Pharmacological features of vascular responses of isolated rat common carotid arteries to vasoactive substances revealed by the cannula inserting method

Using the cannula inserting method, we studied vascular responses of isolated rat common carotid arteries to 15 vasoactive substances. Intraluminal injections of phenylephrine, norepinephrine (NE), angiotensin II, 5-hydroxytryptamine (5-HT) and prostaglandin F2 alpha (PGF2 alpha) induced a strong vasoconstriction in such a way that a maximum increase in perfusion pressure amounted to approximately 50 mmHg. The order of their potencies were as follows: Phenylephrine greater than or equal to NE greater than or equal to angiotensin II greater than PGF2 alpha greater than or equal to 5-HT much greater than KCl. The selective alpha-2 agonists, xylazine and clonidine, induced nor or a slight vasoconstriction. Tyramine, ATP and acetaldehyde induced only a slight constriction. Histamine, adenosine and acetylcholine (ACh) induced no effect. In preparations preconstricted by a high dose of NE, ACh and isoproterenol induced a vasodilation in a dose-related manner. It is concluded that the cannula inserting method is useful for investigating vascular responses in isolated and perfused rat vessels.     

Isolated, blood-perfused canine arteries: different vasoconstrictor responses of internal and external carotid arteries to 5-hydroxytryptamine

Canine carotid arteries were isolated, suspended in a bath and perfused under a constant flow rate with arterial blood led from a support dog. Resting perfusion pressure was set at a constant level over 50 mm Hg. Drugs were administered into the endothelial side of the artery through a cannulated tubing and the response was obtained as pressure changes. This preparation was proved to be superior to usual isolated arterial strips, since its reactivity was kept in a stable condition over 5-10 hours at 37 degrees C. Using this preparation, effects of 5-hydroxytryptamine (5-HT) on the internal and external carotid arteries were investigated in comparison with effects of norepinephrine. In the internal carotid artery, 5-HT caused a much more potent vasoconstriction than norepinephrine. On the other hand, in the external carotid artery, 5-HT caused only slight vasoconstriction, while norepinephrine produced a marked vasoconstriction.     

Regional differences of responses to adrenoceptor agonists in isolated and perfused canine coronary arteries

Regional differences of the responses to adrenoceptor agonists were investigated in isolated canine coronary arteries by use of a cannula inserting method. Acetylcholine induced a dose-dependent vasodilation. Norepinephrine and epinephrine produced a vasoconstriction followed by a strong vasodilation in large coronary arteries and only a weak vasodilation in small coronary arteries. Phenylephrine (a selective alpha-1 agonist) induced a strong vasoconstriction in both arteries. The threshold dose and ED50 value for phenylephrine in small coronary arteries were much larger than those in large coronary arteries, although the vasoconstrictions by KCl and prostaglandin F2 alpha were not different between in large and small coronary arteries. Clonidine and xylazine (selective alpha-2 agonists) produced a slight vasoconstriction but not dose-dependently and a vasodilation with extremely large doses. ED50 value of vasodilation for salbutamol (a selective beta-2 agonist) was approximately 80 times greater than that for isoproterenol (a non-selective beta-agonist) in large coronary arteries, but was approximately 20 times in small coronary arteries. The maximal dilator response to salbutamol was about the same as that to isoproterenol in small coronary arteries, whereas it was much smaller than that to isoproterenol in large coronary arteries. These results suggest that adrenoceptors are heterogeneous according to the distance from the coronary orifice in canine epicardial coronary arteries.     

