Stimulation of intracellular chloride accumulation by noradrenaline and hence potentiation of its depolarization of rat arterial smooth muscle in vitro

Increased intracellular Cl- concentration in pulmonary arterial myocytes is associated with chronic hypoxic pulmonary hypertension

Chloride channels play an important role in regulating smooth muscle contraction and proliferation, and contribute to the enhanced constriction of pulmonary arteries (PAs) in pulmonary hypertension (PH). The intracellular Cl^- concentration ([Cl^-]_i), tightly regulated by various Cl^- transporters, determines the driving force for Cl^- conductance, thereby the functional outcome of Cl^- channel activation. This study characterizes for the first time the expression profile of Cl^- transporters/exchangers in PA smooth muscle and provides the first evidence that the intracellular Cl^- homeostasis is altered in PA smooth muscle cells (PASMCs) associated with chronic hypoxic PH (CHPH). Quantitative RT-PCR revealed that the endothelium-denuded intralobar PA of rats expressed Slc12a gene family-encoded Na-K-2Cl cotransporter 1 (NKCC1), K-Cl cotransporters (KCC) 1, 3, and 4, and Slc4a gene family-encoded Na⁺-independent and Na⁺-dependent Cl^-/HCO₃^- exchangers. Exposure of rats to chronic hypoxia (10% O₂, 3 wk) caused CHPH and selectively increased the expression of Cl^--accumulating NKCC1 and reduced the Cl^--extruding KCC4. The intracellular Cl^- concentration ([Cl^-]_i) averaged at 45 mM and 47 mM in normoxic PASMCs as determined by fluorescent indicator MEQ and by gramicidin-perforated patch-clamp technique, respectively. The ([Cl^-]_i was increased by ∼10 mM in PASMCs of rats with CHPH. Future studies are warranted to further establish the hypothesis that the altered intracellular Cl^- homeostasis contributes to the pathogenesis of CHPH.

Role of the Cation-Chloride-Cotransporters in Cardiovascular Disease

The SLC12 family of cation-chloride-cotransporters (CCCs) is comprised of potassium chloride cotransporters (KCCs), which mediate Cl- extrusion and sodium-potassium chloride cotransporters (N[K]CCs), which mediate Cl- loading. The CCCs play vital roles in cell volume regulation and ion homeostasis. The functions of CCCs influence a variety of physiological processes, many of which overlap with the pathophysiology of cardiovascular disease. Although not all of the cotransporters have been linked to Mendelian genetic disorders, recent studies have provided new insights into their functional role in vascular and renal cells in addition to their contribution to cardiovascular diseases. Particularly, an imbalance in potassium levels promotes the pathogenesis of atherosclerosis and disturbances in sodium homeostasis are one of the causes of hypertension. Recent findings suggest hypothalamic signaling as a key signaling pathway in the pathophysiology of hypertension. In this review, we summarize and discuss the role of CCCs in cardiovascular disease with particular emphasis on knowledge gained in recent years on NKCCs and KCCs.

Spontaneous and α-adrenoceptor-induced contractility in human collecting lymphatic vessels require chloride

