Influence of methazolamide on the human control of breathing: A comparison to acetazolamide.

NEW FINDINGS: What is the central question of this study? Acetazolamide and methazolamide both reduce hypoxic pulmonary vasoconstriction equally, but methazolamide does not impair skeletal muscle function. The effect of methazolamide on respiratory control in humans is not yet known. What is the main finding and its importance? Similar to acetazolamide after chronic oral administration, methazolamide causes a metabolic acidosis and shifts the ventilatory CO2 response curve leftwards without reducing O2 sensitivity. The change in ventilation over the change in log PO2 provides a more accurate measure of hypoxic sensitivity than the change in ventilation over the change in arterial oxyhaemoglobin saturation. ABSTRACT: Acetazolamide is used to prevent/treat acute mountain sickness and both central and obstructive sleep apnoea. Methazolamide, like acetazolamide, reduces hypoxic pulmonary vasoconstriction, but has fewer side-effects, including less impairment of skeletal muscle function. Given that the effects of methazolamide on respiratory control in humans are unknown, we compared the effects of oral methazolamide and acetazolamide on ventilatory control and determined the ventilation-log  PO2 relationship in humans. In a double-blind, placebo-controlled, randomized cross-over design, we studied the effects of acetazolamide (250 mg three times daily), methazolamide (100 mg twice daily) and placebo in 14 young male subjects who were exposed to 7 min of normoxic hypercapnia and to three levels of eucapnia and hypercapnic hypoxia. With placebo, methazolamide and acetazolamide, the CO2 sensitivities were 2.39 ± 1.29, 3.27 ± 1.82 and 2.62 ± 1.79 l min-1  mmHg-1 (n.s.) and estimated apnoeic thresholds 32 ± 3, 28 ± 3 and 26 ± 3 mmHg, respectively (P < 0.001, placebo versus methazolamide and acetazolamide). The relationship between ventilation ( V̇I ) and log  PO2 (using arterialized venous PO2 in hypoxia) was linear, and neither agent influenced the relationship between hypoxic sensitivity ( ΔV̇I/ΔlogPO2 ) and arterial [H+ ]. Using ΔV̇I/ΔlogPO2 rather than Δ V̇I /Δ arterial oxyhaemoglobin saturation enables a more accurate estimation of oxygenation and ventilatory control in metabolic acidosis/alkalosis when right- or leftward shifts of the oxyhaemoglobin saturation curve occur. Given that acetazolamide and methazolamide have similar effects on ventilatory control, methazolamide might be preferred for indications requiring the use of a carbonic anhydrase inhibitor, avoiding some of the negative side-effects of acetazolamide.

The ventilatory response to hypoxia in mammals: mechanisms, measurement, and analysis.

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Erythropoietin does not have effects on the ventilatory and pulmonary vascular response to acute hypoxia in men and women.

NEW FINDINGS: What is the central question of this study? Does a clinically relevant intravenous dose of erythropoeitin affect the hypoxic ventilatory response and/or hypoxic pulmonary vasoconstriction in healthy humans? What is the main finding and its importance? Erythropoeitin does not influence the ventilatory and pulmonary vascular responses to acute hypoxia in men or women. Sustained and chronic hypoxia lead to an increase in pulmonary ventilation (hypoxic ventilatory response, HVR) and to an increase in pulmonary vascular resistance (hypoxic pulmonary vasoconstriction, HPV). In this study, we examined the effect of a clinical i.v. dose of recombinant human erythropoietin (50 IU kg-1 ) on the isocapnic HVR and HPV in seven male and seven female subjects by exposing them to hypoxia for 20 min (end-tidal PO2  âˆ¼50 mmHg) while measuring their ventilation and estimating pulmonary arterial pressure from the maximal velocity of the regurgitant jet over the tricuspid valve during systole (ΔPmax ) with echocardiography. In the placebo session, after 5 and 20 min men responded with an increase in ventilation by 0.0056 and 0.0023 l min-1  kg-1   %SpO2-1 , respectively, indicating the presence of hypoxic ventilatory depression. In women, the increase in ventilation was 0.0067 and 0.0047 l min-1  kg-1   %SpO2-1 , respectively. In both sexes, erythropoietin did not alter these responses significantly. In the placebo session, mean ΔPmax increased by 6.1 ± 0.7 mmHg in men (P = 0.035) and by 8.4 ± 1.4 mmHg in women (P = 0.020) during the hypoxic exposure, whereby women had a âˆ¼5 mmHg lower end-tidal PCO2 . Erythropoietin did not alter these responses; in men, ΔPmax increased by 7.5 ± 1.1 mmHg (n.s. versus placebo) and in women by 9.7 ± 2.2 mmHg (n.s. versus placebo). We conclude that women tended to have a greater HPV in placebo conditions and that a clinical dose of erythropoietin has no effect on the HVR and HPV in either sex.

