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Effects of Perinatal Hyperoxia on Carotid Body Chemoreceptor Activity in Vitro
Abstract
The arterial carotid body (CB) chemoreceptors are highly vascularized sensory organs located in the proximity of the carotid artery bifurcation and formed by clusters of parenchymal cells. The cell clusters are penetrated by sensory fibers of the carotid sinus nerve (CSN) which form synapses with the parenchymal chemoreceptor cells. Functionally, the CBs are the origin of a regulatory loop devoted to restore O2 availability in situations of hypoxia (Richalet, 1997). To achieve this function chemoreceptor cells detect blood PO2, being activated by hypoxia. Hypoxia increases the rate of the release of neurotransmitters from the cells augmenting the action potential frequency in the CSN; this increased activity stimulates the brainstem regulators of respiration and hyperventilation and increased arterial blood PO2 ensue (Gonzalez et al., 1994).
... 3) The attenuation of the HVR can be limited by alternating the chronic hyperoxia with periodic hypercapnia or by replacing chronic by intermittent hyperoxia, suggesting that intermittently increasing carotid body activity reduces the effect (43). 4) Central (carotid sinus to phrenic nerve) integration remains unimpaired, suggesting that the depressing effect originates from the carotid bodies (450). 5) The carotid bodies show degeneration, hypoplasia, changes in membrane properties, and a reduced number of unmyelinated fibers that altogether lead to reduced O 2 sensitivity of glomus cells and the carotid body in vitro (189,220,622,838). 6) Petrosal ganglion neurons also show degenerative changes (loss of tyrosine hydroxylase expression; Ref. 220). ...
The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.
... It is not known whether developmental hyperoxia also causes long-lasting changes in the O 2 sensitivity of individual glomus cells, although the lack of hypoxic sensitivity in single-unit carotid body recordings in chronically hyperoxic kittens suggests changes in glomus cell hypoxic chemotransduction, or changes at the synapse, immediately after hyperoxia (20). Moreover, a recent study found reduced hypoxic dopamine release from carotid bodies isolated from adult rats after developmental hyperoxia (37), consistent with reduced hypoxic chemosensitivity. Thus hyperoxia may interfere with the normal postnatal maturation of carotid body O 2 sensi- Fig. 5. Phrenic responses to hypoxia (PaO 2 ϭ 40 Torr) in 3-to 5-moold and 14-to 15-mo-old rats exposed to 60% O2 for the first month of life (month 60% O2) or in untreated control rats. ...
Developmental hyperoxia (1-4 wk of 60% O2) causes long-lasting impairment of hypoxic phrenic responses in rats. We hypothesized that shorter or less severe hyperoxic exposures would produce similar changes. Hypoxic phrenic responses were measured in 3- to 5-mo-old, urethane-anesthetized rats exposed to 60% O2 for postnatal day 1 or week 1 or to 30% O2 for postnatal week 1. Whereas 1 day of 60% O2 had no lasting effects (P > 0.05 vs. control), both 1 wk of 60% O2 and 1 wk of 30% O2 decreased adult hypoxic phrenic responses (P < 0.05 vs. control), although the effects of 30% O2 were smaller. Hypoxic ventilatory responses (expressed as the ratio of minute ventilation to metabolic CO2 production) were also reduced in unanesthetized rats (5-10 mo old) exposed to 1 wk of 60% O2 during development (P < 0.05). An age-dependent increase toward normal hypoxic phrenic responses was observed in rats exposed to 1 wk of 60% O2 (P < 0.05), suggesting a degree of spontaneous recovery not observed after 1 mo of 60% O2. These data indicate that long-lasting effects of developmental hyperoxia depend on the level and duration of hyperoxic exposure.
... Thus, perinatal hypoxia seems to produce a long-lasting alteration of the ventilatory response to hypoxia in all species, including humans (Gauda & Lawson 2000), but the maturational alteration of the CB appears to be species-specific (Hanson & Kumar 1994). Similarly, perinatal hyperoxia produces profound and permanent alterations in the CSN response to hypoxia or asphyxia, which was found to be totally absent (Eden & Hanson 1986) or greatly attenuated (Ling et al. 1997a; Prieto-Lloret et al. 2003). In the experiments of Eden & Hanson (1986) the ventilatory response to hypoxia was normal and developed earlier following perinatal hyperoxia, leading the authors to conclude that the response originated outside the CB. ...
