Studies of reflexogenic effects of capsaicin and neuropeptides on neural afferents in the dog parietal pericardium.

Stimulation of neural afferents in the parietal pericardium of anaesthetized, open-chest dogs by local application of capsaicin (0.1-100 micrograms) consistently induced dose-related pressor effects and tachycardia, whereas the application (0.1-1 microgram) of neuropeptides substance P (SP), neurokinin A (NKA), neurokinin B (NKB) or calcitonin gene-related peptide (CGRP) had no cardiovascular effect. Capsaicin-induced reflex responses were not affected by vagotomy, but were abolished by bilateral sectioning of the upper thoracic (T1-T4) white rami communicantes and stellectomy. Capsaicin-induced reflex tachycardia could also be abolished by a beta-adrenoceptor blockade with propranolol (0.5 mg/kg, IV), while ganglionic blockade with pentolinium (0.5 mg/kg, IV) eliminated both the tachycardia and pressor effects. Intravenous treatment with the cyclo-oxygenase inhibitors, indomethacin (5 mg/kg) or aspirin (100 mg/kg) had no effect on reflex pressor and heart rate responses to pericardial capsaicin. Also local treatment of the pericardium with either indomethacin (1 microgram/ml) or dual cyclooxygenase/lipoxygenase inhibitor, BW755C (10 micrograms/ml) failed to affect the responses to capsaicin. We conclude that (i) capsaicin-sensitive afferents which are present in the dog pericardium have a spinal origin and can initiate sympathetically-mediated reflex cardiovascular changes; (ii) the reflexogenic action of capsaicin on pericardial afferents does not depend on local production of eicosanoids; (iii) neuropeptides appear to be without reflexogenic effects on neural afferents in the dog parietal pericardium.

Effects of neuropeptides, ruthenium red and neuraminidase on chemoreflexes mediated by afferents in the dog epicardium.

1. Experiments were performed on anaesthetized, open-chest dogs to determine reflex effects on blood pressure and heart rate produced by stimulation of neural afferents of the left ventricular epicardium by local application of capsaicin, bradykinin, nicotine and the neuropeptides substance P (SP), neurokinin A (NKA), neurokinin B (NKB) and calcitonin gene-related peptide (CGRP). 2. Studies also included assessing whether reflexogenic actions of capsaicin, bradykinin and nicotine are influenced by epicardial treatment with either neuropeptides, Ruthenium Red or neuraminidase. 3. Epicardial application of either capsaicin (0.1-10 micrograms) or bradykinin (0.1-1 micrograms), consistently resulted in dose-related increases in blood pressure and heart rate, whereas reflex bradycardia and hypotensive effects were initiated by the application of nicotine (30-50 micrograms). 4. SP, NKA, NKB and CGRP caused marked hypotensive effects and tachycardia when injected intravenously (1 microgram), but failed to produce any cardiovascular response when applied to the epicardium of the left ventricle (0.1-1 microgram). Treatment of the heart surface with these neuropeptides (0.05-0.5 micrograms min-1) was also without any effect on the magnitude of reflex responses evoked by epicardial application of either capsaicin, bradykinin or nicotine. 5. Superfusion of the ventricular epicardium with Ruthenium Red (10-30 microM), a cationic dye known to have sialic acid as a molecular target, antagonized the reflexogenic effects of capsaicin but not those of bradykinin or nicotine. The reflex effects of capsaicin, but not those of bradykinin, were also sensitive to inhibition by epicardial treatment with neuraminidase, an enzyme which cleaves sialic acid residues from glycosides and sialoglycoproteins. 6. We conclude that neuropeptides which may be released from the peripheral endings of some cardiac sensory neurons neither directly activate nor sensitize spinal sympathetic and vagal afferents in the dog heart to the reflexogenic action of bradykinin, nicotine or capsaicin. 7. We further suggest that activation of the cardiac sympathetic chemoreflex by capsaicin involves its interaction with calcium-binding sialic acid moieties present on the surface of axons and/or terminals of chemosensitive sympathetic afferents distributed in the dog ventricular epicardium.

