Methylene blue induced O2 consumption is not dependent on mitochondrial oxidative phosphorylation: Implications for salvage pathways during acute mitochondrial poisoning

While administration of the cyclic redox agent methylene blue (MB) during intoxication by mitochondrial poisons (cyanide, hydrogen sulfide, rotenone) increases survival, the mechanisms behind these antidotal properties remain poorly understood. The objective of the studies presented in this paper was to characterize the interactions between the redox properties of MB, the intermediate metabolism and the mitochondrial respiration. We first show that intra-venous administration of micromolar levels of methylene blue in sedated and mechanically ventilated rats, increases not only resting oxygen consumption but also CO₂ production (by ~ 50%), with no change in their ratio. This hypermetabolic state could be reproduced in a cellular model, where we found that the rate of electron transfer to MB was of the same order of magnitude as that of normal cellular metabolism. Notably, the large increase in cellular oxygen consumption caused by MB was relatively indifferent to the status of the mitochondrial respiratory chain: oxygen consumption persisted even when the respiratory chain was inhibited or absent (using inhibitors and cells deficient in mitochondrial oxidative phosphorylation); yet MB did not impede mitochondrial ATP production in control conditions. We present evidence that after being reduced into leuco-methylene blue (LMB) in presence of reducing molecules that are physiologically found in cells (such as NADH), the re-oxidation of LMB by oxygen can account for the increased oxygen consumption observed in vivo. In conditions of acute mitochondrial dysfunction, these MB redox cycling properties allow the rescue of the glycolysis activity and Krebs cycle through an alternate route of oxidation of NADH (or other potential reduced molecules), which accumulation would have otherwise exerted negative feedback on these metabolic pathways. Our most intriguing finding is that re-oxidization of MB by oxygen ultimately results in an in vivo matching between the increase in the rate of O₂ consumed, by MB re-oxidation, and the rate of CO₂, produced by the intermediate metabolism, imitating the fundamental coupling between the glycolysis/Krebs cycle and the mitochondrial respiration.

Effects of infusion of human methemoglobin solution following hydrogen sulfide poisoning

RATIONALE

We have recently reported that infusion of a solution containing methemoglobin (MetHb) during exposure to hydrogen sulfide results in a rapid and large decrease in the concentration of the pool of soluble/diffusible H2S in the blood. However, since the pool of dissolved H2S disappears very quickly after H2S exposure, it is unclear if the ability of MetHb to "trap" sulfide in the blood has any clinical interest and relevance in the treatment of sulfide poisoning.

METHODS

In anesthetized rats, repetition of short bouts of high level of H2S infusions was applied to allow the rapid development of an oxygen deficit. A solution containing MetHb (600 mg/kg) or its vehicle was administered 1 min and a half after the end of H2S intoxication.

RESULTS

The injection of MetHb solution increased methemoglobinemia to about 6%, almost instantly, but was unable to affect the blood concentration of soluble H2S, which had already vanished at the time of infusion, or to increase combined H2S. In addition, H2S-induced O2 deficit and lactate production as well as the recovery of carotid blood flow and blood pressure were similar in treated and control animals.

CONCLUSION

Our results do not support the view that administration of MetHb or drugs-induced methemoglobinemia during the recovery phase following severe H2S intoxication in sedated rats can restore cellular oxidative metabolism, as the pool of diffusible sulfide, accessible to MetHb, disappears rapidly from the blood after H2S exposure.

Hypocapnia-dependent facilitation of augmented breaths: observations in awake vs. anesthetized rats

We investigated whether commonly used injectable laboratory anesthetics alter the regulation of augmented breaths (ABs) in different respiratory backgrounds. Male rats were studied on three separate experimental days, receiving one of three injections in randomized order: ethyl carbamate ('urethane'; 1.2mgkg(-1)), ketamine/xylazine (ket/xyl; 80/10mgkg(-1)), or normal saline. Following each of the three interventions, breathing was monitored during 15min exposures to normoxia (room air), hypoxia (10% O(2)) and hypoxia+CO(2) (10% O(2), 5% CO(2)). Urethane anesthesia completely eliminated ABs from the breathing rhythm in room air conditions (p<0.001), and decreased the hypocapnia-dependent component of this response (p<0.001). ket/xyl left the normal incidence of ABs in room air breathing intact but significantly suppressed the hypoxia-induced facilitation of ABs (p=0.0015). These results provide the first clear evidence that laboratory anesthesia can profoundly alter the regulation of ABs including the hypocapnia-dependent component of their facilitation.

Dyspnée sévère chez un patient présentant une bronchite chronique

INTRODUCTION

Dyspnoea is the major symptomatic complaint of patients with chronic obstructive pulmonary disease (COPD). Awareness of an unpleasant respiratory sensation, occurs through a complex system of ventilatory control where, as in pain perception, supra-bulbar structures play a major role. In the hyperventilation syndrome, dramatic episodes of dyspnea can occur without any change in peripheral afferent signals and thus do not predictably represent a real deterioration in pulmonary function in patients who also have COPD.

CASE REPORT

We report the observation of a patient with GOLD stage 0 COPD who demonstrated a major limitation of daily activities due to supra-bulbar influences on respiratory control which triggered repetitive periods of hyperventilation and generated almost constant dyspnea.

CONCLUSIONS

Hyperventilation syndrome can be difficult to diagnose in patients with respiratory disease, although, in most cases, the discrepancy between the severity of symptomatic complaint compared to the degree of alteration of lung function in combination with specific tests may suggest the diagnosis.

