Adrenergic mechanisms and chemoreception in the carotid body of the cat and rabbit

Loss of ganglioglomerular nerve input to the carotid body impacts the hypoxic ventilatory response in freely-moving rats

The carotid bodies are the primary sensors of blood pH, pO2 and pCO2. The ganglioglomerular nerve (GGN) provides post-ganglionic sympathetic nerve input to the carotid bodies, however the physiological relevance of this innervation is still unclear. The main objective of this study was to determine how the absence of the GGN influences the hypoxic ventilatory response in juvenile rats. As such, we determined the ventilatory responses that occur during and following five successive episodes of hypoxic gas challenge (HXC, 10% O2, 90% N2), each separated by 15 min of room-air, in juvenile (P25) sham-operated (SHAM) male Sprague Dawley rats and in those with bilateral transection of the ganglioglomerular nerves (GGNX). The key findings were that 1) resting ventilatory parameters were similar in SHAM and GGNX rats, 2) the initial changes in frequency of breathing, tidal volume, minute ventilation, inspiratory time, peak inspiratory and expiratory flows, and inspiratory and expiratory drives were markedly different in GGNX rats, 3) the initial changes in expiratory time, relaxation time, end inspiratory or expiratory pauses, apneic pause and non-eupneic breathing index (NEBI) were similar in SHAM and GGNX rats, 4) the plateau phases obtained during each HXC were similar in SHAM and GGNX rats, and 5) the ventilatory responses that occurred upon return to room-air were similar in SHAM and GGNX rats. Overall, these changes in ventilation during and following HXC in GGNX rats raises the possibility the loss of GGN input to the carotid bodies effects how primary glomus cells respond to hypoxia and the return to room-air.

The Role of Pharmacological Treatment in the Chemoreflex Modulation

From a physiological point of view, peripheral chemoreceptors (PCh) are the main sensors of hypoxia in mammals and are responsible for adaptation to hypoxic conditions. Their stimulation causes hyperventilation—to increase oxygen uptake and increases sympathetic output in order to counteract hypoxia-induced vasodilatation and redistribute the oxygenated blood to critical organs. While this reaction promotes survival in acute settings it may be devastating when long-lasting. The permanent overfunctionality of PCh is one of the etiologic factors and is responsible for the progression of sympathetically-mediated diseases. Thus, the deactivation of PCh has been proposed as a treatment method for these disorders. We review here physiological background and current knowledge regarding the influence of widely prescribed medications on PCh acute and tonic activities.

Loss of Cervical Sympathetic Chain Input to the Superior Cervical Ganglia Affects the Ventilatory Responses to Hypoxic Challenge in Freely-Moving C57BL6 Mice

The cervical sympathetic chain (CSC) innervates post-ganglionic sympathetic neurons within the ipsilateral superior cervical ganglion (SCG) of all mammalian species studied to date. The post-ganglionic neurons within the SCG project to a wide variety of structures, including the brain (parenchyma and cerebral arteries), upper airway (e.g., nasopharynx and tongue) and submandibular glands. The SCG also sends post-ganglionic fibers to the carotid body (e.g., chemosensitive glomus cells and microcirculation), however, the function of these connections are not established in the mouse. In addition, nothing is known about the functional importance of the CSC-SCG complex (including input to the carotid body) in the mouse. The objective of this study was to determine the effects of bilateral transection of the CSC on the ventilatory responses [e.g., increases in frequency of breathing (Freq), tidal volume (TV) and minute ventilation (MV)] that occur during and following exposure to a hypoxic gas challenge (10% O₂ and 90% N₂) in freely-moving sham-operated (SHAM) adult male C57BL6 mice, and in mice in which both CSC were transected (CSCX). Resting ventilatory parameters (19 directly recorded or calculated parameters) were similar in the SHAM and CSCX mice. There were numerous important differences in the responses of CSCX and SHAM mice to the hypoxic challenge. For example, the increases in Freq (and associated decreases in inspiratory and expiratory times, end expiratory pause, and relaxation time), and the increases in MV, expiratory drive, and expiratory flow at 50% exhaled TV (EF₅₀) occurred more quickly in the CSCX mice than in the SHAM mice, although the overall responses were similar in both groups. Moreover, the initial and total increases in peak inspiratory flow were higher in the CSCX mice. Additionally, the overall increases in TV during the latter half of the hypoxic challenge were greater in the CSCX mice. The ventilatory responses that occurred upon return to room-air were essentially similar in the SHAM and CSCX mice. Overall, this novel data suggest that the CSC may normally provide inhibitory input to peripheral (e.g., carotid bodies) and central (e.g., brainstem) structures that are involved in the ventilatory responses to hypoxic gas challenge in C57BL6 mice.

Autonomic innervation of the carotid body as a determinant of its sensitivity: implications for cardiovascular physiology and pathology

The motivation for this review comes from the emerging complexity of the autonomic innervation of the carotid body (CB) and its putative role in regulating chemoreceptor sensitivity. With the carotid bodies as a potential therapeutic target for numerous cardiorespiratory and metabolic diseases, an understanding of the neural control of its circulation is most relevant. Since nerve fibres track blood vessels and receive autonomic innervation, we initiate our review by describing the origins of arterial feed to the CB and its unique vascular architecture and blood flow. Arterial feed(s) vary amongst species and, unequivocally, the arterial blood supply is relatively high to this organ. The vasculature appears to form separate circuits inside the CB with one having arterial venous anastomoses. Both sympathetic and parasympathetic nerves are present with postganglionic neurons located within the CB or close to it in the form of paraganglia. Their role in arterial vascular resistance control is described as is how CB blood flow relates to carotid sinus afferent activity. We discuss non-vascular targets of autonomic nerves, their possible role in controlling glomus cell activity, and how certain transmitters may relate to function. We propose that the autonomic nerves sub-serving the CB provide a rapid mechanism to tune the gain of peripheral chemoreflex sensitivity based on alterations in blood flow and oxygen delivery, and might provide future therapeutic targets. However, there remain a number of unknowns regarding these mechanisms that require further research that is discussed.

G-Protein-Coupled Receptor (GPCR) Signaling in the Carotid Body: Roles in Hypoxia and Cardiovascular and Respiratory Disease

The carotid body (CB) is an important organ located at the carotid bifurcation that constantly monitors the blood supplying the brain. During hypoxia, the CB immediately triggers an alarm in the form of nerve impulses sent to the brain. This activates protective reflexes including hyperventilation, tachycardia and vasoconstriction, to ensure blood and oxygen delivery to the brain and vital organs. However, in certain conditions, including obstructive sleep apnea, heart failure and essential/spontaneous hypertension, the CB becomes hyperactive, promoting neurogenic hypertension and arrhythmia. G-protein-coupled receptors (GPCRs) are very highly expressed in the CB and have key roles in mediating baseline CB activity and hypoxic sensitivity. Here, we provide a brief overview of the numerous GPCRs that are expressed in the CB, their mechanism of action and downstream effects. Furthermore, we will address how these GPCRs and signaling pathways may contribute to CB hyperactivity and cardiovascular and respiratory disease. GPCRs are a major target for drug discovery development. This information highlights specific GPCRs that could be targeted by novel or existing drugs to enable more personalized treatment of CB-mediated cardiovascular and respiratory disease.

