Endothelium-dependent relaxation resistant to NG-nitro-L-arginine in rat aorta

Elucidation of the profound antagonism of contractile action of phenylephrine in rat aorta effected by an atypical sympathomimetic decongestant

Vasoconstrictive properties of sympathomimetic drugs are the basis of their widespread use as decongestants and possible source of adverse responses. Insufficiently substantiated practice of combining decongestants in some marketed preparations, such are those containing phenylephrine and lerimazoline, may affect the overall contractile activity, and thus their therapeutic utility. This study aimed to examine the interaction between lerimazoline and phenylephrine in isolated rat aortic rings, and also to assess the substrate of the obtained lerimazoline-induced attenuation of phenylephrine contraction. Namely, while lower concentrations of lerimazoline (10⁻⁶ M and especially 10⁻⁷ M) expectedly tended to potentiate the phenylephrine-induced contractions, lerimazoline in higher concentrations (10⁻⁴ M and above) unexpectedly and profoundly depleted the phenylephrine concentration-response curve. Suppression of NO with NO synthase (NOS) inhibitor N^w-nitro-L-arginine methyl ester (L-NAME; 10⁻⁴ M) or NO scavanger OHB₁₂ (10⁻³ M), as well as non-specific inhibition of K⁺-channels with tetraethylammonium (TEA; 10⁻³ M), have reversed lerimazoline-induced relaxation of phenylephrine contractions, while cyclooxygenase inhibitor indomethacin (10⁻⁵ M) did not affect the interaction between two vasoconstrictors. At the receptor level, non-selective 5-HT receptor antagonist methiothepin reversed the attenuating effect of lerimazoline on phenylephrine contraction when applied at 3×10⁻⁷ and 10⁻⁶ M, but not at the highest concentration (10⁻⁴ M). Neither the 5-HT_1D-receptor selective antagonist BRL 15572 (10⁻⁶ M) nor 5-HT₇ receptor selective antagonist SB 269970 (10⁻⁶ M) affected the lerimazoline-induced attenuation of phenylephrine activity. The mechanism of lerimazoline-induced suppression of phenylephrine contractions may involve potentiation of activity of NO and K⁺-channels and activation of some methiothepin-sensitive receptors, possibly of the 5-HT_2B subtype.

Mepivacaine-induced contraction is attenuated by endothelial nitric oxide release in isolated rat aorta

Mepivacaine is an aminoamide-linked local anesthetic with an intermediate duration that intrinsically produces vasoconstriction both in vivo and in vitro. The aims of this in-vitro study were to examine the direct effect of mepivacaine in isolated rat aortic rings and to determine the associated cellular mechanism with a particular focus on endothelium-derived vasodilators, which modulate vascular tone. In the aortic rings with or without endothelium, cumulative mepivacaine concentration-response curves were generated in the presence or absence of the following antagonists: N(ω)-nitro-L-arginine methyl ester [L-NAME], indomethacin, fluconazole, methylene blue, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one [ODQ], verapamil, and calcium-free Krebs solution. Mepivacaine produced vasoconstriction at low concentrations (1 × 10(-3) and 3 × 10(-3) mol/L) followed by vasodilation at a high concentration (1 × 10(-2) mol/L). The mepivacaine-induced contraction was higher in endothelium-denuded aortae than in endothelium-intact aortae. Pretreatment with L-NAME, ODQ, and methylene blue enhanced mepivacaine-induced contraction in the endothelium-intact rings, whereas fluconazole had no effect. Indomethacin slightly attenuated mepivacaine-induced contraction, whereas verapamil and calcium-free Krebs solution more strongly attenuated this contraction. The vasoconstriction induced by mepivacaine is attenuated mainly by the endothelial nitric oxide - cyclic guanosine monophosphate pathway. In addition, mepivacaine-induced contraction involves cyclooxygenase pathway activation and extracellular calcium influx via voltage-operated calcium channels.

Indigo carmine enhances phenylephrine-induced contractions in an isolated rat aorta

BACKGROUND

The intravenous administration of indigo carmine has been reported to produce transiently increased blood pressure in patients. The goal of this in vitro study was to examine the effect of indigo carmine on phenylephrine-induced contractions in an isolated rat aorta and to determine the associated cellular mechanism with particular focus on the endothelium-derived vasodilators.

METHODS

The concentration-response curves for phenylephrine were generated in the presence or absence of indigo carmine. Phenylephrine concentration-response curves were generated for the endothelium-intact rings pretreated independently with a nitric oxide synthase inhibitor, N(ω)-nitro-L-arginine methyl ester (L-NAME), a cyclooxygenase inhibitor, indomethacin, and a low-molecular-weight superoxide anion scavenger, tiron, in the presence or absence of indigo carmine. The fluorescence of oxidized dichlorofluorescein was measured in rat aortic vascular smooth muscle cells cultured in the control, indigo carmine alone and tiron plus indigo carmine.

RESULTS

Indigo carmine (10(-5) M) increased the phenylephrine-induced maximum contraction in the endothelium-intact rings with or without indomethacin, whereas indigo carmine produced a slight leftward shift in the phenylephrine concentration-response curves in the endothelium-denuded rings and L-NAME-pretreated endothelium-intact rings. In the endothelium-intact rings pretreated with tiron (10(-2) M), indigo carmine did not alter phenylephrine concentration-response curves significantly. Indigo carmine (10(-5) M) increased the fluorescence of oxidized dichlorofluorescein in the vascular smooth muscle cells, whereas tiron abolished the indigo carmine-induced increase in oxidized dichlorofluorescein fluorescence.

