Membrane Permeability Is Required for the Vasodilatory Effect of Carbonic Anhydrase Inhibitors in Porcine Retinal Arteries.

It has been demonstrated previously that a variety of carbonic anhydrase inhibitors (CAIs) can induce vasodilation in pre-contracted retinal arteriolar segments although with different efficacy and potency. Since the CAIs tested so far are able to permeate cell membranes and inhibit both intracellular and extracellular isoforms of the enzyme, it is not clear whether extra- or intracellular isoforms or mechanisms are mediating their vasodilatory effects. By means of small wire myography, we have tested the effects of four new CAIs on wall tension in pre-contracted retinal arteriolar segments that demonstrably do not enter cell membranes but have high affinity to both cytosolic and membrane-bound isoforms of CA. At concentrations between 10-6 M to 10-3 M, none of the four membrane impermeant CAIs had any significant effect on arteriolar wall tension, while the membrane permeant CAI benzolamide (10-3 M) fully dilated all arteriolar segments tested. This suggests that CAI act as vasodilators through cellular mechanisms located in the cytoplasm of vascular cells.

Carbonic Anhydrase Inhibitors of Different Structures Dilate Pre-Contracted Porcine Retinal Arteries.

Carbonic anhydrase inhibitors (CAIs), such as dorzolamide (DZA), are used as anti-glaucoma drugs to lower intraocular pressure, but it has been found that some of these drugs act as vasodilators of retinal arteries. The exact mechanism behind the vasodilatory effect is not yet clear. Here we have addressed the issue by using small vessel myography to examine the effect of CAIs of the sulfonamide and coumarin type on the wall tension in isolated segments of porcine retinal arteries. Vessels were pre-contracted by the prostaglandin analog U-46619, and CAIs with varying affinity for five different carbonic anhydrase (CA) isoenzymes found in human tissue tested. We found that all compounds tested cause a vasodilation of pre-contracted retinal arteries, but with varying efficacy, as indicated by the calculated mean EC50 of each compound, ranging from 4.12 µM to 0.86 mM. All compounds had a lower mean EC50 compared to DZA. The dilation induced by benzolamide (BZA) and DZA was additive, suggesting that they may act on separate mechanisms. No clear pattern in efficacy and affinity for CA isoenzymes could be discerned from the results, although Compound 5, with a low affinity for all isoenzymes except the human (h) CA isoform IV, had the greatest potency, with the lowest EC50 and inducing the most rapid and profound dilation of the vessels. The results suggest that more than one isozyme of CA is involved in mediating its role in controlling vascular tone in retinal arteries, with a probable crucial role played by the membrane-bound isoform CA IV.

Cardioprotection of benzolamide in a regional ischemia model: Role of eNOS/NO.

BACKGROUND: Recent studies from our laboratory show the cardioprotective action of benzolamide (BZ, carbonic anhydrase inhibitor) against ischemia-reperfusion injury. However, the mechanisms involved have not been fully elucidated. OBJECTIVE: To examine the participation of the endothelial nitric oxide synthase (eNOS)/nitric oxide (NO) in the effects of BZ in a model of regional ischemia. METHODS: Isolated rat hearts perfused by Langendorff technique were submitted to 40 min of coronary artery occlusion followed by 60 min of reperfusion (IC). Other hearts received BZ during the first 10 min of reperfusion in absence or presence of L-NAME, NOS inhibitor. The infarct size (IS) and the post-ischemic recovery of myocardial function were measured. Oxidative/nitrosative damage were assessed by reduced glutathione (GSH) content, thiobarbituric acid reactive substances (TBARS) and 3-nitrotyrosine levels. The expression of phosphorylated forms of Akt, p38MAPK and eNOS, and the concentration of inducible nitric oxide synthase (iNOS) were also determined. RESULTS: BZ significantly decreased IS (6.2 ±â€¯0.5% vs. 34 ±â€¯4%), improved post-ischemic contractility, preserved GSH levels and diminished TBARS and 3-nitrotyrosine. In IC hearts, P-Akt, P-p38MAPK and P-eNOS decreased and iNOS increased. After BZ addition the levels of P-kinases and P-eNOS increased and iNOS decreased. Except for P-Akt, P-p38MAPK and iNOS, the effects of BZ were abolished by L-NAME. CONCLUSIONS: Our data demonstrate that the treatment with BZ at the onset of reperfusion was effective to reduce cell death, contractile dysfunction and oxidative/nitrosative damage produced by coronary artery occlusion. These BZ-mediated beneficial actions appear mediated by eNOS/NO-dependent pathways.

