discharge as arterial PO2 (PaO2) decreased. The response of 35 single chemoreceptor fibers to an isocapnic [arterial PCO2 (PaCO2) 38-40 Torr)] decrease in PaO2 of from 100 +/- 1.7 to 40.7 +/- 0.5 (SE) Torr was an increase in mean discharge frequency from 1.7 +/- 0.2 to 5.8 +/- 0.4 impulses. During sustained isocapnic steady-state hypoxia (PaO2 39.8 +/- 0.5 Torr, PaCO2, 38.4 +/- 0.4 Torr) chemoreceptor afferent discharge frequency remained constant for the first hour of hypoxic exposure. Thereafter, single-fiber chemoreceptor afferents exhibited a progressive, time-related increase in discharge (1.3 +/- 0.2 impulses.s-1.h-1, P less than 0.01) during sustained hypoxia of up to 4-h duration. These data suggest that increased carotid chemoreceptor activity contributes to ventilatory acclimatization to hypoxia.     

Function of the rat carotid body chemoreceptors in ageing

Some age-related deficits in the ventilatory responses have been attributed to a decline in the functionality of the carotid body (CB) arterial chemoreceptors, but a systematic study of the CB function in ageing is lacking. In rats aged 3-24 months, we have performed quantitative morphometry on specific chemoreceptor tissue, assessed the function of chemoreceptor cells by measuring the content, synthesis and release of catecholamines (a chemoreceptor cell neurotransmitter) in normoxia and hypoxia, and determined the functional activity of the intact organ by measuring chemosensory activity in the carotid sinus nerve (CSN) in normoxia, hypoxia and hypercapnic acidosis. We found that with age CBs enlarge, but at the same time there is a concomitant decrease in the percentage of chemoreceptor tissue. CB content and turnover time for their catecholamines increase with age. Hypoxic stimulation of chemoreceptor cells elicits a smaller release of catecholamines in rats after 12 months of age, but a non-specific depolarizing stimulus elicits a comparable release at all ages. In parallel, there was a marked decrease in the responsiveness to hypoxia, but not to an acidic-hypercapnic stimulus, assessed as chemosensory activity in the CSN. We conclude that in aged mammals chemoreceptor cells become hypofunctional, leading to a decreased peripheral drive of ventilation.