Effect of chronic hypoxia on purinergic synaptic transmission in rat carotid body.

Recent studies indicate that chemoafferent nerve fiber excitation in the rat carotid body is mediated by acetylcholine and ATP, acting at nicotinic cholinergic receptors and P2X2 purinoceptors, respectively. We previously demonstrated that, after a 10- to 14-day exposure to chronic hypoxia (CH), the nicotinic cholinergic receptor blocker mecamylamine no longer inhibits rat carotid sinus nerve (CSN) activity evoked by an acute hypoxic challenge. The present experiments examined the effects of CH (9-16 days at 380 Torr) on the expression of P2X2 purinoceptors in carotid body and chemoafferent neurons, as well as the effectiveness of P2X2 receptor blocking drugs on CSN activity evoked by hypoxia. In the normal carotid body, immunocytochemical studies demonstrated a dense plexus of P2X2-positive nerve fibers penetrating lobules of type I cells. In addition, type I cells were lightly stained, indicating P2X2 receptor expression. After CH, the intensity of P2X2 receptor immunostaining was maintained in chemosensory type I cells and in the soma of chemoafferent neurons. P2 receptor expression on type I cells was confirmed by demonstrations of ATP-evoked increased intracellular Ca2+; this response was modulated by simultaneous exposure to hypoxia. In normal preparations, CSN activity evoked by hypoxia in vitro was 65% inhibited in the presence of specific P2X2 receptor antagonists. However, unlike the absence of mecamylamine action after CH, P2X2 antagonists remained effective against hypoxia-evoked activity after CH. Our findings indicate that ATP acting at P2X2 receptors contributes to adjusted chemoreceptor activity after CH, indicating a possible role for purinergic mechanisms in the adaptation of the carotid body in a chronic low-O2 environment.     

Synaptic mechanisms during re-innervation of rat arterial chemoreceptors in co-culture.

The carotid body plays a key role in the control of ventilation during hypoxia, a stimulus that releases catecholamines and other neurotransmitters from chemoreceptor (type 1) cells. Using co-cultures of rat type 1 clusters and 'juxtaposed' petrosal neurons (JPN), we recently showed that hypoxic chemotransmission is mediated via co-release of ACh and ATP. Recordings from JPN at functional, regenerated 'synapses' in vitro revealed spontaneous activity consisting of random e.p.s.p.'s and/or action potentials. This activity depended on chemical transmission since it was inhibited by extracellular solutions containing low Ca2+/high Mg2+, or blockers of nicotinic (e.g. 1-2 microM mecamylamine) and/or P2 purinergic (suramin or reactive blue 2; 10-50 microM) receptors. These solutions also inhibited hypoxia-evoked responses in JPN. The newly formed 'synapses' appeared stable, allowing repeated demonstration of hypoxic chemotransmission in the same JPN after at least a approximately 24-h re-incubation period. Immunofluorescence studies in situ revealed positive staining of P2X2 and P2X3 purinoceptor subunits in chemoafferent nerve terminals, but not type 1 cells; in contrast, both elements were immunopositive for the synaptic vesicle antigen (SV2). These data further support a co-transmitter role for ATP and the involvement of heteromeric P2X2/P2X3 purinoceptors in carotid body function.     

Are the carotid bodies of the guinea-pig functional?

We have previously observed that the guinea-pig appears to have a relatively poor ventilatory (V (E)) response to hypoxia, compared to other mammals. Therefore, in this study, we questioned the ability of the carotid bodies (primary peripheral chemoreceptors) in the guinea-pig to detect hypoxia. The ventilatory responses to poikilocapnic hypoxia (8% O(2)), poikilooxic hypercapnia (8% CO(2)), hyperoxia (100% O(2)) and cyanide (NaCN - 200 mug/kg, i.v.) were assessed before and after carotid body denervation (CBD) in anaesthetized guinea-pigs. Although CBD attenuated the V (E) responses to hypercapnia and cyanide, it had no effect on normoxic breathing or the V (E) responses to hypoxia or hyperoxia. In a separate group of guinea-pigs, nerve activity was recorded from single or few-fibre preparations of the carotid sinus nerve (CSN). Basal chemoreceptor activity could not be detected from any of the nerve preparations. NaCN and hypercapnia consistently provoked an increase in neural activity. In contrast, hypoxia never clearly increased activity in any of the single or few-fibre preparations isolated from the CSN. In conclusion, although the carotid bodies of the guinea-pig, like those of other mammals, are able to detect hypercapnia and histotoxic hypoxia and elicit a reflex increase in V (E), they are essentially hypoxia-insensitive. The latter may explain, at least in part, the relatively poor V (E) response to hypoxia shown by the guinea-pig.     