Extracellular H+ fluxes from tiger salamander Müller (glial) cells measured using self-referencing H+-selective microelectrodes.

Self-referencing H+-selective electrodes were used to measure extracellular H+ fluxes from Müller (glial) cells isolated from the tiger salamander retina. A novel chamber enabled stable recordings using H+-selective microelectrodes in a self-referencing format using bicarbonate-based buffer solutions. A small basal H+ flux was observed from the end foot region of quiescent cells bathed in 24 mM bicarbonate-based solutions, and increasing extracellular potassium induced a dose-dependent increase in H+ flux. Barium at 6 mM also increased H+ flux. Potassium-induced extracellular acidifications were abolished when bicarbonate was replaced by 1 mM HEPES. The carbonic anhydrase antagonist benzolamide potentiated the potassium-induced extracellular acidification, while 300 µM DIDS, 300 µM SITS, and 30 µM S0859 significantly reduced the response. Potassium-induced extracellular acidifications persisted in solutions lacking extracellular calcium, although potassium-induced changes in intracellular calcium monitored with Oregon Green were abolished. Exchange of external sodium with choline also eliminated the potassium-induced extracellular acidification. Removal of extracellular sodium by itself induced a transient alkalinization, and replacement of sodium induced a transient acidification, both of which were blocked by 300 µM DIDS. Recordings at the apical portion of the cell showed smaller potassium-induced extracellular H+ fluxes, and removal of the end foot region further decreased the H+ flux, suggesting that the end foot was the major source of acidifications. These studies demonstrate that self-referencing H+-selective electrodes can be used to monitor H+ fluxes from retinal Müller cells in bicarbonate-based solutions and confirm the presence of a sodium-coupled bicarbonate transporter, the activity of which is largely restricted to the end foot of the cell.NEW & NOTEWORTHY The present study uses self-referencing H+-selective electrodes for the first time to measure H+ fluxes from Müller (glial) cells isolated from tiger salamander retina. These studies demonstrate bicarbonate transport as a potent regulator of extracellular levels of acidity around Müller cells and point toward a need for further studies aimed at addressing how such glial cell pH regulatory mechanisms may shape neuronal signaling.

Autocrine boost of NMDAR current in hippocampal CA1 pyramidal neurons by a PMCA-dependent, perisynaptic, extracellular pH shift.

The plasma membrane Ca(2+)-ATPase (PMCA) is found near postsynaptic NMDARs. This transporter is a Ca(2+)-H(+) exchanger that raises cell surface pH. We tested whether the PMCA acts in an autocrine fashion to boost pH-sensitive, postsynaptic NMDAR currents. In mouse hippocampal slices, NMDAR EPSCs in a singly activated CA1 pyramidal neuron were reduced when buffering was augmented by exogenous carbonic anhydrase (XCAR). This effect was blocked by the enzyme inhibitor benzolamide and mimicked by the addition of HEPES buffer. Similar EPSC reduction occurred when PMCA activation was prevented by dialysis of BAPTA or the PMCA inhibitor carboxyeosin. Using HEPES, BAPTA, or carboxyeosin, the effect of XCAR was completely occluded. XCAR similarly curtailed NMDAR EPSCs of minimal amplitude, but had no effect on small AMPAR responses. These results indicate that a significant fraction of the postsynaptic NMDAR current is reliant on a perisynaptic extracellular alkaline shift generated by the PMCA.

GPI-anchored carbonic anhydrase IV displays both intra- and extracellular activity in cRNA-injected oocytes and in mouse neurons.

