Role of K+ channels in the vasodilator response to bradykinin in the rat heart

The cellular mechanisms by which adenosine evokes release of nitric oxide from rat aortic endothelium

Adenosine and nitric oxide (NO) are important local mediators of vasodilatation. The aim of this study was to elucidate the mechanisms underlying adenosine receptor-mediated NO release from the endothelium. In studies on freshly excised rat aorta, second-messenger systems were pharmacologically modulated by appropriate antagonists while a NO-sensitive electrode was used to measure adenosine-evoked NO release from the endothelium. We showed that A1-mediated NO release requires extracellular Ca2+, phospholipase A2 (PLA2) and ATP-sensitive K+ (KATP) channel activation whereas A2A-mediated NO release requires extracellular Ca2+ and Ca2+-activated K+ (KCa) channels. Since our previous study showed that A1- and A2A-receptor-mediated NO release requires activation of adenylate cyclase (AC), we propose the following novel pathways. The K+ efflux resulting from A1-receptor-coupled KATP-channel activation facilitates Ca2+ influx which may cause some stimulation of endothelial NO synthase (eNOS). However, the increase in [Ca2+]i also stimulates PLA2 to liberate arachidonic acid and stimulate cyclooxygenase to generate prostacyclin (PGI2). PGI2 acts on its endothelial receptors to increase cAMP, so activating protein kinase A (PKA) to phosphorylate and activate eNOS resulting in NO release. By contrast, the K+ efflux resulting from A2A-coupled KCa channels facilitates Ca2+ influx, thereby activating eNOS and NO release. This process may be facilitated by phosphorylation of eNOS by PKA via the action of A2A-receptor-mediated stimulation of AC increasing cAMP. These pathways may be important in mediating vasodilatation during exercise and systemic hypoxia when adenosine acting in an endothelium- and NO-dependent manner has been shown to be important.

Nitric oxide-epoxygenase interactions and arachidonate-induced dilation of rat renal microvessels

Nitric oxide (NO) is an inhibitor of hemoproteins including cytochrome P-450 enzymes. This study tested the hypothesis that NO inhibits cytochrome P-450 epoxygenase-dependent vascular responses in kidneys. In rat renal pressurized microvessels, arachidonic acid (AA, 0.03-1 microM) or bradykinin (BK, 0.1-3 microM) elicited NO- and prostanoid-independent vasodilation. Miconazole (1.5 microM) or 6-(2-propargyloxyphenyl)hexanoic acid (30 microM), both of which are inhibitors of epoxygenase enzymes, or the fixing of epoxide levels with 11,12-epoxyeicosatrienoic acid (11,12-EET; 1 and 3 microM) inhibited these responses. Apamin (1 microM), which is a large-conductance Ca2+-activated K+ (BKCa) channel inhibitor, or 18alpha-glycyrrhetinic acid (30 microM), which is an inhibitor of myoendothelial gap junctional electromechanical coupling, also inhibited these responses. NO donors spermine NONOate (1 and 3 microM) or sodium nitroprusside (0.3 and 3 microM) but not 8-bromo-cGMP (100 microM), which is an analog of cGMP (the second messenger of NO), blunted the dilation produced by AA or BK in a reversible manner without affecting that produced by hydralazine. However, the non-NO donor hydralazine did not affect the dilatory effect of AA or BK. Spermine NONOate did not affect the dilation produced by 11,12-EET, NS-1619 (a BKCa channel opener), or cromakalim (an ATP-sensitive K+ channel opener). AA and BK stimulated EET production, whereas hydralazine had no effect. On the other hand, spermine NONOate (3 microM) attenuated basal (19 +/- 7%; P < 0.05) and AA stimulation (1 microM, 29 +/- 9%; P < 0.05) of renal preglomerular vascular production of all regioisomeric EETs: 5,6-; 8,9-; 11,12-; and 14,15-EET. These results suggest that NO directly and reversibly inhibits epoxygenase-dependent dilation of rat renal microvessels without affecting the actions of epoxides on K+ channels.

The nitric oxide- and prostaglandin-independent component of the renal vasodilator effect of thimerosal is mediated by epoxyeicosatrienoic acids

Epoxyeicosatrienoic acids (EETs) are cytochrome P450-derived metabolites of arachidonic acid that elicit vasodilation via activation of K(+) channels. They have been implicated as endothelium-derived hyperpolarizing factors (EDHFs), mediating the effect of some endothelium-dependent vasodilator agents such as bradykinin in some vascular tissues. We reasoned that an agent that increases the availability of free arachidonic acid should also elicit cytochrome P450-dependent vasodilation that is associated with increased release of EETs and attenuated by agents that inhibit the synthesis or action of EETs. Thus, we used thimerosal as an inhibitor of reacylation of arachidonic acid and determined the contribution of prostaglandins, nitric oxide, and EETs to the vasodilator effect in the isolated, perfused, preconstricted kidney of the rat. Thimerosal elicited vasodilator responses that were unaffected by inhibition of cyclooxygenase with indomethacin but were reduced by the further inhibition of nitric oxide synthesis. The vasodilator activity that remained after inhibition of cyclooxygenase and nitric oxide synthase was reduced by inhibition of K(+) channels with tetraethylammonium and was associated with increased release of EETs measured by gas chromatography-mass spectroscopy following hydrolysis to the corresponding diols. Inhibition of cytochrome P450 with miconazole or epoxygenase with N-methylsulfonyl-6-(2-propargyloxyphenyl)hexamide reduced the nitric oxide- and prostaglandin-independent vasodilator effect of thimerosal and attenuated the increase in the release of EETs. We conclude that thimerosal causes vasodilation of the isolated perfused kidney via nitric oxide-dependent and -independent mechanisms. The nitric oxide-independent component of the response involves activation of K(+) channels and is likely mediated by EETs, possibly acting as EDHFs.

Peroxynitrite inhibits Ca2+-activated K+ channel activity in smooth muscle of human coronary arterioles

We examined the hypothesis that ONOO-, a product of the interaction between superoxide (O2*-) and nitric oxide (NO), inhibits calcium-activated K+ (KCa) channel activity in vascular smooth muscle cells (VSMCs) of human coronary arterioles (HCAs), thereby reducing hyperpolarization-mediated vasodilation. HCAs were dissected from right atrial appendages. The interaction of ONOO- with microvessels was determined by immunohistochemistry using a nitrotyrosine antibody. Strong staining was observed in arteries exposed to authentic ONOO- or to sodium nitroprusside (SNP)+xanthine (XA)+xanthine oxidase (XO). Dilation to 10(-8) mol/L bradykinin (BK) was abolished in vessels exposed to ONOO- (-2.5+/-8%; P<0.05) but not DC-ONOO- (65+/-8%). Reduced dilation to BK was also observed after application of XO and SNP. Dilation to NS1619 (KCa channel opener) was reduced in endothelial denuded arterioles treated with ONOO-. In isolated VSMCs, whole-cell peak K+ current density was reduced by ONOO- (control 65+/-15 pA/pF; ONOO- 42+/-9 pA/pF; P<0.05). Iberiotoxin had no further effect on whole-cell K+ current. In inside-out patches, ONOO- but not DC-ONOO- decreased open state probability (NP(o)) of KCa channel by 50+/-12%. O2*- generated by XA+XO had no effect on BK-induced dilation and NP(o) of KCa channels. These results suggest that ONOO-, but not O2*-, inhibits KCa channel activity in VSMCs possibly by a direct effect. This mechanism may contribute to impaired EDHF-mediated dilation in conditions such as ischemia/reperfusion where increased activity of NO synthase occurs in the presence of excess of O2*-.

