Conditions permitting suppression of stretch-induced and vasoconstrictor tone by basal nitric oxide activity in porcine cerebral artery

Interaction of prostanoid EP₃ and TP receptors in guinea-pig isolated aorta: contractile self-synergism of 11-deoxy-16,16-dimethyl PGE₂

BACKGROUND AND PURPOSE

Surprisingly high contractile activity was reported for 11-deoxy-16,16-dimethyl prostaglandin E₂ (DX-DM PGE₂) on pig cerebral artery when used as a selective EP₃ receptor agonist. This study investigated the selectivity profile of DX-DM PGE₂, focusing on the interaction between its EP₃ and TP (thromboxane A₂-like) agonist activities.

EXPERIMENTAL APPROACH

Contraction of guinea-pig trachea (EP₁ system) and aorta (EP₃ and TP systems) was measured in conventional organ baths.

KEY RESULTS

Strong contraction of guinea-pig aorta to sulprostone and 17-phenyl PGE₂ (EP₃ agonists) was only seen under priming with a second contractile agent such as phenylephrine, histamine or U-46619 (TP agonist). In contrast, DX-DM PGE₂ induced strong contraction, which on the basis of treatment with (DG)-3ap (EP₃ antagonist) and/or BMS-180291 (TP antagonist) was attributed to self-synergism arising from co-activation of EP₃ and TP receptors. EP₃/TP self-synergism also accounted for contraction induced by PGF(2α) and its analogues (+)-cloprostenol and latanoprost-FA. DX-DM PGE₂ also showed significant EP₁ agonism on guinea-pig trachea as defined by the EP₁ antagonists SC-51322, (ONO)-5-methyl-1 and AH-6809, although AH-6809 exhibited poor specificity at concentrations ≥3 µM.

CONCLUSIONS AND IMPLICATIONS

EP₃/TP self-synergism, as seen with PGE/PGF analogues in this study, may confound EP₃ agonist potency comparisons and the characterization of prostanoid receptor systems. The competitive profile of a TP antagonist may be distorted by variation in the silent/overt contraction profile of the EP₃ system in different studies. The relevance of self-synergism to in vivo actions of natural prostanoid receptor agonists is discussed.

Functional serotonin and histamine receptor subtypes in porcine ciliary artery in comparison with middle cerebral artery

Functional serotonin (5-HT) and histamine receptor subtypes were investigated in porcine middle cerebral and ciliary arteries. An H(1) antagonist, mepyramine, antagonized histamine-induced responses with pK(B) values of 8.91-9.10. In the presence of 1 muM mepyramine, however, histamine caused dilation through H(2) receptors in the middle cerebral but not in the ciliary artery. A 5-HT(2A) antagonist, ketanserin, antagonized 5-HT-induced responses, causing rightward shifts in the concentration-response curves with pK(B) values of 8.52-8.71. A 5-HT(1B) antagonist, SB224289, produced rightward shifts of the concentration-response curves to sumatriptan with pK(B) values (6.66) only in the middle cerebral artery. In contrast, a 5-HT(1D) antagonist, BRL15572, had no effect in either artery. An RT-PCR study demonstrated the gene expression of the mRNAs of all three receptors (5HT(1B), 5HT(1D) and 5HT(2A)) in both arteries. These results suggest that histamine-induced contraction is mediated only through functional H(1) receptor in these arteries. Interestingly, there are functional 5-HT(2A) and 5-HT(1B) receptor subtypes in the middle cerebral artery, whereas the only functional receptor is 5-HT(2A) in the ciliary artery. The difference may be important for treatment with 5-HT(1B/1D) agonists (e.g. for migraine) without ocular side effect.

