Acetazolamide attenuates transvascular fluid flux in equine lungs during intense exercise

The effect of systemic acetazolamide administration on intraocular pressure in healthy horses-A preliminary study

OBJECTIVES

In equine glaucoma, topical treatment with carbonic anhydrase inhibitors (CAIs) is recommended. Oral acetazolamide, a systemic CAI, is used in horses with hyperkalemic periodic paralysis. Information regarding its effect on equine intraocular pressure (IOP) is scarce. The aim of the study was to determine the effect of oral acetazolamide treatment on IOP in horses, in a case-control study.

ANIMALS

Ten healthy horses.

PROCEDURES

Horses were treated with oral acetazolamide (4.4 mg/kg) BID for 1 week. Serum acetazolamide concentrations were determined by liquid chromatography/tandem mass spectrometry, and IOP were measured before treatment, daily during treatment, and at 48 and 72 h after treatment.

RESULTS

Acetazolamide serum levels reached steady state at 72 h after the first oral dose. In a mixed effect model logistic regression, there was a significant decrease in IOP on the third treatment day, of 2.4 mmHg (p = .012) and 2.7 mmHg (p = .006) in the left (OS) and right eye (OD), respectively. On the seventh day, there was a decrease in 2.5 mmHg (p = .008) and 2.7 mmHg (p = .007) OS and OD, respectively. A significant increase occurred 48 h following treatment discontinuation (3.6 mmHg, p < .001 and 3.5 mmHg, p < .001 OS and OD, respectively). The area under the concentration versus time curve (AUC(0-10h)) was 1.1 ± 0.5 μg/mL*h, mean residence time 6.7 ± 4.3 h, peak plasma concentration (Cmax) 0.4 ± 0.4 μg/mL and time to reach Cmax 1.8 h. There was a significant increase in serum concentrations 1, 2, 48, 72, and 156 h following the first drug administration (p < .05).

CONCLUSIONS

Further studies are required to determine whether acetazolamide is a potential treatment for equine glaucoma.

Lessons from NATURE: methods for traumatic brain injury prevention

Multiple species obtain repetitive head collisions throughout the course of their lifetimes with minimal neurologic deficit. Nature has allowed the unique development of multiple protective mechanisms to help prevent neurotrauma. In this review, we examine the concept of rapid brain movement within the skull 'Slosh' and what nature teaches on how to prevent this from occurring. We look at individual animals and the protective mechanisms at play. Marching from macroscopic down to the molecular level, we pinpoint key elements of neuroprotection that are likely contributing. We also introduce new concepts for neuroprotection and address avenues of further discovery.

Pulmonary Circulation Transvascular Fluid Fluxes Do Not Change during General Anesthesia in Dogs

General anesthesia (GA) can cause abnormal lung fluid redistribution. Pulmonary circulation transvascular fluid fluxes (J_VA) are attributed to changes in hydrostatic forces and erythrocyte volume (EV) regulation. Despite the very low hydraulic conductance of pulmonary microvasculature it is possible that GA may affect hydrostatic forces through changes in pulmonary vascular resistance (PVR), and EV through alteration of erythrocyte transmembrane ion fluxes (_ionJ_VA). Furosemide (Fur) was also used because of its potential to affect pulmonary hydrostatic forces and _ionJ_VA. A hypothesis was tested that J_VA, with or without furosemide treatment, will not change with time during GA. Twenty dogs that underwent castration/ovariectomy were randomly assigned to Fur (n = 10) (4 mg/kg IV) or placebo treated group (Con, n = 10). Baseline arterial (BL) and mixed venous blood were sampled during GA just before treatment with Fur or placebo and then at 15, 30 and 45 min post-treatment. Cardiac output (Q) and pulmonary artery pressure (P_AP) were measured. J_VA and _ionJ_VA were calculated from changes in plasma protein, hemoglobin, hematocrit, plasma and whole blood ions, and Q. Variables were analyzed using random intercept mixed model (P < 0.05). Data are expressed as means ± SE. Furosemide caused a significant volume depletion as evident from changes in plasma protein and hematocrit (P < 0.001). However; Q, P_AP, and J_VA were not affected by time or Fur, whereas erythrocyte fluid flux was affected by Fur (P = 0.03). Furosemide also affected erythrocyte transmembrane K⁺ and Cl⁻, and transvascular Cl⁻ metabolism (P ≤ 0.05). No other erythrocyte transmembrane or transvascular ion fluxes were affected by time of GA or Fur. Our hypothesis was verified as J_VA was not affected by GA or ion metabolism changes due to Fur treatment. Furosemide and 45 min of GA did not cause significant hydrostatic changes based on Q and P_AP. Inhibition of Na⁺/K⁺/2Cl⁻ cotransport caused by Fur treatment, which can alter EV regulation and J_VA, was offset by the Jacobs Stewart cycle. The results of this study indicate that the Jacobs Stewart cycle/erythrocyte Cl⁻ metabolism can also act as a safety factor for the stability of lung fluid redistribution preserving optimal diffusion distance across the blood gas barrier.