CONCLUSIONS

Indigo carmine increases the phenylephrine-induced contraction mainly through an endothelium-dependent mechanism involving the inactivation of nitric oxide caused by the increased production of reactive oxygen species.

A mathematical model of vasoreactivity in rat mesenteric arterioles: I. Myoendothelial communication

To study the effect of myoendothelial communication on vascular reactivity, we integrated detailed mathematical models of Ca(2+) dynamics and membrane electrophysiology in arteriolar smooth muscle (SMC) and endothelial (EC) cells. Cells are coupled through the exchange of Ca(2+), Cl(-), K(+), and Na(+) ions, inositol 1,4,5-triphosphate (IP(3)), and the paracrine diffusion of nitric oxide (NO). EC stimulation reduces intracellular Ca(2+) ([Ca(2+)](i)) in the SMC by transmitting a hyperpolarizing current carried primarily by K(+). The NO-independent endothelium-derived hyperpolarization was abolished in a synergistic-like manner by inhibition of EC SK(Ca) and IK(Ca) channels. During NE stimulation, IP(3) diffusing from the SMC induces EC Ca(2+) release, which, in turn, moderates SMC depolarization and [Ca(2+)](i) elevation. On the contrary, SMC [Ca(2+)](i) was not affected by EC-derived IP(3). Myoendothelial Ca(2+) fluxes had no effect in either cell. The EC exerts a stabilizing effect on calcium-induced calcium release-dependent SMC Ca(2+) oscillations by increasing the norepinephrine concentration window for oscillations. We conclude that a model based on independent data for subcellular components can capture major features of the integrated vessel behavior. This study provides a tissue-specific approach for analyzing complex signaling mechanisms in the vasculature.

Diazepam Attenuates Phenylephrine-Induced Contractions in Rat Aorta

In this in vitro study we examined the effects of diazepam on a phenylephrine-induced contraction in rat aorta and determined the associated cellular mechanism focusing on the endothelium-derived vasodilators. The concentration-response curves for phenylephrine and potassium chloride were generated in the presence or absence of diazepam. Phenylephrine concentration-response curves were generated from the endothelium-intact rings pretreated independently with N(W)-nitro-L-arginine methyl ester, PK 11195, tetraethylammonium, and indomethacin in the presence or absence of diazepam. Diazepam (7 x 10(-7) M) attenuated the phenylephrine-induced contraction in the endothelium-intact rings, whereas a large dose (5 x 10(-6) M) of diazepam attenuated the phenylephrine-induced contraction in the aortic rings with or without the endothelium. A pretreatment with the N(W)-nitro-L-arginine methyl ester completely abolished the diazepam (7 x 10(-7) M)-induced attenuation of the phenylephrine concentration-response curve, as well as the diazepam (5 x 10(-6) M)-induced attenuation of the maximal contractile response to phenylephrine. The N(W)-nitro-L-arginine methyl ester (10(-4) M)-induced contraction was enhanced in the rings pretreated with diazepam (5 x 10(-6) M). These results indicate that a supraclinical concentration of diazepam attenuates phenylephrine-induced contraction by increasing endothelial nitric oxide activity and directly affecting vascular smooth muscle.

Differential effects of sulphonylureas on the vasodilatory response evoked by K(ATP) channel openers

The potency of three sulphonylureas, glibenclamide, glimepiride and gliclazide in antagonizing the vasorelaxant action of openers of adenosine triphosphate (ATP)-regulated K+ channel (KATP) was studied in vivo and in vitro in micro- and macrovessels, respectively. In the hamster cheek pouch, the vasodilatation and the increase in vascular diameter and blood flow induced by diazoxide were markedly reduced by the addition of either glibenclamide or glimepiride (0.8 microm) while they were not affected by gliclazide up to 12 microm. Similarly, in rat and guinea-pig isolated aortic rings, glibenclamide, glimepiride and gliclazide reduced the vasodilator activity of cromakalim. However, the inhibitory effect of gliclazide was considerably less when compared with either glimepiride or glibenclamide. These results suggest that, in contrast to glibenclamide and glimepiride, therapeutically relevant concentrations of gliclazide do not block the vascular effects produced by KATP channel openers in various in vitro and in vivo animal models.

Arterial relaxation mediated by endothelium-derived hyperpolarizing factor in hypertension induced by chronic inhibition of nitric oxide synthesis

The aim of this study was to evaluate arterial relaxation mediated by endothelium-derived hyperpolarizing factor (EDHF) during chronic inhibition of nitric oxide (NO) synthase. We measured the isometric tension of isolated mesenteric arteries of Wistar rats administered Nomega-nitro-L-arginine methyl ester (L-NAME, 100 mg/Kg/day) for 3 weeks. Relaxation to acetylcholine (ACh) was reduced in L-NAME treated rats (maximum relaxation, 52% versus 79% ). After acute superfusion of 1x10(-4) M L-NAME, half the relaxation was inhibited in controls, while the relaxation was not changed in L-NAME treated rats. In contrast, relaxation to nitroprusside was normal in L-NAME treated rats. Superfusion of 1x10(-6) M apamin, which inhibits the effects of EDHF, reduced the relaxation. The relaxation inhibited by apamin was not significantly different between the two groups. These findings suggested that in endothelial cells, the synthesis of EDHF is unchanged during a chronic deficiency of relaxation influence of NO.

