Selected contribution: chemoreflex responses to CO2 before and after an 8-h exposure to hypoxia in humans.

The ventilatory sensitivity to CO2, in hyperoxia, is increased after an 8-h exposure to hypoxia. The purpose of the present study was to determine whether this increase arises through an increase in peripheral or central chemosensitivity. Ten healthy volunteers each underwent 8-h exposures to 1) isocapnic hypoxia, with end-tidal PO2 (PET(O2)) = 55 Torr and end-tidal PCO2 (PET(CO2)) = eucapnia; 2) poikilocapnic hypoxia, with PET(O2) = 55 Torr and PET(CO2) = uncontrolled; and 3) air-breathing control. The ventilatory response to CO2 was measured before and after each exposure with the use of a multifrequency binary sequence with two levels of PET(CO2): 1.5 and 10 Torr above the normal resting value. PET(O2) was held at 250 Torr. The peripheral (Gp) and the central (Gc) sensitivities were calculated by fitting the ventilatory data to a two-compartment model. There were increases in combined Gp + Gc (26%, P < 0.05), Gp (33%, P < 0.01), and Gc (23%, P = not significant) after exposure to hypoxia. There were no significant differences between isocapnic and poikilocapnic hypoxia. We conclude that sustained hypoxia induces a significant increase in chemosensitivity to CO2 within the peripheral chemoreflex.     

Physiological and Genomic Consequences of Intermittent Hypoxia: Selected Contribution: Role of spleen emptying in prolonging apneas in humans

This study addressed the interaction between short-termadaptation to apneas with face immersion and erythrocyte release from the spleen. Twenty healthy volunteers, including ten splenectomized subjects, participated. After prone rest, they performed five maximal-duration apneas with face immersion in 10°C water, with 2-minintervals. Cardiorespiratory parameters and venous blood samples werecollected. In subjects with spleens, hematocrit and hemoglobinconcentration increased by 6.4% and 3.3%, respectively, over theserial apneas and returned to baseline 10 min after the series. A delayof the physiological breaking point of apnea, by 30.5% (17 s), wasseen only in this group. These parameters did not change in thesplenectomized group. Plasma protein concentration, preapneic alveolarPCO₂, inspired lung volume, and divingbradycardia remained unchanged throughout the series in both groups.Serial apneas thus triggered the hematological changes that have been previously observed after long apneic diving shifts; they were rapidlyreversed and did not occur in splenectomized subjects. This suggeststhat splenic contraction occurs in humans as a part of the divingresponse and may prolong repeated apneas.

     

Physiological and Genomic Consequences of Intermittent Hypoxia: Selected Contribution: Phrenic long-term facilitation requires 5-HT receptor activation during but not following episodic hypoxia

Episodic hypoxia evokes a sustained augmentation of respiratorymotor output known as long-term facilitation (LTF). Phrenic LTF isprevented by pretreatment with the 5-hydroxytryptamine (5-HT) receptorantagonist ketanserin. We tested the hypothesis that 5-HT receptoractivation is necessary for the induction but not maintenance ofphrenic LTF. Peak integrated phrenic nerve activity (Phr) wasmonitored for 1 h after three 5-min episodes of isocapnic hypoxia(arterial PO₂ = 40 ± 2 Torr; 5-minhyperoxic intervals) in four groups of anesthetized, vagotomized,paralyzed, and ventilated Sprague-Dawley rats [1) control(n = 11), 2) ketanserin pretreatment (2 mg/kg iv; n = 7), and ketanserin treatment 0 and 45 minafter episodic hypoxia (n = 7 each)]. Ketanserintransiently decreased Phr, but it returned to baseline levels within10 min. One hour after episodic hypoxia, Phr was significantlyelevated from baseline in control and in the 0- and 45-min posthypoxia ketanserin groups. Conversely, ketanserin pretreatment abolished phrenic LTF. We conclude that 5-HT receptor activation is necessary toinitiate (during hypoxia) but not maintain (following hypoxia) phrenic LTF.

