Inhibitors of the cytochrome P450-mono-oxygenase and endothelium-dependent hyperpolarizations in the guinea-pig isolated carotid artery

N-arachidonylethanolamide relaxation of bovine coronary artery is not mediated by CB1 cannabinoid receptor

It has been reported that the endogenous cannabinoid N-arachidonylethanolamide (AEA), commonly referred to as anandamide, has the characteristics of an endothelium-derived hyperpolarizing factor in rat mesenteric artery. We have carried out studies to determine whether AEA affects coronary vascular tone. The vasorelaxant effects of AEA were determined in isolated bovine coronary artery rings precontracted with U-46619 (3 x 10(-9) M). AEA decreased isometric tension, producing a maximal relaxation of 51 +/- 9% at a concentration of 10(-5) M. Endothelium-denuded coronary arteries were not significantly affected by AEA. The CB1 receptor antagonist SR-141716A (10(-6)M) failed to reduce the vasodilatory effects of AEA, suggesting that the CB1 receptor is not involved in this action of AEA. Because AEA is rapidly converted to arachidonic acid and ethanolamine in brain and liver by a fatty acid amide hydrolase (FAAH), we hypothesized that the vasodilatory effect of AEA results from its hydrolysis to arachidonic acid followed by enzymatic conversion to vasodilatory eicosanoids. In support of this hypothesis, bovine coronary arteries incubated with [3H]AEA for 30 min hydrolyzed 15% of added substrate; approximately 9% of the radiolabeled product was free arachidonic acid, and 6% comigrated with the prostaglandins (PGs) and epoxyeicosatrienoic acids (EETs). A similar result was obtained in cultured bovine coronary endothelial cells. Inhibition of the FAAH with diazomethylarachidonyl ketone blocked both the metabolism of [3H]AEA and the relaxations to AEA. Whole vessel and cultured endothelial cells prelabeled with [3H]arachidonic acid synthesized [3H]PGs and [3H]EETs, but not [3H]AEA, in response to A-23187. Furthermore, SR-141716A attenuated A-23187-stimulated release of [3H]arachidonic acid, suggesting that it may have actions other than inhibition of CB1 receptor. These experiments suggest that AEA produces endothelium-dependent vasorelaxation as a result of its catabolism to arachidonic acid followed by conversion to vasodilatory eicosanoids such as prostacyclin or the EETs.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

Cytochrome P-450 pathway in acetylcholine-induced canine coronary microvascular vasodilation in vivo

In the canine coronary microcirculation, acetylcholine (ACh)-induced vasodilation of large (> or = 100 microns) epicardial arterioles (LgA), but not small (< 100 microns) epicardial arterioles (SmA), is blocked by nitric oxide (NO) synthase inhibitors in vivo. We hypothesized that the ACh-induced vasodilation of SmA is mediated by a cytochrome P-450 metabolite of arachidonic acid (AA). Epicardial coronary microvascular diameters in dogs were measured at baseline and after treatment with topically applied ACh (1, 10, and 100 microM), AA (1, 5, and 10 microM), or sodium nitroprusside (SNP; 10-100 microM). Coronary microvascular diameters were compared among control dogs (group OO); dogs pretreated with N omega-nitro-L-arginine (L-NNA; 70 microM topically) (group NO); dogs pretreated with L-NNA plus clotrimazole (Clo; 1.6 microM topically) or 17-octadecynoic acid (ODYA; 2 microM topically), cytochrome P-450 monooxygenase inhibitors (groups NC and NY, respectively); dogs pretreated with Clo alone (group OC); and dogs pretreated with L-NNA plus Clo with AA as the agonist (group AA). ACh-induced vasodilation of LgA was abolished by L-NNA alone, whereas in SmA, L-NNA was without effect. Clo alone did not inhibit ACh-induced dilation in either SmA or LgA. However, the combinations of L-NNA plus either Clo or ODYA abolished ACh- and AA-induced dilation of SmA (100 microM ACh: NC, 3 +/- 5%; NY, 8 +/- 2%; 10 microM AA: 6 +/- 3%) but did not affect responses to SNP. These results suggest that the ACh-induced vasodilation of SmA is mediated in part by cytochrome P-450 metabolites of AA and provide the first evidence that the cytochrome P-450 pathway contributes to the regulation of coronary resistance vessels in vivo.

