Methacholine provocation of Prinzmetal's variant angina pectoris: a revised perspective

Alpha(1L)-, but not alpha(1H)-, adrenoceptor antagonist prevents allergic bronchoconstriction in guinea pigs in vivo

alpha-Adrenoceptors have been classified into alpha(1)- and alpha(2)-adrenoceptors. Recently, the alpha(1)-adrenoceptors were divided into two subtypes: alpha(1L) with low affinity and alpha(1H) with high affinity for prazosin. Little is known concerning the role of each subtype of alpha(1)-adrenoceptor in asthma. We investigated the effects of specific antagonists of alpha(1)- and alpha(2)-, alpha(1H)-, alpha(1L)-, and alpha(2)-adrenoceptors, namely moxisylyte, prazosin, 3-[N-[2-(4-hydroxy-2-isopropyl-5-methylphenoxy) ethyl]-N-methylaminomethyl]-4-methoxy-2, 5, 6-trimethylphenol hemifumarate (JTH-601), and yohimbine, respectively, on antigen-induced airway reactions in guinea pigs. Fifteen minutes after intravenous administration of moxisylyte (0.01, 0.1 or 1 mg/kg), prazosin (0.01, 0.1, 1 or 10 mg/kg), JTH-601 (1, 3, 6 or 10 mg/kg) or yohimbine (0.1 or 1 mg/kg), passively sensitized and artificially ventilated animals received an aerosolized antigen challenge. Bronchial responsiveness to inhaled methacholine was assessed as the dose of methacholine required to produce a 200% increase in the pressure at the airway opening (PC(200)) in non-sensitized animals. JTH-601 and moxisylyte, but not prazosin or yohimbine, dose dependently inhibited antigen-induced bronchoconstriction. None of the tested drugs altered PC(200). JTH-601 significantly reduced leukotriene C(4) levels in bronchoalveolar lavage fluid obtained 5 min after antigen challenge, but prazosin did not. These results indicate that prevention of antigen-induced bronchoconstriction by blockade of alpha-adrenoceptors is due to the inhibition of mediator release via alpha(1L)-adrenoceptor antagonism.

Neurally mediated negative inotropic effect impairs myocardial function during cholinergic coronary vasoconstriction in pigs

BACKGROUND

Myocardial dysfunction elicited during cholinergic coronary vasospasm is generally ascribed to myocardial ischemia. However, when pericoronary nerves extend to the ventricles, efferent vagal discharges that elicit coronary vasoconstriction may, at the same time, depress myocardial contractility by releasing acetylcholine into the myocardium. We analyzed whether a neurally mediated effect contributes to impairment of ventricular function during cholinergic coronary vasoconstriction.

METHODS AND RESULTS

Left ventricular (LV) function, ECG mapping, and coronary angiography were analyzed during pericoronary application of methacholine (MCh) to the left anteriors. descending coronary artery in 53 chloralose-anesthetized openchest pigs. MCh induced local coronary vasoconstriction and depressed coronary blood flow (13.3 +/- 3.7 to 6.0 +/- 4.6 mL/min [54%]; ANOVA, P < .001), systolic LV pressure (105 +/- 10 to 91 +/- 10 mm Hg [13%], P < .001), peak LV (+)dP/dt (2890 +/- 524 to 2270 +/- 447 mm Hg/s [21%], P < .001), peak LV (-)dP/dt (1446 +/- 484 to 1048 +/- 276 mm Hg/s [27%], P < .001), and regional systolic segment shortening (0.32 +/- 0.09% to 0.18 +/- 0.14% [43%], P = .02) and caused local ST-segment elevation (0.9 +/- 0.8 to 8.4 +/- 3.9 mV, P < .001). These changes were not preceded by heart rate variations, were reproducible, and were inhibited by atropine. MCh during perfusion of nitroglycerin (2 micrograms.kg-1.min-1 IV) continued to depress LV pressure (9%, P = .002), LV (+)dP/dt (16%, P < .01), LV (-)dP/dt (20%, P < .01), and segment shortening (18%, P = .03) even though coronary blood flow drop was markedly attenuated (7% versus 54%; ANOVA, P < .01). Disruption of pericoronary nerves with phenol attenuated MCh-induced LV pressure fall (P < .05). Histological data show that cholinergic and adrenergic pericoronary nerves extend into the ventricular myocardium.

