Propranolol induces contractions of canine small and large coronary arteries

Perioperative coronary artery spasms in patients undergoing catheter ablation of atrial fibrillation

PURPOSE

Catheter ablation (CA) is an established treatment for atrial fibrillation (AF). Although coronary artery spasms (CAS) during or after ablation procedures have been described as a rare complication in some case reports, the incidence and characteristics of this complication have not been fully elucidated. The present observational study aimed to clarify the CAS in a large number of patients experiencing AF ablation.

METHODS

A total of 2913 consecutive patients (male: 78%, mean 66 ± 10 years) who underwent catheter ablation of AF were enrolled.

RESULTS

Nine patients (0.31%, mean 66 ± 10 years, 7 males) had transient ST-T elevation (STE). Eight out of the 9 patients had STE in the inferior leads. STE occurred after the transseptal puncture in 7 patients, after the sheath was pulled out of the left atrium in 1, and 2 h after the ablation procedure in 1. Six patients had definite angiographic CAS without any sign of an air embolization on the emergent coronary angiography. In the3 other patients, the STE improved either directly after an infusion of nitroglycerin or spontaneously before the CAG. The patients with CAS had a higher frequency of a smoking habit (89% vs. 53%; P = .04), smaller left atrial diameter (36 ± 6 vs. 40 ± 7; P = .07), and lower CHADS2 score (0.6 ± 0.5 vs. 1.3 ± 1.1; P = .004) than those without.

CONCLUSIONS

Although the incidence was rare (0.31%), CAS should be kept in mind as a potentially life-threatening complication throughout an AF ablation procedure especially performed under conscious sedation.

Angina due to coronary artery spasm (variant angina): diagnosis and intervention strategies

INTRODUCTION

Since Prinzmetal first described a 'variant' form of angina pectoris, with predominantly resting episodes of pain and cyclic severity variations, it has gradually become apparent that this clinical presentation is caused by episodes of coronary artery spasm (CAS) involving focal or diffuse changes in large and/or small coronary arteries in the presence or absence of 'fixed' coronary artery stenoses. However, most clinicians have only limited understanding of this group of disorders.

AREAS COVERED

We examine the clinical presentation of CAS, associated pathologies outside the coronary vasculature, impediments to making the diagnosis, provocative diagnostic tests, available and emerging treatments, and the current understanding of pathogenesis.

EXPERT OPINION

CAS is often debilitating and substantially under-diagnosed and occur mainly in women. Many patients presenting with CAS crises have non-diagnostic ECGs and normal serum troponin concentrations, but CAS can be suspected on the basis of history and association with migraine, Raynaud's phenomenon and Kounis syndrome. Definitive diagnosis requires provocative testing at coronary angiography. Treatment still centers around the use of calcium antagonists, but with greater understanding of pathogenesis, new management options are emerging.

A Rare Case of Vasospastic Angina Presenting with Inferior Lead ST-segment Elevation and Ventricular Fibrillation in the Absence of Coronary Obstruction: A Case Report

Vasospastic angina (VSA) is a variant form of angina pectoris, which occurs at night or at rest, with transient electrocardiogram modifications and preserved exercise capacity. Its association with stable angina, sudden cardiac death, acute coronary syndrome, arrhythmia, and syncope has previously been established. Its presentation can occur with or without existing coronary artery disease and may present with focal or diffuse alteration and dysfunction of the coronary vasculature. VSA diagnosis involves patient response to nitrates, transient ischemic electrocardiogram (ECG) changes, and coronary artery spasms. The mechanisms proposed to constitute the substrate for susceptibility to VSA include vascular smooth muscle cell hyperreactivity, endothelial dysfunction, magnesium deficiency, low-grade inflammation, altered autonomic nervous system response, hypothyroidism, and oxidative stress. Herein, we present the rare case of a patient with ST-segment elevation in the inferior leads, increased troponin, and an episode of ventricular fibrillation initially thought to be due to lateral wall ST-elevation myocardial infarction (STEMI), although it was revealed to be vasospastic angina. We will also review the literature. Vasospastic angina remains underdiagnosed and a timely diagnosis is crucial to prevent major cardiac events. In patients with diffuse ST-segment elevation on ECG (independently of angiographic findings), VSA should be considered as one of the differential diagnoses and treated if found to be the cause of pathological changes.