Hypoxia increases the release of neurotransmitters from chemoreceptor cells of the carotid body (CB) and the activity in the carotid sinus nerve (CSN) sensory fibers, elevating ventilatory drive. According to previous reports, perinatal hyperoxia causes CSN hypotrophy and varied diminishment of CB function and the hypoxic ventilatory response. The present study aimed to characterize the presumptive hyperoxic damage. Hyperoxic rats were born and reared for 28 days in 55%-60% O2; subsequent growth (to 3.5-4.5 months) was in a normal atmosphere. Hyperoxic and control rats (born and reared in a normal atmosphere) responded with a similar increase in ventilatory frequency to hypoxia and hypercapnia. In comparison with the controls, hyperoxic CBs showed (1) half the size, but comparable percentage area positive to tyrosine hydroxylase (chemoreceptor cells) in histological sections; (2) a twofold increase in dopamine (DA) concentration, but a 50% reduction in DA synthesis rate; (3) a 75% reduction in hypoxia-evoked DA release, but normal high [K+]0-evoked release; (4) a 75% reduction in the number of hypoxia-sensitive CSN fibers (although responding units displayed a nearly normal hypoxic response); and (5) a smaller percentage of chemoreceptor cells that increased [Ca2+]1 in hypoxia, although responses were within the normal range. We conclude that perinatal hyperoxia causes atrophy of the CB-CSN complex, resulting in a smaller number of chemoreceptor cells and fibers. Additionally, hyperoxia damages O2-sensing, but not exocytotic, machinery in most surviving chemoreceptor cells. Although hyperoxic CBs contain substantially smaller numbers of chemoreceptor cells/sensory fibers responsive to hypoxia they appear sufficient to evoke normal increases in ventilatory frequency.
Hypoxic ventilatory and phrenic responses are reduced in adult rats (3–5 months old) exposed to hyperoxia for the first month of life (hyperoxia treated). We previously reported that hypoxic phrenic responses were normal in a small sample of 14- to 15-month-old hyperoxia-treated rats, suggesting slow, spontaneous recovery. Subsequent attempts to identify the mechanism(s) underlying this spontaneous recovery of hypoxic phrenic responses led us to re-evaluate our earlier conclusion. Experiments were conducted in two groups of aged Sprague-Dawley rats (14–15 months old) which were anaesthetized, vagotomized, neuromuscularly blocked and ventilated: (1) a hyperoxia-treated group raised in 60 % O2 for the first 28 postnatal days; and (2) an age-matched control group raised in normoxia. Increases in minute phrenic activity and integrated phrenic nerve amplitude (∫Phr) during isocapnic hypoxia (arterial partial pressures of O2, 60, 50 and 40 ± 1 mmHg) were greater in aged control (n= 15) than hyperoxia-treated rats (n= 11; P 0.01). Phrenic burst frequency during hypoxia was not different between groups. To examine the central integration of carotid chemoafferent inputs, steady-state relationships between carotid sinus nerve (electrical) stimulation frequency and phrenic nerve activity were compared in aged control (n= 7) and hyperoxia-treated rats (n= 7). Minute phrenic activity, ∫Phr and burst frequency were not different between groups at any stimulation frequency between 0.5 and 20 Hz. Carotid body chemoreceptor function was examined by recording whole carotid sinus nerve responses to cessation of ventilation or injection of cyanide in aged control and hyperoxia-treated rats. Electrical activity of the carotid sinus nerve did not change in five out of five hyperoxia-treated rats in response to stimuli that evoked robust increases in carotid sinus nerve activity in five out of five control rats. Estimates of carotid body volume were lower in aged hyperoxia-treated rats (4.4 (± 0.2) × 106μm3) compared to controls (17.4 (± 1.6) × 106μm3; P <0.01). We conclude that exposure to hyperoxia for the first month of life causes life-long impairment of carotid chemoreceptor function and, consequently, blunted phrenic responses to hypoxia.