Kinin receptors mediating the effects of bradykinin on the coronary circulation in anaesthetized greyhounds.

Bradykinin (BK, 0.05 micrograms/kg) or glyceryl trinitrite (5 micrograms/kg) injected into the left circumflex coronary artery of anaesthetized, open-chest greyhounds, caused pronounced increases in large coronary artery diameter (CD) and coronary blood flow (CBF), whereas des-Arg9-BK (0.05-0.3 micrograms/kg), a selective bradykinin B1 agonist, dose dependently elevated CBF but had little effect on CD. BK-induced increases in CD and CBF were not affected by the intracoronary infusion of a selective B1 receptor antagonist, des-Arg9-[Leu8]BK (40 micrograms/min), but were significantly reduced by the infusion of a selective B2 receptor antagonist, D-Arg0-[Hyp3,Thi5,8,D-Phe7] BK (10-12 micrograms/min). The antagonism was reversible and specific for BK since responses to glyceryl trinitrate were not affected. Bilateral vagotomy (n = 3) or autonomic blockade with atropine (0.1 mg/kg i.v.) and propranolol (1 mg/kg i.v.) (n = 5) resulted in significant attenuation of BK-induced increases in CBF but not that of CD. It is concluded that BK is a potent dilator of both conductance and resistance coronary vessels in anaesthetized greyhounds. The dilatation of conductance vessels appears to involve a selective interaction with B2 receptors, while BK-induced increase in CBF may be mediated by both B1 and B2 receptors and involve participation of neuroreflex mechanisms.

Cardiac nociceptive reflexes: role of kinins, prostanoids and capsaicin-sensitive afferents.

Spinal sympathetic afferent fibres with endings in the heart are essential for signalling cardiac pain. In anesthetized, open-chest dogs, stimulation of these cardiac afferents with the algesic agents, bradykinin or capsaicin, results in reflex increases in arterial pressure and cardiac rate. The reflex responses induced by bradykinin are enhanced by concomitant application of prostaglandins of the E type or prostacyclin, and reduced by indomethacin. In contrast, the reflex effects of capsaicin are not influenced by either indomethacin or prostaglandin E1 treatment. Evidence is presented that reflexogenic effects of bradykinin involve its interaction with specific B2-receptors for kinins and that proteolysis of endogenous kininogen precursor to kinins by epicardially applied tissue kallikrein can initiate reflex cardiovascular effects.

A role for cyclic GMP in the initiation of cardiac pressor reflexes by bradykinin and capsaicin.

Reflexogenic effects of bradykinin on the cardiac sympathetic afferents supplying the dog epicardium were inhibited by treatment of the heart surface with either methylene blue or the compound LY-83583 (6-anilino-5,8-quinolinedione), which are known to lower cellular levels of cyclic GMP. Stimulation of cyclic GMP production by epicardial treatment with the NO-containing synonimine compound, SIN-1, potentiated bradykinin-induced reflex effects. In contrast, the sympathetic cardiac pressor response induced by capsaicin was not influenced by epicardial treatment with either methylene blue, LY-83583 or SIN-1. These findings indicate the importance of cyclic GMP for the reflexogenic activity of bradykinin, but not that of capsaicin on the sympathetic afferents supplying the dog heart.

Participation of the kallikrein-kinin-receptor system in reflexes arising from neural afferents in the dog epicardium.

