Relaxation Of Cerebral Arteries Research Articles

Relaxant effects of 17-beta-estradiol in cerebral arteries through Ca(2+) entry inhibition.

Estrogens account for gender differences in the incidence and outcome of stroke, but it remains unclear to what extent neuroprotective effects of estrogens are because of parenchymal or vascular actions. Because reproductive steroids have vasoactive properties, the authors assessed the effects and mechanisms of action of 17-beta-estradiol in rabbit isolated basilar artery. Cumulative doses of 17-beta-estradiol (0.3 micromol/L to 0.1 mmol/L) induced concentration-dependent relaxation that was larger in basilar than carotid artery, in male than female basilar artery, and in KCl-precontracted than UTP-precontracted male basilar artery. Endothelium removal did not modify relaxation induced by 17-beta-estradiol in basilar artery, whereas relaxation induced by acetylcholine (1 nmol/L to 0.1 mmol/L) was almost abolished. Neither the estrogen receptor antagonist ICI 182,780 (1 micromol/L), nor the protein synthesis inhibitor cycloheximide (1 micromol/L) affected 17-beta-estradiol-induced relaxations. Relaxations induced by the K(+) channel openers NS1619 and pinacidil in the same concentration range were greater and lower, respectively, when compared with relaxation to 17-beta-estradiol, which was not significantly modified by incubation with the K(+) channel blockers charybdotoxin (1 nmol/L and 0.1 micromol/L) or glibenclamide (10 nmol/L and 1 micromol/L). Preincubation with 17-beta-estradiol (3 to 100 micromol/L) produced concentration-dependent inhibition of CaCl(2)-induced contraction, with less potency than the Ca(2+) entry blocker nicardipine (0.01 to 10 nmol/L). The authors conclude that 17-beta-estradiol induces endothelium-independent relaxation of cerebral arteries with tissue and gender selectivity. The relaxant effect is because of inhibition of extracellular Ca(2+) influx to vascular smooth muscle, but activation of estrogen receptors, protein synthesis, or K(+) efflux are not involved. Relatively high pharmacologic concentrations of 17-beta-estradiol causing relaxation preclude acute vascular effects of physiologic circulating levels on the cerebral circulation.

Mechanisms that produce nitric oxide-mediated relaxation of cerebral arteries during atherosclerosis.

The first goal of the present study was to examine the hypothesis that relaxation of cerebral arteries to nitric oxide in primates is dependent on activation of soluble guanylate cyclase (sGC). The second goal was to determine whether the role of sGC in mediating responses to nitric oxide is altered in atherosclerosis. Basilar arteries from normal and atherosclerotic monkeys were studied in vitro. After precontraction with prostaglandin F(2alpha) (0.1 to 1 micromol/L), concentration-response curves to authentic nitric oxide (1 nmol/L to 1 micromol/L), sodium nitroprusside (10 nmol/L to 10 micromol/L; a nitric oxide donor), and papaverine (10 nmol/L to 10 micromol/L; a non-nitric oxide, non-sGC-dependent stimulus) were generated in the presence and absence of 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 1 and 10 micromol/L; an inhibitor of sGC). The effect of ODQ on basal tone of basilar arteries from normal and atherosclerotic monkeys was also examined. Nitric oxide, sodium nitroprusside, and papaverine produced relaxation that was similar (P:>0.05) in normal and atherosclerotic monkeys. ODQ produced marked inhibition (P:<0.05) of vasorelaxation in response to nitric oxide and nitroprusside but not papaverine. For example, relaxation of the basilar artery in response to nitric oxide (0.1 micromol/L) was inhibited by approximately 85% and 73% by ODQ (1 micromol/L) in normal and atherosclerotic monkeys, respectively. ODQ produced contraction of the basilar arteries, and the increase in tension to ODQ was greater in normal (2.7+/-0.3 g; mean+/-SE) than in atherosclerotic monkeys (1.4+/-0.4 g; P:<0.05). In contrast, contraction to prostaglandin F(2alpha) was similar in the basilar artery from normal and atherosclerotic monkeys. These findings suggest that (1) relaxation of cerebral arteries in primates in response to nitric oxide is normally dependent, in large part, on activation of sGC and (2) the influence of sGC (via reduced production and/or activity of basal nitric oxide) on cerebral vascular tone is reduced in atherosclerosis.

