VCO2 and VE kinetics during moderate- and heavy-intensity exercise after acetazolamide administration

The Impact of Acetazolamide and Methazolamide on Exercise Performance in Normoxia and Hypoxia

Doherty, Connor J., Jou-Chung Chang, Benjamin P. Thompson, Erik R. Swenson, Glen E. Foster, and Paolo B. Dominelli. The impact of acetazolamide and methazolamide on exercise performance in normoxia and hypoxia. High Alt Med Biol. 24:7-18, 2023.-Carbonic anhydrase (CA) inhibitors are commonly prescribed for acute mountain sickness (AMS). In this review, we sought to examine how two CA inhibitors, acetazolamide (AZ) and methazolamide (MZ), affect exercise performance in normoxia and hypoxia. First, we briefly describe the role of CA inhibition in facilitating the increase in ventilation and arterial oxygenation in preventing and treating AMS. Next, we detail how AZ affects exercise performance in normoxia and hypoxia and this is followed by a discussion on MZ. We emphasize that the overarching focus of the review is how the two drugs potentially affect exercise performance, rather than their ability to prevent/treat AMS per se, their interrelationship will be discussed. Overall, we suggest that AZ hinders exercise performance in normoxia, but may be beneficial in hypoxia. Based upon head-to-head studies of AZ and MZ in humans on diaphragmatic and locomotor strength in normoxia, MZ may be a better CA inhibitor when exercise performance is crucial at high altitude.

Acetazolamide reduces exercise capacity following a 5-day ascent to 4559 m in a randomised study

Objective

To assess whether acetazolamide (Az), used prophylactically for acute mountain sickness (AMS), alters exercise capacity at high altitude.

Methods

Az (500 mg daily) or placebo was administered to 20 healthy adults (aged 36±20 years, range 21–77), who were paired for age, sex, AMS susceptibility and weight, in a double-blind, randomised manner. Participants ascended over 5 days to 4559 m, then exercised to exhaustion on a bicycle ergometer, while recording breath-by-breath gas measurements. Comparisons between groups and matched pairs were done via Mann-Whitney U and Pearson’s χ² tests, respectively.

Results

Comparing paired individuals at altitude, those on Az had greater reductions in maximum power output (P_max) as a percentage of sea-level values (65±14.1 vs 76.6±7.4 (placebo); P=0.007), lower VO_2max (20.7±5.2 vs 24.6±5.1 mL/kg/min; P<0.01), smaller changes from rest to P_max for VO₂ (9.8±6.2 vs 13.8±4.9 mL/kg/min; P=0.04) and lower heart rate at P_max (154±25 vs 167±16, P<0.01) compared with their placebo-treated partners. Correlational analysis (Pearson’s) indicated that with increasing age P_max (r=−0.83: P<0.005) and heart rate at P_max (r=−0.71, P=0.01) reduced more in those taking Az.

Conclusion

Maximum exercise performance at altitude was reduced more in subjects taking Az compared with placebo, particularly in older individuals. The age-related effect may reflect higher tissue concentrations of Az due to reduced renal excretion. Future studies should explore the effectiveness of smaller Az doses (eg, 250 mg daily or less) in older individuals to optimise the altitude–Az–exercise relationships.

Acetazolamide during acute hypoxia improves tissue oxygenation in the human brain