Inhibition of midbrain-evoked tonic and rhythmic motor activity by cutaneous stimulation in decerebrate cats

The effect of mechanical and electrical stimulation of cervical cutaneous afferents was analysed on both the centrally induced tonic and rhythmic activities in hindlimb antagonist muscle nerves of 16 decerebrate paralysed cats. Electrical stimulation of dorsal midbrain evoked in the nerve to the tibialis anterior muscle (TAn) either rhythmic discharges (n=14), associated with tonic discharges in ten cats, or only tonic discharges (n=4). Centrally induced activity in the ipsilateral nerve to gastrocnemius medialis (GMn) occurred in fewer cats (n=12) and displayed similar patterns as in TAn. Manual traction of the scruff of the neck reduced the TAn tonic and rhythmic discharges (n=6) by 73% (P<0.05) and 71% (P<0.05), respectively, and reduced only the tonic component of GMn discharges (by 41%, n=3). Electrical stimulation (impulses 0.1-0.5 ms, 50 Hz) of cervical nerves belonging to C5 or C6 dermatomes, the intensity (0.4-4 mA) of which induced minimal inhibition of both TAn and GMn discharges, reduced significantly the tonic component of TAn discharges (by 39%, n=4). At higher intensities of electrical cervical nerve stimulation (2-6 mA) inducing maximal inhibitory effect, both tonic and rhythmic activities in TAn and GMn were both significantly reduced by, respectively, 81% and 94% in TAn (n=7), and by 49% and 43% in GMn (n=7). Electrical cervical nerve stimulation consistently reduced the isolated tonic discharge in TAn by 66% (n=4, P<0.05) and in GMn by 23% (n=3) when present. Thus the tonic component was more sensitive to inhibition than the rhythmic component of hindlimb muscle nerve activity.

Effect of exercise on lung-perfusion scanning in patients with bronchogenic carcinoma

The aim of this study was to determine whether perfusion-scintillation scanning, used as a predictive pre-operative index of lung functionality in patients with lung cancer, is affected by the level of pulmonary blood flow (PBF). Twenty patients with primary lung cancer underwent spirometry and a radionuclide-perfusion scan (macroaggregated albumin particles labelled with 99mTechnetium) both at rest and during the last minute of a ramp-like increase in work rate until exhaustion. On average, the perfusion of the lung with the tumour was significantly reduced by the same magnitude at rest and during exercise (mean+/-SD: -9+/-6% versus -10+/-4% of the cardiac output), regardless of the extent of the tumour. However, subject-by-subject analysis revealed that in two patients, a larger decrease in the perfusion of the lung with the tumour was observed during exercise than at rest (-11% and -17%, respectively). This leads to an underestimation of predictive postoperative functional parameters if resting values are used in these patients. The use of perfusion scintigraphy at rest therefore gives a clear picture of the functionality of the lung before resection in most patients requiring surgery.

Control of breathing and muscle perfusion in humans

Very brief and intense exercise triggers a biphasic metabolic and respiratory response with a second phase that occurs after the cessation of the muscular activity. The effects on minute ventilation (V(E)) produced by manipulation of the peripheral circulation in metabolically active muscles could thus be studied without the confounding effects of painful contractions. The second phase of breath-by-breath V(E) and pulmonary gas exchange responses to a brief change in work rate (400 W for 12 s) were studied in six healthy male subjects on four occasions (24 tests). An upper thigh cuff inflation was randomly applied either above or below the systolic blood pressure (200 or 90 Torr, respectively) for 90 s just after the cessation of the contractions prior to the delayed rise in pulmonary gas exchange (eight tests in each subject). Total occlusion produced a significant reduction in the delayed rise in V(E) (-29 +/- 3 %) which normally occurred 20-25 s after the cessation of the contractions. In contrast, cuff inflation at a level predominantly impeding venous return while partially maintaining the arterial supply reduced the rise in pulmonary gas exchange in similar proportion to that during total obstruction but with a slight but not significant reduction in ventilation (-9 +/- 5 %). V(E) during partial occlusion was if anything higher than in control tests with similar oxygen uptake (280 W), despite a higher blood pressure (BP) during occlusion (+7 Torr). It is concluded that the factors resulting from a reduction in venous return or from the involvement of the arterial baroreflex are not responsible for the changes in V(E) produced by the obstruction of the circulation to and from metabolically active muscles. It is proposed that factors related to the level of the perfusion pressure in hyperaemic muscles, possibly located at the venular end of the microcirculation, could account for the changes in V(E) observed.

Neurophysiological basis for neurogenic-mediated articular cartilage anabolism alteration

This study was designed to investigate the pathways involved in neurogenic-mediated articular cartilage damage triggered by a nonsystemic distant subcutaneous or intra-articular inflammation. The cartilage damage was assessed 24 h after subcutaneous or intra-articular complete Freund's adjuvant (CFA) injection measuring patellar proteoglycan (PG) synthesis (ex vivo [Na(2)(35)SO(4)] incorporation) in 96 Wistar rats. Unilateral subcutaneous or intra-articular injection of CFA induced significant decrease (25-29%) in PG synthesis in both patellae. Chronic administration of capsaicin (50 mg. kg(-1). day(-1) during 4 days), which blunted the normal response of C fiber stimulation, prevented the bilateral significant decrease in cartilage synthesis. Similarly, intrathecal injection of MK-801 (10 nmol/day during 5 days), which blocked the glutamatergic synaptic transmission at the dorsal horn of signal originating in primary afferent C fibers, eliminated the CFA-induced PG synthesis decrease in both patellae. Chemical sympathectomy, induced by guanethidine (12.5 mg. kg(-1). day(-1) during 6 wk), also prevented PG synthesis alteration. Finally, compression of the spinal cord at the T3-T5 level had a similar protective effect on the reduction of [Na(2)(35)SO(4)] incorporation. It is concluded that the signal that triggers articular cartilage synthesis damage induced by a distant local inflammation 1) is transmitted through the afferent C fibers, 2) makes glutamatergic synaptic connections with the preganglionic neurons of the sympathetic system, and 3) involves spinal and supraspinal pathways.

