The paraganglia within the carotid bifurcation regions of young and old spontaneously hypertensive rats (SHR) after exposure to chronic hypobaric hypoxia. I. The carotid bodies

Carotid body volumes and the histological appearance of these chemoreceptors were studied using light microscopic methods in 10 groups of spontaneously hypertensive rats (SHR). The aim of this study was to clarify the influence of chronic hypobaric hypoxia on the carotid bodies of SHR depending on the age of the rats, on the duration of exposure to hypoxia, and on different salt intake, respectively different blood pressure. We found that: 1. The carotid bodies of chronically hypoxic SHR are enlarged. 2. The degree of carotid body enlargement is dependent on the duration of exposure to hypoxia. 3. In old SHR the increase of carotid body volume was smaller than in young SHR. 4. Old chronically hypoxic SHR exhibited more distinct vascular changes in the carotid bodies than age-matched normoxic controls as well as younger chronically hypoxic and normoxic SHR. 5. The influence of different levels of systemic arterial blood pressure on the carotid body volumes was rather small compared with the effects of chronic hypobaric hypoxia.     

The carotid body—a quantitative assessment in children

The volume of the glomic tissue in the carotid bodies of 100 infants between the ages of 29 weeks gestation and 9 1/2 years has been determined by serial sections and point counting. There is a 10-fold variation in the size of the carotid body and the volume of glomic tissue at all ages studied. Forty of the infants died as unexpected deaths at home. No increase in size of the carotid body is apparent in the majority of these deaths but there is a probability that enlargement occurs in some children dying at the age of one year or older. Ranges of values and means are presented for the carotid body glomic tissue volume from birth to 9 1/2 years.     

Mast cells in the human carotid body.

Mast cell counts were carried out on sections of human carotid bodies from 39 subjects showing one of four stages of histological change associated with aging, and in five subjects showing different forms of histopathology in the carotid body associated with disease. There was no relation between mast cell density and age or the histological changes associated with aging of glomic tissue. The normal range of mast cell density calculated in terms of the 80% confidence limits was 18.5 to 67.5/mm2. In three middle aged subjects with carotid bodies of normal histological appearance there was an abnormally high density of 83 to 96/mm2. In two elderly subjects showing age changes of fibrosis and accumulation of lymphocytes there was an abnormally low density of 12/mm2 or less. Mast cell density was not related to different types of carotid body hyperplasia. The mast cells were essentially stromal in location, usually closely applied to the walls of small glomic blood vessels, and were rarely found in intimate association with glomic chief cells. This suggests that mast cells are not directly concerned with the functions of glomic cells but does not preclude the possibility that they may have some effect on regulating glomic blood vessels and thus participate in the distribution of blood supply within the carotid body.     

低氧性肺动脉高压大鼠红细胞的变形性,膜流动性及外形变化

为探讨红细胞变形性变化在低氧性肺动脉高压形成中的作用及低氧时红细胞变形性变化机制，本文观察了低氧1、3、5周大鼠红细胞的变形性、膜流动性、红细胞形态、红细胞平均体积（MCV）及红细胞平均血红蛋白浓度（MCHC）变化。结果：（1）低氧1周红细胞变形性即明显下降（即红细胞滤过指数FI增高），低氧时间越长红细胞变形性下降越明显；且与肺动脉压、右心室压升高及右心室肥厚呈负相关。（2）低氧3周及5周大鼠异常形态红细胞数明显增多。（3）低氧5周大鼠红细胞膜流动性明显下降。提示：低氧大鼠红细胞形态异常及红细胞膜流动性下降可使红细胞变形性下降。红细胞变形性下降可能在低氧性肺动脉高压及右室肥厚的发生及发展中起一定的作用。     