Biological evaluation of the nitric oxide-trapping agent, N-methyl-D-glucamine dithiocarbamate-Fe2+, as a probe of nitric oxide activity released from control and diabetic rat endothelium

We utilized the nitric oxide (NO) scavenger N-methyl-D-glucamine dithiocarbamate-Fe2+ (MGD-Fe) to characterize the role of NO in basal and acetylcholine (ACh)-stimulated relaxation arising from the endothelium of control vs diabetic rat aortic rings. In phenylephrine-contracted rings, MGD-Fe produced an additional increment in tension that was indomethacin-insensitive (i.e., excluding a role of prostanoids in this action). This MGD-Fe-sensitive component was more pronounced in control rings than diabetic rings and of the same magnitude achieved in rings without MGD-Fe treatment after removal of endothelium or treatment with the NO synthase inhibitor L-nitroarginine (L-NA). This suggests complete scavenging of basal NO by MGD-Fe and supports reduced basal NO in diabetic rings. ACh fully relaxed both control and diabetic rings. This relaxation was abolished by removal of the endothelium and was inhibited by L-NA (by 100% and 90% in control and diabetic rings, respectively). In contrast, MGD-Fe only partially inhibited ACh-induced relaxation in control (65+/-5% inhibition) and diabetic (41+/-11% inhibition) rings. The MGD-Fe-resistant component was not further modified by indomethacin. Addition of L-arginine (L-ARG) (but not D-arginine (D-ARG) enhanced the ACh-induced relaxation of MGD-Fe-treated diabetic (but not control) rings. These data provide evidence about endothelium-dependent relaxation in control and diabetic rings which cannot be discerned by use of L-NA alone. This study suggests that ACh produces a NO synthase-dependent vasodilation, a portion of which is due to free NO radical (*NO) or due to NO in a form or location that is unavailable for scavenging by MGD-Fe.

Biphasic relaxation caused by electrical field stimulation of the mesenteric arteries of rats

Electrical field stimulation (EFS) caused biphasic relaxation, first transient and then sustained, of rat mesenteric arteries precontracted by prostaglandin (PG) F2alpha. The transient relaxation was reduced about 45%, and the sustained relaxation was not observed after endothelium denudation of the arteries. N(omega)-Nitro-L-arginine (L-NOARG) inhibited the biphasic relaxation induced by EFS. At 1 - 100 microM, L-NOARG inhibited the transient relaxation more than the sustained relaxation. Methylene blue inhibited the biphasic relaxation and at 100 microM, L-NOARG abolished the transient relaxation. These results suggest that the transient relaxation mainly involves nitric oxide (NO), whereas the sustained relaxation involves both NO and some other factor(s).

In vitro simultaneous measurements of relaxation and nitric oxide concentration in rat superior mesenteric artery

1. The relationship between nitric oxide (NO) concentration measured with an NO-specific microelectrode and endothelium-dependent relaxation was investigated in isolated rat superior mesenteric artery contracted with 1 microM noradrenaline. 2. Acetylcholine (10 microM) induced endothelium-dependent simultaneous increases in luminal NO concentration of 21 +/- 6 nM, and relaxations with pD2 values and maximum of 6.95 +/- 0.32 and 97.5 +/- 0.7 % (n = 7), respectively. An inhibitor of NO synthase, N G-nitro-L-arginine (L-NOARG, 100 microM) inhibited the relaxations and increases in NO concentration induced by acetylcholine. 3. Oxyhaemoglobin (10 microM) reversed the relaxations and increases in NO concentrations induced by acetylcholine, S-nitroso-N-acetylpenicillamine (SNAP) and S-morpholino-sydnonimine (SIN-1), but not the relaxations induced with forskolin. Oxyhaemoglobin also decreased the NO concentration below baseline level. 4. In the presence of L-NOARG (100 microM), a small relaxation to acetylcholine (10 microM) of noradrenaline-contracted segments was still seen; oxyhaemogobin inhibited this relaxation and decreased the NO concentration by 14 +/- 4 nM (n = 4). 5. The NO concentration-relaxation relationship for acetylcholine resembled that for SNAP and SIN-1 more than for authentic NO. Thus while 7-17 nM NO induced half-maximal relaxations in response to SNAP or SIN-1, 378 +/- 129 nM NO (n = 4) was needed for half-maximal relaxation to authentic NO. 6. The present study provides direct evidence that the relaxation of the rat superior mesenteric artery with the endothelium-dependent vasodilator acetylcholine is correlated to the endogeneous release of NO. The study also suggests that NO mediates the L-NOARG-resistant relaxations in this artery, and that there is a basal NO release.

Endothelium-dependent hyperpolarization in isolated arteries taken from animals treated with NO-synthase inhibitors

To study the effects of chronic in vivo inhibition of NO synthase on endothelium-dependent hyperpolarization, cell-membrane potential (in individual vascular smooth-muscle cells) and changes in tension (in isolated rings) were recorded from isolated canine coronary arteries and guinea-pig carotid arteries and aortas. In coronary arteries taken from control dogs and contracted with U46619, acetylcholine- and bradykinin-induced endothelium-dependent relaxations, which were unaffected by short-term in vitro exposure to indomethacin but were inhibited partially by L-nitro-arginine (LNA). In coronary arteries taken from dogs treated over the long term in vivo with LNA (30 mg/kg on the first day and 20 mg/kg the 7 following days, i.v.), the response to acetylcholine and bradykinin was inhibited when compared with arteries from control dogs. Short-term in vitro exposure to LNA or indomethacin or both did not influence the effects of either agonist. In these arteries, the hyperpolarizing response to acetylcholine, observed in the presence of LNA and indomethacin, was enhanced, whereas that to bradykinin was partially inhibited. In the guinea pig isolated aorta, the relaxation to bradykinin was abolished by long-term in vivo treatment with L-nitro-arginine-methyl-ester (L-NAME; 1.5 mg/ml, in the drinking water for > or =4 days). In the isolated guinea pig carotid artery studied in the presence of LNA and indomethacin, acetylcholine induced a hyperpolarization that was not significantly affected by long-term in vivo treatment with L-NAME. These findings indicate that endothelium-dependent hyperpolarizations are maintained during long-term inhibition of NO synthase and probably act as a back-up mechanism to elicit endothelium-dependent relaxations.