     

Physiological and Genomic Consequences of Intermittent Hypoxia: Selected Contribution: Altered vascular reactivity in arterioles of chronic intermittent hypoxic rats

Recurrentepisodic hypoxia (EH) is a feature of sleep apnea that may beresponsible for some chronic cardiovascular sequelae such as systemichypertension. Chronic EH (8 h/day for 35 days) causes elevation ofdiurnal resting (unstimulated) mean arterial blood pressure (MAP) inthe rat. We used in vivo video microscopy to examine arteriolarreactivity in the cremaster muscle of male Sprague-Dawley ratssubjected to 35 days of EH. Cremaster muscles of EH (n = 6) and control (n = 6) rats were exposed to varying doses of norepinephrine (NE) (10¹⁰ to 10⁵M), ACh (10⁹ to 10⁵ M), and endothelin-1(10¹² to 10⁸ M). In a separate experiment,EH (n = 5) and control (n = 6) ratswere given one dose of a nitric oxide synthase (NOS) inhibitor N^G-nitro-L-arginine methylester (L-NAME; 10⁵ M). We also examinedendothelial NOS mRNA from the kidneys of EH-stimulated and control(unstimulated) rats. Telemetry-monitored EH rats showed a 16-mmHgincrease in MAP over 35 days, whereas control rats showed no change.The response to NE and endothelin-1 were similar for EH and controlrats. ACh vasodilatation of arterioles in EH rats was significantlyattenuated compared with that of controls. The degree ofvasoconstriction in response to blockade of the nitric oxide system byL-NAME was significantly less (83% of baseline diameterwith L-NAME) for arterioles of EH rats compared with thatfor controls (61% of baseline diameter), implying lower basal restingnitric oxide release in the EH rats. Whole kidney mRNA endothelial NOSlevels were not different between groups. These data support thehypothesis that chronic elevation of blood pressure associated with EHinvolves increased peripheral resistance from decreased basal releaseor production of nitric oxide after 35 days of EH.

     

Physiological and Genomic Consequences of Intermittent Hypoxia: Selected Contribution: Variation in acute hypoxic ventilatory response is linked to mouse chromosome 9

Genetic determinants confervariation among inbred mouse strains with respect to the magnitude andpattern of breathing during acute hypoxic challenge. Specifically,inheritance patterns derived from C3H/HeJ (C3) and C57BL/6J (B6)parental strains suggest that differences in hypoxic ventilatoryresponse (HVR) are controlled by as few as two genes. The present studydemonstrates that at least one genetic determinant is located on mousechromosome 9. This genotype-phenotype association was established byphenotyping 52 B6C3F₂ (F₂) offspring for HVRcharacteristics. A genome-wide screen was performed usingmicrosatellite DNA markers (n = 176) polymorphicbetween C3 and B6 mice. By computing log-likelihood values (LODscores), linkage analysis compared marker genotypes with minuteventilation (E), tidal volume (VT), andmean inspiratory flow (VT/TI, whereTI is inspiratory time) during acute hypoxic challenge(inspired O₂ fraction = 0.10, inspired CO₂fraction = 0.03 in N₂). A putative quantitative traitlocus (QTL) positioned in the vicinity of D9Mit207 wassignificantly associated with hypoxic E (LOD = 4.5), VT (LOD = 4.0), andVT/TI (LOD = 5.1). For each of the threeHVR characteristics, the putative QTL explained more than 30% of thephenotypic variation among F₂ offspring. In conclusion,this genetic model of differential HVR characteristics demonstratesthat a locus ~33 centimorgans from the centromere on mouse chromosome9 confers a substantial proportion of the variance inE, VT, and VT/TIduring acute hypoxic challenge.

     

Cardiorespiratory and cerebrovascular responses to acute poikilocapnic hypoxia following intermittent and continuous exposure to hypoxia in humans.

We tested the hypothesis that intermittent hypoxia (IH) and/or continuous hypoxia (CH) would enhance the ventilatory response to acute hypoxia (HVR), thereby altering blood pressure (BP) and cerebral perfusion. Seven healthy volunteers were randomly selected to complete 10-12 days of IH (5-min hypoxia to 5-min normoxia repeated for 90 min) before ascending to mild CH (1,560 m) for 12 days. Seven other volunteers did not receive any IH before ascending to CH for the same 12 days. Before the IH and CH, following 12 days of CH and 12-13 days post-CH exposure, all subjects underwent a 20-min acute exposure to poikilocapnic hypoxia (inspired fraction of O(2), 0.12) in which ventilation, end-tidal gases, arterial O(2) saturation, BP, and middle cerebral artery blood flow velocity (MCAV) were measured continuously. Following the IH and CH exposures, the peak HVR was elevated and was related to the increase in BP (r = 0.66 to r = 0.88, respectively; P < 0.05) and to a reciprocal decrease in MCAV (r = 0.73 to r = 0.80 vs. preexposures; P < 0.05) during the hypoxic test. Following both IH and CH exposures, HVR, BP, and MCAV sensitivity to hypoxia were elevated compared with preexposure, with no between-group differences following the IH and/or CH conditions, or persistent effects following 12 days of sea level exposure. Our findings indicate that IH and/or mild CH can equally enhance the HVR, which, by either direct or indirect mechanisms, facilitates alterations in BP and MCAV.     