Stimulation of ventilation by normobaric hyperoxia in exercising dogs

In order to describe the factors which, during hyperoxic exercise, can counteract the chemoreceptor-mediated inhibition of ventilation by O2, minute ventilation (VE) and the pulmonary gas exchange were studied breath-by-breath in four dogs running on a treadmill (5 km x h(-1)) for 10 min during and following exposure to O2 of different durations. We found that a brief inhalation of O2 applied during the steady state of the VE response provoked a reduction in VE by 6.5 +/- 0.9 l x min(-1) whereas hyperoxia applied 2 min before the onset of exercise and maintained for 2.5 min during the running tests had a significantly weaker effect on VE (-1.8 +/- 0.2 l x min(-1), P < 0.05). The rise in pulmonary CO2 output (VCO2) during the prolonged O2 exposure was less than in normoxic exercise leading to a deficit of CO2 eliminated by the lungs of 181 ml. The return to air breathing provoked a rise in VE, which reached within 73 s a much higher level than the control tests (22.9 +/- 3.6 vs. 19.5 +/- 2.2 l x min(-1), P < 0.05); VE then subsided to control levels with a long exponential decline. The CO2 deficit during O2 breathing, was fully compensated after recovery in air within 6 min. No stimulatory effect on ventilation was observed at rest at the cessation of a similar exposure to O2 despite a higher end-tidal PCO2 (+4 +/- 1 mmHg) than in exercise. In conclusion, the stimulatory effect of O2 during exercise can be clearly revealed after recovery in air and seems to operate through a more complex mechanism than that thought to be involved at rest. We propose that the changes in CO2 stores in the exercising muscles could contribute to O2-induced stimulation during exercise, possibly through stimulation of muscle afferents responding to local circulatory changes. Finally, the observation that during continuous dopamine (DA) infusion (5 microg x kg(-1) x min(-1)) the VE response to recovery in air was only a slow decrease, suggests that the arterial chemoreceptors potentiate O2-induced hyperventilation, or that the vascular actions of DA counteract part of the effects provoked by CO2 accumulation in the exercising muscles.

Corticospinal pathway and exercise hyperpnea: lessons from a patient with Arnold Chiari malformation

The study of a patient with an Arnold Chiari malformation gave us the opportunity to test the hypothesis that the motor cortex contributes significantly to respiratory control during muscular exercise through the corticospinal pathway. The patient was a 25 years old woman who exhibited a severe impairment of the 'automatic' ventilatory control due to a type I Arnold Chiari malformation. Since she never complained of being breathlessness even on exertion, the breath-by-breath ventilatory (VE) and pulmonary gas exchange responses to a three minute bout of constant work rate exercise at 60 W, 90 W and 120 W were studied before then 16 and 23 months after posterior fossa decompression. The VE response to the three different levels of exercise was dramatically blunted so that the expected vertical relationship between PET(CO(2)) and VE during moderate exercise was replaced by an almost horizontal relationship with a slope ranging from 0.15 to 0.17 l/min/Torr. The reduced VE response was associated with a total lack of respiratory sensation during and following the exercise bouts. This abnormal ventilatory response to exercise persisted despite posterior fossa decompression. There was however no evidence of an alteration of the corticospinal pathway. Indeed, not only was there no sign of motor deficit but the patient was able both to mobilize 96% of her expected vital capacity and to voluntarily increase her ventilation to the level expected in a normal subject during exercise. This observation suggests that during exercise, motor control of respiratory muscles via a direct corticospinal pathway does not play a major role in adjusting phrenic motoneuron activity to the magnitude of the motor inputs to the exercising skeletal muscles.

Responses of group III and IV muscle afferents to distension of the peripheral vascular bed

This study was undertaken to test the hypothesis that group III and IV afferents with endings in skeletal muscle signal the distension of the peripheral vascular network. The responses of these slowly conducting afferents to pharmacologically induced vasodilation and to acute obstruction of the venous drainage of the hindlimbs were studied in barbiturate-anesthetized cats. Afferent impulses arising from endings in the triceps surae muscles were recorded from the L(7) and S(1) dorsal roots. Fifteen of the 48 group IV and 3 of the 19 group III afferents tested were stimulated by intra-aortic injections of papaverine (2-2.5 mg/kg). Sixty-two percent of the afferents that responded to papaverine also responded to isoproterenol (50 microg/kg). Seven of the 36 group IV and 2 of the 12 group III afferents tested were excited by acute distension of the hindlimb venous system. Four of the seven group IV afferents responding to venous distension also responded to papaverine (57 vs. 13% for the nonresponding). Finally, we observed that most of the group IV afferents that were excited by dynamic contractions of the triceps surae muscles also responded either to venous distension or to vasodilatory agents. These results are consistent with the histological findings that a large number of group IV endings have their receptive fields close to the venules and suggest that they can be stimulated by the deformation of these vascular structures when peripheral conductance increases. Moreover, such a mechanism offers the possibility of encoding both the effects of muscle contraction through intramuscular pressure changes and the distension of the venular system, thereby monitoring the activity of the veno-muscular pump.