Human lymphatic vessels are myogenically active and respond to sympathetic stimulation. The role of various cations in this behavior has recently been investigated, but whether the anion Cl^- is essential is unclear. With ethical approval and informed consent, human thoracic duct and mesenteric lymphatic vessels were obtained from surgical patients. Spontaneous or norepinephrine-induced isometric force production from isolated vessels was measured by wire myography; the transmembrane Cl^- gradient and Cl^- channels were investigated by substitution of extracellular Cl^- with the impermeant anion aspartate and inhibition of Cl^- transport and channels with the clinical diuretics furosemide and bendroflumethiazide as well as DIDS and 5-nitro-2-(3-phenylpropylamino)benzoic acid. The molecular expression of Ca²⁺-activated Cl^- channels was investigated by RT-PCR, and proteins were localized using immunoreactivity. Spontaneous and norepinephrine-induced contractility in human lymphatic vessels was highly abrogated after Cl^- substitution with aspartate. About 100-300 µM DIDS or 5-nitro-2-(3-phenylpropylamino)benzoic acid inhibited spontaneous contractile behavior. Norepinephrine-stimulated tone was furthermore markedly abrogated by 200 µM DIDS. Furosemide lowered only spontaneous constrictions, whereas bendroflumethiazide had nonspecific inhibitory effects. Consistent expression of transmembrane member 16A [TMEM16A (anoctamin-1)] was found in both the thoracic duct and mesenteric lymphatic vessels, and immunoreactivity with different antibodies localized TMEM16A to lymphatic smooth muscle cells and interstitial cells. The significant change in contractile function observed with inhibitors and anion substitution suggests that Cl^- movement over the plasma membrane of lymphatic myocytes is integral for spontaneous and α-adrenoceptor-evoked contractility in human collecting lymphatic vessels. Consistent detection and localization of TMEM16A to myocytes suggests that this channel could play a major functional role. NEW & NOTEWORTHY In this study, we report the first observations of Cl^- being a critical ionic component of spontaneous and agonist-evoked contractility in human lymphatics. The most consistently expressed Ca²⁺-activated Cl^- channel gene in the human thoracic duct and mesenteric lymphatic vessels appears to be transmembrane member 16A, suggesting that this channel plays a major role.

New selective inhibitors of calcium-activated chloride channels - T16A(inh) -A01, CaCC(inh) -A01 and MONNA - what do they inhibit?

BACKGROUND AND PURPOSE

T16A(inh)-A01, CaCC(inh)-A01 and MONNA are identified as selective inhibitors of the TMEM16A calcium-activated chloride channel (CaCC). The aim of this study was to examine the chloride-specificity of these compounds on isolated resistance arteries in the presence and absence (±) of extracellular chloride.

EXPERIMENTAL APPROACH

Isolated resistance arteries were maintained in a myograph and tension recorded, in some instances combined with microelectrode impalement for membrane potential measurements or intracellular calcium monitoring using fura-2. Voltage-dependent calcium currents (VDCC) were measured in A7r5 cells with voltage-clamp electrophysiology using barium as a charge carrier.

KEY RESULTS

Rodent arteries preconstricted with noradrenaline or U46619 were concentration-dependently relaxed by T16A(inh) -A01 (0.1-10 μM): IC50 and maximum relaxation were equivalent in ±chloride (30 min aspartate substitution) and the T16A(inh) -A01-induced vasorelaxation ±chloride were accompanied by membrane hyperpolarization and lowering of intracellular calcium. However, agonist concentration-response curves ±chloride, with 10 μM T16A(inh) -A01 present, achieved similar maximum constrictions although agonist-sensitivity decreased. Contractions induced by elevated extracellular potassium were concentration-dependently relaxed by T16A(inh)-A01 ±chloride. Moreover, T16A(inh) -A01 inhibited VDCCs in A7r5 cells in a concentration-dependent manner. CaCC(inh) -A01 and MONNA (0.1-10 μM) induced vasorelaxation ±chloride and both compounds lowered maximum contractility. MONNA, 10 μM, induced substantial membrane hyperpolarization under resting conditions.

CONCLUSIONS AND IMPLICATIONS

T16A(inh) -A01, CaCC(inh) -A01 and MONNA concentration-dependently relax rodent resistance arteries, but an equivalent vasorelaxation occurs when the transmembrane chloride gradient is abolished with an impermeant anion. These compounds therefore display poor selectivity for TMEM16A and inhibition of CaCC in vascular tissue in the concentration range that inhibits the isolated conductance.