Soluble cytosolic carbonic anhydrases (CAs) are well known to participate in pH regulation of the cytoplasm of mammalian cells. Membrane-bound CA isoforms--such as isoforms IV, IX, XII, XIV, and XV--also catalyze the reversible conversion of carbon dioxide to protons and bicarbonate, but at the extracellular face of the cell membrane. When human CA isoform IV was heterologously expressed in Xenopus oocytes, we observed, by measuring H(+) at the outer face of the cell membrane and in the cytosol with ion-selective microelectrodes, not only extracellular catalytic CA activity but also robust intracellular activity. CA IV expression in oocytes was confirmed by immunocytochemistry, and CA IV activity measured by mass spectrometry. Extra- and intracellular catalytic activity of CA IV could be pharmacologically dissected using benzolamide, the CA inhibitor, which is relatively slowly membrane-permeable. In acute cerebellar slices of mutant mice lacking CA IV, cytosolic H(+) shifts of granule cells following CO(2) removal/addition were significantly slower than in wild-type mice. Our results suggest that membrane-associated CA IV contributes robust catalytic activity intracellularly, and that this activity participates in regulating H(+) dynamics in the cytosol, both in injected oocytes and in mouse neurons.

Dihalogenated sulfanilamides and benzolamides are effective inhibitors of the three ß-class carbonic anhydrases from Mycobacterium tuberculosis.

A series of halogenated sulfanilamides and halogenated benzolamide derivatives have been investigated as inhibitors of three ß-carbonic anhydrases (CAs, EC 4.2.1.1) from the bacterial pathogen Mycobacterium tuberculosis, mtCA 1 (Rv1284), mtCA 2 (Rv3588c) and mtCA 3 (Rv3273). All three enzymes were inhibited with efficacies between the submicromolar to the micromolar one, depending on the substitution pattern at the sulfanilamide moiety/fragment of the molecule. Best inhibitors were the halogenated benzolamides (K(I)s in the range of 0.12-0.45 µM) whereas the halogenated sulfanilamides were slightly less inhibitory (K(I)s in the range of 0.41-4.74 µM). This class of ß-CA inhibitors may have the potential for developing antimycobacterial agents with a diverse mechanism of action compared to the clinically used drugs for which many strains exhibit multi-drug/extensive multi-drug resistance.

Carbonic anhydrases CA4 and CA14 both enhance AE3-mediated Cl--HCO3- exchange in hippocampal neurons.

Carbonic anhydrase (CA) activity in the brain extracellular space is attributable mainly to isoforms CA4 and CA14. In brain, these enzymes have been studied mostly in the context of buffering activity-dependent extracellular pH transients. Yet evidence from others has suggested that CA4 acts in a complex with anion exchangers (AEs) to facilitate Cl(-)-HCO(3)(-) exchange in cotransfected cells. To investigate whether CA4 or CA14 plays such a role in hippocampal neurons, we studied NH(4)(+)-induced alkalinization of the cytosol, which is mitigated by Cl(-) entry and HCO(3)(-) exit. The NH(4)(+)-induced alkalinization was enhanced when the extracellular CAs were inhibited by the poorly permeant CA blocker, benzolamide, or by inhibitory antibodies specific for either CA4 or CA14. The NH(4)(+)-induced alkalinization was also increased with inhibition of anion exchange by 4,4*-diisothiocyanostilbene-2,2*-disulfonic acid, or by eliminating Cl(-) from the medium. No effect of benzolamide was seen under these conditions, in which no Cl(-)-HCO(3)(-) exchange was possible. Quantitative PCR on RNA from the neuronal cultures indicated that AE3 was the predominant AE isoform. Single-cell PCR also showed that Slc4a3 (AE3) transcripts were abundant in isolated neurons. In hippocampal neurons dissociated from AE3-null mice, the NH(4)(+)-induced alkalinization was much larger than that seen in neurons from wild-type mice, suggesting little or no Cl(-)-HCO(3)(-) exchange in the absence of AE3. Benzolamide had no effect on the NH(4)(+)-induced alkalinization in the AE3 knock-out neurons. Our results indicate that CA4 and CA14 both play important roles in the regulation of intracellular pH in hippocampal neurons, by facilitating AE3-mediated Cl(-)-HCO(3)(-) exchange.

CO2 chemosensing in rat oesophagus.