Adrenaline activation of the carotid body: Key to CO2 and pH homeostasis in hypoglycaemia and potential pathological implications in cardiovascular disease

Ventilatory and neuroendocrine counter-regulatory responses during hypoglycaemia are essential in order to maintain glycolysis and prevent rises in P_aCO₂ leading to systemic acidosis. The mammalian carotid body has emerged as an important driver of hyperpnoea and glucoregulation in hypoglycaemia. However, the adequate stimulus for CB stimulation in hypoglycaemia has remained controversial for over a decade. The recent finding that adrenaline is a physiological activator of CB in hypoglycaemia raises the intriguing possibility that CB stimulation and hyperpnoea may be necessary to maintain pH in other adrenaline-related hypermetabolic states such as exercise. This review will therefore focus on 1) The important functional contribution of the CB in the counter-regulatory and ventilatory response to hypoglycaemia, 2) the proposed mechanisms that cause CB stimulation in hypoglycaemia including hormonal activation by adrenaline and direct low glucose sensing and 3) the possible pathological consequences of repetitive CB activation by adrenaline that could potentially be targeted to reduce CB-mediated cardiovascular disease.

Adrenaline release evokes hyperpnoea and an increase in ventilatory CO2 sensitivity during hypoglycaemia: a role for the carotid body

KEY POINTS

Hypoglycaemia is counteracted by release of hormones and an increase in ventilation and CO2 sensitivity to restore blood glucose levels and prevent a fall in blood pH. The full counter-regulatory response and an appropriate increase in ventilation is dependent on carotid body stimulation. We show that the hypoglycaemia-induced increase in ventilation and CO2 sensitivity is abolished by preventing adrenaline release or blocking its receptors. Physiological levels of adrenaline mimicked the effect of hypoglycaemia on ventilation and CO2 sensitivity. These results suggest that adrenaline, rather than low glucose, is an adequate stimulus for the carotid body-mediated changes in ventilation and CO2 sensitivity during hypoglycaemia to prevent a serious acidosis in poorly controlled diabetes.

ABSTRACT

Hypoglycaemia in vivo induces a counter-regulatory response that involves the release of hormones to restore blood glucose levels. Concomitantly, hypoglycaemia evokes a carotid body-mediated hyperpnoea that maintains arterial CO2 levels and prevents respiratory acidosis in the face of increased metabolism. It is unclear whether the carotid body is directly stimulated by low glucose or by a counter-regulatory hormone such as adrenaline. Minute ventilation was recorded during infusion of insulin-induced hypoglycaemia (8-17 mIU kg(-1)  min(-1) ) in Alfaxan-anaesthetised male Wistar rats. Hypoglycaemia significantly augmented minute ventilation (123 ± 4 to 143 ± 7 ml min(-1) ) and CO2 sensitivity (3.3 ± 0.3 to 4.4 ± 0.4 ml min(-1)  mmHg(-1) ). These effects were abolished by either β-adrenoreceptor blockade with propranolol or adrenalectomy. In this hypermetabolic, hypoglycaemic state, propranolol stimulated a rise in P aC O2, suggestive of a ventilation-metabolism mismatch. Infusion of adrenaline (1 μg kg(-1)  min(-1) ) increased minute ventilation (145 ± 4 to 173 ± 5 ml min(-1) ) without altering P aC O2 or pH and enhanced ventilatory CO2 sensitivity (3.4 ± 0.4 to 5.1 ± 0.8 ml min(-1)  mmHg(-1) ). These effects were attenuated by either resection of the carotid sinus nerve or propranolol. Physiological concentrations of adrenaline increased the CO2 sensitivity of freshly dissociated carotid body type I cells in vitro. These findings suggest that adrenaline release can account for the ventilatory hyperpnoea observed during hypoglycaemia by an augmented carotid body and whole body ventilatory CO2 sensitivity.

Hypoxia increases exercise heart rate despite combined inhibition of β-adrenergic and muscarinic receptors

Hypoxia increases the heart rate response to exercise, but the mechanism(s) remains unclear. We tested the hypothesis that the tachycardic effect of hypoxia persists during separate, but not combined, inhibition of β-adrenergic and muscarinic receptors. Nine subjects performed incremental exercise to exhaustion in normoxia and hypoxia (fraction of inspired O2 = 12%) after intravenous administration of 1) no drugs (Cont), 2) propranolol (Prop), 3) glycopyrrolate (Glyc), or 4) Prop + Glyc. HR increased with exercise in all drug conditions ( P < 0.001) but was always higher at a given workload in hypoxia than normoxia ( P < 0.001). Averaged over all workloads, the difference between hypoxia and normoxia was 19.8 ± 13.8 beats/min during Cont and similar (17.2 ± 7.7 beats/min, P = 0.95) during Prop but smaller ( P < 0.001) during Glyc and Prop + Glyc (9.8 ± 9.6 and 8.1 ± 7.6 beats/min, respectively). Cardiac output was enhanced by hypoxia ( P < 0.002) to an extent that was similar between Cont, Glyc, and Prop + Glyc (2.3 ± 1.9, 1.7 ± 1.8, and 2.3 ± 1.2 l/min, respectively, P > 0.4) but larger during Prop (3.4 ± 1.6 l/min, P = 0.004). Our results demonstrate that the tachycardic effect of hypoxia during exercise partially relies on vagal withdrawal. Conversely, sympathoexcitation either does not contribute or increases heart rate through mechanisms other than β-adrenergic transmission. A potential candidate is α-adrenergic transmission, which could also explain why a tachycardic effect of hypoxia persists during combined β-adrenergic and muscarinic receptor inhibition.

Revisiting cAMP signaling in the carotid body

Chronic carotid body (CB) activation is now recognized as being essential in the development of hypertension and promoting insulin resistance; thus, it is imperative to characterize the chemotransduction mechanisms of this organ in order to modulate its activity and improve patient outcomes. For several years, and although controversial, cyclic adenosine monophosphate (cAMP) was considered an important player in initiating the activation of the CB. However, its relevance was partially displaced in the 90s by the emerging role of the mitochondria and molecules such as AMP-activated protein kinase and O₂-sensitive K⁺ channels. Neurotransmitters/neuromodulators binding to metabotropic receptors are essential to chemotransmission in the CB, and cAMP is central to this process. cAMP also contributes to raise intracellular Ca²⁺ levels, and is intimately related to the cellular energetic status (AMP/ATP ratio). Furthermore, cAMP signaling is a target of multiple current pharmacological agents used in clinical practice. This review (1) provides an outline on the classical view of the cAMP-signaling pathway in the CB that originally supported its role in the O₂/CO₂ sensing mechanism, (2) presents recent evidence on CB cAMP neuromodulation and (3) discusses how CB activity is affected by current clinical therapies that modify cAMP-signaling, namely dopaminergic drugs, caffeine (modulation of A_2A/A_2B receptors) and roflumilast (PDE4 inhibitors). cAMP is key to any process that involves metabotropic receptors and the intracellular pathways involved in CB disease states are likely to involve this classical second messenger. Research examining the potential modification of cAMP levels and/or interactions with molecules associated with CB hyperactivity is currently in its beginning and this review will open doors for future explorations.