Effects of different tetra-n-alkylammonium ions on acetylcholine-induced endothelium-dependent hyperpolarization in rat mesenteric artery

The effects of a series of symmetric tetra-n-alkylammonium (TAA) compounds with alkyl side chains of one to six carbons in length on acetylcholine-induced endothelium-dependent hyperpolarization were examined in rat mesenteric artery. All TAA compounds caused a concentration-dependent inhibition of the hyperpolarizing response to acetylcholine. The potency of TAAs showed a general trend to increase with the lengths of the alkyl side chains. The inhibitory effects of TAAs, excepting the smallest compound, on the acetylcholine response were reversible. However, TAAs with long alkyl side chains may act as antagonists at muscarinic receptors, because the suppressive effect on A23187-induced endothelium-dependent hyperpolarization was more marked with TAAs having smaller alkyl side chains. Conversely, the hyperpolarizing response to pinacidil, an ATP-sensitive K+ channel opener, was significantly prevented only by TAA compounds with long alkyl side chains. TAA compounds with one-to three-carbon alkyl side chains caused a modest and reversible depolarization of the membrane, whereas the depolarizing effects of the compounds with four-to six-carbon alkyl side chains were marked and irreversible. These results suggest that TAAs could gain access to the target K+ channels for endothelium-derived hyperpolarizing factor, the ATP-sensitive K+ channels, and the K+ channels responsible for the regulation of the resting membrane potential in different ways.

Protease-activated receptor-2 activation causes EDHF-like coronary vasodilation: selective preservation in ischemia/reperfusion injury: involvement of lipoxygenase products, VR1 receptors, and C-fibers

Activation of protease-activated receptor (PAR)-2 has been proposed to be protective in myocardial ischemia/reperfusion (I/R) injury, an effect possibly related to an action on the coronary vasculature. Therefore, we investigated the effects of PAR2 activation on coronary tone in isolated perfused rat hearts and elucidated the mechanisms of any observed effects. Although having a negligible effect on ventricular contractility, the PAR2 activating peptide SLIGRL produced an endothelium-dependent coronary vasodilatation (ED(50)=3.5 nmol). Following I/R injury, the response to SLIGRL was selectively preserved, whereas the dilator response to acetylcholine was converted to constriction. Trypsin also produced a vasodilator dose-response curve that was biphasic in nature (ED(50-1)=0.36 U, ED(50-2)=38.71 U). Desensitization of PAR2 receptors indicated that the high potency phase was mediated by PAR2. Removal of the endothelium but not treatment with L-NAME (300 micromol/L), indomethacin (5 micromol/L), or oxyhemoglobin (10 micromol/L) inhibited the response to SLIGRL and trypsin. Treatment with the K(+)-channel blockers TEA (10 mmol/L), charybdotoxin (20 nmol/L)/apamin (100 nmol/L), or elevated potassium (20 mmol/L) significantly suppressed responses. Similarly, inhibition of lipoxygenase with nordihydroguaiaretic acid (1 micromol/L), eicosatetraynoic acid (1 micromol/L), or baicalein (10 micromol/L), desensitization of C-fibers using capsaicin (1 micromol/L, 20 minutes), or blockade of vanilloid (VR1) receptors using capsazepine (3 micromol/L) inhibited the responses. This study shows, for the first time, that PAR2 activation causes endothelium-dependent coronary vasodilation that is preserved after I/R injury and is not mediated by NO or prostanoids, but involves the release of an endothelium-derived hyperpolarizing factor (EDHF), possibly a lipoxygenase-derived eicosanoid, and activation of VR1 receptors on sensory C-fibers.

Altered coronary dilation in deoxycorticosterone acetate-salt hypertension

OBJECTIVE

To compare coronary dilation in uninephrectomized hypertensive deoxycorticosterone acetate (DOCA)-salt rats (HTRs), treated for 2 or 4 weeks, with age-matched uninephrectomized normotensive rats (NTRs). DESIGN AND METHODS Coronary perfusion pressure was recorded in isolated hearts perfused at a constant flow rate to evaluate coronary resistance.

RESULT

A decreased vasoconstriction due to NG-nitro-Larginine (NNLA, 30 pmol/I) in hearts from HTRs suggested a reduced basal nitric oxide (NO) release. In contrast, coronary vasodilation due to the NO donor, sodium nitroprusside (3 pmol/I), remained unaffected in 2-week HTRs, and was enhanced in 4-week HTRs. Cumulative dose-response curves to bradykinin induced an important vasodilation in NTRs, with a maximal response that remained unaffected in the presence of either NNLA (30 pmol/I), indomethacin (10 pmol/l) or the two combined. In contrast, hearts from HTRs showed a diminished maximal relaxation to bradykinin, suggesting an altered endothelium-dependent relaxation. The presence of NNLA or indomethacin had no effect on the weak relaxation observed in HTRs. However, NNLA and indomethacin combined unmasked an important relaxation due to bradykinin in HTRs. The addition of clotrimazole (1 pmol/I) to NNLA and indomethacin blunted the relaxation due to bradykinin in both NTRs and HTRs. Perfusion with superoxide dismutase (120 IU/ml) restored most of the coronary relaxation due to bradykinin in hearts from HTRs. Bradykinin-induced prostaglandin 12 (PGI2) and E2 (PGE2) production was unaffected by hypertension. No increase in thromboxane A2 (TXA2) due to bradykinin was detected. Finally, reduced reactivity to papaverine and forskolin was observed in hearts from HTRs.

CONCLUSION

DOCA-salt hypertension is associated with alterations in coronary reactivity. Basal NO formation appears to be reduced in HTRs, but the intact relaxation to exogenous NO suggests a preserved guanylate cyclase pathway. In addition, alteration in adenylate cyclase activity, and not in prostaglandin production, may explain the blunted cAMP-mediated responses in HTRs. The combined nitric-oxide synthase (NOS) and cyclo-oxygenase (COX) inhibition unmasked an endothelium-derived hyperpolarizing factor (EDHF) involvement in the coronary dilation due to bradykinin in hearts from HTRs, suggesting that endothelial NO and PGI2, although unable to induce coronary smooth-muscle relaxation, can inhibit EDHF production in HTRs. Impairment in the adenylate cyclase pathway and the suppression of NO by free radicals may explain the blunted vasodilation in DOCA-salt hypertension.

Nitric oxide exerts feedback inhibition on EDHF-induced coronary arteriolar dilation in vivo

We tested the hypothesis that nitric oxide (NO) inhibits endothelium-derived hyperpolarizing factor (EDHF)-induced vasodilation via a negative feedback pathway in the coronary microcirculation. Coronary microvascular diameters were measured using stroboscopic fluorescence microangiography. Bradykinin (BK)-induced dilation was mediated by EDHF, when NO and prostaglandin syntheses were inhibited, or by NO when EDHF and prostaglandin syntheses were blocked. Specifically, BK (20, 50, and 100 ng. kg(-1). min(-1) ic) caused dose-dependent vasodilation similarly before and after administration of N(G)-monomethyl-L-arginine (L-NMMA) (3 micromol/min ic for 10 min) and indomethacin (Indo, 10 mg/kg iv). The residual dilation to BK with L-NMMA and Indo was completely abolished by suffusion of miconazole or an isosmotic buffer containing high KCl (60 mM), suggesting that this arteriolar vasodilation is mediated by the cytochrome P-450 derivative EDHF. BK-induced dilation was reduced by 39% after inhibition of EDHF and prostaglandin synthesis, and dilation was further inhibited by combined blockade with L-NMMA to a 74% reduction in the response. This suggests an involvement for NO in the vasodilation. After dilation to BK was assessed with L-NMMA and Indo, sodium nitroprusside (SNP, 1-3 microgram. kg(-1). min(-1) ic), an exogenous NO donor, was administered in a dose to increase the diameter to the original control value. Dilation to BK was virtually abolished when administered concomitantly with SNP during L-NMMA and Indo (P < 0.01 vs. before SNP), suggesting that NO inhibits EDHF-induced dilation. SNP did not affect adenosine- or papaverine-induced arteriolar dilation in the presence of L-NMMA and Indo, demonstrating that the effect of SNP was not nonspecific. In conclusion, our data are the first in vivo evidence to suggest that NO inhibits the production and/or action of EDHF in the coronary microcirculation.