BACKGROUND: Acid in the oesophageal lumen is often sensed as heartburn. It was hypothesised that luminal CO(2), a permeant gas, rather than H(+), permeates through the epithelium, and is converted to H(+), producing an afferent neural signal by activating chemosensors. METHODS: The rat lower oesophageal mucosa was superfused with pH 7.0 buffer, and pH 1.0 or pH 6.4 high CO(2) (P(CO2) = 260 Torr) solutions with or without the cell-permeant carbonic anhydrase (CA) inhibitor methazolamide (MTZ, 1 mM), the cell-impermeant CA inhibitor benzolamide (BNZ, 0.1 mM), the transient receptor potential vanilloid 1 (TRPV1) antagonist capsazepine (CPZ, 0.5 mM) or the acid-sensing ion channel (ASIC) inhibitor amiloride (0.1 mM). Interstitial pH (pH(int)) was measured with 5',6'-carboxyfluorescein (5 mg/kg intravenously) loaded into the interstitial space, and blood flow was measured with laser-Doppler. RESULTS: Perfusion of a high CO(2) solution induced hyperaemia without changing pH(int), mimicking the effect of pH 1.0 perfusion. Perfused MTZ, BNZ, CPZ and amiloride all inhibited CO(2)-induced hyperaemia. CA XIV was expressed in the prickle cells, with CA XII in the basal cells. TRPV1 was expressed in the stratum granulosum and in the muscularis mucosa, whereas all ASICs were expressed in the prickle cells, with ASIC3 additionally in the muscularis mucosa. CONCLUSIONS: The response to CO(2) perfusion suggests that CO(2) diffuses through the stratum epithelium, interacting with TRPV1 and ASICs in the epithelium or in the submucosa. Inhibition of the hyperaemic response to luminal CO(2) by CA, TRPV1 and ASIC inhibitors implicates CA and these chemosensors in transduction of the luminal acid signal. Transepithelial CO(2) permeation may explain how luminal H(+) equivalents can rapidly be transduced into hyperaemia, and the sensation of heartburn.

Endogenous alkaline transients boost postsynaptic NMDA receptor responses in hippocampal CA1 pyramidal neurons.

In hippocampus, activation of the Schaffer collaterals generates an extracellular alkaline transient both in vitro and in vivo. This pH change may provide relief of the H+ block of NMDA receptors (NMDARs) and thereby increase excitability. To test this hypothesis, we augmented extracellular buffering in mouse hippocampal slices by adding 2 microM bovine type II carbonic anhydrase to the superfusate. With addition of enzyme, the alkaline transient elicited by a 10 pulse, 100 Hz stimulus train was reduced by 33%. At a holding potential (V(H)) of -30 mV, the enzyme decreased the half-time of decay and charge transfer of EPSCs by 32 and 39%, respectively, but had no effect at a V(H) of -80 mV. In current clamp, a 10 pulse, 100 Hz stimulus train gave rise to an NMDAR-dependent afterdepolarization (ADP). Exogenous enzyme curtailed the ADP half-width and voltage integral by 20 and 25%, respectively. Similar reduction of the ADP was noted with a brief 12 Hz stimulus train. The effect persisted in the presence of GABAergic antagonists or the L-type Ca2+ channel blocker methoxyverapamil hydrochloride but was absent in the presence of the carbonic anhydrase inhibitor benzolamide or when the exogenous enzyme was heat inactivated. The effects of the enzyme in voltage and current clamp were noted in 0 Mg2+ media but were abolished when (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine maleate was included in the patch pipette. These results provide strong evidence that endogenous alkaline transients are sufficiently large in the vicinity of the synapse to augment NMDAR responses.

Role of carbonic anhydrase IV in corneal endothelial HCO3- transport.

PURPOSE: Carbonic anhydrase activity has a central role in corneal endothelial function. The authors examined the role of carbonic anhydrase IV (CAIV) in facilitating CO(2) flux, HCO(3)(-) permeability, and HCO(3)(-) flux across the apical membrane. METHODS: Primary cultures of bovine corneal endothelial cells were established on membrane-permeable filters. Apical CAIV was inhibited by benzolamide or siRNA knockdown of CAIV. Apical CO(2) fluxes and HCO(3)(-) permeability were determined by measuring pH(i) changes in response to altering the CO(2) or HCO(3)(-) gradient across the apical membrane. Basolateral to apical (B-to-A) HCO(3)(-) flux was determined by measuring the pH of a weakly buffered apical bath in the presence of basolateral bicarbonate-rich Ringer solution. In addition, the effects of benzolamide and CAIV knockdown on steady state DeltapH (apical-basolateral compartment pH) after 4-hour incubation in DMEM were measured. RESULTS: CAIV expression was confirmed, and CAIV was localized exclusively to the apical membrane by confocal microscopy. Both 10 microM benzolamide and CAIV siRNA reduced apparent apical CO(2) flux by approximately 20%; however, they had no effect on HCO(3)(-) permeability or HCO(3)(-) flux. The steady state apical-basolateral pH gradient at 4 hours was reduced by 0.12 and 0.09 pH units in benzolamide- and siRNA-treated cells, respectively, inconsistent with a net cell-to-apical compartment CO(2) flux. CONCLUSIONS: CAIV does not facilitate steady state cell-to-apical CO(2) flux, apical HCO(3)(-) permeability, or B-to-A HCO(3)(-) flux. Steady state pH changes, however, suggest that CAIV may have a role in buffering the apical surface.