Importance of endothelium-derived hyperpolarizing factor in human arteries

The endothelium plays an important role in maintaining the vascular homeostasis by releasing vasodilator substances, including prostacyclin (PGI2), nitric oxide (NO), and endothelium-derived hyperpolarizing factor (EDHF). Although the former two substances have been investigated extensively, the importance of EDHF still remains unclear, especially in human arteries. Thus we tested our hypothesis that EDHF plays an important role in human arteries, particularly with reference to the effect of vessel size, its vasodilating mechanism, and the influences of risk factors for atherosclerosis. Isometric tension and membrane potentials were recorded in isolated human gastroepiploic arteries and distal microvessels (100-150 microm in diameter). The contribution of PGI2, NO, and EDHF to endothelium-dependent relaxations was analyzed by inhibitory effects of indomethacin, NG-nitro- L-arginine, and KCl, respectively. The nature of and hyperpolarizing mechanism by EDHF were examined by the inhibitory effects of inhibitors of cytochrome P450 pathway and of various K channels. The effects of atherosclerosis risk factors on EDHF-mediated relaxations were also analyzed. The results showed that (a) the contribution of EDHF to endothelium-dependent relaxations is significantly larger in microvessels than in large arteries; (b) the nature of EDHF may not be a product of cytochrome P450 pathway, while EDHF-induced hyperpolarization is partially mediated by calcium-activated K channels; and (c) aging and hypercholesterolemia significantly impair EDHF-mediated relaxations. These results demonstrate that EDHF also plays an important role in human arteries.

Studies on the effects of anandamide in rat hepatic artery

1. The effects of anandamide on K+ currents and membrane potential have been examined in freshly-isolated smooth muscle cells from rat hepatic artery and the results compared with the effects of this arachidonic acid derivative on tension and membrane potential changes in segments of whole artery. 2. In the presence of 0.3 mM L-NOARG and 10 microM indomethacin, anandamide (0.1-100 microM) and endothelium-derived hyperpolarizing factor (EDHF; liberated by acetylcholine, 0.01-10 microM) each relaxed endothelium-intact segments of hepatic artery precontracted with phenylephrine. These effects of anandamide, but not those of EDHF, were antagonized by the cannabinoid receptor antagonist, SR141716A (3 microM). 3. The relaxant effects of anandamide were unaffected by a toxin combination (apamin plus charybdotoxin, each 0.3 microM) which abolishes EDHF relaxations and were essentially unchanged in endothelium-denuded arteries. The relaxant effects of anandamide in endothelium-intact arteries were significantly reduced in a physiological salt solution containing 30 mM KCl and abolished when the K+ concentration was raised to 60 mM. 4. Anandamide (10 microM), acetylcholine (1 microM, via release of EDHF) and levcromakalim (10 microM) each markedly hyperpolarized the membrane potential of the smooth muscle cells of endothelium-intact arteries. However, when the endothelium was removed, the hyperpolarizing effects of both anandamide (10 microM) and acetylcholine were essentially abolished whereas those of levcromakalim (10 microM) were unaffected. 5. Under voltage-clamp conditions, anandamide (10 microM) abolished spontaneous transient outward currents (STOCs) in freshly-isolated single hepatic artery cells held at 0 mV but had no effect on the holding current at this potential. In current-clamp mode, the spontaneous hyperpolarizing potentials which corresponded to the STOCs were abolished with no significant change in basal membrane potential. 6. Anandamide (10 microM) abolished the iberiotoxin-sensitive K+ current (IBK(Ca)) produced by caffeine and the corresponding hyperpolarizations generated by this xanthine derivative in current-clamp mode. In contrast, anandamide had no effect on IBK(Ca) generated on exposure to NS1619 (30 microM). 7. It was concluded that anandamide is not EDHF in the rat hepatic artery. Anandamide-induced hyperpolarization is exerted indirectly and requires the presence of the endothelium. Anandamide also acts on the smooth muscle cells to inhibit processes which require functional intracellular calcium stores. This direct action seems more important than membrane hyperpolarization in relaxing phenylephrine-contracted vessels.

