Endothelium-dependent relaxation induced by angiotensin II and histamine in isolated arteries of dog.

In helical strips of dog renal and mesenteric arteries pre-contracted with prostaglandin F2 alpha (PGF2 alpha), endothelium-dependent relaxations were investigated. Removal of the endothelium was shown histologically by staining with silver nitrate and functionally by testing the inability of acetylcholine to induce arterial relaxations. When the endothelium was removed, relaxation of renal arteries to angiotensin (Ang) II was markedly suppressed, whereas relaxations induced by PGI2 or isoprenaline were attenuated only slightly. Removal of the endothelium attenuated the relaxant response of mesenteric arteries to histamine but did not significantly alter the response to PGI2. Treatment with indomethacin caused an additional attenuation of the relaxant response to histamine or a reversal of the Ang II-induced relaxation to a contraction in the arterial strips, from which the endothelium had been removed. Relaxation of renal arteries induced by Ang II and of mesenteric arteries induced by histamine is postulated to result from PGI2 released from the arterial wall. Therefore, it appears that the endothelium is a major site but not the only site responsible for drug-induced release of PGI2.     

Endothelium-dependent and -independent responses to protease-activated receptor-2 (PAR-2) activation in mouse isolated renal arteries

Protease-activated receptors (PARs) are receptors which require proteolytic cleavage to be self-activated by newly exposed N-terminal `tethered ligands'', and hence serve as sensors for protelytic enzymes. While both the thrombin receptor (PAR-1) and PAR-2 (activated by tryptic enzymes) have been shown to mediate endothelium-dependent vasorelaxation, only PAR-1 has been shown to cause direct vascular smooth muscle contraction. In this study, we report that trypsin and the PAR-2 selective peptide ligand SLIGRL-NH₂ not only caused endothelium-dependent relaxation of mouse renal arteries but also direct smooth muscle contraction if endothelial nitric oxide synthase was inhibited or if the endothelium was removed.     

Peroxynitrite-induced relaxation in isolated canine cerebral arteries and mechanisms of action

The present study was undertaken to determine the vascular actions of peroxynitrite (ONOO(-)), the product of superoxide and nitric oxide (NO), in isolated canine cerebral arteries and to gain insight into its potential mechanisms of action. In the absence of any vasoactive agent, ONOO(-) (from 10(-7) to 10(-6) M) was able to reduce the basal tension. In prostaglandin F2alpha-precontracted canine basilar arterial rings, ONOO(-) elicited concentration-dependent relaxation at concentrations from 10(-8) to 10(-5) M. The effective concentrations producing approximately 50% maximal relaxation (EC(50)) to ONOO(-) were 4.06 x 10(-6) and 4.12 x 10(-6) M in intact and denuded rings, respectively (P > 0.05). No significant differences in relaxation responses were found in ring preparations with or without endothelium (P > 0.05). The presence of either 5 microM methylene blue (MB) or 5 microM 1H-[1,2,4]oxadiazolo-[4,3-alpha]quinoxalin-1-one (ODQ) significantly inhibited the relaxations induced by ONOO(-). Tetraethylammonium chloride (T-2265) significantly decreased the ONOO(-)-induced relaxations in a concentration-dependent manner. However, ONOO(-) had no effect on rings precontracted by high KCL (P > 0.05). Addition of low concentrations of calyculin A (50 nM) was able to abolish the ONOO(-)-induced relaxation. Furthermore, ONOO(-) significantly inhibited calcium-induced contractions of K(+)-depolarized canine cerebral rings in a concentration-related manner. Lastly, a variety of pharmacological agents and antagonists including L-NMMA, l-arginine, indomethacin, atropine, naloxone, diphenhydramine, cimetine, glibenclamide, haloperidol, etc., did not influence the relaxant effects of ONOO(-) on the rings. Our new results suggest that ONOO(-)-triggered relaxation, on canine cerebral arteries, is mediated by elevation of cyclic guanosine monophosphate (cGMP) levels, membrane hyperpolarization via K+ channel activation, activation of myosin light chain phosphatase activity, and interference with calcium movement and cellular membrane Ca(2+) entry.     