Post-valvular surgery multi-vessel coronary artery spasm - A literature review

Coronary artery spasm (CAS) refers to the spontaneous or stimuli-induced transient, often localized and intense subtotal or total constriction/occlusion of the epicardial coronary artery, usually concomitant with angina pectoris with associated elevation of the ST segment on electrocardiogram (ECG). In this article, we present a literature review on post-valvular surgery CAS and report the clinical case of a 77 year-old man who experienced severe early post-aortic surgery chest pain and hemodynamic instability. Emergent coronary angiography revealed severe occlusion of multiple branches of both coronary arteries. The CAS was alleviated with intracoronary infusion of nitroglycerin.

Prognostic stratification of patients with vasospastic angina: a comprehensive clinical risk score developed by the Japanese Coronary Spasm Association

OBJECTIVES

The present study aimed to develop a comprehensive clinical risk score for vasospastic angina (VSA) patients.

BACKGROUND

Previous studies demonstrated various prognostic factors of future adverse events in VSA patients. However, to apply these prognostic factors in clinical practice, the assessment of their accumulation in individual patients is important.

METHODS

The patient database of the multicenter registry study by the Japanese Coronary Spasm Association (JCSA) (n = 1,429; median 66 years; median follow-up 32 months) was utilized for score derivation.

RESULTS

Multivariable Cox proportional hazard model selected 7 predictors of major adverse cardiac events (MACE). The integer score was assigned to each predictors proportional to their respective adjusted hazard ratio; history of out-of-hospital cardiac arrest (4 points), smoking, angina at rest alone, organic coronary stenosis, multivessel spasm (2 points each), ST-segment elevation during angina, and beta-blocker use (1 point each). According to the total score in individual patients, 3 risk strata were defined; low (score 0 to 2, n = 598), intermediate (score 3 to 5, n = 639) and high (score 6 or more, n = 192). The incidences of MACE in the low-, intermediate-, and high-risk patients were 2.5%, 7.0%, and 13.0%, respectively (p < 0.001). The Cox model for MACE between the 3 risk strata also showed prognostic utility of the scoring system in various clinical subgroups. The average prediction rate of the scoring system in the internal training and validation sets were 86.6% and 86.5%, respectively.

CONCLUSIONS

We developed a novel scoring system, the JCSA risk score, which may provide the comprehensive risk assessment and prognostic stratification for VSA patients.

The mystery of coronary artery spasm

Coronary artery spasm is an important cause of chest pain and myocardial ischaemia. It can be defined as an exaggerated contractile response of epicardial coronary artery smooth muscle to various stimuli but the underlying mechanism is not well understood. Recent studies have shown that the loss of endothelial vasodilatory function in conjunction with an increase in vascular smooth muscle constrictor sensitivity to calcium are the likely predisposing conditions for coronary spasm. This review highlights current understanding of the pathophysiology, predisposing factors, diagnostic and therapeutic approaches for coronary spasm.

Akrinor-induced relaxation of pig coronary artery in vitro is transformed into alpha1-adrenoreceptor-mediated contraction by pretreatment with propranolol

Akrinor (AKR), a mixture of theodrenaline (TDR) and cafedrine (CDR), is a sympathomimetic agent used to counter transitory hypotension. Although some cases of vascular complications associated with AKR have been reported there are no experimental data about its direct effects on coronary arteries. The effects of AKR, TDR, CDR, and ephedrine (EDR) were studied on the isometric contraction of the ring preparations of pig coronary arteries precontracted with KCl. The influence of endothelium removal and preincubation with nonselective beta-adrenoreceptor antagonist propranolol (PROP), alpha(1)-adrenoreceptor antagonist prazosin, dopamine receptor antagonist SCH 23390, and adenosine receptor antagonist CGS 15943 were also tested. AKR, TDR, and CDR produced relaxation of the preparations. Preparations without endothelium were more sensitive to AKR relaxing effects. EDR produced an increase of vascular ring tonus. AKR, TDR, and EDR produced contraction in preparations pretreated with PROP. Higher concentrations of AKR relaxed PROP-pretreated preparations. AKR-induced contraction could be prevented by pretreatment with prazosin. Dopamine and adenosine receptor antagonists did not influence relaxing effects of AKR. In conclusion, AKR and its constituents induce the relaxation of pig coronary artery preparations precontracted with KCl. The observed contraction in the preparations pretreated with PROP was probably due to stimulation of unmasked alpha(1)-adrenoreceptors.