Benzolamide perpetuates acidic conditions during reperfusion and reduces myocardial ischemia-reperfusion injury.

During ischemia, increased anaerobic glycolysis results in intracellular acidosis. Activation of alkalinizing transport mechanisms associated with carbonic anhydrases (CAs) leads to myocardial intracellular Ca2+ increase. We characterize the effects of inhibition of CA with benzolamide (BZ) during cardiac ischemia-reperfusion (I/R). Langendorff-perfused isolated rat hearts were subjected to 30 min of global ischemia and 60 min of reperfusion. Other hearts were treated with BZ (5 µM) during the initial 10 min of reperfusion or perfused with acid solution (AR, pH 6.4) during the first 3 min of reperfusion. p38MAPK, a kinase linked to membrane transporters and involved in cardioprotection, was examined in hearts treated with BZ in presence of the p38MAPK inhibitor SB202190 (10 µM). Infarct size (IZ) and myocardial function were assessed, and phosphorylated forms of p38MAPK, Akt, and PKCε were evaluated by immunoblotting. We determined the rate of intracellular pH (pHi) normalization after transient acid loading in the absence and presence of BZ or BZ + SB202190 in heart papillary muscles (HPMs). Mitochondrial membrane potential (ΔΨm), Ca2+ retention capacity and Ca2+-mediated swelling after I/R were also measured. BZ, similarly to AR, reduced IZ, improved postischemic recovery of myocardial contractility, increased phosphorylation of Akt, PKCε, and p38MAPK, and normalized ΔΨm and Ca2+ homeostasis, effects abolished after p38MAPK inhibition. In HPMs, BZ slowed pHi recovery, an effect that was restored after p38MAPK inhibition. We conclude that prolongation of acidic conditions during reperfusion by BZ could be responsible for the cardioprotective benefits of reduced infarction and better myocontractile function, through p38MAPK-dependent pathways. NEW & NOTEWORTHY Carbonic anhydrase inhibition by benzolamide (BZ) maintains acidity, decreases infarct size, and improves postischemic myocardial dysfunction in ischemia-reperfusion (I/R) hearts. Protection afforded by BZ mimicked the beneficial effects elicited by an acidic solution (AR). Increased phosphorylation of p38MAPK occurs in I/R hearts reperfused with BZ or with AR. Mitochondria from I/R hearts possess abnormal Ca2+ handling and a more depolarized membrane potential compared with control hearts, and these changes were restored by treatment with BZ or AR.

Temporal adjustment of the juxtaglomerular apparatus during sustained inhibition of proximal reabsorption.

Tubuloglomerular feedback (TGF) stabilizes nephron function by causing changes in single-nephron GFR (SNGFR) to compensate for changes in late proximal flow (VLP). TGF responds within seconds and reacts over a narrow range of VLP that surrounds normal VLP. To accommodate sustained increases in VLP, TGF must reset around the new flow. We studied TGF resetting by inhibiting proximal reabsorption with benzolamide (BNZ; administered repeatedly over a 24-hour period) in Wistar-Froemter rats. BNZ acutely activates TGF, thereby reducing SNGFR. Micropuncture was performed 6-10 hours after the fourth BNZ dose, when diuresis had subsided. BNZ caused glomerular hyperfiltration, which was prevented with inhibitors of macula densa nitric oxide synthase (NOS). Because of hyperfiltration, BNZ increased VLP and distal flow, but did not affect the basal TGF stimulus (early distal salt concentration). BNZ slightly blunted normalized maximum TGF response and the basal state of TGF activation. BNZ sensitized SNGFR to reduction by S-methyl-thiocitrulline (SMTC) and caused the maximum TGF response to be strengthened by SMTC. Sensitization to type I NOS (NOS-I) blockers correlated with increased macula densa NOS-I immunoreactivity. Tubular transport measurements confirmed that BNZ affected TGF within the juxtaglomerular apparatus. During reduced proximal reabsorption, TGF resets to accommodate increased flow and SNGFR through a mechanism involving macula densa NOS.