Heart rate dynamics during sinusoidal exercise: comparison of the control system between children and adults

The aim was to model the dynamics of heart rate (HR) response to sinusoidal work rate (WR) forcing in children and adults. Seven pre-pubertal boys (aged 10-13) and five adult males (aged 22-37) were studied. Continuous ECG recordings were obtained during the following physiological manoeuvres: five constant amplitude ergometer exercises with WR varying sinusoidally with periods of 0.75, 1, 2, 3.5, and 5 min duration, and one step exercise at a constant WR equal to the midpoint of the sinusoid amplitude. The amplitude ratio (AR; standardized by WR) of the fundamental harmonic of the HR response and the phase shift (phi) between the WR to HR were calculated by Fourier analysis. The HR dynamic parameters (gain and time constant (tau)) of a first order model with or without delay (Td) were also estimated. The AR in children was always higher than that in adults, in absolute terms, but not as a function of body weight. The phi was more delayed in the children than the adults only for the shortest period, i.e. 0.75 min. The tau for the first order model, either without or with Td, was found to be no difference between children and adults (44.7 vs. 45.9 s (without Td), 34.9 vs. 42.3 s (with Td)). Td, however, was longer in the children (6.6 vs. 2.3 s). The goodness of fit for the first order model with Td was better than that without Td in children, i.e. due to the difference of phi for 0.75 min period, whereas the HR dynamics in adults was appropriately described by first order model without Td. It is concluded that the fundamental control of HR to sinusoidal exercise between children and adults was not appreciably different, except for a small Td difference at high sinusoidal frequency.

[Value of pulmonary gas exchange study during exercise in the diagnosis of a muscular glycogenosis]

BACKGROUND

The diagnosis of muscular glycogen storage disease is usually difficult to demonstrate as symptoms normally consist of muscular cramps and exercise intolerance. Informations obtained from the study of the pulmonary gas exchange during exercise in a young patient with a glycogen storage disease are reported.

CASE REPORT

The ventilatory and gas exchange responses to a cyclo-ergometer exercise were studied in a 17-year-old girl during a ramp-like test (5 W/min). The temporal profile of CO2 production (VCO2) response was clearly abnormal: VCO2 was always lower than oxygen consumption throughout the test, reflecting the lack of lactate buffering by the bicarbonates due to the absence of lactate production. The respiratory ratio was still around 0.75 at the peak of the test. In contrast, responses were perfectly normal in the other members of the family, allowing rejection of the diagnosis of glycogen storage disease without any blood sampling.

CONCLUSION

This case illustrates the benefit of studying pulmonary gas exchange during exercise for a non-invasive diagnosis of muscular glycogen storage disease and detection of the disease in the other members of family.

The V(O2) slow component for severe exercise depends on type of exercise and is not correlated with time to fatigue

The purpose of this study was to examine the influence of the type of exercise (running vs. cycling) on the O2 uptake V(O2) slow component. Ten triathletes performed exhaustive exercise on a treadmill and on a cycloergometer at a work rate corresponding to 90% of maximal VO2 (90% work rate maximal V(O2)). The duration of the tests before exhaustion was superimposable for both type of exercises (10 min 37 s +/- 4 min 11 s vs. 10 min 54 s +/- 4 min 47 s for running and cycling, respectively). The V(O2) slow component (difference between V(O2) at the last minute and minute 3 of exercise) was significantly lower during running compared with cycling (20.9 +/- 2 vs. 268.8 +/- 24 ml/min). Consequently, there was no relationship between the magnitude of the V(O2) slow component and the time to fatigue. Finally, because blood lactate levels at the end of the tests were similar for both running (7.2 +/- 1.9 mmol/l) and cycling (7.3 +/- 2.4 mmol/l), there was a clear dissociation between blood lactate and the V(O2) slow component during running. These data demonstrate that 1) the V(O2) slow component depends on the type of exercise in a group of triathletes and 2) the time to fatigue is independent of the magnitude of the V(O2) slow component and blood lactate concentration. It is speculated that the difference in muscular contraction regimen between running and cycling could account for the difference in the V(O2) slow component.

Mechanisms of ventilatory inhibition by exogenous dopamine in cats

Intravenous injection of dopamine (DA) has consistently been shown to depress minute ventilation (VE). Whereas at low dosage (</=10 microgram/kg) this effect may be accounted for by inhibition of the carotid sinus nerve chemosensory discharge (CSNCD), other mechanisms appear to be involved with large dosage (>/=50 microgram/kg). The purpose of this study was to elucidate the mechanisms of DA-induced VE depression. The effects of intravenous injection of DA doses ranging from 1 to 200 microgram/kg were studied in 18 anesthetized cats. DA was injected during air and O2 breathing, after alpha-adrenergic blockade by phenoxybenzamine and after baro- and chemodenervation. VE and CSNCD were also simultaneously recorded on four occasions. In contrast to that with use of low-dose DA, VE depression induced by high-dose DA was dissociated from CSNCD, persisted during 100% O2 breathing, and was significantly correlated with the rise in arterial blood pressure. Although blunted, VE depression was still present after complete chemo- and barodenervation but was suppressed by blocking of the concomitant vasoconstriction with phenoxybenzamine. It is concluded that reflexes of circulatory origin contribute to the VE depression induced by large-dose DA, in addition to its effects on arterial chemoreceptors. The contribution of baroreceptor stimulation and peripheral vasoconstriction is discussed.