To define the role of environmental oxygen in regulating postnatal maturation of the carotid body afferent pathway, light and electron microscopic methods were used to compare chemoafferent neurone survival and carotid body development in newborn rats reared from birth in normoxia (21 % O 2 ) or chronic hyperoxia (60 % O 2 ).
Four weeks of chronic hyperoxia resulted in a significant 41 % decrease in the number of unmyelinated axons in the carotid sinus nerve, compared with age‐matched normoxic controls. In contrast, the number of myelinated axons was unaffected by hyperoxic exposure.
Chemoafferent neurones, located in the glossopharyngeal petrosal ganglion, already exhibited degenerative changes following 1 week of hyperoxia from birth, indicating that even a relatively short hyperoxic exposure was sufficient to derange normal chemoafferent development. In contrast, no such changes were observed in the vagal nodose ganglion, demonstrating that the effect of high oxygen levels was specific to sensory neurones in the carotid body afferent pathway. Moreover, petrosal ganglion neurones were sensitive to hyperoxic exposure only during the early postnatal period.
Chemoafferent degeneration in chronically hyperoxic animals was accompanied by marked hypoplasia of the carotid body. In view of previous findings from our laboratory that chemoafferent neurones require trophic support from the carotid body for survival after birth, we propose that chemoafferent degeneration following chronic hyperoxia is due specifically to the loss of target tissue in the carotid body.
Of the many possible functions of the macaque monkey primary visual cortex (striate cortex, area 17) two are now fairly well understood. First, the incoming information from the lateral geniculate bodies is rearranged so that most cells in the striate cortex respond to specifically oriented line segments, and, second, information originating from the two eyes converges upon single cells. The rearrangement and convergence do not take place immediately, however: in layer IVc, where the bulk of the afferents terminate, virtually all cells have fields with circular symmetry and are strictly monocular, driven from the left eye or from the right, but not both; at subsequent stages, in layers above and below IVc, most cells show orientation specificity, and about half are binocular. In a binocular cell the receptive fields in the two eyes are on corresponding regions in the two retinas and are identical in structure, but one eye is usually more effective than the other in influencing the cell; all shades of ocular dominance are seen. These two functions are strongly reflected in the architecture of the cortex, in that cells with common physiological properties are grouped together in vertically organized systems of columns. In an ocular dominance column all cells respond preferentially to the same eye. By four independent anatomical methods it has been shown that these columns have the from of vertically disposed alternating left-eye and right-eye slabs, which in horizontal section form alternating stripes about 400 mu m thick, with occasional bifurcations and blind endings. Cells of like orientation specificity are known from physiological recordings to be similarly grouped in much narrower vertical sheeet-like aggregations, stacked in orderly sequences so that on traversing the cortex tangentially one normally encounters a succession of small shifts in orientation, clockwise or counterclockwise; a 1 mm traverse is usually accompanied by one or several full rotations through 180 degrees, broken at times by reversals in direction of rotation and occasionally by large abrupt shifts. A full complement of columns, of either type, left-plus-right eye or a complete 180 degrees sequence, is termed a hypercolumn. Columns (and hence hypercolumns) have roughly the same width throughout the binocular part of the cortex. The two independent systems of hypercolumns are engrafted upon the well known topographic representation of the visual field. The receptive fields mapped in a vertical penetration through cortex show a scatter in position roughly equal to the average size of the fields themselves, and the area thus covered, the aggregate receptive field, increases with distance from the fovea. A parallel increase is seen in reciprocal magnification (the number of degrees of visual field corresponding to 1 mm of cortex). Over most or all of the striate cortex a movement of 1-2 mm, traversing several hypercolumns, is accompanied by a movement through the visual field about equal in size to the local aggregate receptive field. Thus any 1-2 mm block of cortex contains roughly the machinery needed to subserve an aggregate receptive field. In the cortex the fall-off in detail with which the visual field is analysed, as one moves out from the foveal area, is accompanied not by a reduction in thickness of layers, as is found in the retina, but by a reduction in the area of cortex (and hence the number of columnar units) devoted to a given amount of visual field: unlike the retina, the striate cortex is virtually uniform morphologically but varies in magnification. In most respects the above description fits the newborn monkey just as well as the adult, suggesting that area 17 is largely genetically programmed. The ocular dominance columns, however, are not fully developed at birth, since the geniculate terminals belonging to one eye occupy layer IVc throughout its length, segregating out into separate columns only after about the first 6 weeks, whether or not the animal has visual experience. If one eye is sutured closed during this early period the columns belonging to that eye become shrunken and their companions correspondingly expanded. This would seem to be at least in part the result of interference with normal maturation, though sprouting and retraction of axon terminals are not excluded.
