CO2 dialysis in nucleus tractus solitarius region of rat increases ventilation in sleep and wakefulness.

To evaluate the function of widely distributed central chemoreceptors during sleep and wakefulness in the rat, we focally stimulate single chemoreceptor sites during naturally occurring sleep-wake cycles by microdialysis of artificial cerebrospinal fluid equilibrated with 25% CO2. In retrotrapezoid nucleus, this increased ventilation (tidal volume) by 24% only in wakefulness (Li A, Randall M, and Nattie E. J Appl Physiol 87: 910-919, 1999). In caudal medullary raphé, it increased ventilation (frequency) by 15-20% only in sleep (Nattie EE and Li A. J Appl Physiol 90: 1247-1257, 2001). Here, in nucleus tractus solitarius (NTS), focal acidification significantly increased ventilation by 11% in sleep and 7% in wakefulness rostrally (n = 5) and by 16% in sleep and 28% in wakefulness caudally (n = 5). The sleep-wake cycle was unaltered. Dialysis with 5% CO2 had no effect. Dialysis with 50% CO2 caudally did not further stimulate ventilation but did disrupt sleep. Central chemoreceptors in the NTS affect breathing in both sleep and wakefulness. The threshold for arousal in caudal NTS is greater than that for the stimulation of breathing.     

CO2 dialysis in the medullary raphe of the rat increases ventilation in sleep.

Central chemoreceptors are widespread within the brain stem. We hypothesize that function at different sites varies with arousal state. In unanesthetized rats, we produced focal acidification at single sites by means of microdialysis using artificial cerebrospinal fluid equilibrated with 25% CO2. Tissue acidosis, measured under anesthesia, is equivalent to that observed with 63 Torr end-tidal PCO2 and is limited to 600 microm. Focal acidification of the retrotrapezoid nucleus increased ventilation by 24% only in wakefulness via an increase in tidal volume (Li A, Randall M, and Nattie E. J Appl Physiol 87: 910-919, 1999). In this study of the medullary raphe, the effect of such focal acidification was in sleep (defined by electroencephalographic and electromyographic criteria): ventilation and frequency increased by 15-20% in non-rapid eye movement sleep, and frequency increased by 15% in rapid eye movement sleep. There was no effect in wakefulness. Chemoreception in the medullary raphe appears to be responsive in sleep. Central chemoreceptors at two different locations appear to vary in effectiveness with arousal state.     

Focal CO2 dialysis in raphe obscurus does not stimulate ventilation but enhances the response to focal CO2 dialysis in the retrotrapezoid nucleus.

Simultaneous inhibition of the retrotrapezoid nucleus (RTN) and raphe obscurus (ROb) decreased the systemic CO(2) response by 51%, an effect greater than inhibition of RTN (-24%) or ROb (0%) alone, suggesting that ROb modulates chemoreception by interaction with the RTN (19). We investigated this interaction further by simultaneous dialysis of artificial cerebrospinal fluid equilibrated with 25% CO(2) in two probes located in or adjacent to the RTN and ROb in conscious adult male rats. Ventilation was measured in a whole body plethysmograph at 30 degrees C. There were four groups (n = 5): 1) probes correctly placed in both RTN and ROb (RTN-ROb); 2) one probe correctly placed in RTN and one incorrectly placed in areas adjacent to ROb (RTN-peri-ROb); 3) one probe correctly placed in ROb and one probe incorrectly placed in areas adjacent to RTN (peri-RTN-ROb); and 4) neither probe correctly placed (peri-RTN-peri-ROb). Focal simultaneous acidification of RTN-ROb significantly increased ventilation (Ve) up to 22% compared with baseline, with significant increases in both breathing frequency and tidal volume. Focal acidification of RTN-peri-ROb increased Ve significantly by up to 15% compared with baseline. Focal acidification of ROb and peri-RTN had no significant effect. The simultaneous acidification of regions just outside the RTN and ROb actually decreased Ve by up to 11%. These results support a modulatory role for the ROb with respect to central chemoreception at the RTN.     

Ventilatory effects of impaired glial function in a brain stem chemoreceptor region in the conscious rat.

