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Objective: The effects of capsaicin on sensory neurons are mediated by its interaction with a specific membrane receptor and opening of a non-selective cation channel. In the rat heart, capsaicin-sensitive nerve endings are known to be activated by oxygen radicals. We investigated the possibility that free oxygen radicals stimulate sensory nerve endings by acting upon the capsaicin receptor., Methods: We studied the effects of capsaicin (0.16-16.0 nmol), bradykinin (0.1-10 nmol), H2O2 (1.5-30 mumol), and xanthine + xanthine oxidase (X + XO, 1 mumol + 0.03 mU) applied to the surface of the rat heart for 30 s on the activity of cardiac, capsaicin-sensitive, vagal and sympathetic afferent fibers before and after blockade of capsaicin receptors with capsazepine (200 micrograms/kg, i.v.), a specific antagonist for the capsaicin receptor., Results: Application of capsaicin (0.32-16.0 nmol), H2O2 (9-30 mumol), bradykinin (1-10 nmol), and X + XO increased cardiac vagal and sympathetic afferent activity. Administration of capsazepine had no effect on the baseline activity of either vagal or sympathetic cardiac afferents, but it abolished the response of the afferent fibers to all doses of capsaicin, H2O2, and X + XO tested. Capsazepine had no effect on afferent activation by bradykinin. Administration of another capsaicin receptor blocker, ruthenium red (780 micrograms/kg, i.v.), had similar effects., Conclusions: The results of these experiments indicate that blockade of capsaicin receptors inhibits activation of vagal and sympathetic cardiac afferent fibers by free oxygen radicals. The fact that capsazepine and ruthenium red did not affect the afferent response to bradykinin suggests that this effect of the blockers was specific for capsaicin receptors. The possible functional implications of this interaction are discussed.

Objective: To compare the efficacy of percutaneous transtracheal ventilation (PTV) in the unparalyzed state with that in the paralyzed state using a sedated nonobstructed canine model., Methods: Eight mongrel dogs (16.8-32 kg) were anesthetized, instrumented, and placed in a volume plethysmograph. Anesthesia was achieved with pentobarbital sodium (up to 30 mg/kg). The spontaneous respiratory drive was kept intact. PTV was performed using a 13-ga transtracheal catheter and compressed air at 45 psi at an I:E ratio of 1:3 (15 breaths/min). Each dog was sequentially ventilated in both the paralyzed and unparalyzed states. The paralyzed/unparalyzed sequence was alternated among the animals to avoid sequence bias. Paralysis was achieved with succinylcholine (0.1 mg/kg bolus and 0.01 mg/kg/min drip). Reversal of paralysis was achieved by discontinuing the succinylcholine infusion. Key variables, including arterial blood gas, tidal volume, and pulmonary mechanics, were measured and compared for the paralyzed and unparalyzed states., Results: Gas exchanges and lung mechanics were similar between the unparalyzed and paralyzed states. There was no significant difference in mean pH, pCO2, pO2, tidal volume, or peak inspiratory transpulmonary pressure. There was also no significant difference in pulmonary resistance or pulmonary compliance., Conclusion: In a sedated nonobstructed canine model, PTV is as efficacious in the unparalyzed state as it is in the paralyzed state. The lung mechanics are also similar in the 2 states. These data suggest that it may be unnecessary to induce paralysis when using PTV for emergency ventilation in the heavily sedated state.

The effects of the simple phospholipid mediator lysophosphatidic acid (LPA) on the contractile responsiveness of isolated tracheal rings from rabbits and cats were assessed. In both species, LPA increased the contractile response to the muscarinic agonist methacholine, but LPA did not induce contraction on its own. Conversely, LPA decreased the relaxation response to the beta-adrenergic-agonist isoproterenol in both species. Concentrations of LPA as low as 10(-8) M were effective, and the effects of LPA were rapidly reversed on washing. Phosphatidic acid was much less effective, requiring higher concentrations and producing only a minimal effect. Contractions induced by serotonin and by substance P also were enhanced by LPA, but KCl-induced contractions were unaffected. LPA inhibited the isoproterenol-induced relaxation of KCl-precontracted rings, similar to its effects on methacholine-precontracted rings, and relaxation induced by the direct adenylyl cyclase activator forskolin was inhibited in a manner similar to that induced by isoproterenol. Epithelium removal did not alter the contraction-enhancing effect of LPA. The ability of LPA to both enhance contraction and inhibit relaxation of airway smooth muscle suggests that LPA could contribute to airway hypercontractility in asthma, airway inflammation, or other types of lung injury.

There is good evidence that the cardiopulmonary and arterial baroreflexes are blunted in chronic heart failure (HF). Other evidence, however, suggests that the cardiac chemoreflex is enhanced during HF. In the present study, we sought to determine whether HF alters the sensitivity of cardiac vagal chemosensitive endings to bradykinin (BK), an endogenous mediator that activates ventricular C fiber afferents. We measured the activity of cardiac vagal single fibers and compared the afferent responses to left atrial injections of BK and capsaicin in sham-operated and pacing-induced HF dogs. The capsaicin-sensitive endings did not respond to changes in cardiac pressures evoked by vascular snares and were C fiber endings (0.8-2.1 m/s). Most were located in the left heart. There was no difference in rate or pattern of resting discharge of the cardiac vagal fibers between HF and sham groups (1.5 +/- 0.5 vs. 1.3 +/- 0.3 impulses/s, respectively). The afferent response to BK (0.001-1 microgram/kg), but not capsaicin (1-10 micrograms/kg), was greater in HF compared with sham dogs. Captopril (2 mg/kg i.v.) significantly enhanced resting discharge (P < 0.05) from cardiac chemosensitive vagal afferents in HF but not sham dogs. The afferent response to BK in both groups was significantly (P < 0.05) and similarly enhanced. Indomethacin (5 mg/kg i.v.) significantly inhibited resting discharge (P < 0.05) and nearly abolished the afferent responses to lower doses of BK in HF, but did not affect resting discharge and less effectively attenuated responses to BK in sham dogs. Responses to capsaicin did not differ between HF and sham animals. From these results, we conclude that 1) resting discharge from cardiac vagal chemosensitive endings is not altered in HF, 2) these vagal endings exhibit an enhanced sensitivity to exogenous BK but not to capsaicin in the HF state, 3) angiotensin-converting enzyme activity inhibits resting discharge from these afferents in HF, and 4) the cyclooxygenase system contributes to the enhanced BK responsiveness of cardiac chemosensitive endings in HF.

