Injection Of Phenylbiguanide Research Articles

Activation of 5-HT3 receptors in the medulla oblongata is involved in the phasic control of urinary bladder

The control of micturition depends on reflex mechanisms, however, it undergoes modulation from cortex, pons and medullary areas. This study investigated if the activation of 5-HT3 receptors in the medulla influences the urinary bladder (UB) regulation in rats. Isoflurane female Wistar rats were submitted to catheterization of the femoral artery and vein for mean arterial pressure (MAP) and heart rate (HR) recordings and injection of drugs, respectively. The UB was cannulated for intravesical pressure (IP) measurement. The Doppler flow probe was placed around the left renal artery for renal conductance (RC) recordings. Phenylbiguanide (PB) and granisetron (GN) were injected into the 4th brain ventricle in rats with guide cannulas implanted 5 days prior to the experiments; or PB and GN were randomly injected intravenously or applied topically (in situ) on the UB. PB injection into 4th V significantly increased IP (68.67 ± 11.70%) and decreased MAP (−29 ± 6 mmHg) compared to saline (0.34 ± 0.64% and −2 ± 2 mmHg), with no changes in the HR and RC. GN injection into the 4th V did not significantly change the IP and RC compared to saline, nevertheless, significantly increased MAP (25 ± 4 mmHg) and heart rate (36 ± 9 bpm) compared to saline. Intravenous PB and GN only produced cardiovascular effects, whilst PB but not GN in situ on the UB evoked increase in IP (111.60 ± 30.36%). Therefore, the activation of 5HT-3 receptors in medullary areas increases the intravesical pressure and these receptors are involved in the phasic control of UB. In contrast, 5-HT3 receptors in the medulla oblongata are involved in the pathways of the tonic control of the cardiovascular system. The activation of 5-HT3 receptors in the bladder cause increase in intravesical pressure and this regulation seem to be under phasic control as the blockade of such receptors elicits no changes in baseline intravesical pressure.

Chronic hypoxic incubation blunts a cardiovascular reflex loop in embryonic American alligator (Alligator mississippiensis)

Hypoxia is a naturally occurring environmental challenge for embryonic non-avian reptiles, and this study is the first to investigate the impact of chronic hypoxia on a possible chemoreflex loop in a developing non-avian reptile. We measured heart rate and blood pressure in normoxic and hypoxic-incubated (10% O(2)) American alligator embryos (Alligator mississippiensis) at 70 and 90/95% of development. We hypothesized that hypoxic incubation would blunt embryonic alligators' response to a reflex loop stimulated by phenylbiguanide (PBG), a 5-HT(3) receptor agonist that stimulates vagal pulmonary C-fiber afferents. PBG injection caused a hypotensive bradycardia in 70 and 95% of development embryos (paired t tests, P<0.05), a response similar to mammals breathing inspired air (all injections made through occlusive catheter in tertiary chorioallantoic membrane artery). Hypoxic incubation blunted the bradycardic response to PBG in embryos at 95% of development (two-way ANOVA, P<0.01). We also demonstrated that the vagally mediated afferent limb of this reflex can be partially or completely blocked in ovo with a 5-HT(3) receptor blockade using ondansetron hydrochloride dihydrate (OHD), with a ganglionic blockade using hexamethonium, or with a cholinergic blockade using atropine. Atropine eliminated the hypotensive and bradycardic responses to PBG, and OHD and hexamethonium significantly blunted these responses. This cardiovascular reflex mediated by the vagus was affected by hypoxic incubation, suggesting that reptilian sympathetic and parasympathetic reflex loops have the potential for developmental plasticity in response to hypoxia. We suggest that the American alligator, with an extended length of time between each developmental stage relative to avian species, may provide an excellent model to test the cardiorespiratory effects of prolonged exposure to changes in atmospheric gases. This extended period allows for lengthy studies at each stage without the transition to a new stage, and the natural occurrence of hypoxia and hypercapnia in crocodilian nests makes this stress ecologically and evolutionarily relevant.