Glia are thought to regulate ion homeostasis, including extracellular pH; however, their role in modulating central CO2 chemosensitivity is unclear. Using a push-pull cannula in chronically instrumented and conscious rats, we administered a glial toxin, fluorocitrate (FC; 1 mM) into the retrotrapezoid nucleus (RTN), a putative chemosensitive site, during normocapnia and hypercapnia. FC exposure significantly increased expired minute ventilation (VE) to a value 38% above the control level during normocapnia. During hypercapnia, FC also significantly increased both breathing frequency and expired VE. During FC administration, maximal ventilation was achieved at approximately 4% CO2, compared with 8-10% CO2 during control hypercapnic trials. RTN perfusion of control solutions had little effect on any ventilatory measures (VE, tidal volume, or breathing frequency) during normocapnic or hypercapnic conditions. We conclude that unilateral impairment of glial function in the RTN of the conscious rat results in stimulation of respiratory output.     

Increased propensity for apnea via dopamine-induced carotid body inhibition in sleeping dogs.

We determined the effects of specific carotid body chemoreceptor inhibition on the propensity for apnea during sleep. We reduced the responsiveness of the carotid body chemoreceptors using intravenous dopamine infusions during non-rapid eye movement sleep in six dogs. Then we quantified the difference in end-tidal Pco(2) (Pet(CO(2))) between eupnea and the apneic threshold, the "CO(2) reserve," by gradually reducing Pet(CO(2)) transiently with pressure support ventilation at progressively increased tidal volume until apnea occurred. Dopamine infusions decreased steady-state eupneic ventilation by 15 +/- 6%, causing a mean CO(2) retention of 3.9 +/- 1.9 mmHg and a brief period of ventilatory instability. The apneic threshold Pet(CO(2)) rose 5.1 +/- 1.9 Torr; thus the CO(2) reserve was narrowed from -3.9 +/- 0.62 Torr in control to -2.7 +/- 0.78 Torr with dopamine. This decrease in the CO(2) reserve with dopamine resulted solely from the 20.5 +/- 11.3% increase in plant gain; the slope of the ventilatory response to CO(2) below eupnea was unchanged from normal. We conclude that specific carotid chemoreceptor inhibition with dopamine increases the propensity for apnea during sleep by narrowing the CO(2) reserve below eupnea. This narrowing is due solely to an increase in plant gain as the slope of the ventilatory response to CO(2) below eupnea was unchanged from normal control. These findings have implications for the role of chemoreceptor inhibition/stimulation in the genesis of apnea and breathing periodicity during sleep.     

The apneic threshold during non-REM sleep in dogs: sensitivity of carotid body vs. central chemoreceptors.

The relative importance of peripheral vs. central chemoreceptors in causing apnea/unstable breathing during sleep is unresolved. This has never been tested in an unanesthetized preparation with intact carotid bodies. We studied three unanesthetized dogs during normal sleep in a preparation in which intact carotid body chemoreceptors could be reversibly isolated from the systemic circulation and perfused. Apneic thresholds and the CO(2) reserve (end-tidal Pco(2) eupneic - end-tidal Pco(2) apneic threshold) were determined using a pressure support ventilation technique. Dogs were studied when both central and peripheral chemoreceptors sensed transient hypocapnia induced by the pressure support ventilation and again with carotid body isolation such that only the central chemoreceptors sensed the hypocapnia. We observed that the CO(2) reserve was congruent with4.5 Torr when the carotid chemoreceptors sensed the transient hypocapnia but more than doubled (>9 Torr) when only the central chemoreceptors sensed hypocapnia. Furthermore, the expiratory time prolongations observed when only central chemoreceptors were exposed to hypocapnia differed from those obtained when both the central and peripheral chemoreceptors sensed the hypocapnia in that they 1) were substantially shorter for a given reduction in end-tidal Pco(2), 2) showed no stimulus: response relationship with increasing hypocapnia, and 3) often occurred at a time (>45 s) beyond the latency expected for the central chemoreceptors. These findings agree with those previously obtained using an identical pressure support ventilation protocol in carotid body-denervated sleeping dogs (Nakayama H, Smith CA, Rodman JR, Skatrud JB, Dempsey JA. J Appl Physiol 94: 155-164, 2003). We conclude that hypocapnia sensed at the carotid body chemoreceptor is required for the initiation of apnea following a transient ventilatory overshoot in non-rapid eye movement sleep.     

