Responses of the cardiac sympathetic receptors to various substances

Acute adenosine increases cardiac vagal and reduces sympathetic efferent nerve activities in rats

Adenosine is the first drug of choice in the treatment of supraventricular arrhythmias. While the effects of adenosine on sympathetic nerve activity (SNA) have been investigated, no information is available on the effects on cardiac vagal nerve activity (VNA). We assessed in rats the responses of cardiac VNA, SNA and cardiovascular variables to intravenous bolus administration of adenosine. In 34 urethane-anaesthetized rats, cardiac VNA or cervical preganglionic sympathetic fibres were recorded together with ECG, arterial pressure and ventilation, before and after administration of three doses of adenosine (100, 500 and 1000 μg kg(-1)). The effects of adenosine were also assessed in isolated perfused hearts (n = 5). Adenosine induced marked bradycardia and hypotension, associated with a significant dose-dependent increase in VNA (+204 ± 56%, P < 0.01; +275 ± 120%, P < 0.01; and +372 ± 78%, P < 0.01, for the three doses, respectively; n = 7). Muscarinic blockade by atropine (5 mg kg(-1), i.v.) significantly blunted the adenosine-induced bradycardia (-56.0 ± 4.5%, P < 0.05; -86.2 ± 10.5%, P < 0.01; and -34.3 ± 9.7%, P < 0.01, respectively). Likewise, adenosine-induced bradycardia was markedly less in isolated heart preparations. Previous barodenervation did not modify the effects of adenosine on VNA. On the SNA side, adenosine administration was associated with a dose-dependent biphasic response, including overactivation in the first few seconds followed by a later profound SNA reduction. Earliest sympathetic activation was abolished by barodenervation, while subsequent sympathetic withdrawal was affected neither by baro- nor by chemodenervation. This is the first demonstration that acute adenosine is able to activate cardiac VNA, possibly through a central action. This increase in vagal outflow could make an important contribution to the antiarrhythmic action of this substance.

New look to an old symptom: angina pectoris

At the turn of this century, it was proposed that ischemic cardiac pain might be related to distension of the ventricular wall ("mechanical hypothesis"). Three decades later, it was hypothesized that ischemic pain might be elicited by the intramyocardial release of pain-producing substances induced by ischemia ("chemical hypothesis"). Studies carried out in the past 10 years have given strong support to the chemical hypothesis, because they have consistently shown that adenosine is a mediator of ischemic cardiac pain. Adenosine-induced ischemic cardiac pain is mediated primarily by stimulation of A1 receptors located in cardiac nerve endings and is potentiated by substance P. Conversely, the magnitude and rate of left ventricular dilation during ischemia do not predict the severity of angina. It is worth noting, however, that stretching of epicardial coronary arteries appears to potentiate the severity of angina caused by myocardial ischemia. The nervous activity generated by myocardial ischemia is modulated in intrinsic cardiac, mediastinal, and thoracic ganglia. Then it is further modulated in the central nervous system and projects bilaterally to the cortex, as demonstrated in humans by positron emission tomography, where it is decoded as a painful sensation. The causes responsible for the lack of angina during myocardial ischemia are probably different in patients who present both pain-free and painful myocardial ischemia, in patients with predominantly painless ischemia, and in diabetic patients.

Autonomic neural control of cardiac function: modulation by adenosine and adenosine 5'-triphosphate

Adenosine and adenosine 5'-triphosphate (ATP) are found in every cell of the human body. These molecules are released from cells into the extracellular fluid under physiologic and pathophysiologic conditions. Outside of cells, adenosine and ATP act as physiologic regulators of cells, tissues, and organs. In the heart, extracellular adenosine and ATP exert pronounced inotropic, lusitropic, electrophysiologic, and metabolic effects, which are mediated by specific cell surface receptors. In addition, both compounds can modulate sympathetic and parasympathetic input to the heart by interacting with neural elements within and without the heart, thereby modulating autonomic neural control of cardiac functions. This article briefly reviews these indirect, neurally-mediated actions of adenosine and ATP.

Purinergic modulation of neural control of cardiac function

1. The purine nucleotide adenosine 5'-triphosphate (ATP) and its related nucleoside, adenosine (Ado), exert pronounced electrophysiologic, inotropic, lusitropic and metabolic effects in the mammalian heart. 2. These effects are the result of direct actions of these compounds on cardiac myocytes and endothelial cells, mediated by cell surface receptors. 3. In addition, ATP and Ado can stimulate neural elements inside and outside the heart and thereby modulate neural control of cardiac function. These latter actions of ATP and Ado are briefly reviewed and their hypothetical physiological role is outlined.

Adenosine activates cardiac sympathetic afferent fibers and potentiates the excitation induced by coronary occlusion