Indian red scorpion venom-induced augmentation of cardio-respiratory reflexes and pulmonary edema involve the release of histamine

Pulmonary edema is a consistent feature of Mesobuthus tamulus (MBT) envenomation. Kinins, prostaglandins and other inflammatory mediators are implicated in it. Since, histamine also increases capillary permeability, this study was undertaken to evaluate whether MBT venom utilizes histamine to produce pulmonary edema and augmentation of cardio-respiratory reflexes evoked by phenylbiguanide (PBG). Blood pressure, respiratory excursions and ECG were recorded in urethane anaesthetized adult rats. Injection of PBG (10 μg/kg) produced apnoea, hypotension and bradycardia and the responses were augmented after exposure to venom (100 μg/kg). There was increased pulmonary water content in these animals. Pretreatment with pheniramine maleate (H1 antagonist, 3 mg/kg) blocked both venom-induced augmentation of PBG response and pulmonary edema. In another series, compound 48/80 (mast cell depletor) was treated for 4 days then the PBG responses were elicited as before. At the end of the experiments, mast cells were counted from the peritoneal fluid. The venom-induced pulmonary edema and the augmentation of PBG reflex were not observed in compound 48/80 treated animals. Further, mast cells in the peritoneal fluid were absent in this group as compared to vehicle treated group (29 ± 7.9 cells/mm3). These observations indicate that venom-induced pulmonary edema and augmentation of PBG reflexe are mediated through mast cells by involving H1 receptors.

Modulatory Effect of Fentanyl on Bronchopulmonary C‐fiber (PCF)‐mediated Ventilatory (VE) Responses Is Age‐dependent in Anesthetized Rats

In adult rats, opioids administered into the pulmonary circulation produces an apnea and given systemically turns the phenylbiguanide (PBG)‐evoked rapid shallow breaths (RSBs) to an apnea by stimulating and facilitating PCFs, respectively. Fentanyl, an opioid μ‐receptor agonist, clinically has a lesser inhibitory effect on ventilation in children (&lt; 3 years old) than in adults (Barker, Pediatric Anesthesia 2007). Thus, we tested if the modulatory effect of opioids on the PCF‐mediated VE responses in rats were age‐dependent. VE responses to a right atrial bolus injection of PBG (before and after systemic administration of fentanyl) or fentanyl were recorded in anesthetized rat pups (3 week‐old) and adults. We found that: 1) PBG (5 μg/kg) evoked 5–10 RSBs in adults, but caused no change or 2–3 rapid breaths in pups; 2) systemic fentanyl (6 μg/kg, iv) reduced VE more greatly in adults (60%) than pups (27%, P &lt; 0.01) and switched the PBG‐induced RSB to an apnea in adults, but failed to alter the PBG effects on VE in pups; 3) right atrial injection of fentanyl (2 μg/kg) alone produced an apnea in adults but not in pups; and 4) the VE responses to right atrial injection of PBG or fentanyl were eliminated by bilateral vagotomy. Our data suggest that the PCF‐mediated RSBs and inspiratory inhibition in response to PBG and fentanyl are age‐dependent in rats, indicating a plasticity of PCFs as development. (ALA RT‐83131‐N).

Cardiac vagal preganglionic neurones in the intermediate zone of the brainstem in anaesthetized cats

Cardiac vagal preganglionic neurones (CVPNs) show respiratory modulation in the nucleus ambiguus but not in the dorsal vagal nucleus. Both types of neurones can be activated by pulmonary C fibre afferents. Another brainstem area that has been identified as containing CVPNs is the intermediate zone between the dorsal vagal nucleus and nucleus ambiguus. Experiments were carried out in alpha-chloralose-anaesthetized cats to determine the physiological properties of these CVPNs and their responses to pulmonary C fibre afferent activation. Seven CVPN axons in the right cardiac vagal branches were identified and found to be localized in the intermediate zone with a conduction velocity of between 1.2 and 1.6 m s(-1), in the C fibre range. These seven CVPNs [either showing spontaneous activity (n = 1) or having activity induced by dl-homocysteic acid applied ionophoretically (n = 3)] were neither respiratory modulated nor did they receive a baroreceptor input, thus being similar to those found in the dorsal vagal nucleus. Right atrial injections of phenylbiguanide excited all four CVPNs tested. In conclusion, CVPNs located in the intermediate zone have similar properties to those in the dorsal vagal nucleus but not the nucleus ambiguus.

