Different activation of L-arginine pathway by bradykinin, serotonin, and clonidine in coronary arteries

Differential expression of α2D-adrenoceptor and eNOS in aortas from early and later stages of diabetes in Goto-Kakizaki rats

The aim of the present study was to compare vascular dysfunction between the early (12 wk old) and later (36 wk old) stages of spontaneous diabetes in Goto-Kakizaki (GK) rats. We also evaluated the aortic expression of the α2D-adrenoceptor and endothelial nitric oxide synthase (eNOS). Vascular reactivity was assessed in thoracic aortas from age-matched control rats and 12- and 36-wk GK rats. Using RT-PCR and immunoblots, we also examined the changes in expression of the α2D-adrenoceptor and eNOS. In aortas from GK rats (vs. those from age-matched control rats): 1) the relaxation response to ACh was enhanced at 12 wk but decreased at 36 wk; 2) the relaxation response to sodium nitroprusside was decreased at both 12 and 36 wk, 3) norepinephrine (NE)-induced contractility was decreased at 12 wk but not at 36 wk, 4) the expressions of α1B- and α1D-adrenoceptors were unaffected, whereas those of α2D-adrenoceptor and eNOS mRNAs were increased at both 12 and 36 wk; and 5) NE- and ACh-stimulated NOx (nitrite and nitrate) levels were increased at 12 wk, although at 36 wk ACh-stimulated NOx was lower, whereas NE-stimulated NOx showed no change. These results clearly demonstrate that enhanced ACh-induced relaxation and impaired NE-induced contraction, due to NO overproduction via eNOS and increased α2D-adrenoceptor expression, occur in early-stage GK rats and that the impaired ACh-induced relaxation in later-stage GK rats is due to reductions in both NO production and NO responsiveness (but not in eNOS expression).

Sphingosine 1-phosphate and control of vascular tone

Sphingosine-1-phosphate (S1P) is a platelet-derived lipid mediator that activates the endothelial isoform of nitric oxide synthase (eNOS) in endothelial cells. However, the role of S1P in endothelium-dependent vasodilation and the signaling pathways elicited by S1P in intact vessels are largely unknown. We found that S1P induces dose-dependent transient relaxation of isolated pressurized mesenteric arterioles (EC(50) 10 +/- 3 nM); maximal vasodilation (55 +/- 8%) is seen approximately 2 min after S1P addition and returns to baseline by 5 min. S1P promotes comparable responses in arterioles from wild-type but not eNOS(null) mice. S1P-induced vasodilation is abrogated by removal of endothelium or by the addition of the NOS inhibitor N(omega)-monomethyl-l-arginine but is not affected by the cyclooxygenase inhibitor indomethacin, nor by the blockade of K(+) channels by using 4-aminopyridine. S1P-induced vasodilation is attenuated by pertussis toxin, by the phosphoinositide 3-kinase (PI3-kinase) inhibitor wortmannin, and by the calcium chelator BAPTA. With the use of high-sensitivity protein immunoblots in extracts from single pressurized vessels, we found that S1P, but not bradykinin, promotes the phosphorylation of eNOS at Ser(1179). Maximum S1P-induced eNOS Ser(1179) phosphorylation was reached at the time of maximum vasorelaxation, but enzyme phosphorylation persisted for several minutes after vasodilation had resolved. Thus regulatory pathways distinct from eNOS Ser(1179) dephosphorylation serve to terminate agonist-promoted vasorelaxation. Taken together, our findings demonstrate that S1P, an important intercellular mediator of platelet-vessel wall interactions, is a effective arteriolar vasodilator that acts via G protein-dependent, calcium-sensitive, and PI3-kinase-modulated signaling pathways.

Acute and chronic NOS inhibition enhances alpha(2)- adrenoreceptor-stimulated RhoA and Rho kinase in rat aorta

We demonstrated that arteries from rats made hypertensive with chronic nitric oxide (NO) synthase (NOS) inhibition (N(omega)-nitro-L-arginine in drinking water, LHR) have enhanced contractile sensitivity to alpha(2)-adrenergic receptors (alpha(2)-AR) agonist UK-14304 compared with arteries from normotensive rats (NR). NO may regulate vascular tone in part through suppression of RhoA and Rho kinase (ROK). We hypothesized that enhanced RhoA and ROK activity augments alpha(2)-AR contraction in LHR aortic rings. Y-27632 eliminated UK-14304 contraction in LHR and NR aortic rings. The order of increasing sensitivity to Y-27632 was the following: endothelium-intact NR, LHR, and endothelium-denuded NR. UK-14304 stimulated RhoA translocation to the membrane fraction in LHR and denuded NR but not in intact NR aorta. Basally, more RhoA was present in the membrane fraction in denuded NR than in intact NR or LHR aorta. Relaxation to S-nitroso-N-acetyl-penicillamine and Y-27632 in denuded ionomycin-permeabilized rings was greater in NR than in LHR. Together these studies indicate alpha(2)-AR contraction depends on ROK activity more in NR than LHR aorta. Additionally, endogenous NO may regulate RhoA activation, whereas chronic NOS inhibition appears to cause RhoA desensitization.