CONCLUSIONS

A neurally mediated effect, in addition to ischemia, impairs LV function during cholinergic coronary constriction in a model with pericoronary nerves extending into the ventricles.

Acetylcholine-induced constriction of angiographically normal coronary arteries is not time dependent in transplant recipients. Effects of stepwise infusion at 1, 6, 12 and more than 24 months after transplantation

OBJECTIVES

The aim of this study was to evaluate whether acetylcholine may be a useful tool for detection of early angiographically undetectable coronary atherosclerosis in heart transplant recipients.

BACKGROUND

Coronary artery disease is the main determinant of long-term prognosis in transplant recipients. Acetylcholine-induced constriction of angiographically normal coronary arteries in heart transplant recipients could be due to early atherosclerosis, and acetylcholine has been proposed for early detection of coronary artery disease.

METHODS

The responses of large coronary arteries to stepwise intracoronary infusion of acetylcholine (10(-8) to 10(-5) mol/liter) were compared in five control subjects and in four groups of transplant recipients 1, 6, 12 and > 24 months postoperatively (group 1, n = 6; group 2, n = 7; group 3, n = 6; group 4, n = 6, respectively). All patients had normal coronary arteriographic findings. Vessel dimensions were measured in four segments in each patient.

RESULTS

In control subjects, acetylcholine increased diameters significantly at 10(-8), 10(-7) and 10(-6) mol/liter (all p < 0.001 vs. basal value). No significant variation was observed at 10(-5) mol/liter. Intracoronary isosorbide dinitrate increased diameters of all segments (p < 0.001). In transplant recipients, vessel diameters did not vary significantly from baseline at 10(-8) and 10(-7) mol/liter concentrations in groups 1 and 3 and at 10(-8) mol/liter in group 4. Vessels constricted significantly in all the other cases. Comparisons of each group with control subjects showed that responses were significantly different for all concentrations but 10(-8) mol/liter in groups 3 and 4. Intracoronary isosorbide dinitrate elicited coronary vasodilation similar to that of control subjects in all groups of transplant recipients.

CONCLUSIONS

This study indicates that the acetylcholine response is persistently abnormal in transplant recipients compared with that in normal control subjects and that this abnormality may not be related simply to the presence of atherosclerosis. Thus, acetylcholine may not be a useful tool for early detection of coronary atherosclerosis in heart transplant recipients.

Cholinergic constriction in the general circulation and its role in coronary artery spasm

The release of acetylcholine from autonomic nerves in those tissues that receive a cholinergic innervation is widely believed to dilate blood vessels. Exogenously administered acetylcholine in vivo does dilate vascular beds and produce hypotension; however, this latter effect is indirect and probably the result of liberation of endothelium-derived relaxing factor (EDRF) from endothelial cells. Some blood vessels contain a substantial population of medial constrictor receptors for acetylcholine, and the implications of this presence for vascular control systems has been largely ignored, although it needs to be considered. A survey of the evolution of vasomotor control systems indicates that acetylcholine serves principally as an excitatory transmitter to blood vessels. Neurally mediated cholinergic constriction and not dilation is found in fish, amphibians, reptiles, and birds, with responses initiated by medial muscarinic receptors. Acetylcholine constricts many vascular preparations from these lower animals, but some vessels relax, reflecting the emergence of an EDRF responsive to acetylcholine. An examination of cholinergic responses in mammalian vessels reveals that cholinergic (neurogenic) dilation is limited to a very few vascular beds and to only a few species. Both experimental evidence and evolutionary considerations support the likelihood that cholinergic (neural) constriction operates in some vascular regions in mammals and, in particular, in the coronary circulation of some species, including humans. In fact, constriction, and not dilation, may be the dominant vascular response to activation of the cholinergic axis in most mammals, including humans. The complications and contradictions introduced by the simultaneous presence of both EDRF and a cholinergic constrictor innervation involving medial muscarinic receptors are discussed. A variety of evidence is also presented that implicates cholinergic constriction in at least some instances of coronary artery spasm and sudden death.