Antioxidants prevent ethanol-induced contractions of canine cerebral vascular smooth muscle: relation to alcohol-induced brain injury

The present study was designed to test the hypothesis that α-tocopherol (Vit. E) and pyrrolidine dithiocarbamate (PDTC) might exert direct effects on alcohol-induced contractions of canine basilar cerebral arteries. After precontraction of arterial ring segments with ethanol, PDTC (10 −8–10 −6 M) and Vit. E (10 −6–10 −4 M) induced concentration-dependent relaxations of cerebral arteries, compared to untreated controls. The effective concentrations producing approximately 50% of the maximal relaxation responses (EC 50 values) were about 2.48±0.09 × 10 −7 M for PDTC, and 1.87±0.10 × 10 −5 mM for Vit. E, respectively. Preincubation of these arterial rings with EC 50’s of PDTC or Vit. E for 40 min attenuate markedly the contractions produced by alcohol, at concentrations of 1–400 mM. However, both PDTC and Vit.E do not relax equi-potent precontractions induced by either KCl or prostaglandin F 2α (PGF 2 α ) or inhibit their contractions. These data suggest that alcohol-induced contractions of cerebral arteries are mediated via excitation-contraction coupling pathways different from those used by KCl or receptor-mediated agonists such as PGF 2 α . The present results, when viewed in light of other recently published data, suggest that antioxidants may prove useful in the amelioration and treatment of alcohol-induced brain damage and strokes.

Adventitia-dependent relaxations of canine basilar arteries transduced with recombinant eNOS gene.

We recently reported that expression of recombinant endothelial nitric oxide (NO) synthase (eNOS) gene in adventitial fibroblasts restores NO formation in canine cerebral arteries without endothelium in response to bradykinin ex vivo and in vivo. The present study was designed to further characterize the stimuli that can activate recombinant eNOS enzyme expressed in the adventitia of cerebral arteries. To stimulate recombinant eNOS, we used serum (0. 1-10%), substance P (10(-11)-3 x 10(-9) M), and ANG II (10(-7)-10(-5) M) because they increase intracellular calcium concentrations in fibroblasts. Endothelium-denuded segments of canine basilar arteries were incubated with an adenoviral vector encoding beta-galactosidase gene or eNOS gene for 30 min at 37 degrees C. After 24 h, vasomotor activity and cGMP formation in eNOS or beta-galactosidase arteries were examined by isometric force recording and by radioimmunoassay, respectively. In control arteries and beta-galactosidase gene-transduced arteries, serum caused concentration-dependent contractions, whereas in recombinant eNOS gene-transduced arteries, serum produced concentration-dependent relaxations. Substance P and ANG II had no effect on vascular tone in control and beta-galactosidase arteries but caused concentration-dependent relaxations as well as a significant increase in cGMP levels in eNOS arteries. These relaxations were blocked by the NOS inhibitor NG-nitro-L-arginine methyl ester. Chemical treatment or mechanical inactivation of adventitial function significantly attenuated substance P-induced relaxations and ANG II-induced relaxations. These findings demonstrate that serum, substance P, and ANG II cause adventitia-dependent relaxations in cerebral arteries expressing the recombinant eNOS gene. This mechanism of vasodilatation may have beneficial effects in the prevention and treatment of vascular disorders characterized by the diminished bioavailability of NO, such as cerebral vasospasm.

Characterization of the relaxant action of urocortin, a new peptide related to corticotropin-releasing factor in the rat isolated basilar artery.