Consequences of reduced production of NO on vascular reactivity of porcine coronary arteries after angioplasty: importance of EDHF

1 The consequences of the reduced production of nitric oxide (NO) by cells from regenerated endothelium were investigated by measuring membrane potential of smooth muscle cells (SMCs), isometric tension and cyclic nucleotides content in porcine coronary arteries with intimal thickening, four weeks following angioplasty. 2 Under basal conditions, SMCs of coronary arteries with regenerated endothelium were depolarized by 10 mV. This depolarization was associated with 82% decreased level of cGMP without alteration in cAMP. 3 Sodium nitroprusside (SNP, 1 micro M) repolarized SMCs of the previously denuded coronary arteries. This repolarization was abolished by 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 micro M) and not suppressed by glibenclamide (10 micro M), iberiotoxin (IbTX, 100 nM) and the combination of charybdotoxin (ChTX, 40 nM) plus apamin (100 nM). 4 Four-aminopyridine (4-AP, 1-5 mM) generated spontaneous rhythmic activities only in coronary arteries with regenerated endothelium which were abolished by SNP. Nevertheless, 4-AP did not suppress the repolarization induced by SNP. 5 In vascular segments with regenerated endothelium, contracted with prostaglandin F(2alpha) (PGF(2alpha)), relaxation to bradykinin (BK, 30 nM) was unaltered despite a reduced production of cGMP (-70%). Indomethacin (10 micro M) plus N(omega)-nitro-L-arginine (L-NA, 30 micro M) reduced relaxation (-12% and -35% for native and regenerated endothelium, respectively) but did not abolish it. 6 The hyperpolarizations induced by BK were not altered by the presence of indomethacin and L-NA and were unchanged in segments with regenerated endothelium. 7 These data are consistent with a contribution of impairment in NO production to the depolarization of SMCs. Nevertheless, EDHF responses to BK are sufficient to maintain a normal relaxation after angioplasty.

Alpha(2)-adrenergic-mediated tubular NO production inhibits thick ascending limb chloride absorption

Stimulation of alpha(2)-adrenergic receptors inhibits transport in various nephron segments, and the thick ascending limb of the loop of Henle (THAL) expresses alpha(2)-receptors. We hypothesized that selective alpha(2)-receptor activation decreases NaCl absorption by cortical THALs through activation of NOS and increased production of NO. We found that the alpha(2)-receptor agonist clonidine (10 nM) decreased chloride flux (J(Cl)) from 119.5 +/- 15.9 to 67.4 +/- 13.8 pmol. mm(-1). min(-1) (43% reduction; P < 0.02), whereas removal of clonidine from the bath increased J(Cl) by 20%. When NOS activity was inhibited by pretreatment with 5 mM N(G)-nitro-L-arginine methyl ester, the inhibitory effects of clonidine on THAL J(Cl) were prevented (81.7 +/- 10.8 vs. 71.6 +/- 6.9 pmol. mm(-1). min(-1)). Similarly, when the NOS substrate L-arginine was deleted from the bath, addition of clonidine did not decrease THAL J(Cl) from control (106.9 +/- 11.6 vs. 132.2 +/- 21.3 pmol. mm(-1). min(-1)). When we blocked the alpha(2)-receptors with rauwolscine (1 microM), we found that the inhibitory effect of 10 nM clonidine on THAL J(Cl) was abolished, verifying that alpha(2), rather than I(1), receptors mediate the effects of clonidine in the THAL. We investigated the mechanism of NOS activation and found that intracellular calcium concentration did not increase in response to clonidine, whereas pretreatment with 150 nM wortmannin abolished the clonidine-mediated inhibition of THAL J(Cl), indicating activation of phosphatidylinositol 3-kinase and the Akt pathway. We found that pretreatment of THALs with 10 microM LY-83583, an inhibitor of soluble guanylate cyclase, blocked clonidine-mediated inhibition of THAL J(Cl). In conclusion, alpha(2)-receptor stimulation decreases THAL J(Cl) by increasing NO release and stimulating guanylate cyclase. These data suggest that alpha(2)-receptors act as physiological regulators of THAL NO synthesis, thus inhibiting chloride transport and participating in the natriuretic and diuretic effects of clonidine in vivo.

Melatonin potentiates contractile responses to serotonin in isolated porcine coronary arteries

The present study was designed to determine the effects of melatonin on coronary vasomotor tone. Porcine coronary arteries were suspended in organ chambers for isometric tension recording. Melatonin (10(-10)-10(-5) M) itself caused neither contraction nor relaxation of the tissues. Serotonin (10(-9)-10(-5) M) caused concentration-dependent contractions of coronary arteries, and in the presence of melatonin (10(-7) M) the maximal response to serotonin was increased in rings with but not without endothelium. In contrast, melatonin had no effect on contractions produced by the thromboxane A(2) analog U-46619 (10(-10)-10(-7) M). The melatonin-receptor antagonist S-20928 (10(-6) M) abolished the potentiating effect of melatonin on serotonin-induced contractions in endothelium-intact coronary arteries, as did treatment with 1H-[1, 2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10(-5) M), methylene blue (10(-5) M), or N(G)-nitro-L-arginine (3 x 10(-5) M). In tissues contracted with U-46619, serotonin caused endothelium-dependent relaxations that were inhibited by melatonin (10(-7) M). Melatonin also inhibited coronary artery relaxation induced by sodium nitroprusside (10(-9)-10(-5) M) but not by isoproterenol (10(-9)-10(-5) M). These results support the hypothesis that melatonin, by inhibiting the action of nitric oxide on coronary vascular smooth muscle, selectively potentiates the vasoconstrictor response to serotonin in coronary arteries with endothelium.

