Glycogen metabolism protects against metabolic insult to preserve carotid body function during glucose deprivation

Acute effects of insulin and insulin-induced hypoglycaemia on carotid body chemoreceptor activity and cardiorespiratory responses in dogs

NEW FINDINGS

What is the central question of this study? What are the effects of insulin and insulin-induced hypoglycaemia on carotid body chemoreceptor activity in vivo and how do carotid body chemoreceptor stimulation-mediated cardiorespiratory responses in beagle dogs compare during euglycaemia and insulin-induced hypoglycaemia? What is the main finding and its importance? Intracarotid insulin administration leads to sustained increase in carotid body chemoreceptor activity and respiratory response with significant cardiovascular effects. Insulin-induced hypoglycaemia exacerbated NaCN-mediated carotid body chemoreceptor activity and respiratory response with enhanced cardiovascular reflex response. These findings suggest that insulin-induced hypoglycaemia augments the carotid body chemoreceptors to initiate the adaptive counter-regulatory responses to restore the normoglycaemic condition.

ABSTRACT

The carotid body chemoreceptors (CBC) play an important role in the adaptive counter-regulatory response to hypoglycaemia by evoking the CBC-mediated sympathetic neuronal system to restore normoglycaemia. Ex vivo studies have shown varied responses of insulin-induced hypoglycaemia on CBC function, and several in vivo studies have indirectly established the role of CBCs in restoring normoglycaemia in both animals and humans. However, a direct effect of insulin and/or insulin-induced hypoglycaemia on CBC activity is not established in animal models. Therefore, the aim of this study was to evaluate in vivo effects of insulin and insulin-induced hypoglycaemia on CBC activity and cardiorespiration in a preclinical large animal model. The carotid sinus nerve (CSN) activity and cardiorespiratory responses to sodium cyanide (NaCN; 25 µg/kg) were compared before (euglycaemic) and after (hypoglycaemic) intracarotid administration of insulin (12.5-100 µU/dogs) in beagle dogs. Insulin administration increased CSN activity and minute ventilation (V ̇ $\dot V$ E ) with significant (P < 0.0001) effects on heart rate and blood pressure. Insulin-mediated effects on CSN and cardiorespiration were sustained and the change inV ̇ $\dot V$ E was driven by tidal volume only. Insulin significantly (P < 0.0001) lowered blood glucose level. NaCN-mediated CSN activity andV ̇ $\dot V$ E were significantly (P < 0.0001) augmented during insulin-induced hypoglycaemia. The augmentedV ̇ $\dot V$ E was primarily driven by respiratory frequency and partially by tidal volume. The cardiovascular reflex response mediated through CBC stimulation was significantly (P < 0.0001) exacerbated during insulin-induced hypoglycaemia. Collectively, these results demonstrate direct effects of insulin and insulin-induced hypoglycaemia on CBC chemosensitivity to potentiate CBC-mediated neuroregulatory pathways to initiate adaptive neuroendocrine and cardiorespiratory counter-regulatory responses to restore normoglycaemia.

Are Multiple Mitochondrial Related Signalling Pathways Involved in Carotid Body Oxygen Sensing?

It is generally acknowledged that the carotid body (CB) type I cell mitochondria are unique, being inhibited by relatively small falls in PaO2 well above those known to inhibit electron transport in other cell types. This feature is suggested to allow for the CB to function as an acute O2 sensor, being stimulated and activating systemic protective reflexes before the metabolism of other cells becomes compromised. What is less clear is precisely how a fall in mitochondrial activity links to type I cell depolarisation, a process that is required for initiation of the chemotransduction cascade and post-synaptic action potential generation. Multiple mitochondrial/metabolic signalling mechanisms have been proposed including local generation of mitochondrial reactive oxygen species (mitoROS), a change in mitochondrial/cellular redox status, a fall in MgATP and an increase in lactate. Although each mechanism is based on compelling experimental evidence, they are all not without question. The current review aims to explore the importance of each of these signalling pathways in mediating the overall CB response to hypoxia. We suggest that there is unlikely to be a single mechanism, but instead multiple mitochondrial related signalling pathways are recruited at different PaO2s during hypoxia. Furthermore, it still remains to be determined if mitochondrial signalling acts independently or in partnership with extra-mitochondrial O2-sensors.