Hemorrhagic shock-induced endothelial cell activation in a spontaneous breathing and a mechanical ventilation hemorrhagic shock model is induced by a proinflammatory response and not by hypoxia.

INTRODUCTION: The interaction between neutrophils and activated endothelium is essential for the development of multiple organ dysfunction in patients with hemorrhagic shock (HS). Mechanical ventilation frequently is used in patients with HS. The authors sought to investigate the consequences of mechanical ventilation of mice subjected to HS on microvascular endothelial activation in the lung and kidney. METHODS: Anesthetized wild type C57BL/6 male mice were subjected to controlled hemorrhage; subgroups of mice were mechanically ventilated during the HS insult. To study the effect of acute hypoxia on the mice, the animals were housed in hypoxic cages. Gene expression levels was assessed by quantitative real-time polymerase chain reaction. Protein expression was assessed by immunohistochemistry and enzyme-linked immunosorbent assay. RESULTS: Ninety minutes after the shock induction, a vascular bed-specific, heterogeneous proinflammatory endothelial activation represented by E-selectin, vascular cell adhesion molecule 1, and intercellular adhesion molecule 1 expression was seen in kidney and lung. No differences in adhesion molecules between the spontaneously breathing and mechanically ventilated mice were found. Concentrations of the proinflammatory cytokines chemokine (C-X-C motif) ligand 1 (11.0-fold) and interleukin-6 (21.7-fold) were increased after 90 min of HS. Two hours of 6% oxygen did not induce the expression of E-selectin, vascular cell adhesion molecule 1, and intercellular adhesion molecule 1 in the kidneys and the lung. CONCLUSIONS: Hemorrhagic shock leads to an early and reversible proinflammatory endothelial activation in kidney and lung. HS-induced endothelial activation is not changed by mechanical ventilation during the shock phase. Hypoxia alone does not lead to endothelial activation. The observed proinflammatory endothelial activation is mostly ischemia- or reperfusion-dependent and not related to hypoxia.

Arterial [H+] and the ventilatory response to hypoxia in humans: influence of acetazolamide-induced metabolic acidosis.

In this study, we investigated possible separate effects of H+ ions and CO2 on hypoxic sensitivity in humans. We also examined whether hypoxic sensitivity, conventionally defined as the ratio of (hypoxic - normoxic) ventilation over (hypoxic - normoxic) Hb oxygen saturation can also be estimated by taking the ratio (hypoxic - normoxic) ventilation over (logPa(O2) hypoxia - logPa(O2) normoxia), enabling one to measure the hypoxic response independently from potential confounding influences of changes in position of the Hb oxygen saturation curve. We used acetazolamide to induce a metabolic acidosis. To determine the acute hypoxic response (AHR), we performed step decreases in end-tidal Po2 to approximately 50 Torr lasting 5 min each at three different constant end-tidal Pco2 levels. Nine subjects ingested 250 mg of acetazolamide or placebo every 8 h for 3 days in a randomized double-blind crossover design. The metabolic acidosis was accompanied by a rise in ventilation, a substantial fall in Pa(CO2), and a parallel leftward shift of the ventilatory CO2 response curve. In placebo, CO2 induced equal relative increases in hypoxic sensitivity (O2-CO2 interaction) regardless of the way it was defined. Acetazolamide shifted the response line representing the relationship between hypoxic sensitivity and arterial [H+] ([H+](a)) to higher values of [H+](a) without altering its slope, indicating that it did not affect the O2-CO2 interaction. So, in contrast to an earlier belief, CO2 and H+ have separate effects on hypoxic sensitivity. This was also supported by the finding that infusion of bicarbonate caused a leftward shift of the hypoxic sensitivity-[H+](a) response lines in placebo and acetazolamide. A specific inhibitory effect of acetazolamide on hypoxic sensitivity was not demonstrated.

Methazolamide does not impair respiratory work performance in anesthetized rabbits.