1. Newborn rats and their mothers were exposed from birth in a normobaric environmental chamber to an inspired O2 fraction (FI,O2) of 0.13-0.15 for 5-10 weeks. 2. The respiratory response to reducing FI,O2 to 0.12 or 0.08 for 6 min was measured in the conscious chronically hypoxic rat pups on post-natal days 5 and 14 and again at 5-10 weeks of age. On days 5 and 14 the responses were compared to those of normoxic control pups also exposed acutely to an FI,O2 of 0.12 or 0.08. 3. No significant respiratory response to the acute reduction in FI,O2 was found in the chronically hypoxic pups on post-natal days 5 and 14, whereas the normoxic pups showed a "biphasic" respiratory response on day 5 and a sustained ('adult') response on day 14. A "biphasic" respiratory response to an FI,O2 of 0.08, but not to an FI,O2 of 0.12, was seen in the chronically hypoxic pups at 5-10 weeks of age. 4. At 5-10 weeks recordings from the carotid sinus nerve were made in anaesthetized, paralysed and artificially ventilated rats. The isocapnic hypoxic response curves of chronically hypoxic rats were not significantly different from those of age-matched normoxic rats. 5. Our results suggest that limiting the rise in arterial O2 pressure which usually occurs at birth alters the ventilatory response to acute hypoxia by post-natal day 5 and this alteration is still evident at 5-10 weeks of age.(ABSTRACT TRUNCATED AT 250 WORDS)
1. Rabbit carotid bodies were pre‐loaded with [ ³ H]dopamine (DA) synthesized from [ ³ H]tyrosine and then mounted in a vertical drop‐type superfusion chamber which permitted simultaneous collection of released [ ³ H]DA and recording of chemoreceptor discharge from the carotid sinus nerve.
2. The time course of the spontaneous release of [ ³ H]DA (superfusion with media equilibrated with 100% O 2 ) in the presence of monoamine oxidase inhibitors exhibited two linear components, an initial steep phase followed after 3‐4 hr by a later slower phase of release.
3. When a 5 min low O 2 stimulus was delivered during the initial steep linear component of resting [ ³ H]DA release, there was an abrupt increase in release, the magnitude of which was stimulus‐dependent.
4. The efflux of total radioactivity from the preparation declined exponentially with time; under resting conditions it was principally non‐metabolized [ ³ H]tyrosine. During stimulation, however the efflux increased, and 60‐80% of the radioactivity could be attributed to [ ³ H]DA.
5. For a given low O 2 stimulus, the ratio of [ ³ H]DA release during the stimulus period over that in the preceding control period remained approximately the same throughout a single experiment. Ratios for different low O 2 stimuli (50, 40, 30, 20, 10 and 0% O 2 in N 2 ) yielded a parabolic relationship when plotted against stimulus intensity.
6. Transection of the carotid sinus nerve or removal of the superior cervical ganglion 12‐15 days prior to the experiment did not affect the release of [ ³ H]DA at moderate stimulus intensities (superfusion with media equilibrated with 30% or 10% O 2 in N 2 ) but both procedures significantly depressed release at the highest stimulus intensity (100% N 2 ).
7. Chemoreceptor discharge and [ ³ H]DA release were simultaneously monitored in experiments using superfusion media free of monoamine oxidase inhibitors. In these experiments, the efflux of [ ³ H]dihydroxyphenyl acetic acid (DOPAC) was also measured. The increase in peak chemosensory discharge was closely correlated with the increase in total release ([ ³ H]DA + [ ³ H]DOPAC) during stimulation with a series of low O 2 stimuli.