Inhibition of peptidases within the lungs not only potentiates the effects of neuropeptides released from C-fibers but also the effects of bradykinin and capsaicin both of which stimulate C-fibers. To determine if peptidase inhibition potentiates C-fiber activation, we challenged pulmonary C-fibers in rats with capsaicin or bradykinin before and after inhibition of neutral endopeptidase (NEP) or angiotensin converting enzyme (ACE). Inhibition of NEP by phosphoramidon (10 mg/kg, i.v.) potentiated the effect of capsaicin (0.5-1 micrograms, i.v.) on C-fiber activity but did not change the response to bradykinin (1-2 micrograms, i.v.). Inhibition of ACE by captopril (5 mg/kg, i.v.) potentiated C-fiber activation by either bradykinin or capsaicin. Aerosol administration of either phosphoramidon (1 x 10(-5) M, 2 min) or captopril (4.6 x 10(-3) M, 2 min) potentiated C-fiber activation by capsaicin aerosol (1.6 x 10(-4) M, 1 min) but not by bradykinin aerosol (9.4 x 10(-5) M, 1 min). Therefore, inhibition of NEP or ACE may potentiate airway obstructive mechanisms initiated by C-fiber stimulation.

We studied the role played by prostaglandins and oxygen-derived free radicals in mediating reflex changes in renal sympathetic nerve activity (RSNA) during myocardial ischemia and reperfusion. Ligation of the left coronary artery for 20 min and reperfusion for 10 min were performed in anesthetized rats with sinoaortic denervation and with intact cardiac afferent nerves (control, n = 7), with cardiac sympathetic denervation (SD, n = 6), with vagal denervation (VD, n = 7), and with combined SD + VD (n = 6). In control rats, RSNA decreased by 10 +/- 3% from baseline (P < 0.05) during the first minute of ischemia and increased above baseline after 5 min of ischemia, with the maximum increase at the first minute of reperfusion. In rats with SD, RSNA decreased by 19 +/- 4% from baseline (P < 0.05) at the first minute of ischemia and remained depressed during the entire ischemic and reperfusion periods. In rats with VD, RSNA increased by 26 +/- 5% from baseline (P < 0.05) at the first minute of ischemia, and the increase in RSNA at the end of the ischemic period and at reperfusion was greater than in control rats. No changes in RSNA during ischemia and reperfusion were observed with combined SD + VD. Reflex changes in RSNA that occurred at the onset of ischemia in both VD (n = 7) and SD (n = 7) rats were abolished by indomethacin (5 mg/kg i.v., 20 min before ischemia). Reflex changes in RSNA after prolonged ischemia (> 10 min) and during reperfusion in both VD (n = 7) and SD (n = 7) rats were abolished by the antioxidant deferoxamine (20 mg/kg i.v., 20 min before ischemia). Deferoxamine also diminished the increase of RSNA at the onset of ischemia in VD rats. Thus, in rats, the vagal afferent reflex predominates during early ischemia and the sympathetic afferent reflex predominates during prolonged ischemia and reperfusion. Reflex changes in RSNA that occur at the onset of ischemia are mediated by activation of vagal and sympathetic afferent endings by prostaglandins. Reflex changes in RSNA after prolonged ischemia and during reperfusion are mediated by activation of vagal and sympathetic afferent endings by oxygen-derived free radicals.

1. Myocardial ischaemia and reperfusion can evoke excitation of cardiac vagal afferent nerve endings and activation of a cardiogenic depressor reflex (Bezold-Jarisch effect). We postulate that oxygen free radicals, which are well known to be produced during ischaemia and reperfusion, contribute to this excitation. 2. Activity from vagal afferent fibres in rats, whose endings were located in the walls of all four chambers of the heart, was recorded in response to topical application of pro-oxidant chemicals to the surface of the heart. Activity was also recorded from vagal afferent fibres, whose endings were located in the left ventricle, in response to occlusion of the left anterior coronary artery (LAC) for 30 min and subsequent reperfusion. A majority of the recorded fibres were classified as chemosensitive C-fibre endings due to their irregular discharge under resting conditions, their activation in response to the topical application of capsaicin (1-10 micrograms) to the surface of the heart encompassing the receptive field and their conduction velocities. 3. Topical application of either H2O2 or xanthine/xanthine oxidase to the heart activated 50% of the chemosensitive endings and did not directly affect cardiac mechanoreceptors. This effect was reproducible, dose-dependent and was not due to [H+]. 4. Administration of the superoxide radical scavenging enzyme, superoxide dismutase (20000 U/kg, i.v.), decreased the response of fibres to xanthine/xanthine oxidase but had no effect on the activation caused by H2O2. The antioxidants deferoxamine (20 mg/kg, i.v.) or dimethylthiourea (10 mg/kg, i.v.), which scavenge the hydroxyl radical, abolished the responses to xanthine/xanthine oxidase and H2O2. Administration of indomethacin (5 mg/kg, i.v.) had no effect on the afferent response to H2O2. 5. In response to ligation of the left anterior coronary (LAC), the activity of chemosensitive endings within the ischaemic zone increased within the first 2 min of occlusion. Endings outside the ischaemic zone were not affected at the beginning of ischaemia. Reperfusion activated only chemosensitive endings responsive to topical H2O2. These reperfusion-sensitive endings were located both within and outside the ischaemic zone of the left ventricle. 6. Indomethacin (5 mg/kg, i.v.) prevented activation of chemosensitive endings at the beginning of LAC occlusion regardless of their sensitivity to H2O2 but had no effect on the response to reperfusion. Conversely, deferoxamine (20 mg/kg, i.v.) had no effect on the activation of chemosensitive fibres at the onset of ischaemia, whereas it completely prevented activation at reperfusion. 7. We propose that there are two different mechanisms that activate chemosensitive afferent vagal fibres in the rat heart during ischaemia and reperfusion. The first causes excitation of these endings at the onset of ischaemia and is mediated by prostaglandin synthesis within the ischaemic zone. The second mechanism leads to a more widespread activation of chemosensitive afferents in the left ventricle during prolonged ischaemia and at the moment of reperfusion and is mediated by oxygen free radical formation.