INVOLVEMENT OF Na+-K+-2Cl−COTRANSPORTERS IN HYPERTONICITY-INDUCED RISE IN INTRACELLULAR CALCIUM CONCENTRATION

A hypertonic saline containing propylene glycol facilitates calcium (Ca(2+)) influx through voltage-dependent Ca(2+) channels. The present study performed experiments to elucidate the mechanism by which Na(+)-K(+)-2Cl(-) cotransporters participate in the rise in the intracellular calcium concentration ([Ca(2+)]i) under the hypertonic condition. Both furosemide and ethacryonic acid significantly decreased the [Ca(2+)]i raised by hypertonicity. Similarly, Na(+)-, K(+)-, or Cl(-)-free saline also reduced it. Both norepinephrine and dopamine significantly enhanced the rise in [Ca(2+)]i. In conclusion, the findings obtained indicate that the Na+-K+-2Cl- cotransporters evoke cell depolarization and that this depolarization raises the [Ca(2+)]i by activating voltage-dependent Ca(2+) channels.

Regulation of proliferation and membrane potential by chloride currents in rat pulmonary artery smooth muscle cells

Pulmonary artery smooth muscle cell (PASMC) proliferation contributes to increased pulmonary vascular resistance and pulmonary hypertension. Because proliferation depends on membrane potential (V(m)) and because V(m) is, in part, determined by Cl(-) currents (I(Cl)), we examined the effects of I(Cl) inhibition with 4,4;-diisothiocyanatostilbene-2,2;-disulfonic acid (DIDS) on cultured rat PASMCs. DIDS (30 mumol/L) reduced cell numbers, decreased 5-bromodeoxyuridine incorporation and delayed cell cycle progression. I(Cl) inhibition with 5-Nitro-2-(3-phenylpropylamino) benzoic acid (100 mumol/L) also reduced cell numbers of cultured rat PASMCs. To test the possible involvement of I(Cl) in the regulation of PASMC proliferation, we measured V(m) and I(Cl) in both cultured (proliferating) and acutely dissociated (nonproliferating) rat PASMCs. V(m) (-39.3+/-1.4 mV) was close to the equilibrium potential of Cl(-) (-39 mV) in proliferating PASMCs but differed from equilibrium potential of Cl(-) in acutely dissociated cells (-45.3+/-0.9 mV). DIDS and substitution of extracellular Cl(-) with I(-) induced V(m) hyperpolarization in proliferating but not nonproliferating PASMCs. Consistent with V(m) recordings, DIDS-sensitive baseline and swelling-activated (Ca(2+)-independent) I(Cl)s, recorded with low Ca(2+) (<1 nmol/L) pipette solutions, were approximately 5-fold greater in proliferating than in nonproliferating PASMCs. By contrast, Ca(2+)-activated I(Cl) did not differ between proliferating and nonproliferating PASMCs. Ca(2+)-independent I(Cl)s were also increased in proliferating PASMCs acutely dissociated from rats exposed to hypoxia (10% O(2); 7 days). These findings are consistent with the conclusion that I(Cl)s regulate proliferation of PASMCs and suggest that selective I(Cl) inhibition may be useful in treating pulmonary hypertension.

Effect of the Na-K-2Cl cotransporter NKCC1 on systemic blood pressure and smooth muscle tone

Studies in rat aorta have shown that the Na-K-2Cl cotransporter NKCC1 is activated by vasoconstrictors and inhibited by nitrovasodilators, contributes to smooth muscle tone in vitro, and is upregulated in hypertension. To determine the role of NKCC1 in systemic vascular resistance and hypertension, blood pressure was measured in rats before and after inhibition of NKCC1 with bumetanide. Intravenous infusion of bumetanide sufficient to yield a free plasma concentration above the IC(50) for NKCC1 produced an immediate drop in blood pressure of 5.2% (P < 0.001). The reduction was not prevented when the renal arteries were clamped, indicating that it was not due to a renal effect of bumetanide. Bumetanide did not alter blood pressure in NKCC1-null mice, demonstrating that it was acting specifically through NKCC1. In third-order mesenteric arteries, bumetanide-inhibitable efflux of (86)Rb was acutely stimulated 133% by phenylephrine, and bumetanide reduced the contractile response to phenylephrine, indicating that NKCC1 influences tone in resistance vessels. The hypotensive effect of bumetanide was proportionately greater in rats made hypertensive by a 7-day infusion of norepinephrine (12.7%, P < 0.001 vs. normotensive rats) but much less so when hypertension was produced by a fixed aortic coarctation (8.0%), again consistent with an effect of bumetanide on resistance vessels rather than other determinants of blood pressure. We conclude that NKCC1 influences blood pressure through effects on smooth muscle tone in resistance vessels and that this effect is augmented in hypertension.