Ventilatory and gas exchange response during walking in severe peripheral vascular disease

It has long been recognized that at the onset of a dynamic muscular exercise the ventilatory and the circulatory (blood flow) responses appear to be matched, thereby maintaining arterial blood gas homeostasis. Such a coupling has recently been suggested to rely upon ventilatory reflex triggered by mechanoreceptors encoding changes in muscle blood flow or, more likely, blood volume. The aim of this study was to investigate whether patients with severe peripheral blood flow limitation to the lower extremities have a normal ventilatory response during a light intensity exercise. The ventilatory and gas exchange temporal response characteristics were studied during a 6 min walking test in seven patients with severe ischemic peripheral vascular disease and in six normal age-matched subjects. The magnitude of the overall ventilatory and Vo2 increment at the end of the tests was similar in both groups. However, in contrast to the control subjects, who presented an almost rectangular response, the patients had a considerably slowed response dynamics (t50 = 33 +/- 4 vs. 9 +/- 3 sec for Vo2 and 37 +/- 5 vs. 10 +/- 8 sec for VE) with a dramatic reduction in the magnitude of the initial 20 sec of the responses. Although the slow Vo2 dynamics in patients presumably reflected the impeded perfusion of the working muscles. the accompanying sluggishness of the V1 course implies that either muscular ischemia actually inhibits ventilatory response to exercise or, more likely, that this response is strongly linked to the magnitude of the hyperemia in the exercising muscles.

Laryngeal reflex apnea is blunted during and after hindlimb muscle contraction in sheep

This study was carried out on seven chloralose-anesthetized sheep and was designed to investigate the role of muscular afferent fiber stimulation on the duration of reflex apnea triggered by laryngeal stimulation (LS). In six animals, injection of distilled water onto the laryngeal mucosa provoked a 15.7 +/- 1.0 s (mean +/- SE) apnea associated with a rise in systemic blood pressure (+7 +/- 0.8 Torr). Electrically induced contractions (EIC) of the hindlimb muscles doubled the metabolic rate and ventilation and reduced the duration of the apnea produced by LS to 7.4 +/- 1.0 s (P < 0.01). Apnea duration was still reduced during the first minute after the cessation of EIC (7.2 +/- 1.1 s, P < 0.01) but returned to control after a 5-min recovery period (16.7 +/- 1.6 s). The apnea triggered by LS was also reduced during EIC when the venous return was impeded by occluding the inferior vena cava (5.2 +/- 1.1 s, P < 0.01), despite a profound hypocapnia (20.7 +/- 0.3 Torr). The duration of apnea was not significantly affected (14.2 +/- 1.4 s) by breathing a 6% CO2-14% O2 in N2 gas mixture that roughly mimicked the alveolar gas composition when the apnea turned off. These results suggest that chemical drive has a negligible role in the fast reinitiation of breathing after LS during muscular stimulation. Stimulation of muscle afferent fibers does, however, appear to be a potent source of ventilatory reflexes capable of counteracting the inhibition of breathing resulting from laryngeal stimulation. Conversely, it is postulated that any reduction in somatic afferent traffic during this type of reflex apnea, including that resulting from the LS-induced systemic vasoconstriction, may delay the termination of apnea.

Papaverine injection into the hindlimb circulation stimulates ventilation in sheep

To test the hypothesis, previously suggested by Huszczuk et al. (1993), that distention of the peripheral microvascular network could, per se, stimulate ventilation, the ventilatory effects of papaverine-induced muscular vasodilation were studied in ten anaesthetized sheep. Because systemic action of papaverine may involve the arterial baro- and chemoreceptors, the animals were surgically prepared for a reversible isolation of the hindlimb circulation. Papaverine injection (1-2 mg/kg) into the arterial inflow of the isolated limbs provoked a 13 +/- 6 sec-delayed increase in VE by 1.8 +/- 0.2 L min-1 (p < 0.01) with a concomitant decrease in peripheral vascular resistance and no decrease in the systemic arterial blood pressure. Identical control injection into a jugular vein prior to the hindlimb circulatory separation yielded an increase of VE by 4.95 +/- 0.58 L min-1 with a latency of 21 +/- 2 sec and a coinciding moderate decrease of the systemic arterial pressure. The present data suggest that papaverine injection into the hindlimb circulation can stimulate ventilation independently of its possible effects on the arterial baro- or chemoreceptors, supporting the hypothesis that muscular vasodilation could contribute to the control of breathing through a neural link.

Improved recovery from laryngeal-stimulation-induced apnea by alpha-adrenergic blockade in adult rabbits

The apnea triggered by laryngeal stimulation (LS) is associated with a redistribution of blood flow towards the heart and the brain that depends upon peripheral vasoconstriction. The circulatory component of the response to LS has been shown to be reduced following injection of large dose of beta-adrenergic agonist that more intriguingly blunted the ventilatory response as well. To test whether the reduction of LS-induced apnea could be related to the decreased magnitude of the peripheral vasoconstruction, the effects of the alpha-adrenergic antagonist urapidil on the ventilatory and blood pressure responses to LS were studied in 6 adult rabbits. alpha-Adrenergic blockade reduced both the rise in blood pressure and the magnitude of hypopnea induced by LS. These results suggest that the circulatory component of the laryngeal chemoreflex, i.e. the peripheral vasoconstruction, contributes to the magnitude of the ventilatory depression.