Time-dependent enhancement of xylazine-induced, alpha-2 adrenoceptor-mediated vasoconstriction in isolated and perfused canine pulmonary veins

By using the cannula inserting method, vasoconstrictor responses to norepinephrine (a mixed alpha-1 and alpha-2 adrenoceptor agonist), phenylephrine (a selective alpha-1 adrenoceptor agonist), clonidine and xylazine (selective alpha-2 adrenoceptor agonists) were investigated in the isolated and perfused canine pulmonary vein. The segment of vessels was perfused by Krebs-Ringer bicarbonate solution at a constant flow rate at 37 degrees C. Two hours after setting up the perfusion preparation, norepinephrine and phenylephrine induced dose-dependent increases in perfusion pressure. Xylazine and clonidine, however, did not induce any significant responses at this time. Although the vasoconstrictor responses to norepinephrine and phenylephrine did not change statistically during 11 hr after setting up, xylazine-induced responses were perfusion-time-dependently and significantly enhanced 5, 8 and 11 hr after setting up. On the other hand, clonidine induced no significant vascular responses during 11 hr. Xylazine-induced responses were antagonized by DG-5128 (a selective alpha-2 adrenoceptor antagonist), but not by bunazosin (a selective alpha-1 adrenoceptor antagonist). Furthermore, even in the endothelium-removed preparation by treatment with saponin, the enhancement of xylazine-induced responses were similarly observed in a time-dependent manner. These findings suggest that, in the isolated and perfused canine pulmonary vein, xylazine-induced alpha-2 adrenoceptor-mediated vasoconstrictor response was time-dependently enhanced with the presence and absence of the endothelium. In contrast with xylazine, clonidine did not induce significant vasoconstriction during the experiments. This result suggests that clonidine has little activity at the alpha-2 adrenoceptors which mediate latent xylazine-induced vasoconstriction in the isolated, long-time-perfused canine pulmonary veins.     

二氢石蒜碱对家兔脑血管的选择性作用特点

背景:二氢石蒜碱在外周能抑制交感神经末梢释放儿茶酚胺,阻断α,β肾上腺素能受体,具有扩血管、降血压,抗缺糖、缺氧作用.目的:应用重酒石酸去甲肾上腺素和KCl引起血管收缩反应,观察二氢石蒜碱对家兔脑基底动脉、胸主动脉及心室乳头肌的选择性作用.设计:观察性对比实验.单位:郧阳医学院药理教研室.材料:实验于2001-03/07在郧阳医学院药理教研室的科研室进行,选择46只成年健康新西兰白兔.方法:新西兰白兔在麻醉下耳缘静脉注射10 mL空气处死,迅速取出脑、胸主动脉及心脏.将基底动脉、胸主动脉制成4.0～5.0mm的血管环,打开心脏分离心室乳头肌,分别连接张力换能器.实验观察:①记录重酒石酸去甲肾上腺素或KCl单用时收缩曲线的变化:胸主动脉给予重酒石酸去甲肾上腺素0.1 mmol/L或KCl60mmol/L引起血管亚最大收缩,待收缩曲线平稳后,分别用累积法加入不同浓度的二氢石蒜碱或尼莫地平.基底动脉给予重酒石酸去甲肾上腺素0.1 mmol/L或KCl 60 mmol/L,待血管收缩达峰值后,冲洗液每20 min换1次,共3次.再用不同浓度二氢石蒜碱或尼莫地平,20 min后再加入重酒石酸去甲肾上腺素或KCl,待曲线平稳后,观察每单剂药物所致收缩曲线的变化.②记录电刺激心室乳头肌时收缩的幅度变化:1次/s,波宽3 ms,阈电压的120%强度电刺激以引发心室乳头肌同步收缩活动,待收缩曲线平稳后,分别用累加法加入二氢石蒜碱或尼莫地平.主要观察指标:①离体基底动脉、胸主动脉及心室乳头肌收缩张力变化.②二氢石蒜碱或尼莫地平对重酒石酸去甲肾上腺素或KCl所致家兔血管环收缩的半数有效浓度值.结果:46只家兔离体基底动脉、胸主动脉及心室乳头肌标本均进入实验分析.①二氢石蒜碱使基底动脉静息张力升高,尼莫地平则使其降低,对胸主动脉无影响.②重酒石酸去甲肾上腺素、KCl所致基底动脉及胸主动脉收缩,二氢石蒜碱使之呈剂量依赖性松弛,其半数有效浓度值:对基底动脉分别为(6.69±3.12)×10-,(3.41±1.52)×10-3 mmol/L;对胸主动脉分别为(1.49±0.59)×10-3,(2.91±0.99)×10-3 mmol/L.拮抗重酒石酸去甲肾上腺素所致基底动脉收缩作用明显强于KCl.尼莫地平对KCl所致收缩的抑制显著强于对重酒石酸去甲肾上腺素.③重酒石酸去甲肾上腺素或尼莫地平对电刺激诱发心室乳头肌的收缩均有剂量依赖性抑制作用,二氢石蒜碱对电刺激诱发心室乳头肌收缩的半数有效浓度显著高于重酒石酸去甲肾上腺素所致基底动脉.结论:二氢石蒜碱对家兔基底动脉有明显的选择性作用,在基底动脉可能存在使该血管平滑肌收缩的α1受体亚型和使之舒张的β受体.二氢石蒜碱作用可能与对这些受体的阻断作用相关.其选择性作用可能有助于改善缺血区的血液供给.     

