Presynaptic regulation of isolated neonatal rat carotid body type I cells by histamine

Hypoxia releases S-nitrosocysteine from carotid body glomus cells-relevance to expression of the hypoxic ventilatory response

We have provided indirect pharmacological evidence that hypoxia may trigger release of the S-nitrosothiol, S-nitroso-L-cysteine (L-CSNO), from primary carotid body glomus cells (PGCs) of rats that then activates chemosensory afferents of the carotid sinus nerve to elicit the hypoxic ventilatory response (HVR). The objective of this study was to provide direct evidence, using our capacitive S-nitrosothiol sensor, that L-CSNO is stored and released from PGCs extracted from male Sprague Dawley rat carotid bodies, and thus further pharmacological evidence for the role of S-nitrosothiols in mediating the HVR. Key findings of this study were that 1) lysates of PGCs contained an S-nitrosothiol with physico-chemical properties similar to L-CSNO rather than S-nitroso-L-glutathione (L-GSNO), 2) exposure of PGCs to a hypoxic challenge caused a significant increase in S-nitrosothiol concentrations in the perfusate to levels approaching 100 fM via mechanisms that required extracellular Ca2+, 3) the dose-dependent increases in minute ventilation elicited by arterial injections of L-CSNO and L-GSNO were likely due to activation of small diameter unmyelinated C-fiber carotid body chemoafferents, 4) L-CSNO, but not L-GSNO, responses were markedly reduced in rats receiving continuous infusion (10 μmol/kg/min, IV) of both S-methyl-L-cysteine (L-SMC) and S-ethyl-L-cysteine (L-SEC), 5) ventilatory responses to hypoxic gas challenge (10% O2, 90% N2) were also due to the activation of small diameter unmyelinated C-fiber carotid body chemoafferents, and 6) the HVR was markedly diminished in rats receiving L-SMC plus L-SEC. This data provides evidence that rat PGCs synthesize an S-nitrosothiol with similar properties to L-CSNO that is released in an extracellular Ca2+-dependent manner by hypoxia.

Neurochemical Plasticity of the Carotid Body

A striking feature of the carotid body (CB) is its remarkable degree of plasticity in a variety of neurotransmitter/modulator systems in response to environmental stimuli, particularly following hypoxic exposure of animals and during ascent to high altitude. Current evidence suggests that acetylcholine and adenosine triphosphate are two major excitatory neurotransmitter candidates in the hypoxic CB, and they may also be involved as co-transmitters in hypoxic signaling. Conversely, dopamine, histamine and nitric oxide have recently been considered inhibitory transmitters/modulators of hypoxic chemosensitivity. It has also been revealed that interactions between excitatory and inhibitory messenger molecules occur during hypoxia. On the other hand, alterations in purinergic neurotransmitter mechanisms have been implicated in ventilatory acclimatization to hypoxia. Chronic hypoxia also induces profound changes in other neurochemical systems within the CB such as the catecholaminergic, peptidergic and nitrergic, which in turn may contribute to increased ventilatory and chemoreceptor responsiveness to hypoxia at high altitude. Taken together, current data suggest that complex interactions among transmitters markedly influence hypoxia-induced transmitter release from the CB. In addition, the expression of a wide variety of growth factors, proinflammatory cytokines and their receptors have been identified in CB parenchymal cells in response to hypoxia and their upregulated expression could mediate the local inflammation and functional alteration of the CB under hypoxic conditions.