A comparison of the blood supply of the carotid body in rats and rabbits

Light-microscopic findings regarding the arterial blood supply of the carotid bodies in 145 Wistar rats, in 175 spontaneously hypertensive rats (SHR) and in 14 rabbits were compared. In rats, only one glomic artery was found in each of 637 carotid bifurcations. In 2 bifurcations 2 carotid body arteries supplied the glomus caroticum, the glomic artery being of the muscular type. At the origin of the carotid body artery almost regularly intraarterial cushions were observed in rats. They were also demonstrable at the origin of first or second order branches of the glomic artery. Concerning the origin of the carotid body artery as well as presence and form of the intraarterial cushions some differences between the 2 strains of rats studied were detected. In rabbits the glomic artery was of the elastic type. In 2 out of 28 carotid bifurcations 2 carotid body arteries were found. In all other cases the glomus caroticum was supplied by one artery only. The glomic artery did not terminate in the glomus caroticum in rats as well as in rabbits. Many similarities between the light-microscopic picture of the internal carotid artery and the carotid body artery were observed in rabbits. Therefore a baroreceptor function of the glomic artery seems to be possible.     

The carotid bodies of renal hypertensive rats

The carotid bodies of renal hypertensive rats (one kidney wrap model) were studied by light-microscopic and morphometric methods. Rats with established hypertension showed massive intraglomic vascular alterations, such as exudation of plasma, subendothelial fibrinoid deposits and fibrinoid necroses of the intima and media. Additionally a granuloma-like perivascular proliferation of fibroblasts and histiocytes was seen. The total carotid body volume was enlarged but the volume of the specific glomic tissue was reduced in comparison with normotensive controls. In rats with borderline hypertension similar pathological changes were found but in a more reduced extension. Additionally in these rats some intraglomic vessels showed an accumulation of acid mucopolysaccharides and an hyperplasia of mediocytes. Rats with such vessel alterations also exhibited a small enlargement of specific glomic tissue. In general the pathological changes of the carotid bodies in renal hypertensive rats are different in comparison with those in the glomera carotici of rats with spontaneous hypertension (SHR, GH rats). This study suggests that an elevated blood pressure does not solely cause an increment of the specific chemoreceptive tissue mass of the carotid bodies.     

Restoration of reflex ventilatory response to hypoxia after removal of carotid bodies in the cat

In mammals there are two sets of peripheral arterial chemoreceptors, the carotid bodies innervated by the sinus branch of the glossopharyngeal nerve and the aortic bodies innervated by the vagus nerves. The afferent impulse discharge from both receptors increases during hypoxia and there is a reflexly mediated increase in ventilation (hypoxic hyperventilation). In the present study we tested this response by exposing anesthetized cats to decreased inspired O₂ concentration before and up to 315 days after bilateral resection of the carotid bodies. Acutely after removing the carotid bodies, hypoxic hyperventilation was abolished. This observation supports the view that the reflex pathway from the aortic body receptors normally contributes minimally to hypoxic hyperventilation. Subsequently, there was a restoration of hypoxic hyperventilation. Restoration was significant 30–43 days after removing the carotid bodies, it reached 70% of the preoperative value at 93–111 days and was essentially complete in terminal experiments 260–315 days after carotid body resection. In terminal experiments, hypoxic hyperventilation was not affected by recutting the regenerated carotid sinus nerves but was abolished completely by bilateral transection of the cervical vagosympathetic trunks. The restored ventilatory response was due predominantly to an increase in rate of breathing while an increase in tidal volume was predominant before carotid body resection. Resting ventilation breathing room air was not consistently decreased after carotid body resection while expired CO₂ was elevated from day 20 to day 111 and at the preoperative level in terminal experiments.It is concluded that restoration of hypoxic hyperventilation in the cat after carotid body resection is mediated by the reflex pathway from aortic body chemoreceptors. The possible contribution of chemo-receptive regenerated carotid sinus nerve axons was excluded. It is suggested that restoration may be a consequence of the central reorganization of chemoreceptor afferent pathways consequent to interruption of the carotid body reflex pathway and that as a result the ‘gain’ of the aortic body ventilatory chemoreflex is enhanced.     