Mitochondrial Succinate Metabolism and Reactive Oxygen Species Are Important but Not Essential for Eliciting Carotid Body and Ventilatory Responses to Hypoxia in the Rat

Reflex increases in breathing in response to acute hypoxia are dependent on activation of the carotid body (CB)—A specialised peripheral chemoreceptor. Central to CB O₂-sensing is their unique mitochondria but the link between mitochondrial inhibition and cellular stimulation is unresolved. The objective of this study was to evaluate if ex vivo intact CB nerve activity and in vivo whole body ventilatory responses to hypoxia were modified by alterations in succinate metabolism and mitochondrial ROS (mitoROS) generation in the rat. Application of diethyl succinate (DESucc) caused concentration-dependent increases in chemoafferent frequency measuring approximately 10–30% of that induced by severe hypoxia. Inhibition of mitochondrial succinate metabolism by dimethyl malonate (DMM) evoked basal excitation and attenuated the rise in chemoafferent activity in hypoxia. However, approximately 50% of the response to hypoxia was preserved. MitoTEMPO (MitoT) and 10-(6′-plastoquinonyl) decyltriphenylphosphonium (SKQ1) (mitochondrial antioxidants) decreased chemoafferent activity in hypoxia by approximately 20–50%. In awake animals, MitoT and SKQ1 attenuated the rise in respiratory frequency during hypoxia, and SKQ1 also significantly blunted the overall hypoxic ventilatory response (HVR) by approximately 20%. Thus, whilst the data support a role for succinate and mitoROS in CB and whole body O₂-sensing in the rat, they are not the sole mediators. Treatment of the CB with mitochondrial selective antioxidants may offer a new approach for treating CB-related cardiovascular–respiratory disorders.

The inevitability of ATP as a transmitter in the carotid body

Atmospheric oxygen concentrations rose markedly at several points in evolutionary history. Each of these increases was followed by an evolutionary leap in organismal complexity, and thus the cellular adaptions we see today have been shaped by the levels of oxygen within our atmosphere. In eukaryotic cells, oxygen is essential for the production of adenosine 5'-triphosphate (ATP) which is the 'Universal Energy Currency' of life. Aerobic organisms survived by evolving precise mechanisms for converting oxygen within the environment into energy. Higher mammals developed specialised organs for detecting and responding to changes in oxygen content to maintain gaseous homeostasis for survival. Hypoxia is sensed by the carotid bodies, the primary chemoreceptor organs which utilise multiple neurotransmitters one of which is ATP to evoke compensatory reflexes. Yet, a paradox is presented in oxygen sensing cells of the carotid body when during periods of low oxygen, ATP is seemingly released in abundance to transmit this signal although the synthesis of ATP is theoretically halted because of its dependence on oxygen. We propose potential mechanisms to maintain ATP production in hypoxia and summarise recent data revealing elevated sensitivity of purinergic signalling within the carotid body during conditions of sympathetic overactivity and hypertension. We propose the carotid body is hypoxic in numerous chronic cardiovascular and respiratory diseases and highlight the therapeutic potential for modulating purinergic transmission.

β-Adrenoceptor blockade prevents carotid body hyperactivity and elevated vascular sympathetic nerve density induced by chronic intermittent hypoxia