Characterisation of c-Fos expression in the central nervous system of mice following right atrial injections of the 5-HT 3 receptor agonist phenylbiguanide

Cardiopulmonary receptors relay signals to the central nervous system via vagal and spinal visceral afferents. To date there are no detailed topographical studies in mice indicating the distribution of central neurones activated following stimulation of cardiopulmonary afferents. In anaesthetised mice, we injected the 5-HT 3 receptor agonist phenylbiguanide (PBG), a drug that is known to stimulate cardiopulmonary afferent C-fibres, into the right atrium of the heart and mapped c-Fos expression within specific regions of the central nervous system. Intra-atrial injection of PBG produced a reflex cardiorespiratory response including a pronounced bradycardia and a respiratory depression. Using immunohistochemical detection of the protein product of the immediate–early gene c-fos, we mapped the brain regions affected by cardiopulmonary 5-HT 3 receptor stimulation. Within the nucleus of the solitary tract (nTS) of PBG-injected mice, we detected an increased number of c-Fos-positive nuclei in the dorsolateral and gelatinous parts at the level of the area postrema (− 7.48 mm bregma) but not at more rostral or caudal levels (− 7.76, − 7.20, − 6.84 and − 6.36 mm bregma) relative to vehicle-injected control mice. In addition, c-Fos expression in the crescent part of the lateral parabrachial nucleus was decreased in PBG-injected mice whereas no significant differences were detected between PBG-injected and control mice in the number of c-Fos-positive nuclei in the dorsal part of the lateral parabrachial nucleus. PBG injections had no significant effects on the number of c-Fos-positive catecholaminergic neurones within the C1/A1, C2/A2, A5, A6 and A7 cell groups. Likewise, PBG injections had no significant effects on c-Fos expression in other central regions involved in cardiorespiratory control or cardiorespiratory reflexes (selected non-catecholaminergic nuclei in the medulla and midbrain periaqueductal gray, the paraventricular nucleus of the hypothalamus and the central nucleus of the amygdala). Identification of specific regions of the nTS complex involved in relaying signals from afferent cardiopulmonary C-fibres to the central nervous system will be useful for future studies aimed at understanding neural mechanisms underlying cardiopulmonary reflexes and physiological responses to cardiopulmonary disease.

Endogenous nitrosyl factors may inhibit the desensitization of 5-HT 3 receptors on vagal cardiopulmonary afferents

The pronounced tachyphylaxis to the Bezold–Jarisch reflex (BJR) responses elicited by systemic injections of the 5-HT 3 receptor (5-HT 3R) agonists such as phenylbiguanide (PBG) may involve desensitization and/or reduced rate of resensitization of 5-HT 3Rs on vagal cardiopulmonary afferents. The presence of nitric oxide synthase (NOS) in vagal afferents raises the possibility that endogenous nitrosyl factors regulate the status of 5-HT 3Rs in these afferents. Accordingly, the aim of this study was to determine whether the inhibition of NOS alters the development of tachyphylaxis to the BJR responses elicited by PBG in conscious rats. The first injection of PBG (100 μg/kg, i.v.) elicited robust reductions in heart rate (HR), diastolic arterial blood pressure (BP D), and cardiac output (CO) but minor changes in total peripheral resistance in saline-treated rats. Subsequent injections elicited progressively smaller responses such that the sixth injections elicited minor responses only. The first injection of PBG (100 μg/kg, i.v.) in rats treated with the NOS inhibitor, l-NAME (25 μmol/kg, i.v.) elicited similar reductions in HR, BP D, and CO as in saline-treated rats. However, the rate of development of tachyphylaxis to PBG was markedly faster in the l-NAME-treated rats. The BJR responses elicited by 5-HT (40 μg/kg, i.v.) were markedly attenuated after the development of tachyphylaxis to PBG in saline- and in l-NAME-treated rats whereas the BJR responses elicited by the S-nitrosothiol, l- S-nitrosocysteine (5 μmol/kg, i.v.), were not attenuated in either group. These findings suggest that tachyphylaxis to PBG was not due to the loss of central or efferent processing of the BJR. Taken together, these findings suggest NOS exists in vagal cardiopulmonary afferents mediating the BJR and that nitrosyl factors influence 5-HT 3R function.