alpha(2)-adrenergic receptor-mediated increase in NO production buffers renal medullary vasoconstriction

The present study was designed to investigate the role of nitric oxide (NO) in modulating the adrenergic vasoconstrictor response of the renal medullary circulation. In anesthetized rats, intravenous infusion of norepinephrine (NE) at a subpressor dose of 0.1 microgram. kg(-1). min(-1) did not alter renal cortical (CBF) and medullary (MBF) blood flows measured by laser-Doppler flowmetry nor medullary tissue PO(2) (P(m)O(2)) as measured by a polarographic microelectrode. In the presence of the NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME) in the renal medulla, intravenous infusion of NE significantly reduced MBF by 30% and P(m)O(2) by 37%. With the use of an in vivo microdialysis-oxyhemoglobin NO-trapping technique, we found that intravenous infusion of NE increased interstitial NO concentrations by 43% in the renal medulla. NE-stimulated elevations of tissue NO were completely blocked either by renal medullary interstitial infusion of L-NAME or the alpha(2)-antagonist rauwolscine (30 microgram. kg(-1). min(-1)). Concurrently, intavenous infusion of NE resulted in a significant reduction of MBF in the presence of rauwolscine. The alpha(1)-antagonist prazosin (10 microgram. kg(-1). min(-1) renal medullary interstitial infusion) did not reduce the NE-induced increase in NO production, and NE increased MBF in the presence of prazosin. Microdissection and RT-PCR analyses demonstrated that the vasa recta expressed the mRNA of alpha(2B)-adrenergic receptors and that medullary thick ascending limb and collecting duct expressed the mRNA of both alpha(2A)- and alpha(2B)-adrenergic receptors. These subtypes of alpha(2)-adrenergic receptors may mediate NE-induced NO production in the renal medulla. We conclude that the increase in medullary NO production associated with the activation of alpha(2)-adrenergic receptors counteracts the vasoconstrictor effects of NE in the renal medulla and may play an important role in maintaining a constancy of MBF and medullary oxygenation.

Femtomolar bradykinin-induced relaxation of isolated bovine coronary arteries, mediated by endothelium-derived nitric oxide

We reported previously that bradykinin induces endothelium-dependent relaxation at nanomolar (nM) concentrations in isolated bovine coronary arteries with an intact endothelium. Recently we have found that in the presence of 10 microM indomethacin, femtomolar (fM) concentrations of bradykinin induce endothelium-dependent relaxation in some bovine coronary arteries (approximately 10% of the coronary arteries examined). The present study was designed to characterize the relaxation induced by fM bradykinin. Relaxation was completely abolished by repeated application of fM bradykinin, by 100 microM N(omega)-nitro-L-arginine methyl ester and by 10 microM methylene blue. Relaxation induced by nM bradykinin was partly affected by these treatments. Relaxation induced by both concentrations of bradykinin was inhibited by a B2-kinin receptor antagonist, [Thi5,8, D-Phe7]-bradykinin, in a concentration-dependent manner, but not by a B1-kinin receptor antagonist, des-Arg9, [Leu8]-bradykinin. In the presence of 10 microM captopril, an angiotensin-converting enzyme (ACE) inhibitor, all coronary arteries examined in this experiment showed endothelium-dependent relaxation to fM bradykinin. These results show that some bovine coronary arteries relax in response to fM bradykinin, and this response is mediated predominantly by the release of nitric oxide via stimulation of endothelial B2-kinin receptors. The relaxation may be dependent on ACE activity.

Exogenous L-arginine does not affect angiotensin II-induced renal vasoconstriction in man

AIMS

It has been suggested that provision of the substrate of nitric oxide (NO) synthesis, L-arginine, might influence the effects of renal vasoconstrictors. We have therefore studied the effects of pretreatment or concomitant administration of L-arginine on angiotensin II (ANG II)-increased renovascular resistance.

METHODS

The study was conducted in a double-blind, randomized, cross-over design. Eight healthy subjects were assigned to placebo or a continuous intravenous coinfusion of ANG II (5.0 ng kg[-1] min[-1], infusion period 75 min) with L-arginine (17 mg kg[-1] min[-1], infusion period 30 min). Nine further subjects received a continuous infusion of ANG II with or without pretreatment of L-arginine. Changes in renal plasma flow (RPF) were estimated by the steady state clearance of PAH.

RESULTS

L-arginine alone increased RPF to 110 +/- 10% over baseline (P < 0.003). The ANG II-induced decrease in RPF was not affected by pretreatment or coinfusion of L-arginine.

CONCLUSIONS

Our results demonstrate that a counterregulatory response of the renal vasculature to high levels of ANG II does not depend on exogenous L-arginine. In healthy subjects, this lack of functional antagonism at the renal vasculature is therefore not a result of NO substrate availability.