In addition to its well established neuroendocrine and neurotransmitter effects, corticotropin releasing factor (CRF) exerts a potent vasorelaxant action. Recently, a CRF-related peptide, urocortin, has been identified in the mammalian brain. In the present study, the cerebral vasomotor action of this peptide and the mechanism underlying its relaxant effect are characterized. Ring segments obtained from the rat basilar artery were used for measurement of isometric force. The relaxant action of urocortin, CRF and sauvagine was studied in segments with a functionally intact endothelium. In segments precontracted with prostaglandin F2alpha, urocortin, CRF and sauvagine induced concentration-related relaxation. The order of potency was as follows (pD2+/-s.e.m. given in brackets): urocortin (9.32+/-0.07) > sauvagine (9.08+/-0.08) > CRF (7.50+/-0.07). Complete relaxation was achieved with each agonist. Relaxation was not affected by removal of the endothelium but was markedly attenuated in segments precontracted with 50 mM K+ Krebs solution. The relaxant effect of urocortin was inhibited by astressin in an apparently competitive manner. A pA2 value of 7.52 was estimated for astressin. Inhibition of urocortin-induced relaxation was also observed in the presence of the adenylate cyclase inhibitor SQ22536 (pD2 in the presence of 300 microM SQ22536, 9.36+/-0.05) and the K+ channel blockers tetraethylammonium (10 mM; pD2, 8.65+/-0.07), iberiotoxin (100 nM; pD2, 8.88+/-0.08) and apamin (10 nM; pD2, 8.94+/-0.07). However, the inhibitory actions of SQ22536 and apamin or iberiotoxin were not additive. The results suggest that urocortin induces relaxation of cerebral arteries by activating CRF-R2 receptors present in the vascular wall. Relaxation appears to be mediated by adenylate cyclase stimulation and activation of Ca2+-dependent K+ channels.

Subarachnoid hemorrhage and the role of potassium channels in relaxations of canine basilar artery to nitrovasodilators.

This study was designed to determine the effect of subarachnoid hemorrhage (SAH) on potassium (K+) channels involved in relaxations of cerebral arteries to nitrovasodilators. The effects of K+ channel inhibitors on relaxations to 3-morpholinosydnonimine (SIN-1) and sodium nitroprusside (SNP) were studied in rings of basilar arteries obtained from untreated dogs and dogs exposed to SAH. The levels of cyclic GMP were measured by radioimmunoassay. In rings without endothelium, concentration-dependent relaxations to SIN-1 (10(-9)-10(-4) mol/L) and SNP (10(-9)-10(-4) mol/L) were not affected by SAH, whereas increase in cyclic GMP production stimulated by SIN-1 (10(-6) mol/L) was significantly suppressed after SAH. The relaxations to SIN-1 and SNP were reduced by charybdotoxin (CTX: 10(-7) mol/L), a selective Ca(2+)-activated K+ channel inhibitor, in both normal and SAH arteries; however, the reduction of relaxations by CTX was significantly greater in SAH arteries. By contrast, the relaxations to these nitrovasodilators were not affected by glyburide (10(-5) mol/L), an ATP-sensitive K+ channel inhibitor, in both normal and SAH arteries. These findings suggest that in cerebral arteries exposed to SAH, CA(2+)-activated K+ channels may play a compensatory role in mediation of relaxations to nitric oxide. This may help to explain mechanisms of relaxations to nitrovasodilators in arteries with impaired production of cyclic GMP.

The effect of 1H-[1,2,4]oxadiazolo[4,3- a]quinoxalin-1-one (ODQ) and charybdotoxin (CTX) on relaxations of isolated cerebral arteries to nitric oxide

The mechanism underlying smooth muscle relaxations of cerebral arteries in response to nitric oxide is still not completely understood. The present study was designed to determine the role of soluble guanylate cyclase in the relaxations to a nitric oxide/nucleophile complex, diethylaminodiazen-1-ium-1,2-dioate (DEA-NONOate). Rings of canine middle cerebral arteries without endothelium were suspended in Krebs–Ringer bicarbonate solution for isometric tension recording. The levels of guanosine 3′,5′-cyclic monophosphate (cyclic GMP) were measured by radioimmunoassay technique. During contractions to uridine 5′-triphosphate (UTP), DEA-NONOate (10 −10 to 10 −5 M) caused concentration-dependent relaxations. Measurements of cyclic GMP levels in cerebral arterial wall demonstrated that DEA-NONOate is a potent stimulator of guanylate cyclase and subsequent formation of cyclic GMP. Increasing concentrations of a selective soluble guanylate cyclase inhibitor, 1H-[1,2,4]-oxadiazolo[4,3- a]quinoxalin-1-one (ODQ), caused concentration-dependent reduction of both cyclic GMP production and relaxations to DEA-NONOate. Interestingly, in the presence of the highest concentration (3×10 −6 M) of ODQ, production of cyclic GMP in response to 10 −6 M of DEA-NONOate was abolished, whereas the same concentration of DEA-NONOate caused almost complete relaxation, suggesting that mechanisms independent of cyclic GMP production may mediate relaxing effect of high concentration of a nitric oxide donor. A selective Ca 2+-activated potassium channel blocker charybdotoxin (CTX) significantly reduced relaxations to DEA-NONOate resistant to ODQ, supporting the idea that in cerebral arteries nitric oxide may activate potassium channels independently of cyclic GMP. The results of our study suggest that under physiological conditions, guanylate cyclase is a key mediator of cerebral arterial relaxations to nitric oxide. However, under pathological conditions associated with induction of nitric oxide synthase and increased biosynthesis of nitric oxide (e.g., cerebral ischemia, inflammation, sepsis), mechanisms other than formation of cyclic GMP may be activated.