Acute Oxygen-Sensing via Mitochondria-Generated Temperature Transients in Rat Carotid Body Type I Cells

The Carotid Bodies (CB) are peripheral chemoreceptors that detect changes in arterial oxygenation and, via afferent inputs to the brainstem, correct the pattern of breathing to restore blood gas homeostasis. Herein, preliminary evidence is presented supporting a novel oxygen-sensing hypothesis which suggests CB Type I cell “hypoxic signaling” may in part be mediated by mitochondria-generated thermal transients in TASK-channel-containing microdomains. Distances were measured between antibody-labeled mitochondria and TASK-potassium channels in primary rat CB Type I cells. Sub-micron distance measurements (TASK-1: 0.33 ± 0.04 µm, n = 47 vs TASK-3: 0.32 ± 0.03 µm, n = 54) provided evidence for CB Type I cell oxygen-sensing microdomains. A temperature-sensitive dye (ERthermAC) indicated that inhibition of mitochondrial activity in isolated cells caused a rapid and reversible inhibition of mitochondrial thermogenesis and thus temperature in these microdomains. Whole-cell perforated-patch current-clamp electrophysiological recordings demonstrated sensitivity of resting membrane potential (Vm) to temperature: lowering bath temperature from 37°C to 24°C induced consistent and reversible depolarizations (Vm at 37°C: 48.4 ± 4.11 mV vs 24°C: 31.0 ± 5.69 mV; n = 5; p < 0.01). These data suggest that hypoxic inhibition of mitochondrial thermogenesis may play an important role in oxygen chemotransduction in the CB. A reduction in temperature within cellular microdomains will inhibit plasma membrane ion channels, influence the balance of cellular phosphorylation–dephosphorylation, and may extend the half-life of reactive oxygen species. The characterization of a thermosensory chemotransduction mechanism, that may also be used by other oxygen-sensitive cell types and may impact multiple other chemotransduction mechanisms is critical if we are to fully understand how the CBs, and potentially other oxygen-sensitive cells, respond to hypoxia.

Ethanol and opioids do not act synergistically to depress excitation in carotid body type I cells

OBJECTIVE

The combination of opioids and ethanol can synergistically depress breathing and the acute ventilatory response to hypoxia. Multiple studies have shown that the underlying mechanisms for this may involve calcium channel inhibition in central neurons. But we have previously identified opioid receptors in the carotid bodies and shown that their activation inhibits calcium influx into the chemosensitive cells. Given that the carotid bodies contribute to the drive to breathe and underpin the acute hypoxic ventilatory response, we hypothesized that ethanol and opioids may act synergistically in these peripheral sensory organs to further inhibit calcium influx and therefore inhibit ventilation.

METHODS

Carotid bodies were removed from 56 Sprague-Dawley rats (1021 days old) and then enzymatically dissociated to allow calcium imaging of isolated chemosensitive type I cells. Cells were stimulated with high K+ in the presence and absence of the µ-opioid agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) (10 µM), a maximal sublethal concentration of ethanol (3 g L-1, 65.1 mM) or a combination of both.

RESULTS

DAMGO alone significantly inhibited Ca2+ influx but this effect was not potentiated by the high concentration of ethanol.

CONCLUSION

These results indicate for the first time that while opioids may suppress breathing via an action at the level of the carotid bodies, ethanol is unlikely to potentiate inhibition via this pathway. Thus, the synergistic effects of ethanol and opioids on ventilatory parameters are likely mediated by central rather than peripheral actions.

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.

Selective mu and kappa Opioid Agonists Inhibit Voltage-Gated Ca2+ Entry in Isolated Neonatal Rat Carotid Body Type I Cells

It is known that opioids inhibit the hypoxic ventilatory response in part via an action at the carotid body, but little is known about the cellular mechanisms that underpin this. This study's objectives were to examine which opioid receptors are located on the oxygen-sensing carotid body type I cells from the rat and determine the mechanism by which opioids might inhibit cellular excitability.Immunocytochemistry revealed the presence of μ and κ opioid receptors on type I cells. The μ-selective agonist DAMGO (10 μM) and the κ-selective agonist U50-488 (10 μM) inhibited high K(+) induced rises in intracellular Ca(2+) compared with controls. After 3 h incubation (37 °C) with pertussis toxin (150 ng ml(-1)), DAMGO (10 μM) and U50-488 (10 μM) had no significant effect on the Ca(2+) response to high K(+).These results indicate that opioids acting at μ and κ receptors inhibit voltage-gated Ca(2+) influx in rat carotid body type I cells via G(i)-coupled mechanisms. This mechanism may contribute to opioid's inhibitory actions in the carotid body.