Role of endothelin and endothelin A-type receptor in adaptation of the carotid body to chronic hypoxia

Chronic exposure in a low-PO(2) environment (i.e., chronic hypoxia, CH) elicits an elevated hypoxic ventilatory response and increased hypoxic chemosensitivity in arterial chemoreceptors in the carotid body. In the present study, we examine the hypothesis that changes in chemosensitivity are mediated by endothelin (ET), a 21-amino-acid peptide, and ET(A) receptors, both of which are normally expressed by O(2)-sensitive type I cells. Immunocytochemical staining showed incremental increases in ET and ET(A) expression in type I cells after 3, 7, and 14 days of CH (380 Torr). Peptide and receptor upregulation was confirmed in quantitative RT-PCR assays conducted after 14 days of CH. In vitro recordings of carotid sinus nerve activity after in vivo exposure to CH for 1-16 days demonstrated a time-dependent increase in chemoreceptor activity evoked by acute hypoxia. In normal carotid body, the specific ET(A) antagonist BQ-123 (5 microM) inhibited 11% of the nerve discharge elicited by hypoxia, and after 3 days of CH the drug diminished the hypoxia-evoked discharge by 20% (P < 0.01). This inhibitory effect progressed to 45% at day 9 of CH and to nearly 50% after 12, 14, and 16 days of CH. Furthermore, in the presence of BQ-123, the magnitude of the activity evoked by hypoxia did not differ in normal vs. CH preparations, indicating that the increased activity was the result of endogenous ET acting on an increasing number of ET(A). Collectively, our data suggest that ET and ET(A) autoreceptors on O(2)-sensitive type I cells play a critical role in CH-induced increased chemosensitivity in the rat carotid body.     

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.     

Regulation of chemoreceptor sensitivity in the carotid body: the role of presynaptic sensory nerves

Several neural and vascular mechanisms regulate the sensitivity of carotid body chemoreceptors to hypoxia, hypercapnia, and acidosis. Factors that control blood flow and oxygen delivery in the carotid body along with those that augment or diminish catecholamine release from glomus cells can have major effects on chemoreceptor function. In addition, the sensory nerves themselves may participate in the regulation of chemoreceptor sensitivity. A portion of the carotid body's sensory nerves are presynaptic to glomus cells. In response to stimulation, the sensory nerve terminals exhibit ultrastructural changes that resemble changes associated with increased release of transmitter from motor nerves: 1) the number of small (synaptic) vesicles decreases; and 2) coated vesicles and coated regions of cisternal membrane increase in number during stimulation. If sensory nerves of the carotid body release a neurotransmitters, sensory nerve activity could influence glomus cell secretion of catecholamines or other substances tha modify chemoreceptor sensitivity. Such an effect could be produced in the carotid body by hypoxia and other conditions that stimulate the sensory nerves or it could result from antidromic activity evoked in the sensory nerves by primary afferent depolarization of their terminals in the CNS.     

白细胞介素-1β对实验性急性缺氧诱导的大鼠窦神经放电的影响

最近的研究显示,颈动脉体（carotid body,CB）除具有缺氧等化学感受功能外,还对白细胞介素-1B（IL-1β）的刺激起反应。但是,IL-1β刺激对颈动脉体的缺氧感受功能有何影响还不清楚。本研究运用在体（in vivo）细胞外神经干电位记录的方法,利用麻醉大鼠,观察了CB局部给予IL-1β对实验性急性缺氧（experimental acute hypoxia,EAH）诱导的CB传入神经窦神经（carotid sinus nerve,CSN）放电频率的影响。结果发现,EAH可以诱导麻醉状态下大鼠的CSN放电频率增高;颈动脉体局部给予ATP（0、1mmol／L）和ACh（0,5mmol／L）在一定程度上可模拟缺氧诱导的CSN放电;局部给予ILlp（40μg／L）可诱导窦神经放电频率增加。但同时给予IL-1B和EAH,所引起的放电频率增高效应与单独给予EAH或IL-1β所诱导的放电频率的增高效应间无显著性差别,且IL-1β对ATP和ACh诱导的窦神经放电的增高效应也无显著影响。这些结果提示,IL-1β对EAH诱导的窦神经放电无调节作用。     