Studies on the tubulo-glomerular feedback system in the rat. The mechanism of reduction in filtration rate with benzolamide.

The specific mechanism whereby superficial nephron glomerular filtration rate (sngfr) is reduced after the administration of benzolamide, a carbonic anhydrase inhibitor with a primary inhibitory effect in the proximal tubule, have been examined by measuring pertinent pressures, flows, and glomerular permeabilities in the hydropenic Munich-Wistar rat, a strain with surface glomeruli. Because benzolamide decreases absolute proximal reabsorptive rate, the rate of delivery of tubular fluid to the distal nephron should be at least transiently increased and may reduce sngfr by activating the tubulo-glomerular feedback system. Sngfr fell from 29.2+/2.0 to 2.1+/3.1 nl/min (P less than 0.01) after benzolamide (group 1), a percentage reduction equal to kidney glomerular filtration rate and similar to sngfr obtained in collections from distal tubules. Separate studies (group 2) revealed that if transient increases in distal nephron delivery were prevented by insertion of a long oil block in proximal tubules before control, the decrease in sngfr was prevented (30.3+/1.0 vs. 30.3+/1.8 nl/min, P greater than 0.9). In paired "unblocked" nephrons in the same rats, sngfr fell in group 2 (33.0+/1.0 vs. 25.2+/2.3 nl/min, P less than 0.01). In "blocked" nephrons in which sngfr reduction was prevented, the rate of fluid leaving the proximal tubule increased from 16.9+/ to 23.1+/1.0 nl/min (P less than 0.01). In group 1 studies in which sngfr fell and transient increases in flow out of the last segment of the proximal tubule (distal delivery) (approximately equal to 8 nl/min) were not prevented, steady-state distal delivery was unchanged by benzolamide (13.9+/1.1 vs. 14.2+/2.2 nl/min). Also, sngfr returned toward control, pre-benzolamide values, when a proximal oil block was placed for 15 min and the rate of distal delivery reduced after benzolamide administration, which suggests that this activation was reversible. These data suggest that activation of tubulo-glomerular feedback by transient increases in distal delivery was responsible for decreases in sngfr. Analysis of all determinants of glomerular ultra-filtration revealed that the efferent mechanism leading to reduced sngfr after benzolamide was decreased nephron plasma flow (101+/13 vs. 66+/13 nl/min, P less than 0.01). Hydrostatic pressure and the glomerular permeability coefficient did not contribute to reductions in sngfr with benzolamide. Because the rate of distal delivery remained constant in spite of large changes in both sngfr and absolute proximal reabsorptive rate, it is suggested that the rate of distal delivery may be the physiologic entity that is regulated by the tubulo-glomerular feedback system via alterations in sngfr.

Effects of carbonic anhydrase inhibition on ventilation-perfusion matching in the dog lung.

Lung carbonic anhydrase (CA) permits rapid pH responses when changes in regional ventilation or perfusion alter airway and alveolar PCO2. These pH changes affect airway and vascular resistances and lung compliance to optimize the balance of regional ventilation (VA) and perfusion (Q) in the lung. To test the hypothesis that these or other CA-dependent mechanisms contribute to VA/Q matching, we administered acetazolamide (25 mg/kg intravenously) to six anesthetized and paralyzed dogs and measured VA/Q relationships before and after CA inhibition by the multiple inert gas elimination technique. Four other groups of dogs were studied to control for possible confounding effects of time under anesthesia and nonselective CA inhibition by acetazolamide: (a) saline placebo as a control for duration of anesthesia, (b) 4% CO2 inhalation to mimic systemic CO2 retention, (c) 1 mg/kg benzolamide (a selective renal CA inhibitor) or 0.5 meq/kg HCl to mimic systemic metabolic acidosis, and (d) 500 mg/kg 4,4'-dinitrostilbene-2,2'-disulfonate (an inhibitor of red cell band 3 protein) to mimic the respiratory acidosis arising from an intracapillary block to rapid mobilization of plasma HCO3- in CO2 exchange. Acetazolamide increased VA/Q mismatch and reduced arterial PO2 measured at equilibrium but these did not occur in the control group. There was no deterioration in VA/Q matching when systemic respiratory acidosis produced either by CO2 inhalation or 4,4'-dinitrostilbene-2,2'-disulfonate or metabolic acidosis (benzolamide or HCl) were imposed to mimic the effects of acetazolamide apart from its inhibition of lung CA. These results support the concept that lung CA subserves VA/Q matching in the normal lung.