Adenosine is a possible mediator of cardiac pain during myocardial ischemia; however, little is known about the influence of adenosine on cardiac sympathetic afferent activity and thereby on its algogenic mechanism. In 20 anaesthetized, decerebrated, curarized and artificially ventilated cats, we studied the impulse activity of 20 single afferent sympathetic fibers with a left ventricular receptive field in relation to epicardial applications of adenosine, coronary artery occlusions and arterial pressure rises. All fibers increased their impulse activity (from 1.2 +/- 0.2 to 2.6 +/- 0.5 imp/s; P < 0.001) during slight (20 +/- 8%) rises in aortic pressure, thus exhibiting low-threshold receptor characteristics. In 10 cats, epicardial applications of three different doses of adenosine (0.1, 1 and 10 mg/ml) caused a brief increase in neural activity with dose-related responses. This response was abolished by aminophylline, a P1 purinergic inhibitor. In the other group of 10 cats, four subsequent 30-s occlusions of the coronary arterial vessel supplying the receptive fields of the fibers were performed, in control conditions and 30 s, 3 and 7 min, respectively, after the end of excitation induced by adenosine (1 mg/ml) application. During the control coronary occlusion the impulse activity increased from 1.1 +/- 0.1 to 5.5 +/- 0.7 imp/s (P < 0.0001). A similar activation was present during the second occlusion initiated 30 s after the end of adenosine-induced activation. In contrast, a significant potentiation of the response was observed (8.8 +/- 1.2 vs. 5.3 +/- 0.9 imp/s; P < 0.001) during the occlusion initiated 3 min after the end of excitation by adenosine. This effect was no longer present during the last occlusion performed after 7 min. When the protocol was repeated substituting adenosine with saline (n = 5) or after i.v. administration of aminophylline (n = 5), no potentiation was observed, even though the excitatory response to coronary occlusion was preserved. These data show that adenosine can activate cardiac sympathetic afferent fibers in a dose-related manner, and potentiate their responses to coronary occlusion, while leaving unaffected the responsiveness to a hemodynamic stimulus. The excitatory effects are likely to involve the P1 purinergic receptors. The potentiation phenomenon might play a role in the genesis of an algogenic code.

Reflex ventilatory effects of KCl stimulation of lung receptors with sympathetic afferents

The purpose of this study was to determine the reflex ventilatory effects produced by lung receptors with sympathetic afferent nerves. Seven dogs were anesthetized with sodium pentobarbital, placed on a ventilator, and vagotomized. The chest was opened through a mid-sternal incision. Diaphragm EMG (D-EMG), right and left triangularis sterni EMG (TS-EMG), systemic arterial blood pressure (BP), and tracheal pressure were recorded before and after the application of 2 M KCl to the right or left lung near the venous hilum. The reflex effects produced by KCl applied to the superior vena cava (SVC) and of mechanical distortion of the lungs were also studied. KCl applied to the right or left lung or right or left lung distortion produced significant increases in peak TS-EMG ipsilateral to the applied stimulus with no significant effects on the contralateral TS-EMG. BP, inspiratory time, expiratory time, or peak D-EMG were not significantly affected, except for a decrease in inspiratory time when KCl was applied to the left lung and a decrease in BP with distortion of the left lung. KCl applied to the SVC produced a significant reflex increase in the right TS-EMG. The primary reflex response to chemical stimulation of lung receptors with sympathetic afferents was an increase in expiratory muscle activity. Additionally, mechanical stimulation may also activate these receptors. Furthermore, receptors which produce similar reflex responses are located on the SVC. That the reflex responses produced by intrathoracic receptors with sympathetic afferents were unilateral suggests that spinal mechanisms are involved.

Cardiopulmonary sympathetic afferent input to lower thoracic spinal neurons

Spinal neuronal responses to stimulation of cardiopulmonary sympathetic afferent (CPS) fibers were studied in 25 alpha-chloralose-anesthetized cats. Eighty-two neurons located in the T7-T9 segments were tested for responses to electrical stimulation of CPS fibers. Activity of 55 neurons was altered; 37 were excited, 10 were inhibited, and 8 were both excited and inhibited. All 55 cells with CPS input also responded to stimulation of somatic receptors and the left greater splanchnic nerve (SPL). Somatic receptive fields were primarily located on the upper portion of the abdomen and left lower rib cage. Short and long latency responses occurred following CPS and SPL stimulation. Latencies of responses to CPS stimulation were significantly longer than latencies of responses to SPL stimulation (P less than 0.05). Early responses to CPS stimulation were significantly less in magnitude compared to early responses to SPL stimulation (P less than 0.05). Cell responses to CPS stimulation were reduced in magnitude for as long as 300 ms when a conditioning stimulus was applied to SPL. Inhibitory responses of 10 cells to CPS fiber stimulation were best observed during repetitive stimulation. Eight of the cells were also inhibited by repetitive stimulation of SPL. Injection of bradykinin (4 micrograms/kg) into the left atrium increased activity of 16/30 cells from 8 +/- 2 to 22 +/- 5 spikes/s. The results demonstrate that CPS fiber stimulation alters activity of lower thoracic spinal neurons but not as intensely as SPL stimulation. These neurons may participate in cardiac-abdominal visceral reflexes or the pain of cardiac origin that is referred to the abdomen.

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.

Left ventricular receptors inhibit brain serotonin neurons during coronary artery occlusion

Acute coronary artery ligation in pargyline-treated rats decreased serotonin and increased 5-hydroxyindoleacetic acid in the medulla and posterior hypothalamus. Lidocaine applied topically to the left ventricle completely prevented these alterations. No changes in serotonin were observed in the other brain regions examined. These data suggest a reflex inhibition of bulbar and hypothalamic serotonergic nerves by left ventricular receptors following acute coronary artery occlusion in the rat.

Painless ST-segment depression in patients with angina pectoris. Correlation with daily activities and cigarette smoking

Ambulatory electrocardiographic monitoring was employed in 33 patients with angina pectoris and abnormal stress tests to determine the frequency with which myocardial ischemia manifested by painless ST-segment depression occurred during normal activity. ST-segment depression occurred in 24 patients during the monitoring period; and in 21, it occurred either solely in the absence of pain or both with and without pain. Of 109 recorded episodes of ST-segment depression, 61 percent were painless. The frequency of painless ST-segment depression was independent of activity other than automobile driving, during which all episodes were painless. In patients who smoked cigarettes, ST-segment depression was more common while smoking, but the incidence of painless ST-segment depression was not altered. The study indicates that ST-segment depression occurs more commonly in the absence than in the presence of chest pain and that ambulatory electrocardiographic monitoring is a useful method of determining the frequency of myocardial ischemia during normal daily activity.