Rho-kinase contributes to hypoxia/reoxygenation-induced cerebral endothelial dysfunction

Hypoxia/reoxygenation (H/R) in vitro induced cerebral endothelial dysfunction is mediated by superoxide production. However, the intracellular pathways involved remain unclear. The present study was designed to investigate the involvement of Rho-kinase and its interaction with nitric oxide (NO) in cerebral endothelial dysfunction after H/R. Arterial diameter and intraluminal pressure were simultaneously measured in vitro on rat posterior cerebral arteries. Vascular NO production was determined by measuring stable NO metabolites nitrate/nitrite. H/R selectively inhibited cerebral vasodilation to the endothelium-dependent agonist acetylcholine (ACh, 0.01 to 10 micromol/L) and caused NO deficiency. H/R-impaired vasodilation to ACh was reversed by Y27632 (1 micromol/L), a specific inhibitor of Rho-kinase, but not by chelerythrine (1 micromol/L), a selective inhibitor of protein kinase C. Y27632 had no protective effect in the presence of N-nitro-L-arginine methyl ester (L-NAME; 100 micromol/L), a specific endothelial NO synthase inhibitor. L-NAME (100 micromol/L) alone failed to modulate H/R-impaired vasodilation, so did L-arginine (3 mmol/L), a substrate for NO synthase. However, a stable NO donor diethylenetetra amine-NONOate (5 micromol/L) normalized H/R-impaired dilation to ACh. In conclusion, H/R-induced endothelial dysfunction is associated with activation of Rho-kinase-dependent pathway and NO deficiency. Pretreatment with either Y27632 or the stable NO donor profoundly prevented H/R-mediated cerebral endothelial dysfunction.

Possible Role for K + in Endothelium-Derived Hyperpolarizing Factor–Linked Dilatation in Rat Middle Cerebral Artery

Background and Purpose— Endothelium-derived hyperpolarizing factor (EDHF) and K + are vasodilators in the cerebral circulation. Recently, K + has been suggested to contribute to EDHF-mediated responses in peripheral vessels. The EDHF response to the protease-activated receptor 2 ligand SLIGRL was characterized in cerebral arteries and used to assess whether K + contributes as an EDHF.

Methods— Rat middle cerebral arteries were mounted in either a wire or pressure myograph. Concentration-response curves to SLIGRL and K + were constructed in the presence and absence of a variety of blocking agents. In some experiments, changes in tension and smooth muscle cell membrane potential were recorded simultaneously.

Results— SLIGRL (0.02 to 20 μmol/L) stimulated concentration and endothelium-dependent relaxation. In the presence of N G -nitro- l -arginine methyl ester, relaxation to SLIGRL was associated with hyperpolarization and sensitivity to a specific inhibitor of IK Ca , 1-[(2-chlorophenyl)diphenylmethyl]-1 H -pyrazole (1μmol/L), reflecting activation of EDHF. Combined inhibition of K IR with Ba 2+ (30μmol/L) and Na + /K + -ATPase with ouabain (1 μmol/L) markedly attenuated the relaxation to EDHF. Raising extracellular [K + ] to 15 mmol/L also stimulated smooth muscle relaxation and hyperpolarization, which was also attenuated by combined application of Ba 2+ and ouabain.

Conclusions— SLIGRL evokes EDHF-mediated relaxation in the rat middle cerebral artery, underpinned by hyperpolarization of the smooth muscle. The profile of blockade of EDHF-mediated hyperpolarization and relaxation supports a pivotal role for IK Ca channels. Furthermore, similar inhibition of responses to EDHF and exogenous K + with Ba 2+ and ouabain suggests that K + may contribute as an EDHF in the middle cerebral artery.

Modulation of Intrinsic Cavernous Tone and Nitric Oxide Production by Arginase in Rabbit Corpus Cavernosum

PURPOSE

Arginase shares the common substrate L-arginine with nitric oxide synthase (NOS). We investigated the roles of NOS and arginase for modulating intrinsic and vasoconstrictor tone in rabbit corpus cavernosum (RCC).

MATERIALS AND METHODS

Isolated RCC tissues were used for isometric tension experiments, and NOS and arginase activities. The endothelium lining RCC lacunar spaces was disrupted and/or removed by saponin treatment.