Possible role of P-450-derived metabolites in endothelium-dependent relaxation of rat small mesenteric arteries

We reported previously that acetylcholine (ACh)-induced endothelium-dependent relaxation of rat mesenteric microvessels depended both on nitric oxide (NO) and on a charybdotoxin (CTX)-sensitive endothelium-derived hyperpolarizing vasodilator. Cytochrome P450 (CYP)-dependent arachidonic acid metabolites act in some systems as hyperpolarizing vasodilators. We sought to quantitate contributions of such metabolites to the CTX-sensitive component of ACh-induced vasodilation in isolated rat mesenteric resistance arteries. ACh relaxed these vessels nearly completely (93.3+/-1.2%, n = 71); cyclooxygenase inhibition with indomethacin did not diminish this response (94.3+/-11.4%, n = 9). NO synthase inhibition with Nitro-L-arginine (NNLA) reduced relaxation by 30% (n = 54, p<0.05). Pretreatment of vessels with CYP inhibitors, either clotrimazole (CTM) or 17-octadecynoic acid (17-ODYA), or with selective K+ channel inhibitors, either tetraethyammonium acetate (TEA) or CTX, each led to similar small reductions in maximal relaxation (17%, 22%, 16%, and 9% respectively, n = 3-6). Combined pretreatment with NNLA + either (CTM or 17-ODYA) or (TEA or CTX) each led to similar maximal relaxations (52.2+/-4.8%, 50.6+/-9.2, 37.6+/-8.6%, and 44.1+/-11.5%, respectively, n = 6-35; p<0.05 for NNLA+[CTM or TEA or CTX] vs NNLA alone). Combined pretreatment with NNLA+CTM+(CTX or TEA) led to similar maximal relaxations (43.0+/-7.3%, 43.7+/-15%, n = 6-11) that did not differ from values in vessels pretreated with either NNLA+CTM or NNLA+(CTX or TEA). We conclude that the ACh-induced vasodilation was insensitive to cyclooxygenase inhibition, partially sensitive to NO synthase inhibition, and that the K+ channel blockers TEA and CTX identified the same minor component of ACh relaxation as did the CYP inhibitor CTM.

Contribution of nitric oxide and K+ channel activation to vasorelaxation of isolated rat aorta induced by procaine

The endothelium-dependent and -independent relaxant effect of procaine was examined in isolated rat aortic rings. Procaine induced relaxation of arteries precontracted with phenylephrine or with 60 mM K+ in a concentration-dependent manner (0.01-3 mM). Procaine (1 mM) inhibited the transient contraction induced by caffeine (10 mM) in Ca2+-free Krebs solution. Removal of the endothelium caused a rightward shift of the concentration-response curve for procaine. N(G)-Nitro-L-arginine (L-NNA, 10-100 microM), N(G)-nitro-L-arginine methyl ester (L-NAME, 100 microM) and methylene blue (1-10 microM) significantly attenuated the procaine-induced relaxation without affecting the maximal response. L-Arginine (1 mM) partially but significantly antagonized the effect of L-NAME (100 microM). Pretreatment of endothelium-intact aortic rings with procaine (1 mM) or with acetylcholine (10 microM) significantly elevated the tissue contents of cyclic GMP and this increase was inhibited in the presence of 100 microM L-NNA. Tetrapentylammonium ions (1-3 microM) reduced the procaine-induced relaxation in both endothelium-intact and -denuded arteries. Tetrapentylammonium ions (3 microM) did not affect the procaine-induced relaxation of 60 mM K+-contracted arteries. Tetraethylammonium ions (3 mM) inhibited the procaine-induced relaxation. In contrast, iberiotoxin (100 nM), glibenclamide (3 microM), 4-aminopyridine (3 mM) and indomethacin (10 microM) had no effect. These results indicate that the procaine-induced relaxation may be mediated through multiple mechanisms. A substantial portion of the procaine-induced relaxation in rat aorta was caused by nitric oxide but not by other endothelium-derived factors. The activation of tetrapentylammonium- and tetraethylammonium-sensitive K+ channels contributes in part to the procaine-induced vasorelaxation. Besides, procaine may directly inhibit both external Ca2+ entry and internal Ca2+ release in aortic smooth muscle cells.

Role of non-nitric oxide non-prostaglandin endothelium-derived relaxing factor(s) in bradykinin vasodilation

The most conspicuous effect of bradykinin following its administration into the systemic circulation is a transient hypotension due to vasodilation. In the present study most of the available evidence regarding the mechanisms involved in bradykinin-induced arterial vasodilation is reviewed. It has become firmly established that in most species vasodilation in response to bradykinin is mediated by the release of endothelial relaxing factors following the activation of B2-receptors. Although in some cases the action of bradykinin is entirely mediated by the endothelial release of nitric oxide (NO) and/or prostacyclin (PGI2), a large amount of evidence has been accumulated during the last 10 years indicating that a non-NO/PGI2 factor accounts for bradykinin-induced vasodilation in a wide variety of perfused vascular beds and isolated small arteries from several species including humans. Since the effect of the non-NO/PGI2 endothelium-derived relaxing factor is practically abolished by disrupting the K+ electrochemical gradient together with the fact that bradykinin causes endothelium-dependent hyperpolarization of vascular smooth muscle cells, the action of such factor has been attributed to the opening of K+ channels in these cells. The pharmacological characteristics of these channels are not uniform among the different blood vessels in which they have been examined. Although there is some evidence indicating a role for KCa or KV channels, our findings in the mesenteric bed together with other reports indicate that the K+ channels involved do not correspond exactly to any of those already described. In addition, the chemical identity of such hyperpolarizing factor is still a matter of controversy. The postulated main contenders are epoxyeicosatrienoic acids or endocannabinoid agonists for the CB1-receptors. Based on the available reports and on data from our laboratory in the rat mesenteric bed, we conclude that the NO/PGI2-independent endothelium-dependent vasodilation induced by BK is unlikely to involve a cytochrome P450 arachidonic acid metabolite or an endocannabinoid agonist.

Endothelium-derived hyperpolarizing factor--a critical appraisal

Endothelium-derived hyperpolarizing factor is defined as that substance which produces vascular smooth muscle hyperpolarization which cannot be explained by nitric oxide or by a cyclo-oxygenase product such as prostacyclin. The possibility that the factor is an epoxyeicosatrienoic acid or a cannabinoid agonist such as anandamide continues to be investigated, but definitive evidence in favour of either is lacking. The sensitivity of EDHF-mediated responses to charybdotoxin, to apamin or to mixtures of these two toxins may indicate the opening of more than one smooth muscle K-channel, but the possibility that these are located on the vascular endothelium is discussed.

Role of Ca2+-activated K+ channels in the protective effect of ACE inhibition against ischemic myocardial injury

Angiotensin-converting enzyme (ACE) inhibitors increase the production of nitric oxide (NO) and prostacyclin and open Ca2+-activated K+ channels. The effects of these actions of ACE inhibitors on infarct size were investigated in open-chest dogs subjected to myocardial ischemia and reperfusion. Infarct size was assessed 6 hours after the onset of reperfusion, subsequent to 90 minutes of occlusion of the left anterior descending coronary artery. The ACE inhibitor cilazaprilat was administered into the coronary artery 10 minutes before coronary occlusion, and infusion was continued until 1 hour after reperfusion. The bradykinin and NO concentrations in coronary venous blood 10 minutes after the onset of reperfusion were significantly higher in dogs treated with cilazaprilat (3 microg x kg(-1) x min(-1)) than in control animals. Although there were no significant differences in collateral flow during ischemia, infarct size in the cilazaprilat group was smaller than that in the control group (15.1+/-3.0% versus 46.7+/-4.2% of the area at risk, P<0.0001). The infarct size-limiting effect of cilazaprilat was partially reduced by either N(G)-nitro-L-arginine methyl ester (an inhibitor of NO synthase) or iberiotoxin (a blocker of Ca2+-activated K+ channels) and was abolished by N(G)-nitro-L-arginine methyl ester plus iberiotoxin. Indomethacin (an inhibitor of cyclooxygenase) had no effect on the beneficial action of cilazaprilat. Inhibition of ACE thus reduced myocardial infarct size, an effect that was mediated by NO and the opening of Ca2+-activated K+ channels in canine hearts.