Human ventilatory response to CO2 after 8h of isocapnic or poikilocapnic hypoxia

Duringventilatory acclimatization to hypoxia (VAH), the relationship betweenventilation (E) and end-tidalPCO₂ (PET_CO2) changes.This study was designed to determine 1) whether these changes can be seenearly in VAH and 2) if these changesare present, whether the responses differ between isocapnic andpoikilocapnic exposures. Ten healthy volunteers were studied by usingthree 8-h exposures: 1) isocapnichypoxia (IH), end-tidal PO₂(PET_O2) = 55 Torr andPET_CO2 held at thesubject's normal prehypoxic value;2) poikilocapnic hypoxia (PH),PET_O2 = 55 Torr; and3) control (C), air breathing. TheE-PET_CO2relationship was determined in hyperoxia (PET_O2 = 200 Torr) beforeand after the exposures. We found a significant increase in theslopes ofE-PET_CO2 relationship after both hypoxic exposures compared with control (IH vs.C, P < 0.01; PH vs. C,P < 0.001; analysis of covariance with pairwise comparisons). This increase was not significantly different between protocols IH andPH. No significant changes in theintercept were detected. We conclude that 8 h of hypoxia, whetherisocapnic or poikilocapnic, increases the sensitivity of the hyperoxicchemoreflex response to CO₂.

     

The effects of intermittent exposure to hypoxia during endurance exercise training on the ventilatory responses to hypoxia and hypercapnia in humans

The present study was performed to investigate the effects of a combination of intermittent exposure to hypoxia during exercise training for short periods on ventilatory responses to hypoxia and hypercapnia (HVR and HCVR respectively) in humans. In a hypobaric chamber at a simulated altitude of 4,500 m (barometric pressure 432 mmHg), seven subjects (training group) performed exercise training for 6 consecutive days (30 min · day⁻¹), while six subjects (control group) were inactive during the same period. The HVR, HCVR and maximal oxygen uptake (V˙O_{2 max}) for each subject were measured at sea level before (pre) and after exposure to intermittent hypoxia. The post exposure test was carried out twice, i.e. on the 1st day and 1 week post exposure. It was found that HVR, as an index of peripheral chemosensitivity to hypoxia, was increased significantly (P < 0.05) in the control group after intermittent exposure to hypoxia. In contrast, there was no significant increase in HVR in the training group after exposure. The HCVR in both groups was not changed by intermittent exposure to hypoxia, while V˙O_{2 max} increased significantly in the training group. These results would suggest that endurance training during intermittent exposure to hypoxia depresses the increment of chemosensitivity to hypoxia, and that intermittent exposure to hypoxia in the presence or absence of exercise training does not induce an increase in the chemosensitivity to hypercapnia in humans. Accepted: 18 March 1998     

Cardiorespiratory responses of the woodchuck and porcupine to CO2 and hypoxia

Summary The burrow-dwelling woodchuck (Marmota monax) (mean body wt.=4.45±1 kg) was compared to a similar-sized (5.87±1.5 kg) but arboreal rodent, the porcupine (Erithrizon dorsatum), in terms of its ventilatory and heart rate responses to hypoxia and hypercapnia, and its blood characteristics.V _T,f,T _I andT _E were measured by whole-body plethysmography in four awake individuals of each species. The woodchuck has a longerT _E/T _TOT (0.76±0.03) than the porcupine (0.61±0.03). The woodchuck had a higher threshold and significantly smaller slope to its CO₂ ventilatory response compared to the porcupine, but showed no difference in its hypoxic ventilatory response. The woodchuck P₅₀ of 27.8 was hardly different from the porcupine value of 30.7, but the Bohr factor, –0.72, was greater than the porcupine's, –0.413. The woodchuck breathing air has PaCO₂=48 (±2) torr, PaO₂=72 (±6), pHa=7.357 (±0.01); the porcupine blood gases are PaCO₂=34.6 (±2.8), PaO₂=94.9 (±5), pHa=7.419 (±0.03), suggesting a difference in PaCO₂/pH set points. The woodchuck exhibited no reduction in heart rate with hypoxia, nor did it have the low normoxic heart rate observed in other burrowing mammals.     

