Reflex inhibition of efferent renal sympathetic nerve activity by 5-hydroxytryptamine and nicotine is elicited by different epicardial receptors

Intrapericardial administration of 5-hydroxytryptamine (5-HT) induced reflex effects consisting in an inhibition of renal sympathetic nerve activity (RSNA), bradycardia and a fall in blood pressure. Nicotine caused the same reflex effects as 5-HT. The reflex effects of both 5-HT and nicotine were abolished by vagotomy. MDL 72222, an antagonist at 5-HT M-receptors, abolished or attenuated the decreases in RSNA, heart rate and blood pressure induced by 5-HT, leaving the reflex effects of nicotine unchanged. In the absence of MDL 72222 the reflex bradycardia partially concealed a positive chronotropic response to 5-HT. After blockade of the bradycardia response by MDL 72222, 5-HT elicited a significant tachycardia, which was not altered by propranolol and phentolamine, but was prevented by phenoxybenzamine. 5-HT probably reaches the sinoatrial node and activates 5-HT receptors that mediate directly the increase in heart rate. The nicotine receptor antagonist hexamethonium selectively abolished or attenuated the reflex effects of nicotine without interfering with those of 5-HT. We conclude that 5-HT and nicotine elicit similar reflex effects in epicardial vagal nerve endings by stimulation of M-receptors or nicotine receptors, respectively.

Adenosine, dipyridamole and isosorbide dinitrate are ineffective to prevent the sympathetic initiation of poststenotic myocardial ischemia

An activation of cardiac sympathetic nerves increases coronary vascular resistance distal to severe stenoses and induces ischemia of the dependent myocardium. The selective alpha 2-adrenoceptor antagonist rauwolscine and the calcium antagonist nifedipine prevent both poststenotic vasoconstriction and ischemia. To exclude the possibility that the beneficial action of nifedipine is based on unspecific coronary dilation rather than a functional antagonism against alpha 2-adrenoceptor mediated poststenotic vasoconstriction we now tested coronary dilatory drugs with a different underlying mechanism. The left ventrolateral cervical cardiac sympathetic nerve was stimulated in 12 anesthetized, vagotomized dogs. A severe stenosis of left circumflex coronary artery was defined by the absence of a postocclusive reactive hyperemia. Sympathetic stimulation increased end-diastolic poststenotic resistance from 0.45 +/- 0.10 to 0.83 +/- 0.18 mmHg X min X 100 g/ml and induced a net lactate production of the poststenotic myocardium. Adenosine (50 micrograms/kg X min i.c., n = 5), dipyridamole (0.2 mg/kg i.v., n = 3) and isosorbide-dinitrate (1 mg i.c., n = 4) did not prevent the increase in resistance and the net lactate production. Thus the effectiveness to prevent alpha 2-adrenergic poststenotic coronary constriction appears to be specific for alpha 2-antagonists and calcium antagonists.

Sympathetic mechanisms in poststenotic myocardial ischemia

A coronary stenosis can be compensated by dilation of the poststenotic vascular bed. Activation of cardiac sympathetic nerves can disturb this balance. Experiments in anesthetized, vagotomized dogs revealed that electrical stimulation of cardiac sympathetic nerves as well as reflex sympathetic activation by bilateral carotid occlusion induce vasoconstriction distal to severe coronary stenoses, which were defined by the lack of any postocclusive reactive hyperemia. The poststenotic vasoconstriction is mediated by alpha 2-adrenoceptors and, in concert with the beta-adrenoceptor mediated increase in myocardial oxygen demand, it induces an acute ischemia of the poststenotic myocardium. This ischemia was evidenced by contractile dysfunction, net lactate production, the occurrence of ventricular fibrillation in 10% of dogs, and the development of subendocardial necroses after repetitive sympathetic stimulation. The alpha-antagonist phentolamine, the selective alpha 2-antagonist rauwolscine, and the calcium antagonist nifedipine prevent both poststenotic vasoconstriction and ischemia, whereas the beta-antagonist propranolol enhances even poststenotic vasoconstriction, but still prevents ischemia. Poststenotic ischemia can thus result from sympathetic activation but, in turn, also activates cardiac sympathetic nerves by a spinal reflex. This positive feedback provides a basis for a progressive poststenotic vasoconstriction and aggravation of myocardial ischemia. This vicious cycle can be interrupted by rauwolscine and nifedipine, as well as by segmental epidural anesthesia with procaine at segments C6-T7. The poststenotic ischemia induced by sympathoexcitatory reflexes can also be prevented by blocking the sympathoexcitation at the central nervous level by clonidine. Our studies emphasize the role of cardiac sympathetic nerves and alpha 2-mediated poststenotic vasoconstriction in the genesis and aggravation of poststenotic myocardial ischemia.