Role of potassium channels in relaxations of canine middle cerebral arteries induced by nitric oxide donors.

The mechanisms underlying smooth muscle relaxations of cerebral arteries in response to nitric oxide (NO) and cyclic GMP (cGMP) are still not completely understood. The present study was designed to determine the role of potassium channels in the relaxations to NO donors 3-morpholinosydnonimine (SIN-1) and sodium nitroprusside (SNP), as well as 8-bromo-3',5' -cGMP (a synthetic analogue of cGMP) and zaprinast (a selective cGMP phosphodiesterase inhibitor). Rings of canine middle cerebral asteries without endothelium were suspended in Krebs-Ringer bicarbonate solution for isometric tension recording. The levels of cGMP were measured by radioimmunoassay. Relaxations to NO donors 8-bromo-cGMP and zaprinast were studied in the presence and in the absence of K+ channel blockers charybdotoxin (large-conductance Ca(2+)-activated K+ channels), glyburide (ATP-sensitive K+ channels), 4-aminopyridine (delayed rectifier K+ channels), and BaCl2 (multiple types of K+ channels). Concentration-dependent relaxations caused by NO donors (SIN-1 and SNP) were significantly reduced in arteries treated with BaCl2 (3 x 10(-4) mol/L) or charybdotoxin (3 x 10(-8) mol/L). Relaxations to 8-bromo-cGMP were not affected by the same concentrations of BaCl2 and charybdotoxin; however, they were reduced by higher concentrations of BaCl2 (3 x 10(-3) mol/L) or charybdotoxin (10(-7) mol/L). Zaprinast-induced relaxations were significantly reduced by BaCl2 (3 x 10(-4) mol/L) or charybdotoxin (3 x 10(-8) mol/L). Glyburide (10(-5) mol/L) and 4-aminopyridine (10(-3) mol/L) did not alter the relaxations to SIN-1 or SNP. The production of cGMP stimulated by SIN-1 in the vascular smooth muscle was not affected by BaCl2 (3 x 10(-3) mol/L) or charybdotoxin (10(-7) mol/L). These results indicate that in canine middle cerebral arteries, a significant portion of relaxations to NO liberated from nitrovasodilators is mediated by large-conductance Ca(2+)-activated K+ channels. Other K+ channels, sensitive to BaCl2, may also be involved in the mechanism of relaxations induced by NO.

Characterization of the potassium channels involved in EDHF-mediated relaxation in cerebral arteries.

1. In the presence of NG-nitro-L-arginine (L-NOARG, 0.3 mM) and indomethacin (10 microM), the relaxations induced by acetylcholine and the calcium (Ca) ionophore A23187 are considered to be mediated by endothelium-derived hyperpolarizing factor (EDHF) in the guinea-pig basilar artery. 2. Inhibitors of adenosine 5'-triphosphate (ATP)-sensitive potassium (K)-channels (KATP; glibenclamide, 10 microM), voltage-sensitive K-channels (Kv; dendrotoxin-1, 0.1 microM or 4-aminopyridine, 1 mM), small (SKCa; apamin, 0.1 microM) and large (BKCa; iberiotoxin, 0.1 microM) conductance Ca-sensitive K-channels did not affect the L-NOARG/indomethacin-resistant relaxation induced by acetylcholine. 3. Synthetic charybdotoxin (0.1 microM), an inhibitor of BKCa and Kv, caused a rightward shift of the concentration-response curve for acetylcholine and reduced the maximal relaxation in the presence of L-NOARG and indomethacin, whereas the relaxation induced by A23187 was not significantly inhibited. 4. A combination of charybdotoxin (0.1 microM) and apamin (0.1 microM) abolished the L-NOARG/ indomethacin-resistant relaxations induced by acetylcholine and A23187. However, the acetylcholine-induced relaxation was not affected by a combination of iberiotoxin (0.1 microM) and apamin (0.1 microM). 5. Ciclazindol (10 microM), an inhibitor of Kv in rat portal vein smooth muscle, inhibited the L-NOARG/ indomethacin-resistant relaxations induced by acetylcholine and A23187, and the relaxations were abolished when ciclazindol (10 microM) was combined with apamin (0.1 microM). 6. Human pial arteries from two out of four patients displayed an L-NOARG/indomethacin-resistant relaxation in response to substance P. This relaxation was abolished in both cases by pretreatment with the combination of charybdotoxin (0.1 microM) and apamin (0.1 microM), whereas each toxin had little effect alone. 7. The results suggest that Kv, but not KATP and BKCa, is involved in the EDHF-mediated relaxation in the guinea-pig basilar artery. The synergistic action of apamin and charybdotoxin (or ciclazindol) could indicate that both Kv and SKCa are activated by EDHF. However, a single type of K-channel, which may be structurally related to Kv and allosterically regulated by apamin, could also be the target for EDHF.