Role of endothelial carbon monoxide in attenuated vasoreactivity following chronic hypoxia

Chronic hypoxic exposure has been previously demonstrated to attenuate systemic vasoconstrictor activity to a variety of agents. This attenuated responsiveness is observed not only in conscious animals but in isolated vascular preparations as well. Because hypoxia has been documented to increase heme oxygenase (HO) levels and the subsequent production of the vasodilator CO in vitro, we hypothesized that the blunted reactivity observed with chronic hypoxia (CH) may be in part due to increased HO activity. In thoracic aortic rings from CH rats, cumulative dose-response curves to phenylephrine (PE) in the presence of the nitric oxide (NO) synthase inhibitor Nomega-nitro-L-arginine (L-NNA) and the HO inhibitor zinc protoporphyrin 9 (ZnPPIX) elicited increased contractility compared with CH rings treated with only L-NNA. Similar results were observed in rings incubated overnight with the HO-inducing agent sodium m-arsenite. In contrast, contractile responses in rings from control rats were unaffected by the HO inhibitor. Furthermore, endothelium-denuded rings from either control or CH rats did not exhibit an increase in reactivity to PE following ZnPPIX incubation. ZnPPIX had no effect on relaxant responses to the NO donor S-nitroso-N-penicillamine, suggesting that its actions were specific to HO inhibition. Finally, aortic rings exhibited dose-dependent relaxant responses to exogenous CO that were endothelium independent and blocked by an inhibitor of soluble guanylyl cyclase. The other products of HO enzyme activity, iron and biliverdin, were without effect on vasoreactivity. Thus we conclude that the attenuated vasoreactivity to PE following CH is likely to involve the induction of endothelial HO and the subsequent enhanced production of CO.

Acetylcholine-induced systemic vasodilation resistant to NG-nitro-L-arginine in an anaesthetized rats

1. The effects of NG-nitro-L-arginine (L-NNA; 20 mg/kg bodyweight (BW), i.v.) and metyrapone (300 mg/kg BW, s.c.) on acetylcholine (ACh)-induced depressor responses were investigated in anaesthetized rats. 2. Acetylcholine (0.05, 0.5, 5 micrograms/kg BW, i.v.) dose-dependently evoked a sharp fall in mean blood pressure (BP) followed by a slow recovery under control conditions. 3. Basal BP level was elevated when rats were treated with L-NNA, indicating endogenous nitric oxide (NO) participated in BP regulation. However, pretreatment with L-NNA did not attenuate but rather augmented the ACh-induced maximum vasodilation. In contrast, the time for recovery of mean BP to the pre-ACh administration level was shortened by L-NNA. These observations suggested that ACh-induced vasodilation consisted of two phases: a sharp and transient fall (phase 1) that was resistant to L-NNA followed by a longer depressor response (phase 2) that was suppressed by L-NNA. 4. To examine whether augmentation of phase 1 by L-NNA resulted from the elevation of basal BP, an appropriate dose of phenylephrine was infused to obtain similar BP elevation. Phenylephrine infusion augmented the phase 1 in a similar manner to L-NNA pretreatment but showed little effect on phase 2, supporting the selective inhibition of phase 2 by L-NNA. 5. The s.c. pretreatment with metyrapone for 3 days failed to attenuate phase 1. Thus, the involvement of endothelium-derived hyperpolarizing factor that could be formed by a metyrapone-sensitive oxidase in phase 1 was unlikely. 6. These results suggest that some factor(s), which is not inhibitable by L-NNA or metyrapone, may induce the phase 1 depressor response to ACh while NO is responsible for the phase 2 response. The mechanism inducing the phase 1 response remains to be identified.

Quantitative changes in pharmacodynamic parameters of noradrenaline in different rat aorta preparations: influence of endogenous EDRF