3,4-Diaminopyridine-induced impairment in frog motor nerve terminal response to high frequency stimulation

The refractory period of the presynaptic Na+ current (INa) of the frog neuromuscular junction before and after the block of the presynaptic delayed rectifier K+ conductance by 3,4-diaminopyridine (3,4-DAP) was studied by the perineurial recording technique. Application of 3,4-DAP 0.45 mM greatly prolonged the refractory period of the last nodes of Ranvier of frog motor axons. Suppression of the repetitive activity caused by 3,4-DAP by 3-aminobenzoic acid ethyl ester (tricaine) 0.46 mM (a local anesthetic) decreased the refractory period back towards normal values. These results indicate that 3,4-DAP impairs conduction of high frequency nerve impulses along the last nodes of Ranvier due to its block of presynaptic K+ conductance. The spontaneous activation of the most excitable, last nerve segments seemed to be the main factor causing such impairment. This phenomenon could explain in part the adverse motor effects shown by some patients treated with high doses of 3,4-DAP.

Systemic administration of the potassium channel activator cromakalim attenuates cerebral vasospasm after experimental subarachnoid hemorrhage

OBJECTIVE

Cerebral vasospasm is a primary complication after aneurysmal subarachnoid hemorrhage (SAH). Recent evidence indicates that the activation of potassium (K+) channels may be of benefit in relieving spastic constriction. The present study examined the effects of systemic administration of a K+ channel activator, cromakalim, on cerebral vasospasm after experimental SAH.

METHODS

Experimental SAH was performed in rabbits by injecting autologous blood into the cisterna magna. Intravenous injections of cromakalim or vehicle were administered twice daily with the first injection administered 1 hour after induction of SAH. Animals were killed by perfusion-fixation 48 hours after SAH. Basilar arteries were removed and sectioned, and the luminal cross-sectional areas were measured.

RESULTS

Experimental SAH induced cerebral vasospasm in untreated and vehicle-treated animals. Cromakalim attenuated cerebral vasospasm in a dose-dependent manner. This effect achieved statistical significance at doses of 0.1 and 0.3 mg/kg.

CONCLUSION

These results support the concept that targeting vascular K+ channels can be of benefit in preventing the development of cerebral vasospasm. The findings also indicate that cromakalim represents a potential therapeutic agent for the treatment of cerebrovascular pathophysiology after SAH.

Endothelium-derived hyperpolarizing factor, but not nitric oxide, is reversibly inhibited by brefeldin A

The subcellular localization of the enzymes synthesizing endothelium-derived vasodilator autacoids has been proposed to play a role in determining the ability of endothelial cells to enhance autacoid production in response to stimulation. We therefore investigated the effects of brefeldin A-induced disruption of the Golgi apparatus and Golgi-plasma membrane trafficking on the production of nitric oxide (NO), prostacyclin, and the endothelium-derived hyperpolarizing factor (EDHF) by native and cultured endothelial cells. In porcine coronary artery segments, brefeldin A (35 micromol/L, 90 minutes) did not affect relaxations to sodium nitroprusside or the K+ channel opener cromakalim but elicited a rightward shift in the concentration-response curve to bradykinin without altering the maximum vasodilator response (Rmax). Brefeldin A failed to attenuate the bradykinin-induced, NO-mediated relaxation under depolarizing conditions but inhibited the bradykinin response under conditions of combined cyclooxygenase/NO synthase blockade, suggesting that this agent selectively interferes with the production of EDHF. Indeed, incubation of porcine coronary arteries with brefeldin A, which did not affect the bradykinin-induced accumulation of either cyclic GMP or 6-keto-prostaglandin F1alpha, markedly and reversibly attenuated the EDHF-mediated hyperpolarization of detector smooth muscle cells in a patch-clamp bioassay system. The microtubule destabilizer nocodazole also affected both the EC50 and Rmax to bradykinin in porcine coronary arteries. Since EDHF is thought to be a cytochrome P450-derived metabolite of arachidonic acid and both brefeldin A and nocodazole are known to interfere with the targeting of cytochrome P450 from the Golgi apparatus to the plasma membrane, it is conceivable that brefeldin A inhibits EDHF formation by preventing the targeting of the EDHF-synthesizing enzymes to the plasma membrane.

Hyperpolarizing factors

There is now overwhelming evidence for factors, other than nitric oxide (NO), that mediate endothelium-dependent vasodilation by hyperpolarizing the underlying smooth muscle via activation of Ca2+-activated K+ channels. Although the identity of endothelium-derived hyperpolarizing factor (EDHF) remains to be established, cytochrome P450 (CYP)-dependent metabolites of arachidonic acid (AA), namely, the epoxides, fulfill several of the criteria required for consideration as putative mediators of endothelium-dependent hyperpolarization. They are produced by the endothelium, released in response to vasoactive hormones, and elicit vasorelaxation via stimulation of Ca2+-activated K+ channels. Our studies in the rat indicate that, of the epoxides, 5,6-epoxyeicosatrienoic acid (5,6-EET) is the most likely mediator of NO-independent, but CYP-dependent coronary vasodilation in response to bradykinin. Studies in the rat kidney, however, support the existence of additional EDHFs as acetylcholine also exhibits NO-independent vasodilation that is unaffected by CYP inhibitors in concentrations that attenuate responses to bradykinin. In some blood vessels, NO may tonically suppress the expression of CYP-dependent EDHF. In the event of impaired NO synthesis, therefore, a CYP-dependent vasodilator mechanism may serve as a backup to a primary NO-dependent mechanism, although they may act in concert. In other vessels, particularly microvessels, an EDHF may constitute the major vasodilator mechanism for hormones and other physiological stimuli. EDHFs appear to be important regulators of vascular tone; alterations in this system can be demonstrated in hypertension and diabetes, conditions associated with altered endothelium-dependent vasodilator responsiveness.

Signalling pathways in bradykinin- and nitric oxide-induced hypotension in the normotensive rat; role of K+-channels

1. Bradykinin and nitric oxide (NO) are potent hypotensive agents. In the present study, the role of K+-channels in the signalling pathways responsible for their hypotensive action was investigated in normotensive, anaesthetized rats. The rats were treated with ion-channel inhibitors before administration of bradykinin (2.8, 5.6, 28 and 56 nmol kg(-1), i.v.) followed in some of the protocols by nitroprusside (1.1, 3.5, 7, 14, and 28 nmol kg(-1), i.v.). 2. No attenuation of the hypotensive response to bradykinin was detected for inhibitors of the Na-K-Cl-cotransporter (30 micromol kg(-1) furosemide), the ATP-sensitive K+-channel (40 micromol kg(-1) glibenclamide), high conductance Ca2+-activated K+-channel (180 micromol kg(-1) tetraethylammonium, 54 micromol kg(-1) tetrabutylammonium, 35 nmol kg(-1) iberiotoxin, 35 nmol kg(-1) charybdotoxin) or the low conductance Ca2+-activated K+-channel (74 nmol kg(-1) apamin). 3. However, the voltage-sensitive K+-channel (I(A)) inhibitor 4-aminopyridine (4.05-40.5 micromol kg(-1)) induced a concentration-dependent (P<0.0001) attenuation of the hypotensive response (P<0.0001). Bradykinin had no effect on heart rate in anaesthetized rats and this observation was not altered by pretreatment with 4-aminopyridine. 4. 4-Aminopyridine (53 micromol kg(-1)) also significantly attenuated the hypotensive response to nitroprusside (P<0.0003) without altering the heart rate concentration-response curve. Of the two Ca2+-activated K+-channel inhibitors tested on nitroprusside-induced hypotension, tetrabutylammonium induced a slight attenuation (P<0.0101), whereas iberiotoxin had no effect. 5. We therefore concluded that, although the acute hypotensive response to bradykinin in the normotensive rat is not mediated through nitric oxide synthesis, the hypotensive response to both agents was mediated through opening of voltage-sensitive K+-channels (I(A)), resulting in a decrease in peripheral vascular resistance.