Hypoxic intensity: a determinant for the contribution of ATP and adenosine to the genesis of carotid body chemosensory activity

Excitatory effects of adenosine and ATP on carotid body (CB) chemoreception have been previously described. Our hypothesis is that both ATP and adenosine are the key neurotransmitters responsible for the hypoxic chemotransmission in the CB sensory synapse, their relative contribution depending on the intensity of hypoxic challenge. To test this hypothesis we measured carotid sinus nerve (CSN) activity in response to moderate and intense hypoxic stimuli (7 and 0% O(2)) in the absence and in the presence of adenosine and ATP receptor antagonists. Additionally, we quantified the release of adenosine and ATP in normoxia (21% O(2)) and in response to hypoxias of different intensities (10, 5, and 2% O(2)) to study the release pathways. We found that ZM241385, an A(2) antagonist, decreased the CSN discharges evoked by 0 and 7% O(2) by 30.8 and 72.5%, respectively. Suramin, a P(2)X antagonist, decreased the CSN discharges evoked by 0 and 7% O(2) by 64.3 and 17.1%, respectively. Simultaneous application of both antagonists strongly inhibited CSN discharges elicited by both hypoxic intensities. ATP release by CB increased in parallel to hypoxia intensity while adenosine release increased preferably in response to mild hypoxia. We have also found that the lower the O(2) levels are, the higher is the percentage of adenosine produced from extracellular catabolism of ATP. Our results demonstrate that ATP and adenosine are key neurotransmitters involved in hypoxic CB chemotransduction, with a more relevant contribution of adenosine during mild hypoxia, while vesicular ATP release constitutes the preferential origin of extracellular adenosine in high-intensity hypoxia.     

Dependency of hypoxic chemotransduction in cat carotid body on voltage-gated calcium channels.

The hypothesis that the entry of extracellular calcium ions into some compartment, quite possibly the type I cells, through voltage-gated calcium channels (VGCC) is essential for hypoxic chemotransduction in the cat carotid body was tested using an in situ perfusion technique. The neural output of the carotid body of anesthetized, paralyzed, and artificially ventilated cats in response to perfusions with Krebs-Ringer bicarbonate solution (KRB), calcium-free KRB, KRB containing calcium channel blockers, or KRB containing BAY K 8644 was recorded. Selective perfusion of the carotid body with hypoxic calcium-free KRB significantly decreased carotid chemoreceptor activity, suggesting that extracellular calcium is essential for hypoxic chemotransduction. Selective perfusion of the carotid body with hypoxic KRB containing verapamil (10-100 microM), diltiazem (10-100 microM), or nifedipine (10-100 microM) dose dependently attenuated the increase in chemoreceptor activity produced by hypoxia, suggesting that VGCC need to be activated for hypoxic chemotransduction. The carotid body response to hyperoxic KRB containing the calcium channel agonist BAY K 8644 (10 microM) was 267 +/- 87% of hyperoxic control KRB, suggesting that an enhanced influx of calcium ions through VGCC stimulates carotid chemoreceptor activity. Selective perfusion of the carotid body with severely hypoxic KRB containing BAY K 8644 did not increase chemoreceptor activity above that produced by severe hypoxia alone. This suggests that severe hypoxia increases intracellular calcium in some compartment of the carotid body to achieve stimulatory maximum response and that further increase in intracellular calcium does not produce further elevation of neural activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

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

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

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.     

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

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

Acetylcholine release from the carotid body by hypoxia: evidence for the involvement of autoinhibitory receptors.

