(Hypertension. 1999;33:1043-1048.)
© 1999 American Heart Association, Inc.
Scientific Contributions |
Inhibition of Carbonic Anhydrase Accounts for the Direct Vascular Effects of Hydrochlorothiazide
From the Department of Pharmacology, University Hospital Nijmegen, Nijmegen, Netherlands (P.P., P.S.), and Department of Clinical Pharmacology, National Heart and Lung Institute, Queen Elizabeth the Queen Mother Wing, St. Mary's Hospital, Imperial College of Science, Technology, and Medicine, London, UK (R.S.G., M.S., A.D.H.).
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Abstract |
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Abstract—Hydrochlorothiazide has been shown to exert direct vasodilator effects by activation of calcium-activated potassium (KCa) channels in human and guinea pig isolated resistance arteries. Since hydrochlorothiazide binds to and inhibits the enzyme carbonic anhydrase and because KCa channel activation is pH sensitive, we investigated the role of intracellular and extracellular carbonic anhydrase in the vascular effects of thiazide diuretics. Small arteries were isolated from guinea pig mesentery and studied by use of a microvascular myograph technique. In some experiments, tone and intracellular pH (pHi) were measured simultaneously with 2',7'-bis(2-carboxyethyl)-5(6)'-carboxyfluorescein (BCECF-AM). Bendroflumethiazide, a thiazide diuretic with minimal inhibitory effects on carbonic anhydrase, had little effect on noradrenaline-induced tone (16±8% relaxation) compared with hydrochlorothiazide (74±12% relaxation). In contrast to hydrochlorothiazide, the action of bendroflumethiazide was unaffected by 100 nmol/L charybdotoxin, a selective blocker of KCa channels. All inhibitors of carbonic anhydrase relaxed noradrenaline-induced tone in a concentration-dependent manner, and this effect was blocked by charybdotoxin. Hydrochlorothiazide and the inhibitors of carbonic anhydrase failed to relax tone induced by a depolarizing potassium solution. Acetazolamide and hydrochlorothiazide increased pHi by 0.27±0.07 and 0.21±0.04, respectively, whereas bendroflumethiazide had a much smaller effect: 0.06±0.03. The rise in pHi induced by any agent was not inhibited by charybdotoxin. The vasorelaxant effect of hydrochlorothiazide is shared by other inhibitors of carbonic anhydrase. Inhibitors of carbonic anhydrase, but not bendroflumethiazide, cause intracellular alkalinization, which is associated with KCa channel opening. These data suggest that the vasodilator effect of thiazide diuretics results primarily from inhibition of vascular smooth muscle cell carbonic anhydrase, which results in a rise in pHI, leading to KCa channel activation and vasorelaxation.
Key Words: hydrochlorothiazide • carbonic anhydrase inhibition • muscle, smooth, vascular • pHi • potassium channels
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Introduction |
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Thiazide diuretics were developed in the 1950s by chemical modification of carbonic anhydrase inhibitors. Although their blood pressure–lowering effects have been well documented, their mechanism of action is still not fully resolved. The principal site of action of thiazides is the early segment of the distal nephron, where they inhibit a luminal transmembrane–coupled NaCl transport system. In the long term, thiazides act by reducing peripheral resistance rather than by their diuretic effects,1 and therefore a direct vascular effect has been proposed.
Previous studies in isolated human and guinea pig resistance arteries have established a direct vasodilator activity of hydrochlorothiazide. This vasorelaxant response to hydrochlorothiazide was abolished by charybdotoxin and iberiotoxin, both selective blockers of large-conductance Ca2+-activated potassium (KCa) channels, but not by inhibitors of other vascular K+ channels.2 On the basis of the fact that thiazide-like drugs such as cicletanine and diazoxide lead to hyperpolarization in vascular smooth muscle cells3 and the fact that hydrochlorothiazide increases 86Rb efflux as a marker of K+ efflux,2 4 it was proposed that hydrochlorothiazide opens KCa channels, thereby leading to K+ efflux and membrane hyperpolarization. The resultant closure of voltage-dependent Ca2+ channels leads to a fall in [Ca2+]i and vasorelaxation.5
In addition to [Ca2+]i, the open state probability of the KCa channel is also modulated by intracellular pH (pHi). Channel opening is inhibited by intracellular acidosis in carotid body cells,6 while in isolated blood vessels intracellular alkalinization leads to relaxation associated with hyperpolarization of the cell membrane and a consequent fall of [Ca2+]i.7 At present, how hydrochlorothiazide opens the KCa channel is unknown; it could act by direct interaction with the channel or involve an intermediate intracellular biochemical effect.