Evidence of endothelial dysfunction in angiographically normal coronary arteries of patients with coronary artery disease

Acetylcholine causes endothelium-dependent dilation of normal arteries in most animal species. The effect of acetylcholine on normal human coronary arteries is controversial. Pathologic studies and epicardial echocardiography have shown that diffuse atherosclerosis is often present despite angiographic evidence of discrete coronary artery disease (CAD). Therefore, we postulated that acetylcholine would cause vasoconstriction of coronary arteries that are angiographically normal in patients with CAD. Coronary artery diameter, measured by automated quantification of digitized cineangiograms, was determined before and after the intracoronary infusion of 0.2 mM acetylcholine at 0.8-1.6 ml/min. The diameter of stenotic or irregular segments of six atherosclerotic coronary arteries decreased from 1.80 +/- 0.42 mm before acetylcholine to 1.26 +/- 0.46 mm after acetylcholine (p = 0.0025). Acetylcholine had a significantly different effect on the diameter of two groups of coronary arteries that are angiographically normal. Acetylcholine caused a 0.16 +/- 0.09-mm increase in the diameter of 14 normal coronary arteries in patients without CAD, whereas it caused a 0.26 +/- 0.12-mm decrease in the diameter of 14 normal coronary arteries in patients with CAD (p less than 0.01). Thus, the normal response to intracoronary acetylcholine is vasodilation, suggesting that endothelium-derived relaxing factor is released from normal human coronary endothelium. The vasoconstrictive effect of acetylcholine in the angiographically normal coronary arteries of patients with CAD suggests the presence of a diffuse abnormality of endothelial function.

Paradoxical vasoconstriction induced by acetylcholine in atherosclerotic coronary arteries

Acetylcholine is believed to dilate normal blood vessels by promoting the release of a vasorelaxant substance from the endothelium (endothelium-derived relaxing factor). By contrast, if the endothelium is removed experimentally, acetylcholine constricts blood vessels. We tested the hypothesis that muscarinic cholinergic vasodilation is impaired in coronary atherosclerosis. Graded concentrations of acetylcholine and, for comparison, the nonendothelial-dependent vasodilator nitroglycerin were infused into the left anterior descending artery of eight patients with advanced coronary stenoses (greater than 50 percent narrowing), four subjects with angiographically normal coronary arteries, and six patients with mild coronary atherosclerosis (less than 20 percent narrowing). Vascular responses were evaluated by quantitative angiography. In several segments each of four normal coronary arteries, acetylcholine caused a dose-dependent dilation from a control diameter of 1.94 +/- 0.16 mm to 2.16 +/- 0.15 mm with the maximal acetylcholine dose (P less than 0.01). In contrast, all eight of the arteries with advanced stenoses showed dose-dependent constriction, from 1.05 +/- 0.05 to 0.32 +/- 0.16 mm at the highest concentration of acetylcholine (P less than 0.01), with temporary occlusion in five. Five of six vessels with minimal disease also constricted in response to acetylcholine. All vessels dilated in response to nitroglycerin, however. We conclude that paradoxical vasoconstriction induced by acetylcholine occurs early as well as late in the course of coronary atherosclerosis. Our preliminary findings suggest that the abnormal vascular response to acetylcholine may represent a defect in endothelial vasodilator function, and may be important in the pathogenesis of coronary vasospasm.

Mechanisms of methacholine-induced coronary vasospasm in an experimental model of variant angina in the anesthetized rat

Methacholine (Mch), when injected near the ostia of the coronary arteries, induces an intense coronary vasospasm which can be measured by the degree of S wave elevation monitored from an electrocardiogram (ECG). The observed ECG changes resemble those occurring in patients with variant angina. The effects of Mch were blocked by atropine, but not by d-tubocurarine, hexamethonium, adrenergic receptor blockade, or prior reserpinization, indicating that Mch is acting directly on muscarinic receptors to produce a vasoconstriction of the coronary arteries. This model of Mch-induced coronary vasospasm appears to be useful for testing spasmolytic agents which might be of benefit in variant angina.