Combined effects of carbonic anhydrase inhibitor and adenosine A1 receptor antagonist on hemodynamic and tubular function in the kidney.

BACKGROUND: Carbonic anhydrase inhibitors (CAI) reduce proximal reabsorption, activating tubuloglomerular feedback (TGF) and reducing glomerular filtration rate (GFR). Adenosine A(1) receptors (A(1)R) mediate the TGF response and stimulate proximal reabsorption. METHODS: Clearance and micropuncture studies were performed in Wistar rats to determine whether blockade of A(1)R (KW3902 0.3 mg/kg i.v.) would prevent CAI (benzolamide 5 mg/kg i.v.) from lowering GFR, whether CAI and KW3902 exert additive effects on sodium excretion, and to what extent such interactions depend on events in the glomerulus, proximal tubule, or distal nephron. RESULTS: KW3902 raised GFR and prevented CAI from lowering GFR. KW3902 and CAI caused additive diuresis and natriuresis. KW3902 and CAI increased lithium clearance, but their effects were redundant. CAI increased the dependence of proximal reabsorption on active chloride transport. KW3902, alone, did likewise, but to a lesser extent than CAI. Adding KW3902 to CAI lessened the shift toward active chloride transport. CONCLUSIONS: The data reveal that A(1)R mediate glomerular vascular resistance whether or not TGF is activated, that additive effects of CAI and KW3902 on salt excretion occur, in part, because KW3902 inhibits reabsorption downstream from the macula densa, and that KW3902 likely inhibits proximal reabsorption by interfering with apical sodium-hydrogen exchange.

Carbonic anhydrase inhibitors reduce cardiac dysfunction after sustained coronary artery ligation in rats.

BACKGROUND: Two potent carbonic anhydrase (CA) inhibitors with widely differing membrane permeability, poorly diffusible benzolamide (BZ), and highly diffusible ethoxzolamide (ETZ) were assessed to determine whether they can reduce cardiac dysfunction in rats subjected to coronary artery ligation (CAL)-induced myocardial infarction. METHODS AND RESULTS: Rats with evidence of heart failure (HF) at 32 weeks following a permanent left anterior coronary artery occlusion were treated with placebo, BZ, or ETZ (4 mg kgday-1) for 4 weeks at which time left ventricular function and structure were evaluated. Lung weight/body weight (LW/BW) ratio increased in CAL rats by 17±1% vs. control, suggesting pulmonary edema. There was a trend for BZ and ETZ to ameliorate the increase in LW/BW by almost 50% (9±5% and 9±8%, respectively, versus CAL) (P=.16, NS). Echocardiographic assessment showed decreased left ventricular midwall shortening in HF rats, 21±1% vs. control 32±1%, which was improved by BZ to 29±1% and ETZ to 27±1%, and reduced endocardial shortening in HF rats 38±3% vs. control 62±1%, partially restored by BZ and ETZ to ~50%. Expression of the hypoxia-inducible membrane-associated CAIX isoform increased by ~60% in HF rat hearts, and this effect was blocked by ETZ. CONCLUSIONS: We conclude that CAL-induced myocardial interstitial fibrosis and associated decline in left ventricular function were diminished with BZ or ETZ treatment. The reductions in cardiac remodeling in HF with both ETZ and BZ CA inhibitors suggest that inhibition of a membrane-bound CA appears to be the critical site for this protection.

Interstitial carbonic anhydrase (CA) activity in brain is attributable to membrane-bound CA type IV.