1. The aim of the present study was to assess the role of endothelial cells in the modulation of vasocontractile responses to noradrenaline in rat isolated aorta when cut as standard helical strips or as ring segments. 2. Noradrenaline-potency in helical strip preparations evaluated as -logEC50 was greater than that obtained in endothelium-intact ring preparations (9.45 +/- 0.28 versus 8.69 +/- 0.09, respectively) (P < 0.05). The maximum contractile response of helical strips was significantly higher than the response of ring preparations (P < 0.05). 3. Subsequent experiments were performed on helical strips and ring preparations where the endothelium was removed by rubbing the luminal surface of the aorta with filter paper. Removal of the endothelium potentiated the noradrenaline-induced contraction in ring preparations, but not in the helical strips. 4. The nitric oxide synthase inhibitors L-NAME (3 x 10(-5)-3 x 10(-4) M) or L-NNA (1 x 10(4)-3 x 10(-4) M) which were added to the tissue bath potentiated the noradrenaline-induced contraction in the endothelium-intact ring preparations, although only L-NNA induced a statistically significant potentiation. Both L-NAME and L-NNA had no effect on the noradrenaline-contraction induced in rings without endothelium, or in helical strips with or without endothelium. 5. Vascular acetylcholine-induced relaxation is dependent on endothelium derived relaxing factor (nitric oxide). Acetylcholine (10(-9)-10(-6) M) induced a concentration-dependent relaxation in noradrenaline preconstricted intact rings. The relaxant response was strongly reduced by L-NAME (3 x 10(-5)-1 x 10(-4) M). The relaxant response to acetylcholine was very weak in ring and helical strip preparations without endothelium, but also, surprisingly, in unrubbed standard helical strips. 6. The present results suggest that the endothelium of standard helical strip preparations may be greatly damaged, a view confirmed by morphological studies. The structural and functional damage of the endothelium induced very important changes in pharmacodynamic parameters such as in the potency and the maximal responses of vascular preparations to noradrenaline. Therefore, caution must be observed when the potency and intrinsic activity of agonists evaluated on different preparations are compared, even if these come from the same vascular segment.

Nitric oxide and carbon monoxide: parallel roles as neural messengers

Nitric oxide is now appreciated to be a molecule with important signaling functions in the body. The purification and cloning of the first NO synthesizing enzyme, NO synthase (NOS), from brain has led to the characterization of the roles of NO in normal physiology and in pathogenic states. NO synthesis is regulated in a complex manner, involving the association of activatory and inhibitory proteins. The body appears to use at least one other, highly related gas in a signaling function, carbon monoxide (CO). The enzyme responsible for CO biosynthesis in brain, heme oxygenase-2 (HO2), is rapidly regulated by neurotransmitter stimulation. The role for CO as neurotransmitter is suggested by the altered intestinal motility in mice harboring a genomic deletion of HO2.

Sex differences in the relative contributions of nitric oxide and EDHF to agonist-stimulated endothelium-dependent relaxations in the rat isolated mesenteric arterial bed

1. We have used the isolated, buffer-perfused, superior mesenteric arterial bed of male and female rats to assess the relative contributions of nitric oxide (NO) and the endothelium-derived hyperpolarizing factor (EDHF) to endothelium-dependent relaxations to carbachol. 2. Carbachol caused dose-related relaxations of methoxamine-induced tone in mesenteric vascular beds from male rats described by an ED50(M) of 0.43+/-0.15 nmol and a maximum relaxation (Rmax(M) of 89.6+/-1.2% (n=28) which were not significantly different from those observed in mesenteries from female rats (ED50(F)=0.72+/-0.19 nmol and Rax(F)=90.7+/-0.9%; n=22). 3. In the males, the addition of 100 microM NG-nitro-L-arginine methyl ester (L-NAME) caused the dose-response curve to carbachol to be significantly (P<0.001) shifted to the right 15 fold (ED50(M)=6.45+/-3.53 nmol) and significantly (P<0.01) reduced Rmax(M) (79.7+/-2.8%, n=13). By contrast, L-NAME had no effect on vasorelaxation to carbachol in mesenteries from female rats (ED50(f)= 0.89+/-0.19 nmol, Rmax(F)=86.9+/-2.3%, n=9). 4. Raising tone with 60 mM KCl significantly reduced the maximum relaxation to carbachol in mesenteries from male rats 2 fold (Rmax(M)=40.3+/-9.2%, n=4; P<0.001) and female rats by 1.5 fold (Rmax(F)=55.3+/-3.3%, n=6; P<0.001), compared with methoxamine-induced tone. The potency of carbachol was also significantly reduced 1.2 fold in preparations from males (ED50(M)=0.87+/-0.26 nmol; P<0.01) but not the females (ED50(F)=4.04+/-1.46 nmol). In the presence of both 60 mM KCl and L-NAME, the vasorelaxation to carbachol was completely abolished in mesenteries from both groups. 5. The cannabinoid receptor antagonist SR141716A (1 microM), which is also a putative EDHF antagonist, had no significant effect on the responses to carbachol in mesenteries from males or females (ED50(M)=1.41+/-0.74 nmol, Rmax(M)=89.4+/-2.5%, n=7; ED50(F)=2.17+/-0.95 nmol, Rmax(F)=89.9+/-1.8%, n=9). In mesenteries from male rats, in the presence of 100 microM L-NAME, SR141716A significantly (P<0.05) shifted the dose-response curve to carbachol 8 fold further to the right than that seen in the presence of L-NAME alone (ED50(M)= 53.8+/-36.8 nmol) without affecting Rmax(M) (72.4+/-4.8%, n=10). In mesenteries from female rats, the combined presence of L-NAME and SR141716A, significantly (P < 0.01) shifted the dose-response curve to carbachol 7.5 fold, (ED50(F)=6.66+/-2.46 nmol), as compared to L-NAME alone and significantly (P<0.001) decreased Rmax(F) (70.1+/-5.5%, n=8). 6. Vasorelaxations to the nitric oxide donor sodium nitroprusside (SNP), to the endogenous cannabinoid, anandamide (a putative EDHF) and to the ATP-sensitive potassium channel activator, levcromakalim, did not differ significantly between male and female mesenteric vascular beds. 7. The continuous presence of sodium nitroprusside (SNP; 20-60 nM) had no effect on vasorelaxation to carbachol in mesenteries from either males or females. In the presence of L-NAME, SNP significantly (P<0.05) reduced the potency of carbachol 6 fold, without affecting the maximal relaxation in mesenteries from male rats (ED50(M)=40.9+/-19.6 nmol, Rmax(M)=79.4+/-2.5%, n=11). Similarly in mesenteries from female rats, the ED50(F) was also significantly (P<0.01) increased 7 fold (6.24+/-2.02 nmol), while the Rmax(F) was unaffected (81.9+/-11.0%; n=4). 8 The results of the present investigation demonstrate that the relative contributions of agonist-stimulated NO and EDHF to endothelium-dependent relaxations in the rat isolated mesenteric arterial bed, differ between males and females. Specifically, although both NO and EDHF appear to contribute towards endothelium-dependent relaxations in males and females, blockade of NO synthesis alone has no effect in the female. This suggests that EDHF is functionally more important in females; one possible explanation for this is that in the absence of NO, the recently identified ability of EDHF to compensate for the loss of NO, is functio