The impact of acute and chronic catecholamines on respiratory responses to hypoxic stress in the rat

Chronic catecholamine production is associated with desensitisation and down-regulation of adrenergic receptors and occurs in conditions, such as heart failure and myocardial infarction. The effects of further acute adrenergic stimulation, which may occur during exercise, and their subsequent effects on chemosensitivity and ventilation are unclear. Chronic isoprenaline (ISO) increased ventilation by 50 % (P < 0.05) yet the sensitivity to graded hypoxia was preserved. Acute noradrenaline (NA) in control animals led to a doubling of ventilation in hyperoxia (P < 0.001), and this difference was preserved in graded hypoxia (P < 0.001). Yet, combination of NA + ISO did not increase ventilation beyond ISO at baseline or in hypoxia. ISO, NA, and NA + ISO all induced a metabolic acidosis (P < 0.05) with enhanced ventilation in partial compensation. Carotid sinus nerve (CSN) section led to a partial loss of catecholamine-induced augmentation in ventilation (P < 0.05), yet direct recording from CSN in vitro suggests catecholamine is inhibitory for CSN discharge. These observations suggest that chronic catecholamine exposure may result in decreased exercise performance as a direct consequence of the hyperpnea to compensate for an increased metabolic rate coupled with acidosis and leading to increased central chemosensitivity. A limited contribution from peripheral chemoreceptors was noted but was not a consequence of catecholamine stimulation of the carotid body.

Methamphetamine causes sustained depression in cerebral blood flow

The use prevalence of the highly addictive psychostimulant methamphetamine (MA) has been steadily increasing over the past decade. MA abuse has been associated with both transient and permanent alterations in cerebral blood flow (CBF), hemorrhage, cerebrovascular accidents and death. To understand MA-induced changes in CBF, we exposed C56BL/6 mice to an acute bolus of MA (5mg/kg MA, delivered IP). This elicited a biphasic CBF response, characterized by an initial transient increase (~ 5 minutes) followed by a prolonged decrease (~ 30 minutes) of approximately 25% relative to baseline CBF--as measured by laser Doppler flowmetry over the somatosensory cortex. To assess if this was due to catecholamine derived vasoconstriction, phentolamine, an α-adrenergic antagonist was administered prior to MA treatment. This reduced the initial increase in CBF but failed to prevent the subsequent, sustained decrease in CBF. Consistent with prior reports, MA caused a transient increase in mean arterial blood pressure, body temperature and respiratory rate. Elevated respiratory rate resulted in hypocapnia. When respiratory rate was controlled by artificially ventilating mice, blood PaCO(2) levels after MA exposure remained unchanged from physiologic levels, and the MA-induced decrease in CBF was abolished. In vivo two-photon imaging of cerebral blood vessels revealed sustained MA-induced vasoconstriction of pial arterioles, consistent with laser Doppler flowmetry data. These findings show that even a single, acute exposure to MA can result in profound changes in CBF, with potentially deleterious consequences for brain function.

Dopaminergic metabolism in carotid bodies and high-altitude acclimatization in female rats

We tested the hypothesis that ovarian steroids stimulate breathing through a dopaminergic mechanism in the carotid bodies. In ovariectomized female rats raised at sea level, domperidone, a peripheral D2-receptor antagonist, increased ventilation in normoxia (minute ventilation = +55%) and acute hypoxia (+32%). This effect disappeared after 10 daily injections of ovarian steroids (progesterone + estradiol). At high altitude (3,600 m, Bolivian Institute for High-Altitude Biology-IBBA, La Paz, Bolivia), neutered females had higher carotid body tyrosine hydroxylase activity (the rate-limiting enzyme for catecholamine synthesis: +129%) and dopamine utilization (+150%), lower minute ventilation (-30%) and hypoxic ventilatory response (-57%), and higher hematocrit (+18%) and Hb concentration (+21%) than intact female rats. Consistent signs of arterial pulmonary hypertension (right ventricular hypertrophy) also appeared in ovariectomized females. None of these parameters was affected by gonadectomy in males. Our results show that ovarian steroids stimulate breathing by lowering a peripheral dopaminergic inhibitory drive. This process may partially explain the deacclimatization of postmenopausal women at high altitude.

Ventilatory effects of 8 h of isocapnic hypoxia with and without beta-blockade in humans

This study investigated whether changing sympathetic activity, acting via beta-receptors, might induce the progressive ventilatory changes observed in response to prolonged hypoxia. The responses of 10 human subjects to four 8-h protocols were compared: 1) isocapnic hypoxia (end-tidal PO2 = 50 Torr) plus 80-mg doses of oral propranolol; 2) isocapnic hypoxia, as in protocol 1, with oral placebo; 3) air breathing with propranolol; and 4) air breathing with placebo. Exposures were conducted in a chamber designed to maintain end-tidal gases constant by computer control. Ventilation (VE) was measured at regular intervals throughout. Additionally, the subjects' ventilatory hypoxic sensitivity and their residual VE during hyperoxia (5 min) were assessed at 0, 4, and 8 h by using a dynamic end-tidal forcing technique. beta-Blockade did not significantly alter either the rise in VE seen during 8 h of isocapnic hypoxia or the changes observed in the acute hypoxic ventilatory response and residual VE in hyperoxia over that period. The results do not provide evidence that changes in sympathetic activity acting via beta-receptors play a role in the mediation of ventilatory changes observed during 8 h of isocapnic hypoxia.

Dopaminergic modulation of respiratory motor output in peripherally chemodenervated goats

We examined the ventilatory effects of exogenous dopamine (DA) and norepinephrine (NE) administration in chloralose-anesthetized, paralyzed, artificially ventilated adult goats before and after carotid body denervation (CBD). Intravenous (iv) DA bolus injections and slow iv infusions caused dose-dependent inhibition of phrenic nerve activity (PNA) in carotid body (CB)-intact animals during normoxia and hyperoxia but not during hypercapnia. NE administration in CB-intact goats caused dose-dependent inhibition of PNA of similar magnitude to DA trials. The DA D2-receptor agonists quinelorane and quinpirole inhibited PNA, whereas the DA D1-receptor agonist SKF-81297 had no effect. After CBD, the ventilatory depressant effects of DA persisted, but responses were significantly attenuated compared with CB-intact trials. CBD abolished the inhibitory effect of low-dose NE administration but did not alter ventilatory responses to high-dose NE injection. The peripheral DA D2-receptor antagonist domperidone substantially attenuated the inhibitory effects of DA bolus injections and infusions and reversed the inhibitory ventilatory effect of high-dose DA administration to excitation in some animals. The alpha-adrenoceptor antagonist phentolamine had no effect on DA-induced ventilatory depression. Beta-Adrenoceptor stimulation with isoproterenol produced similar hemodynamic effects to DA administration but had no effect on PNA. We conclude that DA and NE exert both CB-mediated and non-CB-mediated inhibitory effects on respiratory motor output in anesthetized goats. The ventilatory depressant effects that persist in peripherally chemodenervated animals are DA D2-receptor mediated, but their exact location remains speculative.