Effects on breathing in awake and sleeping goats of focal acidosis in the medullary raphe.

Our aim was to determine the effects of focal acidification in the raphe obscurus (RO) and raphe pallidus (RP) on ventilation and other physiological variables in both the awake and sleep states in adult goats. Through chronically implanted microtubules, 1) a focal acidosis was created by microdialysis of mock cerebrospinal fluid (mCSF), equilibrated with various levels of CO2, and 2) medullary extracellular fluid (ECF) pH was measured by using a custom-made pH electrode. Focal acidosis in the RO or RP, by dialyzing either 25 or 80% CO2 (mCSF pH approximately 6.8 or 6.3), increased (P < 0.05) inspiratory flow by 8 and 12%, respectively, while the animals were awake during the day, but not at night while they were awake or in non-rapid eye movement sleep. While the animals were awake during the day, there were also increases in heart rate and blood pressure (P < 0.05) but no significant change in metabolic rate or arterial Pco2. Dialysis with mCSF equilibrated with 25 or 80% CO2 reduced ECF pH by the same amount (25%) or three times more (80%) than when inspired CO2 was increased to 7%. During CO2 inhalation, the reduction in ECF pH was only 50% of the reduction in arterial pH. Finally, dialysis in vivo only decreased ECF pH by 19.1% of the change during dialysis in an in vitro system. We conclude that 1) the physiological responses to focal acidosis in the RO and RP are consistent with the existence of chemoreceptors in these nuclei, and 2) local pH buffering mechanisms act to minimize changes in brain pH during systemic induced acidosis and microdialysis focal acidosis and that these mechanisms could be as or more important to pH regulation than the small changes in inspiratory flow during a focal acidosis.     

Chronic fluoxetine microdialysis into the medullary raphe nuclei of the rat, but not systemic administration, increases the ventilatory response to CO2.

In conscious rats, focal CO2 stimulation of the medullary raphe increases ventilation, whereas interference with serotonergic function here decreases the ventilatory response to systemic hypercapnia. We sought to determine whether repeated administration of a selective serotonin reuptake inhibitor in this region would increase the ventilatory response to hypercapnia in unanesthetized rats. In rats instrumented with electroencephalogram-electromyogram electrodes, 250 or 500 microM fluoxetine or artificial cerebrospinal fluid (aCSF) was microdialyzed into the medullary raphe for 30 min daily over 15 days. To compare focal and systemic treatment, two additional groups of rats received 10 mg x kg(-1) x day(-1) fluoxetine or vehicle systemically. Ventilation was measured in normocapnia and in 7% CO2 before treatment (day 0), acutely (days 1 or 3), on day 7, and on day 15. There was no change in normocapnic ventilation in any treatment group. Rats that received 250 microM fluoxetine microdialysis showed a significant 13% increase in ventilation in wakefulness during hypercapnia on day 7, due to an increase in tidal volume. In rats microdialyzed with 500 microM fluoxetine, there were 16 and 32% increases in minute ventilation during hypercapnia in wakefulness and sleep on day 7, and 20 and 28% increases on day 15, respectively, again due to increased tidal volume. There was no change in the ventilatory response to CO2 in rats microdialyzed with aCSF or in systemically treated rats. Chronic fluoxetine treatment in the medullary raphe increases the ventilatory response to hypercapnia in an unanesthetized rat model, an effect that may be due to facilitation of chemosensitive serotonergic neurons.     