Evaluation of acute stress-induced regional myocardial ischemia: comparison of regional contractile and metabolic indices to easily accessible global left ventricular indices

In clinical studies stress-induced myocardial ischemia in patients with coronary artery disease is used to evaluate the severity of this disease. The discussion on the importance of some parameters measured during this intervention is controversial, other parameters are difficult to obtain. On the basis of an experimental model of stress-induced myocardial ischemia, we tried to find an index that reflects best this cardiac state. We therefore compared in eight anesthetized open-chest dogs control conditions with three other hemodynamic states with increasing imbalance between myocardial oxygen demand and oxygen supply: severe stenosis on circumflex coronary artery, 60 s cardiac sympathetic nerve stimulation (CSNS) with a severe stenosis on circumflex coronary artery, and 60 s complete occlusion of circumflex coronary artery. Using a one-way analysis of variance, we found two significantly changed parameters during the stress-induced ischemia: Regional lactate extraction was turned to production (32 +/- 4 vs. -4 +/- 1%) and the ratio of dP/dtmin to dP/dtmax was decreased (1.13 +/- 0.05 vs. 0.67 +/- 0.05; control vs. CSNS with a severe stenosis on circumflex coronary artery). We conclude that the ratio of dP/dtmin to dP/dtmax is an easily accessible, sensitive, and dynamic index for characterization of an stress-induced myocardial ischemia.

Alpha 2-adrenoceptor-mediated coronary vasoconstriction persists after exhaustion of coronary dilator reserve

The effect of intracoronary administration of the selective alpha 2-adrenoceptor agonist B-HT 920 on coronary resistance was tested in 14 open-chest dogs before and after exhaustion of coronary dilator reserve. Under both conditions B-HT 920 increased coronary resistance to the same extent: 27 +/- 4% before and 32 +/- 3% after. Administration of B-HT 920 did not affect cardiovascular haemodynamics significantly, although after exhaustion of coronary dilator reserve left ventricular dP/dtmax decreased slightly by 12 +/- 5%. While efferent cardiac sympathetic discharge remained constant during control conditions it increased by 27 +/- 7% when B-HT 920 was administered into a stenotic coronary artery. We conclude that, in contrast to observations made on alpha 1-adrenoceptors, activation of coronary alpha 2-adrenoceptors may have deleterious effects after exhaustion of coronary dilator reserve because these receptors mediate a comparable constriction in well- and under-perfused coronary vessels. Thus they are unlikely to favour a redistribution of coronary blood flow into under-perfused myocardial regions. In addition, this unfavourable effect of B-HT 920 on coronary perfusion after coronary dilator reserve has been exhausted may be further exacerbated by an increase in efferent sympathetic discharge.

Cardiac sympathetic nerve activity and progressive vasoconstriction distal to coronary stenoses: feed-back aggravation of myocardial ischemia

This study tested the hypothesis that the relative ischemia distal to a severe coronary stenosis increases the activity of cardiac sympathetic nerves which in turn results in poststenotic vasoconstriction and an aggravation of ischemia. An acute severe stenosis which reduced coronary blood flow to 50% of control was produced in 23 anesthetized, vagotomized dogs and maintained for 20 min. The activity of postganglionic cardiac sympathetic nerves increased by 23 +/- 4% within 20 min. In parallel, poststenotic coronary resistance increased from 0.48 +/- 0.03 to 0.61 +/- 0.03 mm Hg X min X 100 g/ml resulting in a net lactate production after 15 min. Pretreatment with aspirin (6 mg/kg i.v.; n = 5) was without any influence on these reactions. The selective alpha 2-adrenoceptor antagonist rauwolscine (0.2 mg/kg i.v.; n = 6) and the calcium antagonist nifedipine (10 micrograms/kg i.v.; n = 6) prevented the progressive increase in poststenotic resistance and lactate production, but still permitted an increase in sympathetic activity. Segmental anesthesia of cardiac sympathetic nerves by epidural infiltration of procaine at segments C7-T6 (n = 6) prevented the sympathetic activation, the progressive increase in poststenotic resistance and the resulting myocardial ischemia. Sympathetic activation and a concomitant increase in poststenotic resistance resulting in myocardial ischemia were also found in 6 dogs with intact vagus nerves. These data support the hypothesis of a vicious cycle between poststenotic coronary vasoconstriction and sympathetic activation resulting in severe myocardial ischemia.

Alpha 1- and alpha 2-adrenoceptor-mediated vasoconstriction of large and small canine coronary arteries in vivo

The role of alpha-adrenoceptor subtypes mediating vasoconstriction of large epicardial and small resistive coronary arteries was investigated in 26 open-chest dogs. The left circumflex coronary artery was perfused at a constant pressure. Large vessel vasomotion was determined by measurement of circumflex coronary arterial diameter (ultrasonic transit-time technique); the vasomotion of the small resistive coronary arteries was determined from calculated end-diastolic circumflex artery resistance. beta-Receptors were blocked by propranolol (2 mg/kg i.v.) and both vagal nerves were cut. In eight dogs, the intracoronary administration of the alpha 1-adrenoceptor agonist methoxamine (500 micrograms) increased calculated large vessel resistance by 18.8 +/- 5.7% and end-diastolic resistance by 9.5 +/- 0.3%. Intracoronary administration of the alpha 2-adrenoceptor agonist BHT 920 (500 micrograms) did not affect large vessel resistance, but increased end-diastolic resistance by 37.5 +/- 5.6%. In an additional 18 dogs, left cardiac sympathetic nerve stimulation induced an increase in large vessel resistance by 11.1 +/- 0.9% and in end-diastolic resistance by 31.8 +/- 3.0%. The increase in large vessel resistance was prevented by the alpha 1-adrenoceptor antagonist prazosin (1.2 mg/kg i.v.), which still permitted an increase in end-diastolic resistance by 23.0 +/- 3.2%. The increase in end-diastolic resistance was prevented by the alpha 2-adrenoceptor antagonist rauwolscine (0.2 mg/kg i.v.), which still permitted an increase in large vessel resistance by 11.9 +/- 2.4%. The calcium antagonist nifedipine (20 micrograms/kg i.v.) prevented both the increase in large vessel resistance and end-diastolic resistance during sympathetic stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

