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

Acute and long-term effects of acetazolamide in presumed high loop gain sleep apnea

BACKGROUND

The acute effect during positive pressure titration and long term efficacy of acetazolamide (AZT) in high loop gain sleep apnea (HLGSA) is inadequately assessed. We predicted that AZT may improve HLGSA in both conditions.

METHODS

A retrospective analysis of polysomnograms from patients with presumed HLGSA and residual respiratory instability administered AZT (125 or 250 mg) about 3 h into an initially drug-free positive pressure titration. A responder was defined as ≥ 50% reduction of the apnea hypopnea index(AHI 3% or arousal) before and after AZT. A multivariable logistic regression model estimated responder predictors. Long term efficacy of AZT was assessed by comparing both auto-machine (aREI_FLOW) and manually scored respiratory events (sREI_FLOW) extracted from the ventilator, prior to and after 3 months of AZT, in a subset.

RESULTS

Of the 231 participants (median age of 61[51-68] years) and 184 (80%) males in the acute effect testing: 77 and 154 patients were given 125 mg and 250 mg AZT. Compared to PAP alone, PAP plus AZT was associated with a lower breathing related arousal index (8 [3-16] vs. 5 [2-10], p < 0.001), and AHI3% (19 [7-37] vs. 11 [5-21], p < 0.001); 98 patients were responders. The non-rapid eye movement sleep (NREM) AHI3% (OR 1.031, 95%CI [1.016-1.046], p < 0.001) was a strong predictor for responder status with AZT exposure. In the 109 participants with 3-month data, both aREI_FLOW and sREI_FLOWwere significantly reduced after AZT.

CONCLUSIONS

AZT acutely and chronically reduced residual sleep apnea in presumed HLGSA; NREM AHI3% is a response predictor. AZT was well tolerated and beneficial for at least 3 months.

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 P O 2 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  P O 2 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 ( V ̇ I ) and log  P O 2 (using arterialized venous P O 2 in hypoxia) was linear, and neither agent influenced the relationship between hypoxic sensitivity ( Δ V ̇ I / Δ log P O 2 ) and arterial [H⁺ ]. Using Δ V ̇ I / Δ log P O 2 rather than Δ V ̇ I /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.

Control of Ventilation in Health and Disease

Control of ventilation occurs at different levels of the respiratory system through a negative feedback system that allows precise regulation of levels of arterial carbon dioxide and oxygen. Mechanisms for ventilatory instability leading to sleep-disordered breathing include changes in the genesis of respiratory rhythm and chemoresponsiveness to hypoxia and hypercapnia, cerebrovascular reactivity, abnormal chest wall and airway reflexes, and sleep state oscillations. One can potentially stabilize breathing during sleep and treat sleep-disordered breathing by identifying one or more of these pathophysiological mechanisms. This review describes the current concepts in ventilatory control that pertain to breathing instability during wakefulness and sleep, delineates potential avenues for alternative therapies to stabilize breathing during sleep, and proposes recommendations for future research.

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.

Effects of acetazolamide on cerebrovascular function and breathing stability at 5050 m

One of the many actions of the carbonic anhydrase inhibitor, acetazolamide (ACZ), is to accelerate acclimatisation and reduce periodic breathing during sleep. The mechanism(s) by which ACZ may improve breathing stability, especially at high altitude, remain unclear. We tested the hypothesis that acute I.V. ACZ would enhance cerebrovascular reactivity to CO₂ at altitude, and thereby lower ventilatory drive and improve breathing stability during wakefulness. We measured arterial blood gases, minute ventilation (˙VE) and middle cerebral artery blood flow velocity (MCAv) before and 30 min following ACZ administration (I.V. 10 mg kg⁻¹) in 12 healthy participants at sea level and following partial acclimatisation to altitude (5050 m).Measures were made at rest and during changes in end-tidal PCO₂ and PO₂ (isocapnic hypoxia). At sea level, ACZ increased resting MCAv and its reactivity to both hypocapnia and hypercapnia (P < 0.05), and lowered resting VE, arterial O₂ saturation (Sa,O₂ ) and arterial PO₂ (Pa,O₂) (P < 0.05); arterial PCO₂ (Pa,CO₂ ) was unaltered (P > 0.05). At altitude, ACZ also increased resting MCAv and its reactivity to both hypocapnia and hypercapnia (resting MCAv and hypocapnia reactivity to a greater extent than at sea level). Moreover, ACZ at altitude elevated Pa,CO₂ and again lowered resting Pa,O₂ and Sa,O₂ (P <0.05). Although the ˙VE sensitivity to hypercapnia or isocapnic hypoxia was unaltered following ACZ at both sea level and altitude (P > 0.05), breathing stability at altitude was improved (e.g. lower incidence of ventilatory oscillations and variability of tidal volume; P < 0.05). Our data indicate that I.V. ACZ elevates cerebrovascular reactivity and improves breathing stability at altitude, independent of changes in peripheral or central chemoreflex sensitivities. We speculate that Pa,CO₂-mediated elevations in cerebral perfusion and an enhanced cerebrovascular reactivity may partly account for the improved breathing stability following ACZ at high altitude.