Endothelium-dependent relaxation in isolated renal arteries of diabetic rabbits

1 In this study, we have investigated the vasodilator response to acetylcholine under diabetes conditions in isolated renal arteries of rabbits. We have also examined the contribution of endothelium-derived nitric oxide (EDNO) and endothelium-derived hyperpolarizing factor (EDHF) to the endothelium-dependent relaxation caused by acetylcholine in the renal arteries of alloxan-induced diabetic rabbits. 2 Acetylcholine (10(-10) - 10(-4) M) produced cumulative concentration-response curve in the renal arteries of both control and diabetic rabbits. The EC50 values and maximal responses to acetylcholine were not significantly different relative to diabetic conditions. In order to isolate the EDHF component of acetylcholine-induced vasodilator response, L-nitro-methyl arginine ester (L-NAME, 10(-4) M) and indomethacin (10(-6) M) were added to the Krebs' solution throughout the experiment. Under these conditions, acetylcholine induced vasodilatation in the isolated renal arteries from both control and diabetic rabbits. The vasodilator response to acetylcholine was not affected under diabetic conditions. 3 Sodium nitroprusside (SNP)-induced relaxation was increased in the diabetic rabbits compared with the control animals. 4 Tetrabutyl ammonium (TBA, 0.5 mM) produced a significant reduction in acetylcholine-induced vasodilatation in both preparations from control and diabetic animals, consistent with involvement of K+ channels in mediating this response. Glibenclamide (1 microM) attenuated acetylcholine-induced vasodilatation in preparations from control animals only, while iberiotoxin (0.05 microM) significantly reduced the vasodilator response to acetylcholine in preparations from both control and diabetic animals. 5 The role of EDNO in mediating acetylcholine-induced vasodilatation was examined. The vascular preparations were incubated with 20 mM K(+)-Krebs' solution to inhibit the EDHF contribution to acetylcholine-induced vasodilatation. Under this condition, acetylcholine induced a vasodilator response in both preparations from control and diabetic rats. Pretreatment with L-NAME (10(-4) M) attenuated acetylcholine-induced vasodilatation in both preparations, indicating an nitric oxide-mediated vasodilator response. 6 Our results indicated that acetylcholine-induced vasodilatation in the isolated renal arteries of alloxan-induced diabetic rabbits was not affected under diabetic conditions. Acetylcholine-induced vasodilatation is mediated by two vasodilator components; namely, EDHF and EDNO. The contribution of EDHF and EDNO to acetylcholine-induced vasodilatation was not affected under diabetic conditions and there was no indication of endothelial dysfunction associated with diabetes. EDHF component was found to act mainly through high conductance Ca(2+)-activated K+ channels under normal and diabetic conditions, while the adenosine triphosphate-dependent K+ channels were involved in mediating acetylcholine vasodilator response in the control preparations only.     

Mechanisms of action of eperisone on isolated dog saphenous arteries and veins

Effects of eperisone, an antispasmodic in skeletal muscle, were investigated in helical strips of dog saphenous artery and vein. Eperisone relaxed saphenous arteries and veins previously contracted with norepinephrine, serotonin, acetylcholine, K+, or Ba2+; but in contrast, it produced contractions in the blood vessels contracted with prostaglandin (PG) F2 alpha. Treatment with eperisone attenuated the contractions induced by norepinephrine and serotonin in the arteries and those by clonidine and phenylephrine in the veins. Eperisone inhibited angiotensin II-induced relaxations, mediated possibly by endogenous PGI2, but did not alter relaxations caused by exogenous PGI2. Treatment with eperisone (10(-5) M) potentiated the contractile response to electrical stimulation of adrenergic nerves; the potentiating effect was suppressed by yohimbine. The eperisone-induced contraction in PGF2 alpha-contracted arteries was inhibited by treatment with indomethacin or aspirin, although cyclooxygenase activity was not inhibited by eperisone. These results may indicate that eperisone blocks postjunctional alpha 1- and alpha 2-adrenergic, muscarinic, serotonergic receptors and prejunctional alpha 2 adrenoceptors and reduces PGI2 synthesis via a mechanism other than cyclooxygenase inhibition.     