Functional evidence for the rapid desensitization of 5-HT 3 receptors on vagal afferents mediating the Bezold–Jarisch reflex

The aim of this study was to determine whether 5-hydroxytryptamine (5-HT) 3 receptors on cardiopulmonary afferents mediating the Bezold–Jarisch reflex (BJR) desensitize upon repeated exposure to selective agonists. BJR-mediated falls in heart rate, diastolic arterial blood pressure and cardiac output elicited by the 5-HT 3-receptor agonists, phenylbiguanide (100 μg/kg, i.v.) or 2-methyl-5-HT (100 μg/kg, i.v.), progressively diminished upon repeated injection in conscious rats. The BJR responses elicited by 5-HT (40 μg/kg, i.v.) were markedly reduced in rats which had received the above injections of phenylbiguanide or 2-methyl-5-HT whereas the BJR responses elicited by l- S-nitrosocysteine (10 μmol/kg, i.v.) were similar before and after the injections of the 5-HT 3 receptor agonists. These findings suggest that tachyphylaxis to 5-HT 3 receptor agonists may be due to the desensitization of 5-HT 3 receptors on cardiopulmonary afferents rather than the impairment of the central or peripheral processing of the BJR.

Effect of pulmonary C-fibre afferent stimulation on cardiac vagal neurones in the nucleus ambiguus in anaesthetized cats.

It has been demonstrated previously that the vagal bradycardia evoked by activation of pulmonary C-fibres is not respiratory modulated. Experiments were carried out in alpha-chloralose anaesthetized cats to determine if these cardiac vagal preganglionic neurones (CVPNs) in the nucleus ambiguus (NA), which have respiratory modulated activity, can be activated when pulmonary C-fibre afferents are stimulated by right atrial injections of phenylbiguanide (PBG). Eleven CVPNs with B-fibre axons in the right cardiac vagal branches were identified and found to be localized within or ventrolateral to the nucleus ambiguus. Ionophoretic application of a high current of dl-homocysteic acid (DLH) induced a vagally mediated bradycardia and hypotension in six of eight sites from which CVPNs were recorded. The activity of B-fibre CVPNs, whether spontaneous (n = 4) or induced by ionophoresis of DLH (n = 7) was respiratory modulated, firing perferentially during post-inspiration and stage 2 expiration. This activity also correlated with the rising phase of the arterial blood pressure wave consistent with these CVPNs receiving an arterial baroreceptor input. Right atrial injections of PBG excited nine of eleven CVPNs tested. In eight of these activated neurones the onset latency of the excitation was within the pulmonary circulation time, consistent with being activated only by pulmonary C-fibre afferents. In two neurones the PBG-evoked excitation still occurred when central inspiratory drive was inhibited, as indicated by the disappearance of phrenic nerve activity. In conclusion, B-fibre respiratory modulated CVPNs can be activated following stimulation of pulmonary C-fibre afferents.

Pattern of cardiorespiratory afferent convergence to solitary tract neurons driven by pulmonary vagal C-fiber stimulation in the mouse.