Since it is known that most thiazide diuretics bind to and inhibit carbonic anhydrase,8 we hypothesized that a rise in pHi by inhibition of carbonic anhydrase could represent the mechanism of action by which thiazide diuretics open vascular KCa channels and relax resistance arteries. We were able to examine the influence of carbonic anhydrase inhibitor activity since different thiazide compounds exert different degrees of carbonic anhydrase inhibitor activity.9 It was also possible to distinguish between an effect on intracellular or extracellular membrane–bound forms of carbonic anhydrase by the use of lipophilic and hydrophilic carbonic anhydrase inhibitors.9
The vasodilator effects of thiazides may contribute to their antihypertensive properties, and opening of KCa channels by these agents may represent a novel mode of action of these drugs in the vasculature.
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Methods |
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Tissue and Myograph Procedure
After approval of our ethics committee, guinea pig mesentery was removed from male animals (250 to 300 g) killed by cervical dislocation. Mesenteric resistance arteries (n=44 in total; ID, 320±1.9 µm) were dissected free of surrounding tissue and mounted as ring segments in an isometric microvascular myograph. The myograph contained 10 mL physiological saline solution (PSS) (in mmol/L: NaCl 118, KCl 4.7, CaCl2 · 6H2O 2.5, MgSO4 · 7H2O 1.17, NaHCO3 25.0, NaH2PO4 · 2H2O 1.0, Na2EDTA 0.03, glucose 5.5) maintained at 37°C and aerated with 95% O2 and 5% CO2. The vessels were allowed to equilibrate for 1 hour and then were set at a "normalized" internal circumference of 0.9L100, estimated to be 90% of the circumference they would maintain if relaxed and exposed to 100 mm Hg transmural pressure. This was calculated for each individual artery on the basis of the passive length-tension characteristics of the artery and the LaPlace relationship.10 At this setting near-maximal force development can be obtained, and the internal diameters referred to were derived from this calculation.
Before start of the studies, vessels were tested for viability with the use of a depolarizing potassium solution (KPSS: PSS with equimolar substitution of 118 mmol/L KCl for NaCl) and noradrenaline (10 µmol/L). Those vessels failing to produce a tension equivalent to 90 mm Hg to these stimulants were discarded.
Effects of Carbonic Anhydrase Inhibitors on
Vascular Tone
Vessels were precontracted with noradrenaline
(10 µmol/L), and once stable tone was attained,
concentration-response curves (n=4 for each agent;
10-9 to 10-4.5 mol/L)
were constructed for acetazolamide, benzolamide
(hydrophilic), or ethoxzolamide (lipophilic). Because benzolamide has a
lower ether/water partition coefficient and a markedly lower
pKa value (3.2) than acetazolamide
(7.4),9 it is generally assumed that it permeates into
cells very slowly and therefore more or less specifically inhibits the
activation of extracellular carbonic anhydrase.
Interaction Between Carbonic Anhydrase Inhibitors and
Vascular Potassium Channels
It was previously demonstrated5 11 that the
vascular action of hydrochlorothiazide was absent in
vessels precontracted with a depolarizing high-potassium solution and
also was inhibited by charybdotoxin, but not by antagonists
of other potassium channels such as glibenclamide (ATP-dependent
K+ channel) and apamin (small-conductance
KCa channel).2 To demonstrate that
carbonic anhydrase inhibitors (n=4 for each agent; 30
µmol/L) exert direct vasoactivity by a mechanism similar to that of
hydrochlorothiazide, we precontracted some vessels with
noradrenaline in the presence of KPSS. Under these
depolarized conditions, potassium channel activation will have a
negligible effect on membrane potential and therefore should not reduce
calcium entry and vascular tone. If the vasodilation induced by
carbonic anhydrase inhibitors is evoked by
hyperpolarization of vascular smooth muscle due to
increased K+ conductance, its action should be
inhibited by depolarized conditions. Additionally, the vasorelaxant
properties of the carbonic anhydrase inhibitors (30
µmol/L) were compared before and after incubation with charybdotoxin
(n=5; 20 minutes; 100 nmol/L) or glibenclamide (n=5; 20 minutes;
100 µmol/L) in noradrenaline-contracted vessels.