Endothelium-dependent vasomotor effects of telmisartan in isolated rat femoral arteries

AT₁ receptor antagonists (ARBs) are drugs widely used for preventing and/or treating major cardiovascular diseases. Some of these drugs also show AT₁ receptor-independent effects that may have patho-physiological significance, such as Peroxisome Proliferator-Activated Receptors gamma (PPARγ) stimulation. Here we investigated the effect of telmisartan (that also stimulates PPARγ) on vasomotor responses of femoral arteries isolated from rat, in comparison to losartan. Femoral artery segments were mounted in a wire myograph and challenged with cumulative concentrations of phenylephrine (PE) and acetylcholine (ACh) after 30-min incubation in the absence or presence of 30 μM telmisartan or 30 μM losartan. Vasomotor responses were not significantly changed by losartan, whereas telmisartan reduced vasoconstriction to PE and increased vasodilatation to ACh. Incubation with 0.1 mM N^G-nitro-l-arginine abolished relaxation to ACh in untreated controls as well as in losartan-treated preparations, but did not in telmisartan-treated preparations (were 20% relaxation subsisted); this residual relaxing effect was abolished by indomethacin and by endothelium removal. Incubation with 30 μM GW9662 (PPARγ antagonist), 10 μM PD123319 (AT₂ antagonist) or 30 μM A779 (angiotensin(1-7)/Mas antagonist) did not change the effect of telmisartan on vasomotor responses in preparations with intact endothelium. We conclude that telmisartan modifies constriction and dilatation of isolated arteries in an endothelium-dependent manner, involving both nitric oxide and prostanoid production. The present effect of telmisartan, however, does not seem to involve PPARγ, AT₂ or angiotensin(1-7)/Mas.     

Regional differences in the response to vasoconstrictor agents of dog and monkey isolated coronary arteries.

Contractile responses to vasoconstrictor agents were compared in helical strips of dog and monkey epicardial coronary arteries of different sizes. Contractions of large, medium and small arteries induced by KCl ( 30mM ) were virtually identical. Contractions induced by 5-hydroxytryptamine (5-HT) (10(-9) - 2 X 10(-6) M) were in the order of large greater than medium greater than small arteries in dogs, and large = medium greater than small arteries in monkeys. Cinanserin suppressed these responses. In contrast, contractions produced by angiotensin II (AII) (10(-7) M) were in the order of small greater than medium greater than large arteries in dogs, and small greater than medium = large arteries in monkeys. Sar1-Ala8-angiotensin II markedly attenuated the peptide-induced contractions. Contractions induced by prostaglandin F2 alpha (PGF2 alpha) were significantly greater in large and medium sized arteries than in small arteries in dogs, while those of the arteries of different sizes isolated from monkeys did not differ. Contractions induced by carbocyclic thromboxane A2 (c-TXA2), in arteries of different sizes in dogs and monkeys, did not differ. These results suggest that the sensitivity and/or the population of 5-HT receptors are greater in proximal coronary arteries than in distal arteries, while, in contrast, the sensitivity and/or population of AII receptors are greater in distal coronary arteries. Receptors for PGF2 alpha and c-TXA2 appear to react to a similar degree in monkey arteries of different sizes, although receptors for PGF2 alpha appear to be fewer in distal coronary arteries in dogs.     

Pharmacological features of vascular responses of isolated dog and monkey lingual arteries to vasoactive substances.

Using a perfusion technique of isolated vessels, vasoconstrictor responses to alpha-adrenoceptor agonists (norepinephrine [NE], phenylephrine [PE], clonidine, xylazine and tyramine) and KCl were investigated in isolated, perfused dog and monkey lingual arteries. A stainless steel cannula was inserted into the lingual artery segment and perfused with Krebs-Henseleit solution at a constant flow rate. In dog lingual arteries, the agonists induced vasoconstrictions with the following order of potency: NE greater than PE greater than tyramine much greater than clonidine greater than xylazine greater than KCl. In monkey preparations, the order was NE greater than PE much greater than clonidine greater than or equal to tyramine greater than xylazine greater than KCl. In both preparations, NE- and PE-induced constrictions were blocked by bunazosin (an alpha-1 adrenoceptor antagonist), but not influenced by midaglizole (a potent alpha-2 antagonist). Diltiazem (a Ca entry blocker) significantly attenuated NE-induced vasoconstrictions in dog lingual arteries, but did not significantly influence these in monkey preparations. These results suggest that: [1] these arteries contain mostly alpha-1 but scarcely any alpha-2 adrenoceptors; [2] in dog preparations, tyramine induced a marked vasoconstriction which may contribute to investigation on the mechanisms of catecholamine releases from sympathetic nerve terminals; and [3] different blocking effects of diltiazem may indicate that extracellular Ca++ influx may have varying degrees of importance in alpha-1 adrenoreceptor-mediated constrictions in different species, although participation of an intracellular mechanism might not be ruled out.     