Chronic nitric oxide blockade modulates renal Na–K–2Cl cotransporters

OBJECTIVE

The Na-K-2Cl cotransporter (NKCC2 isoform) of the thick ascending limb of Henle's loop (TAL) plays an important role in renal sodium handling, and the vascular isoform (NKCC1) participates in the response to vasoconstrictors. Both isoforms appear to be regulated by nitric oxide. This study aimed to analyze the effect of chronic nitric oxide deficiency on tubular and vascular Na-K-2Cl cotransporters in kidney and their potential role in the development of N-nitro-L-arginine-methyl ester (L-NAME) hypertension.

METHODS

Wistar rats were given L-NAME (vehicle, 10, 35 and 80 mg/100 ml drinking water) for 4 weeks. Blood pressure was measured by the tail-cuff method. NKCC2 activity was estimated as the bumetanide-sensitive Rb influx in fresh isolated TAL tubules. NKCC1-contractile function was estimated as the bumetanide-sensitive vasocontractile response to phenylephrine in isolated perfused kidneys. Acute effects of L-NAME and endothelium removal were also evaluated. NKCC2 and NKCC1 protein expression were assessed by western blot analysis.

RESULTS

Chronic L-NAME administration increased, in a dose-dependent manner, both blood pressure and NKCC2 activity, and these changes significantly correlated (r2 = 0.89, P < 0.01). NKCC1-contractile activity decreased with the highest dose of L-NAME (80 mg/100 ml drinking water group) but it was not affected by acute nitric oxide blockade or endothelium removal. This 80 mg group showed increased NKCC2 expression in the renal medulla and decreased NKCC1 expression in aorta.

CONCLUSIONS

Chronic nitric oxide deficiency stimulates tubular Na-K-2Cl cotransporter, suggesting that NKCC2 hyperactivity contributes to the inability to excrete sodium, and hence to the development of L-NAME hypertension. In contrast, L-NAME hypertension develops independently of vascular NKCC1-contractile activity.

Effects of chloride substitution on electromechanical responses in the pulmonary artery of Dahl normotensive and hypertensive rats

1. We have investigated the in vitro interaction between chloride ions and endothelium as revealed by alterations in vascular contractility and smooth muscle cell membrane potential in isolated pulmonary arteries from Dahl salt-resistant normotensive and salt-sensitive hypertensive rats. 2. Exposure to nitro-l-arginine methyl ester (l-NAME) of tissues from normotensive but not hypertensive rats augmented contractions to cirazoline. While chloride removal did not alter cirazoline-induced contractions, it completely abolished the augmentation by l-NAME in normotensive rats. However, in hypertensive rats, removal of chloride ions significantly attenuated contractions elicited by cirazoline, and l-NAME effectively reversed this inhibition. 3. Methacholine-induced endothelium-dependent relaxations of the same magnitude were evident in both normotensive and hypertensive rats. However, basal cyclic GMP levels were found to be significantly higher (7.8-fold) in blood vessels of normotensive rats compared to hypertensive rats. 4. The resting membrane potential in pulmonary arteries of hypertensive rats (-52.1+/-1.04 mV) revealed a significant hyperpolarisation when compared with that of normotensive rats (-46.4+/-1.58 mV). Cirazoline did not produce a significant depolarisation in blood vessels of either normotensive or hypertensive rats. Perfusion with chloride-free solution resulted in a modest but significant hyperpolarisation (-8.0 mV) in the blood vessels of hypertensive but not in normotensive rats. 5. We conclude that salt-dependent hypertension in Dahl rats is accompanied by functional and biochemical changes in low-pressure blood vessels. These changes can, in part, be attributed to impairment in the basal, but not methacholine-stimulated, release of nitric oxide, and to altered chloride ion handling.