Activation of upper airway muscles before onset of inspiration in normal humans

Animal studies have shown activation of upper airway muscles prior to inspiratory efforts of the diaphragm. To investigate this sequence of activation in humans, we measured the electromyogram (EMG) of the alae nasi (AN) and compared the time of onset of EMG to the onset of inspiratory airflow, during wakefulness, stage II or III sleep (3 subj), and CO2-induced hyperpnea (6 subj). During wakefulness, the interval between AN EMG and airflow was 92 +/- 34 ms (mean +/- SE). At a CO2 level of greater than or equal to 43 Torr, the AN EMG to airflow was 316 +/- 38 ms (P < 0.001). During CO2-induced hyperpnea, the AN EMG to airflow interval and AN EMG magnitude increased in direct proportion to CO2 levels and minute ventilation. During stages II and III of sleep, the interval between AN EMG and airflow increased when compared to wakefulness (P < 0.005). We conclude that a sequence of inspiratory muscle activation is present in humans and is more apparent during sleep and during CO2-induced hyperpnea than during wakefulness.     

Correlation between ventilation and brain blood flow during hypoxic sleep

Ventilation and brain blood flow (BBF) were simultaneously measured during carbon monoxide (CO) inhalation in awake and sleeping goats up to HbCO levels of 40%. Unilateral BBF, which was continuously measured with an electromagnetic flow probe placed around the internal maxillary artery, progressively increased with CO inhalation in the awake and both sleep stages. The increase in BBF with CO inhalation during rapid-eye-movement (REM) sleep (delta BBF/delta arterial O2 saturation = 1.34 +/- 0.27 ml X min-1 X %-1) was significantly greater than that manifested during wakefulness (0.87 +/- 0.14) or slow-wave sleep (0.92 +/- 0.13). Ventilation was depressed by CO inhalation during both sleep stages but was unchanged from base-line values in awake goats. In contrast to slow-wave (non-REM) sleep, the ventilatory depression of REM sleep was primarily due to a reduction in tidal volume. Since tidal volume is more closely linked to central chemoreceptor function, we believe that these data suggest a possible role of the increased cerebral perfusion during hypoxic REM sleep. Induction of relative tissue alkalosis at the vicinity of the medullary chemoreceptor may contribute to the ventilatory depression exhibited during this sleep period.     

Ventilatory effects of gap junction blockade in the RTN in awake rats

We tested the hypothesis that carbenoxolone, a pharmacological inhibitor of gap junctions, would reduce the ventilatory response to CO(2) when focally perfused within the retrotrapezoid nucleus (RTN). We tested this hypothesis by measuring minute ventilation (V(E)), tidal volume (V(T)), and respiratory frequency (F(R)) responses to increasing concentrations of inspired CO(2) (Fi(CO(2)) = 0-8%) in rats during wakefulness. We confirmed that the RTN was chemosensitive by perfusing the RTN unilaterally with either acetazolamide (AZ; 10 microM) or hypercapnic artificial cerebrospinal fluid equilibrated with 50% CO(2) (pH approximately 6.5). Focal perfusion of AZ or hypercapnic aCSF increased V(E), V(T), and F(R) during exposure to room air. Carbenoxolone (300 microM) focally perfused into the RTN decreased V(E) and V(T) in animals <11 wk of age, but V(E) and V(T) were increased in animals >12 wk of age. Glyzyrrhizic acid, a congener of carbenoxolone, did not change V(E), V(T), or F(R) when focally perfused into the RTN. Carbenoxolone binds to the mineralocorticoid receptor, but spironolactone (10 microM) did not block the disinhibition of V(E) or V(T) in older animals when combined with carbenoxolone. Thus the RTN is a CO(2) chemosensory site in all ages tested, but the function of gap junctions in the chemosensory process varies substantially among animals of different ages: gap junctions amplify the ventilatory response to CO(2) in younger animals, but appear to inhibit the ventilatory response to CO(2) in older animals.     