The central integration of signals from pulmonary vagal C-fibers (or type-J receptors) with those arising from cardiac, peripheral chemoreceptor, and baroreceptor afferents to neurons within the nucleus of the solitary tract (NTS) was studied in an arterially perfused working heart-brain stem preparation of adult mouse. Pulmonary vagal C-fibers were excited by right atrial injection of phenylbiguanide (PBG) while cardiac receptors were stimulated by left ventricular injection of veratridine (1-3 micrograms/kg) or mechanically by distension of the left ventricle (20-50 microl perfusate) using an indwelling cannula. Carotid body chemoreceptors were activated by aortic injection of Na cyanide, whereas baroreceptors were stimulated by increasing arterial perfusion pressure. Stimulation of pulmonary C-fibers and cardiac, chemo-, and baroreceptors all produced a reflex bradycardia (23-133 bpm). Central respiratory activity, as recorded from the phrenic nerve, was depressed by stimulating pulmonary C-fibers and cardiac and baroreceptors but enhanced in amplitude and frequency during chemoreceptor stimulation. Twenty-seven NTS neurons were excited and three were inhibited after pulmonary C-fiber stimulation displaying decrementing discharges with a peak firing frequency of up to 42 Hz (15 +/- 2.2 Hz, mean +/- SE) that lasted for 8.8 +/- 0.9 s. These responses occurred <1 s from the end of the PBG injection that was within the pulmonary circulation time. None of these cells responded to increases in right atrial pressure. All cells excited by PBG were also driven synaptically after electrical stimulation of the ipsilateral cervical vagus nerve at a latency of 32.9 +/- 3.2 ms (range 20-62 ms). None of these neurons had ongoing activity related to central respiratory activity. Convergence from cardiorespiratory afferents to 21 neurons driven by pulmonary C-fibers was tested. Twenty-five percent of cells were selectively excited by chemical stimulation of cardiac receptors alone, 19% were driven by peripheral chemoreceptors, and 38% responded to both cardiac and chemoreceptor activation. In contrast, only 13% of the cells activated by PBG injection responded to stimulation of baroreceptors and only 6% to cardiac mechanoreceptor stimulation. None of these neurons were activated by increasing right atrial pressure. The data indicate a high proportion of afferent convergence from pulmonary C-fibers, cardiac receptors, and peripheral chemoreceptors in the NTS. However, these neurons appear not to integrate inputs from cardiovascular mechanoreceptors. The significance of the data is discussed in relation to pathological disease states such as pulmonary congestion and cardiac failure.

Evidence that 5-HT 3 receptors in the nucleus tractus solitarius and other brainstem areas modulate the vagal bradycardia evoked by activation of the von Bezold–Jarisch reflex in the anesthetized rat

The effects of intracisternal (i.c.) application of the 5-HT 3 receptor antagonist granisetron (0.016–0.16 μg kg −1) and the agonist phenylbiguanide (0.3–3 μg kg −1) on reflex bradycardia evoked by injection of phenylbiguanide (i.v.; 10 μg kg −1) were investigated in urethane anesthetized atenolol-pretreated rats. The effect of bilateral microinjection of granisetron (10 nmol per side, 100 nl) into the nucleus tractus solitarius (NTS) on the reflex was also investigated. Intracisternal administration of granisetron dose-dependently (0.016–0.16 μg kg −1) and significantly attenuated the reflex bradycardia whilst the highest dose given i.v. had no significant effect on the reflex bradycardia. Phenylbiguanide given i.c. only caused significant potentiation at the middle dose (1 μg kg −1), having no significant effects at the other doses. Neither granisetron nor phenylbiguanide given i.c. affected resting heart rate or blood pressure. Granisetron microinjected bilaterally into the NTS also significantly attenuated both reflex bradycardia and hypotension. It is concluded that excitation of cardiac vagal motoneurones evoked by cardiopulmonary afferents involves activation of 5-HT 3 receptors located in the nucleus tractus solitarius and other brainstem areas.

Activity of C fibre cardiac vagal efferents in anaesthetized cats and rats.