c-Fos expression in ouabain-treated vascular smooth muscle cells from rat aorta: evidence for an intracellular-sodium-mediated, calcium-independent mechanism

In this study, we examined the effect of Na(+)-K(+) pump inhibition on the expression of early response genes in vascular smooth muscle cells (VSMC) as possible intermediates of the massive RNA synthesis and protection against apoptosis seen in ouabain-treated VSMC in our previous experiments. Incubation of VSMC with ouabain resulted in rapid induction of c-Fos protein expression with an approximately sixfold elevation after 2 h of incubation. c-Jun expression was increased by approximately fourfold after 12 h, whereas expression of activating transcription factor 2, cAMP/Ca(2+) response element binding protein (CREB)-1 and c-Myc was not altered. Markedly augmented c-Fos expression was also observed under Na(+)-K(+) pump inhibition in potassium-depleted medium. Na(+)-K(+) pump inhibition triggered c-Fos expression via elevation of the [Na(+)](i)/[K(+)](i) ratio. This conclusion follows from experiments showing the lack of effect of ouabain on c-Fos expression in high-potassium-low-sodium medium and from the comparison of dose responses of Na(+)-K(+) pump activity, [Na(+)](i) and [K(+)](i) content and c-Fos expression to ouabain. A fourfold increment of c-Fos mRNA was revealed 30 min following addition of ouabain to the incubation medium. At this time point, treatment with ouabain resulted in an approximately fourfold elevation of [Na(+)](i) but did not affect [K(+)](i). Augmented c-Fos expression was also observed under VSMC depolarization in high-potassium medium. Increments in both c-Fos expression and (45)Ca uptake in depolarized VSMC were abolished under inhibition of L-type Ca(2+) channels with 0.1 microM nicardipine. Ouabain did not affect the free [Ca(2+)](i) or the content of exchangeable [Ca(2+)](i). Ouabain-induced c-Fos expression was also insensitive to the presence of nicardipine and [Ca(2+)](o), as well as chelators of [Ca(2+)](o) (EGTA) and [Ca(2+)](i) (BAPTA). The effect of ouabain and serum on c-Fos expression was additive. In contrast to serum, however, ouabain failed to activate the Elk-1, serum response factor, CREB and activator protein-1 transcription factors identified within the c-Fos promoter. These results suggest that Na(+)-K(+) pump inhibition triggers c-Fos expression via [Na(+)](i)-sensitive [Ca(2+)](i)-independent transcription factor(s) distinct from factors interacting with known response elements of this gene promoter.

Manipulation of chloride flux affects histamine-induced contraction in rabbit basilar artery

Cl(-) efflux induces depolarization and contraction of smooth muscle cells. This study was undertaken to explore the role of Cl(-) flux in histamine-induced contraction in the rabbit basilar artery. Male New Zealand White rabbits (n = 16) weighing 1.8-2.5 kg were euthanized by an overdose of pentobarbital sodium. The basilar arteries were removed for isometric tension recording. Histamine produced a concentration-dependent contraction that was attenuated by the H(1) receptor antagonist chlorpheniramine (10(-8) M) but not by the H(2) receptor antagonist cimetidine (3 x 10(-6) M) in normal Cl(-) Krebs-Henseleit bicarbonate solution (123 mM Cl(-)). The histamine-induced contraction was reduced by the following manipulations: 1) inhibition of Na(+)-K(+)-2Cl(-) cotransporter with bumetanide (3 x 10(-5) and 10(-4) M), 2) bicarbonate-free HEPES solution to disable Cl(-)/HCO exchanger, and 3) blockade of Cl(-) channels with the use of niflumic acid, 5-nitro-2-(3-phenylpropylamino) benzoic acid, and indoleacetic acid 94 R-(+)-methylindazone. In addition, substitution of extracellular Cl(-) (10 mM) with methanesulfonate acid (113 mM) transiently enhanced histamine-induced contraction. Manipulation of Cl(-) flux affects histamine-induced contraction in the rabbit basilar artery.