Experiments were conducted with chloralose-urethan anesthetized rats to assess the effects of 1) bilateral stimulation of the cervical vagus nerves and 2) parasympathomimetic and sympathomimetic agents. Transpulmonary pressure (Ptp) was used as an index of airway smooth muscle tone, and peak inspiratory Ptp (Ptppeak) values were used for a comparison of responses. In untreated animals, vagal stimulation elicited an increase in Ptppeak of 155%. Cooling of the vagus nerves to 15 degrees C abolished the response of Ptppeak to vagal stimulation. Although isoproterenol (1-10 micrograms/kg i.v.) did not alter resting Ptppeak, it did prevent vagal stimulation from evoking an increase in Ptppeak. Nadolol (1.5 mg/kg i.v.) augmented the increase in Ptppeak elicited by vagal stimulation. Vagal stimulation did not evoke any change in Ptppeak after the administration of both nadolol and atropine or after combined administration of nadolol, atropine, and either serotonin aerosol or prostaglandin F2 alpha. In rats pretreated with capsaicin 1 wk before the experiment, vagal stimulation evoked an increase in Ptppeak that was not statistically different from that of untreated control animals. Therefore, nonadrenergic noncholinergic systems did not appear to play an independent role in the response of the airways to the activation of the vagus nerves.

Objectives: Ischemia of the myocardium stimulates cardiac sensory nerve endings resulting in the local release of neuropeptides. The significance of the release of neuropeptides, such as substance P (SP), for the function of the heart during ischemia and reperfusion is not known. We examined the effects of both chronic and acute neuropeptide depletion and of SP administration on contractile function and rhythmicity of the isolated rat heart during global ischemia and subsequent reperfusion., Methods: Experiments were conducted on the isolated perfused heart from vehicle and capsaicin-pretreated rats (100 mg/kg) to deplete neuropeptides from peripheral nerve terminals. The hearts were perfused with Krebs-Henseleit solution (95%O2 + 5% CO2, 37 degrees C, at constant pressure of 90 cmH2O). Left ventricular developed and diastolic pressures (LVDevP and LVEDP), heart rate (HR) and coronary flow (CF) were measured. Hearts were subjected to 20 min global no-flow ischemia and 30 min reperfusion., Results: Prior to interrupting coronary flow, LV pressures, HR and CF did not differ between vehicle and capsaicin-pretreated rats. However, throughout the reperfusion period, the recovery of LVDevP, HR and CF in hearts from capsaicin-pretreated rats was consistently less than in control hearts (P < 0.05), and the incidence of fibrillation during reperfusion was higher (P < 0.05). In other experiments, acute perfusion of isolated hearts with capsaicin (10(-6) M) for 5 min before ischemia had a similar limiting effect during reperfusion. Administration of SP (10(-6)-10(-9) M) to capsaicin-pretreated hearts before ischemia restored their ability to recover contractile function and CF during reperfusion. Administration of SP to untreated hearts before ischemia also improved their recovery above normal during reperfusion and decreased the incidence of fibrillation without affecting postischemic CF. The beneficial effects of SP were abolished by an NK-1 receptor antagonist, CP-96,345 (10(-6) M)., Conclusions: These data indicate that sensory neuropeptides play a role in protection of the isolated heart against ischemic damage and suggest a role of SP in the resistance of the myocardium to ischemia and reperfusion injury.

One of the hallmarks of chronic congestive heart failure is an increase in sympathetic tone to the peripheral circulation and to the heart. A correlation between plasma norepinephrine and the severity of the heart failure state has been demonstrated. One mechanism that has been proposed to account for this sympathoexcitation is a depression in the baroreflex and, perhaps, cardiac reflex control of sympathetic nerve activity. This review summarizes work from several laboratories, including our own, that documents a depressed baroreflex control of heart rate and sympathetic nerve activity in both animals and humans with heart failure. The mechanism of the depressed baroreflex most likely is caused by reduced baroreceptor sensitivity as well as enhanced input to the central nervous system from cardiac receptors that are chemosensitive. Although sympathetic tone and arterial baroreflex sensitivity are altered in heart failure, there have been no studies showing a cause-and-effect relationship. Increases in plasma norepinephrine are similar in baroreceptor-denervated and intact dogs paced into heart failure. This latter observation cells into question the traditional concept of baroreceptor-mediated increases in sympathetic tone in heart failure.

Myocardial ischemia and reperfusion can evoke excitation of cardiac vagal nerve endings and activation of a cardiogenic depressor reflex (Bezold-Jarisch effect). We postulate that oxygen-derived free radicals, which are known to be produced during prolonged ischemia and reperfusion, contribute to this afferent excitation. We recorded activity from 47 chemosensitive vagal afferent fibers in 31 rats; the endings of these fibers were located in the left ventricle. Chemosensitive endings were identified with topical applications of capsaicin (10 micrograms) to the surface of the heart. Reactivity of the endings to oxygen-derived free radicals was assessed by topical application of H2O2 (3 to 9 mumol). Activity of the vagal fibers was recorded during 30 minutes of occlusion of the left anterior descending coronary artery (LAD) and 10 minutes of subsequent reperfusion. The activity of chemosensitive endings within the ischemic zone increased in the first 2 minutes of LAD occlusion from 2.2 +/- 0.4 to 4.3 +/- 0.9 impulses per second (107 +/- 30% increase, P < .05). This increased activity waned after 3 to 5 minutes of occlusion. Endings outside the ischemic zone did not increase, their activity at the beginning of ischemia. Reperfusion caused a rapid elevation of activity only in chemosensitive fibers whose endings were found to respond to topical H2O2. The reperfusion-sensitive endings were located both within and outside the ischemic zone of the left ventricle. Indomethacin (5 mg/kg i.v., 20 minutes before occlusion) effectively prevented activation of chemosensitive afferent endings at the beginning of LAD occlusion regardless of their sensitivity to H2O2 but had no effect on the activation at reperfusion.