Effects on the rabbit carotid body of stimulation by almitrine, natural high altitude, and experimental normobaric hypoxia

Rabbits were given intraperitoneal injections of almitrine in ascending doses for 5 weeks. They were compared with a control group and with a group of rabbits which had been exposed from birth to the natural hypobaric hypoxia found at Cerro de Pasco (433 m) in the Peruvian Andes. A further group of animals was placed in an experimental normobaric chamber for either 3 or 6 months to subject them to the same degree of hypoxia as that occurring in Cerro. The carotid bodies of the rabbits in all these groups were processed for light and electron microscopy, and examined both qualitatively and quantitatively. The carotid bodies in the group given almitrine showed no changes in their size or in the population of their glomic cells when compared with controls. In contrast, the carotid bodies of Peruvian rabbits were greatly enlarged with a disproportionate increase in the population of the light variant of chief cell. Rabbits from the hypoxic chamber also had enlarged carotid bodies but those killed after 3 months showed an increase in the dark variant of chief cell, whereas after 6 months this cell was reduced in number. There was also intense cytoplasmic vacuolation. Election microscopy confirmed these changes and revealed that dark cells had larger, more pleomorphic granules than the light variant. Vacuolation of the granules in light cells was most pronounced in Peruvian rabbits, but was uncommon in animals exposed to hypoxia for 3 months. We suggest that the dark cell responds to the early stages of hypoxia but later matures into the light variant of chief cell.     

The earliest histopathological response to hypobaric hypoxia in rabbits in the rifugio torino (3370 M) on Monte Bianco

Twelve Dutch rabbits were kept on Monte Bianco at an altitude of 3370 m. Half of the animals were killed after 3 months, the remainder after 6 months, and a further six animals maintained at sea-level acted as controls. The carotid bodies of all the rabbits were processed for light and electron microscopy and examined qualitatively and quantitatively. The lungs were processed for light microscopical assessment of small pulmonary arterial vessels; the thickness of the pulmonary trunks and aortas were measured; and the hearts were dissected to obtain ratios of the ventricular weight. There was a slight increase in the right ventricular weight in the hypoxic rabbits but no change in the thickness of the pulmonary trunk compared with that of the aorta. In particular, there was no hypoxic remodelling of the pulmonary vasculature such as muscularization of pulmonary arterioles or intimal longitudinal muscie in pulmonary arteries. The earliest histopathological response to hypoxia occurred in the carotid bodies in the form of an increase in the count of the dark variant of chief cell after 3 months which returned to normal after 6 months. It is concluded that the carotid body of the rabbit responds with a change in its population of dark chief cells to a level of hypoxia which is insufficient to affect the pulmonary arterioles. Changes in the cardiopulmonary system can no longer be considered to be the earliest histopathological response to hypobaric hypoxia.     

Comparative pathology of the enlarged carotid body

A histological study was made of the carotid bodies of man and various animal species from low and high altitudes. The animals studied were the alpaca, llama, cattle, guinea-pig, rabbit, dog, rat and man. As well as a qualitative microscopic study, a differential cell count was carried out to determine the percentage of the light and dark variants of chief cells and of sustentacular cells present. The investigation showed that the carotid bodies enlarge in cattle, guinea-pigs and rabbits living in the hypobaric hypoxia of high altitude to which they have to acclimatize. The carotid bodies are not enlarged in llamas and alpacas which show Darwinian adaptation to high altitude. There is no single histopathological appearance to be found with enlargement of the carotid body; on the contrary, there appears to be a characteristic histological reaction for different species. Thus, man shows hyperplasia of sustentacular cells, cattle show focal dark cell proliferation and rabbits, guinea-pigs, and dogs show striking hyperplasia and vacuolation of chief cells. In the rat, the enlargement of the carotid body is not characterized by the differential proliferation of any specific element and, as a result, it does not appear to be a good model for the human organ. In man and rat, carotid body enlargement occurs in response to systemic hypertension as well as to chronic hypoxaemia and the histological response to the 2 stimuli is the same, depending on the species. The normal rabbit carotid body is more reminiscent of that of man but, in this species, the reaction of glomic tissue differs from that of human glomic tissue.     