Nitric oxide-independent dilation of conductance coronary arteries to acetylcholine in conscious dogs

NO and prostacyclin formation cannot entirely account for receptor-operated endothelium-dependent dilation of coronary vessels, since vasodilator responses are not completely suppressed by inhibitors of these agents. Therefore, we considered that another factor, such as an endothelium-derived hyperpolarizing factor described in vitro, may participate in NO- and prostacyclin-independent coronary dilator responses. In conscious instrumented dogs, intracoronary acetylcholine (ACh, 30.0 ng.kg-1.min-1) increased the external epicardial coronary diameter (CD) by 0.18 +/- 0.03 mm (from 3.44 +/- 0.11 mm) when increases in coronary blood flow (CBF) were prevented and increased the CD by 0.20 +/- 0.05 when CBF was allowed to increase. After the administration of intracoronary N omega-nitro-L-arginine methyl ester (L-NAME), CBF responses to ACh were abolished, but CD responses (0.23 +/- 0.05 from 3.22 +/- 0.09 mm) were maintained. Blockade of NO formation was confirmed by reduced CD baselines and blunted flow-dependent CD responses caused by adenosine and transient coronary artery occlusions after L-NAME administration. ACh-induced CD increases resistant to L-NAME and indomethacin were reduced after the administration of intracoronary quinacrine, an inhibitor of phospholipase A2, or proadifen, an inhibitor of cytochrome P-450. Quinacrine or proadifen alone (without L-NAME) did not alter CD responses to ACh, but L-NAME given after proadifen blunted ACh-induced increases in CD. The increases in CD caused by arachidonic acid given after L-NAME + indomethacin were antagonized by proadifen but not altered by quinacrine. Thus, a cytochrome P-450 metabolite of arachidonic acid accounts for L-NAME-resistant and indomethacin-resistant dilation of large epicardial coronary arteries to ACh. Conversely, NO formation is the dominant mechanism of ACh-induced dilation after blockade of the cytochrome P-450 pathway.

Influence of cytochrome P-450 inhibitors on endothelium-dependent nitro-L-arginine-resistant relaxation and cromakalim-induced relaxation in rat mesenteric arteries

In several blood vessels, endothelium-dependent vasorelaxation is in part mediated by an endothelium-derived hyperpolarizing factor (EDHF), the nature of which is as yet unknown. However, some evidence suggests that EDHF might be a cytochrome P-450-dependent monooxygenase metabolite of arachidonic acid. By using isometric tension measurements on rat main mesenteric arteries, the influence of four structurally and mechanistically different cytochrome P-450 inhibitors (proadifen, miconazole, 1-amino-benzotriazole, and 17-octadecynoic acid) was investigated on relaxations elicited by EDHF, assessed as the nitro-L-arginine-resistant component of acetylcholine-induced relaxation, and on relaxations provoked by the endothelium-independent potassium channel opener cromakalim. Proadifen (30 microM) inhibited the EDHF- as well as the cromakalim-induced relaxation, but not that elicited by nitroprusside. Also miconazole (30 microM) inhibited both the EDHF and the cromakalim-induced relaxation. On the other hand, 17-octadecynoic acid (5 microM) had no influence, and 1-aminobenzotriazole (1 mM) even potentiated EDHF- and cromakalim-induced relaxations. We conclude that the EDHF, released from the rat mesenteric artery by acetylcholine, is unlikely to be a cytochrome P-450-dependent monooxygenase metabolite of arachidonic acid and that proadifen and miconazole interfere with the action of cromakalim.

Characterization and modulation of EDHF-mediated relaxations in the rat isolated superior mesenteric arterial bed