Effects of adenosine on ventilatory responses to hypoxia and hypercapnia in humans

Adenosine infusion (100 micrograms X kg-1 X min-1) in humans stimulates ventilation but also causes abdominal and chest discomfort. To exclude the effects of symptoms and to differentiate between a central and peripheral site of action, we measured the effect of adenosine infused at a level (70-80 micrograms X kg-1 X min-1) below the threshold for symptoms. Resting ventilation (VE) and progressive ventilatory responses to isocapnic hypoxia and hyperoxic hypercapnia were measured in six normal men. Compared with a control saline infusion given single blind on the same day, adenosine stimulated VE [mean increase: 1.3 +/- 0.8 (SD) l/min; P less than 0.02], lowered resting end-tidal PCO2 (PETCO2) (mean fall: -3.9 +/- 0.9 Torr), and increased heart rate (mean increase: 16.1 +/- 8.1 beats/min) without changing systemic blood pressure. Adenosine increased the hypoxic ventilatory response (control: -0.68 +/- 0.4 l X min-1 X %SaO2-1, where %SaO2 is percent of arterial O2 saturation; adenosine: -2.40 +/- 1.2 l X min-1 X %SaO2-1; P less than 0.01) measured at a mean PETCO2 of 38.3 +/- 0.6 Torr but did not alter the hypercapnic response. This differential effect suggests that adenosine may stimulate ventilation by a peripheral rather than a central action and therefore may be involved in the mechanism of peripheral chemoreception.     

Hemodynamics and muscle sympathetic nerve activity after 8 h of sustained hypoxia in healthy humans

Hemodynamics, muscle sympathetic nerve activity (MSNA), and forearm blood flow were evaluated in 12 normal subjects before, during (1 and 7 h), and after ventilatory acclimatization to hypoxia achieved with 8 h of continuous poikilocapnic hypoxia. All results are means +/- SD. Subjects experienced mean oxygen saturation of 84.3 +/- 2.3% during exposure. The exposure resulted in hypoxic acclimatization as suggested by end-tidal CO(2) [44.7 +/- 2.7 (pre) vs. 39.5 +/- 2.2 mmHg (post), P < 0.001] and by ventilatory response to hypoxia [1.2 +/- 0.8 (pre) vs. 2.3 +/- 1.3 l x min(-1).1% fall in saturation(-1) (post), P < 0.05]. Subjects exhibited a significant increase in heart rate across the exposure that remained elevated even upon return to room air breathing compared with preexposure (67.3 +/- 15.9 vs. 59.8 +/- 12.1 beats/min, P < 0.008). Although arterial pressure exhibited a trend toward an increase across the exposure, this did not reach significance. MSNA initially increased from room air to poikilocapnic hypoxia (26.2 +/- 10.3 to 32.0 +/- 10.3 bursts/100 beats, not significant at 1 h of exposure); however, MSNA then decreased below the normoxic baseline despite continued poikilocapnic hypoxia (20.9 +/- 8.0 bursts/100 beats, 7 h Hx vs. 1 h Hx; P < 0.008 at 7 h). MSNA decreased further after subjects returned to room air (16.6 +/- 6.0 bursts/100 beats; P < 0.008 compared with baseline). Forearm conductance increased after exposure from 2.9 +/- 1.5 to 4.3 +/- 1.6 conductance units (P < 0.01). These findings indicate alterations of cardiovascular and respiratory control following 8 h of sustained hypoxia producing not only acclimatization but sympathoinhibition.     