8. Release of [ ³ H]DA was almost completely abolished during superfusion with Ca ²⁺ ‐free, high Mg ²⁺ (2·1 mM) media, and the stimulus‐related efflux of [ ³ H]DOPAC was significantly reduced. However, chemoreceptor discharge was diminished by only 55%. These data are discussed with respect to their implications for DA as a chemosensory transmitter in rabbit carotid body.
1. This study was designed to test the hypothesis that perinatal suppression of peripheral arterial chemoreceptor inputs attenuates the hypoxic ventilatory response in adult rats. Perinatal suppression of peripheral chemoreceptor activity was achieved by exposing rats to hyperoxia throughout the first month of life. 2. Late-gestation pregnant rats were housed in a 60% O2 environment, exposing the pups to hyperoxia from several days prior to birth until they were returned to normoxia on postnatal day 28. These perinatally treated rats were then reared to adulthood (3-5 months old) in normoxia. In addition to the mother rats, adult male rats were also exposed to hyperoxia, creating an adult-treated control group. Two to four months after the hyperoxic exposure, treated rats were compared with untreated male rats of similar age. 3. A whole-body, flow-through plethysmograph was used to measure hypoxic and hypercapnic ventilatory responses of the unanaesthetized adult rats. In moderate hypoxia (arterial oxygen partial pressure, Pa,O2 approximately 48 mmHg). VE (minute ventilation) and the ratio VE/VCO2 (ventilation relative to CO2 production) increased by 16.7 +/- 4.0 and 35.4 +/- 3.4%, respectively, in perinatal-treated rats (means +/- S.E.M.), but increased more in untreated control rats (51.4 +/- 2.8 and 83.1 +/- 4.3%; both P < 10(-6)). 4. In contrast to the impaired hypoxic ventilatory response, ventilatory responses to hypercapnia (5% CO2) were similar between untreated control and perinatal-treated rats. 5. Impaired hypoxic responsiveness was unique to the perinatal-treated rats since hypoxic ventilatory responses were not attenuated in adult-treated rats. 6. The results indicate that ventilatory responses to hypoxaemia are greatly attenuated in adult rats that had experienced hyperoxia during their first month of life, and suggest that normal hypoxic ventilatory control mechanisms are susceptible to developmental plasticity.
1. Hypoxic ventilatory responses are greatly attenuated in adult rats exposed to moderate hyperoxia (60% O2) during the first month of life (perinatal treated rats). The present study was designed to test the hypothesis that perinatal hyperoxia impairs central integration of carotid chemoreceptor afferent inputs, thereby diminishing the hypoxic ventilatory response. 2. Time-dependent phrenic nerve responses to electrical stimulation of the carotid sinus nerve (CSN) and steady-state relationships between CSN stimulation frequency and phrenic nerve output were compared in control and perinatal treated rats. The rats were urethane anaesthetized, vagotomized, paralysed and artificially ventilated. End-tidal CO2 was monitored and maintained at isocapnic levels; arterial blood gases were determined. 3. Two stimulation protocols were used: (1) three 2 min episodes of CSN stimulation (20 Hz, 0.2 ms duration, 3 x threshold), separated by 5 min intervals; and (2) nine 45 s episodes of CSN stimulation with stimulus frequencies ranging from 0.5 to 20 Hz (0.2 ms duration, 3 x threshold), separated by 4 min intervals. 4. The mean threshold currents to elicit phrenic responses were similar between groups. Burst frequency (f, burst min-1), peak amplitude of integrated phrenic activity (integral of Phr), and minute phrenic activity (integral of Phr x f) during and after CSN stimulation were not distinguishable between groups in either protocol at any time or at any stimulus intensity (P > 0.05). 5. Perinatal hyperoxia does not alter temporal or steady-state phrenic responses to CSN stimulation, suggesting that the central integration of carotid chemoreceptor afferent inputs is not impaired in perinatal treated rats. It is speculated that carotid chemoreceptors per se are impaired in perinatal treated rats.