Low doses of the carbonic anhydrase inhibitor acetazolamide provides accelerated acclimatization to high-altitude hypoxia and prevention of cerebral and other symptoms of acute mountain sickness. We previously observed increases in cerebral O2 metabolism (CMRO2 ) during hypoxia. In this study, we investigate whether low-dose oral acetazolamide (250 mg) reduces this elevated CMRO2 and in turn might improve cerebral tissue oxygenation (PtiO2 ) during acute hypoxia. Six normal human subjects were exposed to 6 h of normobaric hypoxia with and without acetazolamide prophylaxis. We determined CMRO2 and cerebral PtiO2 from MRI measurements of cerebral blood flow (CBF) and cerebral venous O2 saturation. During normoxia, low-dose acetazolamide resulted in no significant change in CBF, CMRO2 , or PtiO2 . During hypoxia, we observed increases in CBF [48.5 (SD 12.4) (normoxia) to 65.5 (20.4) ml·100 ml(-1)·min(-1) (hypoxia), P < 0.05] and CMRO2 [1.54 (0.19) to 1.79 (0.25) μmol·ml(-1)·min(-1), P < 0.05] and a dramatic decline in PtiO2 [25.0 to 11.4 (2.7) mmHg, P < 0.05]. Acetazolamide prophylaxis mitigated these rises in CBF [53.7 (20.7) ml·100 ml(-1)·min(-1) (hypoxia + acetazolamide)] and CMRO2 [1.41 (0.09) μmol·ml(-1)·min(-1) (hypoxia + acetazolamide)] associated with acute hypoxia but also reduced O2 delivery [6.92 (1.45) (hypoxia) to 5.60 (1.14) mmol/min (hypoxia + acetazolamide), P < 0.05]. The net effect was improved cerebral tissue PtiO2 during acute hypoxia [11.4 (2.7) (hypoxia) to 16.5 (3.0) mmHg (hypoxia + acetazolamide), P < 0.05]. In addition to its renal effect, low-dose acetazolamide is effective at the capillary endothelium, and we hypothesize that local interruption in cerebral CO2 excretion accounts for the improvements in CMRO2 and ultimately in cerebral tissue oxygenation during hypoxia. This study suggests a potentially pivotal role of cerebral CO2 and pH in modulating CMRO2 and PtiO2 during acute hypoxia.

The abuse of diuretics as performance-enhancing drugs and masking agents in sport doping: pharmacology, toxicology and analysis

Diuretics are drugs that increase the rate of urine flow and sodium excretion to adjust the volume and composition of body fluids. There are several major categories of this drug class and the compounds vary greatly in structure, physicochemical properties, effects on urinary composition and renal haemodynamics, and site and mechanism of action. Diuretics are often abused by athletes to excrete water for rapid weight loss and to mask the presence of other banned substances. Because of their abuse by athletes, diuretics have been included on The World Anti-Doping Agency's (WADA) list of prohibited substances; the use of diuretics is banned both in competition and out of competition and diuretics are routinely screened for by anti-doping laboratories. This review provides an overview of the pharmacology and toxicology of diuretics and discusses their application in sports. The most common analytical strategies currently followed by the anti-doping laboratories accredited by the WADA are discussed along with the challenges laboratories face for the analysis of this diverse class of drugs.

Effect of acetazolamide on pulmonary and muscle gas exchange during normoxic and hypoxic exercise

Acetazolamide (ACZ) is used to prevent acute mountain sickness at altitude. Because it could affect O2 transport in several different and potentially conflicting ways, we examined its effects on pulmonary and muscle gas exchange and acid-base status during cycle exercise at approximately 30, 50 and 90% VO2max in normoxia (F(IO2) = 0.2093) and acute hypoxia (F(IO2) = 0.125). In a double-blind, order-balanced, crossover design, six healthy, trained men (normoxic VO2max= 59 ml kg(-1) min(-1)) exercised at both F(IO2) values after ACZ (3 doses of 250 mg, 8 h apart) and placebo. One week later this protocol was repeated using the other drug (placebo or ACZ). We measured cardiac output (QT), leg blood flow (LBF), and muscle and pulmonary gas exchange, the latter using the multiple inert gas elimination technique. ACZ did not significantly affect VO2, QT, LBF or muscle gas exchange. As expected, ACZ led to lower arterial and venous blood [HCO3-], pH and lactate levels (P < 0.05), and increased ventilation (P < 0.05). In both normoxia and hypoxia, ACZ resulted in higher arterial P(O2) and saturation and a lower alveolar-arterial P(O2) difference (AaD(O2)) due to both less VA/Q mismatch and less diffusion limitation (P < 0.05). In summary, ACZ improved arterial oxygenation during exercise, due to both greater ventilation and more efficient pulmonary gas exchange. However, muscle gas exchange was unaffected.