1. Bradycardia can be evoked by stimulation of both myelinated and non-myelinated vagal efferent fibres. The on-going activity and synaptic inputs to cardiac vagal preganglionic neurones with myelinated axons have been studied in detail, but there is little information regarding cardiac preganglionic neurones with non-myelinated axons. In the present study, the on-going discharge and afferent inputs to cardiac vagal efferents with non-myelinated axons were studied in anaesthetized rats and cats. 2. Extracellular recordings were made from vagal preganglionic neurones in the dorsal vagal motor nucleus (DVMN) of anaesthetized rats following electrical stimulation of the cervical vagus nerve. Based on calculated axonal conduction velocities, fifty-six neurones had non-myelinated axons. Sixteen of these C fibre neurones had on-going activity but this showed no relationship to central respiratory drive, lung inflation or the cardiac cycle. Activity of twenty-one of these fifty-six neurones was increased at short latency following right atrial injections of phenylbiguanide (PBG). 3. Eight presumed cardiac vagal preganglionic neurones were also recorded in the DVMN. Five of these were activated by PBG administration, and the one neurone with resting activity showed no indication of respiratory-related activity. 4. Finally, twenty-one single C fibres were recorded in peripheral branches of the cardiac vagus. They had a low rate of on-going activity, and in twelve fibres this on-going discharge was analysed in detail. On-going activity in seven of these fibres showed no relationship to central respiratory drive, lung inflation or the cardiac cycle, whereas the other five had prominent relationships with both central respiratory drive and lung inflation, and two of these also showed a relationship to heart period. PBG administration evoked an increased activity in fourteen of the fifteen fibres tested, and the latencies of nine of these responses (1.3 +/- 0.5 s) were within the pulmonary circulation time. 5. In anaesthetized cats, extracellular recordings were made in the DVMN from thirty-three cardiac vagal preganglionic neurones with C fibre axons. The on-going activity of these neurones, when present, never exhibited an on-going- or reflexly induced respiratory rhythm, nor any indication of an input from the arterial baroreceptors. Right atrial injection of PBG evoked a short-latency response in four of the five spontaneously active neurones but was without effect on the eight neurones tested which did not exhibit on-going activity. 6. In conclusion, the majority of cardiac vagal preganglionic neurones located in the DVMN have C fibre axons, show no obvious input from central or peripheral respiratory- or cardiac-related inputs, but are activated by stimulation of pulmonary C fibre afferent fibres.

Effect of cardiopulmonary C fibre activation on the firing activity of ventral respiratory group neurones in the rat.

1. Cardiopulmonary C fibre receptor stimulation elicits apnoea and rapid shallow breathing, but the effects on the firing activity of central respiratory neurones are not well understood. This study examined the responses of ventral respiratory group neurones: decrementing expiratory (Edec), augmenting expiratory (Eaug), and inspiratory (I) neurones during cardiopulmonary C fibre receptor-evoked apnoea and rapid shallow breathing. 2. Extracellular neuronal activity, phrenic nerve activity and arterial pressure were recorded in urethane-anaesthetized rats. Cardiopulmonary C fibre receptors were stimulated by right atrial injections of phenylbiguanide. Neurones were tested for antidromic activation from the contra- and ipsilateral ventral respiratory group (VRG), spinal cord and cervical vagus nerve. 3. Edec neurones discharged tonically during cardiopulmonary C fibre-evoked apnoea and rapid shallow breathing, displaying increased burst durations, number of impulses per burst, and mean impulse frequencies. Edec neurones recovered either with the phrenic nerve activity (25 s) or much later (3 min). 4. By contrast, the firing activity of Eaug and most I neurones was decreased, featuring decreased burst durations and number of impulses per burst and increased interburst intervals. Eaug activity recovered in approximately 3 min and inspiratory activity in approximately 1 min. 5. The results indicate that cardiopulmonary C fibre receptor stimulation causes tonic firing of Edec neurones and decreases in Eaug and I neuronal activity coincident with apnoea or rapid shallow breathing.

Effects of moderate hypothermia on baroreflex and pulmonary chemoreflex heart rate response in decerebrate ferrets.

The effects on the baroreceptor reflexes and pulmonary reflexes of moderate immersion hypothermia (core temperature, 30.2 +/- 0.07 degrees C) have been investigated in the decerebrate ferret. Baroreflex sensitivity was estimated from the relationship between change in cardiac interval and change in systolic blood pressure following i.v. bolus injection of phenylephrine. The responses to phenylephrine in ferrets were best fitted by two linear slopes with an initial shallow slope, a, followed by a steeper slope, b. The slope or sensitivity of b was increased significantly by moderate cooling (n = 6 animals, 36 responses, P < 0.05). Pulmonary J-receptor reflex sensitivity was assessed by i.v. injection of phenylbiguanide (PBG), which evokes a dose-dependent bradycardia. The bradycardic response was unaltered by the hypothermia (n = 6 animals, 88 responses). Electrical stimulation of the right peripheral vagal nerve was employed to assess effects on the efferent components of the reflexes. The bradycardia, in response to stimulation, was significantly increased by moderate cooling at all stimulation frequencies (n = 8 animals, 88 responses, P < 0.001). Thus the results suggested that baroreceptor heart rate reflex sensitivity was enhanced by the moderate hypothermia. At least one component of the enhanced baroreflex response may be the result of changes in the efferent pathway of the reflex response. However, the absence of effect of PBG may indicate a differential afferent and efferent organization of pulmonary J-receptors compared with baroreceptors.