Oxygen sensing is a determinant function of mammals, especially humans, to maintain their activity under acute or chronic exposure to hypoxia. True O2 sensors (chemoreceptors, erythropoietin secreting cells) are involved in regulation loops, which aim to restore O2 availability to the cells. Pseudo O2 sensors are cells activated by the lack of oxygen but not clearly involved in regulation processes. Potassium channels in the carotid bodies have been suspected to be O2 sensitive and could mediate the chemosensitivity to hypoxia. Na,K-ATPase related ion transport in alveolar pneumocytes could be sensitive to O2 availability and regulate the flux of water and sodium in the alveolar space. Signal transduction in G-protein-dependent receptor systems is modified in hypoxia, such as in cardiac beta-receptors and adenosinergic and muscarinic receptors. Recent studies have provided some evidence to the possible role of hypoxia inducible factors (HIF-1) in the regulation of protein synthesis at the transcriptional level. Similarities between O2-sensing mechanisms in erythropoiesis and in the synthesis of vascular endothelial growth factor were recently evidenced. Both genes are upregulated in hypoxia. However, the precise structure (heme-linked enzyme?) of all these O2-sensitive sites is not known, either in the erythropoietic system or in the chemoreceptor function. An adequate balance between hypoxia-induced upregulation and downregulation processes is necessary for optimal survival in a hypoxic environment.
This paper will describe recent studies concerning the existence of developmental plasticity in the hypoxic ventilatory control system and the locus of the functional impairment following perinatal sensory suppression. Suppression of peripheral arterial chemoreceptor activity was achieved by exposing rats to hyperoxia (60% O2) for the first month of life; all measurements were conducted 2-5 months after the exposure (perinatal treated rats). Hypoxic (but not hypercapnic) ventilatory responses were severely attenuated in awake perinatal treated rats, but not in rats exposed to hyperoxia as adults, indicating that the persistent effect is unique to development and is not the nonspecific result of O2 toxicity. Impairments of the hypoxic ventilatory response due to changes in pulmonary mechanics, gas exchange or central integration of carotid chemoafferent inputs were all ruled out as primary causal factors. However, a persistent impairment of carotid chemotransduction in perinatal treated rats was apparent. These studies suggest that the hypoxic ventilatory response is susceptible to developmental plasticity, and that a carotid chemoreceptor deficit is the primary cause. These findings may have important clinical implications for patients subjected to excessive O2 therapy during neonatal intensive care.
1. Carotid chemoreceptor sensitivity is minimal immediately after birth and increases with postnatal age. In the present study we have investigated the peri- and postnatal developmental time course of [Ca2+]i responses to hypoxia in clusters of type I cells isolated from near-term fetal rats and rats that were 1, 3, 7, 11, 14 and 21 days old, using the Ca2+-sensitive fluoroprobe fura-2. 2. In type I cells from all age groups a graded increase in [Ca2+]i occurred in response to lowering the PO2 from 150 mmHg to 70, 35, 14, 7, 2 and 0 mmHg. The graded [Ca2+]i response to hypoxia was hyperbolic at all ages. 3. Type I cells from rats near-term fetal to 1 day old exhibited small [Ca2+]i responses, mainly to the most severe levels of hypoxia. After day 1, an increase in the [Ca2+]i responses to submaximal hypoxia stimulation resulted in a rightward shift in the O2 response curve. Using the Delta[Ca2+]i between 35 and 2 mmHg PO2 as an index of O2 sensitivity, type I cell O2 sensitivity increased approximately 4- to 5-fold between near-term fetal to 1 day old and 11 to 14 days of age. 4. Exposure to elevated extracellular potassium (10, 20 and 40 mM K+) caused a dose-dependent [Ca2+]i rise in type I cells from all age groups. There were no age-related changes in [Ca2+]i responses to any level of K+ between near-term fetal and 21 days. 5. We conclude that the maximal type I cell [Ca2+]i response to anoxia, as well as the sensitivity to submaximal hypoxic stimulation, of rats aged from near-term fetal to 21 days depends on the level of postnatal maturity. The lack of an age-related increase in the [Ca2+]i response to elevated K+ during the timeframe of maximal development of O2 sensitivity suggests that resetting involves maturation of O2 sensing, rather than non-specific developmental changes in the [Ca2+]i rise resulting from depolarization.