In human medicine, the carbonic anhydrase (CA) inhibitor acetazolamide is used to treat irregular breathing disorders. Previously, we demonstrated in the rabbit that this substance stabilized closed-loop gain properties of the respiratory control system, but concomitantly weakened respiratory muscles. Among others, the highly diffusible CA-inhibitor methazolamide differs from acetazolamide in that it fails to activate Ca(2+)-dependent potassium channels in skeletal muscles. Therefore, we aimed to find out, whether or not methazolamide may exert attenuating adverse effects on respiratory muscle performance as acetazolamide. In anesthetized spontaneously breathing rabbits (n = 7), we measured simultaneously the CO(2) responses of tidal phrenic nerve activity, tidal transpulmonary pressure changes, and tidal volume before and after intravenous application of methazolamide at two mean (+/- SE) cumulative doses of 3.5 +/- 0.1 and 20.8 +/- 0.4 mg/kg. Similar to acetazolamide, low- and high-dose methazolamide enhanced baseline ventilation by 52 +/- 10% and 166 +/- 30%, respectively (P < 0.01) and lowered the base excess in a dose-dependent manner by up to 8.3 +/- 0.9 mmol/l (P < 0.001). The transmission of a CO(2)-induced rise in phrenic nerve activity into volume and/or pressure and, hence, respiratory work performance was 0.27 +/- 0.05 ml x kg(-1) x kPa x unit(-1) under control conditions, but remained unchanged upon low- or high-dose methazolamide, at 0.30 +/- 0.06 and 0.28 +/- 0.07 ml x kg(-1) x kPa x unit(-1), respectively. We conclude that methazolamide does not cause respiratory muscle weakening at elevated levels of ventilatory drive. This substance (so far not used for medication of respiratory diseases) may thus exert stabilizing influences on breathing control without adverse effects on respiratory muscle function.

Differential effect of morphine and morphine-6-glucuronide on the control of breathing in the anesthetized cat.

BACKGROUND: Morphine's metabolite, morphine-6-glucuronide (M6G), activates the mu-opioid receptor. Previous data suggest that M6G activates a unique M6G receptor that is selectively antagonized by 3-methoxynaltrexome (3mNTX). The authors compared the effects of M6G and morphine on breathing in the anesthetized cat and assessed whether 3mNTX reversal was selective for M6G. METHODS: Step changes in end-tidal carbon dioxide concentration were applied in cats anesthetized with alpha-chloralose-urethane. In study 1, the effect of the 0.15 mg/kg morphine followed by 0.2 mg/kg 3mNTX and next 0.8 mg/kg M6G was assessed in six cats. In study 2, the effect of 0.8 mg/kg M6G followed by 0.2 mg/kg 3mNTX and 0.15 mg/kg morphine was tested in another six cats. The ventilatory carbon dioxide responses were analyzed with a two-compartment model of the ventilatory controller, which consists of a fast peripheral and a slow central component. RESULTS: Both opioids shifted the ventilatory carbon dioxide responses to higher end-tidal carbon dioxide levels. Morphine had a preferential depressant effect within the central chemoreflex loop. In contrast, M6G had a preferential depressant effect within the peripheral chemoreflex loop. Irrespective of the opioid, 3mNTX caused full reversal of and prevented respiratory depression. CONCLUSIONS: In anesthetized cats, the mu-opioids morphine and M6G induce respiratory depression at different sites within the ventilatory control system. Because 3mNTX caused full reversal of the respiratory depressant effects of both opioids, it is unlikely that a 3mNTX-sensitive unique M6G receptor is the cause of the differential respiratory behavior of morphine and M6G.

Bilateral carotid sinus nerve transection exacerbates morphine-induced respiratory depression.

Opioid-induced respiratory depression (OIRD) involves decreased sensitivity of ventilatory control systems to decreased blood levels of oxygen (hypoxia) and elevated levels of carbon dioxide (hypercapnia). Understanding the sites and mechanisms by which opioids elicit respiratory depression is pivotal for finding novel therapeutics to prevent and/or reverse OIRD. To examine the contribution of carotid body chemoreceptors OIRD, we used whole-body plethysmography to evaluate hypoxic (HVR) and hypercapnic (HCVR) ventilatory responses including changes in frequency of breathing, tidal volume, minute ventilation and inspiratory drive, after intravenous injection of morphine (10 mg/kg) in sham-operated (SHAM) and in bilateral carotid sinus nerve transected (CSNX) Sprague-Dawley rats. In SHAM rats, morphine produced sustained respiratory depression (e.g., decreases in tidal volume, minute ventilation and inspiratory drive) and reduced the HVR and HCVR responses. Unexpectedly, morphine-induced suppression of HVR and HCVR were substantially greater in CSNX rats than in SHAM rats. This suggests that morphine did not compromise the function of the carotid body-chemoafferent complex and indeed, that the carotid body acts to defend against morphine-induced respiratory depression. These data are the first in vivo evidence that carotid body chemoreceptor afferents defend against rather than participate in OIRD in conscious rats. As such, drugs that stimulate ventilation by targeting primary glomus cells and/or chemoafferent terminals in the carotid bodies may help to alleviate OIRD.