Pulmonary chemoreflex and phrenic sympathetic efferents

The pulmonary chemoreflex components such as reactions of phrenic sympathetic neuron (PhSN) activity, phrenic nerve activity, heart rate and blood pressure were tested in chloralose-anesthetized, paralyzed cats. 10 μg to 160 μg phenylbiguanide (PBG) in 0.9% NaCl was injected into the pulmonary circulation. PBG injected into the right atrium (in 11 of 19 experiments) and into the pulmonary artery (in 5 of 8 experiments), evoked short-latency (1–1.4 sec) dose-dependent increase in PhSN activity accompanied by increase in blood pressure, and followed by decrease in these two variables. In all experiments, activity of the phrenic nerve was depressed, and bradycardia occurred after PBG injection. All responses to PBG injection into the pulmonary artery were abolished following bilateral vagotomy. In the same procedure related to the right atrium after vagotomy, the increases in PhSN activity and blood pressure were also abolished, although a decrease in heart rate, PhSN activity and in the amplitude of phrenic nerve discharges together with an increase in their frequency persisted. Our results suggest that short-latency increase in PhSN activity is a component of pulmonary chemoreflex.

Pulmonary C-fibers elicit both apneusis and tachypnea in the rabbit

The effects of phenylbiguanide (PBG) on phrenic nerve and pulmonary C-fibers were studied in anesthetized spontaneously breathing rabbits with unilateral vagotomy. Right atrial injections of PBG at low (10 μg/kg) and high (100 μg/kg) dose resulted in a shallow tachypnea and apneusis followed by tachypnea, respectively, and these effects were blocked by procaine treatment of the vagus nerve. Also, the injection of PBG (100 μg/kg) still evoked the rapid shallow breathing preceded by apneusis in carotid chemoreceptor-denervated animals. Vigorous stimulation of pulmonary C-fibers by PBG (100 μg/kg) coincided with apneusis and the response was followed by a more modest increase in activity associated with tachypnea. Administration of PBG (10 μg/kg) into the right atrium caused an increase in pulmonary C-fiber activity associated with tachypnea. However, a small dose of PBG injected into the aortic circulation had no effect on the C-fiber activity but did inhibit respiration. These results suggest that the stimulation of pulmonary C-fibers via PBG injection can produce both inspiratory apnea and tachypnea.

Some reflex cardioinhibitory responses in the cat and their modulation by central inspiratory neuronal activity.

1. Cats were anaesthetized with a mixture of chloralose and urethane, and were artifically ventilated. 2. An open pneumothorax was provided by two large-bore tubes which were sealed in the sixth intercostal space on each side. They were connected to a Fleisch pneumotachograph. Phasic changes in central inspiratory neuronal activity were measured quantitatively as changes in the volume of the pneumothorax during temporary interruption of artificial respiration, the volume of the lungs being held constant at their end-expiratory level. In this way the activity of slowly adapting pulmonary stretch receptors was maintained constant. 3. Reflex cardioinhibitory responses were elicited by stimulation of (a) the carotid body chemoreceptors by intracarotid injections of cyanide; (b) the arterial baroreflex by controlled elevations of the blood pressure; (c) cardiac receptors by left atrial injections of veratridine; and (d) pulmonary C fibres (including J receptors) by right atrial injections of phenylbiguanide. 4. The effects of central inspiratory neuronal activity on pulse interval were assessed by comparing the values observed during the inspiratory and expiratory phases of the respiratory cycle in the control state and during stimulation of each cardiovascular receptor group. 5. The carotid chemoreceptor-induced bradycardia measured during the expiratory phase of respiration was reduced during inspiration to a value of about 15% of control. The central inspiratory drive was less effective in altering the reflex responses from the arterial baroreceptors and cardiac receptors, the corresponding values being 42 and 51% respectively. 6. In contrast, the bradycardia evoked by pulmonary C fibre stimulation was not significantly affected by the central inspiratory drive. 7. The differential nature of the modulation by the central inspiratory drive occurred independently of the integrity of the sympathetic nerve supply to the heart indicating that the cardiac efferents involved were largely fibres in the vagus nerves. 8. The possible explanation of these results in terms of central mechanisms is discussed.