Charybdotoxin is a selective inhibitor of
KCa channels,12 while glibenclamide
is a selective blocker of KATP
channels.13
Interaction Between Carbonic Anhydrase Inhibitors, the
Eicosanoid System, and the Endothelium
The vasorelaxant effect of acetazolamide (30
µmol/L) was determined with or without 30 minutes of preincubation
with 20 µmol/L indomethacin.
Indomethacin is a potent NSAID, and it has been well
established that NSAIDs inhibit prostaglandin synthesis by
blocking the enzyme cyclooxygenase, which is
involved in the generation of prostaglandin from
arachidonic acid.14 In addition, the
effect of endothelial removal was examined in 4
vessels. Endothelium was removed from vessels mounted
in the myograph by passing a hair through the lumen of the
vessel.15 The efficacy of this procedure was confirmed by
abolition of relaxation to endothelium-dependent
vasodilators acetylcholine (10 µmol/L) or substance P (100
nmol/L).
Effects of Thiazide Diuretics on Vascular Tone and Their
Interaction With KCa Channels
If the ability of hydrochlorothiazide to
activate KCa channels and relax
resistance arteries is dependent on its carbonic anhydrase–inhibiting
activity, any vascular effects of bendroflumethiazide, a thiazide that
practically lacks carbonic anhydrase–inhibiting
activity,9 should not be associated with
KCa channel activation. To test this hypothesis,
we compared the vascular effects of each drug (n=8 to 12; 30
µmol/L) and determined whether these effects were inhibited by
charybdotoxin (20 minutes; 100 nmol/L). Because it was previously
established that the vasorelaxant effect of
hydrochlorothiazide is dose dependent,2 11
we used the concentration that elicited the maximal effect.
Measurements of pHi
In some vessels, measurements of pHi were
obtained as described previously.7 16 In brief, vessel
segments were set up in a single-channel myograph dedicated to
fluorescence measurements and incubated with 10 µmol/L
of the acetoxymethyl ester of the pH-sensitive dye
2',7'-bis(carboxyethyl)5(6)'-carboxyfluorescein
(BCECF-AM). Fluorescence was measured with a
Deltascan spectrofluorimeter (Photon Technology International)
connected to an inverted Axiovert 35 fluorescence microscope
(Carl Zeiss) using only quartz objectives (Ultrafluor x10).
pHi was assessed on the basis of the ratio of
fluorescence emission measured at 510 nm, which was evoked by
excitation at 450- and 495-nm light. Emission signals and vascular tone
were measured simultaneously at 1 Hz and acquired online
with an analog/digital interface (Photon Technology International)
connected to an IBM computer. Data were stored on an optical disk and
later analyzed offline with commercially available software
(Photon Technology International). At the end of each experiment, the
ratio was calibrated with 4 solutions (K+/HEPES,
in mmol/L: KCl 140, MgCl2 1.0,
CaCl2 1.6, EDTA 0.026, glucose 10, HEPES 10.0) in
the pH range of 6.8 to 7.4 containing nigericin (10 µmol/L), as
described previously.16 Nigericin is a
K+/H+ ionophore that will
equilibrate intracellular and extracellular pH in high-potassium
buffers. The first solution was applied to the myograph for 7 minutes,
and the subsequent solutions were added for 5 minutes each. With the
use of this technique, a linear regression line could be calculated and
the other intensity ratios could be evaluated to give true pH
readings.
Effect of Acetazolamide,
Hydrochlorothiazide, and Bendroflumethiazide on
pHi
Vessels were prepared as described above, and the effect of
acetazolamide (n=10; 30 µmol/L),
hydrochlorothiazide (n=10; 30 µmol/L), or
bendroflumethiazide (n=10; 30 µmol/L) on
pHi was compared. The effect of
acetazolamide and hydrochlorothiazide on
pHi was also examined after incubation with
charybdotoxin (n=6; 100 nmol/L; 20 minutes).
Drugs and Solutions
Acetazolamide, bendroflumethiazide,
hydrochlorothiazide, indomethacin,
nigericin, and substance P were obtained from Sigma Chemical Company.
Pluronic and preweighed aliquots of BCECF were purchased from Molecular
Probes; one fresh aliquot was used for each experiment. Charybdotoxin
was purchased from Calbiochem. Benzolamide and ethoxzolamide were a
generous gift from Professor Thomas Maren (University of Florida,
Gainesville, Fla). Thiazides and carbonic anhydrase
inhibitors were dissolved in dimethyl sulfoxide. All serial
dilutions were made in distilled water. The final concentration of
dimethyl sulfoxide of 0.1% (vol/vol) had no effect on vessel
reactivity.