Effects of acetazolamide on transient K+ currents and action potentials in nodose ganglion neurons of adult rats

The aim of the present study was to determine whether acetazolamide (AZ) contributes to the inhibition of the fast inactivating transient K(+) current (I(A) ) in adult rat nodose ganglion (NG) neurons. We have previously shown that pretreatment with either AZ or 4-AP attenuated or blocked the CO(2) -induced inhibition of slowly adapting pulmonary stretch receptors in in vivo experiments. The patch-clamp experiments were performed by using the isolated NG neurons. In addition to this, the RT-PCR of mRNA and the expression of voltage-gated K(+) (Kv) 1.4, Kv 4.1, Kv 4.2, and Kv 4.3 channel proteins from nodose ganglia were examined. We used NG neurons sensitive to the 1 mM AZ application. The application of 1 mM AZ inhibited the I(A) by approximately 27% and the additional application of 4-AP (1 mM) further inhibited I(A) by 48%. The application of 0.1 μM α-dendrotoxin (α-DTX), a slow inactivating transient K(+) current (I(D) ) blocker, inhibited the baseline I(A) by approximately 27%, and the additional application of 1 mM AZ further decreased the I(A) by 51%. In current clamp experiments, AZ application (1 mM) increased the number of action potentials due to the decreased duration of the depolarizing phase of action potentials and/or due to a reduction in the resting membrane potential. Four voltage-gated K(+) channel proteins were present, and most (80-90%) of the four Kv channels immunoreactive neurons showed the co-expression of carbonic anhydrase-II (CA-II) immunoreactivity. These results indicate that the application of AZ causes the reduction in I(A) via the inhibition of four voltage-gated K(+) channel (Kv) proteins without affecting I(D).

The hypoxia-induced facilitation of augmented breaths is suppressed by the common effect of carbonic anhydrase inhibition

The typical respiratory response to hypoxia includes a dramatic facilitation of augmented breaths (ABs) or 'sighs' in the breathing rhythm. We recently found that when acetazolamide treatment is used to promote CO(2) retention and counteract alkalosis during exposure to hypoxia, then the hypoxia-induced facilitation of ABs is effectively prevented. These results indicate that hyperventilation-induced hypocapnia/alkalosis is an essential factor involved in the hypoxia-induced facilitation of augmented breaths. However, acetazolamide is also known to decrease the sensitivity of the arterial chemoreceptors. Therefore, the question remains as to whether acetazolamide prevents the facilitation of ABs during hypoxia by offsetting the effects of respiratory alkalosis, or alternatively by suppressing carotid body afferent activity. In the present study, we addressed this question by studying the effects of treatment with an alternative carbonic anhydrase inhibitor, methazolamide, which has been reported to leave carotid body responsiveness to hypoxia intact. Respiratory variables were monitored before, during and after 2 days of methazolamide treatment (10 mg kg(-1) IP, bid) in unsedated and unrestrained adult male rats. Pre-treatment, the number of ABs observed in a 5 min observation window was 1.2 + or - 0.8 and 17.4 + or - 3.8 in room air and hypoxia, respectively. During methazolamide treatment, the facilitation of ABs in hypoxia was rapidly and reversibly suppressed such that ABs we no longer significantly more frequent than they were in room air. The present results demonstrate that the hypoxia-induced facilitation of ABs can be suppressed via the general effects of carbonic anhydrase inhibition, which are common to both acetazolamide and methazolamide. We discuss these results as they pertain to the mechanisms regulating augmented breath production, and the possible association between hypocapnia/alkalosis and sleep disordered breathing.

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 Ca2+-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 CO2 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 CO2-induced rise in phrenic nerve activity into volume and/or pressure and, hence, respiratory work performance was 0.27 ± 0.05 ml·kg−1·kPa·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·kg−1·kPa·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.