CO2 permeability and carbonic anhydrase activity of rat cardiomyocytes

AIM

To determine the CO₂ permeability (P_CO2 ) of plasma membranes of cardiomyocytes. These cells were chosen because heart possesses the highest rate of O₂ consumption/CO₂ production in the body.

METHODS

Cardiomyocytes were isolated from rat hearts using the Langendorff technique. Cardiomyocyte suspensions exhibited a vitality of 2-14% and were studied by the previously described mass spectrometric ¹⁸ O-exchange technique deriving P_CO2 . We showed by mass spectrometry and by carbonic anhydrase (CA) staining that non-vital cardiomyocytes are free of CA and thus do not contribute to the mass spectrometric signal, which is determined exclusively by the fully functional vital cardiomyocytes.

RESULTS

Lysed cardiomyocytes yielded an intracellular CA activity for vital cells of 5070; that is, the rate of CO₂ hydration inside the cell is accelerated 5071-fold. Using this number, analyses of the mass spectrometric recordings from cardiomyocyte suspensions yield a P_CO2 of 0.10 cm s^-1 (SD ± 0.06, n = 15) at 37 °C.

CONCLUSION

In comparison with the P_CO2 of other cells, this value is quite high and about identical to that of the human red cell membrane. As no major protein CO₂ channels such as aquaporins 1 and 4 are present in rat cardiac sarcolemma, the high P_CO2 of this membrane is likely due to its low cholesterol content of about 0.2 (mol cholesterol)·(mol total membrane lipids)^-1 . Previous work predicted a P_CO2 of ≥0.1 cm s^-1 from this level of cholesterol. We conclude that the low cholesterol establishes a P_CO2 high enough to render the membrane resistance to CO₂ diffusion almost negligible, even under conditions of maximal O₂ consumption of the heart.

Acetazolamide and inhaled carbon dioxide reduce periodic breathing during exercise in patients with chronic heart failure

BACKGROUND

Periodic breathing (PB) during sleep and exercise in heart failure (HF) is related to respiratory acid-base status, CO2 chemosensitivity, and temporal dynamics of CO2 and O2 sensing. We studied inhaled CO2 and acetazolamide to alter these factors and reduce PB.

METHODS AND RESULTS

We measured expired and arterial gases and PB amplitude and duration in 20 HF patients during exercise before and after acetazolamide given acutely (500 mg intravenously) and prolonged (24 hours, 2 g orally), and we performed overnight polysomnography. We studied CO2 inhalation (1%-2%) during constant workload exercise. PB disappeared in 19/20 and 2/7 patients during 2% and 1% CO2. No changes in cardiorespiratory parameters were observed after acute acetazolamide. With prolonged acetazolamide at rest: ventilation +2.04 ± 4.0 L/min (P = .001), tidal volume +0.11 ± 1.13 L (P = .003), respiratory rate +1.24 ± 4.63 breaths/min (NS), end-tidal PO2 +4.62 ± 2.43 mm Hg (P = .001), and end-tidal PCO2 -2.59 ± 9.7 mm Hg (P < .001). At maximum exercise: Watts -10% (P < .02), VO2 -61 ± 109 mL/min (P = .04) and VCO2 101 ± 151 mL/min (P < .02). Among 20 patients, PB disappeared in 1 and 7 subjects after acute and prolonged acetazolamide, respectively. PB was present 80% ± 26, 65% ± 28, and 43% ± 39 of exercise time before and after acute and prolonged acetazolamide, respectively. Overnight apnea/hypopnea index decreased from 30.8 ± 83.8 to 21.1 ± 16.9 (P = .003).

CONCLUSIONS

In HF, inhaled CO2 and acetazolamide reduce exercise PB with additional benefits of acetazolamide on sleep PB.