Statistics
All data are expressed as mean±SEM, with the number of
observations in parentheses. Statistical significance of values was
tested with a 2-tailed paired Student's t test.
Concentration-response data were fitted to a logistic function by
nonlinear regression, and pD2, the concentration
of drug producing half-maximal response, was calculated.
Concentration-response data were compared in terms of
-log(pD2) and maximum response by Student's
t test for paired data. P<0.05 was
considered significant.
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Results |
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Effects of Carbonic Anhydrase Inhibitors on Vascular Tone
All 3 carbonic anhydrase inhibitors induced a concentration-dependent relaxation of guinea pig vessels (Figure 1). The pD2 values of acetazolamide, benzolamide, and ethoxzolamide were 5.7±0.1, 6.3±0.3, and 7.0±0.3 (n=4 for all), respectively. The vasorelaxant effect was found not to be dependent on the presence of endothelium, and responses to acetazolamide were unaffected by preincubation with indomethacin (20 µmol/L) (maximum relaxation to acetazolamide, 74±8%; in the presence of indomethacin, 82±2%; n=6). The vasorelaxant effect was probably not due to inhibition of intracellular carbonic anhydrase, since the hydrophilic benzolamide exerted a similar response to the lipophilic ethoxzolamide.
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), ethoxzolamide (lipophilic, 
), or
benzolamide (hydrophilic, 
) after noradrenaline
preconstriction. Each value represents the mean±SE percentage
of control response to noradrenaline of 4 arteries.
Interaction Between Carbonic Anhydrase Inhibitors and
Potassium Channels
All carbonic anhydrase inhibitors failed to relax
KPSS-induced tone (n=4). The effect of incubation with charybdotoxin,
an inhibitor of KCa channels, on the
relaxation of the carbonic anhydrase inhibitors is shown in
Figure 2. Incubation with charybdotoxin
(100 nmol/L) had no effect on the subsequent contraction to
noradrenaline. The vasorelaxant effect of all 3 carbonic
anhydrase inhibitors was significantly inhibited by
charybdotoxin. Substance P (100 nmol/L) was used as a control, and its
vasorelaxant effect was not inhibited by charybdotoxin.
Acetazolamide-induced relaxation was unaffected by the
KATP-selective antagonist
glibenclamide; relaxation to acetazolamide was 73.8±8.2%
and 77.5±7.7% in the absence and presence of glibenclamide,
respectively (P=NS).
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)
of charybdotoxin (CTx) (100 nmol/L). Substance P (Sub P) (100 nmol/L)
was used as a control for charybdotoxin. Each value represents
the mean±SE percentage of control response to
noradrenaline of 4 arteries. *Significantly different from
control (P<0.001).
Effects of Thiazide Diuretics on Vascular Tone and Their
Interaction With KCa Channels
In agreement with previous reports,2 5 11 ,
hydrochlorothiazide (30 µmol/L) relaxed guinea
pig vessels (74±12%; P<0.001), and this effect was almost
totally abolished by charybdotoxin (P<0.001). In contrast,
bendroflumethiazide had little effect on vascular tone (relaxation
16±8%; n=12). The small relaxation seen in response to
bendroflumethiazide was not significantly inhibited by charybdotoxin
(Figure 3).
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Effects of Carbonic Anhydrase Inhibitors and Thiazides
on pHi
Resting pHi in isolated guinea pig
mesenteric arteries was 7.18±0.19 (n=15). As shown in Figure 4, the vasorelaxant effect of
acetazolamide and hydrochlorothiazide was
associated with a rise in pHi.
Bendroflumethiazide caused a small rise in pHi,
but this was not statistically significant. In vessels incubated in
charybdotoxin, the acetazolamide- and
hydrochlorothiazide-induced rises in
pHi were not significantly affected (Figure 5).