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.

Perivascular sensory nerve Ca2+ receptor and Ca2+-induced relaxation of isolated arteries

The present study tested two hypotheses: (1) that a receptor for extracellular Ca2+ (Ca2+ receptor [CaR]) is located in the perivascular sensory nerve system and (2) that activation of this receptor by physiological concentrations of extracellular Ca2+ results in the release of vasodilator substance that mediates Ca2+-induced relaxation. Reverse transcription-polymerase chain reaction using primers derived from rat kidney CaR cDNA sequence showed that mRNA encoding a CaR is present in dorsal root ganglia but not the mesenteric resistance artery. Western blot analysis using monoclonal anti-CaR showed that a 140-kD protein that comigrates with the parathyroid CaR is present in both the dorsal root ganglia and intact mesenteric resistance artery. Immunocytochemical analysis of whole mount preparations of mesenteric resistance arteries showed that the anti-CaR-stained perivascular nerves restricted to the adventitial layer. Biophysical analysis of mesenteric resistance arteries showed that cumulatively raising Ca2+ from 1 to 1.25 mol/L and above relaxes precontracted arteries with an ED50 value of 2.47+/-0.17 mmol/L (n=12). The relaxation is endothelium independent and is unaffected by blockade of nitric oxide synthase but is completely antagonized by acute and subacute phenolic destruction of perivascular nerves. A bioassay showed further that superfusion of Ca2+ across the adventitial surface of resistance arteries releases a diffusible vasodilator substance. Pharmacological analysis indicates that the relaxing substance is not a common sensory nerve peptide transmitter but is a phospholipase A2/cytochrome P450-derived hyperpolarizing factor that we have classified as nerve-derived hyperpolarizing factor. These data demonstrate that a CaR is expressed in the perivascular nerve network, show that raising Ca2+ from 1 to 1.25 mol/L and above causes nerve-dependent relaxation of resistance arteries, and suggest that activation of the CaR induces the release of a diffusible hyperpolarizing vasodilator. We propose that this system could serve as a molecular link between whole-animal Ca2+ balance and arterial tone.

N(G)-nitro-L-arginine- and indomethacin-resistant endothelium-dependent relaxation in the rabbit renal artery: effect of hypercholesterolemia

Studies were designed to compare the N(G)-nitro-L-arginine- and indomethacin-resistant, endothelium-dependent relaxation to acetylcholine in isolated renal artery rings from normal and cholesterol-fed rabbits. It was assumed that the resistant part in response to acetylcholine is mediated by the endothelial-derived hyperpolarizing factor (EDHF). Rabbits were fed normal (n = 15) or cholesterol enriched chow (n = 13, 1% cholesterol for 4 weeks, 0.5% for 12 weeks). In organ chamber experiments, renal artery rings were precontracted with 0.1-1 microM phenylephrine or 35 mM KCl, and relaxed with acetylcholine (0.001-10 microM) in the presence of 10 microM indomethacin. Studies were performed in the presence or absence of: 100 microM N(G)-nitro-L-arginine (L-NOARG) to inhibit the nitric oxide pathway, 100 nM charybdotoxin (CTX) or 1 mM tetrabutylammonium (TBA) to inhibit Ca2+-activated K+ channels, and 100 microM SKF 525a to inhibit cytochrome P450 monoxygenase pathway. In normal arteries, L-NOARG partially inhibited acetylcholine-induced relaxation. The resistant part was almost abolished when the arteries were depolarized with KCl, or when L-NOARG was combined with either CTX, TBA or SKF 525a. In arteries from hypercholesterolemic animals, the relaxation to acetylcholine was only slightly impaired as compared to normal animals. However, in comparison to arteries from normal animals, the L-NOARG-resistant part of acetylcholine-induced endothelium-dependent relaxation was enhanced. It is speculated that differences in the balance between nitric oxide (NO)- and EDHF-mediated control of vascular tone may maintain acetylcholine-induced vasodilatation of the renal artery in hypercholesterolemia.