Intracisternal DIDS enhances ventilatory response to rebreathing CO2 in rats

In 11 anesthetized rats, we tested the hypothesis that carrier-mediated anion transport in part determines the medullary chemoreceptor response to acute hypercapnia by infusing the transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) in mock cerebrospinal fluid (CSF) into the cisterna magna. In five additional rats with sham CSF infusion, we found no effect of mock CSF on the response to rebreathing CO2. Dye infused into the cistern stained the putative chemoreceptor areas on the ventral surface of the medulla. DIDS, at 10 to 1,000 nM, increased the respiratory response to CO2 in a dose-related manner but had no effect on arterial pressure or heart rate. At 1,000 nM, the hypercapnic minute ventilation response was almost doubled because of both volume and rate of breathing. We conclude that the net effect of anion transport is to mitigate the stimulus to the medullary chemoreceptors during acute hypercapnia.     

Neurokinin-1 receptor-expressing neurons in the ventral medulla are essential for normal central and peripheral chemoreception in the conscious rat.

Neurokinin-1 receptor immunoreactive (NK1R-ir) neurons and processes are widely distributed within the medulla, prominently at central chemoreceptor sites. Focal lesions of NK1R-ir neurons in the medullary raphe or the retrotrapezoid nucleus partially reduced the CO(2) response in conscious rats. We ask if NK1R-ir cells and processes over a wide region of the ventral medulla are essential for central and peripheral chemoreception by cisterna magna injection of SSP-SAP, a high-affinity version of substance P-saporin. After 22 days, NK1R-ir cell loss was -79% in the retrotrapezoid nucleus and -65% in the A5 region, which lie close to the ventral surface, and -38% in the medullary raphe and -49% in the pre-B?tzinger complex/rostral ventral respiratory group, which lie deeper. Dorsal chemoreceptor sites, the caudal nucleus tractus solitarius and the A6 region, were unaffected. At 8 and 22 days, these lesions produced 1) hypoventilation during air breathing in wakefulness ( approximately 8%) and in non-rapid eye movement (NREM) ( approximately 9%) and rapid eye movement ( approximately 14%) sleep, as measured over a 4-h period; 2) a substantially reduced ventilatory response to 7% CO(2) by 61% in wakefulness and 46-57% in NREM sleep; and 3) a decreased ventilatory response to 12% O(2) by 40% in wakefulness and 35% in NREM sleep at 8 days, with partial recovery by 22 days. NK1R-ir neurons in the ventral medulla are essential for normal central chemoreception, provide a drive to breathe, and modulate the peripheral chemoreceptor responses. These effects are not state dependent.     

Lesions in retrotrapezoid nucleus decrease ventilatory output in anesthetized or decerebrate cats.

Kainic acid (KA) injections into the retrotrapezoid nucleus (RTN) of anesthetized deafferented cats profoundly decreased phrenic activity (PA) and CO2 sensitivity (J. Appl. Physiol. 68: 1157-1166, 1990). In this study small electrolytic lesions of the RTN produced the same results, indicating that the KA destroyed cells. We then asked whether anesthetic depression or the absence of peripheral chemoreceptors could explain the degree of respiratory depression observed. In decerebrate cats electrolytic lesions of the RTN resulted in a decrease in PA similar to that seen under anesthesia. CO2 sensitivity was decreased by RTN lesions that extended into the caudal RTN but less so than under anesthesia. KA injections resulted in an initial increase in PA followed by a continuous decrease, a pattern similar to that seen under anesthesia but with a slower time course. CO2 sensitivity was essentially absent. Peripheral chemodenervation produced a small further decrease in PA and a downward shift of the CO2 response without change in slope. Blood pressure was unaffected by RTN lesions but was decreased by more-caudal lesions without respiratory effects. The RTN appears to be necessary for the maintenance of eupneic phrenic activity and CO2 sensitivity even in decerebrate cats with intact peripheral chemoreceptors.     