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Discussion |
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Our experiments were designed to determine to what extent inhibition of carbonic anhydrase by thiazide diuretics accounts for their direct vasodilator effects. We have shown that at clinically relevant concentrations, the vasorelaxant effect of hydrochlorothiazide attributable to KCa channel opening is shared by other agents that inhibit carbonic anhydrase. In contrast, even a high concentration of bendroflumethiazide, a thiazide that practically lacks carbonic anhydrase–inhibiting activity, only minimally affected vascular tone. Furthermore, both hydrochlorothiazide and acetazolamide increased pHi in association with opening the KCa channel, while bendroflumethiazide had minimal effects on either pHi or tone. Since the effect of hydrochlorothiazide and acetazolamide on pHi was also present in vessels preincubated with charybdotoxin, a blocker of the KCa channel, we conclude that the rise in pHi is likely to be a cause and not a consequence of KCa channel activation and/or vasorelaxation. Since we have previously reported that charybdotoxin also inhibits the fall in intracellular calcium induced by hydrochlorothiazide,5 it is also unlikely that changes in intracellular calcium account for the rise in pHi. Therefore, we conclude that inhibition of carbonic anhydrase and the associated rise in pHi may play a primary role in the vasodilator effect of hydrochlorothiazide. Although our findings concentrate on the carbonic anhydrase–inhibiting properties and not on inhibition of the NaCl cotransporter by thiazide diuretics, these mechanisms may be interrelated, since studies in rat distal colon demonstrated that thiazide-induced inhibition of NaCl absorption is directly due to inhibition of mucosal carbonic anhydrase.17 However, to our knowledge NaCl cotransport has not been demonstrated in vascular smooth muscle cells.
Carbonic Anhydrase Inhibition and pHi
Relatively few studies have focused on the effects of inhibition
of carbonic anhydrase on pHi and tone in vascular
smooth muscle cells. In agreement with our findings,
acetazolamide has also been reported to increase
pHi in turtle bladder cells,18
kidney cells,19 choroid plexus epithelial
cells,20 and the mandibular gland.21 Under
different conditions and in different cells, acetazolamide
has also been reported to decrease22 or have no
effect23 on pHi. The mechanism of
action of the acetazolamide-induced rise in
pHi is not completely understood, but most
reports focus on an intracellular accumulation of
HCO324 25 due to inhibition of
Cl/HCO3 exchange. In addition, an
acetazolamide-sensitive inward chloride pump, different
from the Cl/HCO3 exchange and NaKCl
cotransporter, has been reported in rat arterial vascular
smooth muscle cells.26 Others found the same
acetazolamide-induced inhibition of renal
Cl/HCO3 exchange in vivo and suggested that in
the presence of acetazolamide, H+
extrusion continues, but the rate of reaction of
OH- with CO2 is diminished
as a result of carbonic anhydrase inhibition.19 27 In our
experiments the hydrophilic benzolamide was approximately as effective
as acetazolamide and ethoxzolamide, indicating that
inhibition of the extracellular membrane–bound form of carbonic
anhydrase is responsible for the intracellular alkalinization,
KCa channel activation, and vasorelaxation. It is
unclear how inhibition of this enzyme can mediate changes in
pHi, but it seems possible that this
extracellular enzyme might modulate Cl/HCO3
exchange, resulting in an attenuated HCO3
extrusion and increase in pHi. The present
study does not allow definite conclusions on the mechanism of the
acetazolamide-induced increase in
pHi, but our observation that the vascular action
and the pHi effect are shared by a thiazide
diuretic that also exerts carbonic anhydrase–inhibiting
activity and not by a thiazide that lacks this effect suggests that
both effects may be due to inhibition of carbonic anhydrase.
pHi and Vascular Tone
One of the various ways (for review, see Reference 2828 ) in which
changes of pHi could alter the force development
in smooth muscle cells is through potassium channel modulation, since
marked effects of pHi on the
KCa channel have been reported in various
tissues.29 30 31 Since the vasorelaxant action of
hydrochlorothiazide is inhibited by charybdotoxin, we
hypothesized that a pHi change, due to the
carbonic anhydrase–inhibiting activity of the drug, was the trigger
for KCa channel activation. In isolated type I
cells of the neonatal rat carotid body, the K+
current that was inhibited by intracellular acidosis was also inhibited
by charybdotoxin and not by apamin,6 suggesting that the
K+ current is carried through large-conductance
KCa channels. In accordance, the vascular effects
of hydrochlorothiazide have also been reported to be
inhibited by charybdotoxin and iberiotoxin2 5 11 but not
by apamin.2 11
Direct Vasoactivity of Carbonic Anhydrase Inhibitors
Although we used the carbonic anhydrase inhibitors in
the present study as a tool to elucidate the mechanism of action of
thiazide diuretics, the observation that
acetazolamide is a direct vasodilator at clinically
relevant concentrations in isolated resistance arteries is an
interesting finding in itself. The vascular effects of
acetazolamide have been well studied, especially on the
cerebral vasculature,32 but our finding that the
vasorelaxant effect of acetazolamide is associated with a
rise in pHi and KCa channel
activation is completely novel. We found that the dose-dependent
vasorelaxant effect of acetazolamide is caused by opening
of KCa channels and not mediated by other
K+ channels, the eicosanoid system, or the
endothelium.