Role of potassium channels in relaxations of isolated canine basilar arteries to acidosis.

Concentration of hydrogen ions is an important regulator of cerebral arterial tone under physiological and pathological conditions. Previous studies demonstrated that in cerebral arteries, relaxations to hypercapnia are due to decrease in extracellular pH. The present study was designed to determine the role of potassium channels in mediation of cerebral arterial relaxations induced by extracellular acidosis. Rings of canine basilar arteries without endothelium were suspended for isometric force recording. Acidosis (pH 7.3 to 7.0) was produced by incremental addition of hydrochloric acid (1.0N). The concentration of hydrogen ions was continuously monitored with a pH meter. During contractions to UTP, acidosis (pH 7.3 to 7.0) induced pH-dependent relaxations. These relaxations were abolished in arteries contracted by potassium chloride (20 mmol/L). A nonselective potassium channel inhibitor, BaCl2 (10(-4) and 10(-4) mol/L), and an ATP-sensitive potassium channel inhibitor, glyburide (5 x 10(-6) mol/L), significantly reduced relaxations to acidosis. Furthermore, BaCl2 (10(-4) mol/L) and glyburide (5 x 10(-6) mol/L) abolished relaxations to an ATP-sensitive potassium channel opener, cromakalim (10(-8) to 3 x 10(-5) mol/L). However, these potassium channel inhibitors did not affect relaxations to a voltage-dependent calcium channel inhibitor, diltiazem (10(-8) to 10(-4) mol/L), and glyburide (5 x 10(-6) mol/L) did not alter relaxations to a nitric oxide donor, SIN-1 (10(-9) to 10(-4) mol/L). A calcium-activated potassium channel inhibitor, charybdotoxin (10(-7) mol/L), and a delayed rectifier potassium channel inhibitor, 4-aminopyridine (10(-3) mol/L), did not affect relaxations to acidosis. These results suggest that extracellular acidosis causes relaxations of cerebral arteries in part by activation of potassium channels. ATP-sensitive potassium channels appear to contribute to acidosis-induced decrease in cerebral arterial tone.

Potassium channels and the cerebral circulation.

1. Hyperpolarization of vascular muscle in response to activation of potassium channels is a major mechanism of vasodilatation. 2. In cerebral blood vessels, two potassium channels have received considerable study recently: ATP-sensitive and calcium-dependent potassium channels. Activation of these potassium channels appears to play a major role in the relaxation of cerebral arteries and arterioles in response to diverse stimuli, including receptor-mediated agonists, intracellular second messengers, reactive oxygen species and hypoxia. 3. The functional influence of ATP-sensitive and calcium-dependent potassium channels may be altered in disease states, including hypertension, diabetes and subarachnoid haemorrhage.

Effects of losartan on cerebral arteries in stroke-prone spontaneously hypertensive rats.