Coronary vasospasm and the regulation of coronary blood flow

Under physiologic conditions, epicardial arteries contribute minimally to coronary vascular resistance. However, in the presence of endothelial dysfunction, stimuli that normally produce vasodilation may instead cause constriction. Examples include neural release of acetylcholine or norepinephrine, platelet activation and production of serotonin and thrombin, and release of local factors such as bradykinin. This shift from a primary endothelial-mediated vasodilator influence to one of endothelial dysfunction and unchecked vasoconstriction is precisely the milieu in which coronary vasospasm is observed. This condition, which typically occurs during periods of relatively sedentary activity, is associated with focal and transient obstruction of an epicardial arterial segment resulting in characteristic echocardiographic changes and symptoms of myocardial ischemia. This review highlights the current understanding of mechanisms regulating the coronary circulation during health and examines the pathophysiologic changes that occur with coronary spasm. Genetic and other predisposing conditions are addressed, as well as novel therapies based on recent mechanistic insights of the coronary contractile dysfunction associated with coronary spasm.

Attenuation of the serotonin-induced increase in intracellular calcium in rat aortic smooth muscle cells by sarpogrelate

Although serotonin (5-HT) induced proliferation of vascular smooth muscle cells is considered to involve changes in intracellular Ca2+([Ca2+]i), the mechanism of Ca2+mobilization by 5-HT is not well defined. In this study, we examined the effect of 5-HT on rat aortic smooth muscle cells (RASMCs) by Fura-2 microfluorometry for [Ca2+]imeasurements. 5-HT was observed to increase the [Ca2+]iin a concentration- and time-dependent manner. This action of 5-HT was dependent upon the extracellular concentration of Ca2+([Ca2+]e) and was inhibited by both Ca2+channel antagonists (verapamil and diltiazem) and inhibitors of sarcoplasmic reticular Ca2+pumps (thapsigargin and cyclopia zonic acid). The 5-HT-induced increase in [Ca2+]iwas blocked by sarpogrelate, a 5-HT2A-receptor antagonist, but not by different agents known to block other receptor sites. 5-HT-receptor antagonists such as ketanserin, cinanserin, and mianserin, unlike methysergide, were also found to inhibit the 5-HT-induced Ca2+mobilization, but these agents were less effective in comparison to sarpogrelate. On the other hand, the increase in [Ca2+]iin RASMCs by ATP, angiotensin II, endothelin-1, or phorbol ester was not affected by sarpogrelate. These results indicate that Ca2+mobilization in RASMCs by 5-HT is mediated through the activation of 5-HT2Areceptors and support the view that the 5-HT-induced increase in [Ca2+]iinvolves both the extracellular and intracellular sources of Ca2+.Key words: sarpogrelate, serotonin, vascular smooth muscle cells, intracellular Ca2+.

Effects of thoracic epidural analgesia on coronary hemodynamics and myocardial metabolism in coronary artery bypass surgery

OBJECTIVE

A possible influence of thoracic epidural analgesia on coronary hemodynamics and myocardial metabolism in coronary artery bypass grafting was investigated.

DESIGN

The study was prospective and randomized.

SETTING

The study was performed in a university hospital.

PARTICIPANTS

Thirty male patients less than 65 years of age and with ejection fraction greater than 0.5 participated. They were randomized into 3 groups: the high fentanyl (HF) group receiving high-dose fentanyl (55 micrograms/kg) anesthesia, the HF + thoracic epidural analgesia (TEA) group receiving the same general anesthesia plus thoracic epidural analgesia, and the low-fentanyl (LF) + TEA group receiving low-dose fentanyl (15 micrograms/kg) anesthesia plus thoracic epidural analgesia.

INTERVENTIONS

A thoracic epidural catheter, a peripheral and central venous catheter, a radial artery catheter, a thermodilution pulmonary artery catheter, and a coronary sinus reverse thermodilution catheter were inserted.

MEASUREMENTS AND MAIN RESULTS

Coronary circulatory parameters, myocardial oxygenation, and myocardial substrate utilization were investigated before bypass and for 9 hours after bypass. Before bypass, the most striking finding was a reduction in myocardial lactate extraction in all groups, but also coronary flow and myocardial oxygen consumption decreased compared with baseline. After bypass, the only significant finding was a lower coronary vascular resistance early postoperatively in the epidural groups, but coronary blood flow was adequate in all groups. Myocardial metabolism was essentially unchanged both with and without epidural analgesia after bypass.

CONCLUSION

With regard to the coronary circulation and myocardial metabolism, no hard data supporting the use of thoracic epidural analgesia in coronary artery bypass grafting were found.