1. Reflexogenic effects of bradykinin, lysyl-bradykinin and endogenously formed kinins on neural afferents of the left ventricular epicardium were studied in anaesthetized, open-chest dogs. 2. Epicardial application of either bradykinin (0.01-10 micrograms), lysyl-bradykinin (0.01-10 micrograms) or tissue kallikrein (0.003-1 U) consistently resulted in dose-related increases in blood pressure and heart rate. The pressor and heart rate responses to epicardial kallikrein were slower in onset and longer lasting than those evoked by bradykinin or lysyl-bradykinin. The effects of kallikrein, but not those of exogenous kinins, were subject to tachyphylaxis. The application of higher doses of kallikrein (0.1 or 1 U) also resulted in long-lasting desensitization of the epicardium to the effects of bradykinin. 3. Treatment of the epicardium with a proteinase inhibitor, aprotinin, prevented the reflexogenic effects of kallikrein but not those of bradykinin or lysyl-bradykinin. Treatment with aprotinin also counteracted post-kallikrein desensitization of sensory receptors of the ventricular epicardium to the reflexogenic effect of bradykinin. 4. Superfusion of the epicardium with a selective B2 receptor antagonist, D-Arg[Hyp3,Thi5,8,D-Phe7]-bradykinin, was equally effective in antagonizing the reflexogenic effects of kallikrein, bradykinin and lysyl-bradykinin. 5. We conclude that the response to epicardial application of kallikrein indicates an ample presence of endogenous substrate for local formation of bradykinin and/or related kinins. These then initiate reflex activation of the cardiovascular system by interacting with specific B2 receptors associated with sympathetic afferent neurones in the dog epicardium. We suggest that the kallikrein-kinin-receptor system has a role in the reflex function of the cardiac sympathetic afferents in both physiological and pathological conditions.

Specific receptors for bradykinin-induced cardiac sympathetic chemoreflex in the dog.

Bradykinin (BK, 0.03-1 microgram), capsaicin (1 microgram) or potassium chloride (KCl, 13 mumol) applied to the epicardium of the left ventricle of anaesthetized, open-chest dogs, caused reflex tachycardia and pressor effects, whereas des-Arg9-BK (1-100 micrograms), a selective bradykinin B1-receptor agonist, failed to produce any cardiovascular response. Superfusion of the epicardium with a selective B1-receptor antagonist, des-Arg9-[Leu8]BK (50-100 micrograms/min) had no effect on reflex responses to epicardial BK (0.03-0.1 microgram). However, the selective B2-receptor antagonist, D-Arg-[Hyp3,Thi5,8,D-Phe7]BK (10-25 micrograms/min) abolished the reflex effects of 0.03 and 0.1 microgram BK and reduced by 50 to 70% the responses to 1 microgram BK. Another B2-receptor antagonist [Thi6,9,D-Phe8]kallidin (10-50 micrograms/min) also reduced (30-70%) responses to 1 microgram BK. The antagonism was reversible and specific for BK since reflex responses to epicardial application of either capsaicin or KCl were not affected. The results indicate that BK interacts with B2-receptors, probably located on terminals and/or axons of sympathetic afferents supplying the dog heart, to activate a cardiac sympathetic chemoreflex.

Modification by diltiazem, a calcium antagonist, of the pulmonary vagal and cardiac sympathetic chemoreflexes in the dog.

1. Experiments were performed on anaesthetized, open-chest dogs to determine the effects of diltiazem on: the pulmonary vagal chemoreflex evoked by intravenous (i.v.) injection of capsaicin; cardiac sympathetic chemoreflexes activated by epicardial application of bradykinin or capsaicin; and baroreflex-mediated changes in heart rate resulting from both pressor and depressor effects produced by i.v. injections of noradrenaline and bradykinin, respectively. 2. Diltiazem infused i.v. at a rate of 10-30 micrograms/kg per min (mean cumulative dose 0.53 +/- 0.05 mg/kg, n = 9), did not affect basal heart rate, despite significant (P less than 0.001) reduction of resting blood pressure. 3. Diltiazem treatment did not affect the pressor responses to i.v. noradrenaline (0.3 micrograms/kg) or the hypotensive effects of i.v. bradykinin (0.3 micrograms/kg), but reduced significantly both the baroreflex-mediated bradycardia (P less than 0.01) and tachycardia (P less than 0.05) occurring with noradrenaline and bradykinin, respectively. 4. In contrast, diltiazem greatly enhanced reflex bradycardia (P less than 0.001) and systemic hypotension (P less than 0.01) resulting from activation of the afferent vagal pulmonary receptors by i.v. capsaicin (3-5 micrograms/kg). 5. Reflex pressor responses evoked by activation of the afferent cardiac sympathetic neurons by epicardial application of bradykinin (1 microgram) or capsaicin (10-20 micrograms) were not affected by diltiazem, but the corresponding reflex increases in heart rate evoked by both substances were significantly (P less than 0.01) reduced. 6. The results indicate that diltiazem, while reducing the influence of sinoaortic baroreceptors on heart rate, facilitates the reflex vagal control of the cardiac pacemaker by the afferent cardiopulmonary vagal receptors. These nervous reflex mechanisms, which include attenuation of positive chronotrophic effects that may result from ischaemia-induced activation of the afferent cardiac sympathetic neurons, may play an important role in the protective action of diltiazem in ischaemic heart disease.