To investigate in young salt-loaded stroke-prone spontaneously hypertensive rats (SHR-SP) the effects of a long-term administration of the angiotensin II AT1 receptor antagonist losartan [1 mg/kg (L1) and 10 mg/kg (L10) per day, from 5 to 20 weeks of age] on the structural and functional characteristics of the middle cerebral artery. Morphological measurements and isometric tension recordings (myograph, contractile responses to potassium chloride and serotonin, relaxant responses to bradykinin and sodium nitroprusside) were performed on isolated vessels from randomly selected control and losartan-treated SHR-SP and age-matched Wistar-Kyoto (WKY) rats killed at ages 6-7, 10-11 and 16-17 weeks. Whereas all control SHR-SP had died within 18 weeks of being born, losartan at both doses afforded full protection against stroke and mortality. Losartan limited malignant hypertension development dose-dependently. Age-related increases in cerebral arterial wall thickness and wall:lumen ratio were not affected (L1) or limited slightly (L10) by losartan. In control SHR-SP, contractile responses of cerebral arteries to agonists decreased with ageing and stroke occurrence and were significantly smaller than those of age-matched WKY rat arteries. Losartan limited the cerebrovascular contractility impairment dose-dependently in SHR-SP but did not affect the WKY rat cerebral artery contractility. In addition, losartan limited the age-related alteration of the endothelium-dependent relaxation of cerebral arteries observed in control SHR-SP dose-dependently. In SHR-SP, losartan prevented stroke and improved the cerebral artery's smooth muscle and endothelial cell functions, which are altered during ageing and impaired even more dramatically by stroke occurrence.

Role of potassium channels in cerebral blood vessels.

Hyperpolarization of vascular muscle in response to activation of potassium channels is a major mechanism of vasodilatation. In cerebral blood vessels, four different potassium channels have been described: ATP-sensitive potassium channels, calcium-activated potassium channels, delayed rectifier potassium channels, and inward rectifier potassium channels. Activation of ATP-sensitive and calcium activated potassium channels appears to play a major role in relaxation of cerebral arteries and arterioles in response to diverse stimuli, including receptor-mediated agonists, intracellular second messengers, and hypoxia. Both calcium-activated and delayed rectifier potassium channels may contribute to a negative feedback system that regulates tone in large cerebral arteries. The influence of ATP-sensitive and calcium-activated potassium channels is altered in disease states such as hypertension, diabetes, and atherosclerosis. Activation of potassium channels is a major mechanism of cerebral vasodilatation. Alteration of activity of potassium channels and impairment of vasodilatation may contribute to the development or maintenance of cerebral ischemia or vasospasm.

Endothelin-3-induced relaxation of isolated rat basilar artery is mediated by an endothelial ETB-type endothelin receptor.

The endothelin (ET) receptor mediating relaxation of cerebral arteries was characterized using ring segments obtained from the rat basilar artery. Under resting tension, ET-3 (> 10(-8) M) but not the specific ETB receptor agonist IRL 1620 induced contraction. In ring segments precontracted with 3 x 10(-6) M prostaglandin (PG) F2 alpha, ET-3 (10(-12) - 10(-8) M) and IRL 1620 (10(-14) - 10(-6) M) induced concentration-related relaxation. IRL 1620 was more potent than ET-3, the pD2 (-log10EC50) values being 10.002 +/- 0.751 (mean +/- SD) for IRL 1620 and 8.836 +/- 0.415 for ET-3. Relaxation was abolished after preincubation with the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine (10(-5) M) as well as in segments devoid of a functionally intact endothelium. At a concentration above 10(-8) M, ET-3 resulted in a further increase of PGF2 alpha-induced contraction that was not observed with IRL 1620. The presumably specific ETB receptor antagonist IRL 1038 (10(-7) - 3 x 10(-6) M) diminished or even abolished (3 x 10(-6) M) the relaxation induced by ET-3 or IRL 1620. IRL 1038 did not exert any vasomotor effect by itself, and it did not significantly affect ET-3-induced contraction. These results indicate that in the rat isolated basilar artery, the ET-3-induced relaxation is probably due to activation of an ETB-type receptor located on the endothelial cells and mediated by release of nitric oxide.

Effects of isocarbacyclin, a stable prostacyclin analogue, on monkey isolated cerebral and peripheral arteries.