Propranolol blocks the stimulatory effects of naloxone on ventilation and oxygen consumption in hamsters

The purposes of these studies were: 1) to determine the effects of various doses of propranolol, a nonspecific beta-adrenergic antagonist, on ventilation, oxygen consumption, and body temperature in hamsters, and 2) to test the hypothesis that in hamsters the stimulatory effects of naloxone, an opioid receptor antagonist, on ventilation and oxygen consumption occur, at least in part, through the release of catecholamines that act via beta-adrenergic receptors. Propranolol, a non-specific beta adrenergic receptor antagonist, at a 20 mg/kg depressed body temperature, oxygen consumption, tidal volume, and ventilation relative to saline. The lower dose of 10 mg/kg had only transitory effects on tidal volume at 60 min and ventilation at 30 min post-injection-Naloxone (1 mg/kg) relative to saline stimulated ventilation and oxygen consumption. These effects were blocked by propranolol pretreatment. The results of these experiments demonstrate that in the hamster, 1) body temperature, oxygen consumption, and ventilation appear to be modulated by beta-adrenergic receptors, and 2) the stimulatory effects of naloxone on oxygen consumption and ventilation may occur through the interaction of endogenous opioids and beta-adrenergic receptor systems.

Potassium and breathing in exercise

The increase in ventilation caused by exercise is controlled by a combination of neural and chemical events, although the precise contribution and relative importance of these signals is still debated. It is generally agreed that the genesis of exercise hyperpnoea lies within the central nervous system and that peripheral reflexes, both chemical and neural, modulate central drive. Recently, attention has once again focused on the idea that circulating factors, in particular potassium, may play an important role in this modulation by stimulating known areas of peripheral chemoreception. Arterial chemoreceptors, muscle chemoreflex and slowly adapting pulmonary stretch receptors are all excited by hyperkalaemia. When potassium is raised to mimic exercise concentrations it increases ventilation in anaesthetised animals. This response is abolished by surgical denervation of the arterial chemoreceptors and is markedly reduced by chemical denervation with hyperoxia. Hypoxia enhances the ventilatory response to hyperkalaemia, and the stimulatory effects of potassium are further increased when combined with lactic acid or raised concentrations of noradrenaline. Hyperkalaemia can also increase the hypoxic sensitivity of the arterial chemoreflex in exercise. There is a close temporal relationship between potassium and ventilation during exercise, but changes in potassium are not proportionally related to changes in ventilation. When all data are taken together, there is good evidence that potassium has a supporting role in the control of exercise hyperpnoea, predominantly through modulation of the arterial chemoreflex.

Mechanisms of alpha2-adrenoceptor-mediated inhibition in rabbit carotid body

We have used the in vitro preparation of the intact carotid body (CB) and isolated chemoreceptor cells to elucidate the distribution and function of alpha2-adrenoreceptors. The significance of the study lies in the fact that norepinephrine (NE), being the neurotransmitter of the sympathetic innervation to the CB, is also abundant in chemoreceptor cells. In intact CB whose catecholamine (CA) deposits had been labeled by prior incubation with the CA precursor [3H]tyrosine, the alpha2-antagonist yohimbine (10 microM) potentiated the low-PO2 (33 and 60 mmHg)-induced release of [3H]CA by 100 and 53%, respectively. Yohimbine (10 microM) and SKF-86466 (50 microM; another alpha2-antagonist) reversed the inhibition of the release of [3H]CA produced by the alpha2-receptor agonists clonidine and UK-14304 (10 microM). The increase in adenosine 3',5'-cyclic monophosphate produced by low PO2 was further augmented by yohimbine and nearly halved by UK-14304 and clonidine. In isolated chemoreceptor cells, UK-14304 and NE inhibited voltage-dependent Ca2+ currents by 28 and 32%, respectively. These results indicate that alpha2-receptors are present in chemoreceptor cells, where they reduce the release of [3H]CA. Inhibition of adenylate cyclase(s) and Ca2+ channels may be involved in this effect. Using intact CB from normal and chronically sympathectomized animals, we demonstrated a specific accumulation of [3H]NE in intraglomic sympathetic endings. Hypoxia (PO2 approximately 33 mmHg) did not elicit release of [3H]NE from the sympathetic endings, but high extracellular K+ (K+(e)) induced a release of [3H]NE that was inhibited by alpha2-agonists and augmented by alpha2-antagonists. These findings demonstrate that alpha2-receptors are also present in the sympathetic endings of the CB, where they modulate the release of NE. As a whole, this work provides a more detailed understanding of the role of the sympathetic innervation in the control of the CB chemoreceptor function, including the cellular mechanisms of the action of NE.

Decreased carotid body hypoxic sensitivity in chronic hypoxia: role of dopamine

Previously we showed that prolonged exposure to severe hypoxia produces decreased peripheral chemoreceptor responsiveness to hypoxia and attenuates central nervous system (CNS) chemosensory translation, which together may contribute to the decreased hypoxic ventilatory response (HVR) in chronic hypoxia. In this study, we sought to determine whether the central or peripheral activity of endogenous dopamine modulates this decreased HVR. We examined the effects of peripheral and central dopamine receptor blockade on HVR and carotid sinus nerve (CNS) response to hypoxia in controls and in cats exposed to a simulated altitude of 5500 m for 3 weeks. Domperidone increased CSN response to hypoxia in hypoxic cats to levels similar to those observed in controls. HVR was also augmented by domperidone in hypoxic cats, but remained below that of controls. As a result, the CNS chemosensory translation remained reduced in hypoxic animals. We further treated animals with haloperidol. However, this combined treatment with domperidone and haloperidol led to no further increase in CSN or ventilatory responses to hypoxia, or in CNS chemosensory translation in hypoxic cats. Thus, decreased HVR in hypoxic cats is mediated both by depression of hypoxic sensitivity of the carotid body, which is largely dopaminergic, and by decreased CNS chemosensory translation which must involve non-dopaminergic mechanisms.

Cardio-ventilatory control in rainbow trout: I. Pharmacology of branchial, oxygen-sensitive chemoreceptors

The effects of various neurochemicals on O2-sensitive chemoreceptor afferent discharge in the glossopharyngeal nerve (cranial nerve IX) were examined in an isolated, perfused first gill arch preparation from rainbow trout. Afferent neural activity from O2 receptors in the first gill arch increased in response to hypoxic perfusate and NaCN. Adrenergic agonists (epinephrine, norepinephrine and isoproterenol) had little effect on neural activity. Dopamine and 5-hydroxytryptamine (serotonin) caused a brief, small burst in chemoreceptor activity followed by a mild inhibition of receptor discharge. Acetylcholine and nicotine were potent neurochemical stimulants; muscarine had only a slight effect. While atropine completely blocked the effects of acetylcholine on receptor discharge, it only slightly inhibited responses to hypoxia and NaCN. Thus, although cholinergic mechanisms appear more likely than either adrenergic or serotonergic mechanisms to alter cardiovascular and ventilatory reflexes in fishes through their effects on O2-sensitive chemoreceptor activity, the transduction process involved in O2-chemoreception appears to be complex and not dependent on any single one of the neurochemicals tested.

Acute effects of inhaled salbutamol on the metabolic rate of normal subjects

BACKGROUND

This study was designed to investigate the contribution of inhaled salbutamol to the increase in resting metabolic rate found in patients with chronic airflow limitation who were receiving bronchodilator therapy.

METHODS

The resting metabolic rate of 10 normal subjects (age 20-47 years, weight 42-105 kg, seven men) was studied after inhalations of salbutamol or placebo. An open canopy method of indirect calorimetry was used to measure resting oxygen consumption (VO2) and resting carbon dioxide production (VCO2). Subjects inhaled two, four, eight, or 12 puffs (100 micrograms/puff) of salbutamol or placebo in a double blind manner. Recordings of VO2 and VCO2 were made after inhalation of the four doses of salbutamol or placebo, integrated over one hour, and compared.