K+ channels which contribute to the acetylcholine-induced hyperpolarization in smooth muscle of the guinea-pig submucosal arteriole

1. Membrane potentials were recorded from submucosal arterioles (diameter, 30-80 microns) of the guinea-pig small intestine, using conventional microelectrode techniques. In control solution the resting membrane potential was about -73 mV, and the addition of 0.5 mM Ba2+ depolarized the membrane to about -43 mV. 2. ACh (10 nM to 10 microM), or substance P (0.1 microM), caused a membrane hyperpolarization in preparations which had been depolarized by Ba2+ but not in control preparations. ACh produced a sustained hyperpolarization, whereas substance P produced a transient hyperpolarization, without being affected by either nitroarginine (0.1 mM) or indomethacin (10 microM). 3. In the presence of 50 microM BAPTA (acetoxymethyl ester form), the membrane potentials were not altered in the control solution or in the presence of Ba2+, but Ba2+ caused a smooth depolarization of the membrane. Following this procedure, both ACh and substance P caused membrane depolarization instead of hyperpolarization, suggesting that the ACh- and substance P-induced hyperpolarization in arteriolar smooth muscle are intracellular [Ca2+] dependent. 4. In short segments (200-500 microns) of arteriole, the time constant of electrotonic potentials produced by passing current pulses through the recording electrode was about 75 ms. The addition of Ba2+ increased both the input resistance and the time constant. 5. The hyperpolarizations produced by ACh or substance P were associated with a reduction in the amplitude and the time constant of electrotonic potential. 6. The reversal potential for the ACh-induced hyperpolarization, estimated from the current-voltage relationship, was about -86 mV, a value close to the equilibrium potential for K+. 7. In the presence of 50 nM charybdotoxin the hyperpolarization produced by ACh became transient and was reduced in amplitude: the residual response was further reduced by apamin (0.1 microM). The response produced by substance P was also reduced by 50 nM charybdotoxin: again the residual response was sensitive to 0.1 microM apamin. The hyperpolarizations produced by either ACh or substance P were insensitive to glibenclamide (10 microM) and 4-aminopyridine (1 mM). 8. It is suggested that in submucosal arterioles of the guinea-pig ileum, ACh- or substance P-induced hyperpolarizations of smooth muscle result from activation of both charybdotoxin-sensitive and apamin-sensitive K+ channels, with the former being predominant. The results are discussed in relation to the possible involvement of one or more endothelium-dependent hyperpolarizing factors in ACh- and substance P-induced hyperpolarization.

Characterization of endothelium-derived relaxing factors released by bradykinin in human resistance arteries

1. Relaxing factors released by the endothelium and their relative contribution to the endothelium-dependent relaxation produced by bradykinin (BK) in comparison with different vasodilator agents were investigated in human omental resistance arteries. 2. BK produced an endothelium-dependent relaxation of arteries pre-contracted with the thromboxane A2 agonist, U46619. The B2 receptor antagonist, Hoe 140 (0.1, 1 and 10 microM), produced a parallel shift to the right of the concentration-response curve to BK with a pA2 of 7.75. 3. Neither the cyclo-oxygenase inhibitor, indomethacin (10 microM) alone, the nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME, 300 microM) alone, the nitric oxide scavenger, oxyhaemoglobin (Hb, 10 microM) alone, nor the combination of L-NAME plus Hb affected the concentration-response curve to BK. Conversely, the combination of indomethacin with either L-NAME or Hb attenuated but did not abolish the BK-induced relaxation. By contrast, the relaxations produced by the Ca2+ ionophore, calcimycin (A23187), and by the inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, thapsigargin (THAPS), were abolished in the presence of indomethacin plus L-NAME. Also, the presence of indomethacin plus L-NAME produced contraction of arteries with functional endothelium. 4. The indomethacin plus L-NAME resistant component of BK relaxation was abolished in physiological solution (PSS) containing 40 mM KCl and vice versa. However, in the presence of KCl 40 mM, indomethacin plus L-NAME did not affect the nitric oxide donor, S-N-acetylpenicillamine-induced relaxation. 5. The indomethacin plus L-NAME resistant component of the relaxation to BK was significantly attenuated by the K+ channel blocker tetrabutylammonium (TBA, 1 mM). However, it was not affected by other K+ channel blockers such as apamin (10 microM), 4-aminopyridine (100 microM), glibenclamide (10 microM), tetraethylammonium (10 mM) and charybdotoxin (50 nM). 6. In the presence of indomethacin plus L-NAME, the relaxation produced by BK was not affected by the phospholipase A2 inhibitor, quinacrine (10 microM) or by the inhibitor of cytochrome P450, SKF 525a (10 microM). Another cytochrome P450 inhibitor, clotrimazole (10 microM) which also inhibits K+ channels, inhibited the relaxation to BK. 7. These results show that BK induces endothelium-dependent relaxation in human small omental arteries via multiple mechanisms involving nitric oxide, cyclo-oxygenase derived prostanoid(s) and another factor (probably an endothelium-derived hyperpolarizing factor). They indicate that nitric oxide and cyclo-oxygenase derivative(s) can substitute for each other in producing relaxation and that the third component is not a metabolite of arachidonic acid, formed through the cytochrome P-450 pathway, in these arteries.

Evidence that mechanisms dependent and independent of nitric oxide mediate endothelium-dependent relaxation to bradykinin in human small resistance-like coronary arteries

1. The effects of the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine (L-NOARG), the NO scavenger, oxyhaemoglobin (HbO) and high extracellular K+ upon endothelium-dependent relaxation to bradykinin were investigated in human isolated small coronary arteries. 2. Endothelium-dependent relaxations to bradykinin were compared in vessels contracted to approximately 50% of their maximum contraction to 124 mM KCl Krebs solution, regardless of treatments, with the thromboxane A2 mimetic, U46619 and acetylcholine. All relaxations were expressed as percentage reversal of the initial level of active force. 3. L-NOARG (100 microM) caused a small but significant, 12% (P < 0.01), decrease in the maximum relaxation (Rmax: 91.5 +/- 5.4%) to bradykinin but did not significantly affect the sensitivity (pEC50: 8.08 +/- 0.17). Increasing the concentration of L-NOARG to 300 microM had no further effect on the pEC50 or Rmax to bradykinin. HbO (20 microM) and a combination of HbO (20 microM) and L-NOARG (100 microM) reduced Rmax to bradykinin by 58% (P < 0.05) and 54% (P < 0.05), respectively. HbO (20 microM) and L-NOARG (100 microM, combined but not HbO (20 microM) alone, caused a significant 11 fold (P < 0.05) decrease in sensitivity to bradykinin. HbO (20 microM) decreased the sensitivity to the endothelium-independent NO donor, S-nitroso-N-acetylpenicillamine (SNAP), approximately 17 fold (P < 0.05). 4. Raising the extracellular concentration of K+ isotonically to 30 mM, reduced the Rmax to bradykinin from 96.6 +/- 3.1% to 43.9 +/- 10.1% (P < 0.01) with no significant change in sensitivity. A combination of HbO, L-NOARG and high K+ (30 mM) abolished the response to bradykinin. High K+ did not change either the sensitivity or maximum relaxation to SNAP. 5. In conclusion, L-NOARG does not completely inhibit endothelial cell NO synthesis in human isolated small coronary arteries. By comparison, HbO appeared to block all the effects of NO in this tissue and revealed that most of the relaxation to bradykinin was due to NO. The non-NO -dependent relaxation to bradykinin in the human isolated small coronary arteries appeared to be mediated by a K(+)-sensitive vasodilator mechanism, possibly endothelium-derived hyperpolarizing factor (EDHF).