A modified cannula inserting method for measuring vascular responsiveness of the common carotid artery of the dog

A modified cannula inserting method was developed to investigate effects of either intra- or extraluminal application of vasoactive drugs on the isolated vessel which was cannulated and suspended in an organ bath and perfused under a constant flow rate with modified Krebs solution. By inserting a newly developed stainless steel cannula into the vessel, the space between the internal surface of the vessel and the cannula was filled with perfusate and perfused by a pump. Drugs were administered intraluminally through a cannula or extraluminally in the bath. The perfusate after passing through the inside of the vessel was introduced to the tubing and then drained to the outside. Although a lot of methods have been used to observe contractile responses of isolated vessels, this method may be superior to them as follows: (1) drugs act selectively on the inner or outer surface of the vessel; (2) vascular responses to drugs, not only constriction, but also dilatation, are readily observed in a relatively larger artery (1-3 mm I.D.); 3) the vascular reactivity is kept in a stable condition over 7-8 hr at 37 degrees C because reproducible responses were consistently observed. We used this preparation to examine the constrictive and dilatory responses of the canine common carotid artery to extraluminally administered oxyhemoglobin, and intraluminally injected 5-hydroxytryptamine, norepinephrine and ACh.     

Enhancement of vasoconstrictor responses to 5-HT but no to methoxamine by cooling in isolated dog lingual and mesenteric arteries

The effect of temperature on submaximal vasoconstrictions to an intraluminal injection of serotonin (5-HT) and methoxamine was investigated in isolated and perfused canine lingual and mesenteric arteries, using the cannula insertion method. In both arteries cooling (from 37 degrees C to 27 degrees C) caused a remarkable enhancement of vasoconstriction to 5-HT, but did not to methoxamine. In lingual arteries, methoxamine-induced constrictions were strongly depressed, although those were slightly depressed in mesenteric arteries. It is assumed that 5-HT produces an important role to modulate vascular tonicity in low temperature conditions.     

Relationship between receptors mediating serotonin (5-HT) contractions in the canine basilar artery to 5-HT1, 5-HT2 and rat stomach fundus 5-HT receptors

The receptors responsible for contraction to serotonin (5-HT) in the canine basilar artery have not been definitively established to date. Several selective 5-HT2 receptor antagonists (spiperone, ketanserin and LY53857) did not inhibit markedly 5-HT-induced contractions in the canine basilar artery in doses higher than required for substantial inhibition of 5-HT2 receptor-mediated responses. These data suggest that the receptors mediating 5-HT-induced contractions in the basilar artery are not 5-HT2 receptors. Using a series of 5-HT antagonists with relatively high affinity at 5-HT1 sites, over a 1000-fold difference occurred in their ability to block 5-HT receptors in the canine basilar artery, in spite of the similar and high affinity of the antagonists at 5-HT1 binding sites. These data support the contention that 5-HT receptors in the canine basilar artery are not 5-HT1 receptors as defined by ligand binding studies in brain cortical membranes. Similarity of the contractile effects of 5-HT in the rat stomach fundus and in the basilar artery coupled to the previous observations that receptors mediating 5-HT-induced contractions in the fundus were not 5-HT1, 5-HT1A, 5-HT1B or 5-HT2 led us to consider the possibility that 5-HT receptors in the canine basilar artery may resemble those in the rat stomach fundus. The affinity of several 5-HT antagonists determined in the canine basilar artery correlated extremely well (correlation coefficient = 0.96) with the affinities obtained for the same antagonists in the rat stomach fundus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Existence of two types of postjunctional alpha adrenoceptors in the isolated canine intermediate auricular artery