Myocardial ischemia and reperfusion can evoke excitation of cardiac vagal afferent nerve endings and activation of a cardiogenic depressor reflex (Bezold-Jarisch effect). We postulate that oxygen-derived free radicals, which are well known to be produced during prolonged ischemia and reperfusion, contribute to this excitation. Hydroxyl radicals derived from hydrogen peroxide (H2O2) activate abdominal sympathetic afferents and produce reflex excitation of the cardiovascular system. However, it is not known whether inhibitory vagal cardiac afferents are activated by oxygen-derived free radicals. We recorded activity from 52 single vagal afferent fibers in 29 rats; the endings of these fibers were located in the walls of all four chambers of the heart. Thirty-three (63%) of these fibers were classified as chemosensitive C-fiber endings because of their irregular discharge under resting conditions, their activation in response to the topical application of capsaicin (1 to 10 micrograms) to the surface of the heart encompassing the receptive field, and their conduction velocities. Fourteen (27%) of the remaining fibers were found to be mechanoreceptors. Topical application of H2O2 to the heart activated 50% of the chemosensitive endings and did not directly affect cardiac mechanoreceptors. Activity increased by 498% at a dose of 3 mumol (P < .001). This effect was reproducible and dose dependent and was not due to [H+]. Topical application of xanthine/xanthine oxidase (20 mmol/0.03 mU) activated 8 of the 12 chemosensitive fibers tested and had no direct effect on mechanosensitive fibers. Activity increased by 287% (P < .001).(ABSTRACT TRUNCATED AT 250 WORDS)

Prostaglandins F2 alpha and E2 increase release of immunoreactive (irANP) in primary cultures of rat atrial cardiocytes. This effect is independent of cell density in the cultures and does not appear to operate through a cAMP-dependent mechanism. Studies that probed the PGF2 alpha effect with a number of different pharmacological antagonists suggest that it is tied to a calmodulin-dependent step. This latter effect does not appear to be related to increased calcium entry through voltage-gated channels in the plasma membrane nor to mobilization of ryanodine-sensitive intracellular calcium pools. Inhibitors of the lipoxygenase pathway, a second avenue of arachidonate metabolism, resulted in a decrease in irANP release from cultured atrial or ventricular cardiocytes. Leukotriene C4, a lipoxygenase product, had a modest effect to promote irANP release over a 24-h period. However, pretreatment of anesthetized rats with nordihydroguarietic acid, a lipoxygenase inhibitor, had no effect on stretch-dependent release of irANP from the heart in vivo. These findings suggest that the prostaglandins represent the more important group of arachidonate metabolites in regulating irANP release physiologically.

Stimulation of pulmonary C-fibers (PCs) by capsaicin and of rapidly adapting receptors (RARs) by reduced lung compliance reflexly increases airway submucosal gland secretion in dogs. Because both PCs and RARs are stimulated by cigarette smoke (nicotine being the primary stimulus), we performed experiments in anesthetized open-chest artificially ventilated dogs (with aortic nerves cut) to determine whether cigarette smoke reflexly stimulates airway secretion. We measured submucosal gland secretion by counting the hillocks in a 1.2-cm2 field of tracheal epithelium coated with tantalum dust. Secretion was stimulated by delivery of 40-320 ml smoke from high-nicotine cigarettes to the lower trachea, secretion rate increasing from 7.4 +/- 1.3 to 48.1 +/- 5.1 hillocks.cm-2.min-1. Results of cutting the pulmonary vagal branches or carotid sinus nerves or both indicated that the secretory response was initiated by stimulation of lower respiratory vagal afferents and augmented several seconds later by stimulation of carotid chemoreceptors. Results of cooling the cervical vagus nerves to 7 and 0 degrees C indicated that most of the vagally mediated increase in secretion was due to stimulation of afferent lung C-fibers.

In anesthetized rats with sinoaortic denervation, intracerebroventricular (icv) injection of atrial natriuretic peptide (ANP) resulted in decreased mean arterial blood pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) (depressor effects), whereas icv angiotensin II (ANG II) produced increases in these variables (pressor effects). The depressor effects of ANP were slower in onset and longer in duration than the pressor effects of ANG II. Intracerebroventricular injection of the ANG II-receptor blocker sarthran or the ANG II-synthesis inhibitor captopril resulted in a significant reduction in MAP; HR and RSNA were not affected. Both sarthran and captopril abolished the depressor responses to icv ANP. In contrast, injection of an anti-rat ANP antibody, which blocked the depressor effects of icv ANP, did not by itself modify MAP, HR, or RSNA, nor did the antibody affect the pressor responses to icv ANG II. These data suggest that, in this animal model, the depressor effects of icv ANP are mediated by the inhibition of brain ANG II-dependent neural activity. These results also demonstrate that, in this preparation, the endogenous ANG II system actively contributes to the maintenance of basal MAP, whereas the central ANP system, at least in regions accessible to the antirat ANP antibody, plays little role in this maintenance.