Role of Cl- current in endothelin-1-induced contraction in rabbit basilar artery

Cl- efflux induces depolarization and contraction of smooth muscle cells. This study was undertaken to explore the role of Cl- channels in endothelin-1 (ET-1)-induced contraction in rabbit basilar artery. Male New Zealand White rabbits (n = 26), weighing 1.8-2.5 kg, were euthanized by an overdose of pentobarbital. The basilar arteries were removed for isometric tension recording. ET-1 produced a concentration-dependent contraction of the rabbit basilar artery in the normal Cl- Krebs-Henseleit bicarbonate buffer (123 mM Cl-). The ET-1-induced contraction was reduced by the following manipulations: 1) inhibition of Na+-K+-2Cl- cotransporter with bumetanide (3 x 10(-5) and 10(-4) M), 2) bicarbonate-free solution to disable Cl-/HCO exchanger, and 3) preincubation of rings with the Cl- channel blockers niflumic acid, 5-nitro-2-(3-phenylpropylamino)benzoic acid, and indanyloxyacetic acid 94. The ET-1-induced contraction was enhanced by substitution of extracellular Cl- (10 mM) with methanesulfonic acid (113 mM). Cl- channels are involved in ET-1-induced contraction in the rabbit basilar artery.

Measurement of chloride flux associated with the myogenic response in rat cerebral arteries

1. Self-referencing ion-selective (SERIS) electrodes were used to measure the temperature and pressure dependence of Cl(-) efflux, during myogenic contraction of pressurized rat cerebral resistance arteries. 2. At room temperature (18-21 degrees C), a small, pressure-independent Cl(-) efflux was measured. On warming to 37 degrees C, arteries developed pressure-dependent myogenic tone, and this was associated with a pressure-dependent increase in Cl(-) efflux (n = 5). 3. Both myogenic tone and the pressure- and temperature-dependent Cl(-) efflux were abolished on application of 10 microM tamoxifen, a Cl(-) channel blocker (IC(50) 3.75 +/- 0.2 microM). Tamoxifen (10 microM) also prevented contraction to 60 mM K(+), suggesting non-specific effects of tamoxifen (n = 5). 4. Myogenic tone was abolished by 2 microM nimodipine, but Cl(-) efflux was unaffected. In the presence of nimodipine, 10 microM tamoxifen still abolished pressure- and temperature-dependent Cl(-) efflux (n = 3). 5. In summary, a Cl(-) efflux can be measured from rat cerebral arteries, with a temperature dependence that is closely correlated with myogenic contraction. We conclude that Cl(-) efflux through Cl(-) channels contributes to the depolarization associated with myogenic contraction.