Four alpha adrenergic agonists (phenylephrine, xylazine, clonidine and norepinephrine) and two antagonists (prazosin and yohimbine) were used to investigate the characteristics of the postjunctional alpha adrenoceptors involved in contraction of the isolated and perfused canine intermediate auricular artery. All agonists showed almost equal potencies for inducing vasoconstriction in the perfused arterial preparations (i.e., all agonists caused strong vasoconstrictor responses in a dose-related manner; the threshold dose of each agonist was within a dose range of 0.003 to 0.01 micrograms and 0.3 to 1.0 micrograms of each agent caused approximately 200 mm Hg increase in perfusion pressure). Prazosin inhibited phenylephrine-induced vasoconstriction in a competitive manner, but yohimbine did not significantly influence phenylephrine-induced responses. Xylazine-induced responses were inhibited by both prazosin and yohimbine, but the former was less potent than the latter at a low dose and the antagonistic property of prazosin against xylazine was not competitive. The pA2 values of prazosin against phenylephrine, xylazine and clonidine were 7.10, 6.82 and 6.99, respectively, and that of yohimbine against xylazine was 7.16. These results support the theory that not only alpha-1 but also alpha-2 adrenoceptors are involved in the contractile responses of the isolated and perfused canine intermediate auricular artery.     

Vascular effects of ketanserin (R 41 468), a novel antagonist of 5-HT2 serotonergic receptors

The serotonergic receptor antagonist 3-(2-[4-(4-fluorobenzoyl)-1-piperidinyl]ethyl)-2,4-[1H,3H]quinazolinedione Ketanserin (R 41 468) caused a dose-dependent inhibition on the contractile responses to 5-hydroxytryptamine of isolated rat caudal artery, canine basilar, carotid, coronary and gastrosplenic arteries, canine gastrosplenic veins (threshold 10(-10)-10(-9) M) and canine saphenous veins (threshold 10(-8) M). In concentrations up to 2.5 X 10(-5) M, it did not have agonistic properties. From 10(-8) M on, R 41 468 inhibited the contractions of rat caudal arteries and canine saphenous veins caused by postjunctional alpha adrenergic activation. In the rat caudal artery, R 41 468, in concentrations which did not affect the contractile response to norepinephrine, abolished the amplifying effect of low concentrations of 5-hydroxytryptamine on alpha adrenergic activation. In the canine saphenous vein, R 41 468 did not affect the prejunctional inhibitory effect of 5-hydroxytryptamine during sympathetic nerve stimulation. In the perfused guinea-pig stomach, R 41 468 depressed and in certain experiments reversed the vasoconstrictor response to 5-hydroxytryptamine. In isolated perfused kidneys from both normotensive and spontaneously hypertensive rats, R 41 468, in concentrations which did not depress vasoconstrictor responses to exogenous norepinephrine, inhibited those to 5-hydroxytryptamine. The compound caused a dose-related reduction in aortic blood pressure in unanesthetized spontaneously hypertensive rats, which was larger and occurred at lower concentrations, than in control animals. These results demonstrate that R 41 468 is a potent antagonist of the vasoconstrictor effects of 5-hydroxytryptamine, in particular of its amplifying effect on threshold amounts of norepinephrine, which may help explain its antihypertensive properties.     