Physiological concentrations of estradiol attenuate endothelin 1-induced coronary vasoconstriction in vivo

BACKGROUND

Estrogens are cardioprotective hormones and are reported to have antianginal properties. We examined the effect of physiological concentrations of 17beta-estradiol on coronary reactivity in anesthetized female farm pigs.

METHODS AND RESULTS

Epicardial coronary cross-sectional area (CSA) was assessed by two-dimensional intravascular ultrasound, average coronary peak flow velocity (APV) by intravascular Doppler velocimetry, and coronary blood flow (CBF) was calculated. Dose-response curves to intracoronary endothelin-1 (ET-1, 1 pmol/L to 10 nmol/L), the selective ET(B) receptor agonist sarafotoxin (1 pmol/L to 10 nmol/L), and serotonin (0.1 nmol/L to 1 micromol/L) were assessed before and after a 10-minute infusion of intracoronary estradiol (1 nmol/L). Before estradiol administration, ET-1 induced significant dose-dependent decreases in CSA, APV, and CBF. Estradiol attenuated ET-1-induced epicardial vasoconstriction (P<.001) as well as ET-1-induced decreases in APV (P=.05) and CBF (P=.012). In an additional five pigs, vehicle (DMSO) had no effect on ET-1-induced coronary vasoconstriction. Before estradiol administration, sarafotoxin induced no net change in CSA but induced increases in APV and CBF, the extent of which did not change significantly after estradiol. Serotonin induced small decreases in CSA but increased APV and CBF. Estradiol did not influence serotonin-induced changes in CSA, APV, or CBF.

CONCLUSIONS

We conclude that estradiol attenuates ET-1-induced vasoconstriction, possibly through effects on the ET(A) receptor, because selective ET(B) receptor-induced stimulation with sarafotoxin remained unchanged. Such an effect on the ET(A) receptor may relate to the antianginal properties of estrogens.

Effect of 5-hydroxytryptamine on the membrane potential of endothelial and smooth muscle cells in the pig coronary artery

1. Many endothelium-dependent vasodilators hyperpolarize the endothelial cells in blood vessels. It is not known whether these hyperpolarizations are linked to nitric oxide synthesis or to an endothelium-derived hyperpolarizing phenomenon, since most of the vasodilators release both factors. In this context, we first verified that the endothelium-dependent relaxations induced by 5-hydroxytryptamine (5-HT) on pig coronary arteries are due only to the activation of the nitric oxide pathway. Then we studied the effects of 5-HT on membrane potential of endothelial and smooth muscle cells. 2. In the absence of endothelium, 5-HT caused a concentration-dependent contraction of coronary artery strips. No change of the smooth muscle cell membrane potential was observed during contraction to 1 microM 5-HT. 3. In the presence of 1 microM ketanserin to suppress the contractile effect of 5-HT, 5-HT induced concentration-dependent relaxation of endothelium-intact strips precontracted by 10 microM prostaglandin F2 alpha (PGF2 alpha). These relaxations were suppressed by 1 microM NG-nitro-L-arginine, an inhibitor of nitric oxide synthesis, showing that they were produced predominantly by nitric oxide. 4. In the presence of 1 microM ketanserin, 1 microM 5-HT did not change the smooth muscle cell membrane potential of strips precontracted by either 10 microM PGF2 alpha or by 10 microM acetylcholine (ACh). In the same conditions, 1 microM 5-HT caused a weak 2.6 +/- 0.4 mV hyperpolarization, of the endothelial cells. 5. In conclusion, the fact that 5-HT did not change the membrane potential of smooth muscle cells and only weakly hyperpolarized the endothelial cells during relaxations, suggests that in both cell types no electrical events accompany activation of the nitric oxide pathway. This is in contrast to the hyperpolarizations observed in endothelial and smooth muscle cells when the endothelium-derived hyperpolarization factor (EDHF) pathway is activated.

Cytochrome P450-dependent effects of bradykinin in the rat heart

1. Vasodilator responses to bradykinin (BK) in the rat heart are reported to be independent of NO and cyclo-oxygenase/lipoxygenase products of arachidonic acid (AA). 2. We verified that inhibition of NO synthase with L-nitroarginine (50 microM) and cyclo-oxygenase with indomethacin (2.8 microM) were without effect on vasodilator responses to BK (10-1000 ng) in the Langendorff rat heart preparation. 3. L-Nitroarginine elevated perfusion pressure, signifying a crucial role of NO in the maintenance of basal vasculature tone. 4. In hearts treated with L-nitroarginine to eliminate NO and elevate perfusion pressure, vasodilator responses were reduced by inhibitors of cytochrome P450 (P450), clotrimazole (1 microM) and 7-ethoxyresorufin (1 microM). 17-Octadecynoic acid (17-ODYA 2 microM), a mechanism based inhibitor of P450-dependent metabolism of fatty acids, also reduced vasodilator responses to BK. 5. These results confirm that NO and prostaglandins do not mediate vasodilator responses to BK in the rat heart but suggest a major role for a P450-dependent mechanism via AA metabolism.