Electronic Nose Technology Fails to Sniff Out Acute Mountain Sickness.

Berendsen, Remco R., Marieke E. van Vessem, Marcel Bruins, Luc J.S.M. Teppema, Leon P.H.J. Aarts, and Bengt Kayser. Electronic nose technology fails to sniff out acute mountain sickness. High Alt Med Biol. 19:232-236, 2018. AIM: The aim of the study was to evaluate whether an electronic nose can discriminate between individuals with and without acute mountain sickness (AMS) following rapid ascent to 4554 m. RESULTS: We recruited recreational climbers (19 women, 82 men; age 35 ± 10 years, mean ± standard deviation [SD]) upon arrival at 4554 m (Capanna Regina Margherita, Italy) for a proof of concept study. AMS was assessed with the Lake Louise self-report score (LLSRS) and the abbreviated Environmental Symptoms Questionnaire (ESQc); scores ≥3 and ≥0.7 were considered AMS, respectively. Exhaled air was analyzed with an electronic nose (Aeonose; The eNose Company, Netherlands). The collected data were analyzed using an artificial neural network. AMS prevalence was 44% with the LLSRS (mean score of those sick 4.4 ± 1.4 [SD]) and 20% with the ESQc (1.2 ± 0.5). The electronic nose could not discriminate between AMS and no AMS (LLSRS p = 0.291; ESQc p = 0.805). CONCLUSION: The electronic nose technology utilized in this study could not discriminate between climbers with and without symptoms of AMS in the setting of an acute exposure to an altitude of 4554 m. At this stage, we cannot fully exclude that this technology per se is not able to discriminate for AMS. The quest for objective means to diagnose AMS thus continues.

Attenuation of human hypoxic pulmonary vasoconstriction by acetazolamide and methazolamide.

Acetazolamide (AZ), a carbonic anhydrase inhibitor used for preventing altitude illness attenuates hypoxic pulmonary vasoconstriction (HPV) while improving oxygenation. Methazolamide (MZ), an analog of acetazolamide, is more lipophilic, has a longer half-life, and activates a major antioxidant transcription factor. However, its influence on the hypoxic pulmonary response in humans is unknown. The aim of this study was to determine whether a clinically relevant dosing of MZ improves oxygenation, attenuates HPV, and augments plasma antioxidant capacity in men exposed to hypoxia compared with an established dosing of AZ known to suppress HPV. In this double-blind, placebo-controlled crossover trial, 11 participants were randomized to treatments with MZ (100 mg 2× daily) and AZ (250 mg 3× daily) for 2 days before 60 min of hypoxia (FIO2 ≈0.12). Pulmonary artery systolic pressure (PASP), alveolar ventilation (V̇A), blood gases, and markers of redox status were measured. Pulmonary vascular sensitivity to hypoxia was determined by indexing PASP to alveolar PO2. AZ caused greater metabolic acidosis than MZ, but the augmented V̇A and improved oxygenation with hypoxia were similar. The rise in PASP with hypoxia was lower with MZ (9.0 ± 0.9 mmHg) and AZ (8.0 ± 0.7 mmHg) vs. placebo (14.1 ± 1.3 mmHg, P < 0.05). Pulmonary vascular sensitivity to hypoxia (ΔPASP/ΔPAO2) was reduced equally by both drugs. Only AZ improved the nonenzymatic plasma antioxidant capacity. Although AZ had only plasma antioxidant properties, MZ led to similar improvements in oxygenation and reduction in HPV at a dose causing less metabolic acidosis than AZ in humans.NEW & NOTEWORTHY Both acetazolamide and methazolamide are effective in the prevention of acute mountain sickness by inducing an increase in ventilation and oxygenation. Acetazolamide attenuates hypoxic pulmonary vasoconstriction; however, it was previously unknown whether methazolamide has the same effect in humans. This study shows that a dosing of methazolamide causing less metabolic acidosis improves oxygenation while attenuating hypoxic pulmonary vasoconstriction and pulmonary vascular sensitivity to hypoxia. Acetazolamide improved plasma antioxidant capacity better than methazolamide.

Integration of Central and Peripheral Respiratory Chemoreflexes.