Effects of naftidrofuryl on adrenergic nerves, endothelium and smooth muscle in isolated canine blood vessels

Experiments were designed to determine the effects of the vasoactive drug naftidrofuryl on vascular smooth muscle, endothelial cells and adrenergic nerves in isolated canine blood vessels. Naftidrofuryl inhibited contractions of basilar arteries (in a decreasing order of potency), evoked by 5-hydroxytryptamine greater than KCl = anoxia (in rings with endothelium) greater than prostaglandin F2 alpha = uridine-5'-triphosphate. Naftidrofuryl antagonized competitively the contractions evoked by 5-hydroxytryptamine in the femoral artery and the saphenous vein. Naftidrofuryl caused the release of an endothelium-derived relaxing factor(s) from the endothelium of femoral arteries. The compound depressed contractions of saphenous veins evoked by electrical stimulation of the adrenergic nerve endings, but not those caused by the indirect sympathomimetic amine tyramine or exogenous norepinephrine. In saphenous veins incubated previously with [3H]norepinephrine, the drug inhibited the contractions and the release of transmitter evoked by electrical stimulation. Thus, naftidrofuryl acts at different levels in the blood vessel wall to cause: release of endothelium-derived relaxing factor(s); inhibition of S2-serotonergic receptors on vascular smooth muscle; prejunctional inhibition of adrenergic neurotransmission; and nonselective inhibition of the contractile process in vascular smooth muscle, which is particularly pronounced in cerebral arteries.     

Effects of dopamine (DA) and SKF 82526, a selective DA1-receptor agonist, on vascular resistances in the canine forelimb

This study was performed to determine the presence of vascular dopamine (DA1) receptors in the canine forelimb. Local i.a. infusions of DA (2, 4 or 8 micrograms base/min) produced cutaneous and skeletal muscle vasoconstriction in the forelimb comparable to that produced by local i.a. infusions of norepinephrine (0.5, 1 or 4 micrograms base/min). In the cutaneous vasculature, DA produced large artery, small vessel and large vein constrictions. The small vessels and large veins constricted proportionately more than the large arteries. The pattern of constriction produced by DA along the cutaneous vascular tree was similar to that produced by norepinephrine. The forelimb vasoconstriction produced by both DA and norepinephrine was abolished completely by treatment with phentolamine, indicating that both agents produce vasoconstriction mediated by stimulation of alpha adrenoceptors. The failure of DA to produce vasodilation after phentolamine suggests that there are no vascular or DA1-subtype DA receptors in the canine forelimb vasculature. The local i.a. infusion of SKF 82526, a selective DA1-receptor agonist, for 3 min produced cutaneous and skeletal muscle dilation which could be antagonized by either sulpiride or phentolamine. This neurogenic dopaminergic vasodilation produced by SKF 82526 (25, 50 or 100 micrograms base/min) was converted into vasoconstriction after ganglionic blockade. Inasmuch as SKF 82526 does not activate presynaptic or DA2-subtype DA receptors, the neurogenic vasodilation caused by SKF 82526 may be due to a dopaminergic inhibition of ganglionic transmission. Whereas the results of our study fail to provide any evidence for the presence of vascular DA (DA1) receptors in the canine forelimb, they show that SKF 82526, a DA1-receptor agonist, produces neurogenic vasodilation probably by activating ganglionic DA receptors.     

Blockade by saralasin of adrenergic potentiation induced by renin-angiotensin system.

Participation of the renin-angiotensin system in the potentiation of responses to adrenergic nerve stimulation in the pump-perfused dog paw was studied during suprarenal aortic constriction. Vasoconstrictor responses in the paw were elicited by norepinephrine injected intra-arterially and by sympathetic nerve stimulation in a control session and during suprarenal aortic constriction (cephalad to the origin of one or two renal arteries). Aortic constriction decreased renal blood flow by approximately 50% and increased systemic blood pressure and plasma renin activity. The vasoconstrictor responses to norepinephrine and nerve stimulation were not significantly affected when the constriction was cephalad to only one renal artery, but there was a 31% increase in the response to stimulation at 5 Hz during aortic constriction above two renal arteries. There was an approximate 2-fold greater increase in plasma renin activity during the latter than in the former case. Saralasin administered intra-arterially to the paw reversed the adrenergic potentiating effect of aortic constriction. These results indicate that when the renin-angiotensin system is activated by restricting renal blood flow, sufficient circulating endogenous angiotensin is formed to cause a moderate adrenergic potentiating effect in the canine cutaneous circulation of the paw.     