Repeated episodes of myocardial ischaemia during combined thoracic epidural-isoflurane anaesthesia

A patient with a history of coronary artery diseases developed new ST segment depressions in the ECG registration during a low dose (0.7%) isoflurane anaesthesia that was combined with a continuous thoracic epidural analgesia. Simultaneously a small decrease in mean arterial blood pressure (MAP) was noted. During the next 5 min these changes were followed by a severe drop in MAP (from 88 to 60 mmHg) and in cardiac output from 5.5 to 3.2 L/min. When isoflurane was discontinued both the ECG and the cardiovascular changes returned to the previous condition. Later, when 0.5% isoflurane was restarted, the ECG changes reappeared within ten min, but disappeared once again when isoflurane administration was discontinued. Thus, this patient had repeated episodes of myocardial ischaemia which were associated with the use of low dose isoflurane. Although isoflurane-induced "coronary steal" may appear as a likely cause of these ischaemic episodes, it is possible that the thoracic epidural had synergistic action and rendered the patient exceptionally sensitive to minor changes in perfusion pressure.

Possible involvement of ATP-sensitive K+ channels in the inhibition of rat central adrenergic neurotransmission under hypoxia

By using rat brain cortical slices preloaded with [3H]norepinephrine, we examined whether ATP-sensitive K+ channels are involved in altered adrenergic neurotransmission during hypoxia. The tritium overflow evoked by transmural nerve stimulation (TNS) was significantly inhibited at 5 min of hypoxia and reached the maximum inhibition at 20 min. The inhibition of the TNS-evoked tritium overflow under a 20-min hypoxia was reversed by subsequent reoxygenation and was concentration-dependently antagonized by glibenclamide (0.1 and 1 microM). 86Rb+ efflux was increased after introduction of hypoxia and reached the peak value at about 20 min, which was concentration-dependently antagonized by glibenclamide (0.1-10 microM). Hypoxia decreased cortical ATP content. Linear correlations were mutually observed among the changes by hypoxia in the TNS-evoked tritium overflow, tissue ATP content and 86Rb+ efflux. The spontaneous tritium outflow was inhibited only after hypoxic periods of more than 16 min, the inhibition being reversed by reoxygenation and antagonized by 1 microM glibenclamide. These results suggest that the inhibition of rat central adrenergic neurotransmission during hypoxia may be associated with an activation of ATP-sensitive K+ channels.

Effect of intravenous propranolol on coronary vasomotion at rest and during dynamic exercise in patients with coronary artery disease

Coronary vasomotion was studied at rest and during bicycle exercise with biplane quantitative coronary arteriography in 28 patients with coronary artery disease. Patients were divided into two groups; the first 18 patients served as controls (group 1), and the next 10 patients were treated with propranolol 0.1 mg/kg, which was infused intravenously before exercise (group 2). Luminal area of a normal and a stenotic vessel segment was determined at rest, during supine bicycle exercise, and 5 minutes after sublingual administration of 1.6 mg nitroglycerin after exercise. In group 1, the normal vessel showed vasodilation (+16%, p less than 0.001) during exercise, whereas the stenotic vessel segment showed vasoconstriction (-31%, p less than 0.001). After sublingual administration of nitroglycerin, there was coronary vasodilation of both normal (+36%, p less than 0.001 vs. rest) and stenotic (+20%, p less than 0.001) vessel segments. Patients with angina pectoris during supine exercise (n = 10) had significantly (p less than 0.05) more vasoconstriction (-36%) than patients without angina (-23%). In group 2, intravenous administration of propranolol at rest was associated with a decrease in luminal area of both normal (-24%, p less than 0.001) and stenotic (-43%, p less than 0.001) vessel segments; however, during subsequent exercise, both normal (-2%, p = NS vs. rest) and stenotic (-3%, p = NS vs. rest) vessel segments dilated when compared with the measurements after propranolol. Administration of nitroglycerin further increased luminal area of both vessel segments (normal segment, +23%, p less than 0.001; stenotic segment, +46%, p less than 0.001 vs. rest). It is concluded that dynamic exercise in patients with coronary artery disease is associated with coronary vasodilation of the normal and vasoconstriction of the stenotic coronary arteries. Patients with exercise-induced angina had significantly more stenosis vasoconstriction than patients without angina although minimal luminal area at rest was similar. Intravenous administration of propranolol is accompanied by a significant decrease in coronary luminal area of both normal and stenotic vessel segments at rest, which is overridden by dynamic exercise and sublingual nitroglycerin. The reduction in myocardial oxygen consumption and the prevention of exercise-induced stenosis vasoconstriction might explain the beneficial effect of beta-blocker treatment in most patients with coronary artery disease.