RESULTS

VO2 and VCO2 increased in a dose dependent manner after inhaled salbutamol with a maximum effect at five minutes after inhalation. After four puffs, VO2 was 203 and 188 ml/kg/h for salbutamol and placebo respectively. After eight puffs, VO2 was 207 and 185 and VCO2 was 167 and 155 ml/kg/h. After 12 puffs, VO2 was 220 and 190 with a VCO2 of 181 and 168 ml/kg/h. Twelve puffs of salbutamol increased the mean (SE) respiratory quotient from 0.85 (0.01) to 0.93 (0.04) at five minutes indicating an increase in ventilation in excess of metabolic demand. Mean heart rate increased in parallel with VO2.

CONCLUSION

Inhaled salbutamol significantly increases resting metabolic rate in a dose dependent manner.

Carotid body noradrenergic sensitivity in ventilatory acclimatization to hypoxia

Norepinephrine inhibits ventilation in awake goats under normoxic, resting conditions. This inhibition is carotid body (CB) mediated and may be due to stimulation of noradrenergic receptor on the CB. Cao et al. (FASEB J. A118, 1991) recently suggested that CB noradrenergic receptors may be down regulated following 24-36 hours of hypoxic exposure in cats. Our study was aimed at determining whether a change in noradrenergic receptor sensitivity during ventilatory acclimatization to hypoxia (VAH) was responsible for the increased sensitivity of the CB to hypoxia during prolonged exposure to hypoxia in goats. We tested this hypothesis using intracarotid infusions of norepinephrine (NE) (0.5, 1.0, 5.0 micrograms.kg-1.min-1) and dopamine (DA) (1.0 micrograms.kg-1.min-1) in awake goats under control normoxic conditions, during 4 h of isocapnic hypoxia, and upon return to normoxia. NE and DA (1.0 micrograms.kg-1.min-1) both inhibited control ventilation significantly during the intracarotid infusions (56% and 62% decreases, respectively). No significant differences were found between the pre- and post-hypoxic infusions of NE and DA in normoxia. During hypoxia, inhibition of VE during NE and DA infusions was attenuated relative to control. Time-dependent change of the NE response were not apparent during the acclimatization period suggesting that a decreased carotid body chemoreceptor sensitivity to NE and DA is not responsible for the increased drive to breathe characteristic of ventilatory acclimatization to hypoxia.

Intracarotid norepinephrine infusions inhibit ventilation in goats

Plasma norepinephrine (NE) increases from rest to exercise during normoxic exercise, and significantly more during hypoxic exercise in goats. To determine carotid body (CB) mediated effects of increased NE on ventilatory control, we investigated ventilatory responses to intracarotid NE infusions in awake, resting goats. NE was infused (0.5-5.0 micrograms.kg-1 x min-1, 2-3 min) into either a CB intact or contralateral CB-denervated carotid artery in both normoxia and hypoxia (FIO2. = 0.11). PRE-infusion measurements of arterial blood gases, blood pressure and pulmonary ventilation (VI) were compared with values 30-45 sec after beginning NE infusions at 1.0 micrograms.kg-1 x min-1. On the CB-intact side, NE infusions decreased VI by an average of 43% (P < 0.05) and increased PaCO2 4.0 +/- 0.3 mmHg (P < 0.05); ventilatory inhibition preceded an increase in arterial blood pressure. NE infusions on the CB-denervated side had no significant effects on VI or PaCO2, but still increased blood pressure to the same level as infusions on the CB-intact side. In hypoxia, NE infusions on the intact side no longer inhibited VI. NE induced VI inhibition in normoxia was similar in magnitude and time course to dopamine induced VI inhibition. Experiments were repeated following administration the alpha-adrenergic receptor antagonist, phenoxybenzamine (1 mg.kg-1, i.v.) the beta-adrenergic receptor antagonist, propranolol (1 mg.kg-1, i.v.) and the D2-dopamine receptor antagonist, domperidone (1 mg.kg-1, i.v.). Phenoxybenzamine partially blocked NE induced ventilatory depression and domperidone blocked it, but propranolol had no effect. These data indicate that NE inhibits ventilation in goats via effects on carotid chemoreceptors. NE induced inhibition is independent of changes in blood pressure or baroreceptor feedback, and appears to involve both alpha-adrenergic and D2-dopaminergic receptors.

Norepinephrine-containing glomus cells in the rabbit carotid body. II. Immunocytochemical evidence of dopamine-beta-hydroxylase and norepinephrine

The presence of noradrenergic glomus cells in the rabbit carotid body was investigated at the light and electron microscope levels, using dopamine-beta-hydroxylase and norepinephrine immunocytochemistry as well as the chromaffin reaction. Frozen and semi-thin plastic sections showed some dopamine-beta-hydroxylase immunoreactive glomus cells either isolated in the connective tissue or, more frequently, mixed with unreactive cells. At the ultrastructural level immunopositive cells differed from immunonegative ones by the larger size of most of their dense-cored vesicles. Similar observations were made after using anti-norepinephrine antibodies. Immunoreactive cells to anti-dopamine-beta-hydroxylase and anti-norepinephrine antibodies were relatively few although their number varied from carotid body to carotid body. The immunolabelling intensity was very variable from cell to cell. Consecutive frozen sections processed for norepinephrine- and dopamine-immunocytochemistry showed many cell clusters containing both norepinephrine and dopamine-immunoreactive glomus cells. Some chromaffin glomus cells were clearly identifiable by the very strong electron opacity of their dense-cored vesicles; most of these vesicles were characterized by their large size, as the dense-cored vesicles observed in dopamine-beta-hydroxylase- and norepinephrine-immunopositive cells. These results demonstrated that dopamine-beta-hydroxylase and norepinephrine-immunopositive, as well as chromaffin cells, were identical to the cells which take up exogenous norepinephrine, described in part I of this study. However, many intermediate levels were found between norepinephrine-immunonegative and strongly norepinephrine-immunopositive glomus cells, suggesting that the distinction between these two kinds of cells is not clearcut.

Functional characteristics of arterial chemoreceptors in an amphibian (Bufo marinus)

In order to further describe the functional characteristics of arterial chemoreceptors of anuran amphibians, multi-unit chemoreceptor discharge frequency (MCDF) was recorded from the carotid (N = 23) or aortic nerve (N = 2) of pithed, unidirectionally ventilated toads (Bufo marinus). MCDF increased with decreasing PaO2: typically, the threshold PaO2 lay between 40 and 60 mmHg. The MCDF-PaO2 relationship was right-shifted along the PaO2 axis by increasing PaCO2 (N = 5). In three toads, the MCDF-PaO2 relationship was unaffected when CaO2 was reduced 35-89% by hemorrhage. MCDF was also unaffected by occlusion of the outflow from the heart, though it increased upon release of the occlusion. MCDF was stimulated by epinephrine, and inhibited by dopamine. Our results demonstrate that the MCDF responds to the range of PaO2 and PaCO2 values encountered in vivo, suggesting that arterial chemoreceptors may participate in ventilatory control in toads. The receptors do not respond to the rate of oxygen delivery per se, and may be influenced by catecholamines known to exist in the carotid labyrinth.