1. The effects of isocarbacyclin (TEI 7165), a stable prostacyclin analogue, were examined in monkey isolated cerebral and peripheral arteries. 2. Addition of TEI 7165 (0.1 nM-10 microM) produced a dose-dependent relaxation in cerebral arteries pre-contracted with 1 microM 5-hydroxytryptamine (5-HT). High concentrations (more than 1 microM) of TEI 7165 elicited a transient contraction followed by a sustained relaxation. 3. TEI 7165 also elicited a dose-dependent relaxation in the peripheral (except popliteal) arteries. The maximum relaxation induced by 10 microM TEI 7165 was greater (P < 0.05) in the mesenteric artery than in the cerebral artery. The negative logarithm of the EC50 value for the mesenteric, 7.6 +/- 0.3, was greater (P < 0.05) than that for the cerebral artery, 6.4 +/- 0.3. The decreasing order of potency for the TEI 7165-induced relaxation was as follows: mesenteric > renal > cerebral > coronary > popliteal. 4. Removal of the endothelium did not significantly affect TEI 7165-induced relaxations. 5. The transient contraction produced by high concentrations of TEI 7165 was not observed in cerebral arteries precontracted with 1 nM U46619, a stable analogue of thromboxane A2 (TXA2). Furthermore, the TEI 7165-induced contraction was markedly suppressed (P < 0.05) by treatment with 10 nM S1452, a TXA2 blocking agent. 6. These results suggest that TEI 7165 causes an endothelium-independent relaxation in monkey cerebral and peripheral arteries, and that there is a marked regional difference in the TEI 7165-induced relaxations. A high concentration of TEI 7165 also produces a transient contraction which is probably through activation of TXA2 (TP-) receptors.

Different Effects of Mg2+ on Endothelin-1− and 5-Hydroxytryptamine-Elicited Responses in Goat Cerebrovascular Bed

Mg2+ influences the response of cerebral arteries to several agonists, but until now its effects on endothelin-1 (ET-1) had not been studied. We recorded and compared the responses of goat cerebrovascular bed to ET-1 and 5-hydroxytryptamine (5-HT) during various Mg2+ treatments. We performed experiments in vitro by recording isometric tension in isolated goat middle cerebral arteries and in vivo by recording cerebral blood flow (CBF) and other physiologic parameters in conscious goats. Cumulative addition of ET-1 (10(-11)-3 x 10(-8) M) and 5-HT (10(-9) -10(-5) M) contracted cerebral arteries concentration dependently in bath media containing 0 (Mg(2+)-free medium), 1 (control), and 10 mM Mg2+, but the influence of Mg2+ was different: Mg2+ deprivation increased sensitivity (EC50) and Mg2+ overload reduced contractility (Emax) of cerebral arteries to 5-HT, whereas the ET-1 response did not change in these conditions. Cumulative addition of Mg2+ (10(-4)-3 x 10(-2) M) at the active tone induced by ET-1 (10(-9) M) and 5-HT (10(-5) M) elicited concentration-dependent relaxations of cerebral arteries, but the relaxant response was lower at the ET-1 precontraction. Infusions of ET-1 (0.1 nmol/min) and 5-HT (10 micrograms/min) directly into the cerebroarterial supply of the unanesthetized goats elicited a sustained decrease in CBF and an increase in cerebral vascular resistance. Magnesium sulfate, administered as increasing doses (10-300 mg) in the same way increased CBF and decreased cerebral vascular resistance, although this effect was less on ET-1-induced than on 5-HT-induced cerebral vasoconstriction.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by calmodulin antagonists of the neurogenic relaxation in cerebral arteries

The present study was aimed to determine the effect of calmodulin inhibitors on the relaxant response of isolated dog and monkey cerebral arteries to vasodilator nerve stimulation, which is hypothesized to be mediated by nitric oxide (NO) from nerve endings. The relaxations caused by nerve stimulation by electrical pulses in endothelium-denuded arteries were attenuated by treatment with calmidazolium and W-7 ( N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide hydrochloride) and were abolished by N G - nitro- l - arginine , an inhibitor of nitric oxide synthase, and tetrodotoxin. The calmodulin inhibitors also attenuated the relaxations caused by nicotine and substance P, which were endothelium-independent and -dependent, respectively, but did not influence the relexant response to NO. It is concluded that calmodulin is required for activation of the NO synthase present in the vasodilator nerve as well as that in the endothelium.