A debate has raged since the discovery of central and peripheral respiratory chemoreceptors as to whether the reflexes they mediate combine in an additive (i.e., no interaction), hypoadditive or hyperadditive manner. Here we critically review pertinent literature related to O2 and CO2 sensing from the perspective of system integration and summarize many of the studies on which these seemingly opposing views are based. Despite the intensity and quality of this debate, we have yet to reach consensus, either within or between species. In reviewing this literature, we are struck by the merits of the approaches and preparations that have been brought to bear on this question. This suggests that either the nature of combination is not important to system responses, contrary to what has long been supposed, or that the nature of the combination is more malleable than previously assumed, changing depending on physiological state and/or respiratory requirement.

The noncarbonic anhydrase inhibiting acetazolamide analog N-methylacetazolamide reduces the hypercapnic, but not hypoxic, ventilatory response.

Previous studies have shown that the carbonic anhydrase (CA) inhibitors acetazolamide (AZ) and methazolamide (MZ) have inhibiting actions on breathing. Classically these have been attributed to CA inhibition, but other effects unrelated to CA inhibition have been identified in other tissues. To explore this possibility in the control of ventilation by the central nervous system, we investigated whether an AZ-analog without CA inhibiting properties, by virtue of a single methylation on the sulfonamide moiety, N-methylacetazolamide (NMA), would still display similar actions to acetazolamide and methazolamide. NMA (20 mg kg(-1)) was given intravenously to anesthetized cats and we measured the responses to steady-state isocapnic hypoxia and stepwise changes in end-tidal pco2 before and after infusion of this AZ analog using the technique of end-tidal forcing. NMA caused a large decrease in the apneic threshold and CO2 sensitivity very similar to those previously observed with AZ and MZ, suggesting that these effects are mediated independently of CA inhibition. In contrast to acetazolamide, but similar to methazolamide, NMA did not affect the steady-state isocapnic hypoxic response. In conclusion, our data reveal complex effects of sulfonamides with very similar structure to AZ that reveal both CA-dependent and CA-independent effects, which need to be considered when using AZ as a probe for the role of CA in the control of ventilation.

Low-dose acetazolamide reduces the hypoxic ventilatory response in the anesthetized cat.

Low intravenous dose acetazolamide causes a decrease in steady-state CO(2) sensitivity of both the peripheral and central chemoreflex loops. The effect, however, on the steady-state hypoxic response is unknown. In the present study, we measured the effect of 4 mg x kg(-1) acetazolamide (i.v.) on the isocapnic steady-state hypoxic response in anesthetized cats. Before and after acetazolamide administration, the eucapnic steady-state hypoxic response in these animals was measured by varying inspiratory P(O2) levels to achieve steady-state Pa(O2) levels between hyperoxia Pa(O2) approximately 55 kPa, approximately 412 mmHg) and hypoxia (Pa(O2) approximately 7 kPa, approximately 53 mmHg). The hypoxic ventilatory response was described by the exponential function V(I) = G exp (-DP(o2) + A with an overall hypoxic sensitivity G, a shape parameter D and ventilation during hyperoxia A. Acetazolamide significantly reduced G from 3.057 +/- 1.616 to 1.573 +/- 0.8361 min(-1) (mean +/- S D). Parameter A increased from 0.903 +/- 0.257 to 1.193 +/- 0.321 min(-1), while D remained unchanged. The decrease in overall hypoxic sensitivity by acetazolamide is probably mediated by an inhibitory effect on the carotid bodies and may have clinical significance in the treatment of sleep apneas, particularly those cases that are associated with an increased ventilatory sensitivity to oxygen and/or carbon dioxide.

Anesthetics and control of breathing.

An important side effect of general anesthetics is respiratory depression. Anesthetics have multiple membrane targets of which ionotropic receptors such as gamma-aminobutyric acid-A (GABA(A)), glycine, N-methyl-D-aspartate and nicotinic acetylcholinergic (nACh) receptors are important members. GABA, glutamate and ACh are crucial neurotransmitters in the respiratory neuronal network, and the ability of anesthetics to modulate their release and interact with their receptors implies complex effects on respiration. Metabotropic receptors and intracellular proteins are other important targets for anesthetics suggesting complex effects on intracellular signaling pathways. Here we briefly overview the effects of general anesthetics on protein targets as far as these are relevant for respiratory control. Subsequently, we describe some methods with which the overall effect of anesthetics on the control of breathing can be measured, as well as some promising in vivo approaches to study their synaptic effects. Finally, we summarize the most important respiratory effects of volatile anesthetics in humans and animals and those of some intravenous anesthetics in animals.