Chemoreflex drive and the dynamics of ventilation and gas exchange during exercise at hypoxia

We tested the hypothesis that the promotion of hypoxic ventilatory responsiveness (HVR) and/or hypercapnic ventilatory responsiveness (HCVR) mostly acting on the carotid body with a changing work rate can be attributed to faster hypoxic ventilatory dynamics at the onset of exercise. Eleven subjects performed a cycling exercise with two repetitions of 6 minutes while breathing at FIO(2) = 12%. The tests began with unloaded pedaling, followed by three constant work rates of 40%, 60%, and 80% of the subject's ventilatory threshold at hypoxia. Reference data were obtained at the 80% ventilatory threshold work rate during normoxia. Using three inhaled 100% O(2) breath tests, a comparison of hypoxia and normoxia revealed an augmentation of HVR in hypoxia, which then significantly increased proportionally with the increase in work rate. In contrast, HCVR using three inhaled 10% CO(2) breath tests was unaffected by the difference in work rate at hypoxia but did exceed its level at normoxia. The decrease in the half-time of hypoxic ventilation became significant with an increase in work rates and was significantly lower than at normoxia. Using a multiregression equation, HVR was found to account for 63% of the variance of hypoxic ventilatory dynamics at the onset of exercise and HCVR for 9%. O(2) uptake on-kinetics and off-kinetics under hypoxic conditions were significantly slower than under normoxic conditions, whereas they were not altered by the changing work rates at hypoxia. These results suggest that the faster hypoxic ventilatory dynamics at the onset of exercise can be mostly attributed to the augmentation of HVR with an increase in work rates rather than to HCVR. Otherwise, O(2) uptake dynamics are affected by the lower O(2), not by the changing work rates under hypoxic conditions.

Acetazolamide reduces exercise capacity and increases leg fatigue under hypoxic conditions

Acetazolamide (Acz) is used at altitude to prevent acute mountain sickness, but its effect on exercise capacity under hypoxic conditions is uncertain. Nine healthy men completed this double-blind, randomized, crossover study. All subjects underwent incremental exercise to exhaustion with an inspired O(2) fraction of 0.13, hypoxic ventilatory responses, and hypercapnic ventilatory responses after Acz (500 mg twice daily for 5 doses) and placebo. Maximum power of 203 +/- 38 (SD) W on Acz was less than the placebo value of 225 +/- 40 W (P < 0.01). At peak exercise, arterialized capillary pH was lower and Po(2) higher on Acz (P < 0.01). Ventilation was 118.6 +/- 20.0 l/min at the maximal power on Acz and 102.4 +/- 20.7 l/min at the same power on placebo (P < 0.02), and Borg score for leg fatigue was increased on Acz (P < 0.02), with no difference in Borg score for dyspnea. Hypercapnic ventilatory response on Acz was greater (P < 0.02), whereas hypoxic ventilatory response was unchanged. During hypoxic exercise, Acz reduced exercise capacity associated with increased perception of leg fatigue. Despite increased ventilation, dyspnea was not increased.

VE response to VCO2 during exercise is unaffected by exercise training and different exercise limbs

We designed two experiments to investigate the relationship between ventilation (VE) and CO2 output (VCO2) during exercise under the conditions of exercising different limbs, the arms as opposed to the legs (experiment 1), and of different physical training states after undergoing standard exercise training for 90 d (experiment 2). Six healthy young subjects underwent submaximal ramp exercise at an incremental work rate of 15 W/min for the arm and leg, and 11 healthy middle-aged subjects underwent an incremental exercise test at the rate of 30 W/3 min before and after exercise training. We measured pulmonary breath-by-breath VE, VCO2, oxygen uptake (VO2), tidal volume (VT), breathing frequency (bf), and end-tidal O2 and CO2 pressures (PETO2, PETCO2) via a computerized metabolic cart. In experiment 1, arm exercise produced significantly greater VE than did leg exercise at the same work rates, as well as significantly higher VO2, VCO2, and bf. The slopes of the regression lines in the VE-VCO2 relationship were not significantly different: the values were 27.8 +/- 2.1 (SD) during the arm exercise, and 25.3 +/- 3.9 during the leg exercise, with no differences in their intercepts. In experiment 2, the VO2, VCO2, and VE responses at the same work rates were similar in both before and after the 90-d exercise training, whereas the heart rate (HR) and mean blood pressure (MBP) were significantly reduced after training. Exercise training did not alter the VE-VCO2 relationship, the slope of which was 31.9 +/- 4.9 before exercise training and 34.2 +/- 4.4 after exercise training. We concluded that the VE-VCO2 relationship during exercise is unaltered, independent of not only working muscle regions but also exercise training states.