Comparison of endothelium‐dependent responses of monkey cerebral and temporal arteries

1. Endothelium-dependency of vasodilator responses was compared in helical strips of monkey cerebral and superficial temporal arteries contracted with prostaglandin F2 alpha. Acetylcholine produced an endothelium-dependent relaxation in the temporal arteries, but did not consistently alter the tone of cerebral arteries. 2. Adenosine 5'-triphosphate (ATP) produced a transient contraction followed by a relaxation in the temporal and cerebral arteries; removal of the endothelium partially attenuated the relaxation of the cerebral arteries and markedly suppressed the relaxation in the temporal arteries. The dependency of adenosine 5'-diphosphate (ADP)-induced relaxations on the endothelium was also greater in temporal arteries than in cerebral arteries. 3. Histamine-induced relaxations in the temporal arteries were independent of the endothelium and were reversed to contractions by cimetidine. Cerebral arterial relaxations induced by histamine were partly dependent on the endothelium. Relaxations caused by substance P were reversed to contractions by removal of the endothelium in the temporal arteries, whereas the peptide did not consistently alter the tone of cerebral arteries. 4. The Ca2+ ionophore, A23187, relaxed the temporal and cerebral arteries to a similar extent; removal of the endothelium abolished these relaxations. Glyceryl trinitrate elicited similar relaxation of cerebral and temporal arteries, and these were independent of the endothelium. 5. These findings clearly indicate heterogeneity in the endothelium-dependency of several vasodilator responses in monkey intra- and extracranial arteries, although the ability of these arteries to respond to A23187 and glyceryl trinitrate does not appear to differ. The heterogeneous responses observed so far could therefore be due to different distributions of receptors or to variation in receptor-effector coupling in endothelial cells.

Influence of the Endothelium on Histamine-Induced Relaxation of Rat Middle Cerebral Arteries in Vitro

Histamine relaxed PGF2 alpha-precontracted rat isolated middle cerebral arteries (ID approximately 230 microns) concentration dependently with a pD2 of 5.31 (EC50: 5 x 10(-6) M). Cimetidine induced a concentration-dependent rightward shift of the histamine concentration-response curve of endothelium-intact arteries. The slope of the Schild plot was indistinguishable from unity, and the estimated pA2 for cimetidine was 6.14. The selective H2-receptor agonist dimaprit induced a concentration dependent relaxation of the cerebral arteries similar to that induced by histamine. This indicates that the histamine receptor mediating the relaxation in rat middle cerebral arteries belongs to the H2-receptor subtype. 2-Pyridylethylamine, a selective H1-receptor agonist, induced a small concentration-dependent contraction of the arteries with a pD2 of 4.16. Mepyramine, a selective H1-receptor antagonist had no potentiating effect on the relaxation induced by histamine, suggesting either that the contractile effect of 2-pyridylethylamine is nonselective or that H1 receptors mediating contraction are of minor importance for the overall histamine response. The selective H3-receptor agonist, (R)-alpha-methylhistamine, was without effect in a specific concentration range (10(-7)-10(-5) M) excluding participation of H3 receptors in the histamine-induced relaxation of these vessels. Indomethacin did not affect the vessel response to histamine, but removal of the endothelium and treatment of endothelium-intact arteries with 3 x 10(-6) M methylene blue induced a similar 0.5 log rightward shift of the histamine concentration-response curve.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of endothelial cells in the relaxation of cerebral arteries and arachidonic acid metabolism.

Vascular responses to acetylcholine (ACh) and calcium ionophore A23187 were characterized in the rings of canine basilar artery. In preparations precontracted to a stable plateau by 3×10-7 M 5-hydroxytryptamine or 3×10-6 M prostaglandin F2α, A23187 (10-9-10-7 M) elicited significant relaxation of the basilar rings when the endothelium was intact. The ability of a ring to relax in this manner depends upon the presence of the endothelium, according to the evidence obtained by scanning electron microscopy, and the same concentration of A23187 did not relax vascular tissues in which the endothelium was artificially disrupted. On the other hand, ACh (10-7-10-3 M) produced negligible relaxation of canine basilar artery with intact endothelium. Indomethacin pretreatment (1.5×10-5 M) did not affect the A23187-induced relaxation, whereas preincubation with the lipoxygenase inhibitors nordihydroguaiaretic acid (1.5×10-5 M) and AA861 (10-5 M) prevented its relaxant effect. These results suggest that endothelium-dependent relaxation of canine basilar artery by A23187 may be mediated by one or more noncyclooxygenase metabolites.