Persistent increased lung response to methacholine after normobaric hyperoxia in rabbits

This study was performed to determine the occurrence and time course of airway hyperreactivity following exposure to normobaric hyperoxia. Twenty-six rabbits were studied. Twelve served as control (group 1), and 14 were exposed to normobaric hyperoxia (FiO2 > or = 95%) for 48 h: 9 rabbits (group 2) were studied after 1 day of recovery in room air and 5 (group 3) after 7 days. The rabbits were anesthetized, curarized and artificially ventilated. Respiratory resistance (Rrs) and elastance (Ers) and their respective changes induced by cumulative doses of aerosolized methacholine were assessed by the multiple linear regression analysis of airway pressure, tidal flow and volume. Weight-specific Rrs and Ers were significantly higher in group 2 (respectively, 87.7 +/- 6.5 cmH2O.L-1.sec.kg and 1100.2 +/- 87.1 cmH2O.L-1.kg, mean +/- SEM) than in group 1 (respectively, 65.2 +/- 3.2 cmH2O.L-1.sec.kg and 904.4 +/- 49.7 cmH2O.L-1.kg (P < 0.05)), but were not different from group 3 (79.4 +/- 7.9 cmH2O.L-1.sec.kg and 952.3 +/- 125.0 cmH2O.L-1.kg, respectively). The dose of methacholine required to increase Rrs by 50% (PDRrs50) was significantly lower in both treated groups: 0.37 +/- 0.11 mg in group 2 and 0.51 +/- 0.19 mg in group 3 vs 2.07 +/- 0.51 mg in group 1 (P < 0.05)). PDErs50 was significantly lower in group 2 (0.45 +/- 0.15 mg) and 3 (0.75 +/- 0.43 mg) compared with controls (1.11 +/- 0.26 mg (P < 0.05)). These results show that hyperoxia induces an increase in Rrs and Ers, and airway hyperreactivity in the rabbit. The latter is prolonged beyond the immediate post-exposure period.

Vascular distension in muscles contributes to respiratory control in sheep

It has recently been proposed that afferent fibers from skeletal muscle could sense the state of the microvascular circulation, linking ventilation to the degree of peripheral perfusion or vascular distension (Huszczuk et al., Respir. Physiol., 91:207-226, 1993). Ventilatory and circulatory responses to manipulation of peripheral vascular pressures in the hind limbs of anaesthetized (sodium thiopental) sheep were examined. Inflatable balloons were placed at the caudal ends of the abdominal aorta and the vena cava (Vc). Aortic (Ao) occlusion induced a consistent normocapnic decrease in minute ventilation (VE). In contrast, VE increased significantly during vena cava obstruction, leading to hypocapnia. Small changes in systemic blood pressure were observed (+7 mmHg for Ao occlusion and -12 mmHg during Vc obstruction). Moreover, inflation of the caval balloon superimposed on a previously established Ao occlusion, preventing venous drainage of anastomotic inflow, resulted in a significant rise in distal vascular pressures with trivial changes in systolic blood pressure. This led to a gradual rise of VE, despite further reduction of the CO2 flux to the lungs. The subsequent deflation of the aortic balloon, exposing the hindlimb vasculature to aortic pressure, resulted in an even more profound hypocapnic hyperpnea. The concurrent arterial blood pressure changes were too small to possibly involve the ventilatory component of the arterial baroreflex. We therefore hypothesize, that perfusion-related afferent signals within the muscles could contribute to respiratory homeostasis by maintaining ventilation of the lungs commensurate with the circulatory state of the muscular apparatus.

Femoral vascular occlusion and ventilation during recovery from heavy exercise

Ventilation and cardiac output subside gradually following cessation of exercise, which is commonly linked to the slow wash-out of materials from the recovering muscles. The effect of hindering the removal of the metabolic products of heavy cycle exercise on the kinetics of ventilation and gas exchange was studied in 5 subjects by occluding the femoral circulation with cuffs during the first 2 min of recovery (15 tests). Fifteen undisturbed recoveries served as controls. Compared to spontaneous recovery, circulatory obstruction induced an immediate (from the first breath) decrease in minute ventilation (VE), while end-tidal CO2 (PETCO2) as well as lactate and K+ in venous blood at forearm did not change significantly. A ventilatory deficit of 27 +/- 9 L was observed from the 2 min of occlusion. Following cuff deflation, VE rose 2-3 breaths after PETCO2 began to increase in every subject. The mechanisms of the normocapnic reduction of VE during occlusion, as well as the rise of ventilation following cuff release, are still unclear. However, these results argue against any significant role for hyperpnea-inducing intramuscular chemoreception, or point to muscular perfusion as a prerequisite of such a mechanism to operate.

O2 uptake kinetics above and below the lactic acidosis threshold during sinusoidal exercise

O2 uptake (VO2) kinetics at the onset of a constant work rate exercise are difficult to describe for work rates above the lactic acidosis threshold (LAT), because the steady-state level of VO2 response can usually not be identified. To describe the ability of the O2 transport system to deliver and the cells to utilize O2 above the LAT relative to that below the LAT, we applied a fluctuating (sinusoidal) variation of work rate. After 4 min of constant work at the midpoint of the sinusoidal work rate, a fluctuating work rate, at a period of 4 min, was applied below the LAT for the next 16 min. This was repeated in a range of work rates above the LAT with the same sine-wave amplitude. VO2 response appeared to follow a sinusoidal pattern similar to that of work rate for below- and above-LAT exercise. However, the amplitude of the VO2 response was significantly reduced (5.4 +/- 2.6 vs. 7.6 +/- 1.9 ml.min-1 x W-1, P < 0.01), and the phase lag increased above- compared with below-LAT work rate. VO2/heart rate fluctuations were dramatically reduced, whereas heart rate amplitude decreased and phase lag increased, for above-LAT sinusoidal work rate changes. These results suggest that VO2 kinetics are slowed in the work rate domain above the LAT relative to that below the LAT and that VO2 kinetics could be limited by the O2 transport mechanisms to the exercising muscle.