Nitric oxide and endothelium-derived hyperpolarizing factor: formation and interactions

Nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) represent the two major endothelial autacoids involved in the local control of vascular tone. Here we describe a novel pathway leading to the calcium-independent activation of the endothelial NO synthase by shear stress and enhanced cellular tyrosine phosphorylation. In addition we present evidence that NO inhibits the formation of EDHF, which we have characterized as a transferable, beta-naphthoflavone-inducible P450-dependent metabolite of arachidonic acid.

Involvement of a cannabinoid in endothelium-derived hyperpolarizing factor-mediated coronary vasorelaxation

We have recently proposed that an endocannabinoid is the endothelium-derived hyperpolarizing factor (EDHF) and have now tested this hypothesis in the rat isolated perfused heart. In this preparation bradykinin gave rise to nitric oxide- and prostanoid-independent relaxations, assessed as reductions in coronary perfusion pressure (ED50 = 14.9 +/- 5.9 pmol; Rmax = 25.2 +/- 2.2%), which are thought to be mediated by EDHF. These relaxations were antagonised by both the highly selective cannabinoid antagonist, SR141716A (1 microM) (Rmax = 8.3 +/- 1.2%, P < 0.001) and by the calcium-dependent potassium channel blocker tetrabutylammonium (300 microM) (Rmax = 6.7 +/- 3.4%, P < 0.01) and were abolished by the EDHF inhibitor clotrimazole (3 microM). The endogenous cannabinoid, anandamide, similarly caused coronary vasorelaxation (Rmax = 32.3 +/- 2.3%), which was abolished by clotrimazole (3 microM) and antagonised by both 300 microM tetrabutylammonium (Rmax = 18.2 +/- 2.8%, P < 0.01) and 1 microM SR141716A (Rmax = 16.4 +/- 3.3%, P < 0.01). Accordingly, these results suggest that EDHF-mediated responses in the rat coronary vasculature are due to an endogenous cannabinoid and that anandamide causes vasorelaxation through potassium channel activation. These findings are, therefore, consistent with our recent proposal that EDHF is an endogenous cannabinoid.

Comparative pharmacology of endothelium-derived hyperpolarizing factor and anandamide in rat isolated mesentery

We have recently proposed that anandamide, or a related cannabinoid, is the endothelium-derived hyperpolarizing factor (EDHF) and have now compared EDHF-mediated responses (induced by carbachol in the presence of both nitric oxide and prostanoid synthesis inhibitors) with those induced by anandamide in the rat isolated superior mesenteric arterial bed. Both EDHF-mediated and anandamide-induced relaxations were inhibited in the presence of high K+ (60 mM) and opposed by blockade of K+ channels with 10 mM tetraethylammonium. The cytochrome P450 inhibitors, and putative EDHF inhibitors, clotrimazole (10 microM) and proadifen (SKF 525A) (10 microM), opposed both anandamide-induced and EDHF-mediated relaxations and also relaxant responses to the K+ channel activator levcromakalim. Therefore, EDHF-mediated and anandamide-induced vasorelaxations show very similar pharmacological characteristics, with both responses being mediated via K+ channel activation. Further, the actions of EDHF and anandamide are both sensitive to proadifen and clotrimazole, EDHF antagonists which appear to act through K+ channel inhibition. Accordingly, these results support our proposal that an endocannabinoid is an EDHF.

Evidence against the involvement of cytochrome P450 metabolites in endothelium-dependent hyperpolarization of the rat main mesenteric artery