Cyclooxygenase products of arachidonic acid might be some of the substances that accumulate in contracting muscle to cause the reflex increases in arterial pressure and ventilation that are evoked by exercise. Recently, cyclooxygenase blockade has been shown to attenuate the reflex cardiovascular responses to static muscular contraction in anesthetized cats. Group IV afferents are believed to comprise part of the afferent arm of the reflex arc, the activation by which static muscular contraction causes these cardiovascular effects. We therefore examined the effects of indomethacin and aspirin, two cyclooxygenase-blocking agents, on the responses to static contraction of group IV afferents with endings in the triceps surae muscles of anesthetized cats. We found that indomethacin (5 mg/kg iv) decreased the responses to contraction of each of eight group IV afferents tested. Likewise, aspirin (50 mg/kg iv) decreased the responses to contraction of each of four group IV afferents tested. On the other hand, we found that arachidonic acid (2 mg) injected into the femoral artery did not increase the responses to contraction of four group IV afferents that were stimulated by this maneuver. In addition, arachidonic acid injection did not cause any of seven group IV afferents not stimulated by static contraction to become responsive to this maneuver. Nevertheless, arachidonic acid injection with the muscle at rest stimulated five of seven contraction-insensitive and two of four contraction-sensitive group IV afferents. Our data suggest that cyclooxygenase metabolites of arachidonic acid are needed for the full expression of the responses of group IV muscle afferents to static contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

We have examined the effects of several PGs on the synthesis and release of the atrial natriuretic peptide (ANP) in vivo and in vitro. PGF2 alpha infusion in anesthetized rats resulted in a significant increase in plasma immunoreactive (ir) ANP levels in vivo despite effecting only modest changes in hemodynamics. The PGs were also effective at promoting irANP secretion in primary cultures of neonatal rat atrial and ventricular cardiocytes. PGF2 alpha increased irANP release with half-maximal induction seen at approximately 10(-8) M; PGE2 was somewhat less effective and prostacyclin (PGI2) was without effect. The PGs also increased ANP mRNA levels in these cells, suggesting that these agents exert a major effect on the synthesis as well as the secretion of the prohormone. Transient expression analysis of atrial cells transfected with 2,500 bp of human (h) ANP 5' flanking sequence linked to a chloramphenicol acetyltransferase (CAT) reporter demonstrated that PGF2 alpha (10(-5) M) increased hANP promoter activity approximately twofold relative to the control. PGF2 alpha had no effect on the promoterless control (pSV0-lamin CAT). Treatment of cultured atriocytes with high concentrations of a cyclooxygenase inhibitor resulted in a significant suppression of ANP secretion in vitro and a truncation of the plasma ANP response to volume infusion in vivo. Taken together these studies support a role for PGs as regulators of cardiac ANP synthesis and secretion, and suggest an additional mechanism whereby eicosanoids may act to control cardiovascular and renal homeostasis.

Previously, we reported that intravenous (iv) administration of atrial natriuretic peptide (ANP) evokes a decrease in sympathetic outflow. This effect requires an afferent input from vagal C-fibers. Here, we examined the effect of intracerebroventricular (icv) administration of ANP on sympathetic outflow, as well as the potential role of central mechanisms in mediating the sympathoinhibitory effects evoked by systemic administration of the peptide. In anesthetized rats with arterial baroreceptors intact, injection of ANP (100-500 ng) into the third ventricle did not affect renal and least-splanchnic sympathetic nerve activities, heart rate, and mean arterial pressure. After sinoaortic denervation, however, icv injection of ANP (100 ng) decreased these variables by 8 +/- 1 and 9 +/- 2% of control nerve activity, 9 +/- 3 beats/min, and 13 +/- 2 mmHg, respectively (P less than 0.05). The inhibitory sympathetic and cardiovascular effects of icv ANP were dose dependent, with responses seen after doses too small to produce systemic effects (less than 100 ng). Vagal blockade did not abolish the effects evoked by icv ANP. In addition, iv administration of ANP did not alter reflex responses to graded electrical stimulation of afferent vagal C-fibers. Furthermore, central administration of an antiserum directed against rat ANP did not alter the inhibitory sympathetic responses evoked by iv administration of the peptide. Taken together, these results indicate that centrally administered ANP, like systemic ANP, decreases sympathetic outflow, heart rate, and blood pressure; however, the central and peripheral actions of the peptide can be distinguished and appear to be independent.

The afferent arm of the reflex are responsible for the pressor response to static contraction is comprised of group III and IV fibers. The nature of the contraction-induced stimulus activating these fibers remains unclear. Evidence suggests that most group III afferents are sensitive to mechanical stimuli, whereas most group IV afferents are sensitive to metabolic stimuli. Recently, in anesthetized cats, stimulation of group III mechanoreceptors has been shown to have a role in the reflex pressor response to static contraction. In skin, the sensitivity of thin fiber mechanoreceptors to distortion of their receptive fields has been shown to be increased by both cyclooxygenase and lipoxygenase products of arachidonic acid metabolism. Therefore, in barbiturate-anesthetized cats we recorded the responses of group III muscle afferents to static contraction before and after arachidonic acid (1-2 mg ia) and/or indomethacin (5 mg/kg iv). Arachidonic acid increased the responses of group III afferents (n = 11) to contraction by 265% (from 0.17 +/- 0.07 to 0.62 +/- 0.24 impulses/s; P less than 0.025). Indomethacin decreased the responses of group III afferents (n = 9) to contraction by 61% (from 1.00 +/- 0.37 to 0.39 +/- 0.16 impulses/s; P less than 0.025). Arachidonic acid given after indomethacin increased the responses of two of four group III afferents to contraction. We conclude that both cyclooxygenase and lipoxygenase products of arachidonic acid metabolism sensitize group III muscle afferents to static contraction.

We attempted to determine whether stimulation of pulmonary rapidly adapting receptors (RARs) increase tracheal submucosal gland secretion in anesthetized open-chest dogs. Electroneurographic studies of pulmonary afferents established that RARs but not lung C-fibers were stimulated by intermittent lung collapse during deflation, collapse being produced by removing positive end-expiratory pressure (PEEP, 4 cmH2O) or by applying negative end-expiratory pressure (NEEP, -4 cmH2O). We measured tracheal secretion by the "hillocks" method. Removing PEEP or applying NEEP for 1 min increased secretion from a base line of 6.0 +/- 1.1 to 11.8 +/- 1.7 and 22.0 +/- 2.8 hillocks.cm-2.min-1, respectively (P less than 0.005). After PEEP was restored, dynamic lung compliance (Cdyn) was 37% below control, and secretion remained elevated (P less than 0.05). A decrease in Cdyn stimulates RARs but not other pulmonary afferents. Hyperinflation, which restored Cdyn and RAR activity to control, returned secretion rate to base line. Secretory responses to lung collapse were abolished by vagal cooling (6 degrees C), by pulmonary vagal section, or by atropine. We conclude that RAR stimulation reflexly increases airway secretion. We cannot exclude the possibility that reduced input from slowly adapting stretch receptors contributed to the secretory response.

