Inhibition of Carbonic Anhydrase Accounts for the Direct Vascular Effects of Hydrochlorothiazide

Abstract

Abstract-Hydrochlorothiazide has been shown to exert direct vasodilator effects by activation of calcium-activated potassium (K Ca ) channels in human and guinea pig isolated resistance arteries. Since hydrochlorothiazide binds to and inhibits the enzyme carbonic anhydrase and because K Ca 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 (pH i ) were measured simultaneously with 2Ј,7Ј-bis(2-carboxyethyl)-5(6)Ј-carboxyfluorescein (BCECF-AM). Bendroflumethiazide, a thiazide diuretic with minimal inhibitory effects on carbonic anhydrase, had little effect on noradrenaline-induced tone (16Ϯ8% relaxation) compared with hydrochlorothiazide (74Ϯ12% relaxation). In contrast to hydrochlorothiazide, the action of bendroflumethiazide was unaffected by 100 nmol/L charybdotoxin, a selective blocker of K Ca channels. All inhibitors of carbonic anhydrase relaxed noradrenaline-induced tone in a concentration-dependent manner, and this effect was blocked by charybdotoxin. Key Words: hydrochlorothiazide Ⅲ carbonic anhydrase inhibition Ⅲ muscle, smooth, vascular Ⅲ pH i Ⅲ potassium channels T hiazide 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 Ca 2ϩ -activated potassium (K Ca ) channels, but not by inhibitors of other vascular K ϩ channels. 2 On the basis of the fact that thiazide-like drugs such as cicletanine and diazoxide lead to hyperpolarization in vascular smooth muscle cells 3 and the fact that hydrochlorothiazide increases 86 Rb efflux as a marker of K ϩ efflux, 2,4 it was proposed that hydrochlorothiazide opens K Ca channels, thereby leading to K ϩ efflux and membrane hyperpolarization. The resultant closure of voltage-dependent Ca 2ϩ channels leads to a fall in [Ca 2ϩ ] i and vasorelaxation. 5 In addition to [Ca 2ϩ ] i , the open state probability of the K Ca channel is also modulated by intracellular pH (pH i ). Channel opening is inhibited by intracellular acidosis in carotid body cells, 6 while in isolated blood vessels intracellular alkalinization leads to relaxation asso...

How do thiazide and thiazide-like diuretics lower blood pressure?

“…It was therefore suggested that the vasorelaxant effects of hydrochlorothiazide were due to inhibition of carbonic anhydrase and an effect of increased pH i on opening of K Ca . However, while this proposal is consistent with the known vasodilator effect of carbonic anhydrase inhibitors, 91,92 and may explain the acute vasodilator effects of thiazides in vivo at high doses, 90 there are two problems with this as an explanation of the mechanism of the action of thiazides as antihypertensive agents. First, the concentrations of thiazides needed to cause vasorelaxation in all these studies are considerably higher than those achieved therapeutically.…”

“…The authors' proposal that thiazides act via K + channels is consistent with an earlier report of increased (86Rb) efflux from ex vivo tail artery The ability of hydrochlorothiazide to reduce intracellular Ca 2+ and tone in noradrenaline-contracted guinea-pig mesenteric arteries was also attributed to K Ca opening. [87][88][89] Further studies 90 showed that the relaxant effects of hydrochlorothiazide were not shared by bendroflumethiazide, but that inhibitors of carbonic anhydrase also relaxed guinea-pig mesenteric arteries via opening K Ca and both hydrochlorothiazide and acetazolamide increased intracellular pH (pH i ). It was therefore suggested that the vasorelaxant effects of hydrochlorothiazide were due to inhibition of carbonic anhydrase and an effect of increased pH i on opening of K Ca .…”

Mechanism underlying the efficacy of combination therapy with losartan and hydrochlorothiazide in rats with salt-sensitive hypertension

“…Although it has been suggested that thiazide-induced vasodilation is mediated by the opening of Ca 2+ -activated K + channels or by Losartan-hydrochlorothiazide therapy Y Yamada et al inhibition of carbonic anhydrase, 34,35 the precise mechanisms by which thiazide-like diuretics inhibit vasoconstriction and vascular growth are still unclear. The predominant activity of thiazide diuretics is to inhibit a directly coupled Na + -Cl -co-transporter along the distal convoluted tubule of the kidney, and this transporter is not expressed by vascular smooth muscle or cardiac tissue.…”

Antidiuretic Effect of Hydrochlorothiazide in Lithium-Induced Nephrogenic Diabetes Insipidus Is Associated with Upregulation of Aquaporin-2, Na-Cl Co-transporter, and Epithelial Sodium Channel

“…Vasopressin may be required for the thiazide-induced increase in AQP2. In the aspect of pharmacologic properties, HCTZ significantly inhibits carbonic anhydrase activity, whereas bendroflumethiazide lacks the carbonic anhydrase inhibiting activity (35).…”