Chloride in smooth muscle

Interest in the functions of intracellular chloride expanded about twenty years ago but mostly this referred to tissues other than smooth muscle. On the other hand, accumulation of chloride above equilibrium seems to have been recognised more readily in smooth muscle. Experimental data is used to show by calculation that the Donnan equilibrium cannot account for the chloride distribution in smooth muscle but it can in skeletal muscle. The evidence that chloride is normally above equilibrium in smooth muscle is discussed and comparisons are made with skeletal and cardiac muscle. The accent is on vascular smooth muscle and the mechanisms of accumulation and dissipation. The three mechanisms by which chloride can be accumulated are described with some emphasis on calculating the driving forces, where this is possible. The mechanisms are chloride/bicarbonate exchange, (Na+K+Cl) cotransport and a novel entity, "pump III", known only from own work. Their contributions to chloride accumulation vary and appear to be characteristic of individual smooth muscles. Thus, (Na+K+Cl) always drives chloride inwards, chloride/bicarbonate exchange is always present but does not always do it and "pump III" is not universal. Three quite different biophysical approaches to assessing chloride permeability are considered and the calculations underlying them are worked out fully. Comparisons with other tissues are made to illustrate that low chloride permeability is a feature of smooth muscle. Some of the functions of the high intracellular chloride concentrations are considered. This includes calculations to illustrate its depolarising influence on the membrane potential, a concept which, experience tells us, some people find confusing. The major topic is the role of chloride in the regulation of smooth muscle contractility. Whilst there is strong evidence that the opening of the calcium-dependent chloride channel leads to depolarisation, calcium entry and contraction in some smooth muscles, it appears that chloride serves a different function in others. Thus, although activation and inhibition of (Na+K+Cl) cotransport is associated with contraction and relaxation respectively, the converse association of inhibition and contraction has been seen. Nevertheless, inhibition of chloride/bicarbonate exchange and "pump III" and stimulation of (K+Cl) cotransport can all cause relaxation and this suggests that chloride is always involved in the contraction of smooth muscle. The evidence that (Na+K+Cl) cotransport more active in experimental hypertension is discussed. This is a common but not universal observation. The information comes almost exclusively from work on cultured cells, usually from rat aorta. Nevertheless, work on smooth muscle freshly isolated from hypertensive rats confirms that (Na+K+Cl) cotransport is activated in hypertension but there are several other differences, of which the depolarisation of the membrane potential may be the most important.Finally, a simple calculation is made which indicates as much as 40% of the energy put into the smooth muscle cell membrane by the sodium pump is necessary to drive (Na+K+Cl) cotransport. Notwithstanding the approximations in this calculation, this suggests that chloride accumulation is energetically expensive. Presumably, this is related to the apparently universal role of chloride in contraction.

Role of Cl(-) channels in alpha-adrenoceptor-mediated vasoconstriction in the anesthetized rat

In vitro studies have provided evidence that Cl(-) ion currents are important for activation of vascular smooth muscle contraction. The stilbene, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), disrupts Cl(-) metabolism by blocking Cl(-) channels and by inhibiting Cl(-) bicarbonate exchange. The aims of this study were to: (i) characterize the hemodynamic responses produced by DIDS in pentobarbital anesthetized rats, and (ii) examine vasoconstrictor responses to norepinephrine before and after administration of DIDS. DIDS (2.5-50 micromol/kg, 92.5 micromol/kg total dose, i.v.) produced dose-dependent but transient reductions in mean arterial blood pressure and in hindquarter, renal and mesenteric vascular resistances. Prior to the administration of DIDS, norepinephrine (1. 0-5.0 microgram/kg, i.v.) produced dose-dependent increases in mean arterial pressure, renal resistance and mesenteric resistance, but decreases in hindquarter resistance that were inversely related to dose. After administration of DIDS, the peak pressor responses produced by norepinephrine were either slightly diminished (1.0, 2.5 microgram/kg) or unchanged (5.0 microgram/kg). Peak norepinephrine-induced changes in hindquarter and renal vascular resistance were unaffected by DIDS, while increases in mesenteric resistance were augmented. The total norepinephrine-induced increases in mean arterial pressure (mm Hgxs) were markedly reduced by DIDS. These effects of DIDS on norepinephrine-induced responses were similar, but not identical to those of the voltage-sensitive Ca(2+) channel blocker, nifedipine (500 nmol/kg, i.v.). These findings suggest that DIDS may interfere with norepinephrine-induced depolarization of resistance arteries, thereby preventing activation of voltage-sensitive Ca(2+) channels.