Carotid body (CB) hyperactivity promotes hypertension in response to chronic intermittent hypoxia (CIH). The plasma concentration of adrenaline is reported to be elevated in CIH and our previous work suggests that adrenaline directly activates the CB. However, a role for chronic adrenergic stimulation in mediating CB hyperactivity is currently unknown. This study evaluated whether beta-blocker treatment with propranolol (Prop) prevented the development of CB hyperactivity, vascular sympathetic nerve growth and hypertension caused by CIH. Adult male Wistar rats were assigned into 1 of 4 groups: Control (N), N + Prop, CIH and CIH + Prop. The CIH paradigm consisted of 8 cycles h-1, 8 h day-1, for 3 weeks. Propranolol was administered via drinking water to achieve a dose of 40 mg kg-1 day-1. Immunohistochemistry revealed the presence of both β1 and β2-adrenoceptor subtypes on the CB type I cell. CIH caused a 2-3-fold elevation in basal CB single-fibre chemoafferent activity and this was prevented by chronic propranolol treatment. Chemoafferent responses to hypoxia and mitochondrial inhibitors were attenuated by propranolol, an effect that was greater in CIH animals. Propranolol decreased respiratory frequency in normoxia and hypoxia in N and CIH. Propranolol also abolished the CIH mediated increase in vascular sympathetic nerve density. Arterial blood pressure was reduced in propranolol groups during hypoxia. Propranolol exaggerated the fall in blood pressure in most (6/7) CIH animals during hypoxia, suggestive of reduced sympathetic tone. These findings therefore identify new roles for β-adrenergic stimulation in evoking CB hyperactivity, sympathetic vascular hyperinnervation and altered blood pressure control in response to CIH.

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.

Carotid Body and Metabolic Syndrome: Mechanisms and Potential Therapeutic Targets

The carotid body (CB) is responsible for the peripheral chemoreflex by sensing blood gases and pH. The CB also appears to act as a peripheral sensor of metabolites and hormones, regulating the metabolism. CB malfunction induces aberrant chemosensory responses that culminate in the tonic overactivation of the sympathetic nervous system. The sympatho-excitation evoked by CB may contribute to the pathogenesis of metabolic syndrome, inducing systemic hypertension, insulin resistance and sleep-disordered breathing. Several molecular pathways are involved in the modulation of CB activity, and their pharmacological manipulation may lead to overall benefits for cardiometabolic diseases. In this review, we will discuss the role of the CB in the regulation of metabolism and in the pathogenesis of the metabolic dysfunction induced by CB overactivity. We will also explore the potential pharmacological targets in the CB for the treatment of metabolic syndrome.

Ecto-5'-nucleotidase (CD73) regulates peripheral chemoreceptor activity and cardiorespiratory responses to hypoxia

KEY POINTS

Carotid body dysfunction is recognized as a cause of hypertension in a number of cardiorespiratory diseases states and has therefore been identified as a potential therapeutic target. Purinergic transmission is an important element of the carotid body chemotransduction pathway. We show that inhibition of ecto-5'-nucleotidase (CD73) in vitro reduces carotid body basal discharge and responses to hypoxia and mitochondrial inhibition. Additionally, inhibition of CD73 in vivo decreased the hypoxic ventilatory response, reduced the hypoxia-induced heart rate elevation and exaggerated the blood pressure decrease in response to hypoxia. Our data show CD73 to be a novel regulator of carotid body sensory function and therefore suggest that this enzyme may offer a new target for reducing carotid body activity in selected cardiovascular diseases.

ABSTRACT

Augmented sensory neuronal activity from the carotid body (CB) has emerged as a principal cause of hypertension in a number of cardiovascular related pathologies, including obstructive sleep apnoea, heart failure and diabetes. Development of new targets and pharmacological treatment strategies aiming to reduce CB sensory activity may thus improve outcomes in these key patient cohorts. The present study investigated whether ecto-5'-nucleotidase (CD73), an enzyme that generates adenosine, is functionally important in modifying CB sensory activity and cardiovascular respiratory responses to hypoxia. Inhibition of CD73 by α,β-methylene ADP (AOPCP) in the whole CB preparation in vitro reduced basal discharge frequency by 76 ± 5% and reduced sensory activity throughout graded hypoxia. AOPCP also significantly attenuated elevations in sensory activity evoked by mitochondrial inhibition. These effects were mimicked by antagonism of adenosine receptors with 8-(p-sulfophenyl) theophylline. Infusion of AOPCP in vivo significantly decreased the hypoxic ventilatory response (Δ V ̇ E control 74 ± 6%, Δ V ̇ E AOPCP 64 ± 5%, P < 0.05). AOPCP also modified cardiovascular responses to hypoxia, as indicated by reduced elevations in heart rate and exaggerated changes in femoral vascular conductance and mean arterial blood pressure. Thus we identify CD73 as a novel regulator of CB sensory activity. Future investigations are warranted to clarify whether inhibition of CD73 can effectively reduce CB activity in CB-mediated cardiovascular pathology.