Spasm of the coronary arteries: causes and consequences (the scientist's viewpoint)

Both beta 1- and alpha 1-adrenoceptors are present on canine coronary arteries, and they are accessible to norepinephrine released from the sympathetic nerves. Under normal conditions, these arteries relax because of the predominance of the beta 1-adrenoceptors, whereas constriction prevails in the presence of beta 1-adrenoceptor antagonists. The coronary arteries also have cholinergic nerves. When activated, these nerves release acetylcholine, which acts on muscarinic receptors on the sympathetic nerve terminals to reduce the output of norepinephrine and thereby lessen the relaxation mediated by beta 1-adrenoceptors. Thus, muscarinic agonists can precipitate coronary artery spasm. If the smooth muscle cells of the coronary arteries become hypoxic, their responsiveness to beta-adrenergic stimulation is lost and constrictor responses are exaggerated. Cardiac glycosides prevent the predominance of the beta-adrenergic effects of norepinephrine. Therefore, after treatment with ouabain, release of norepinephrine from the sympathetic nerves leads not to relaxation but to further contraction of coronary arteries. The endothelium of the coronary arteries inhibits platelet aggregation by the formation and release of prostacyclin, and it reacts to platelet products by causing relaxation of the underlying smooth muscle. In addition, if any thrombin is formed, it also causes endothelium-mediated relaxation. If the endothelium is damaged, these protective mechanisms are lost. Patients with coronary artery spasm usually have morphologic changes in the artery at the site of the spasm. Platelets can aggregate at this site and release vasoactive substances, which--aided by formation of thrombin--cause contraction. Thus, the blood supply to the myocardium is reduced; the ensuing hypoxia augments the constriction. Acute myocardial ischemia caused by coronary vasospasm may precipitate acute cardiac rhythm disturbances and sudden death by ventricular tachycardia or fibrillation.     

Serotonergic prejunctional inhibition of canine coronary adrenergic nerves

The actions of 5-hydroxytryptamine (5-HT) on the response of isolated canine coronary arteries to adrenergic nerve stimulation and norepinephrine were studied. 5-HT inhibited the beta adrenergic relaxation of left circumflex coronary rings in response to transmural electrical stimulation. The sensitivity to exogenously added norepinephrine was unaffected, suggesting that the effect on the response to electrical stimulation is prejunctional. Inhibition of norepinephrine release by 5-HT was confirmed in strips of coronary artery preincubated in [3H]norepinephrine. Serotonergic antagonists were tested for their ability to block the prejunctional inhibition by 5-HT, as well as their effects on the response of the coronary artery to electrical stimulation and norepinephrine. The nonselective serotonergic antagonists, methiothepin and metergoline, but not the selective 5-HT2 antagonists, ketanserin and LY 53857, prevented the inhibition by 5-HT of the response to electrical stimulation and of the stimulated overflow of [3H]norepinephrine. All of the serotonergic antagonists studied had additional effects on the response of the coronary artery to electrical stimulation or to norepinephrine. However, the alpha adrenergic antagonist, phentolamine, had additional effects similar to the serotonergic antagonists, but did not antagonize prejunctional inhibition caused by 5-HT. Furthermore, methiothepin did not block prejunctional inhibition caused by acetylcholine, suggesting the specificity of the nonselective serotonergic antagonists. Because the prejunctional inhibition by 5-HT was unaffected by neuronal uptake blockade with cocaine, these results suggest specific, non-5-HT2 serotonergic receptors on coronary adrenergic nerves which, when activated, inhibit the stimulated release of norepinephrine.     