The ultrastructure of the carotid body in chronically hypoxic rabbits

1. The ultrastructure of the carotid body in the rabbit has been examined by electron microscopy.2. A comparison was made between the ultrastructure of the carotid bodies in sea level rabbits, in rabbits which had been exposed to hypoxia equivalent to an altitude of 6000 m for 7 days and in rabbits which always had lived at an altitude of 4000-4300 m.3. We could not detect any difference in the ultrastructure between the two groups of hypoxic rabbits.4. When the hypoxic rabbits were compared with sea level rabbits there was a marked increase in the number of dense cored vesicles and mitochondria in the type I cells in the hypoxic rabbits. The Golgi region also appeared to be enlarged in the type I cells in the hypoxic rabbits.5. The finding suggests that in the rabbit the production of amines, probably dopamine, within the type I cells is increased during prolonged hypoxia which might explain the lowered ventilatory response to hypoxia observed in human high altitude residents.6. If the carotid bodies are organs of internal secretion the finding is compatible with an increased production of a hormone produced within the type I cells.     

Augmentation of hypoxia-induced nitric oxide generation in the rat carotid body adapted to chronic hypoxia: an involvement of constitutive and inducible nitric oxide synthases

Acute hypoxia increases the endogenous release of nitric oxide (NO) in rat carotid body and the expression of nitric oxide synthases is modulated by chronic hypoxia. The aim of the study was to examine hypoxia-induced NO generation in rat carotid body adapted to chronic hypoxia with inspired oxygen at 10% for 4 weeks. The concentration of NO was measured electrochemically with a Pt/Nafion/Pd-IrOx/POAP modified electrode inserted into the isolated carotid body superfused with bicarbonate-buffer saline at 35 degrees C. Acute hypoxia increased the concentration of NO by 471.3+/-71.4 nM in the carotid body of chronically hypoxic (CH) rats. The amount of NO release induced by hypoxia was significantly augmented when compared with that of the normoxic control (87.6+/-15.9 nM). The hypoxia-induced NO generation was markedly attenuated by pretreatment with L- NG-nitroarginine methylester (L-NAME; 500 microM), a non-selective nitric oxide synthase (NOS) inhibitor and also by removal of extracellular calcium with the calcium chelator EGTA (5 mM). Additionally, NO generation during hypoxia was reduced by 30% in the CH carotid body treated with S-methylisothiourea (SMT; 50 microM), a specific blocker of inducible NOS (iNOS). Immunohistochemical study revealed that positive iNOS protein immunoreactivity was detected in clusters of glomus cells in the carotid bodies of CH rats, but not in the normoxic group. Thus, chronic hypoxia enhances hypoxia-induced NO generation mediated by calcium-dependent NOSs and iNOS in the carotid body. Extracellular recording of sinus nerve activity of CH carotid bodies showed that L-NAME treatment enhanced the afferent discharge in response to hypoxia, confirming that the generation of NO suppresses the activities of carotid chemoreceptors. Taken together, our results suggest that hypoxia-induced NO production increases in the rat carotid body adapted to chronic hypoxia and that constitutive and inducible NOSs are involved in the NO generation. The enhancement of NO generation may play a physiological role in blunting the hypoxic chemosensitivity during chronic hypoxia.     

Carotid sinus nerve responses and ventilatory acclimatization to hypoxia in adult rats following 2 weeks of postnatal hyperoxia