Acetazolamide suppresses the prevalence of augmented breaths during exposure to hypoxia

Augmented breaths, or “sighs,” commonly destabilize respiratory rhythm, precipitating apneas and variability in the depth and rate of breathing, which may then exacerbate sleep-disordered breathing in vulnerable individuals. We previously demonstrated that hypocapnia is a unique condition associated with a high prevalence of augmented breaths during exposure to hypoxia; the prevalence of augmented breaths during hypoxia can be returned to normal simply by the addition of CO2to the inspired air. We hypothesized that counteracting the effect of respiratory alkalosis during hypocapnic hypoxia by blocking carbonic anhydrase would yield a similar effect. We, therefore, investigated the effect of acetazolamide on the prevalence of augmented breaths in the resting breathing cycle in five awake, adult male rats. We found a 475% increase in the prevalence of augmented breaths in animals exposed to hypocapnic hypoxia compared with room air. Acetazolamide treatment (100 mg/kg ip bid) for 3 days resulted in a rapid and potent suppression of the generation of augmented breaths during hypoxia. Within 90 min of the first dose of acetazolamide, the prevalence of augmented breaths in hypoxia fell to levels that were no greater than those observed in room air. On cessation of treatment, exposure to hypocapnic hypoxia once again caused a large increase in the prevalence of augmented breaths. These results reveal a novel means by which acetazolamide acts to stabilize breathing and may help explain the beneficial effects of the drug on breathing stability at altitude and in patients with central forms of sleep-disordered breathing.

Acetazolamide protects against posthypoxic unstable breathing in the C57BL/6J mouse

Acetazolamide (Acz), a carbonic anhydrase inhibitor, is used to manage periodic breathing associated with altitude and with heart failure. We examined whether Acz would alter posthypoxic ventilatory behavior in the C57BL/6J (B6) mouse model of recurrent central apnea. Experiments were performed with unanesthetized, awake adult male B6 mice (n = 9), ventilatory behavior was measured using flow-through whole body plethysmography. Mice were given an intraperitoneal injection of either vehicle or Acz (40 mg/kg), and 1 h later they were exposed to 1 min of 8% O(2)-balance N(2) (poikilocapnic hypoxia) or 12% O(2)-3% CO(2)-balance N(2) (isocapnic hypoxia) followed by rapid reoxygenation (100% O(2)). Hypercapnic response (8% CO(2)-balance O(2)) was examined in six mice. With Acz, ventilation, including respiratory frequency, tidal volume, and minute ventilation, in room air was significantly higher and hyperoxic hypercapnic ventilatory responsiveness was generally lower compared with vehicle. Poikilocapnic and isocapnic hypoxic ventilatory responsiveness were similar among treatments. One minute after reoxygenation, animals given Acz exhibited posthypoxic frequency decline, a lower coefficient of variability for frequency, and no tendency toward periodic breathing, compared with vehicle treatment. We conclude that Acz improves unstable breathing in the B6 model, without altering hypoxic response or producing short-term potentiation, but with some blunting of hypercapnic responsiveness.

Factors of lowered respiratory CO2 sensitivity by acetazolamide in anaesthetized rabbits

The carbonic anhydrase (CA) inhibitor acetazolamide is a classic drug to treat patients with breathing disorders. Recent studies in rabbits showed that low-dose acetazolamide (not causing appreciable inhibition of red cell CA) significantly weakened respiratory muscle performance, accompanied by diminished ventilatory CO2-sensitivity, which implies stabilizing loop-gain properties. Now is aimed to explore the interaction of these factors under conditions of complete CA-inhibition by acetazolamide in a higher dose-range.

In anesthetized rabbits (N=7), acetazolamide (up to 75 mg·kg−1) distinctly lowered the base excess (to-7.6 ± 0.9mM, mean ± SEM) without respiratory compensation of arterial pH. Ventilatory CO2-sensitivity was nearly abolished to 15.1 ± 5.2% of control, but the transmission of a CO2-mediated rise in tidal phrenic activity into respiratory work was only reduced by 51.6 ± 6.4%, P < 0.001, not very much more than (~38%) already observed at low-doses.

Thus, the large reduction of ventilatory CO2-sensitivity in the high-dose range cannot be ascribed to respiratory muscle weakening, but rather may relate to complete inhibition of red cell CA. Conversely, CA-inhibition may not be the only cause for the weakening effect of acetazolamide on (respiratory) muscles. Adverse effects on respiratory muscles, impaired CO2-transport and acid-base imbalance may limit to make use of stabilizing effects on breathing control functions by high-dose acetazolamide.

Acetazolamide affects breathing differently in ICR and C57 mice

Acetazolamide (ACZ) administration was compared on ventilation in outbred male ICR Swiss Webster (ICR) and inbred C57BL/6J (C57) mice, used in development of transgenic strains. We hypothesized that in both strains ACZ would affect breathing similarly. Mice received intraperitoneally vehicle and the next week ACZ (40 mg/kg), and were exposed to air for 90 min, followed by 5-min exposure to 10% O(2), air for 15 min, and to 5 min of 5% CO(2) in O(2). Ventilation was evaluated using plethysmography. ACZ stimulated ventilation in both stains exposed to air. C57 mice minimally increased frequency and tidal volume, whereas ICR mice markedly increased frequency. Strain differences in the ventilatory pattern in response to hypoxia and hypercapnia occurred. ACZ-treated ICR mice decreased hypoxic responsiveness to 50% of vehicle values, whereas ACZ had no effect in C57 mice. ACZ decreased hypercapnic ventilatory responsiveness in both strains. Differential effects of ACZ breathing in these two strains suggest that genetic factors modulate its effect on breathing.