1. The influence of different inhibitors of cytochrome P450 mono-oxygenase on the endothelium-dependent and -independent hyperpolarization in the isolated rat main mesenteric artery was investigated. 2. Application of acetylcholine (ACh; 1 microM) for 10 min evoked an endothelium-dependent peak hyperpolarization of about 18 mV followed by a partial recovery to a level 7 mV more negative than the resting value (-50.2 +/- 0.5 mV). 3. Proadifen (30 microM) completely and reversibly inhibited the ACh-induced hyperpolarization. Conversely, the imidazole antimycotics clotrimazole (30 microM) and miconazole (100 microM) had less effect on the peak endothelium-dependent hyperpolarization. The suicide substrate inhibitors 17-octadecynoic acid (17-ODYA; 5 microM) and 1-aminobenzotriazole (1-ABT; 2 mM) did not significantly influence endothelium-dependent hyperpolarization. 4. The endothelium-independent hyperpolarization (16 mV) evoked by leveromakalim (300 nM) was completely inhibited by proadifen as well as by clotrimazole and miconazole but was not affected by 17-ODYA or 1-ABT. 5. These results do not support the view that the ACh-induced endothelium-dependent hyperpolarization in the rat mesenteric artery is mediated by cytochrome P450 mono-oxygenase metabolites. Proadifen and imidazole antimycotics impair the activation of ATP-regulated K+ channels in mesenteric artery cells, rendering non-specific inhibition of smooth muscle K+ channel activation an alternative explanation for the inhibitory influence of some (but not all) P450 inhibitors on endothelium-dependent hyperpolarization in this preparation.

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 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.

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.

Hypoxic dilatation of porcine small coronary arteries: role of endothelium and KATP-channels

1. The aim of the present study was to determine the cellular mechanims and potential mediators involved in hypoxic dilatation of porcine small coronary arteries. 2. Small coronary arteries were isolated from a branch of the left anterior descending artery of porcine hearts, cannulated with glass micropipettes and studied in a perfusion myograph system. At a transmural pressure of 40 mmHg, the arteries had an internal diameter of 167.8 +/- 6.6 microns (n = 37). 3. In arteries contracted with acetylcholine (ACh), hypoxia (0% O2, 30 min) caused dilatation (86.9 +/- 6.7% relaxation, n = 6) in vessels with endothelium but constriction in endothelium-denuded vessels. 4. Hypoxic vasodilatation occurring in arteries with endothelium was abolished by the KATP channel inhibitor, glibenclamide (0.44 microM), but was not affected by inhibition of nitric oxide synthase (L-NAME, 44 microM) or cyclo-oxygenase (indomethacin, 4.4 microM). 5. Bradykinin evoked endothelium-dependent relaxation that was inhibited by L-NAME (44 microM) but not glibenclamide 0.44 microM). Cromakalim (0.1-0.3 microM), a KATP channel opener, caused relaxation that was inhibited by glibenclamide, but was not affected by L-NAME (44 microM) and/or indomethacin (4.4 microM). 6. Endothelium-removal inhibited vasodilatation evoked by cromakalim, but increased vasodilator responses to the NO donor, SIN-1 (10(-8) to 10(-5) M). 7. These results indicate that hypoxia acted directly on vascular smooth muscle of small coronary arteries to cause contraction. However, this effect was overwhelmed by endothelium-dependent relaxation in response to hypoxia. This relaxation was most likely mediated by release of an endothelium-derived factor, distinct from nitric oxide or prostacyclin, that activated smooth muscle KATP-channels.

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.

Evidence against a role of cytochrome P450-derived arachidonic acid metabolites in endothelium-dependent hyperpolarization by acetylcholine in rat isolated mesenteric artery