Evidence that central 5‐HT1A‐receptors play a role in the von Bezold‐Jarisch reflex in the rat

1. The effects of intracisternal (i.c.) application of putative 5-hydroxytryptamine (5-HT)1A antagonists on the reflex bradycardia evoked by injection of phenylbiguanide (i.v.) were investigated in anaesthetized, atenolol-pretreated rats. 2. Intracisternal application of spiperone (100 micrograms kg-1) reversibly attenuated the reflex bradycardia whilst the same dose given i.v. had no effect. The bradycardia was also attenuated by i.c. methiothepin (200 micrograms kg-1), (+/-)-pindolol (100 micrograms kg-1) and buspirone (200 micrograms kg-1) but was not attenuated by antagonists selective for alpha 1-adrenoceptors (alfuzosin; 100 micrograms kg-1), 5-HT2-receptors (BW 501C67; 100 micrograms kg-1) or dopamine D2-receptors ((-)-sulpiride; 100 micrograms kg-1) given i.c. 3. It is concluded that the 5-HT1A-receptor antagonist action of intracisternally applied spiperone, methiothepin, (+/-)-pindolol and buspirone is responsible for the ability of these drugs to attenuate reversibly the excitation of cardiac vagal motoneurones caused by activation of the von Bezold-Jarisch reflex.

Rapid and transient excitation of respiration mediated by central chemoreceptor.

We evaluated rapid and transient changes in phrenic nerve (PN) and internal intercostal (IIC) activities when 0.2-0.5 ml of saline saturated with 100% CO2 was injected into the vertebral artery during various respiratory phases in decerebrated spontaneously breathing cats. The injections evoked an initial transient inhibition of ongoing PN or IIC activity with a mean onset latency of 0.17 s, followed by excitation of subsequent respiratory activities with an onset latency ranging from 0.4 to 2.7 s; the average onset latency of expiratory excitation (1.49 s) was significantly longer than that of inspiratory facilitation (0.89 s). The initial inhibitory responses were analogous to reflex effects of injections of phenyl biguanide, indicating that the initial inhibition was due to activation of vascular nociceptors and the subsequent excitation was due to stimulation of the central chemoreceptors. In addition, CO2-saline injections during hypocapnic apnea developed a quick reappearance of respiratory rhythm, and the first facilitatory effect appeared in tonic IIC activity, which became more active before rhythm started. In summary, the present study, by use of a technique of vertebral arterial injections of 100% CO2-saline, revealed dynamic properties of respiratory control system mediated by central chemoreceptors and vascular nociceptors.

Lung reflexes affecting the larynx in the pig, and the effect of pulmonary microembolism.

Laryngeal airflow resistance was measured in anaesthetized pigs during stimulation of lung vagal reflexes by injection of phenylbiguanide and of capsaicin, before and during pulmonary microembolism due to intravenous injection of hardened red blood cells; the microembolism caused pulmonary hypertension. The Breuer-Hering reflex was also assessed before and after pulmonary microembolism. Phenylbiguanide and capsaicin caused apnoea followed by rapid shallow breathing, hypertension, bradycardia and laryngeal constriction in inspiration and expiration. Most of these effects were abolished by bilateral cervical vagotomy. Pulmonary microembolism caused only small changes in breathing pattern, mainly a decrease in inspiratory time. After microembolism the Breuer-Hering reflex was enhanced, and injections of phenylbiguanide and capsaicin caused longer apnoeas by a vagal reflex. The changes in pattern of breathing and in lung reflexes after lung microembolism and during the associated pulmonary hypertension are consistent with an enhancement of pulmonary stretch receptor activity in this condition, but do not indicate any important role for C fibre reflexes.