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.

Moderate inhibition of mitochondrial function augments carotid body hypoxic sensitivity

A functional role for the mitochondria in acute O2 sensing in the carotid body (CB) remains undetermined. Whilst total inhibition of mitochondrial activity causes intense CB stimulation, it is unclear whether this response can be moderated such that graded impairment of oxidative phosphorylation might be a mechanism that sets and modifies the O2 sensitivity of the whole organ. We assessed NADH autofluorescence and [Ca2+]i in freshly dissociated CB type I cells and sensory chemoafferent discharge frequency in an intact CB preparation, in the presence of varying concentrations of nitrite (NO2 −), a mitochondrial nitric oxide (NO) donor and a competitive inhibitor of mitochondrial complex IV. NO2 − increased CB type I cell NADH in a manner that was dose-dependent and rapidly reversible. Similar concentrations of NO2 − raised type I cell [Ca2+]i via L-type channels in a PO2-dependent manner and increased chemoafferent discharge frequency. Moderate inhibition of the CB mitochondria by NO2 − augmented chemoafferent discharge frequency during graded hypoxia, consistent with a heightened CB O2 sensitivity. Furthermore, NO2 − also exaggerated chemoafferent excitation during hypercapnia signifying an increase in CB CO2 sensitivity. These data show that NO2 − can moderate the hypoxia sensitivity of the CB and thus suggest that O2 sensitivity could be set and modified in this organ by interactions between NO and mitochondrial complex IV.

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.

Ecto-5'-Nucleotidase, Adenosine and Transmembrane Adenylyl Cyclase Signalling Regulate Basal Carotid Body Chemoafferent Outflow and Establish the Sensitivity to Hypercapnia

Carotid body (CB) stimulation by hypercapnia causes a reflex increase in ventilation and, along with the central chemoreceptors, this prevents a potentially lethal systemic acidosis. Control over the CB chemoafferent output during normocapnia and hypercapnia most likely involves multiple neurotransmitters and neuromodulators including ATP, acetylcholine, dopamine, serotonin and adenosine, but the precise role of each is yet to be fully established. In the present study, recordings of chemoafferent discharge frequency were made from the isolated in vitro CB in order to determine the contribution of adenosine, derived specifically from extracellular catabolism of ATP, in mediating basal chemoafferent activity and responses to hypercapnia. Pharmacological inhibition of ecto-5'-nucleotidase (CD73), a key enzyme required for extracellular generation of adenosine from ATP, using α,β-methylene ADP, virtually abolished the basal normocapnic single fibre discharge frequency (superfusate PO(2) ~ 300 mmHg, PCO(2) ~ 40 mmHg) and diminished the chemoafferent response to hypercapnia (PCO(2) ~ 80 mmHg). These effects were mimicked by the blockade of adenosine receptors with 8-(p-sulfophenyl) theophylline. The excitatory impact of adenosinergic signalling on CB hypercapnic sensitivity is most likely to be conferred through changes in cAMP. Here, inhibition of transmembrane, but not soluble adenylate cyclases, reduced normocapnic single fibre activity and inhibited the elevation evoked by hypercapnia by approximately 50 %. These data therefore identify a functional role for CD73 derived adenosine and transmembrane adenylate cyclases, in modulating the basal chemoafferent discharge frequency and in priming the CB to hypercapnic stimulation.