1. In rat mesenteric artery, acetylcholine (ACh) causes endothelium-dependent hyperpolarization by releasing endothelium-derived hyperpolarizing factor (EDHF). Recent evidence suggests that EDHF may be a cytochrome P450-derived arachidonic acid metabolite. The aim of the present study was to investigate whether such a metabolite is indeed contributing to ACh-induced hyperpolarization observed in rat mesenteric artery. 2. The phospholipase A2 inhibitor quinacrine (30 microM) nearly completely eliminated ACh-induced hyperpolarization. However, the hyperpolarizing effect of pinacidil was also abolished in the presence of quinacrine. 3. The imidazole antimycotic agents ketoconazole (50 microM), clotrimazole (30 microM) and miconazole (10 microM), which bind to the heme moiety of cytochrome P450, eliminated not only ACh-induced hyperpolarizations but also those induced by pinacidil. SKF525A (30 microM), a prototype inhibitor of the enzyme, also abolished the hyperpolarizing responses to both agents. In contrast, neither 17-octadecynoic acid (10 microM), a mechanism-based inhibitor of cytochrome P450 metabolism of fatty acids, nor eicosatetraynoic acid (20 microM), an inhibitor of all arachidonic acid metabolic pathways, altered ACh-induced hyperpolarization. Furthermore, the hyperpolarization was unaffected by the preferential inhibitors of specific cytochrome P450 isozymes, alpha-naphtoflavone (1 microM), diedthyldithiocarbamate (50 microM), metyrapone (20 microM) and troleandomycin (10 microM). 4. Pretreatment of rats with lipopolysaccharide (2 mg kg-1) and exposure to nitroprusside (10 microM), both of which are expected to inhibit cytochrome P450 activity due to nitric oxide overproduction, were without effect on ACh-induced hyperpolarization. Pretreatment of rats for 3 days with pentobarbitone (80 mg kg-1 day-1), a cytochrome P450 inducer, also did not affect the hyperpolarizing response to ACh. 5. Arachidonic acid in concentrations up to 100 microM had no detectable effect on smooth muscle membrane potential. 11, 12-Epoxyeicosatrienoic acid (EET, 10 microM), one of cytochrome P450-derived epoxygenase metabolites of arachidonic acid, elicited a small endothelium-independent membrane hyperpolarization. The hyperpolarizing response to EET was blocked by glibenclamide (30 microM), in contrast to the response to ACh. 6. These results suggest that the contribution of a cytochrome P450-derived metabolite of arachidonic acid to ACh-induced hyperpolarization via EDHF release is minimal or absent in rat mesenteric artery.

Endothelium- and cytochrome P-450-dependent relaxation induced by isoproterenol in rat aortic rings

In rat aortic rings, the mechanism of endothelium-dependent relaxation induced by isoproterenol is examined. Pretreatment with (+/-)-1-[2,3]-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methyleth yl)amino] -2-butanol (ICI-118,551), a beta 2-adrenoceptor antagonist, or atenolol, a beta 1-adrenoceptor antagonist, partly inhibited the relaxing response to isoproterenol. The relaxing response to isoproterenol in the presence of ICI-118,551 or atenolol was markedly inhibited by removal of endothelium. In the aorta pretreated with ICI-118,551 or atenolol, residual relaxing response to isoproterenol was also inhibited by 2-methyl-1,2-di-3-pyridyl-1-propanone (metyrapone), alpha-naphthoflavone or 8-methoxypsoralen, cytochrome P-450 monoxygenase inhibitors, and methylene blue, but not by indomethacin, a cyclooxygenase inhibitor, 2,3,5-trimethyl-6-(12-hydroxy-5,10-dodecadiynyl)-1, 4-benzoquinone (AA861), a 5-lipoxygenase inhibitor, NG-nitro-L-arginine (NOARG), a nitric oxide synthase inhibitor, Zn protoporphyrin IX, a heme oxygenase inhibitor, or yohimbine, a alpha 2-adrenoceptor antagonist. In the aorta denuded of endothelium, metyrapone did not affect the residual relaxing response to isoproterenol in the presence of atenolol. These results suggest that the cytochrome P-450 system may be involved in the endothelium-dependent relaxation induced by isoproterenol through beta 1- and beta 2-adrenoceptor activation.

Role of K+ channels in EDHF-dependent relaxation induced by acetylcholine in canine coronary artery

To identify the K+ channels responsible for endothelium-derived hyperpolarizing factor (EDHF)-dependent relaxation, we studied the effects of various K+ channel blockers on acetylcholine-induced relaxation, which persists even in the presence of both an inhibitor of nitric oxide synthase and that of cyclooxygenase, in canine coronary artery rings. A nonselective K+ channel blocker, tetrabutylammonium (TBA), a large and intermediate conductance Ca2+-activated K+ channel blocker, charybdotoxin (CTX), and a voltage-dependent K+ channel blocker, 4-aminopyridine (4-AP), significantly inhibited this residual relaxation. A combined treatment with CTX and 4-AP almost completely blocked the relaxation. Neither a large (iberiotoxin) nor a small (apamin) conductance Ca2+-activated K+ channel blocker blocked the relaxation. We also investigated effects of K+ channel blockers on basal tone to determine whether or not EDHF is involved in regulating basal tone. TBA and CTX substantially raised basal tone to a greater degree in endothelium-intact preparations than in endothelium-denuded preparations. These results indicate that EDHF may exert its relaxing action through intermediate conductance Ca2+-activated and voltage-dependent K+ channels in canine coronary arteries. In addition, EDHF may play a role in maintaining basal vascular tone.