Acutely Administered Leptin Increases [Ca2+] i and BK Ca Currents But Does Not Alter Chemosensory Behavior in Rat Carotid Body Type I Cells

Obesity related pathologies are the health care crisis of our generation. The fat cell derived adipokine leptin has been shown to be a central stimulant of respiration. Very high levels of leptin, however, are associated with the depressed ventilatory phenotype observed in obesity hypoventilation syndrome. Leptin receptors have been identified on carotid body type I cells but how their activation might influence the physiology of these cells is not known.The acute application of leptin evoked calcium signaling responses in isolated type I cells. Cells increased their Fura 2 ratio by 0.074 ± 0.010 ratio units (n = 39, P < 0.001). Leptin also increased the peak membrane currents in 6 of 9 cells increasing the peak macroscopic currents at +10 mV by 61 ± 14 % (p < 0.02). Leptin administered in the presence of the selective BK(Ca) channel inhibitor Paxilline (0.5 μM) failed to increase membrane currents (n = 5). Interestingly, leptin did not significantly alter the resting membrane potential of isolated type I cells (n = 9) and anoxic/acidic depolarizations were unaffected by leptin (n = 7, n = 6).These data suggest that leptin receptors are functional in type I cells but that their acute activation does not alter chemosensory properties. Future studies will use chronic models of leptin dysregulation.

Inhibition of adenylate cyclase attenuates muscarinic Ca²(+) signaling by a PKA-independent mechanism in rat carotid body Type I cells

Carotid body (CB) Type I cells respond to hypoxia by releasing excitatory and inhibitory neurotransmitters. This mechanism leads to increased firing of the carotid sinus nerve (CSN) which alters breathing to maintain blood gases within the physiological range. Acetylcholine targets both muscarinic and nicotinic receptors in the rat CB, acting postsynaptically on CSN and presynaptically on Type I cells. Muscarinic Ca²(+) signaling is inhibited by the activation of G(i)-coupled receptors including histamine H3 receptors. Here inhibition of adenylate cyclase with SQ22536 mimicked H3 receptor activation. Using Ca²(+) imaging techniques it was observed that inhibition of muscarinic Ca²(+) signaling was independent of protein kinase A (PKA) as PKA inhibitors H89 and KT5720 were without effect on the muscarinic Ca²(+) response. By contrast the Epac (exchange protein activated by cAMP) inhibitor brefeldin A inhibited muscarinic Ca²(+) signaling whereas the Epac activator 8-pCPT-2'-O-Me-cAMP-AM potentiated Ca²(+) signaling. Thus in Type I cells inhibition of adenylate cyclase inhibited muscarinic Ca²(+) signaling via a PKA-independent pathway that may rely upon modulation of Epac.

Evidence for functional, inhibitory, histamine H3 receptors in rat carotid body Type I cells

The Type I cells are the sensory elements of the carotid bodies and play a critical role in defining the ventilatory response to hypoxia and hypercapnia. Type I cells release multiple neurotransmitters during a chemosensory stimulus resulting in increased firing of the carotid sinus nerve and modification of the breathing pattern. While much is known about the actions of individual neurotransmitters in this system, very little is known about how multiple neurotransmitters may integrate to shape the output of the carotid body. Recent data has indicated that the neurotransmitter histamine does not excite isolated Type I cells despite being released during hypoxia and its receptors being present on the Type I cells. Here the hypothesis that histamine might modulate an excitatory neurotransmitter such as acetylcholine was tested. Using calcium imaging techniques it was found that histamine attenuated calcium signaling events initiated by the muscarinic acetylcholine receptor agonist acetyl-beta-methylcholine via an H3 receptor mediated mechanism. In summary, these results suggest that when acetylcholine and histamine are co-released from Type I cells in response to chemostimuli, histamine may attenuate or modulate the excitatory presynaptic actions of acetylcholine.