TASK channels in arterial chemoreceptors and their role in oxygen and acid sensing

Arterial chemoreceptors play a vital role in cardiorespiratory control by providing the brain with information regarding blood oxygen, carbon dioxide, and pH. The main chemoreceptor, the carotid body, is composed of sensory (type 1) cells which respond to hypoxia or acidosis with a depolarising receptor potential which in turn activates voltage-gated calcium entry, neurosecretion and excitation of adjacent afferent nerves. The receptor potential is generated by inhibition of Twik-related acid-sensitive K(+) channel 1 and 3 (TASK1/TASK3) heterodimeric channels which normally maintain the cells' resting membrane potential. These channels are thought to be directly inhibited by acidosis. Oxygen sensitivity, however, probably derives from a metabolic signalling pathway. The carotid body, isolated type 1 cells, and all forms of TASK channel found in the type 1 cell, are highly sensitive to inhibitors of mitochondrial metabolism. Moreover, type1 cell TASK channels are activated by millimolar levels of MgATP. In addition to their role in the transduction of chemostimuli, type 1 cell TASK channels have also been implicated in the modulation of chemoreceptor function by a number of neurocrine/paracrine signalling molecules including adenosine, GABA, and serotonin. They may also be instrumental in mediating the depression of the acute hypoxic ventilatory response that occurs with some general anaesthetics. Modulation of TASK channel activity is therefore a key mechanism by which the excitability of chemoreceptors can be controlled. This is not only of physiological importance but may also offer a therapeutic strategy for the treatment of cardiorespiratory disorders that are associated with chemoreceptor dysfunction.

Tasting arterial blood: what do the carotid chemoreceptors sense?

The carotid bodies are sensory organs that detect the chemical composition of the arterial blood. The carotid body sensory activity increases in response to arterial hypoxemia and the ensuing chemoreflex regulates vital homeostatic functions. Recent studies suggest that the carotid bodies might also sense arterial blood glucose and circulating insulin levels. This review focuses on how the carotid bodies sense O₂, glucose, and insulin and some potential implications of these sensory functions on physiological regulation and in pathophysiological conditions. Emerging evidence suggests that carbon monoxide (CO)-regulated hydrogen sulfide (H₂S), stemming from hypoxia, depolarizes type I cells by inhibiting certain K⁺ channels, facilitates voltage-gated Ca²⁺ influx leading to sensory excitation of the carotid body. Elevated CO and decreased H₂S renders the carotid bodies insensitive to hypoxia resulting in attenuated ventilatory adaptations to high altitude hypoxia, whereas reduced CO and high H₂S result in hypersensitivity of the carotid bodies to hypoxia and hypertension. Acute hypoglycemia augments the carotid body responses to hypoxia but that a prolonged lack of glucose in the carotid bodies can lead to a failure to sense hypoxia. Emerging evidence also indicates that carotid bodies might sense insulin directly independent of its effect on glucose, linking the carotid bodies to the pathophysiological consequences of the metabolic syndrome. How glucose and insulin interact with the CO-H₂S signaling is an area of ongoing study.

Carotid body, insulin, and metabolic diseases: unraveling the links

The carotid bodies (CB) are peripheral chemoreceptors that sense changes in arterial blood O2, CO2, and pH levels. Hypoxia, hypercapnia, and acidosis activate the CB, which respond by increasing the action potential frequency in their sensory nerve, the carotid sinus nerve (CSN). CSN activity is integrated in the brain stem to induce a panoply of cardiorespiratory reflexes aimed, primarily, to normalize the altered blood gases, via hyperventilation, and to regulate blood pressure and cardiac performance, via sympathetic nervous system (SNS) activation. Besides its role in the cardiorespiratory control the CB has been proposed as a metabolic sensor implicated in the control of energy homeostasis and, more recently, in the regulation of whole body insulin sensitivity. Hypercaloric diets cause CB overactivation in rats, which seems to be at the origin of the development of insulin resistance and hypertension, core features of metabolic syndrome and type 2 diabetes. Consistent with this notion, CB sensory denervation prevents metabolic and hemodynamic alterations in hypercaloric feed animal. Obstructive sleep apnea (OSA) is another chronic disorder characterized by increased CB activity and intimately related with several metabolic and cardiovascular abnormalities. In this manuscript we review in a concise manner the putative pathways linking CB chemoreceptors deregulation with the pathogenesis of insulin resistance and arterial hypertension. Also, the link between chronic intermittent hypoxia (CIH) and insulin resistance is discussed. Then, a final section is devoted to debate strategies to reduce CB activity and its use for prevention and therapeutics of metabolic diseases with an emphasis on new exciting research in the modulation of bioelectronic signals, likely to be central in the future.