Adult rats have decreased carotid body volume and reduced carotid sinus nerve, phrenic nerve, and ventilatory responses to acute hypoxic stimulation after exposure to postnatal hyperoxia (60% O2, PNH) during the first 4 weeks of life. Moreover, sustained hypoxic exposure (12%, 7 days) partially reverses functional impairment of the acute hypoxic phrenic nerve response in these rats. Similarly, 2 weeks of PNH results in the same phenomena as above except that ventilatory responses to acute hypoxia have not been measured in awake rats. Thus, we hypothesized that 2-week PNH-treated rats would also exhibit blunted chemoafferent responses to acute hypoxia, but would exhibit ventilatory acclimatization to sustained hypoxia. Rats were born into, and exposed to PNH for 2 weeks, followed by chronic room-air exposure. At 3-4 months of age, two studies were performed to assess: (1) carotid sinus nerve responses to asphyxia and sodium cyanide in anesthetized rats and (2) ventilatory and blood gas responses in awake rats before (d0), during (d1 and d7), and 1 day following (d8) sustained hypoxia. Carotid sinus nerve responses to i.v. NaCN and asphyxia (10 s) were significantly reduced in PNH-treated versus control rats; however, neither the acute hypoxic ventilatory response nor the time course or magnitude of ventilatory acclimatization differed between PNH and control rats despite similar levels of PaO2 . Although carotid body volume was reduced in PNH rats, carotid body volumes increased during sustained hypoxia in both PNH and control rats. We conclude that normal acute and chronic ventilatory responses are related to retained (though impaired) carotid body chemoafferent function combined with central neural mechanisms which may include brainstem hypoxia-sensitive neurons and/or brainstem integrative plasticity relating both central and peripheral inputs.     

The effect of chronic hypoxia on the number and nuclear diameter of type I cells in the carotid bodies of rats.

We measured the number and nuclear diameter of type I cells in the carotid bodies of 10 normal rats and in 10 rats living in a hypobaric chamber at a pressure of 460 mm Hg for 25 to 96 days. In normal rats, the number of type I rats, the number ranged from 15.92 to 30.77 times 10-3 with a mean of 40.79 times 10-3 which differed significantly from that in the control group. In 5 hypoxic rats the number of type I cells was less than the highest figure in the control group. The mean diameter of the nuclei of type I cells of hypoxic rats (5.5 mu) was greater than that of the controls (5.0 mu). The largest type I nuclei were seen in those rats which had been subjected to hypoxia for the shortest time.     

Arteriovenous anastomoses at the carotid bodies of rats

In 2 out of 390 carotid bodies of rats arteriovenous anastomoses (AVA) have been found. Both cases belonged to a group of 12 glomera carotici which were prepared using the semithin section technique. The AVA were found in the course of branches of the glomic arteries leaving the main vessel before its entrance into the carotid body tissue. Thus the AVA were placed near but outside of the carotid bodies. Histologically they showed a distinct extension of the tunica media caused by epitheloidly modified smooth muscle cells. Functionally these AVA could act as shunt vessels (Sperrarterien) that would be able to control the blood flow and/or flow distribution through their carotid bodies.     

The carotid body in Sudden Infant Death Syndrome

The aim of the present study is to provide a review of cytochemical, clinical and experimental data indicating disruption of perinatal carotid body maturation as one of the possible mechanisms underlying SIDS pathogenesis. SIDS victims have been reported to show alterations in respiratory regulation which may partly be ascribed to peripheral arterial chemoreceptors. Carotid body findings in SIDS victims, although not entirely confirmed by other authors, have included reductions in glomic tissue volume and cytoplamic granules of type I cells, changes in cytological composition (higher percentages of progenitor and type II cells) and increases in dopamine and noradrenaline contents. Prematurity and environmental factors, such as exposure to tobacco smoke, substances of abuse, hyperoxia and continuous or intermittent hypoxia, increase the risk of SIDS and are known to affect carotid body functional and structural maturation adversely, supporting a role for peripheral arterial chemoreceptors in SIDS.     

The carotid bodies in a case of ventricular septal defect

A woman of 62 years with Turner's syndrome died in congestive cardiac failure secondary to a large ventricular septal defect with biventricular hypertrophy. During her last few months the lifelong left-to-right shunt underwent reversal exposing her carotid bodies to hypoxaemia. The carotid bodies were not enlarged, thus demonstrating that hyperplasia of glomic tissue is not brought about by increased myocardial mass per se. They were, however, abnormally cellular with more dark cells ( a variant of chief cells), many of which were abnormally large and showed ultrastructural features of metabolic activity. These changes may represent the earliest histological response of the carotid body to hypoxaemia and later, the dark cells may mature into the more familiar and common light variant. It seems likely that this dark cell activity precedes, probably by a long period, the sustentacular cell hyperplasia and proliferation of nerve axons which we have reported elsewhere as the chronic reaction of the carotid bodies to hypoxaemia.     