Display of the characteristics of endothelium-derived hyperpolarizing factor by a cytochrome P450-derived arachidonic acid metabolite in the coronary microcirculation

1. In addition to nitric oxide (NO) and prostacyclin (PGI2) an endothelium-derived factor, which hyperpolarizes vascular smooth muscle cells via activation of K+ channels, contributes to the dilator effect of bradykinin in different vascular beds. Since this so-called endothelium-derived hyperpolarizing factor (EDHF) also seems to play an important role in the coronary circulation, we investigated its nature and mechanism of action in the rat isolated perfused heart (Langendorff preparation). 2. Bolus injections of bradykinin (1, 10, and 100 pmol) elicited a transient dose-dependent dilator response (e.g., 12 +/- 2% decrease in coronary perfusion pressure (CPP) at 10 pmol bradykinin, n = 41). Administration of the cyclo-oxygenase inhibitor, diclofenac (1 microM), augmented the bradykinin-induced dilation approximately twofold (n = 9 P < 0.01). Combined treatment with the NO synthase inhibitor, NG-nitro-L-arginine (30 microM) and diclofenac (1 microM) significantly reduced the duration, but increased the amplitude of the dilator response to bradykinin (27 +/- 2% decrease in CPP, n = 24, P < 0.01). 3. The abolition of this NG-nitro-L-arginine/diclofenac-insensitive dilator response to bradykinin by tetrabutylammonium (0.3 mM), an inhibitor of Ca(2+)-dependent K+ channels (4 +/- 1% decrease in CPP, n = 6, P < 0.01), supports the view that the dilator compound released in the coronary microcirculation is EDHF. 4. This EDHF-type dilation was reversibly inhibited by the phospholipase A2 inhibitor, quinacrine (3 microM, 9 +/- 3% decrease in CPP, n = 6, P < 0.01) and by the cytochrome P450 inhibitor SKF525a (3 microM, 6 +/- 1% decrease in CPP, n = 6, P < 0.01). 5. Tetrabutylammonium, quinacrine or SKF 525a did not affect the endothelium-independent dilator response to sodium nitroprusside (1 nmol), indicating that these compounds did not affect smooth muscle relaxation in a non-specific manner.6. These findings suggest that in the coronary microcirculation bradykinin stimulates the release of a cytochrome P450-derived arachidonic acid metabolite, which exhibits the characteristic features of EDHF.

The role of myoendothelial cell contact in non-nitric oxide-, non-prostanoid-mediated endothelium-dependent relaxation of porcine coronary artery

1. Experiments were designed to analyse the requirement of myoendothelial junctions by bradykinin-induced endothelium-dependent relaxations resistant to NG-nitro-L-arginine (L-NOARG) and indomethacin porcine coronary arteries. 2. Rings of porcine coronary arteries were contracted with the thromboxane receptor agonist, U46619 and relaxations to bradykinin recorded isometrically. All experiments were performed in the presence of indomethacin. Nitric oxide (NO)-mediated effects were blocked by the NO synthase inhibitor L-NOARG (250 microM) and myoendothelial contacts inhibited by treatment with hypertonic solution containing D-mannitol or sucrose (each 180 mM) or the gap junctional uncoupling agent 1-heptanol (2 mM). High [K+] solutions (40 mM) were used to probe a possible contribution of endothelium-derived hyperpolarizing factor (EDHF). 3. In the presence of endothelium, bradykinin induced concentration-dependent relaxations with a mean EC50 of 3.2 nM and a maximum response of 95 +/- 1% of papaverine-induced relaxation (control curve). 4. In the absence of endothelium, bradykinin failed to induce relaxations. Addition of cultured porcine aortic endothelial cells to the organ bath resulted in some relaxation and restored in part the relaxant effect of bradykinin. This endothelial cell-mediated relaxant effect was completely abolished in the presence of 250 microM L-NOARG. 5. Bradykinin-induced relaxations in endothelium-preserved rings were only slightly suppressed by L-NOARG (86% of control). In vessels partially depolarized by high extracellular [K+] (40 mM) relaxation was reduced to 72% of control. In the presence of L-NOARG, bradykinin failed to relax partially depolarized vessels. 6. In the presence of 2 mM -heptanol, 180 mM mannitol or 180 mM sucrose maximum relaxation to bradykinin was reduced to ~70%, i.e. to the same extent as in the presence of high [K+]. The remaining relaxation was sensitive to blockade by L-NOARG.7. Tissue cyclic GMP content which reflects NO activity, was increased about 4 fold by bradykinin(300 nM). This increase was unaffected by high [K+], heptanol or sucrose but blocked by L-NOARG.8 Our results suggest that non-nitric oxide- and non-prostanoid-mediated endothelium-dependent relaxation of porcine coronary artery requires functionally intact myoendothelial junctions.