Low-dose acetazolamide reduces CO(2)-O(2) stimulus interaction within the peripheral chemoreceptors in the anaesthetised cat

1. Using the technique of end-tidal CO(2) forcing, we measured the effect of the carbonic anhydrase inhibitor acetazolamide (4 mg kg(-1), I.V.) on the CO(2) sensitivities of the peripheral and central chemoreflex loops both during hyperoxia and hypoxia in 10 cats anaesthetised with alpha-chloralose-urethane. 2. In the control situation, going from hyperoxia (arterial P(O2) (P(a,O2)) 47.40 +/- 3.62 kPa, mean +/- S.D.) into moderate hypoxia (P(a,O2) 8.02 +/- 0.30 kPa) led to an almost doubling of the peripheral CO(2) sensitivity (S(P)): a rise from 0.09 +/- 0.07 to 0.16 +/- 0.06 l min(-1) kPa(-1). After acetazolamide, however, lowering the P(a,O2) from 46.95 +/- 5.19 to 8.02 +/- 0.66 kPa did not result in a rise in S(P), indicating the absence of a CO(2)-O(2) stimulus interaction. 3. In hypoxia, acetazolamide reduced S(P) from 0.16 +/- 0.06 to 0.07 +/- 0.05 l min(-1) kPa(-1). In hyperoxia, however, the effect on S(P) was much smaller (an insignificant reduction from 0.09 +/- 0.07 to 0.06 +/- 0.05 l min(-1) kPa(-1)). 4. Acetazolamide reduced both the hyperoxic and hypoxic sensitivities (S(C)) of the central chemoreflex loop: from 0.45 +/- 0.16 to 0.27 +/- 0.13 l min(-1) kPa(-1) and from 0.40 +/- 0.16 to 0.26 +/- 0.13 l min(-1) kPa(-1), respectively. In hyperoxia, the apnoeic threshold B (X-intercept of the ventilatory CO(2) response curve) decreased from 2.91 +/- 0.57 to 0.78 +/- 1.9 kPa (P = 0.005). In hypoxia, B decreased from 1.59 +/- 1.22 to -0.70 +/- 2.99 kPa (P = 0.03). 5. Because acetazolamide abolished the CO(2)-O(2) interaction, i.e. the expected increase in S(P) when going from hyperoxia into hypoxia, we conclude that the agent has a direct inhibitory effect on the carotid bodies. The exact mechanism by which the agent exerts this effect will remain unclear until more detailed information becomes available on the identity of the carbonic anhydrase iso-enzymes within the carotid bodies and their precise subcellular distribution.

Forearm muscle metabolism studied using (31)P-MRS during progressive exercise to fatigue after Acz administration

The effects of acetazolamide (Acz)-induced carbonic anhydrase inhibition (CAI) on muscle intracellular thresholds (T) for intracellular pH (pH(i)) and inorganic phosphate-to-phosphate creatine ratio (P(i)/PCr) and the plasma lactate (La(-)) threshold were examined in nine adult male subjects performing forearm wrist flexion exercise to fatigue. Exercise consisted of raising and lowering (1-s contraction, 1-s relaxation) a cylinder whose volume increased at a rate of 200 ml/min. The protocol was performed during control (Con) and after 45 min of CAI with Acz (10 mg/kg body wt iv). T(pH(i)) and T(P(i)/PCr), determined using (31)P-labeled magnetic resonance spectroscopy (MRS), were similar in Acz (722 +/- 50 and 796 +/- 75 mW, respectively) and Con (855 +/- 211 and 835 +/- 235 mW, respectively). The pH(i) was similar at end-exercise (6.38 +/- 0.10 Acz and 6.43 +/- 0.22 Con), but pH(i) recovery was slowed in Acz. In a separate experiment, blood was sampled from a deep arm vein at the elbow for determination of plasma lactate concentration ([La(-)](pl)) and T(La(-)). [La(-)](pl) was lower (P < 0.05) in Acz than Con (3.7 +/- 1.7 vs. 5.0 +/- 1.7 mmol/l) at end-exercise and in early recovery, but T(La(-)) was higher (1,433 +/- 243 vs. 1,041 +/- 414 mW, respectively). These data suggest that the lower [La(-)](pl) seen with CAI was not due to a delayed onset or rate of muscle La(-) accumulation but may be related to impaired La(-) removal from muscle.