In anesthetized, open-chest dogs we examined the effect of pulmonary edema on the firing frequency of afferent vagal fibers arising from the lung. We recorded impulses from slips of the cervical vagus nerves and infused isotonic Krebs-Henseleit solution (20% of body weight) intravenously to increase net filtration pressure in the lung microvasculature. Measurement of extravascular lung water (6.0 +/- 0.4 g/g dry lung), and morphological examination of lung tissue (revealing various degrees of perivascular and peribronchial cuffing) confirmed that edema was present. At the end of the infusion when the lungs were congested (lung microvascular pressure, 37 cm water) and edematous, the impulse frequency of pulmonary and bronchial C-fibers and rapidly adapting receptors had increased 5-6 times. The only significant change in slowly adapting receptor activity was an increase during deflation. When lung water was still elevated but lung microvascular pressure had been restored to control by withdrawal of blood, impulse activity of rapidly and slowly adapting receptors reverted to or below control. Pulmonary C-fiber activity, although less than during congestion, remained significantly above control, several C-fibers being stimulated by interstitial edema in the absence of alveolar edema. Bronchial C-fibers were stimulated in severely edematous lung showing pronounced peribronchial cuffing and alveolar edema, but were not stimulated in milder grades of edema. Our results support the hypothesis (Paintal, 1969) that pulmonary C-fibers (J-receptors) are stimulated by an increase in interstitial pressure secondary to edema.

Stimulation of bronchial C-fibers evokes a reflex increase in secretion by tracheal submucosal glands, but the influence of pulmonary C-fibers on tracheal gland secretion is uncertain. In anesthetized dogs with open chests, we sprayed powdered tantalum on the exposed mucosa of a segment of the upper trachea to measure the rate of secretion by submucosal glands. Secretions from the gland ducts caused elevations (hillocks) in the tantalum layer. We counted hillocks at 10-s intervals for 60 s before and 60 s after we injected capsaicin (10-20 micrograms/kg) into the right atrium to stimulate pulmonary C-fiber endings. Right atrial injection of capsaicin increased the rate of hillock formation fourfold, but left atrial injection had no significant effect. The response was abolished by cutting the vagus nerves or cooling them to 0 degree C. We conclude that the reflex increase in tracheal submucosal gland secretion evoked by right atrial injection of capsaicin was initiated as capsaicin passed through the pulmonary vascular bed, and hence that pulmonary C-fibers, like bronchial C-fibers, reflexly increase airway secretion.

Bronchodilator prostaglandins E2 and I2 may cause airway irritation and bronchoconstriction in human subjects. These experiments were designed to test the hypothesis that this paradoxical bronchoconstriction is a vagal reflex triggered by stimulation of airway afferents. We recorded smooth muscle tension in an innervated upper tracheal segment in anesthetized dogs and injected prostaglandins into the general circulation or into a bronchial artery or administered them as aerosol to the lungs. Prostaglandins usually caused tracheal contraction, which survived vagal cooling to 5-7 degrees C but was abolished at 0 degrees C. Vagally mediated tracheal contraction was also evoked when prostacyclin was injected into the pulmonary circulation of dogs whose pulmonary and systemic circulations were independently pump perfused. Recordings of afferent vagal impulses indicated that bronchial arterial injection of prostaglandins stimulated bronchial C-fibers; aerosols of prostaglandin stimulated pulmonary and bronchial C-fibers and C-fibers in extrapulmonary airways. We postulate that in susceptible human subjects concentrations of these prostaglandins too low to have direct bronchodilator effects may cause reflex bronchoconstriction by stimulating afferent vagal C-fibers in the lower airways.

We monitored cardiovascular and renal function in conscious dogs with surgically denervated hearts during two experimental procedures: 1) inflation of a balloon in the left atrium and 2) intravascular volume expansion. The results obtained were compared with results from identical experiments on sham-operated control dogs. Left atrial balloon inflation in the sham-operated dogs produced an increase in left atrial pressure, heart rate, urine flow, and sodium excretion; central venous pressure decreased. These changes were absent in the cardiac-denervated dogs. Infusion of 6% dextran in isotonic saline (16% of estimated blood volume) increased the heart rate significantly in the control dogs but not in the cardiac-denervated dogs; other hemodynamic measurements were comparable in the two groups. Urine flow and sodium excretion increased significantly in both the cardiac-denervated and control dogs; the responses did not differ significantly between the two groups. These experiments demonstrate that inflation of a balloon in the left atrium of a conscious dog elicits diuretic and natriuretic responses that are dependent on intact cardiac neural pathways, presumably specifically dependent on afferent neural impulses from left atrial receptors. On the other hand, an increase in circulating blood volume induced by the intravenous infusion of an isotonic, isoncotic solution elicits diuretic and natriuretic responses in the cardiac-denervated dog that are similar to the renal responses produced in a control dog. Thus, although cardiac receptors are capable of eliciting reflex changes in both hemodynamics and renal function, it is not clear what role they play in mediating the renal responses evoked by increases in blood volume.

Administration of atrial natriuretic factor (ANF) to anesthetized rats decreases renal nerve activity (RNA), an effect prevented by vagotomy but not by atropine. We sought to determine whether afferent vagal C-fibers mediate the inhibition of sympathetic outflow. ANF (2.5 micrograms/kg iv) decreased mean arterial pressure (MAP) by 25 +/- 2 mmHg, RNA by 11 +/- 5%, and least splanchnic nerve activity (LSNA) by 10 +/- 4% in anesthetized rats with arterial baroreceptors intact, and by 40 +/- 3 mmHg, 28 +/- 7%, and 23 +/- 4%, respectively, in sinoaortic-denervated rats. Inhibition of RNA and LSNA by ANF was reduced slightly by cooling the vagi to 6 or 7 degrees C, a temperature at which conduction in A-fibers was blocked and that in C-fibers attenuated; inhibition was abolished when C-fibers were blocked by cooling to 0 degrees C. We conclude that the inhibition of RNA and LSNA by ANF was mediated by afferent vagal C-fibers. We also obtained evidence that the aortic nerves contribute to ANF-induced inhibition of RNA. Our results support the notion that ANF evokes a generalized decrease in sympathetic tone that contributes to the hypotension, cardiac inhibition, and natriuresis accompanying systemic administration of the peptide.