The purpose of the present study was to investigate whether hypoxia influences acetylcholine (ACh) release from the rabbit carotid body and, if so, to determine the mechanism(s) associated with this response. ACh is expressed in the rabbit carotid body (5.6 +/- 1.3 pmol/carotid body) as evidenced by electrochemical analysis. Immunocytochemical analysis of the primary cultures of the carotid body with antibody specific to ACh further showed that ACh-like immunoreactivity is localized to many glomus cells. The effect of hypoxia on ACh release was examined in ex vivo carotid bodies harvested from anesthetized rabbits. The basal release of ACh during normoxia ( approximately 150 Torr) averaged 5.9 +/- 0.5 fmol.min-1.carotid body-1. Lowering the Po2 to 90 and 20 Torr progressively decreased ACh release by approximately 15 and approximately 68%, respectively. ACh release returned to the basal value on reoxygenation. Simultaneous monitoring of dopamine showed a sixfold increase in dopamine release during hypoxia. Hypercapnia (21% O2 + 10% CO2) as well as high K+ (100 mM) facilitated ACh release from the carotid body, suggesting that hypoxia-induced inhibition of ACh release is not due to deterioration of the carotid body. Hypoxia had no significant effect on acetylcholinesterase activity in the medium, implying that increased hydrolysis of ACh does not account for hypoxia-induced inhibition of ACh release. In the presence of either atropine (10 microM) or domperidone (10 microM), hypoxia stimulated ACh release. These results demonstrate that glomus cells of the rabbit carotid body express ACh and that hypoxia overall inhibits ACh release via activation of muscarinic and dopaminergic autoinhibitory receptors in the carotid body.     

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.     

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.     

Differential responses of expiratory muscles to chemical stimuli in awake dogs

We assessed respiratory muscle response patterns to chemoreceptor stimuli (hypercapnia, hypoxia, normocapnic hypoxia, almitrine, and almitrine + CO2) in six awake dogs. Mean electromyogram (EMG) activities were measured in the crural (CR) diaphragm, triangularis sterni (TS), and transversus abdominis (TA). Hypercapnia and normocapnic hypoxia caused mild to marked hyperpnea [2-5 times control inspiratory flow (VI)] and increased activity in CR diaphragm, TS, and TA. When hypocapnia was permitted to develop during hypoxia and almitrine-induced moderate hyperpnea, CR diaphragm activity increased, whereas TS and TA activities usually did not change or were reduced below control. Over time in hypercapnia, CR diaphragm, TS, and TA were augmented and maintained at these levels over many minutes; with hypoxic hyperventilation CR diaphragm, TS, and TA were first augmented but then CR diaphragm remained augmented while TS and, less consistently, TA were inhibited over time. Marked hyperpnea (4-5 times control) due to carotid body stimulation increased TA and TS EMG activity despite an accompanying hypocapnia. We conclude that in the intact awake dog 1) carotid body stimulation augments the activity of both inspiratory and expiratory muscles; 2) hypocapnia overrides the augmenting effect of carotid body stimulation on expiratory muscles during moderate hyperpnea, usually resulting in either no change or inhibition; 3) at higher levels of hyperpnea both chemoreceptor stimulation and stimulatory effects secondary to a high ventilatory output favor expiratory muscle activation; these effects override any inhibitory effects of a coincident hypocapnia; and 4) expiratory muscles of the rib cage/abdomen may be augmented/inhibited independently of one another.     

Internal intercostal nerve discharges in the cat: influence of chemical stimuli

We studied the influence of central and peripheral chemoreceptor stimulation on the activities of the phrenic and internal intercostal (iic) nerves in decerebrate, vagotomized, and paralyzed cats with bilateral pneumothoraces. Whole iic nerves of the rostral thorax (T2-T5) usually discharged during neural inspiration, whereas those of the caudal thorax (T7-T11) were primarily active during neural expiration. Filaments of rostral iic nerves that terminated in iic muscles generally discharged during expiration, suggesting that inspiratory activity recorded in whole iic nerves may have innervated other structures, possibly parasternal muscles. All nerves were phasically active at hyperoxic normocapnia and increased their activities systematically with hypercapnia. Isocapnic hypoxia or intra-arterial NaCN injection consistently increased phrenic and inspiratory iic nerve activities. In contrast, expiratory iic nerve discharges were either decreased (10 cats) or increased (7 cats) by hypoxia. Furthermore, expiratory responses to NaCN were highly variable and could not be predicted from the corresponding response to hypoxia. The results show that central and peripheral chemoreceptor stimulation can affect inspiratory and expiratory motoneuron activities differentially. The variable effects of hypoxia on expiratory iic nerve activity may reflect a relatively weak influence of carotid body afferents on expiratory bulbospinal neurons. However, the possibility that the magnitude of expiratory motoneuron activity is influenced by the intensity of the preceding centrally generated inspiratory discharge is also discussed.