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

1. The role of K+ channels in the nitric oxide (NO)-independent coronary vasodilator effect of bradykinin was examined in the Langendorff heart preparation in which nitroarginine was used to inhibit NO synthesis and elevate perfusion pressure; cyclo-oxygenase was inhibited with indomethacin. 2. The K+ channel inhibitors, tetraethylammonium, procaine and charybdotoxin, but not glibenclamide, further increased perfusion pressure suggesting a role for K+ channels, other than ATP-sensitive K+ channels, in the regulation of coronary vascular tone under the experimental conditions adopted here. 3. The non-specific K+ channel inhibitors, tetraethylammonium and procaine, reduced vasodilator responses to bradykinin and cromakalim but not those to nitroprusside in the perfused heart treated with nitroarginine and indomethacin. 4. Glibenclamide, an inhibitor of ATP-sensitive K+ channels, reduced vasodilator responses to cromakalim but did not affect those to bradykinin or nitroprusside. 5. Charybdotoxin, an antagonist of Ca(2+)-activated K+ channels, inhibited responses to bradykinin but did not affect those to cromakalim or nitroprusside. 6. Nifedipine inhibited vasodilator responses to bradykinin and cromakalim without affecting those to nitroprusside. 7. Inhibition of cytochrome P450 with clotrimazole reduced responses to bradykinin but did not modify those to cromakalim or nitroprusside. 8. These results suggest that bradykinin utilizes a Ca(2+)-activated K+ channel to produce vasodilatation in the rat heart.

Local L-NG-monomethyl-arginine attenuates the vasodilator action of bradykinin in the human forearm

1. Studies in animals indicate that bradykinin relaxes blood vessels directly through an action on smooth muscle and indirectly through the release of endothelium-derived mediators. Its precise mechanism of action in the human arterial circulation is not yet known. 2. In this study the effects of a specific inhibitor of nitric oxide synthase, L-NG-monomethyl-arginine (L-NMMA) and noradrenaline on the vasodilator responses to bradykinin were examined in the forearm arterial bed of healthy volunteers. Noradrenaline was used as a control for vasoconstriction by L-NMMA; glyceryl trinitrate (GTN) as a control vasodilator acting independently of the NO synthase enzyme. 3. L-NMMA (4 mumol min-1; 5 min) alone reduced resting forearm blood flow by 44% (P < 0.01; n = 6) confirming that nitric oxide plays an important role in regulating vascular tone. 4. Bradykinin (10 and 100 pmol min-1; 3 min each dose) and GTN (2 and 5 nmol min-1; 3 min each dose) increased forearm blood flow in a dose-dependent manner (percentage changes 171 +/- 17% and 398 +/- 35%, and 176 +/- 21% and 268 +/- 42%, respectively; n = 6). 5. The response to bradykinin, but not that to GTN, was attenuated by L-NMMA compared with noradrenaline (P < 0.05; n = 6), suggesting that bradykinin-induced vasodilatation in the forearm is mediated, at least in part, by stimulating release of nitric oxide.

Evidence for differential roles of nitric oxide (NO) and hyperpolarization in endothelium-dependent relaxation of pig isolated coronary artery

1. The possible roles of endothelial and smooth muscle cell hyperpolarization and nitric oxide (NO) in endothelium-dependent relaxation were examined in isolated rings of pig right coronary artery. 2. The effects of hyperpolarization were prevented with high K+ (30-125 mM), isotonic Krebs solutions. Functional antagonism due to high K(+)-induced smooth muscle contraction was prevented with 0.3 microM nifedipine (in all treatments, for consistency). All rings were contracted with the thromboxane-mimetic U46619, (1-100 nM) to bring them to an initial active force of within 30-50% of maximum contraction. 3. High K+ had no effects on the sensitivity (EC50) or time course of endothelium-dependent (substance P, SP; bradykinin, BK; calcimycin, A23187) and -independent (sodium nitroprusside, SNP) agents. Maximum relaxations (Rmax) to SP, BK and A23187 were reduced significantly by approximately 20% but only with 125 mM K+. 4. In normal K+ Krebs solution (5.9 mM), NG-nitro-L-arginine (L-NOARG; 100 microM) caused 40%, 20% and no reduction in Rmax for SP, BK and SNP respectively. EC50s for SP and BK were decreased significantly by approximately two fold whereas that for SNP was increased significantly by approximately ten fold. At all high K+ concentrations (30-125 mM), L-NOARG (100 microM) caused complete inhibition of relaxations to SP and BK but those to SNP were unaffected. 5. High K+ (30 mM) unmasked potent and concentration-dependent inhibition of relaxations of SP by L-NOARG. At 10 microM L-NOARG, all relaxation responses to SP were abolished and at the higher concentrations of SP (1-10 nM) small but significant contractions were observed. 6. N0-monomethyl-L-arginine (L-NMMA) had similar effects on relaxations to SP in the presence of 30 mM K+ except that maximum inhibition (40%) of Rmax was achieved at 10 MicroM L-NMMA and this was not increased with either 100 or 1000 MicroM L-NMMA. In normal K+, L-NMMA (1000 MicroM) only decreased the EC50 by approximately two fold, without affecting Rmax.7. High choline+ (25, 75 and 125 mM) isotonic Krebs also had no direct effect on the relaxations to SP,but like high K+, enabled L-NOARG (100 MicroM) to inhibit these responses completely. Neither charybdotoxin(30 nM) nor substitution of 25 mM NaCl with 50 mM sucrose had any direct effect on relaxations to SP or on the block of relaxations to SP by L-NOARG (100 MicroM).8. In conclusion, most if not all of the endothelium-dependent relaxation in the pig coronary artery in vitro is due to NO, but hyperpolarization can supplement 60% -80% of this response if NO synthesis is blocked. Multiple endothelium-derived factors could not only explain heterogeneity of the degree of block of endothelium-dependent relaxation responses by L-arginine analogues, but also constitute important 'back-up' mechanisms for control of arterial diameter.