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.

Endothelium-derived hyperpolarizing factor

1. Not all endothelium-dependent relaxations can be fully explained by the release of either nitric oxide (NO) and/or prostacyclin. Another unidentified substance(s) that hyperpolarizes the underlying vascular smooth muscle cells (endothelium-derived hyperpolarizing factor; EDHF) contributes to endothelium-dependent relaxations. 2. In blood vessels from various species these hyperpolarizations are resistant to inhibitors of NO synthase (NOS) and cyclo-oxygenase. In canine, porcine and human blood vessels the hyperpolarization cannot be mimicked by nitrovasodilators or exogeneous NO. However, in other species (rat, guinea-pig, rabbit) endothelium-dependent hyperpolarizations resistant to inhibitors of NOS and cyclo-oxygenase and hyperpolarizations to endothelium-derived or exogeneous NO can be observed in the same vascular smooth muscle cells. 3. In blood vessels where NO causes hyperpolarization, the response is blocked by glibenclamide, suggesting the involvement of ATP-dependent potassium channels. Hyperpolarizations caused by EDHF are insensitive to glibenclamide but, depending on the tissue, are inhibited by relatively small concentrations of tetraethylammonium (TEA) or by apamin or the combination of charybdotoxin plus apamin, indicating that calcium-dependent potassium channels are likely to be involved. 4. Metabolites of arachidonic acid, through the cytochrome P450 mono-oxygenase pathway (epoxyeicosatrienoic acids), are produced by the endothelial cells, increase the open-state probability of calcium-activated potassium channels sensitive to TEA or charybdotoxin, and induce the hyperpolarization of arterial smooth muscle cells, indicating that epoxyeicosatrienoic acids could be EDHF. However, in blood vessels from various species, cytochrome P450 inhibitors do not affect endothelium-dependent hyperpolarizations, indicating that EDHF is not yet identified with certainty. 5. Endothelium-derived hyperpolarizing factor released from cultured endothelial cells reduces the intracellular calcium concentration in vascular smooth muscle cells and the EDHF component of the relaxation is proportionally more important in smaller than larger arteries. In aging animals and in various models of diseases, endothelium-dependent hyperpolarizations are diminished. 6. The identification of EDHF and/or the discovery of specific inhibitors of its synthesis and its action may allow a better understanding of its physiological and pathophysiological role(s).

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.

Bisallylic hydroxylation and epoxidation of polyunsaturated fatty acids by cytochrome P450

Polyunsaturated fatty acids can be oxygenated by cytochrome P450 to hydroxy and epoxy fatty acids. Two major classes of hydroxy fatty acids are formed by hydroxylation of the omega-side chain and by hydroxylation of bisallylic methylene carbons. Bisallylic cytochrome P450-hydroxylases transform linoleic acid to 11-hydroxylinoleic acid, arachidonic acid to 13-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid, 10-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid and 7-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid and eicosapentaenoic acid to 16-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid, 13-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid and 10-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid as major metabolites. The bisallylic hydroxy fatty acids are chemically unstable and decompose rapidly to cis-trans conjugated hydroxy fatty acids during acidic extractive isolation. Bisallylic hydroxylase activity appears to be augmented in microsomes induced by the synthetic glucocorticoid dexamethasone and by some other agents, but the P450 gene families of these hydroxylases have yet to be determined. The fatty acid epoxides, which are formed by cytochrome P450, are chemically stable, but are hydrolyzed to diols by soluble epoxide hydrolases. Epoxidation of polyunsaturated fatty acids is a prominent pathway of metabolism in the liver and the renal cortex and epoxy-genase activity appears to be under homeostatic control in the kidney. Many arachidonate epoxygenases have been identified belonging to the CYP2C gene subfamily. Epoxygenases have also been found in the central nervous system, endocrine organs, the heart and endothelial cells. Epoxides of arachidonic acid have been found to exert pharmacological effects on many cells.

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.