1. We have used the isolated, buffer-perfused, mesenteric arterial bed of the rat (preconstricted with methoxamine or 60 mM K+) to characterize nitric oxide (NO)-independent vasorelaxation which is thought to be mediated by the endothelium-derived hyperpolarizing factor (EDHF). 2. The muscarinic agonists carbachol, acetylcholine (ACh) and methacholine caused dose-related relaxations in preconstricted preparations with ED50 values of 0.18 +/- 0.04 nmol (n = 8), 0.05 +/- 0.02 nmol (n = 6) and 0.26 +/- 0.16 nmol (n = 5), respectively. In the same preparations NG-nitro-L-arginine methyl ester (1-NAME, 100 microM) significantly (P < 0.05) decreased the potency of all the agents (ED50 values in the presence of L-NAME: carbachol, 0.66 +/- 0.11 nmol; ACh, 0.28 +/- 0.10 nmol; methacholine, 1.97 +/- 1.01 nmol). The maximal relaxation to ACh was also significantly (P < 0.05) reduced (from 85.3 +/- 0.9 to 73.2 +/- 3.7%) in the presence of L-NAME. The vasorelaxant effects of carbachol were not significantly altered by the cyclo-oxygenase inhibitor indomethacin (10 microM; n = 4). 3. The K+ channel blocker, tetraethylammonium (TEA, 10 mM) also significantly (P < 0.001) reduced both the potency of carbachol (ED50 = 1.97 +/- 0.14 nmol in presence of TEA) and the maximum relaxation (Rmax = 74.6 +/- 3.2% in presence of TEA, P < 0.05, n = 3). When TEA was added in the presence of L-NAME (n = 4), there was a further significant (P < 0.001) decrease in the potency of carbachol (ED50 = 22.4 +/- 13.5 nmol) relative to that in the presence of L-NAME alone, and Rmax was also significantly (P < 0.05) reduced (74.6 +/- 4.2%). The ATP-sensitive K+ channel inhibitor, glibenclamide (10 microM), had no effect on carbachol-induced relaxation (n = 9). 4. High extracellular K+ (60 mM) significantly (P < 0.01) reduced the potency of carbachol (n = 5) by 5 fold (ED50: control, 0.16 +/- 0.04 nmol; high K+, 0.88 +/- 0.25 nmol) and the Rmax was also significantly (P < 0.01) reduced (control, 83.4 +/- 2.7%; high K+, 40.3 +/- 9.2%). The residual vasorelaxation to carbachol in the presence of high K+ was abolished by L-NAME (100 microM; n = 5). In preparations preconstricted with high K+, the potency of sodium nitroprusside was not significantly different from that in preparations precontracted with methoxamine, though the maximal response was reduced (62.4 +/- 3.4% high K+, n = 7; 83.1 +/- 3.1% control, n = 7). 5. In the presence of the cytochrome P450 inhibitor, clotrimazole (1 microM, n = 5 and 10 microM, n = 4), the dose-response curve to carbachol was significantly shifted to the right 2 fold (P < 0.05) and 4 fold (P < 0.001) respectively, an effect which was further enhanced in the presence of L-NAME. Rmax was significantly (P < 0.01) reduced by the presence of 10 microM clotrimazole alone, being 86.9 +/- 2.5% in its absence and 61.8 +/- 7.8% in its presence (n = 6). 6. In the presence of the cell permeable analogue of cyclic GMP, 8-bromo cyclic GMP (6 microM), the inhibitory effects of L-NAME on carbachol-induced relaxation were substantially enhanced (ED50: L-NAME alone, 0.52 +/- 0.11 nmol, n = 5; L-NAME + 8-bromo cyclic GMP, 1.42 +/- 0.28 nmol, n = 7, Rmax: L-NAME alone, 82.2 +/- 2.4%; L-NAME + 8-bromo cyclic GMP, 59.1 +/- 1.8%. P < 0.001). These results suggest that the magnitude of the NO-independent component of vasorelaxation is reduced when functional cyclic GMP levels are maintained, suggesting that basal NO (via cyclic GMP) may modulate EDHF activity and, therefore, on loss of basal NO production the EDHF component of endothelium-dependent relaxations becomes functionally greater. 7. The present investigation demonstrates that muscaranic receptor-induced vasorelaxation in the rat mesenteric arterial bed is mediated by both NO-dependent and independent mechanisms. The L-NAME-insensitive mechanism, most probably occurs via activation of a K+ conductance and shows the characteristics of EDHF-mediated responses. Finally, the results demonstrate that EDHF activity may become upregulated on inhibition of NO production and this may compensate for the loss of NO.

NO/PGI2-independent vasorelaxation and the cytochrome P450 pathway in rabbit carotid artery