In order to throw light on the mechanism of acute hypertensive baroreceptor resetting, we examined the relationship between aortic baroreceptor firing and aortic wall properties in anaesthetized dogs as pressure was varied in a number of ways. We recorded baroreceptor impulses from the left aortic nerve, and measured aortic pressure with a catheter-tip transducer and external aortic diameter with ultrasonic transit-time transducers. Narrow anticlockwise hysteresis loops were evident in the pressure-diameter relationship of the upper thoracic aorta, both during the rapid pulsatile pressure changes of the cardiac cycle and during the slow excursions of mean pressure imposed for construction of baroreceptor pressure--response curves. In contrast to the 'phase-lag' response of diameter to pressure, the baroreceptor response was 'phaselead' in character, decreasing when stress-induced creep occurred in the aortic wall. When the mean arterial pressure set-point was increased from 100 to 125 mmHg for 20 min, the hysteresis loops relating mean diameter to mean pressure in the range 60-200 mmHg were displaced along the diameter axis in the direction of wall creep. A reduction in the baroreceptor response to pressure (i.e. resetting) always accompanied this displacement. Administration of ouabain (25-35 micrograms/kg) had no consistent effect on baroreceptor resetting. It has been suggested that acute baroreceptor resetting is akin to adaptation. To investigate the possibility that the two processes are accompanied by similar changes in aortic wall properties, we converted the aorta into a closed sac and distended it with a square wave of pressure. Like resetting, adaptation of the baroreceptor response to maintained pressure was associated with a small degree of creep of the aortic wall. Our results are compatible with the hypothesis that acute hypertensive resetting of aortic baroreceptors is similar to adaptation, both phenomena being attributable to relaxation of viscoelastic coupling elements, leading to a reduction of strain at the receptor membrane. Whether viscoelastic processes alone can account for acute resetting, or whether changes in ionic balance are involved also, baroreceptor responsiveness is a function of the stress history of the wall, the pressure-response curve moving along the pressure axis in the direction of the prevailing set-point. Hence, in early hypertension physiological resetting of baroreceptors will precede pathological resetting, and may even promote an upward movement of set-point.

Pulmonary C-fibers initiate the prompt apnea evoked by pulmonary arterial injections of capsaicin, but their role in the subsequent rapid shallow breathing of the pulmonary chemoreflex is disputed. To determine whether this reflex tachypnea is triggered by pulmonary C-fibers rather than by afferents further downstream, we separately perfused the pulmonary and systemic circulation in anesthetized dogs. When the lungs were rhythmically inflated, injection of capsaicin (0.5-20.0 micrograms X kg-1) into the isolated pulmonary circulation evoked an immediate cessation of phrenic nerve firing ("apnea") and when phrenic bursts resumed, a reduction in their amplitude. Phrenic burst frequency was usually entrained to the ventilator cycle and did not increase. By contrast, when entrainment of phrenic bursts was avoided by statically inflating the lungs at a transpulmonary pressure of 3 cmH2O, the injection of capsaicin evoked apnea, followed by a prolonged increase in phrenic burst frequency and a decrease in amplitude ("rapid shallow breathing"). The infusion of capsaicin (10-20 micrograms X kg-1 X min-1) also evoked rapid shallow breathing, but without apnea. Our results are consistent with the hypothesis that in spontaneously breathing animals, stimulation of pulmonary C-fibers evokes rapid shallow breathing.

1. We compared the pressure-response characteristics of C fibre and A fibre baroreceptors in the carotid sinus of anaesthetized dogs, recording impulses from the sinus nerve and varying mean pressure in the vascularly isolated sinus, which was distended with a pulsatile pressure. Functional stimulus-response curves were obtained by gradually increasing sinus pressure above and decreasing it below a set-point of 100 mmHg. Baroreceptors were identified by a pulsatile discharge synchronous with the pulsations in sinus pressure. A and C fibre baroreceptors were identified by the conduction velocities and blocking temperatures of their axons. 2. The pressure-response characteristics of C and A fibre baroreceptors differed in several respects. C fibres had a pulsatile firing threshold 50 mmHg higher than that of A fibres (105.8 +/- 1.8 and 54.6 +/- 2.9 mmHg, respectively), an average maximal sensitivity 35% of that of A fibres (0.39 and 1.12 impulses s-1 mmHg-1, respectively), and a maximal frequency (at 220 mmHg) 29% of that of A fibres (24.5 and 84.3 impulses/s, respectively). Although invariably pulsatile at pressures above threshold, the firing pattern of C fibre baroreceptors tended to be more irregular than that of their A fibre counterparts. 3. Impulses were also recorded from C fibres that were stimulated by increasing sinus pressure but had an irregular, non-pulsatile discharge, a high pressure threshold (averaging 154.1 +/- 7.2 mmHg), and a low maximum frequency (10.8 +/- 2.4 impulses/s). 4. Cooling the sinus nerve progressively attenuated conduction in both A and C fibres, A fibres being blocked between 12 and 4 degrees C (mean 6.8 degrees C) and C fibres between 4 and -1.5 degrees C (mean 1.0 degree C). Although cooling the sinus nerve to 7 degrees C did not block conduction in all A fibres, impulse activity in baroreceptor A fibres at a carotid sinus pressure of 200 mmHg was no greater than that at a pressure of 75 mmHg. By contrast, at 7 degrees C baroreceptor C fibres still provided a signal proportional to sinus pressure. 5. Our results suggest that A and C fibre baroreceptors subserve different reflex functions, the former signalling changes in arterial pressure both above and below the normal set-point, the latter only changes above. They also suggest that differential cold blockade may be a useful tool to determine the contribution of C fibre baroreceptors to cardiovascular reflexes.