Dual responses of carotid chemosensory afferents to dopamine in the newborn kitten

The effects of dopamine and of dopamine D2 receptor blocker haloperidol on the activity of carotid chemoreceptors were studied in 24 anesthetized, paralyzed and artificially ventilated newborn kittens aged 0-17 days. Single or few fiber preparations of chemoreceptors were made from one carotid sinus nerve. The responses of the chemosensory afferents to intravenous injections of dopamine (5-50 micrograms.kg-1) were studied in kittens breathing air and 8% O2 in N2. The effects of haloperidol on the chemosensory activity in air or 100% O2 and on the chemosensory response to hypoxia were studied. Dopamine inhibited the chemosensory discharge in 25/44 studies in normoxia. Of the 9 studies performed in hypoxia, dopamine was excitatory in 5 or had no effect in 4 (P < 0.05 vs normoxia). Inhibition of the chemosensory discharge was observed in 24/37 studies performed in kittens aged more than 3 days and was predominantly excitatory in 6/7 studies in kittens aged 0-3 days (P < 0.01). One minute after haloperidol, the chemosensory discharge had increased significantly in all experiments. The biphasic pattern of chemosensory response to hypoxia (Marchal et al., Respir. Physiol. 87: 183-193, 1992) was not changed by haloperidol. The steady-state chemosensory activity was significantly increased after haloperidol, respectively from 3.9 +/- 0.7 impulses.sec-1 to 9.8 +/- 1.2 impulses.sec-1 in air, and from 13.1 +/- 1.4 impulses.sec-1 to 17.8 +/- 2.4 impulses.sec-1 in hypoxia (mean +/- SEM, P < 0.03). It is concluded that the dopaminergic mechanisms are active in the carotid body of the kitten, and the observed responses to dopamine and haloperidol are qualitatively similar to those reported in the adult cat.

Mechanical properties of the upper airway wall in children and their influence on respiratory impedance measurements

The upper airway wall impedance (Zuaw) may be responsible for a large artifact in the measurement of respiratory system impedance (Zrs) in children. In 17 normal children aged 3.5-13 years Zuaw and Zrs were estimated by varying transrespiratory pressure directly at the mouth (conventional method: Z1) and around the subject's head (head generator method: Z2) from 4 to 32 Hz. Zrs and Zuaw were calculated from Z1 = Zrs.Zuaw/(Zrs+Zuaw) and Z2 = Zrs (1 + Zp/Zuaw), where Zp is the impedance of the pneumotachograph. From the real and imaginary part of Z1, Z2, Zrs, and Zuaw, the corresponding resistance, inertance, compliance and resonant frequency were calculated assuming simple RIC models. No significant difference was found between the mean +/- SE of parameters derived from Zrs (respectively, 6.8 +/- 0.4 cmH2O.L-1.s, 0.034 +/- 0.001 cmH2O.L-1.s2, 10.4 +/- 0.8 m.cmH2O-1, 9.1 +/- 0.3 Hz) and Z2 (6.8 +/- 0.4 cmH2O.L-1.s, 0.038 +/- 0.002 cmH2O.L-1.s2, 10.7 +/- 0.7 ml.cmH2O-1, 8.7 +/- 0.4 Hz). All but the compliance, derived from Z1 were significantly different (P less than 0.01) from those derived from Zrs (5.3 +/- 0.3 cmH2O.L-1.s, 0.008 +/- 0.001 cmH2O.L-1.s2, 11.9 +/- 1.2 ml.cmH2O-1, and 20.3 +/- 1.6 Hz). Respiratory resistance and compliance correlated significantly with height (r = -0.56 and 0.86, respectively), in contrast to upper airway wall resistance (Ruaw) and compliance (Cuaw). Ruaw (8.6 +/- 0.8 cmH2O.L-1.s), Cuaw (1.2 +/- 0.2 m.cmH2O-1), and upper airway wall inertance (0.030 +/- 0.004 cmH2O.L-1.s2) were close to those obtained by direct measurements in adults. The mechanical properties of the upper airway wall are responsible for a significant error in the measurement of Zrs by the conventional method in normal children. Most of the artifact may be corrected for by applying pressure around the child's head.

Carotid chemoreceptor response to natural stimuli in the newborn kitten

The activities of carotid chemoreceptors at three levels of inspired PO2 (55, 145 and 690 Torr) and at two levels of inspired PCO2 (35 and 70 Torr in O2) were studied in 28 anesthetized, mechanically ventilated kittens aged 0-17 days. A biphasic response to hypoxia was found in 46% of them: the chemosensory activity increased to a peak within 30 sec after the initial response to hypoxia and thereafter declined slowly to a stable value. The steady-state single-fiber chemosensory activity at an inspired PO2 of 55 Torr was significantly lower in kittens less than 10 days old (mean +/- SE: 5.8 +/- 0.6 impulses.sec-1) than in the older ones (8.8 +/- 1.3 impulses.sec-1, P less than 0.03). The response curves to arterial PO2 were hyperbolic in both groups but the curve for the younger kittens was displaced to the left of the curve for the older ones. The response to hypercapnia was a progressive increase in chemosensory activity with little evidence of rapid or slow adaptation. The response to hypercapnia was significantly stronger in the older kittens than in the young ones. It is concluded that, in the kitten, the carotid chemoreceptor response to hypoxia may be biphasic. The responses to hypoxia and hypercapnia are already developed but are weak at birth and continue to develop further during the first weeks of postnatal life.