Mechanisms of coronary microvascular dilation induced by the activation of pertussis toxin-sensitive G proteins are vessel-size dependent. Heterogeneous involvement of nitric oxide pathway and ATP-sensitive K+ channels

G proteins are critically important mediators of many signal transduction systems. In the present study, we investigated the effect of direct activation of pertussis toxin (PTX)-sensitive G protein (GPTX) on coronary arterial microvascular tone in 37 open-chest anesthetized dogs in vivo. Coronary arterial microvessels on the surface of the beating left ventricle were visualized by performing fluorescence coronary microangiography using an intravital microscope with a floating objective system. Microvessels were divided into two groups, small microvessels (inner diameter, < or = 130 microns) and large microvessels (inner diameter, > 130 microns). Topically applied mastoparan (G protein activator, 10, 30, and 100 mumol/L) produced homogeneous microvascular dilation in a concentration-dependent manner (10 mumol/L, 7.9 +/- 2.0%; 30 mumol/L, 10.3 +/- 2.4%; and 100 mumol/L, 16.7 +/- 4.5% in small microvessels; 10 mumol/L, 5.3 +/- 1.2%; 30 mumol/L, 9.8 +/- 2.5%; and 100 mumol/L, 15.5 +/- 3.9% in large microvessels). These dilations were reversed to constriction by pretreatment with PTX (300 ng/mL, 2 hours) in both microvessel groups. Blockade of nitric oxide production by NG-nitro-L-arginine (LNNA, 300 mumol/L) offset the mastoparan-induced dilation in large microvessels but not in small microvessels. Cosuperfusion of glibenclamide (10 mumol/L) with LNNA produced constriction of all sizes of microvessels in response to mastoparan, whereas charybdotoxin (10 nmol/L) did not affect the mastoparan effect. Pretreatment with glibenclamide alone reversed mastoparan dilation to constriction in small microvessels, whereas it only offset the dilation without producing constriction in large microvessels. We conclude that the activation of GPTX produces homogeneous coronary arterial microvascular dilation and that the underlining mechanisms of the dilation are vessel size dependent. The L-arginine-nitric oxide pathway mediates the dilation only in large microvessels, whereas ATP-sensitive K+ channel activation plays a central role in the dilation of small microvessels when GPTX is directly activated. ATP-sensitive K+ channels are also involved in the dilation of large microvessels in a synergistic fashion with nitric oxide production.

Actions of neurotransmitters and other messengers on Ca2+ channels and K+ channels in smooth muscle cells

Ion channels play key roles in determining smooth muscle tone by setting the membrane potential and allowing Ca2+ influx. Perhaps not surprisingly, therefore, they also provide targets for neurotransmitters and other messengers that act on smooth muscle. Application of patch-clamp and molecular biology techniques and the use of selective pharmacology has started to provide a wealth of information on the ion channel systems of smooth muscle cells, revealing complexity and functional significance. Reviewed are the actions of messengers (e.g., noradrenaline, acetylcholine, endothelin, angiotensin II, neuropeptide Y, 5-hydroxytryptamine, histamine, adenosine, calcitonin gene-related peptide, substance P, prostacyclin, nitric oxide and oxygen) on specific types of ion channel in smooth muscle, the L-type calcium channel, and the large conductance Ca(2+)-activated, ATP-sensitive, delayed rectifier and apamin-sensitive K+ channels.

Bradykinin, lemakalim and sodium nitroprusside relax the mouse trachea in vitro by different mechanisms

The role of K+ channels in the relaxations induced by bradykinin, lemakalim, an activator of ATP-sensitive K+ channels and sodium nitroprusside (SNP), a nitric oxide (NO) donor was examined in the isolated mouse trachea precontracted by methacholine (1 microM). 4-aminopyridine (4-AP, 0.1-2 mM), an inhibitor of 4-AP sensitive delayed rectifier channels, did not alter relaxations induced by bradykinin, lemakalim or SNP. Glibenclamide and glipizide (10-33 microM), inhibitors of ATP-sensitive K+ channels, inhibited relaxation to lemakalim without affecting responses to bradykinin and SNP. Charybdotoxin (10-100 nM) and iberiotoxin (10-100 nM), inhibitors of large conductance Ca2+-activated K+ channels, failed to inhibit relaxation to bradykinin, lemakalim or SNP. Apamin (0.1-1 microM), an inhibitor of small conductance Ca2+-activated K+ channels, did not alter responses to bradykinin, lemakalim and SNP. The results implicate that the mechanism of relaxation induced by bradykinin and SNP is different from that of lemakalim. Relaxation of the isolated mouse trachea by lemakalim appears to be mediated by ATP-sensitive K+ channels. Bradykinin and SNP induced relaxations are not mediated via Ca2+-activated K+ channels.

Comparison of responses to T-kinin and bradykinin in the mesenteric vascular bed of the cat

Responses to T-kinin and bradykinin were compared in the mesenteric vascular bed of the cat. Under constant-flow conditions, injection of T-kinin and bradykinin into the perfusion circuit induced similar dose-related decreases in perfusion pressure. Responses to T-kinin and bradykinin were inhibited by the kinin B2 receptor antagonist Hoe-140, but were not altered by the B1 receptor antagonist des-Arg9-[Leu8]-BK, the histamine H1 antagonist pyrilamine, the histamine H2 receptor antagonist cimetidine, or the H3 receptor antagonist thioperamide. Vasodilator responses to T-kinin and bradykinin were attenuated by the nitric oxide synthase inhibitor, N omega Nitro-L-arginine methyl ester (L-NAME), but were not altered by the cyclooxygenase inhibitor, sodium meclofenamate, or the K+ ATP channel antagonist, U37883A. These data suggest that vasodilator responses to T-kinin and bradykinin are mediated by kinin B2 receptor stimulated release of nitric oxide from the endothelium, but that the activation of kinin B1 receptors, the release of vasodilator prostaglandins, or the opening of K+ ATP channels are not involved in the response to T-kinin in the mesenteric vascular bed of the cat.

A transferable, beta-naphthoflavone-inducible, hyperpolarizing factor is synthesized by native and cultured porcine coronary endothelial cells

1. The vascular endothelium releases a hyperpolarizing factor (endothelium-derived hyperpolarizing factor, EDHF) tentatively identified as a cytochrome P450-derived arachidonic acid metabolite. However, there is still controversy concerning its transferability and identity. We designed a bioassay system for assessing EDHF release in which the membrane potential was recorded in cultured vascular smooth muscle cells located downstream from donor endothelial cells. 2. Under combined nitric oxide (NO) synthase and cyclo-oxygenase blockade with NG-nitro-L-arginine (100 mumol l-1) and diclofenac (10 mumol l-1), the superfusate from bradykinin (30 mumol l-1)-stimulated, cultured porcine coronary endothelial cells induced a distinct hyperpolarization followed by a depolarization. Direct application of bradykinin to the smooth muscle cells resulted solely in membrane depolarization. Similar results were obtained using bradykinin-stimulated porcine coronary arteries as donor. 3. Single-channel current measurements suggest that this EDHF-induced hyperpolarization was elicited by the activation of Ca(2+)-dependent K+ channels. 4. Increasing the transmural pressure within the donor segment significantly enhanced the duration, but not the amplitude of the hyperpolarization induced by the effluate from bradykinin-stimulated donor segments. 5. Inhibition of P450 oxygenase activity with clotrimazole (3 mumol l-1) or 17-octadecynoic acid (3 mumol l-1) abolished EDHF release from the coronary endothelium, while the P450-derived arachidonic acid metabolite, 5,6-epoxyeicosatrienoic acid, induced a hyperpolarization of detector smooth muscle cells almost identical to that induced by EDHF. Moreover, induction of P450 activity by beta-naphthoflavone (3 mumol l-1, 48 h), significantly increased the bradykinin-induced release of EDHF. 6. These findings suggest that the vascular endothelium releases a transferable hyperpolarizing factor, chemically distinct from NO and prostacyclin, in response to agonists and mechanical stimulation. This beta-naphthoflavone-inducible EDHF appears to be a cytochrome P450-derived metabolite of arachidonic acid, which elicits hyperpolarization by activation of Ca(2+)-dependent K+ channels.

Nitric oxide attenuates the release of endothelium-derived hyperpolarizing factor

BACKGROUND

The contribution of the endothelium-derived hyperpolarizing factor (EDHF), proposed to be a cytochrome P450-derived metabolite of arachidonic acid, to endothelium-dependent dilatation under physiological conditions has yet to be established, because its effect can be detected only after inhibition of NO synthase and cyclooxygenase. The possibility that NO exerts a feedback inhibition on EDHF formation was studied in isolated perfused arterial segments.