We measured hemodynamics and renal function in conscious dogs while partially obstructing blood flow at various sites within the thorax. Inflation of a balloon in the left atrium increased left atrial pressure (LAP) by 9 mmHg and caused a parallel increase in pulmonary arterial pressure (PAP); heart rate, arterial pressure, and total peripheral resistance increased; stroke volume and right atrial pressure decreased; and cardiac output remained unchanged. The increase in LAP was accompanied by a fourfold increase in urine flow and a threefold increase in sodium excretion. Plasma vasopressin (AVP) and renin activity (PRA) decreased. On the other hand, partial occlusion of the pulmonary veins or the main pulmonary artery produced similar increases in PAP without affecting LAP, systemic hemodynamics, renal function, or plasma AVP. Similarly, inflation of a balloon in the right atrium failed to alter renal function, plasma AVP, or PRA. Finally, constriction of the thoracic inferior vena cava decreased LAP and increased PRA. In summary, these data emphasize that inflation of a balloon in the left atrium of the conscious dog produces a composite response consisting of alterations in cardiovascular function, renal function, and circulating hormones. Moreover, our data indicate that the response is mediated by a reflex initiated from receptors located in the left atrium; we detected no evidence that receptors located in the pulmonary vasculature or right heart contribute to this response.

Arterial baroreceptors are known to influence airway smooth muscle tone. Thus, increasing carotid sinus pressure from 20 to 200 mm Hg causes reflex tracheal dilation. However, the effects of changing sinus pressure around a normal arterial pressure set-point of 100 mm Hg have not been examined. In anesthetized, artificially ventilated dogs, we distended the vascularly isolated carotid sinuses with a pulsatile pressure and recorded isometric tension in an upper tracheal segment. The aortic nerves were cut. Increasing mean carotid sinus pressure in steps between 100 and 200 mm Hg decreased tracheal tension, heart rate, and arterial blood pressure; decreasing sinus pressure between 100 and 25 mm Hg had the opposite effect. Changing carotid sinus pressure still evoked tracheal responses when systemic arterial pressure was held constant. Increasing and decreasing carotid sinus pulse pressure around a constant mean pressure evoked similar changes in tracheal tension. All reflex effects were abolished by cutting or cooling (0 degree C) the carotid sinus nerves; tracheal responses were abolished by the carotid baroreflex were of comparable magnitude to those triggered by stimulating pulmonary stretch receptors, laryngeal receptors, and pulmonary C-fibers. Our results indicate that carotid sinus baroreceptors exert a tonic influence on the upper airways by a vagal cholinergic pathway, increasing and decreasing tracheal smooth muscle tension as blood pressure varies around the normal set-point.

To study central and peripheral mechanisms that regulate the level of circulating renin during activation of the carotid sinus baroreflex, both carotid sinuses were isolated and perfused at constant flow in chloralase-anesthetized dogs. Sinus pressure was controlled by an adjustable reservoir while systemic pressure was stabilized using an external chamber. Arterial renin activity, measured by radioimmunoassay, was increased during sinus hypotension only if systemic pressure was held constant. The renin response was eliminated by either sinus or renal denervation. Administration of propranolol (iv) or phentolamine directly into the renal artery totally blocked the increase in renin during activation of the reflex. Perfusion of a beta-adrenergic blocker (propranolol) or an alpha-adrenergic blocker (phentolamine) through the third and fourth cerebroventricles had no effect on the increase in renin during sinus hypotension, whereas centrally administered clonidine, and alpha-agonist, blocked the response. We conclude that the sinus reflex arc affecting renin release involves not only activation of peripheral alpha- and beta-adrenergic receptors in the kidney but also inhibition of central alpha-adrenergic receptors. No evidence for central beta-involvement was found.

The influence of arterial baroreceptors on secretion of catecholamines from the adrenal medulla was evaluated by several methods. Conscious mongrel dogs with surgically denervated hearts were hemorrhaged until an estimated 16% of their blood volume had been removed. On a separate day they were anesthetized and their blood pressure was lowered with intravenous nitroglycerin. Neither of these maneuvers produced appreciable increases in heart rate in these dogs. In contrast, in a group of sham-operated control dogs, hemorrhage induced a mean increase in heart rate of 20 beats/min (P less than 0.05), and nitroglycerin-induced hypotension induced an increase of 50 beats/min (P less than 0.05). In a separate group of conscious dogs with aortic arch denervation but intact cardiac nerves, occlusion of the common carotid arteries for 5 min increased blood pressure and heart rate significantly but elicited only small, insignificant increases in plasma epinephrine and norepinephrine; the peak concentration of epinephrine achieved was considerably less than the amount necessary to cause appreciable effects on blood pressure and heart rate as determined in another experiment by infusing varying amounts of epinephrine into conscious, cardiac-denervated dogs. We conclude that the arterial baroreceptor reflex, within the range of activity likely to occur during most physiological and pathophysiological adjustments in the conscious dog, exerts only minimal effects on the secretion of catecholamines from the adrenal medulla.

The present experiments were designed to measure and characterize direct hemodynamic effects of propranolol on vascularly isolated carotid sinuses. dl-Propranolol, when restricted to the isolated carotid sinuses, inhibited in a dose-dependent manner the reflex increases in heart rate and mean arterial pressure during carotid sinus hypotension. When perfused through isolated sinuses at a concentration of 10 micrograms/ml, the drug totally abolished reflex changes to carotid sinus hypotension. Perfusion of the sinuses with d-propranolol or procaine at the same doses also totally inhibited the response. On the other hand, perfusion of the sinuses with sotalol, an adrenergic blocker without membrane stabilizing effects, did not alter resting levels of arterial pressure or heart rate and did not affect reflex changes during sinus hypotension. These results suggest that propranolol at high doses may affect baroreceptor afferents and inhibit the baroreceptor reflex through membrane-stabilizing and local anesthetic properties rather than through hypothesized beta receptors. It seems unlikely that these pathways contribute to the antihypertensive actions of the drug.