Hypoxia Impairs Vasodilation in the Lung

Alveolar hypoxia causes pulmonary vasoconstriction; we investigated whether hypoxia could also impair pulmonary vasodilation. We found in the isolated perfused rat lung a delay in vasodilation following agonist-induced vasoconstriction. The delay was not due to erythrocyte or plasma factors, or to alterations in base-line lung perfusion pressure. Pretreating lungs with arachidonic acid abolished hypoxic vasoconstriction, but did not influence the hypoxia-induced impairment of vasodilation after angiotensin II, bradykinin, or serotonin pressor responses. Progressive slowing of vasodilation followed angiotensin II-induced constriction as the lung oxygen tension fell progressively below 60 Torr. KCl, which is not metabolized by the lung, caused vasoconstriction; the subsequent vasodilation time was delayed during hypoxia. However, catecholamine depletion in the lungs abolished this hypoxic vasodilation delay after KCl-induced vasoconstriction. In lungs from high altitude rats, the hypoxia-induced vasodilation impairment after an angiotensin II pressor response was markedly less than it was in lungs from low altitude rats. We conclude from these studies that (a) hypoxia impairs vasodilation of rat lung vessels following constriction induced by angiotensin II, serotonin, bradykinin, or KCl, (b) hypoxia slows vasodilation following KCl-induced vasoconstriction probably by altering lung handling of norepinephrine, (c) the effect of hypoxia on vasodilation is not dependent on its constricting effect on lung vessels, (d) high altitude acclimation moderates the effect of acute hypoxia on vasodilation, and (e) the hypoxic impairment of vasodilation is possibly the result of an altered rate of dissociation of agonists from their membrane receptors on the vascular smooth muscle.     

Pb2+ inhibition of sympathetic alpha 7-nicotinic acetylcholine receptor-mediated nitrergic neurogenic dilation in porcine basilar arteries

Chronic exposure to inorganic lead (Pb2+) has been shown to facilitate peripheral vasoconstriction causing hypertension. Effect of lead on cerebral vascular function has not been reported. We have suggested in isolated porcine cerebral arteries that alpha 7-nicotinic acetylcholine receptors (alpha 7-nAChRs) on perivascular sympathetic nerves mediate calcium influx in these neurons, resulting in release of norepinephrine. The released norepinephrine then acts on presynaptic beta2-adrenoceptors located on the neighboring nitrergic nerve terminals, causing nitric oxide (NO) release and vasodilation. Because Pb2+ has been shown to inhibit alpha 7-nAChR-mediated responses in the central nervous system, effects of Pb2+ on alpha 7-nAChR-mediated nitrergic neurogenic dilation in isolated porcine basilar arteries and calcium influx in cultured superior cervical ganglion (SCG) cells of the pig were examined using in vitro tissue bath and confocal microscopic techniques. The results indicated that Pb2+ (but not Cd2+, Zn2+, or Al3+) in a concentration-dependent manner blocked relaxation of endothelium-denuded basilar arterial rings induced by nicotine (100 microM) and choline (1 mM) without affecting relaxation induced by sodium nitroprusside or isoproterenol. Furthermore, significant calcium influx in cultured SCG cells induced by choline and nicotine was attenuated specifically by Pb2+ with IC50 values comparable with those from tissue bath study. These results provide evidence supporting that lead is a likely antagonist for alpha 7-nAChRs that are found on postganglionic sympathetic adrenergic nerve terminals of SCG origin. Furthermore, these results indicate that lead can attenuate dilation of cerebral arteries by blocking sympathetic nerve-mediated release of NO from the perivascular nitrergic nerves.     

Posturally induced microvascular constriction in patients with different stages of leg ischaemia: effect of local skin heating.

1. Skin microcirculation was investigated in 12 asymptomatic subjects and 76 patients, grouped according to their ankle-to-brachial systolic blood pressure index, in order to evaluate to what extent posturally induced microvascular constriction is dependent on the stage of leg ischaemia at different local skin temperatures. 2. Skin microcirculation was assessed in the supine and sitting position by using laser Doppler fluxmetry at unheated skin temperature and at 36 degrees C, and transcutaneous oximetry at 37 degrees C and 44 degrees C. 3. Skin perfusion and oxygenation diminished with decreasing ankle-to-brachial systolic blood pressure index. In healthy control subjects, perfusion and oxygenation were reduced when changing from the supine to the sitting position, but were enhanced in patients with severe leg ischaemia (ankle-to-brachial systolic blood pressure less than 30%), indicating disturbed posturally induced vasoconstriction. 4. Increasing the local skin temperature resulted in a higher perfusion and masked the posturally induced vasoconstriction in healthy subjects. In patients with severe leg ischaemia, however, perfusion was unaltered by the temperature increase, apparently because the microvessels were already maximally dilated. The induction of reactive hyperaemia produced no additional increase in perfusion or oxygenation. 5. It is concluded that posturally induced microvascular constriction in the skin is disturbed in patients with severe leg ischaemia (ankle-to-brachial systolic blood pressure index less than 30%). Disturbed microvascular constriction upon dependency was also seen in healthy subjects after local skin heating. This suggests that posturally induced vasoconstriction is mainly regulated by local mechanisms.     