Carbonic anhydrase inhibition delays plasma lactate appearance with no effect on ventilatory threshold

The effect of carbonic anhydrase (CA) inhibition with acetazolamide (Acz, 10 mg/kg body wt iv) on exercise performance and the ventilatory (VET) and lactate (LaT) thresholds was studied in seven men during ramp exercise (25 W/min) to exhaustion. Breath-by-breath measurements of gas exchange were obtained. Arterialized venous blood was sampled from a dorsal hand vein and analyzed for plasma pH, PCO(2), and lactate concentration ([La(-)](pl)). VET [expressed as O(2) uptake (VO(2)), ml/min] was determined using the V-slope method. LaT (expressed as VO(2), ml/min) was determined from the work rate (WR) at which [La(-)](pl) increased 1.0 mM above rest levels. Peak WR was higher in control (Con) than in Acz sutdies [339 +/- 14 vs. 315 +/- 14 (SE) W]. Submaximal exercise VO(2) was similar in Acz and Con; the lower VO(2) at exhaustion in Acz than in Con (3.824 +/- 0. 150 vs. 4.283 +/- 0.148 l/min) was appropriate for the lower WR. CO(2) output (VCO(2)) was lower in Acz than in Con at exercise intensities >/=125 W and at exhaustion (4.375 +/- 0.158 vs. 5.235 +/- 0.148 l/min). [La(-)](pl) was lower in Acz than in Con during submaximal exercise >/=150 W and at exhaustion (7.5 +/- 1.1 vs. 11.5 +/- 1.1 mmol/l). VET was similar in Acz and Con (2.483 +/- 0.086 and 2.362 +/- 0.110 l/min, respectively), whereas the LaT occurred at a higher VO(2) in Acz than in Con (2.738 +/- 0.223 vs. 2.190 +/- 0.235 l/min). CA inhibition with Acz is associated with impaired elimination of CO(2) during the non-steady-state condition of ramp exercise. The similarity in VET in Con and Acz suggests that La(-) production is similar between conditions but La(-) appearance in plasma is reduced and/or La(-) uptake by other tissues is enhanced after the Acz treatment.

Peripheral chemoreceptor function after carbonic anhydrase inhibition during moderate-intensity exercise

The effect of carbonic anhydrase inhibition with acetazolamide (Acz, 10 mg/kg) on the ventilatory response to an abrupt switch into hyperoxia (end-tidal PO2 = 450 Torr) and hypoxia (end-tidal PO2 = 50 Torr) was examined in five male subjects [30 +/- 3 (SE) yr]. Subjects exercised at a work rate chosen to elicit an O2 uptake equivalent to 80% of the ventilatory threshold. Ventilation (VE) was measured breath by breath. Arterial oxyhemoglobin saturation (%SaO2) was determined by ear oximetry. After the switch into hyperoxia, VE remained unchanged from the steady-state exercise prehyperoxic value (60.6 +/- 6.5 l/min) during Acz. During control studies (Con), VE decreased from the prehyperoxic value (52.4 +/- 5.5 l/min) by approximately 20% (VE nadir = 42.4 +/- 6.3 l/min) within 20 s after the switch into hyperoxia. VE increased during Acz and Con after the switch into hypoxia; the hypoxic ventilatory response was significantly lower after Acz compared with Con [Acz, change (Delta) in VE/DeltaSaO2 = 1.54 +/- 0.10 l. min-1. SaO2-1; Con, DeltaVE/DeltaSaO2 = 2.22 +/- 0.28 l. min-1. SaO2-1]. The peripheral chemoreceptor contribution to the ventilatory drive after acute Acz-induced carbonic anhydrase inhibition is not apparent in the steady state of moderate-intensity exercise. However, Acz administration did not completely attenuate the peripheral chemoreceptor response to hypoxia.