Endothelium modulates anion channel-dependent aortic contractions to iodide

Anion currents contribute to vascular smooth muscle (VSM) membrane potential. The substitution of extracellular chloride (Cl) with iodide (I) or bromide (Br) initially inhibited and then potentiated isometric contractile responses of rat aortic rings to norepinephrine. Anion substitution alone produced a small relaxation, which occurred despite a lack of active tone and minimal subsequent contraction of endothelium-intact rings (4.2 +/- 1.2% of the response to 90 mM KCl). Endothelium-denuded rings underwent a similar initial relaxation but then contracted vigorously (I > Br). Responses to 130 mM I (93.7 +/- 1.9% of 90 mM KCl) were inhibited by nifedipine (10(-6) M), niflumic acid (10(-5) M), tamoxifen (10(-5) M), DIDS (10(-4) M), and HCO(-)(3)-free buffer (HEPES 10 mM) but not by bumetanide (10(-5) M). Intact rings treated with N(omega)-nitro-L-arginine (10(-4) M) responded weakly to I (15.5 +/- 2.1% of 90 mM KCl), whereas hemoglobin (10(-5) M), indomethacin (10(-6) M), 17-octadecynoic acid (10(-5) M), and 1H-[1,2, 4]oxadiazole[4,3-a]quinoxalin-1-one (10(-6) M) all failed to augment the response of intact rings to I. We hypothesize that VSM takes up I primarily via an anion exchanger. Subsequent I efflux through anion channels having a selectivity of I > Br > Cl produces depolarization. In endothelium-denuded or agonist-stimulated vessels, this current is sufficient to activate voltage-dependent calcium channels and cause contraction. Neither nitric oxide nor prostaglandins are the primary endothelial modulator of these anion channels. If they are regulated by an endothelium-dependent hyperpolarizing factor it is not a cytochrome P-450 metabolite.

Vasoconstrictors and nitrovasodilators reciprocally regulate the Na+-K+-2Cl- cotransporter in rat aorta

Little is known about the function and regulation of the Na+-K+-2Cl- cotransporter NKCC1 in vascular smooth muscle. The activity of NKCC1 was measured as the bumetanide-sensitive efflux of 86Rb+ from intact smooth muscle of the rat aorta. Hypertonic shrinkage (440 mosmol/kgH2O) rapidly doubled cotransporter activity, consistent with its volume-regulatory function. NKCC1 was also acutely activated by the vasoconstrictors ANG II (52%), phenylephrine (50%), endothelin (53%), and 30 mM KCl (54%). Both nitric oxide and nitroprusside inhibited basal NKCC1 activity (39 and 34%, respectively), and nitroprusside completely reversed the stimulation by phenylephrine. The phosphorylation of NKCC1 was increased by hypertonic shrinkage, phenylephrine, and KCl and was reduced by nitroprusside. The inhibition of NKCC1 significantly reduced the contraction of rat aorta induced by phenylephrine (63% at 10 nM, 26% at 30 nM) but not by KCl. We conclude that the Na+-K+-2Cl- cotransporter in vascular smooth muscle is reciprocally regulated by vasoconstrictors and nitrovasodilators and contributes to smooth muscle contraction, indicating that alterations in NKCC1 could influence vascular smooth muscle tone in vivo.

Subtypes of functional alpha1- and alpha2-adrenoceptors

In this review, subtypes of functional alpha1- and alpha2-adrenoceptors are discussed. These are cell membrane receptors, belonging to the seven transmembrane spanning G-protein-linked family of receptors, which respond to the physiological agonists noradrenaline and adrenaline. Alpha1-adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A 4th alpha1-adrenoceptor, the alpha1L-, has been postulated to mediate contractions in some tissues, but its relationship to cloned receptors remains to be established. Alpha2-adrenoceptors can be divided into alpha2A-, alpha2B- and alpha2C-adrenoceptors, all of which mediate contractile responses. Prejunctional inhibitory alpha2-adrenoceptors are predominantly of the alpha2A-adrenoceptor subtype (the alpha2D-adrenoceptor is a species orthologue), although alpha2C-adrenoceptors may also occur prejunctionally. Although alpha2-adrenoceptors are linked to inhibition of adenylate cyclase, this may not be the primary signal in causing smooth muscle contraction; likewise, prejunctional inhibitory actions probably involve restriction of Ca2+ entry or opening of K+ channels. Receptor knock-out mice are beginning to refine our knowledge of the functions of alpha-adrenoceptor subtypes.