Hypoxia increases the cyclic AMP content of the cat carotid body in vitro

The cyclic AMP content of cat carotid bodies in vitro measured with a radioimmunoassay under control conditions (PO2: 230 torr) was 0.79 +/- 0.10 pmol/carotid body (n = 10). Lowering medium PO2 to 20 torr for 2 min significantly increased cyclic AMP content to 1.13 +/- 0.14 pmol/carotid body (n = 10). This increase was inhibited neither by propranolol (34 microM) nor by propranolol plus haloperidol (27 microM). Inhibition of the cyclic nucleotide phosphodiesterase with 1-methyl-3-isobutylxanthine (0.8 mM) provoked a fast and large increase in cyclic AMP during both control and hypoxic conditions. The cyclic AMP increase induced by hypoxia was still observed when extracellular Ca2+ was absent. Inhibition of the adenylate cyclase by N-(cis-2-phenylcyclopentyl)azacyclotridecan-2-imine hydrochloride (MDL 12330A; 20-1,000 microM) under zero-Ca2+ conditions irreversibly inhibited the cyclic AMP increase produced by hypoxia. Similarly, inhibition of the Ca2(+)-calmodulin complex by trifluoperazine (0.2 mM) or calmidazolium (R 24571; 50-200 microM) prevented the cyclic AMP response. These results suggest that cyclic AMP may be involved in the PO2-sensing mechanism of the carotid body. Hypoxia appears to activate adenylate cyclase directly and independent of any hormone-receptor interactions.

Effect of oxygen on potassium-excited ventilation in the decerebrate cat

Raising arterial potassium ([K+]a) from ca. 3.5 to 6.5 mM, as occurs in heavy exercise, excites the arterial chemoreceptors and ventilation (VE) in anaesthetised cats. We have previously shown that the excitation of chemoreceptors by potassium is enhanced by hypoxia and abolished by hyperoxia, and here we show, in decerebrate cats, that the potassium-induced increase in VE is also abolished by hyperoxia. 100% oxygen was given abruptly in hypoxia (PETO2 ca. 50 Torr), with inspired gas tensions adjusted to give the same PETO2 and PETCO2 values before all tests on a given animal. Intravenous infusions of 150 mM KCl, which raised [K+]a from 3.9 +/- 0.3 mM to 7.4 +/- 0.3 mM (mean +/- SE), always excited hypoxic VE (42 +/- 8%; P less than 0.01). Hyperoxia, given during KCl infusion, reduced VE to a value not significantly greater (P greater than 0.27) than the hyperoxic value obtained before infusion. These results show that: (i) VE reflects the responses of chemoreceptors to K+, (ii) that abrupt hyperoxia removes the potassium-induced ventilatory drive, and (iii) that, in our experiments, K+ appears to have excited VE only via the peripheral chemoreceptors.

Carotid body chemoreceptor response to prolonged hypoxia in the rabbit: effects of domperidone and propranolol

1. The discharge of single afferent chemoreceptor fibres was recorded from the cut sinus nerve over periods of 60 or 90 min of constant, isocapnic hypoxia (arterial O2 pressure, Pa,O2, 3.13-5.25 kPa), in twenty anaesthetized rabbits, after dividing the sympathetic supply to the carotid body. 2. Under control conditions, discharge after 60 min of hypoxia adapted to a mean (S.E.M.) of 71.95 (2.75)% of that attained at 5 min of hypoxia in twenty-three hypoxic experiments. This adaptation was more pronounced when Pa,O2 was lower than 4 kPa (30 Torr). 3. Domperidone (1 mg kg-1 bolus + 1 mg kg-1 h-1 infusion I.V.), increased normoxic afferent discharge by a mean of 142%. In ten experiments, discharge after 60 min of hypoxia adapted to a mean (S.E.M.) of 56.22 (+/- 3.40)% of that attained at 5 min of hypoxia which was significantly different from control hypoxic runs (P = 0.006). 4. In seven experiments propranolol (1 mg kg-1 bolus + 1 mg kg-1 h-1 infusion I.V.) did not affect the normoxic discharge. The mean adaptation of discharge after 60 min of hypoxia was to 77.43 (3.97)% of discharge attained at 5 min of hypoxia, which was not significantly different from control hypoxic runs (P = 0.34). 5. Under control conditions plasma [K+] increased steadily during 60 min of hypoxia, in fourteen experiments, from a mean of 2.76 (0.14) to 2.85 (0.12) mmol l-1 but this was not significant (P = 0.21). Domperidone (n = 6) did not affect plasma [K+] at any time, but after propranolol (n = 6) it increased by a mean (S.E.M.) of 0.39 (0.09) mmol l-1 (P = 0.01) in normoxia and by a further 0.62 (0.28) mmol l-1 (P = 0.08) at 60 min of hypoxia. 6. The results suggest that the adaptation of chemoreceptor discharge to hypoxia in the rabbit is not mediated by changes in plasma [K+]; in addition, endogenous dopamine, but not noradrenaline, contributes to the maintenance of chemoreceptor discharge in prolonged hypoxia.

Differential stimulus coupling to dopamine and norepinephrine stores in rabbit carotid body type I cells

Recent studies suggest that preneural type I (glomus) cells in the arterial chemoreceptor tissue of the carotid body act as primary transducer elements which respond to natural stimuli (low O2, pH or increased CO2) by releasing chemical transmitter agents capable of exciting the closely apposed afferent nerve terminals. These type I cells contain multiple putative transmitters, but the identity of the natural excitatory agents remains an unresolved problem in carotid body physiology. Characterization of putative transmitter involvement in the response to natural and pharmacological stimuli has therefore become fundamental to further understanding of chemotransmission in this organ. The present study demonstrates that a natural stimulus (hypoxia) evokes the release of dopamine (DA) and norepinephrine (NE) in approximate proportion to their unequal stores in rabbit carotid body (DA release/NE release = 8.2). In contrast, nicotine (100 microM), a cholinomimetic agent thought to act on the nicotinic receptors present on the type I cells, evokes the preferential release of NE (DA release/NE release = 0.17). These findings suggest that distinct mechanisms are involved in a differential mobilization of these two catecholamines from the rabbit carotid body.

Development of the arterial chemoreflex and turnover of carotid body catecholamines in the newborn rat