Heterogeneous patterns of pH regulation in glial cells in the dorsal and ventral medulla

We examined pH regulation in two chemosensitive areas of the brain, the retrotrapezoid nucleus (RTN) and the nucleus tractus solitarius (NTS), to identify the proton transporters involved in regulation of intracellular pH (pHi) in medullary glia. Transverse brain slices from young rats [postnatal day 8 (P8) to P20] were loaded with the pH-sensitive probe 2',7'-bis (2-carboxyethyl)-5,6-carboxyfluorescein after kainic acid treatment removed neurons. Cells were alkalinized when they were depolarized (extracellular K+ increased from 6.24 to 21.24 mM) in the RTN but not in the NTS. This alkaline shift was inhibited by 0.5 mM DIDS. Removal of CO2/HCO3- or Na+ from the perfusate acidified the glial cells, but the acidification after Na+ removal was greater in the RTN than in the NTS. Treatment of the slice with 5-(N-ethyl-N-isopropyl)amiloride (100 microM) in saline containing CO2/HCO3- acidified the cells in both nuclei, but the acidification was greater in the NTS. Restoration of extracellular Cl- after Cl- depletion during the control condition acidified the cells. Immunohistochemical studies of glial fibrillary acid protein demonstrated much denser staining in the RTN compared with the NTS. We conclude that there is evidence of Na+-HCO3- cotransport and Na+/H+ exchange in glia in the RTN and NTS, but the distribution of glia and the distribution of these pH-regulatory functions are not identical in the NTS and RTN. The differential strength of glial pH regulatory function in the RTN and NTS may also alter CO2 chemosensory neuronal function at these two chemosensitive sites in the brain stem.     

State and chemical drive modulate respiratory variability.

The quantification of respiratory variability may provide insight into the integrative control of breathing. To test the hypothesis that sleep and/or increased chemical drive modifies respiratory variability, six male adult Sprague-Dawley rats were instrumented with diaphragm electromyographic (EMG) electrodes and exposed to 0, 2.5, and 5.0% CO2 with a balance of room air during wakefulness and behaviorally determined sleep. Respiratory interval (Ttot), peak diaphragm EMG, and ventilation index (peak diaphragm EMG/Ttot) were calculated for 1,024 sequential breaths. The variability of breathing was quantified with a measurement of signal complexity, the approximate entropy, and two autocorrelation measurements, the autoregressive power spectrum slope and the detrended fluctuation analysis slope. Elevated chemical drive and/or sleep significantly modulated the variability of ventilation index and Ttot. There were also significant interactions between state and CO2 drive in all respiratory parameters. We conclude that state (sleep or wakefulness) and increased chemical drive affect respiratory variability differentially.     

Effect of caffeine on peripheral chemoreceptor activity in premature neonates: interaction with sleep stages.

Caffeine is widely used for the treatment of apnea in premature neonates. However, the localization of caffeine's target site (central nervous system and/or peripheral chemoreceptors) is not well defined, especially for sleeping neonates whose sleep stages interact with respiratory control. The aim of this study was to assess the activity of the peripheral chemoreceptors in relation to sleep stages in premature neonates treated (or not) with caffeine for idiopathic apnea. Peripheral chemoreceptor activity was assessed in 22 neonates (postconceptional age of 36 +/- 1 wk with birth weights ranging from 790 to 1,910 g) by performing a 30-s hyperoxic test during active and quiet sleep. Eleven neonates received caffeine treatment (4.0 +/- 0.5 mg.kg(-1).day(-1)) and 11 served as controls. For all neonates, the decrease in minute ventilation observed during hyperoxia was greater during active than during quiet sleep. Neonates receiving caffeine showed a significantly greater decrease in ventilation during hyperoxia in both sleep stages, compared with controls (caffeine; -29.7 +/- 12.8% vs. control; -22.0 +/- 7.4%; F(1,15) = 4.6, P = 0.04). We conclude that caffeine administration increases the effectiveness of chemoreceptor activity. Because sleep stage durations were not affected by the treatment, it is likely that the decrease in apneic episodes typically observed with caffeine therapy is only related to respiratory processes and is independent of the sleep stage organization.     