Endothelium-dependent relaxation of human epicardial coronary arteries: frequent lack of effect of acetylcholine

Strip preparations of human epicardial coronary arteries (free of atherosclerosis) relaxed in an endothelium-dependent fashion to substance P and Ca2+-ionophore A23187. Acetylcholine generally caused contraction in the same strips. Glyceryl trinitrate and isoproterenol induced relaxation irrespective of the presence or absence of endothelium. Nordihydroguaiaretic acid abolished the relaxation produced by substance P and A23187. Mepacrine only blocked substance P relaxation. Haemoglobin and methylene blue inhibited substance P and A23187 relaxations but also reduced the response to glyceryl trinitrate. These inhibitor experiments indicate that human coronary arteries are relaxed by the endothelium-derived relaxing factor.     

Vasorelaxation to N-oleoylethanolamine in rat isolated arteries: mechanisms of action and modulation via cyclooxygenase activity

Background and purpose:

The endocannabinoid-like molecule N-oleoylethanolamine (OEA) is found in the small intestine and regulates food intake and promotes weight loss. The principal aim of the present study was to evaluate the vascular effects of OEA.

Experimental approach:

Perfused isolated mesenteric arterial beds were pre-contracted with methoxamine or high potassium buffers and concentration-response curves to OEA were constructed. Combinations of inhibitors to block nitric oxide production, sensory nerve activity, cyclooxygenase activity, potassium channels, chloride channels and gap junctions, and a cannabinoid CB₁ receptor antagonist, were used during these experiments. The effects of OEA on caffeine-induced contractions in calcium-free buffer were also assessed. Isolated thoracic aortic rings were used as a comparison.

Key results:

OEA caused concentration-dependent vasorelaxation in rat isolated mesenteric arterial beds and thoracic aortic rings, with a greater maximal response in mesenteric vessels. This relaxation was sensitive to inhibition of sensory nerve activity and endothelial removal in both preparations. The cyclooxygenase inhibitor indomethacin reversed the effects of capsaicin pre-treatment in perfused mesenteric arterial beds and indomethacin alone enhanced vasorelaxation to OEA. The OEA-induced vasorelaxation was inhibited by a CB₁ receptor antagonist only in aortic rings. In mesenteric arteries, OEA suppressed caffeine-induced contractions in calcium-free buffer.

Conclusions and implications:

The vasorelaxant effects of OEA are partly dependent on sensory nerve activity and a functional endothelium in the vasculature. In addition, vasorelaxation to OEA is enhanced following cyclooxygenase inhibition. OEA may also interfere with the release of intracellular calcium in arterial preparations.This article is part of a themed issue on Cannabinoids. To view the editorial for this themed issue visit http://dx.doi.org/10.1111/j.1476-5381.2010.00831.x     

Intra- and extraluminally-applied acetylcholine on the vascular tone or the response to transmural stimulation in dog isolated mesenteric arteries