Ventilatory dynamics in children and adults during sinusoidal exercise

The ventilatory response to sinusoidally varying exercise was studied in five adults and seven prepubertal children to determine whether the faster kinetics of ventilation observed in children during abrupt changes in exercise intensity remained more rapid when exercise intensity varied continuously. Each subject exercised on a cycle ergometer first against a constant load and then against a load fluctuating over six different periods ranging from 0.75 to 10 min. The pedal rate was kept constant for all loads. The inspiratory minute ventilation was determined breath-by-breath. Amplitude (A) and phase angle (phi) of the fundamental component and the first harmonics of the ventilatory response were calculated by Fourier analysis for an integer number of waves for each period. From the relationship between A, phi and frequency, dynamic parameters of a first order model with and without delay were compared between adults and children. Firstly we found that the ventilatory time constant was significantly faster in children: 49.7 (SD 9.1) s vs 74.6 (SD 11.1) s (P less than 0.01). Secondly, the change in A and phi with the frequency was not however characteristic of a first order system without delay in most of the subjects (phi greater than 90 degrees for the shorter periods). Thirdly, even when the ventilatory control system was described as a first order model with a positive delay, time constants remained significantly shorter in children: 45.6 (SD 5.7) s vs 67.4 (SD 13) s (P less than 0.01). The ability to increase ventilation faster in children appeared to be a characteristic of the ventilatory control system during exercise independent of the type of drive used.

Thoracic gas volume at functional residual capacity measured with an integrated-flow plethysmograph in infants and young children

Thoracic gas volume (TGV) was measured with an integrated flow plethysmograph in 15 infants aged 2-34 months. End-expiratory (TGVe) and end-inspiratory (TGVi) airway occlusions were compared, after correction of TGV for the occluded volume above functional residual capacity (FRC). The relationship between pressure at the airway opening (Pao) and volume displaced from the box during airway occlusion (Vg) was studied numerically by: 1) an algorithm including a correction for the drift of Vg and linear regression analysis (LR); and 2) Fourier analysis of the signals (FFT). TGVe was significantly higher than TGVi (256 vs 237 ml, 20.4 (square root of residual variance; p less than 0.002). The correlation coefficient of the Pao-Vg relationship was slightly but significantly higher for TGVi than for TGVe: 0.9968 (0.9937-0.9995) vs 0.9947 (0.9840-0.9990) (means and range). No difference was observed between LR and FFT, although the intra-individual coefficient of variation was lower for LR than FFT: 5.2% (1.6-11.3) vs 7.9% (1.9-21.0) (means and range). Model simulations suggested that the difference between TGVe and TGVi could be mainly attributed to gas compression in the instrumental deadspace and upper airway wall motion and/or to uneven distribution of alveolar and pleural pressure associated with chest wall distortion.

Thoracic gas volume at functional residual capacity measured with an integrated-flow plethysmograph in infants and young children

Thoracic gas volume (TGV) was measured with an integrated flow plethysmograph in 15 infants aged 2-34 months. End-expiratory (TGVe) and end-inspiratory (TGVi) airway occlusions were compared, after correction of TGV for the occluded volume above functional residual capacity (FRC). The relationship between pressure at the airway opening (Pao) and volume displaced from the box during airway occlusion (Vg) was studied numerically by: 1) an algorithm including a correction for the drift of Vg and linear regression analysis (LR); and 2) Fourier analysis of the signals (FFT). TGVe was significantly higher than TGVi (256 vs 237 ml, 20.4 (square root of residual variance; p less than 0.002). The correlation coefficient of the Pao-Vg relationship was slightly but significantly higher for TGVi than for TGVe: 0.9968 (0.9937-0.9995) vs 0.9947 (0.9840-0.9990) (means and range). No difference was observed between LR and FFT, although the intra-individual coefficient of variation was lower for LR than FFT: 5.2% (1.6-11.3) vs 7.9% (1.9-21.0) (means and range). Model simulations suggested that the difference between TGVe and TGVi could be mainly attributed to gas compression in the instrumental deadspace and upper airway wall motion and/or to uneven distribution of alveolar and pleural pressure associated with chest wall distortion.

Respiratory mechanics in spontaneously breathing term and preterm neonates

The compliance (Crs), resistance (Rrs) and passive time constant (tau rs) of the respiratory system were determined by the single-breath method (sb) in 24 healthy term and preterm newborns. In 22 of them, Crssb was compared to the slope of the pressure-volume curve determined by the multiple occlusion technique (mo), which is not dependent on the assumption of a linear flow-volume relationship. Crssb and Rrs correlated significantly with body weight (kg): Crssb = 0.56 x kg + 1.22 (r = 0.67); Rrs = -20.1 x kg + 134.6 (r = -0.68). No difference in Crs and Rrs between prone and supine positions was found. tau rs was not significantly different between premature (0.21 +/- 0.06 s) and full-term infants (0.21 +/- 0.05 s). Crssb was significantly higher than Crsmo in premature babies (2.27 +/- 0.41 ml.cm H2O-1 vs. 1.98 +/- 0.47 ml.cm H2O-1. This difference may be explained by a continuous braking of expiratory airflow after release of the occlusion, or more likely, by a difference in the lung volume at which Crssb and Crsmo are measured. However, the difference between Crssb and Crsmo (approximately 15%) is in the same range as the intrasubject variability, and is meaningless compared to the alterations of respiratory mechanics observed during neonatal ventilatory disorders. Therefore, the single-breath method appears to be a suitable and noninvasive method to measure respiratory mechanics in nonintubated prematures.

Measurement of ventilatory system resistance in infants and young children

The mechanics of the ventilatory system were studied in 29 sleeping infants and young children by the analysis of a passive expiration following an end-inspiratory airway occlusion ('single breath' method). The ventilatory system time constant (tau rs) to compliance ratio yielded the value of ventilatory system resistance (Rrs). The calculated ventilatory system compliance correlated well with the slope of the quasi static pressure-volume curve (r = 0.97). The allometric relationship between Rrs and height (Rrs = 81.9.10(3).Ht (cm)-1.76, r = -0.82) is in agreement with forced oscillation measurements during the first year of life (Wohl et al., 1969). tau rs was found to increase significantly over the first months of life (P less than 0.01).