Peripheral chemoreceptors in respiratory oscillations

The hypothesis that instability of cardiorespiratory control may depend on the response and sensitivity of carotid body chemoreceptors to arterial blood gases was studied in anesthetized cats under three different experimental conditions. 1) Following administration of the peripheral dopamine receptor blocker [domperidone (0.6-0.8 mg X kg-1, iv)], carotid chemoreceptor activity and its sensitivity to CO2 during hypoxia increased, leading to cardiorespiratory oscillations at low arterial PO2 in four of eight cats. Inhalation of 100% O2 promptly decreased chemoreceptor activity and eliminated the oscillations. Inhalation of CO2 stimulated the chemoreceptor activity and ventilation but did not eliminate the oscillations. Bilateral section of carotid sinus nerves abolished the cardiorespiratory oscillations. The implication is that the dopaminergic system in the carotid body keeps chemoreceptor responses to blood gas stimuli suppressed and hence cardiorespiratory oscillations damped. 2) Hypotension and circulatory delay induced by the partial occlusion of venous return led to cardiorespiratory oscillations at low but not at high arterial PO2. 3) A few cats developed cardiorespiratory oscillations without any particular experimental intervention. These oscillations were independent of arterial PO2 and chemoreceptor activity. Thus it is reasonable to conclude that the peripheral chemoreflex can play a critical role in developing cardiorespiratory oscillations in certain instances.     

Effects on breathing of focal acidosis at multiple medullary raphe sites in awake goats.

To gain insight into why there are chemoreceptors at widespread sites in the brain, mircrotubules were chronically implanted at two or three sites in the medullary raphe nuclei of adult goats (n = 7). After >2 wk, microdialysis (MD) probes were inserted into the microtubules to create focal acidosis (FA) in the awake state using mock cerebral spinal fluid (mCSF) equilibrated with 6.4% (pH = 7.3), 50% (pH = 6.5), or 80% CO(2) (pH = 6.3), where MD with 50 and 80% CO(2) reduces tissue pH by 0.1 and 0.18 pH unit, respectively. There were no changes in all measured variables with MD with 6.4% at single or multiple raphe sites (P > 0.05). During FA at single raphe sites, only 80% CO(2) elicited physiological changes as inspiratory flow was 16.9% above (P < 0.05) control. However, FA with 50 and 80% CO(2) at multiple sites increased (P < 0.05) inspiratory flow by 18.4 and 30.1%, respectively, where 80% CO(2) also increased (P < 0.05) tidal volume, heart rate, CO(2) production, and O(2) consumption. FA with 80% CO(2) at multiple raphe sites also led to hyperventilation (-2 mmHg), indicating that FA had effects on breathing independent of an increased metabolic rate. We believe these findings suggest that the large ventilatory response to a global respiratory brain acidosis reflects the cumulative effect of stimulation at widespread chemoreceptor sites rather than a large stimulation at a single site. Additionally, focal acidification of raphe chemoreceptors appears to activate an established thermogenic response needed to offset the increased heat loss associated with the CO(2) hyperpnea.     

Effect of mechanical loading on expiratory and inspiratory muscle activity during NREM sleep

We investigated the effect of acute and sustained inspiratory resistive loading (IRL) on the activity of expiratory abdominal muscles (EMGab) and the diaphragm (EMGdi) and on ventilation during wakefulness and non-rapid-eye-movement (NREM) sleep in healthy subjects. EMGdi and EMGab were measured with esophageal and transcutaneous electrodes, respectively. During wakefulness, EMGdi increased in response to acute loading (18 cmH2O.l-1.s) (+23%); this was accompanied by preservation of tidal volume (VT) and minute ventilation (VE). During NREM sleep, no augmentation was noted in EMGdi or EMGab. Inspiratory time (TI) was prolonged (+5%), but this was not sufficient to prevent a decrease in both VT and VE (-21 and -20%, respectively). During sustained loading (12 cmH2O.l-1 s) in NREM sleep, control breaths (C) were compared with the steady-state loaded breaths (SS) defined by breaths 41-50. Steady-state IRL was associated with augmentation of EMGdi (12%) and EMGab (50%). VT returned to control levels, expiratory time shortened, and breathing frequency increased. The net result was the increase in VE above control levels (+5%, P less than 0.01). No change was noted in end-tidal CO2 or O2. We concluded that 1) wakefulness is a prerequisite for immediate load compensation (in its absence, TI prolongation is the only compensatory response) and 2) during sustained IRL, the augmentation of EMGdi and EMGab can lead to complete ventilatory recovery without measurable changes in chemical stimuli.