Relaxation of porcine coronary artery to bradykinin. Role of arachidonic acid

Bradykinin-induced relaxation of precontracted, porcine coronary artery (PCA) rings is mediated by distinctly different endothelium-derived relaxing factors depending on the contractile agent used. Thus when contracted with KCl, bradykinin-induced relaxation of PCA rings is mediated solely by nitric oxide (NO), whereas when contracted with the thromboxane mimetic U46619, a small component of the relaxation is attributable to NO and a large component is attributable to a non-NO mechanism that is independent of cyclooxygenase activity. We hypothesized that the non-NO component was mediated by arachidonic acid (AA) or by a non-cyclooxygenase product of AA metabolism. Bradykinin-induced relaxations of PCA rings precontracted with U46619 in the presence of indomethacin (10 mumol/L) were moderately attenuated by the NO synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME, 100 mumol/L), whereas when precontracted with KCl, L-NAME abolished the relaxations. AA produced endothelium-dependent relaxations of rings precontracted with U46619 that were unaffected by L-NAME, whereas AA did not relax rings precontracted with KCl. In rings precontracted with U46619, in the presence of L-NAME and indomethacin the phospholipase inhibitors quinacrine (50 mumol/L) and 4-bromophenacyl bromide (10 mumol/L) attenuated bradykinin- but not AA-induced relaxations. Inhibitors of both lipoxygenase (BW 755c [100 mumol/L] and nafazatrom [20 mumol/L]) and cytochrome P-450 (proadifen [10 mumol/L] and clotrimazole [10 mumol/L]) pathways did not eliminate bradykinin- or AA-induced relaxations, although clotrimazole partially attenuated AA-induced relaxations. These findings suggest that bradykinin-induced relaxation of PCA rings is mediated by AA through a mechanism that is not dependent on cyclooxygenase, lipoxygenase, or cytochrome P-450 pathways.

Southwestern Internal Medicine Conference: endothelial dysfunction and vascular disease

Vascular endothelial cells, critically situated at the blood-tissue interface, exert important effects on vascular tone and permeability, regulate the coagulation and fibrinolytic systems, mediate translocation of inflammatory cells to the tissue compartment, and modulate proliferation of vascular smooth muscle cells. As the physiology of the endothelium has been defined, defects in endothelial function have been identified in association with human disease, and a syndrome of dysfunctional endothelium has been described. Although it remains debatable whether a coherent syndrome of endothelial dysfunction exists, disordered endothelial biology appears to contribute to the pathophysiology of human vascular disease. Identification of specific molecular mechanisms offers potential targets for novel therapeutic interventions, including genetic modification of endothelial cells in vivo.

Mechanism underlying substance P-induced relaxation in dog isolated superficial temporal arteries

1. In helical strips of dog superficial temporal arteries with intact endothelium, substance P elicited a concentration-related relaxation with an EC50 of 2.8 (2.4-3.2) x 10(-10) M. 2. The relaxant response to the peptide in low concentrations (1-4 x 10(-10) M) sufficient to produce approximately half maximal relaxation was not inhibited by indomethacin, but was markedly suppressed by NG-nitro-L-arginine (L-NOARG), a nitric oxide (NO) synthase inhibitor, and by endothelium denudation. 3. High concentration (10(-7) M) of substance P produced marked relaxations in endothelium-intact strips. Removal of the endothelium attenuated the relaxation, and indomethacin or tranylcypromine suppressed the endothelium-independent relaxation. In indomethacin-treated strips with intact endothelium, L-NOARG attenuated but did not abolish the relaxation. The residual, L-NOARG-resistant relaxation was not significantly inhibited by ouabain, glibenclamide or tetraethylammonium. 4. Substance P (10(-7) M) increased the levels of cyclic GMP and cyclic AMP. The increase in cyclic GMP was abolished by endothelium denudation and treatment with L-NOARG, whereas the cyclic AMP increment was abolished by indomethacin. 5. Three different mechanisms may be involved in the substance P-induced relaxation: (1) an endothelium-dependent relaxation mediated by the release of NO from the endothelium, resulting in an increase of cyclic GMP (low and high concentrations of the peptide); (2) an endothelium-independent relaxation in association with cyclic AMP increment caused by prostaglandin I2 released from subendothelial tissues (high concentration), and (3) another endothelium-dependent relaxation possibly mediated by unidentified mediator(s) released from the endothelium (high concentration).