1. The nature and cellular mechanisms that are responsible for endothelium-dependent relaxations resistant to indomethacin and NG-nitro-L-arginine methyl ester (L-NAME) were investigated in phenylephrine (PE) precontracted isolated carotid arteries from the rabbit. 2. In the presence of the cyclo-oxygenase inhibitor, indomethacin (10 microM), acetylcholine (ACh) induced a concentration- and endothelium-dependent relaxation of PE-induced tone which was more potent than the calcium ionophore A23187 with pD2 values of 7.03 +/- 0.12 (n = 8) and 6.37 +/- 0.12 (n = 6), respectively. The ACh-induced response was abolished by removal of the endothelium, but was not altered when indomethacin was omitted (pD2 value 7.00 +/- 0.10 and maximal relaxation 99 +/- 3%, n = 6). Bradykinin and histamine (0.01-100 microM) had no effect either upon resting or PE-induced tone (n = 5). 3. In the presence of indomethacin plus the NO synthase inhibitor, L-NAME (30 microM), the response to A23187 was abolished. However, the response to ACh was not abolished, although it was significantly inhibited with the pD2 value and the maximal relaxation decreasing to 6.48 +/- 0.10 and 67 +/- 3%, respectively (for both P < 0.01, n = 8). The L-NAME/indomethacin insensitive vasorelaxation to ACh was completely abolished by preconstriction of the tissues with potassium chloride (40 mM, n = 8). 4. The Ca(2+)-activated K+ (KCa) channel blockers, tetrabutylammonium (TBA, 1 mM, n = 5) and charybdotoxin (CTX, 0.1 microM, n = 5), completely inhibited the nitric oxide (NO) and prostacyclin (PGI2)-independent relaxation response to ACh. However, iberiotoxin (ITX, 0.1 microM, n = 8) or apamin (1-3 microM, n = 6) only partially inhibited the relaxation. 5. Inhibitors of the cytochrome P450 mono-oxygenase, SKF-525A (1-10 microM, n = 6), clotrimazole (1 microM, n = 5) and 17-octadecynoic acid (17-ODYA, 3 microM, n = 7) also reduced the NO/PGI2-independent relaxation response to ACh. 6. In endothelium-denuded rings of rabbit carotid arteries, the relaxation response to exogenous NO was not altered by either KCa channel blockade with apamin (1 microM, n = 5) or CTX (0.1 microM, n = 5), or by the cytochrome P450 mono-oxygenase blockers SKF-525A (10 microM, n = 4) and clotrimazole (10 microM, n = 5). However, the NO-induced response was shifted to the right by LY83583 (10 microM, n = 4), a guanylyl cyclase inhibitor, with the pD2 value decreasing from 6.95 +/- 0.14 to 6.04 +/- 0.09 (P < 0.01). 7. ACh (0.01-100 microM) induced a concentration-dependent relaxation of PE-induced tone in endothelium-denuded arterial segments sandwiched with endothelium-intact donor segments. This relaxation to ACh was largely unaffected by indomathacin (10 microM) plus L-NAME (30 microM), but abolished by the combination of indomethacin, L-NAME and TBA (1 mM, n = 5). 8. These data suggest that in the rabbit carotid artery: (a) ACh can induce the release of both NO and EDHF, whereas A23187 only evokes the release of NO from the endothelium, (b) the diffusible EDHF released by ACh may be a cytochrome P450-derived arachidonic acid metabolite, and (c) EDHF-induced relaxation involves the opening of at least two types of KCa channels, whereas NO mediates vasorelaxation via a guanosine 3': 5'-cyclic monophosphate (cyclic GMP)-mediated pathway, in which a cytochrome P450 pathway and KCa channels do not seem to be involved.

Endothelium-derived factors and hyperpolarization of the carotid artery of the guinea-pig

1. Transmembrane potentials were recorded from isolated carotid arteries of the guinea-pig superfused with modified Krebs-Ringer bicarbonate solution. Smooth muscle cells were impaled from the adventitial side with intracellular glass microelectrodes filled with KCl (30-80 M omega). 2. Acetylcholine (1 microM) in the presence of inhibitors of nitric oxide synthase, (N omega-nitro-L-arginine (L-NOARG) 100 microM) and cyclo-oxygenase, (indomethacin 5 microM) induced an endothelium-dependent hyperpolarization (-18.9 +/- 1.6 mV, n = 15). 3. In the presence of these two inhibitors, S-nitroso-L-glutathione (10 microM), sodium nitroprusside (10 microM), 3-morpholinosydnonimine (SIN-1, 10 microM) and iloprost (0.1 microM) induced endothelium-independent hyperpolarizations of the smooth muscle cells (respectively: -16.0 +/- 2.3, -16.3 +/- 3.4, -12.8 +/- 2.0 and -14.5 +/- 1.5 mV, n = 4-6). 4. The addition of glibenclamide (1 microM) did not influence the acetylcholine-induced L-NOARG/ indomethacin-resistant hyperpolarization (-18.0 +/- 1.8 mV, n = 10). In contrast, the responses induced by S-nitroso-L-glutathione, sodium nitroprusside, SIN-1 and iloprost were abolished (changes in membrane potential: -0.8 +/- 1.1, 1.3 +/- 3.9, 4.5 +/- 4.6 and 0.3 +/- 0.8 mV respectively, n = 4-5). 5. In the presence of NO synthase and cyclo-oxygenase inhibitors, charybdotoxin (0.1 microM) or apamin (0.5 microM) did not influence the hyperpolarization produced by acetylcholine. However, in the presence of the combination of charybdotoxin and apamin, the acetylcholine-induced L-NOARG/indomethacin-resistant hyperpolarization was converted to a depolarization (4.4 +/- 1.2 mV, n = 20) while the endothelium-independent hyperpolarizations induced by S-nitroso-L-glutathione, sodium nitroprusside, SIN-1 and iloprost were not affected significantly (respectively: -20.4 +/- 3.4, -22.5 +/- 4.9, -14.5 +/- 4.7 and -14.5 +/- 0.5 mV, n = 4-5). 6. In the presence of the combination of charybdotoxin and apamin and in the absence of L-NOARG and indomethacin, acetylcholine induced a hyperpolarization (-19.5 +/- 3.7 mV, n = 4). This hyperpolarization induced by acetylcholine was not affected by the addition of indomethacin (-18.3 +/- 4.6 mV, n = 3). In the presence of the combination of charybdotoxin, apamin and L-NOARG (in the absence of indomethacin), acetylcholine, in 5 out of 7 vessels, still produced hyperpolarization which was not significantly smaller (-9.1 +/- 5.6 mV, n = 7) than the one observed in the absence of L-NOARG. 7. These findings suggest that, in the guinea-pig isolated carotid artery, the endothelium-independent hyperpolarizations induced by NO donors and iloprost involve the opening of KATP channels while the acetylcholine-induced endothelium-dependent hyperpolarization (resistant to the inhibition of NO-synthase and cyclo-oxygenase) involves the opening of Ca(2+)-activated potassium channel(s). Furthermore, in this tissue, acetylcholine induces the simultaneous release of various factors from endothelial origin: hyperpolarizing factors (NO, endothelium derived hyperpolarizing factor (EDHF) and prostaglandins) and possibly a depolarizing factor.