Importance of magnesium and potassium concentration on basal tone and 5-HT-induced contractions in canine isolated coronary artery

1. In vitro studies were undertaken to investigate the effects of external potassium ([K+]o) and magnesium ([Mg2+]o) concentration on canine coronary arterial basal tone and on 5-hydroxytryptamine (5-HT)-induced contractions. 2. Acute withdrawal of, or reduction in, [K+]o produced relaxation of basal tone in isolated coronary arteries, whereas acute withdrawal (but not reduction) of [Mg2+]o produced contraction of these blood vessels. 3. The magnitude of coronary contraction obtained on withdrawal of [Mg2+]o was dependent upon the [K+]o; the higher the [K+]o, the greater the contraction. 4. The precise ratio of [K+]o/[Mg2+]o appeared to be important in dictating the degree of contraction (maximum response) and sensitivity (EC50) of canine coronary vascular smooth muscle cells to 5-HT. The EC50 to 5-HT was enhanced by increases in the [K+]o/[Mg2+]o ratio, whereas the ability of 5-HT to induce a maximal contraction was attenuated by decreases in the [K+]o; the latter being modulated by [Mg2+]o. Small changes in [Mg2+]o could effect large changes in the EC50 as [K+]o was lowered. 5. These actions took place over patho-physiological ranges of [K+]o and [Mg2+]o. 6. Maintenance of a constant [K+]o/[Mg2+]o ratio, irrespective of the exact [K+]o and [Mg2+]o, produced similar degrees of maximum tension. 7. Use of intact vascular ring preparations and helically-cut vascular strips produced similar results with varying [K+]o/[Mg2+]o. 8. A variety of pharmacological receptor antagonists (phentolamine, propranolol, atropine, diphenhydramine, cimetidine), as well as a prostaglandin cyclo-oxygenase inhibitor, did not modify the altered contractile responses or basal tone evoked by varying [K+]o/[Mg2+]o ratios. 9. These results suggest: (1) that basal tone and contractility of canine coronary vascular smooth muscle cells appear to be exquisitely sensitive to alterations in extracellular K+ and Mg2+; and (2) 5-HT receptor-operated Ca2+ channels, as well as those Ca2+ channels involved in generation of coronary arterial basal tone are modulated and controlled by the precise concentrations of [K+]o and [Mg2+]o.

Is there coronary vasoconstriction after intracoronary beta-adrenergic blockade in patients with coronary artery disease

Vasomotility of normal and stenosed coronary arteries was studied at rest and during supine bicycle exercise in 10 patients with classical exercise-induced angina pectoris receiving 1 mg intracoronary propranolol before the exercise test (propranolol group). Normal and stenotic coronary lumen areas were determined from biplane coronary arteriograms using a computer-assisted system. Measurements were performed at rest, after 1 mg intracoronary propranolol, during supine exercise (89 W for 3.4 minutes) and 5 minutes after 1.6 mg sublingual nitroglycerin administered at the end of the exercise test. The results were compared with previously obtained data on the effect of dynamic exercise on coronary lumen area in 12 patients receiving no medication (control group) and in 6 patients receiving 0.1 mg intracoronary nitroglycerin before the exercise test (nitroglycerin group). In the control group, coronary stenosis area decreased during exercise to 71% of levels at rest (p less than 0.001) whereas normal coronary lumen area increased to 123% of control (p less than 0.01). In the propranolol group both normal (113%, p less than 0.05 versus rest) and stenotic coronary lumen area (122%, p less than 0.05 versus rest) increased during exercise. A similar increase in both normal and stenotic areas was observed during exercise after pretreatment with 0.1 mg intracoronary nitroglycerin (123%, p less than 0.01 and 114%, p = NS versus rest). Sublingual administration of 1.6 mg nitroglycerin at the end of exercise increased coronary stenosis area to 145% (p less than 0.01 versus rest) in the propranolol group and to 115% in the control group (p = NS versus rest). It is concluded that intracoronary administration of propranolol does not potentiate coronary vasoconstriction of the epicardial vessels at rest and during exercise. In contrast, intracoronary propranolol prevents exercise-induced stenosis narrowing either because of reduced myocardial oxygen demand with a lower coronary blood flow resulting in a smaller transstenotic pressure gradient and, thus, a smaller flow-induced fall in stenosis distending pressure; or because of "local" beta-receptor blockade with unopposed distal arteriolar alpha-receptor tone, resulting in a higher poststenotic pressure and, thus, in a greater stenosis distending pressure; or because of a local anesthetic effect of propranolol with a decrease in calcium influx to the coronary smooth musculature.