We tested the hypothesis that extracellular membrane-bound carbonic anhydrase (CA) type IV is responsible for the regulation of interstitial pH (pH(o)) transients in brain. Rat hippocampal slices were incubated in phosphatidylinositol-specific phospholipase C (PI-PLC), which cleaves the link of CA IV to the external face of plasma membranes. Then evoked alkaline pH(o) shifts were studied in a recording chamber, using pH microelectrodes. Incubation fluid was saved for later analysis. The ability to buffer a rapid alkaline load was reduced markedly in PI-PLC-treated tissue as compared with adjacent, paired control slices. The effect of benzolamide (a poorly permeant CA inhibitor) on evoked pH(o) shifts was diminished greatly in the PI-PLC-treated tissue, consistent with the washout of interstitial CA. Treatment of the incubation fluid with SDS abolished nearly all of the CA activity in fluid from controls, whereas an SDS-insensitive component remained in the fluid from PI-PLC-treated slices. These data suggested that CA type II (which is blocked by SDS) leaked from injured glial cells in both slice preparations, whereas CA type IV (which is insensitive to SDS) was liberated selectively into the fluid from PI-PLC-treated tissue. Western blot analysis was consistent with this interpretation, demonstrating a predominance of CA IV in the incubation fluid from PI-PLC-treated tissue and variable amounts of CA II in fluid from PI-PLC-treated and control slices. These results demonstrate that interstitial CA activity brain is attributable principally to membrane-bound CA IV.

Intrinsic proton modulation of excitatory transmission in rat hippocampal slices.

Recent work suggests that activity-induced alkaline transients within the interstitial space of nervous tissue are largely due to net fluxes of acid-base equivalents across postsynaptic receptor-gated ion channels. In view of the marked pH sensitivity of certain receptor channels, it has been frequently postulated that synaptically-evoked H+ shifts might play a neuromodulatory role. We provide here the first evidence to support the above hypothesis in showing that extracellularly recorded glutamatergic responses in area CA1 of rat hippocampal slices are potentiated upon inhibition of fast extracellular H+ buffering by a poorly-permeant carbonic anhydrase inhibitor, benzolamide (10 microM). Experiments with glutamate receptor antagonists and Mg(2+)-free solutions suggest that the action of benzolamide is mediated by the H+ sensitivity of N-methyl-D-aspartate (NMDA) receptor channels. In further agreement with an intrinsic neuromodulatory role for H+ in excitatory transmission, addition of the H+ buffer HEPES (20 mM) produced a selective attenuation of pharmacologically-isolated NMDA receptor-mediated responses.

Kinetics of CO2 excretion and intravascular pH disequilibria during carbonic anhydrase inhibition.

Inhibition of carbonic anhydrase (CA) activity (activity in red blood cells and activity available on capillary endothelium) results in decrements in CO2 excretion (VCO2) and plasma-erythrocyte CO2-HCO(-3)-H+ disequilibrium as blood travels around the circulation. To investigate the kinetics of changes in blood PCO2 and pH during progressive CA inhibition, we used our previously detailed mathematical model of capillary gas exchange to analyze experimental data of VCO2 and blood-gas/pH parameters obtained from anesthetized, paralyzed, and mechanically ventilated dogs after treatment with acetazolamide (Actz, 0-100 mg/kg i.v.). Arterial and mixed venous blood samples were collected via indwelling femoral and pulmonary arterial catheters, respectively. Cardiac output was measured by thermodilution. End-tidal PCO2, as a measure of alveolar PCO2, was obtained from continuous records of airway PCO2 above the carina. Experimental results were analyzed with the aid of a mathematical model of lung and tissue-gas exchange. Progressive CA inhibition was associated with stepwise increments in the equilibrated mixed venous-alveolar PCO2 gradient (9, 19, and 26 Torr at 5, 20, and 100 mg/kg Actz, respectively). The maximum decrements in VCO2 were 10, 24, and 26% with 5, 20, and 100 mg/kg Actz, respectively, without full recovery of VCO2 at 1 h postinfusion. Equilibrated arterial PCO2 overestimated alveolar PCO2, and tissue PCO2 was underestimated by the measured equilibrated mixed venous blood PCO2. Mathematical model computations predicted hysteresis loops of the instantaneous CO2-HCO(-3)-H+ relationship and in vivo blood PCO2-pH relationship due to the finite reaction times for CO2-HCO(-3)-H+ reactions. The shape of the hysteresis loops was affected by the extent of Actz inhibition of CA in red blood cells and plasma.