Summary Acetylcholine applied extraluminally to isolated, perfused dog mesenteric artery segments produced an endothelium-dependent depressor response when the perfusion pressure was raised by continuous infusion of noradrenaline; the potency was 1/30 to 1/60 that of intraluminal acetylcholine. Contractions induced by transmural electrical stimulation were attenuated by treatment with intra- and extraluminal acetylcholine; the inhibitory effect of intraluminal acetylcholine was greater than that of extraluminal acetylcholine. Removal of endothelium did not significantly alter the inhibitory effect. In mesenteric artery strips with endothelium, treatment with oxyhaemoglobin suppressed the relaxant response to acetylcholine but did not influence the inhibitory effect of acetylcholine on stimulation-evoked contractions. Acetylcholine reduced the ³H-overflow and contraction of superfused mesenteric artery strips, preloaded with ³H-noradrenaline, response to transmural stimulation. By the use of bioassay (dog femoral artery segment with endothelium/coronary artery strip without endothelium), the release of EDRF was first determined in the perfusate, which was introduced to dog mesenteric artery strips loaded with ³H-noradrenaline. The ³H-overflow and contraction caused by the stimulation were not attenuated by EDRF and were also observed following treatment with superoxide dismutase. Inability of the perfusate to reduce the stimulation-evoked ³H-overflow was also observed when the donor and assay tissues were treated with superoxide dismutase. It may be concluded that the inhibition by acetylcholine of the release of neuronal noradrenaline is not dependent on endothelium, Extraluminally applied acetylcholine would reach the endothelium and release EDRF, and intraluminal acetylcholine is presumed to act directly on prejunctional muscarinic receptors; however, acetylcholine appears to cross the medial layer more efficiently from intima to adventitia than in the reverse direction. Send offprint requests to N. Toda at the above address     

Responses of isolated dog coronary arteries to tyramine

In isolated dog coronary arteries contracted with prostaglandin F2 alpha, tyramine in concentrations of 10(-6) and 5 x 10(-6) M caused relaxations, but it produced contractions at 2 x 10(-5) M or higher. The relaxant response to tyramine was attenuated, but the contractile response was enhanced at the second trial as compared with the responses at the first. Relaxations induced by low concentrations of tyramine were reversed to contractions by treatment with propranolol (10(-6) M) or sotalol (10(-5) M), and were abolished by cocaine (3 x 10(-6) M) or bretylium (2 x 10(-5) M). In coronary arteries isolated from reserpine (0.5 mg/kg)-pretreated dogs, tyramine produced only a contraction. Under resting conditions, contractions induced by tyramine (5 x 10(-6) to 2 x 10(-3) M) were potentiated by cocaine and propranolol, and were inhibited by phentolamine. Norepinephrine produced a dose-dependent relaxation in the arteries contracted with prostaglandin F2 alpha. In the presence of propranolol, the arteries under resting conditions were contracted by norepinephrine, the contraction being suppressed by treatment with phentolamine. It may be concluded that relaxations of dog coronary arteries induced by tyramine are mediated by liberation of norepinephrine from adrenergic nerves which stimulates beta-adrenoceptors in the smooth muscle. It seems likely that the tyramine (2 x 10(-5) M or higher)-induced contraction is not mediated by norepinephrine released, but it is partly due to a direct action on alpha-adrenoceptors.     

Two possible mechanisms underlying nitrate tolerance in monkey coronary arteries

1. Previous studies using isolated arteries have demonstrated cross-tolerance between nitric oxide (NO) donors such as nitroglycerin (NTG) and sodium nitroprusside (SNP). However, it remains unclear whether the vasorelaxing effect of atrial natriuretic peptide (ANP), an activator of particulate guanylate cyclase, is affected by treatment with NO donors. To investigate the cross-tolerance and interactions between NTG and ANP in coronary vasorelaxant responses, we used two models of monkey coronary arterial strips (Macaca fuscata). 2. In one model, which was induced by a 1 h treatment with 4.4 x 10(-4) mol/L NTG followed by washout of the agent for 1 h, the vasorelaxing effects of subsequent NTG were markedly attenuated, whereas those of ANP and NO were not affected. These findings suggest that the development of NTG tolerance is associated with a biotransformation process from NTG to NO. In the other model, which did not include washout after exposure to 3 x 10(-6) mol/L NTG, the vasorelaxant responses to 10(-8) mol/L ANP (31.1+/-5.4 vs 5.1+/-2.1%, respectively; P < 0.001), 10(-6) mol/L NO (61.5+/-2.4 vs 29.5+/-8.5%, respectively; P < 0.001) and 10(-8) mol/L SNP (49.4+/-6.4 vs 8.0+/-2.0%, respectively; P < 0.001) were significantly attenuated. The concentration- response curve for 8-bromo-cGMP (8-Br-cGMP) was shifted to the right, whereas responses to papaverine and forskolin were unchanged. These findings suggest that an intracellular process that occurs after the synthesis of cGMP is responsible for this interaction. 3. As a mechanism of NTG tolerance, two possible processes may be impaired: (i) biotransformation from NTG to NO; and (ii) an intracellular process that occurs after the synthesis of cGMP.     