1. The peripheral, arterial chemoreceptors in the carotid body are active and responsive in the fetus. At birth, when oxygenation increases, the chemoreceptors are silenced. Over the next few days the sensitivity is reset toward the adult level and the chemoreceptors influence breathing during normal conditions. In order to investigate the underlying mechanisms of this resetting we examined the strength of the chemoreflex in newborn rats and correlated this to the contents of dopamine and noradrenaline in the carotid bodies of the newborn pups and near-term fetuses. Furthermore, turnover rates of dopamine and noradrenaline were determined in newborn rats up to 1 week of age by analysis of catecholamine decreases after inhibition of synthesis with alpha-methyl-p-tyrosine. 2. Chemoreceptor influence was assessed by the method of 'physiological chemodenervation' with hyperoxia of 15-20 s duration in unanaesthetized rat pups. Relative changes in ventilation elicited by hyperoxia were determined by body plethysmography. We found no change in ventilation on the day of birth either in vaginally born rats or in near-term pups delivered by Caesarean section. After 1 day there was a significant decrease in ventilation of -19.4 +/- 2.3% (mean +/- S.E.M.) and at 7 days of age the decrease was -28.8 +/- 2.2%, suggesting an increasing influence from the peripheral chemoreceptors. 3. The contents of dopamine and noradrenaline were measured by high-performance liquid chromatography. Dopamine increased from 3.7 +/- 0.4 pmol (pair of carotid bodies)-1 in the fetus to a peak of 15.9 +/- 2.6, 6-12 h after birth followed by a decline to 7.1 +/- 0.7 at 7 days of age. Noradrenaline levels increased from 1.3 +/- 0.3 in the fetus to 9.6 +/- 1.1 pmol (pair of carotid bodies)-1 after 4 days. The turnover rate of dopamine decreased from 4.4 pmol (pair of carotid bodies)-1 h-1 0-6 h after birth to 1.0 at 6-12 h of age. The turnover rate of noradrenaline also decreased over the first hours following delivery. 4. Since dopamine is an inhibitory neuromodulator in this system, we suggest that the increase in sensitivity seen after the first day of life is, at least in part, due to a decrease in the release of dopamine and thus a removal of an inhibitory mechanism.

Peripheral and central dopamine receptors in respiratory control

The role of peripheral and central dopaminergic mechanisms in respiratory control was studied in anesthetized cats. In one series, we simultaneously measured carotid chemoreceptor and ventilatory responses to hypoxia and hypercapnia before and after a saturation dose of intravenous domperidone, a peripheral dopamine (D2) receptor antagonist. Both carotid chemoreceptor and ventilatory responses were augmented by domperidone essentially in proportion, suggesting that they reflected the increase of peripheral chemoreceptor activity. Haloperidol which crosses into the brain from blood, given subsequent to domperidone, did not further affect carotid chemoreceptor responses but attenuated ventilatory responses to hypoxia without significantly altering those to hypercapnia. Thus, the additional ventilatory effect of haloperidol is mediated through central dopaminergic mechanisms involving peripheral chemoreflex pathway alone. In another series, the anesthetized cats were paralyzed and artificially ventilated to study carotid chemoreceptor responses to step increases in the end-tidal PCO2 before and after domperidone. Domperidone significantly augmented the responses to CO2. The results support the hypothesis that both peripheral and central dopaminergic mechanisms play a significant modulatory role in chemoreflex respiratory control.

Interactions between hypoxia, acetylcholine and dopamine in the carotid body of rabbit and cat

1. Acetylcholine (ACh) and dopamine (DA) were either infused into the carotid artery or applied directly to the surface of the carotid body of twenty-six rabbits and fifteen cats. Afferent discharge of single chemoreceptor units was recorded at a range of Pa,O2 (arterial O2 pressure) values during drug administration. 2. There were no observable systemic effects of either drug when applied to the surface of the carotid body. 3. Acetylcholine tended to depress afferent discharge when applied to the surface of the rabbit carotid body or when infused into the carotid sinus. In the cat, intracarotid and surface application of ACh had mild and inconsistent effects. DA consistently depressed discharge in both species independent of the route of administration. Antagonists of ACh and DA failed to abolish the chemoreceptor response to hypoxia. 4. The changes in afferent discharge elicited by all drugs were small compared with the range of discharge rates attained with physiological stimuli. The effects of ACh and DA were more marked in hyperoxia than in hypoxia for both routes of administration, disappearing at Pa,O2 values close to 20 Torr (7.5 Torr = 1 kPa). 5. A role for DA in the maintenance of the hypoxic response was investigated in six rabbits. After 15 min of hypoxia (Pa,O2 = 21.8 +/- 1.1 Torr; mean +/- S.E.M.) the discharge of single chemoreceptor fibres adapted moderately (to 79.3 +/- 5.2% of maximum discharge). Following administration of domperidone or haloperidol (1.0-5.3 mg kg-1, I.V.) the same fibres responded with equal magnitude to the onset of the hypoxic stimulus but showed a significantly larger adaptation (to 48.5 +/- 4.4%). 6. It is concluded that endogenous ACh and DA are unlikely to mediate the transduction process of the carotid body, but DA may play a role in preventing adaptation to a prolonged hypoxic stimulus.

Studies on the regenerated carotid sinus nerve of the rabbit

1. The central end of the distally cut left carotid sinus nerve was sutured to the tunica media of the external carotid artery, 1 cm cranial to the carotid bifurcation, in nineteen rabbits. The carotid body was removed in fourteen of these rabbits but left in situ in the remaining five. After 56-165 days of recovery a neuroma was identified at the site of the suture. Ventilatory reflexes mediated by both sinus nerves were tested and afferent activity recorded from the regenerated nerve. 2. Ventilatory reflex responses to hypoxia and sodium cyanide were abolished on sectioning the right sinus nerve, whilst the hypercapnic response was maintained. 3. Electrical stimulation of the regenerated sinus nerve caused hypotension and hyperventilation. These responses were attenuated compared to stimulation of the right sinus nerve. 4. A level of afferent activity equivalent to that found in non-regeneration experiments was recorded from all regenerated sinus nerves. Whole-nerve afferent activity was modulated by changes in carotid sinus blood pressure but not by changes in Pa,O2, Pa,CO2 (arterial O2 and CO2 pressures) or intracarotid injection of sodium cyanide. 5. A minimum of thirty single afferent fibres was identified in each experiment, the vast majority of which were mechanoreceptors. In only nine experiments were chemoreceptor fibres found and only twelve chemoreceptor fibres (1.7% of total) were identified in these nine experiments. In ten experiments no chemoreceptor fibres could be found. Leaving the carotid body in situ increased the incidence of chemoreceptive preparations. A small number of fibres unresponsive to mechanical stimulation and asphyxia was also identified. 6. The responses of regenerated chemoreceptor fibres to physiological and pharmacological stimuli were generally similar to those found in control carotid body preparations. Fibres unresponsive to mechanical stimulation and asphyxia did not respond to sodium cyanide, dopamine or isoprenaline; some of these fibres were excited by nicotine. 7. The receptive fields of mechanosensitive fibres were localized on or up to 2 cm away from the neuroma. Surface application of 20-40 microliters sodium cyanide (200 micrograms ml-1) was used to localize the receptive fields of seven of the twelve chemoreceptor fibres. All seven were localized to the site of the carotid body. 8. The neuroma and site of the carotid body were examined under light and electron microscopy. Glomus tissue was absent from the neuroma but was found at the site of the carotid body. 9. In conclusion, recovery of chemoreceptor function after carotid sinus nerve section appears to be associated with reinnervation of glomus tissue.

The effect of beta adrenergic blockade on the carotid body response to hyperkalaemia in the cat

Arterial chemoreceptor discharge and ventilation are both significantly increased when the concentration of arterial potassium is raised to a level typical of moderate exercise. However, although the plasma potassium level of exercising, beta-blocked patients rises by more than that of normal subjects, this does not show up in their steady-state ventilatory response, i.e. exercising beta-blocked subjects ventilate no more than exercising controls. The present experiments were designed to test the hypothesis that the apparent failure of beta-blocked subjects to respond to the extra hyperkalaemia that they experience might be accounted for by a reduction in the sensitivity of arterial chemoreceptors to potassium. We used eleven pentobarbitone-anaesthetized, thoracotomized, artificially hyperventilated cats, in which arterial potassium was raised from ca. 4.5 to ca. 7 mM before and during beta blockade by propranolol or atenolol. The steady-state relation between chemoreceptor discharge and arterial potassium was curvilinear, discharge becoming more sensitive to potassium as the concentration of the latter was raised. Beta blockade significantly reduced discharge at all levels of plasma potassium (P less than 0.0001). It also significantly reduced (P less than 0.05) the slope of the response of discharge to a given increase of plasma potassium. Our results show that beta blockade decreases the sensitivity of arterial chemoreceptors to increases in arterial potassium. This may explain why exercising beta-blocked subjects breathe no harder than controls, in spite of the fact that they are more hyperkalaemic.