Regulation of large coronary arteries

The majority of studies on the control of coronary artery vasoactivity have examined changes in coronary blood flow and coronary vascular resistance, indices that primarily reflect regulation of small arterioles and precapillary vessels. With the emergence of coronary artery vasospasm as a significant cause of angina pectoris, myocardial infarction, and sudden death, the control of large coronary artery caliber has assumed more significance. It is clear that resistance coronary vessels and large coronary arteries differ in response to both pharmacologic and physiologic stimuli. Vasodilation of large coronary arteries may occur by direct action of agents on the arterial smooth muscle or by the indirect action of receptor occupation, changes in blood flow, or liberation of endothelial factors. These indirect factors appear to contribute also to responses to agents that constrict coronary smooth muscle directly or through the autonomic nervous system. Furthermore, the mechanisms responsible for control of large coronary vessels in the normal circulation are likely to be profoundly different from those in the presence of diseased vessels. For example, several factors associated with coronary artery disease--elevated plasma cholesterol levels, endothelial disruption, atherosclerosis, vascular stenosis, and aggregated platelets--all have important actions on the control of large coronary arteries.

Responses of isolated and perfused dog coronary arteries to acetylcholine, norepinephrine, KCl, and diltiazem before and after removal of the endothelial cells by saponin

The vascular responses to acetylcholine (ACh), norepinephrine (NE), KCl, and diltiazem were examined before and after removal of endothelial cells by an intraluminal bolus injection of saponin (1 mg) in isolated and perfused dog coronary arteries. Without any precontraction, ACh induced a long-lasting vasodilation in small doses (less than 1 microgram), and an initial brief vasoconstriction was occasionally accompanied in large doses. These vascular responses to ACh were not significantly affected by the pretreatment with propranolol (5 X 10(-6) mol/l). The endothelial removal by intraluminal saponin was confirmed electron microscopically. After 20-60 min of saponin treatment, the responses to drugs were observed and compared with the control. The ACh-induced vasodilation was significantly attenuated by saponin (P less than 0.01), but the ACh-induced vasoconstriction was not affected by it. The vasodilation was blocked by atropine. The NE- and KCl-induced vasoconstrictions and diltiazem-induced vasodilation were not affected by saponin treatment. It is suggested that: (1) ACh produced a vasodilation in the nonpreconstricted condition of dog coronary arteries; (2) the vasodilation caused by ACh is mostly endothelium-dependent, which is considered to be mediated by muscarinic receptors; and (3) the vascular responses to NE, KCl, and diltiazem and the vasoconstriction produced by ACh are not influenced by removal of the endothelium in a relatively large epicardial coronary artery of the dog.

The mechanisms of action of cyanide on the rabbit aorta

Pretreatment of strips of rabbit aorta with 10(-3) M sodium cyanide reduced contractions to 10(-8) through 10(-4) M norepinephrine (NE) added cumulatively. This antagonism by cyanide was not altered by 4 X 10(-6) M ouabain or verapamil, suggesting a lack of involvement of Na+, K+ ATPase or of calcium influx in the antagonism. Cyanide potentiated contractions caused by 3 X 10(-2) M potassium, but reduced contractions induced by higher potassium concentrations. Because the antagonism of higher concentrations of potassium and NE were similar, it seems that selective actions on different calcium pools are possibly not involved in the antagonism of agonist-induced contractions. 10(-2) M cyanide contracted rabbit aorta with a mean contraction 16% of that induced by 10(-4) M NE. These contractions were potentiated by pretreatment with 4 X 10(-6) M ouabain and 4 X 10(-6) M verapamil but were unaffected by the serotonin antagonist 2-bromo lysergic acid diethylamide, 10(-4) M (2-BrLSD), the alpha adrenergic antagonist phentolamine, 4 X 10(-5) M, the H1 antihistaminic pyrilamine, 10(-5) M, or the antimuscarinic atropine, 10(-6) M. The contractions were reduced by 10(-4) M 4,4'-di-isothiocyano-2,2'-stilbene disulfonic acid (DIDS) or chlorpromazine. The reduction may be due to a blockade of anionic channel mechanisms facilitating entry of cyanide into the vascular smooth muscle cell, as both of these agents can block anionic channels in other tissues.