Modulation of the hypoxic sensory response of the carotid body by 5-hydroxytryptamine: role of the 5-HT2 receptor

Previous studies have shown that glomus cells of the carotid body express 5-hydroxytryptamine (5-HT). The aim of this study was to elucidate the role of 5-HT on the hypoxic sensory response (HSR) of the carotid body. Sensory activity was recorded from multi-fiber (n=16) and single-fiber (n=8) preparations of ex vivo carotid bodies harvested from anesthetized, adult rats. 5-HT (3 microM) had no significant effect on the magnitude or on the onset of the HSR. However, 5-HT consistently prolonged the time necessary for the sensory activity to return to baseline following the termination of the hypoxic challenge. Ketanserin (40 microM), a 5-HT2 receptor antagonist completely prevented 5-HT-induced prolongation of the HSR, whereas had no effect on the control HSR (onset, magnitude, and time for decay without 5-HT). Carotid bodies expressed 5-HT, but hypoxia did not facilitate 5-HT release. These observations suggest that 5-HT is not critical for the HSR of the rat carotid body, but it modulates the dynamics of the HSR via its action on 5-HT2 receptors.     

Morphological findings at the carotid bodies of humans suffering from different types of systemic hypertension or severe lung diseases

Post-mortem studies of the carotid bodies of 62 humans were carried out using light microscopic and morphometric methods. According to the clinical and autopsic data the subjects were divided into 5 groups: normotensives, essential hypertensives, renal hypertensives, chronically hypoxic persons with severe lung diseases, and people who suffered from both lung diseases and essential hypertension. Carotid body volume showed age-dependence in the normotensive group; the biggest glomera carotici were found at an age of 40-60 years, whereas younger and older people exhibited smaller carotid bodies. In the group with the essential hypertensives only old patients exhibited enlarged carotid bodies. In younger essential hypertensives but also in the renal hypertensives an increase of carotid body size was not demonstrable. The people with severe lung diseases regularly had greater carotid bodies when compared with age-matched normotensive subjects. In addition, chronically hypoxic patients had a proliferation of type II cells, perhaps with involvement of Schwann cells and fibrocytes. This increases of elongated cells was only seldom observed in the other groups. The results are discussed with respect to the alterations known so far of arterial chemoreceptor function and reflex effects in systemic arterial hypertension.     

Ibuprofen blocks time-dependent increases in hypoxic ventilation in rats

Recently, inflammatory processes have been shown to increase O(2)-sensitivity of the carotid body during chronic sustained hypoxia [Liu, X., He, L., Stensaas, L., Dinger, B., Fidone, S., 2009. Adaptation to chronic hypoxia involves immune cell invasion and increased expression of inflammatory cytokines in rat carotid body. Am. J. Physiol. Lung Cell Mol. Physiol. 296, L158-L166]. We hypothesized that blocking inflammation with ibuprofen would reduce ventilatory acclimatization to hypoxia by blocking such increases in carotid body O(2) sensitivity. We tested this in conscious rats treated with ibuprofen (4mg/kg IP daily) or saline during acclimatization to hypoxia ( [Formula: see text] for 7 days). Ibuprofen blocked the increase in hypoxic ventilation observed in chronically hypoxic rats treated with saline; ibuprofen had no effects on ventilation in normoxic control rats. Ibuprofen blocked increases in inflammatory cytokines (IL-1β, IL-6) in the brainstem with chronic hypoxia. The data supports our hypothesis and further analysis indicates that ibuprofen also blocks inflammatory processes in the central nervous system contributing to ventilatory acclimatization to hypoxia. Possible mechanisms linking inflammatory and hypoxic signaling are reviewed.