Endothelium-dependent vasorelaxation independent of nitric oxide and K(+) release in isolated renal arteries of rats

1. We investigated whether K(+) can act as an endothelium-derived hyperpolarizing factor (EDHF) in isolated small renal arteries of Wistar-Kyoto rats. 2. Acetylcholine (0.001 - 3 microM) caused relaxations that were abolished by removal of the endothelium. However, acetylcholine-induced relaxations were not affected by the nitric oxide (NO) synthase inhibitor N:(omega)-nitro-L-arginine methyl ester (L-NAME, 100 microM), by L-NAME plus the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 1 microM) or by L-NAME plus the cyclo-oxygenase inhibitor indomethacin (10 microM). In rings precontracted with high-K(+)(60 mM) physiological salt solution in the presence of L-NAME, acetylcholine-induced relaxations were abolished. 3. L-NAME-resistant relaxations were abolished by the large-conductance Ca(2+)-activated K(+) channel inhibitor charybdotoxin plus the small-conductance Ca(2+)-activated K(+) channel inhibitor apamin, while the inward rectifier K(+) channel inhibitor Ba(2+) or the gap junction inhibitor 18alpha-glycyrrhetinic acid had no effect. Acetylcholine-induced relaxation was unchanged by ouabain (10 microM) but was partially inhibited by a higher concentration (100 microM). 4. In half of the tissues tested, K(+)(10 mM) itself produced L-NAME-resistant relaxations that were blocked by ouabain (10 microM) and partially reduced by charybdotoxin plus apamin, but not affected by 18alpha-glycyrrhetinic acid or Ba(2+). However, K(+) did not induce relaxations in endothelium-denuded tissues. 5. In conclusion, acetylcholine-induced relaxations in this tissue are largely dependent upon hyperpolarization mechanisms that are initiated in the endothelium but do not depend upon NO release. K(+) release cannot account for endothelium-dependent relaxation and cannot be an EDHF in this artery. However, K(+) itself can initiate endothelium-dependent relaxations via a different pathway from acetylcholine, but the mechanisms of K(+)-induced relaxations remain to be clarified.     

Muscarinic suppression of the nicotinic action of acetylcholine on the isolated,blood-perfused atrium of the dog

1. The isolated canine right atrium perfused through the sinus node artery at a constant pressure of 100 mm Hg with blood led from a support dog was suspended in a bath filed with blood and kept at constant temperature. This preparation maintained its constant tension development and rate over 5 hrs in all 5 control experiments. 2. A relatively small amount of acetylcholine (ACh) induced a negative inotropic effect at 0.01 mug and a negative chronotropic effect at 0.1 mug. 3. A relatively large dose of ACh induced a biphasic inotropic response, i.e., initially a negative inotropic response and secondarily a long-lasting positive tropic response. These positive chronotropic and inotropic responses were abolished by propranolol, hexamethonium or tetrodotoxin. 6. In the papillary muscle and atrial muscle preparations isolated from one canine heart, ACh caused negative and positive inotropic effect in both paced papillary and atrial muscle preparations. In contrast to the results obtained with atria, the positive inotropic response of the papillary muscle preparation was completely blocked by treatment with propranolol or tetrodotoxin. 7. From these results, it is suggested that in the canine atrium muscarinic mechanisms predominate over nicotinic ones. This may well be due to the known inhibition of nicotinic responses by stimulation of muscarinic receptors.     