Cardiovascular reflexes mediated by capsaicin sensitive cardiac afferent neurones in the dog.

Reflex circulatory changes mediated by capsaicin sensitive cardiac afferent neurones were studied in anaesthetised, open chest dogs. Application of capsaicin to the epicardium of the left ventricle, either in single doses (0.01-100 micrograms) or by superfusion (20 micrograms X min-1), consistently resulted in dose related increases in blood pressure and heart rate. These responses were not affected by bilateral vagotomy but were abolished or reversed by bilateral sectioning of the upper thoracic (T1-T4) white rami communicantes and stellectomy. Injection of capsaicin (0.3-1 microgram X kg-1) into the left circumflex coronary artery caused either systemic hypotension and bradycardia (70.3% of experiments), a pressor response associated with tachycardia (13.5%), or a biphasic effect with an initial rise and then fall in blood pressure and heart rate (16.2%). With intravenous injections of capsaicin (3-5 micrograms X kg-1) the response was invariably cardioinhibitory and depressor. The reflex bradycardia and hypotension evoked with either intracoronary or intravenous injections of capsaicin were reversed after bilateral vagotomy to increases in cardiac rate and blood pressure. The post-vagotomy tachycardia occurring with intracoronary capsaicin could be abolished by beta adrenoceptor blockade with propranolol (0.5 mg X kg-1 iv), whereas ganglionic transmission blockade with pentolinium (0.5 mg X kg-1 iv) eliminated both the tachycardia and pressor effects. The results indicate that in the dog's heart capsaicin sensitive afferent neurones capable of affecting the circulatory system have both vagal and spinal sympathetic origin. It is suggested that capsaicin induced excitatory cardiogenic reflex is nociceptive in nature and may involve activation of substance P containing afferent fibres incorporated in cardiac sympathetic nerves.

Participation of prostanoids in chemical activation of the pericardial pressor reflex in dogs.

Experiments were performed on anaesthetized, open-chest dogs to determine the reflex effects on systemic blood pressure and heart rate produced by stimulation of the parietal pericardium with bradykinin, prostacyclin, prostaglandin E2 (PGE2), prostaglandin D2 (PGD2) and arachidonic acid. Pericardial application of bradykinin (1 microgram) consistently elicited reflex increases in blood pressure and heart rate, whereas application of prostanoids or arachidonic acid in doses up to 10 micrograms failed to produce any cardiovascular responses. Indomethacin, applied either directly to the parietal pericardium (1 microgram/ml) or given intravenously (5 mg/kg) caused a long lasting reduction of the reflex responses to bradykinin. The reflex effects of bradykinin could be temporarily restored by treatment of the pericardium with either prostacyclin (0.1 microgram/min) or PGE2 (0.1 microgram/min). PGD2 (0.1-1 microgram/min) did not influence the bradykinin induced pericardial reflex. Superfusion of arachidonic acid (3 micrograms/min) over the pericardium amplified the reflex effects of bradykinin when given before, but not when given after indomethacin treatment. The results indicate that locally formed prostanoids, specifically prostacyclin and PGE2, can facilitate activation of the pericardial pressor reflex by bradykinin. The findings may be relevant to the changes in cardiovascular activity occurring during pericardial inflammation.