METHODS AND RESULTS

Under combined blockade of NO synthase and cyclooxygenase, the EDHF-mediated vasodilatation elicited by receptor-dependent agonists in rabbit carotid and porcine coronary arteries was significantly attenuated by the NO donors C87-3786 and CAS 1609. The endothelium-independent dilatation elicited by isoproterenol was not altered by either NO donor. In NG-nitro-L-arginine-treated carotid artery segments, C87-3786 significantly attenuated the acetylcholine-induced increase in 6-keto-prostaglandin F1 alpha release, which was taken as an index of arachidonic acid liberation. In parallel experiments using cultured human endothelial cells, C87-3786 attenuated the Ca2+ response to bradykinin. The release of EDHF from a luminally perfused porcine coronary artery was detected by recording the membrane potential of downstream-situated cultured rat aortic smooth muscle cells. The NO donor C87-3786 had no effect on the hyperpolarization elicited by preformed EDHF but markedly inhibited its release from bradykinin-stimulated donor segments.

CONCLUSIONS

These findings indicate that under physiological conditions, the production of EDHF is damped by NO. Therefore, it follows that when NO synthesis is impaired, alleviation of this intrinsic inhibition may, at least in part, maintain endothelial vasodilator function.

Effects of cytochrome P450 inhibitors on EDHF-mediated relaxation in the rat hepatic artery

1. The possibility that the endothelium-derived hyperpolarising factor (EDHF) in the rat hepatic artery is a cytochrome P450 mono-oxygenase metabolite of arachidonic acid was examined in the present study. In this preparation, acetylcholine elicits EDHF-mediated relaxations in the presence of the nitric oxide (NO) synthase and cyclo-oxygenase inhibitors N omega-nitro-L-arginine (L-NOARG) and indomethacin, respectively. 2. 17-Octadecynoic acid (17-ODYA, 50 microM), a suicide-substrate inhibitor of the cytochrome P450 mono-oxygenases responsible for the production of 5,6-, 8,9-, 11,12- and 14,15-epoxyeicosatrienoic acids (EETs), had no effect on acetylcholine-induced relaxations in the presence of L-NOARG (0.3 mM) plus indomethacin (10 microM). Furthermore, 5,6-, 8,9-, 11,12- and 14,15- EETs failed to relax arteries without endothelium in the presence of L-NOARG plus indomethacin. 3. Proadifen and clotrimazole, which are inhibitors of several isoforms of cytochrome P450 mono-oxygenases, inhibited acetylcholine-induced relaxations in the presence of L-NOARG plus indomethacin. The concentration of acetylcholine which caused half-maximal relaxation was about 3 and 30 times higher in the presence than in the absence of clotrimazole (3 microM) and proadifen (10 microM), respectively. The maximal relaxation was reduced by proadifen but not by clotrimazole. Proadifen (10 microM) also inhibited acetylcholine-induced hyperpolarization in the presence of L-NOARG plus indomethacin. 4. In the presence of 30 mM K+ plus indomethacin (10 microM), acetylcholine induced an L-NOARG-sensitive relaxation mediated via release of NO. Under these conditions, proadifen (10 microM) shifted the acetylcholine concentration-response curve 6 fold to the right without affecting the maximal relaxation. Clotrimazole (3 microM) was without effect on these responses. The relaxant actions of the NO donor, 3-morpholino-sydnonimine, were unaffected by proadifen (10 microM). 5. The relaxant effects of the opener of ATP-sensitive potassium channels, levcromakalim, were abolished by proadifen (10 microM) and strongly attenuated by clotrimazole (3 microM). Proadifen (10 microM) also abolished the hyperpolarization induced by levcromakalim (1 microM). 6. The lack of effect of 17-ODYA on relaxations mediated by EDHF, together with the failure of extracellularly-applied EETs to produce relaxation, collectively suggest that EDHF is not an EET in the rat hepatic artery. It seems likely that inhibition of ion channels in the smooth muscle rather than reduced EDHF formation in the endothelium offers a better explanation for the actions of the cytochrome P450 inhibitors proadifen and clotrimazole.

Contribution of calcium-activated potassium channels to the vasodilator effect of bradykinin in the isolated, perfused kidney of the rat

1. NO- and prostaglandin-independent, endothelium-dependent vasodilator responses to bradykinin are attributed to release of a hyperpolarizing factor. Therefore, the contribution of K+ channels to the renal vasodilator effect of bradykinin was examined in rat perfused kidneys that were preconstricted with phenylephrine and treated with NG-nitro-L-arginine (L-NOARG) and indomethacin to inhibit NO and prostaglandin synthesis. 2. The non-specific K+ channel inhibitors, TEA and TBA reduced vasodilator responses to bradykinin and cromakalim but not those to nitroprusside. 3. Glibenclamide, an inhibitor of ATP-sensitive K+ channels, blocked the vasodilator response to cromakalim without affecting responses to bradykinin. 4. Charybdotoxin, a selective inhibitor of Ca(2+)-activated K+ channels, greatly attenuated vasodilator responses to bradykinin without affecting those to cromakalim or nitroprusside. 5. Iberiotoxin and leiurotoxin, inhibitors of large and small conductance Ca(2+)-activated K+ channels, respectively, were without effect on vasodilator responses to bradykinin, cromakalim or nitroprusside. 6. These results implicate K+ channels, specifically Ca(2+)-activated K+ channels of intermediate conductance, in the renal vasodilator effect of bradykinin and, thereby, support a role for a hyperpolarizing factor.

Paracrine functions of the coronary vascular endothelium

Coronary vascular endothelial cells control vascular tone by modulating the local concentration of circulating vasoactive substances (e.g. adenine nucleotides, biogenic amines and bradykinin) and by synthesising and releasing the vasoactive autacoids nitric oxide (NO) and prostacyclin (PGI2). The fluid shear stress exerted by the streaming blood is the physiologically most important stimulus for a continuous endothelial NO production, which counteracts neuro- and myogenic constriction. This shear stress-dependent NO release represents a highly effective local system for maintaining adequate blood flow to the myocardial tissue. At the transcriptional level endothelium-derived NO modulates the regulation of a number of genes (e.g. monocyte chemoattractant protein-1, P-selectin and vascular cell adhesion molecule-1) most probably by direct and/or indirect interaction with transcription factors. In addition to NO and PGI2, the coronary vascular endothelium is also able to release a factor which causes hyperpolarisation of the underlying smooth muscle. This so-called endothelium-derived hyperpolarising factor (EDHF) displays the characteristics of a cytochrome P450-derived arachidonic acid metabolite. However, since NO is able to attenuate production of this factor, EDHF may contribute to the regulation of vascular tone essentially in situations associated with an apparent dysfunction of the endothelium.

Role of potassium channels in endothelium-dependent relaxation resistant to nitroarginine in the rat hepatic artery