Vasodilator actions of flunarizine in isolated dog cerebral and extracerebral arteries

Flunarizine relaxed isolated canine arteries precontracted with prostaglandin (PG) F2 alpha, epithio-methano-thromboxane A2 and K+; the relaxation was in the order of cerebral greater than renal greater than mesenteric = coronary arteries, when contracted with PGF2 alpha or the thromboxane A2 analogue. Flunarizine-induced relaxation was unaffected by treatment with atropine, propranolol, cimetidine, cimetidine, chlorpheniramine, aminophylline and indomethacin, and by removal of endothelium. Under normoxia, flunarizine attenuated contractions elicited by Ca2+ in the K+-stimulated cerebral and mesenteric arteries that had been previously exposed to Ca2+-free media to a greater extent than that in PGF 2 alpha-stimulated preparations. The Ca2+-induced contraction in cerebral arteries was more sensitive to flunarizine than that in mesenteric arteries. Contractions caused by PGF2 alpha in Ca2+-free media were not influenced by flunarizine. In cerebral and mesenteric arteries that had been previously exposed to Ca2+-free media and severe hypoxia and then stimulated by PGF2 alpha and Ca2+, reoxygenation produced a persistent contraction. Flunarizine suppressed the reoxygenation induced-contraction. It is concluded that flunarizine dilates cerebral arteries predominantly over the other arteries; the vasodilatation appears to derive from an interference with the transmembrane Ca2+ influx that occurs through a voltage-dependent process and, to a lesser extent, receptor-operated channels, but not with the Ca2+ release from stored sites. Contraction induced by reoxygenation is expected to be due mainly to the transmembrane influx of Ca2+, which is also suppressed by flunarizine.     

Muscarinic receptor subtype in isolated dog and monkey facial vein.

Using the cannula inserting method, we investigated vascular responses to ACh and McN-A-343 (a M1-agonist) in isolated and perfused canine and simian facial veins in non-preconstricted conditions. ACh usually induced only a vasoconstriction, but McN-A-343 did not induce any significant vasoconstriction. It is concluded that canine and simian facial veins contain very few receptors of the muscarinic M1-subtype; and according to previous studies, these vessels have abundant M3-receptors.     

Heterogeneity in the response to histamine of isolated dog arteries

Arterial rings were used to evaluate the differences in the effect of histamine (HIS) on canine dorsal pedal (DPA) and coronary arteries. Moreover, the responses of intact coronary arteries (IA) to HIS were compared with those of arteries isolated from sham operated dogs (SA) and from dogs in the late postinfarction period (PA). DPA responded to HIS added cumulatively with contraction which was potentiated by cimetidine (CIM) and reversed by mepyramine (MEP). IA responded with relaxation potentiated by MEP and reversed by CIM. Under resting conditions, coronary arteries responded to a single concentration of HIS with contraction or relaxation (in IA, SA). In PA, only contractions were seen. Responses of PA were significantly higher than those of IA and SA. It appears that in PA the ratio of H1- and H2-receptors is changed.     

Responsiveness and high-energy phosphate metabolism of isolated dog coronary arteries

The relationship between responsiveness and high-energy phosphate metabolism was observed in isolated dog coronary arterial strips before and after equilibration, under aerobic conditions. The strips responded slowly to potassium, norepinephrine, isoproterenol and 5-hydroxytryptamine before being equilibrated but did respond quickly to these drugs after being equilibrated for 1 hour under aerobic conditions. The contents of ATP, creatine phosphate (CP) and lactate were not significantly altered before and after equilibration for 1 hr. It is proposed that, before being equilibrated under aerobic conditions, coronary arteries may be unable to utilize ATP and CP due to injury of contractile or relaxant mechanisms resulting in a weak vascular responsiveness.     

Constrictor responses of isolated and perfused monkey coronary arteries to pindolol

We studied the mechanisms of pindolol-induced vasoconstrictions in isolated and perfused monkey coronary arteries. Phentolamine marginally reduced the vasoconstrictions, but bunazosin and DG 5128 did not. Diltiazem and methysergide significantly inhibited them, but atropine, diphenhydramine and cimetidine did not. Diltiazem significantly suppressed KCl-induced vasoconstrictions, but methysergide did not. These results suggest that pindolol-induced vasoconstrictions of monkey coronary arteries are mediated, at least partly, through 5-hydroxytryptamine receptors, but not through alpha-adrenoceptors, and that these vasoconstrictions appear to be associated with an increase of Ca2+ entry into the smooth muscle cell.