The carbonic anhydrase inhibitors methazolamide and acetazolamide have different effects on the hypoxic ventilatory response in the anaesthetized cat

We compared the effects of the carbonic anhydrase inhibitors methazolamide and acetazolamide (3 mg kg(-1), i.v.) on the steady-state hypoxic ventilatory response in 10 anaesthetized cats. In five additional animals, we studied the effect of 3 and 33 mg kg(-1) methazolamide. The steady-state hypoxic ventilatory response was described by the exponential function: *Vi= G exp(-D P(O2)) + A where *Vi is the inspired ventilation, G is hypoxic sensitivity, D is the shape factor and A is hyperoxic ventilation. In the first group of 10 animals, methazolamide did not change parameters G and D, while A increased from 0.86 +/- 0.33 to 1.30 +/- 0.40 l min(-1) (mean +/- s.d., P = 0.003). However, the subsequent administration of acetazolamide reduced G by 44% (control, 1.93 +/- 1.32; acetazolamide, 1.09 +/- 0.92 l min(-1), P = 0.003), while A did not show a further change. Acetazolamide tended to reduce D (control, 0.20 +/- 0.07; acetazolamide, 0.14 +/- 0.06 kPa(-1), P = 0.023). In the second group of five animals, neither low- nor high-dose methazolamide changed parameters G, D and A. The observation that even high-dose methazolamide, causing full inhibition of carbonic anhydrase in all body tissues, did not reduce the hypoxic ventilatory response is reminiscent of previous findings by others showing no change in magnitude of the hypoxic response of the in vitro carotid body by this agent. This suggests that normal carbonic anhydrase activity is not necessary for a normal hypoxic ventilatory response to occur. The mechanism by which acetazolamide reduces the hypoxic ventilatory response needs further study.

Antioxidants reverse depression of the hypoxic ventilatory response by acetazolamide in man

The carbonic anhydrase inhibitor acetazolamide may have both inhibitory and stimulatory effects on breathing. In this placebo-controlled double-blind study we measured the effect of an intravenous dose (4 mg kg(-1)) of this agent on the acute isocapnic hypoxic ventilatory response in 16 healthy volunteers (haemoglobin oxygen saturation 83-85%) and examined whether its inhibitory effects on this response could be reversed by antioxidants (1 g ascorbic acid i.v. and 200 mg alpha-tocopherol p.o.). The subjects were randomly divided into an antioxidant (Aox) and placebo group. In the Aox group, acetazolamide reduced the mean normocapnic and hypercapnic hypoxic responses by 37% (P < 0.01) and 55% (P < 0.01), respectively, and abolished the O2-CO2 interaction, i.e. the increase in O2 sensitivity with rising Pco2. Antioxidants completely reversed this inhibiting effect on the normocapnic hypoxic response, while in hypercapnia the reversal was partial. In the placebo group, acetazolamide reduced the normo- and hypercapnic hypoxic responses by 33 and 47%, respectively (P < 0.01 versus control in both cases), and also abolished the O2-CO2 interaction. Placebo failed to reverse these inhibitory effects of acetazolamide in this group. We hypothesize that either an isoform of carbonic anhydrase may be involved in the regulation of the redox state in the carotid bodies or that acetazolamide and antioxidants exert independent effects on oxygen-sensing cells, in which both carbonic anhydrase and potassium channels may be involved. The novel findings of this study may have clinical implications, for example with regard to a combined use of acetazolamide and antioxidants at high altitude.

Differential respiratory effects of [Dmt1]DALDA and morphine in mice

H-Dmt-D-Arg-Phe-Lys-NH(2) ([Dmt(1)]DALDA, dDAL), a highly selective mu-opioid peptide, produces potent analgesia without respiratory depression after intrathecal administration. Despite carrying 3+ net charge, dDAL is also a potent analgesic after systemic administration. We compared the respiratory effects of dDAL and morphine after subcutaneous administration in mice using whole body plethysmography. Analgesic doses of 3 and 10 times ED(50) were examined. Both drugs dose-dependently decreased respiratory frequency and minute volume in room air. Tidal volume was increased by the lower dose of morphine, while it was decreased by the higher dose of dDAL. The decrease in minute volume by dDAL and morphine was completely reversed by naloxone. No difference in ventilatory response to CO(2) was observed between dDAL and morphine at three times ED(50). Ventilatory response to hypoxia was significantly diminished by dDAL compared to morphine and saline, and this effect of dDAL was naloxone-irreversible. Thus dDAL likely reduces the sensitivity of the peripheral chemoreflex loop through a non-opioid action.