RESULTS

Following stretch of approximately 1gm NG-nitro-L-arginine methyl ester (L-NAME) as a NOS inhibitor caused endothelium dependent contraction, while the potent and specific arginase inhibitor N omega-hydroxy-nor-L-arginine (nor-NOHA) caused endothelium dependent relaxation. Relaxation with nor-NOHA was reversed by L-NAME. In the presence of 10 mM L-arginine 0.1 mM nor-NOHA was ineffective. Pretreatment with 0.1 mM L-NAME and 0.1 mM nor-NOHA did not significantly affect the vasoconstrictor response to phenylephrine. The magnitude of contraction with 0.1 mM L-NAME and relaxation with 0.1 mM nor-NOHA during contraction induced by phenylephrine were not significantly different from changes with L-NAME and nor-NOHA under intrinsic basal tone. In the enzymatic study NOS and arginase were detectable in cavernous homogenates. Nor-NOHA inhibited arginase but not NOS activity.

CONCLUSIONS

Results indicate that basal nitric oxide production from the endothelium regulates intrinsic cavernous tone and endogenous arginase activity in the endothelium modulates tone by inhibiting nitric oxide production, presumably through competition with constitutive NOS for the common substrate L-arginine.

Myogenic responses and compliance of mesenteric and splenic vasculature in the rat

In the rat, the spleen is a major site of fluid efflux out of the blood. By contrast, the mesenteric vasculature serves as a blood reservoir. We proposed that the compliance and myogenic responses of these vascular beds would reflect their different functional demands. Mesenteric and splenic arterioles ( approximately 150-200 microm) and venules (<250 microm) from rats anesthetized with pentobarbital sodium were mounted in a pressurized myograph. Mesenteric arterial diameter decreased from 146 +/- 6 to 133 +/- 6 microm on raising intraluminal pressures from 80 to 120 mmHg. This response was enhanced in the presence of N(omega)-nitro-l-arginine methyl ester (l-NAME; 139 +/- 6 to 112 +/- 7 microm). There was no such myogenic response in the splenic arterioles, except in the presence of l-NAME (194 +/- 4 to 164 +/- 4.2 microm). We propose that, whereas mesenteric arterioles exhibit myogenic responses, this is normally masked by NO-mediated dilation in the splenic vessels. The mesenteric venules were highly distensible (active, 184 +/- 15 to 320 +/- 30.9 microm; passive in Ca(2+)-free media, 209 +/- 31 to 344 +/- 27 microm; 4-8 mmHg) compared with the splenic vessels (active, 169 +/- 11 to 184 +/- 16 microm; passive, 187 +/- 12 to 207 +/- 17 microm). We conclude that, in response to an increase in perfusion pressure, mesenteric arterial diameter would decrease to limit the changes in flow and microvascular pressure. In addition, mesenteric venous capacitance would increase. By contrast, splenic arterial diameter would increase, while there would be little change in venous diameter. This would enhance the increase in intrasplenic microvascular pressure and increase fluid extravasation.

Chronic hypoxia increases MCA contractile response to U-46619 by reducing NO production and/or activity

Chronic hypoxia alters contractile sensitivity of isolated arteries to alpha-adrenergic stimulation and other agonists. However, most studies have been performed in thoracic aortas or other large vessels making little contribution to vascular resistance in their respective circulations. To determine the effect of chronic hypoxia on the vasoconstrictor response in a small, resistance-sized vessel, we studied second and third generation middle cerebral arteries (MCA; approximately 75-microm internal diameter before mounting). MCA were isolated from normoxic (inspired oxygen = 125 Torr) and hypoxic (8 wk at 3,960 m; inspired oxygen = 90 Torr) guinea pigs, and their vasoconstrictor responses were determined to the thromboxane mimetic U-46619 by using dual-pipette video microscopy. Arteries from hypoxic animals had greater contractile sensitivity to U-46619 compared with those of the normoxic animals (-log EC50 = 7.86 +/- 0.11 vs. 7.62 +/- 0.06, respectively, P < 0.05). Addition of the nitric oxide (NO) inhibitor nitro-L-arginine (200 microM) to the vessel bath eliminated the differences in contractile sensitivity between the MCA from the normoxic and chronically hypoxic groups. Supplementation with L-arginine in the drinking water sufficient to raise plasma L-arginine levels 41% reduced MCA contractile sensitivity to U-46619 in the normoxic group (-log EC50 = 7.22 +/- 0.31, P < 0.05 compared with the nonsupplemented normoxic group) but not in the chronically hypoxic group. These results show that chronic hypoxia increases the sensitivity of the MCA to the vasoconstrictor U-46619, likely because of a reduction in NO production and/or activity.