1. In the presence of indomethacin (IM, 10 microM) and N omega-nitro-L- arginine (L-NOARG, 0.3 mM), acetylcholine (ACh) induces an endothelium-dependent smooth muscle hyperpolarization and relaxation in the rat isolated hepatic artery. The potassium (K) channel inhibitors, tetrabutylammonium (TBA, 1 mM) and to a lesser extent 4-aminopyridine (4-AP, 1 mM) inhibited the L-NOARG/IM-resistant relaxation induced by ACh, whereas apamin (0.1-0.3 microM), charybdotoxin (0.1-0.3 microM), iberiotoxin (0.1 microM) and dendrotoxin (0.1 microM) each had no effect. TBA also inhibited the relaxation induced by the receptor-independent endothelial cell activator, A23187. 2. When combined, apamin (0.1 microM) + charybdotoxin (0.1 microM), but not apamin (0.1 microM) + iberiotoxin (0.1 microM) or a triple combination of 4-AP (1 mM) + apamin (0.1 microM) + iberiotoxin (0.1 microM), inhibited the L-NOARG/IM-resistant relaxation induced by ACh. At a concentration of 0.3 microM, apamin + charybdotoxin completely inhibited the relaxation. This toxin combination also abolished the L-NOARG/ IM-resistant relaxation induced by A23187. 3. In the absence of L-NOARG, TBA (1 mM) inhibited the ACh-induced relaxation, whereas charybdotoxin (0.3 microM) + apamin (0.3 microM) had no effect, indicating that the toxin combination did not interfere with the L-arginine/NO pathway. 4. The gap junction inhibitors halothane (2 mM) and 1-heptanol (2 mM), or replacement of NaCl with sodium propionate did not affect the L-NOARG/IM-resistant relaxation induced by ACh. 5. Inhibition of Na+/K(+)-ATPase by ouabain (1 mM) had no effect on the L-NOARG/IM-resistant relaxation induced by ACh. Exposure to a K(+)-free Krebs solution, however, reduced the maximal relaxation by 13% without affecting the sensitivity to ACh. 6. The results suggest that the L-NOARG/IM-resistant relaxation induced by ACh in the rat hepatic artery is mediated by activation of K-channels sensitive to TBA and a combination of apamin + charybdotoxin. Chloride channels, Na+/K(+)-ATPase and gap junctions are probably not involved in the response. It is proposed that endothelial cell activation induces secretion of an endothelium-derived hyperpolarizing factor(s) (EDHF), distinct from NO and cyclo-oxygenase products, which activates more than one type of K-channel on the smooth muscle cells. Alternatively, a single type of K-channel, to which both apamin and charybdotoxin must bind for inhibition to occur, may be the target for EDHF.

Effects of glibenclamide on the regional haemodynamic actions of alpha-trinositol and its influence on responses to vasodilators in conscious rats

1. In conscious rats, alpha-trinositol (D-myo-inositol-1, 2, 6 triphosphate; 5-80 mg kg-1 h-1 infusion) caused dose-dependent hypotension, tachycardia and hyperaemic dilatation in renal, mesenteric and hindquarters vascular beds. These effects were accompanied by inhibition of the renal vasodilator effects of acetylcholine (ACh), and of the mesenteric vasodilator effects of sodium nitroprusside (SNP) and, particularly, of levcromakalim (LCK). 2. In the light of the latter finding, in a second experiment, we assessed the influence of the KATP channel inhibitor, glibenclamide (20 mg kg-1), on resting haemodynamics, on responses to ACh, bradykinin (BK), SNP and LCK, on the haemodynamic action of alpha-trinositol, and on the effects of the latter on responses to the vasodilators, over a period of 3 days. 3. In the presence of saline, glibenclamide caused a reproducible pressor effect, accompanied by renal, mesenteric, and hindquarters vasoconstrictions on all 3 experimental days; these effects were unrelated to changes in blood glucose. In the presence of glibenclamide, only the hindquarters vasodilator response to BK, and all the cardiovascular actions of LCK were inhibited. 4. On the first experimental day, the hindquarters vasodilator effect of alpha-trinositol was substantially inhibited by glibenclamide, the renal vasodilatation less so, and the mesenteric vasodilatation not at all. However, over the subsequent two days, the mesenteric vasodilator effect of alpha-trinositol became increasingly sensitive to glibenclamide. 5. In the presence of alpha-trinositol and glibenclamide, on the first experimental day, the inhibition of the renal vasodilator effect of ACh was no greater than with alpha-trinositol alone in the first experiment. Moreover, on the third experimental day, both before and after glibenclamide, the inhibition by alpha-trinositol of the renal vasodilator response to ACh was less than on the first experimental day. Similarly, the alpha-trinositol-induced inhibition of the mesenteric vasodilator effect of SNP, and of the hindquarters vasodilator action of BK, waned over the 3 experimental days. The inhibition of the cardiovascular effects of LCK were similar on all 3 experimental days, but no greater in the presence of alpha-trinositol and glibenclamide than with glibenclamide alone. 6. These results indicate that KATP channels are involved in the maintenance of resting vasodilator tone in renal, mesenteric and hindquarters vascular beds. However, although additional activation of KATP channels is responsible for all the haemodynamic effects of LCK, it contributes only to the hindquarters vasodilator action of BK and is not involved in any of the responses to ACh or SNP. The hindquarters, renal and mesenteric vasodilator effects of alpha-trinositol may involve (in the same rank order) activation of KATP channels, probably through an indirect mechanism. However, it is unlikely that direct or indirect interaction of alpha-trinositol with KATP channels explains the ability of the drug to inhibit the renal vasodilator action of ACh, or the mesenteric vasodilator effects of SNP or LCK.

Angiotensin-(1-7) potentiates the hypotensive effect of bradykinin in conscious rats

Treatment with angiotensin-converting enzyme inhibitors increases the angiotensin-(1-7) [Ang-(1-7)] and bradykinin concentrations in plasma and tissue. In this study we evaluated the interaction between these peptides by determining the effect of Ang-(1-7) on the hypotensive action of bradykinin in conscious rats. Administration of Ang-(1-7) (5 nmol) did not change mean arterial pressure or heart rate. However, the hypotensive effect of bradykinin, produced by an intravenous or intra-arterial route, was potentiated by Ang-(1-7) in a dose-dependent manner. The Ang-(1-7) doses necessary to transform the effect of a single dose of bradykinin into that produced by a double dose (potentiating unit) were 2 nmol i.v. and 5 nmol IA. The Ang-(1-7) dose used did not change either the pressor effect of Ang II or the hypotensive effect of sodium nitroprusside. The bradykinin-potentiating Ang-(1-7) activity was significantly attenuated by pretreatment with indomethacin (5 mg/kg IM, n = 4). In an additional group the bradykinin-potentiating activity of Ang-(1-7) was evaluated 30 minutes after treatment with the angiotensin-converting enzyme inhibitor enalaprilat (10 mg/kg i.v., n = 9). Under this condition the bradykinin-potentiating activity of Ang-(1-7) was substantially increased, resulting in a potentiating unit of approximately 0.2 nmol IV. Pretreatment with indomethacin (5 mg/kg IM, n = 7) also attenuated the bradykinin-potentiating activity of Ang-(1-7) in enalaprilat-treated rats. These results show that Ang-(1-7) is a bradykinin-potentiating peptide in vivo. Furthermore, the data obtained with indomethacin suggest that prostaglandins participate in the mechanism of the bradykinin potentiation by Ang-(1-7). More importantly, these data suggest that the interaction between Ang-(1-7) and bradykinin can contribute to the pharmacological effects of angiotensin-converting enzyme inhibitors.

Volatile and intravenous anesthetics selectively attenuate the release of endothelium-derived hyperpolarizing factor elicited by bradykinin in the coronary microcirculation

In addition to nitric oxide (NO) and prostacyclin (PGI2) another endothelium-derived factor, which hyperpolarizes vascular smooth muscle cell via activation of K+ channels, contributes to the vasorelaxant effect of bradykinin in different vascular beds. Preliminary findings suggest that this endothelium-derived hyperpolarizing factor (EDHF)-mediated vasodilatation is attenuated by both volatile and intravenous anesthetics. Since EDHF may play an important role in the coronary microcirculation, we investigated the effects of isoflurane (2 vol.% equivalent to approximately 250 microM), etomidate (30 and 100 microM), phenobarbital (100 microM) and thiopental (30 and 100 microM) on the EDHF-mediated dilator response to bradykinin and on the endothelium-independent dilatation evoked by sodium nitroprusside (SNP) in the isolated saline-perfused rat heart (Langendorff preparation). None of the anesthetics tested affected the dilator response to bradykinin or SNP under basal conditions. However, following inhibition of NO and PGI2 formation with NG-nitro-L-arginine (100 microM) and diclofenac (1 microM) respectively, isoflurane, etomidate and thiopental, but not phenobarbital, significantly attenuated the NO/PGI2-independent, i.e. EDHF-mediated dilator response to bradykinin, while the vasorelaxant effect of SNP remained unaffected. Isoflurane, etomidate and thiopental, but not phenobarbital, display cytochrome P450-inhibiting properties, suggesting that these anesthetics impair the cytochrome P450-dependent synthesis of EDHF in the coronary microcirculation.