Involvement of specific receptors and calcium mechanisms in serotonergic contractile response of isolated cerebral and peripheral arteries from rats

Studies were performed on isolated cerebral and peripheral arterial segments from the rat to define contractile receptors for 5-hydroxytryptamine (5-HT) and to elucidate the responses to calcium channel blockers in relation to their effects on potassium-induced contractions. 5-HT induced strong contraction of the middle cerebral artery, arteries forming the circle of Willis, basilar artery and tail artery in the mentioned order of relative potency and with an intrinsic activity in the brain vessels approximately 70% of that caused by 124 mM potassium in the buffer solution. Ketanserin inhibited the contraction both in the basilar and tail arteries competitively, with pA2 = 9.35 and 9.09, respectively, suggesting mediation by 5-HT2 receptors. The inhibition by cyproheptadine and methysergide (of the response in the basilar artery) was noncompetitive. High potassium in the buffer solution contracted the basilar and tail arteries biphasically, including prazosin-sensitive alpha-adrenoceptor activation in the latter. Cyproheptadine, nimodipine, verapamil and diltiazem inhibited the 5-HT-induced contraction in the mentioned order of potency. Verapamil was more potent than diltiazem, also in the tail artery, but nimodipine inhibited the contraction only by 35%. Also the (tonic) contraction induced by high potassium concentration was attenuated, with the same relative potency as in the presence of 5-HT except for cyproheptadine, which was less efficient than nimodipine. The transient potassium-induced contraction was inhibited less effectively by the calcium antagonists. The IC50 values were characteristically lower in the basilar than in the tail artery, irrespective of whether the contraction had been produced by 5-HT or high potassium.(ABSTRACT TRUNCATED AT 250 WORDS)     

Responses of coronary vessels to adrenergic stimuli

Coronary responses to adrenergic stimuli were determined in the intact beating heart before and after administration of practolol, 4-(2-hydroxy-3-isopropylaminoproproxy) acetanilide, which in low doses blocks myocardial but not vascular beta receptors. The left circumflex coronary artery of dogs was perfused with arterial blood at constant flow, and coronary perfusion pressure was measured.Before practolol, intracoronary injections of isoproterenol and norepinephrine and electrical stimulation of left cardiac sympathetic nerves caused reductions in perfusion pressure or vasodilatation associated with increases in left ventricular dp/dt, heart rate, and systolic pressure.After practolol, the coronary vasodilator response to isoproterenol was reduced by about 30% and occurred without significant changes in dp/dt, heart rate, and pressures. The addition of propranolol blocked completely the coronary responses to isoproterenol. Vascular responses to isoproterenol in the paw were not altered by practolol.Practolol antagonized the increases in dp/dt, heart rate, and systolic pressure and reversed coronary responses to norepinephrine and nerve stimulation from dilatation to constriction. The constriction, in turn, was reduced or reversed by phentolamine, an alpha receptor antagonist. Propranolol did not augment the constriction seen in response to norepinephrine and nerve stimulation after practolol.These results indicate that the coronary vasodilator action of norepinephrine and sympathetic nerve stimulation is indirect and caused by stimulation of myocardial beta receptors. The direct effect of these two stimuli on coronary vessels is minimal and is mediated through stimulation of alpha (vasoconstrictor) receptors. In contrast, the coronary vasodilator response to isoproterenol is both direct and indirect, resulting from stimulation of vascular and myocardial beta receptors; the direct vascular effect predominated in this study.