Alterations of the nitric oxide pathway in cerebral arteries from spontaneously hypertensive rats

Hypertension-associated alterations of the nitric oxide (NO) pathway were analyzed in middle cerebral arteries (MCA) from normotensive (WKY) and hypertensive (SHR) rats. The vasoconstrictor response to prostaglandin F2alpha (PGF(2 alpha), 30 and 100 microM) was smaller in MCA from SHR than from WKY. Endothelium-dependent relaxations to bradykinin (1 nM-10 microM) or acetylcholine (10 microM) were similar in MCA from both strains, whereas the endothelium-independent response to sodium nitroprusside (1 nM-0.1 mM) was smaller in MCA from SHR. L-arginine (L-Arg, 10 microM) similarly inhibited the vasoconstrictor responses in both strains; however, the inhibitory effect of 100 microM of L-Arg was greater in MCA from SHR. N(omega)-nitro-L-arginine methyl ester (L-NAME, 100 microM), but not aminoguanidine (100 microM) or 7-nitroindazole (10 microM), increased basal tone, potentiated the PGF(2 alpha)-induced vasoconstrictor responses and reduced the bradykinin-elicited relaxation in a similar way in MCA from WKY and SHR. N(omega)-nitro-L-arginine methyl ester also antagonized the inhibitory effect of 10 microM of L-Arg. Incubation for 5 h with lipopolysaccharide (10 microg/ml) similarly reduced the response to PGF(2 alpha) in MCA from WKY and SHR; this reduction was antagonized by dexamethasone (1 microM). Cerebral arteries expressed endothelial (eNOS) and neuronal (nNOS) NO synthase similarly in both strains, but inducible NOS (iNOS) expression was more evident in SHR. Lipopolysaccharide increased iNOS expression in both strains to a similar level. The basal constitutive NOS (cNOS) and iNOS activities were similar in arteries from WKY and SHR. Lipopolysaccharide increased iNOS activity only in arteries from SHR. These results indicate that hypertension did not impair endothelial NO production by NOS activation but induced an up-regulation of basal iNOS expression.

Enhancement of phenylephrine-induced contraction in the isolated rat aorta with endothelium by H2O-extract from an Oriental medicinal plant Leonuri herba

Leonuri herba (I-mu-ts'ao, the Chinese motherwort) is an ancient Chinese traditional herb. Although the pharmacological effects of extracts of Leonuri herba have been shown in platelets and uteri, the effect on the vascular system has not been determined. In the present study, we investigated the effects of extracts of Leonuri herba on the contraction of the isolated rat aorta. Although the H20-extract (0.3-3 mg/ml) by itself showed a limited effect, the extract enhanced phenylephrine-induced contraction of the aorta with endothelium, but not without endothelium. The H20-extract, like N(G)-nitro-L-arginine methyl ester (L-NAME, an inhibitor of nitric oxide synthase), significantly inhibited the relaxation induced by acetylcholine in the aorta with endothelium. The inhibitory effect of H20-extract on the relaxation decreased by co-addition with 1 mM L-arginine. The vasoconstrictive effect of H20-extract was not due to leonurine, which is a constituent in Leonuri herba and shows uterotonic activity. Intravenous injection of the H20-extract (1.5 mg/kg) to rats caused an increase in blood pressure for 5 min, like L-NAME (1.35 mg/kg). These findings suggest that there is a component(s) in Leonuri herba, which shows a vasoconstrictive activity in rat aorta in vitro and in vivo and has similar pharmacological profile to that of L-NAME.