The O2 sensitivity of carotid chemoreceptor type I cells is low just after birth and increases with postnatal age. Chronic hypoxia during postnatal maturation blunts ventilatory and carotid chemoreceptor neural responses to hypoxia, but the mechanism remains unknown. We tested the hypothesis that chronic hypoxia from birth impairs the postnatal increase in type I cell O2 sensitivity by comparing intracellular Ca2+ concentration ([Ca2+]i) responses to graded hypoxia in type I cell clusters from rats born and reared in room air or 12% O2. [Ca2+]i levels at 0, 1, 5, and 21% O2, as well as with 40 mM K+, were measured at 3, 11, and 18 days of age with use of fura 2 in freshly isolated cells. The [Ca2+]i response to elevated CO2/low pH was measured at 11 days. Chronic hypoxia from birth abolished the normal developmental increase in the type I cell [Ca2+]i response to hypoxia. Effects of chronic hypoxia on development of [Ca2)]i responses to elevated K+ were small, and [Ca2+]i responses to CO2 remained unaffected. Impairment of type I cell maturation was partially reversible on return to normoxic conditions. These results indicate that chronic hypoxia severely impairs the postnatal development of carotid chemoreceptor O2 sensitivity at the cellular level and leaves responses to other stimuli largely intact.
In order to better understand the post-natal increase in peripheral chemoreceptor responsiveness to hypoxia, chemoreceptors of newborn (1-2 days) and older (10-12 days, 30 days, adult) rabbits were isolated and superfused, in vitro. The free tissue catecholamine concentration was measured using carbon-fiber voltammetry and pauci-fiber nerve activity was recorded from the sinus nerve during stimulation (4 min) with graded hypoxia or increased potassium. Both the peak catecholamine and peak nerve responses to stimulation with 10% and 0% oxygen increased with age, particularly between 10 and 30 days of age. In contrast, peak nerve and peak catecholamine responses to increased potassium did not significantly change with age. For a better understanding of how responsiveness increases with age, the fast Na+ and the Ca2+ currents were measured from isolated glomus cells of newborn and older rabbits, but the magnitude of the currents when normalized to membrane area was not significantly different between ages. We conclude that: (1) rabbit chemoreceptors mature in the newborn period (10-30 days) and part of this maturation is an increase in catecholamine secretion, (2) maturation of hypoxia transduction primarily occurs in steps prior to depolarization since potassium-evoked responses were not affected, and (3) an increase in the magnitude of glomus cell fast Na+ or Ca2+ currents is not a likely mechanism for the maturational change, but changes in the oxygen sensitivity of these currents cannot be excluded.
In recent years, an appreciation of the plastic nature of the respiratory neural control system has emerged (Eden & Hanson, 1986, 1987; Eldridge & Milhorn, 1986; Okubo & Mortolo, 1988, 1990; Mitchell et al. 1990; McCrimmon et al. 1995; Poon, 1996; Ling et al. 1997b). This study indicates that respiratory plasticity in adult rats can be used to offset functional deficits caused by abnormal experiences during development, such as hyperoxia. Katz-Salomon and colleagues examined peripheral chemoreflexes in healthy infants and infants with bronchopulmonary displasia following long-term oxygen therapy (Katz-Salomon & Lagercrantz, 1994; Katz-Salomon et al. 1995, 1996). Their findings suggest that experiences in early life, as well as treatment with supplemental oxygen, can have deleterious effects, because these infants displayed significantly blunted peripheral chemoreflexes. It is not known whether these deficits in peripheral chemosensitivity have a significant impact on ventilation during normal breathing or whether they persist throughout adulthood. However, if deficits in peripheral chemosensitivity do persist throughout adulthood, our data suggest that the inherent plasticity of the respiratory neural control system might allow (at least partial) restoration of chemosensory function. Moreover, because the effects of intermittent hypoxia appear to persist for at least 1 week following treatment, this might represent a viable therapeutic strategy. On the other hand, because some intermittent hypoxia protocols are known to elicit significant pathology, such as systemic hypertension (Greenberg et al. 1999) or hippocampal apoptosis (Gozal et al. 2001), its direct use as a therapeutic tool might be limited. Regardless, an understanding of spontaneous and evoked functional recovery following early life exposure to hyperoxia will advance our understanding of plasticity in respiratory motor control in general.
Function of the carotid body intra-utero and in the postnatal period
- D F Donnelly
- DF Donnelly