Effect of nitro-L-arginine on endothelium-dependent hyperpolarizations and relaxations of pig coronary arteries

1. Endothelium-dependent relaxation is caused by an endothelium-derived relaxing factor (EDRF) identified as nitric oxide (NO). Our objective was to test whether one or several distinct endothelium-dependent relaxing factors exist. 2. In pig coronary arteries, a hyperpolarization accompanied by the relaxation caused by high concentrations of substance P (SP) and bradykinin (BK). 3. To examine the role played by nitric oxide and prostacyclin in the endothelium-dependent relaxations and hyperpolarizations caused by SP and BK on pig coronary arterial strips, the production of NO was inhibited by NG-nitro-L-arginine (L-NNA) and the production of prostacyclin was inhibited by indomethacin, while monitoring smooth muscle membrane potential and isometric tension. 4. Indomethacin had no effect on resting isometric tension nor on SP and BK relaxations of strips precontracted by prostaglandin F2 alpha. 5. L-NNA contracted arterial strips with intact endothelium, without changing the membrane potential of smooth muscles. 6. The inhibitor shifted to the right the concentration-response curve of kinins by 0.2 nM SP and 20 nM BK. It inhibited the maximal relaxations and hyperpolarizations by 30%. 7. The results show that, in pig coronary arteries, EDRF (NO) mainly controls the basal tension, whereas other factor(s) play(s) an important role in hyperpolarizations and relaxations caused by the kinins.

Contribution of NO and cytochrome P450 to the vasodilator effect of bradykinin in the rat kidney

1. Inhibition of nitric oxide generation with Nw-nitro-L-arginine (nitroarginine) reduced vasodilator responses to bradykinin and acetylcholine and enhanced those to nitroprusside in the rat isolated perfused kidney, preconstricted with phenylephrine. 2. Inhibition of cyclo-oxygenase with indomethacin, decreased the vasodilator responses to bradykinin by approximately 25% without affecting those to acetylcholine or nitroprusside. 3. BW755c, a dual inhibitor of cyclo-oxygenase and lipoxygenase, reduced renal vasodilator responses to bradykinin, comparable to the effect of indomethacin suggesting an effect related to inhibition of cyclo-oxygenase rather than lipoxygenase. 4. ETYA, an inhibitor of all arachidonic acid metabolic pathways, markedly reduced vasodilator responses to bradykinin but was without effect on the renal vasodilatation induced by acetylcholine or nitroprusside. 5. Clotrimazole and 7-ethoxyresorufin, inhibitors of cytochrome P450, greatly attenuated vasodilator responses to bradykinin without affecting those to acetylcholine or nitroprusside. 6. These data suggest that the renal vasodilator response to bradykinin is subserved by arachidonic acid metabolites as well as nitric oxide, the former accounting for up to 70% of the vasodilator effect of bradykinin.

Effects of nitric oxide synthase inhibitors, L-NG-nitroarginine and L-NG-nitroarginine methyl ester, on responses to vasodilators of the guinea-pig coronary vasculature

1. The effects of L-NG-nitroarginine (L-NOARG) and L-NG-nitroarginine methyl ester (L-NAME) on vasodilatation induced by ATP, substance P, 5-hydroxytryptamine (5-HT), bradykinin and sodium nitroprusside (SNP) were examined in the guinea-pig coronary bed, by use of a Langendorff technique. The effects of these inhibitors of nitric oxide synthesis were assessed on their ability to inhibit both the amplitude and the area of the vasodilator response. 2. The vasodilator responses evoked by low doses of 5-HT (5 x 10(-10)-10(-8) mol) were almost abolished by L-NAME and L-NOARG (both at 10(-5), 3 x 10(-5) and 10(-4) M), although L-NOARG (3 x 10(-5) M) was significantly less potent than L-NAME (3 x 10(-5) M) as an inhibitor of vasodilator responses to 5-HT (5 x 10(-8) mol). 3. The vasodilator responses evoked by substance P (5 x 10(-12)-5 x 10(-9) mol) were reduced in the presence of L-NAME and L-NOARG (both at 10(-5) and 3 x 10(-5) M). The response to substance P was almost abolished by L-NAME and L-NOARG (both at 10(-4) M). 4. The amplitude of the vasodilator responses to ATP (5 x 10(-11) and 5 x 10(-9)-5 x 10(-7) mol) was little affected by either L-NAME or L-NOARG (both at 10(-5), 3 x 10(-5) and 10(-4) M).7. It is concluded that in the guinea-pig coronary vasculature, the vasodilatation evoked by substance P and low doses of 5-HT is mediated almost exclusively via nitric oxide, whereas the vasodilatations evoked by ATP and bradykinin appear to involve other mechanisms in addition to the release of nitric oxide. L-NAME was a more effective agent than L-NOARG in inhibiting the vasodilator actions of 5-HT and ATP in this preparation.

Endothelial vasomotor regulation in health and disease

The purpose of this review is to provide the anaesthetists with a comprehensive update on the endothelial-cell control of local blood flow. This single cell layer was originally thought to represent only a passive barrier. It is now evident that it plays an active role in a broad variety of biological functions. Since the discovery of the endothelial-derived relaxing factor (EDRF), it has been the subject of a considerable amount of research. It is established that EDRF is secreted continuously at a basal state and that many physical stimuli as well as vasoactive substances can modulate its secretion. Evidence presented indicates that the endogenous vasodilatation produced by EDRF is similar to that of the exogenous nitrovasodilator nitroglycerin and nitroprusside (i.e., nitric oxide). Aside from EDRF, the endothelium produces other vasodilating as well as vasoconstricting factors. A review of the physiology of the endothelium regarding the local control of blood flow is provided along with its influence upon several pathophysiological states. Also included is an overview of the influence of anaesthetic agents on endothelial function. These findings linking vasomotor control to endothelial function will help to explain pathophysiological process and may lead to new therapeutic modalities.