[Significance of the sympathetic nervous system for the coronary circulation]

The activation of sympathetic nerves plays a significant role in the initiation of acute myocardial ischemia. In this review the effects of sympathetic nerves on coronary blood flow are summarized. Under physiological conditions the increase in myocardial performance during sympathetic activation is accompanied by metabolic coronary vasodilation. The resulting increase in coronary blood flow is limited by about 30% by alpha-adrenergic coronary constriction. Extravascular compression and beta-adrenergic coronary dilation are of minor importance during sympathetic activation. With exhausted coronary reserve distal to severe coronary stenoses metabolic vasodilation during sympathetic activation is not possible any more; an alpha 2-adrenoceptor-mediated coronary vasoconstriction now predominates. This coronary constriction induces ischemia of the post-stenotic myocardium. The alpha-antagonists Phentolamine and Rauwolscine as well as the calcium-antagonist Nifedipine prevent poststenotic coronary constriction and myocardial ischemia. Also in the genesis of coronary arterial spasm the sympathetic nervous system may play an important role; the constriction of epicardial coronary arteries is mediated by alpha 1-adrenoceptors. The sympathetic nervous system is not only involved in the initiation of myocardial ischemia, but is also activated by ischemia. This feedback-activation of sympathetic nerves leads to an aggravation of myocardial ischemia.

Vagal and sympathetic reflexes of left ventricular origin on the efferent activity of cardiac and renal nerves on anaesthetized cats

The influence of reflexes of left ventricular origin on the postganglionic sympathetic activity and heart rate was investigated in anaesthetized cats. The experiments were to clarify 1. whether there are regionally different reflex adjustments due to an activation of ventricular receptors, 2. whether an increase of left ventricular diastolic pressure, which is known to activate afferent vagal fibres, causes an inhibition of sympathetic activity, 3. whether a coronary artery occlusion can activate a pressor reflex and a depressor reflex. Left ventricular receptors were stimulated by obstruction of the aortic root, coronary artery occlusion and mechanical stretch of the ventricular wall. In animals with intact CNS, all stimuli led to an inhibition of the activity of the inferior cardiac and renal sympathetic nerves and bradycardia. These reflex effects are initiated by mechanoreceptors and abolished by vagotomy. The inhibition of sympathetic activity was equally pronounced in the cardiac and renal nerves. After coronary artery occlusion and aortic obstruction, inhibition occurred as soon as the ventricular diastolic pressure had risen about 2 mmHg. In spinal animals both stimuli caused a sympathetic activation which was mainly restricted to the cardiac nerve. This activation is not due to mechanical changes, but rather a direct result of myocardial ischaemia. Coronary artery occlusion is able to produce both inhibition and activation of sympathetic fibres, but the activation is normally suppressed and thus seems not to be particularly important for circulatory control.

Activation and inhibition of muscle and cutaneous postganglionic neurones to hindlimb during hypothalamically induced vasoconstriction and atropine-sensitive vasodilation

1. Discharge patterns in postganglionic neurones to muscle and to hairy skin of the hindlimb of chloralose anaesthetized cats were investigated during electrical hypothalamic stimulation which induced either vasoconstriction or atropine sensitive vasodilation in the skeletal muscle. 2. Spontaneously active postganglionic neurones to muscle were activated both during hypothalamically induced vasoconstriction and active vasodilation. Stimulation of the hypothalamic vasodilator area induced mostly a sequence of activation-depression-activation in these neurones. Stimulation of cutaneous Group IV afferents elicited reflexes in these neurones; repetitive high frequency stimulation of large diameter afferents in the vago-depressor nerve produced depression of spontaneous activity followed by a post-inhibitory excitation. The characteristics of these neurones fit those that would be expected of vasoconstrictors. 3. Normally inactive postganglionic neurones to skeletal muscle could only be activated during hypothalamically induced atropine sensitive vasodilation. These neurones exhibit no reflexes on somatic stimulation. The axons of these neurones conduct faster than those of the spontaneously active postganglionic neurones. It is likely that they are cholinergic vasodilator neurones. 4. Most of the cutaneous postganglionic neurones to hairy skin were activated during stimulation of both the hypothalamic vasoconstrictor and the vasodilator areas. These neurones have the characteristics of cutaneous vasoconstrictor neurones. Part of the cutaneous not spontaneously active postganglionic neurones could neither be activated from the hypothalamus nor by somatic stimuli.