In vivo, systemic administration of acetazolamide can produce pronounced hypercapnia. Because hypercapnia is a potent dilator of cerebral blood vessels,33 it is possible that the direct vasodilator mechanism that we describe does not account for the cerebral vasodilation in response to acetazolamide. In our experiments in isolated arteries, the maximal response to acetazolamide is reached within minutes, whereas the maximal vasodilator response in the carotid vascular bed after systemic administration of acetazolamide takes up to 1 hour.32 Furthermore, the acetazolamide-induced vasodilation of cerebral vessels appears to be dependent on prostaglandin synthesis but not on nitric oxide release, since it was found to be inhibited by indomethacin34 but not by NG-nitro-L-arginine,35 an inhibitor of nitric oxide synthase. In contrast, our results indicate that the direct vasorelaxant effect of acetazolamide is independent of both local prostaglandin synthesis and the endothelium. It is assumed that cerebral vessels are sensitive to a fall in extracellular pH,36 whereas our data concentrate on the rise in pHi.
In contrast to the well-studied effects of acetazolamide on the cerebral vasculature, not much is known about its direct vascular effects in other vascular beds. Since vascular effects appear to depend on inhibition of carbonic anhydrase activity, this could account for contradictory reports regarding the vasoactivity of acetazolamide. Vascular carbonic anhydrase activity varies from organ to organ and also between species.37 It has been reported that rabbit aorta does not contain carbonic anhydrase activity,38 and, in agreement, thiazide diuretics that possess carbonic anhydrase–inhibiting activity do not relax isolated rabbit arteries (A.D.H., unpublished data, 1996). It was reported that the direct vasorelaxant effects of hydrochlorothiazide are present in human and guinea pig vessels but not in rat resistance arteries,39 and it is of interest that acetazolamide fails to change pHi in rat mesenteric resistance arteries (Aalkjær C., written communication, 1997).
It is difficult to speculate whether the aforementioned properties of acetazolamide and hydrochlorothiazide play a role during long-term administration of the drugs in humans. In the present study we used rather high concentrations of the thiazide diuretics and carbonic anhydrase inhibitors; however, the concentration-response curves show that direct vascular effects are seen at clinically relevant concentrations.39 Furthermore, the antihypertensive effects of thiazide diuretics take several weeks to reach their maximum and also wear off slowly after termination of therapy. This may suggest that slow accumulation in the target organ takes place, especially since the thiazide-like agent indapamide was found at a 9-fold higher concentration in vascular smooth muscle cells than in the plasma.40 Consequently, despite the high concentrations used in this study, it is conceivable that the mechanism described may be relevant to the actions of these agents in vivo after long-term administration.
Conclusion
We have previously shown that hydrochlorothiazide
relaxes human39 and guinea pig2 5 11 39
isolated arteries by opening KCa channels. This
effect of hydrochlorothiazide is not shared by
bendroflumethiazide and seems related to its activity as an
inhibitor of carbonic anhydrase. At clinically relevant
concentrations, other inhibitors of carbonic anhydrase also
relax vascular smooth muscle by activation of KCa
channels associated with an increase in pHi. In
view of the efficacy of the hydrophilic inhibitor
benzolamide, this effect probably does not involve an effect on
intracellular carbonic anhydrase. As a result of our studies, we
propose that acetazolamide and thiazide diuretics
that inhibit carbonic anhydrase activity produce intracellular
alkalosis of vascular smooth muscle cells. The rise in
pHi as a consequence of inhibition of carbonic
anhydrase appears to activate the KCa
channel, resulting in hyperpolarization of the
vascular smooth muscle cell, reduction of voltage-dependent calcium
channel activity, fall in
[Ca2+]i, and
vasorelaxation. It is possible that this novel mechanism of
vasodilation contributes to the antihypertensive action of thiazides in
vivo.
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Acknowledgments |
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This study was supported by a grant from the Dutch Heart Foundation (grant 94.059) (Dr Pickkers).
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Footnotes |
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Reprint requests to Alun D. Hughes, MBBS, PhD, Department of Clinical Pharmacology, National Heart and Lung Institute, Queen Elizabeth the Queen Mother Wing, St. Mary's Hospital, Imperial College of Science, Technology, and Medicine, London W2 1NY, UK.
Received July 21, 1998; first decision August 21, 1998; accepted December 3, 1998.
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