Effects of beta-adrenergic blockade on the ventilatory responses to hypoxic and hyperoxic exercise in man

1. The ventilatory responses to step changes from rest to 100 W cycling exercise were studied in five healthy human subjects. Exercise was performed in hypoxia (end-tidal O2 pressure, PET,O2, 50-55 mmHg), a condition characterized by a marked enhancement of arterial chemoreceptor activity, and in hyperoxia (PET,O2 greater than 250 mmHg), a condition in which arterial chemoreceptor activity is largely suppressed. The subjects were studied at each O2 level after placebo and after an oral dose of 120 mg propranolol. 2. The magnitude of phase 1, the immediate, rapid ventilatory response at the onset of work, was unaffected by hypoxia and at both oxygen levels it was also unaffected by propranolol. 3. Phase 2, analysed from 20 to 120 s after the onset of exercise, was significantly affected by both O2 level and beta-blockade. The kinetics of the ventilatory changes in this phase were well described in all four conditions by a simple exponential function. The overall mean time constants after placebo were shorter in hypoxia (31.0 s) than in hyperoxia (40.2 s), and at each O2 level longer after propranolol, in hypoxia 61.3 s and in hyperoxia 106.0 s. 4. Continuous analysis of gas sampled at the mouth with a mass spectrometer showed constancy of end-tidal PCO2 throughout the step change in hypoxia both with and without beta-blockade. In contrast, in both hyperoxic conditions PET,CO2 rose, mainly in phase 2, to a value 5-6 mmHg higher than the starting value. 5. The steady-state ventilation was higher in hypoxia than in hyperoxia, and end-tidal CO2 pressure, PET,CO2, correspondingly lower. Neither ventilation nor PCO2 were, however, affected by propranolol in either condition. 6. It is concluded that the arterial chemoreceptors are important for both the rate of adaptation of ventilation to a new rate of metabolism during a step change of work rate, and for the matching of ventilation to CO2 flow which normally ensures isocapnia. The further slowing of the dynamics of the ventilatory response in hyperoxia as well as the preserved isocapnia in hypoxia after beta-blockade argue against any major role of beta-adrenergic mechanisms for these functions of the arterial chemoreceptors. The observed effects are considered to be secondary to the reduced cardiac output and an increased CO2 storage initially during exercise following beta-adrenergic blockade.

Effect of moderate hypoxemia on atrial natriuretic factor and arginine vasopressin in normal man

The object of this study was to assess the effect of moderate acute hypoxemia on plasma concentrations of atrial natriuretic factor (ANF), arginine vasopressin (AVP), plasma renin activity (PRA) and urinary excretion of prostaglandin E2 (UPGE2V). Eight volunteers were exposed for 2 hours to a gas mixture containing 10% O2, 4.5% CO2 and 85.5% N2. Hypoxia increased diastolic blood pressure and free water clearance. Hypoxia did not change the AVP, PRA or UPG2V, although increased ANF from 17.7 +/- 3.4 pg/mL to 27.2 +/- 1.7 pg/mL (p less than 0.005) at 120 minutes. ANF changes were closely associated with the rise in blood pressure.

[Effect of bupranolol on hypoxic respiratory stimulation in healthy probands]

To determine whether hypoxic ventilatory response results, in part, from concomitant systemic sympathoadrenal stimulation, we studied ventilation in 20 healthy subjects before and after administration of a beta-blocking agent. A single oral dose of 100 mg bupranolol (vs placebo) significantly lowered minute ventilation from 9.4 +/- 0.7 to 8.3 +/- 0.2 l/min (mean +/- SEM) during normoxia, and from 10.8 +/- 0.8 to 8.9 +/- 0.2 l/min, when 11% O2 was inhaled. In our study, there were marked oscillations of ventilation on changing from room air to hypoxic breathing and back. They were ascribed to the preceding sampling of specimens for blood gas analysis. However, bupranolol had no influence on these transients. Bupranolol also had only slight cardiocirculatory effects during normoxia and did not prevent significant T-wave flattening, increase in heart rate, and fall in diastolic blood pressure during hypoxia. However, it did block the hypoxic increase in systolic blood pressure. From our results we suggest that (1) in spontaneously breathing conscious subjects, hypoxia-induced hyperventilation is also due to hypoxic sympathoadrenal activity and individual mental state, and (2) these influences do not affect all aspects of the cardiocirculatory response.

DL-propranolol inhibits the vascular changes in the rat carotid body induced by long-term hypoxia

Two groups of rats were exposed to hypoxia (10% O2) for 1 and 3 weeks with or without daily injections of DL-propranolol (0.66 mg.kg-1 ip). The structure of the carotid body was analyzed by light microscopical morphometry and the catecholamine content was assayed by high performance liquid chromatography. Exposure to hypoxia induced enlargement of the carotid body due to enhanced vascularity and hypertrophy of glomic and interstitial tissues. The dopamine and norepinephrine content were increased at both 1 and 3 weeks of hypoxia and reached levels 40-50 times those of the controls. The DL-propranolol treatment abolished the vasodilatory effect of hypoxia within the first week but did not prevent the other structural changes or the rise in catecholamine content. The data suggest that 1. the vasodilation elicited by long-term hypoxia may be controlled by beta-adrenoceptors and 2. the structural and biochemical events occurring in rat carotid body during long-term hypoxia do not influence each other and thus seem to be controlled by different mechanisms.

Role of beta adrenergic receptors in carotid body function of the goat

Previous studies in anesthetized or decerebrate cats and rabbits and awake man have shown conflicting results regarding a potential role for beta-adrenergic receptors in carotid body function. Therefore, we sought to clarify the role of beta-adrenergic receptor activity in carotid body function by assessing: the ventilatory response to intravenous isoproterenol infusion in awake and anesthetized carotid body intact and carotid body denervated goats, the effect of propranolol on the ventilatory response to isocapnic hypoxemia, and carotid sinus nerve chemoreceptor discharge rate response to isoproterenol and hypoxia. Isoproterenol increased ventilation to a similar degree in carotid body intact and denervated awake and anesthetized goats. This ventilatory increase was blocked by propranolol. Propranolol did not alter the hypoxic ventilatory response. Although ventilation increased, carotid sinus nerve chemoreceptor discharge rate was not altered by isoproterenol infusion or bolus IV injection in anesthetized goats. Hypoxia did increase carotid sinus nerve discharge rate. In this study, beta-adrenergic stimulation of ventilation did not occur via the carotid body, and beta-adrenergic blockade did not affect the carotid body hypoxic ventilatory response. Therefore, we found no evidence of functional beta-adrenergic activity within the carotid body of the goat.