Intermittent exposure to aquatic pollutants: assessment,toxicity and sublethal responses in fish and invertebrates

Aquatic animals are often exposed to intermittent, variable poison concentrations during pollution incidents. However, current understanding of ecotoxicology has evolved primarily from continuous exposure studies. This review summarises the relatively dispersed toxicity literature on intermittent exposures. Methodologies used in existing continuous exposure toxicity tests may be adapted to intermittent regimes provided the exposure profile is known and “poison concentration” is defined to give toxicologically relevant lethality estimate. Such tests rely on assuming that continuous and intermittent exposures of equivalent dose have the same toxicity. This assumption is untrue for some chemicals. The toxicity of intermittent events may be assessed by correlating mortality with poison accumulation, biochemical, haematological or physiological response syndromes. Such bioassays can be performed without knowledge of the exposure profile, and are often sufficiently rapid to record short pollution events. Intermittent and continuous exposures of equivalent dose may not have the same toxicities. Intermittent exposures are less toxic than continuous events, but only when peak concentrations of pollutant are the same in each regime. Exceptionally, sulphuric acid, acid/Al and ammonia are much more toxic to fish when administered intermittently. Variations in intermittent exposure frequency or duration do not produce proportional changes in lethality, since apparently large changes in exposure dose may not significantly alter toxicity. The short-lived nature of intermittent exposures suggests that equilibriums in poison concentrations between the external environment and the body compartments of the test species are not achieved. The overall accumulation response depends particularly on the duration of peak concentrations and any “recovery periods” between multiple episodes relative to poison uptake and depuration rates respectively. Transient biochemical and physiological disturbances occur during intermittent exposures. Latent effects include reduced post-exposure growth and reproductive failure in the F₁ generation, or increased deformities in the F₂ generation of fish.     

Influence of ventilation and hypocapnia on sympathetic nerve responses to hypoxia in normal humans

The sympathetic response to hypoxia depends on the interaction between chemoreceptor stimulation (CRS) and the associated hyperventilation. We studied this interaction by measuring sympathetic nerve activity (SNA) to muscle in 13 normal subjects, while breathing room air, 14% O2, 10% O2, and 10% O2 with added CO2 to maintain isocapnia. Minute ventilation (VE) and blood pressure (BP) increased significantly more during isocapnic hypoxia (IHO) than hypocapnic hypoxia (HHO). In contrast, SNA increased more during HHO [40 +/- 10% (SE)] than during IHO (25 +/- 19%, P less than 0.05). To determine the reason for the lesser increase in SNA with IHO, 11 subjects underwent voluntary apnea during HHO and IHO. Apnea potentiated the SNA responses to IHO more than to HHO. SNA responses to IHO were 17 +/- 7% during breathing and 173 +/- 47% during apnea whereas SNA responses to HHO were 35 +/- 8% during breathing and 126 +/- 28% during apnea. During ventilation, the sympathoexcitation of IHO (compared with HHO) is suppressed, possibly for two reasons: 1) because of the inhibitory influence of activation of pulmonary afferents as a result of a greater increase in VE, and 2) because of the inhibitory influence of baroreceptor activation due to a greater rise in BP. Thus in humans, the ventilatory response to chemoreceptor stimulation predominates and restrains the sympathetic response. The SNA response to chemoreceptor stimulation represents the net effect of the excitatory influence of the chemoreflex and the inhibitory influence of pulmonary afferents and baroreceptor afferents.     

Metabolic effects of exposure to hypoxia plus cold at rest and during exercise in humans

To determine effects on metabolic responses, subjects were exposed to four environmental conditions for 90 min at rest followed by 30 min of exercise: breathing room air with an ambient temperature of 25 degrees C (NN); breathing room air with an ambient temperature of 8 degrees C (NC); hypoxia (induced by breathing 12% O2 in N2) with a neutral temperature (HN); and hypoxia in the cold (HC). Hypoxia increased heart rate (HR), systolic blood pressure (SBP), pulmonary ventilation (VE), respiratory exchange ratio (R), blood lactate, and perceived exertion during exercise while depressing rectal temperature (Tre) and O2 uptake (VO2). Cold exposure elevated SBP, diastolic blood pressure (DBP), VE, VO2, blood glucose, and blood glycerol but decreased HR, Tre, and R. Shivering and DBP were higher and Tre was lower in HC compared with NC. HR, SBP, VE, R, and lactate tended to be higher in HC compared with NC, whereas VO2 and blood glycerol tended to be depressed. These results suggest that cold exposure during hypoxia results in an increased reliance on shivering for thermogenesis at rest whereas, during exercise, heat loss is accelerated.