Effects of CO2-HCO3- on catecholamine efflux from cat carotid body

Carotid body plastic behavior: evidence for D2-H3 receptor-receptor interactions

Dopamine and histamine receptors D2R and H3R are G protein-coupled receptors (GPCRs) which can establish physical receptor-receptor interactions (RRIs), leading to homo/hetero-complexes in a dynamic equilibrium. Although D2R and H3R expression has been detected within the carotid body (CB), their possible heterodimerization has never been demonstrated. The aim of this work was to verify D2R and H3R colocalization in the CB, thus suggesting a possible interplay that, in turn, may be responsible of specific D2R-H3R antagonistic functional implications. The CBs of both Sprague-Dawley rats (n = 5) and human donors (n = 5) were dissected, and immunolocalization of D2R and H3R was performed; thereafter, in situ proximity ligation assay (PLA) was developed. According to experimental evidence (immunohistochemistry and double immunofluorescence), all the samples displayed positive D2R/H3R elements; hence, PLA assay followed by confocal microscopy analysis was positive for D2R-H3R RRIs. Additionally, D2R-H3R heterodimers were mainly detected in type I cells (βIII-tubulin-positive cells), but type II cells' involvement cannot be excluded. RRIs may play a role in functional modulation of CB cells; investigating RRIs in the CB may guide toward the comprehension of its plastic changes and fine regulatory role while also unveiling their possible clinical implications.

Carotid body chemoreceptors: physiology, pathology, and implications for health and disease

The carotid body (CB) is the main peripheral chemoreceptor for arterial respiratory gases O2 and CO2 and pH, eliciting reflex ventilatory, cardiovascular, and humoral responses to maintain homeostasis. This review examines the fundamental biology underlying CB chemoreceptor function, its contribution to integrated physiological responses, and its role in maintaining health and potentiating disease. Emphasis is placed on 1) transduction mechanisms in chemoreceptor (type I) cells, highlighting the role played by the hypoxic inhibition of O2-dependent K+ channels and mitochondrial oxidative metabolism, and their modification by intracellular molecules and other ion channels; 2) synaptic mechanisms linking type I cells and petrosal nerve terminals, focusing on the role played by the main proposed transmitters and modulatory gases, and the participation of glial cells in regulation of the chemosensory process; 3) integrated reflex responses to CB activation, emphasizing that the responses differ dramatically depending on the nature of the physiological, pathological, or environmental challenges, and the interactions of the chemoreceptor reflex with other reflexes in optimizing oxygen delivery to the tissues; and 4) the contribution of enhanced CB chemosensory discharge to autonomic and cardiorespiratory pathophysiology in obstructive sleep apnea, congestive heart failure, resistant hypertension, and metabolic diseases and how modulation of enhanced CB reactivity in disease conditions may attenuate pathophysiology.

Petrosal ganglion: a more complex role than originally imagined

The petrosal ganglion (PG) is a peripheral sensory ganglion, composed of pseudomonopolar sensory neurons that innervate the posterior third of the tongue and the carotid sinus and body. According to their electrical properties PG neurons can be ascribed to one of two categories: (i) neurons with action potentials presenting an inflection (hump) on its repolarizing phase and (ii) neurons with fast and brisk action potentials. Although there is some correlation between the electrophysiological properties and the sensory modality of the neurons in some species, no general pattern can be easily recognized. On the other hand, petrosal neurons projecting to the carotid body are activated by several transmitters, with acetylcholine and ATP being the most conspicuous in most species. Petrosal neurons are completely surrounded by a multi-cellular sheet of glial (satellite) cells that prevents the formation of chemical or electrical synapses between neurons. Thus, PG neurons are regarded as mere wires that communicate the periphery (i.e., carotid body) and the central nervous system. However, it has been shown that in other sensory ganglia satellite glial cells and their neighboring neurons can interact, partly by the release of chemical neuro-glio transmitters. This intercellular communication can potentially modulate the excitatory status of sensory neurons and thus the afferent discharge. In this mini review, we will briefly summarize the general properties of PG neurons and the current knowledge about the glial-neuron communication in sensory neurons and how this phenomenon could be important in the chemical sensory processing generated in the carotid body.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

Differences in tyrosine hydroxylase expression after short-term hypoxia, hypercapnia or hypercapnic hypoxia in rat carotid body

In the carotid body (CB), it has been reported that the expressions of tyrosine hydroxylase (TH) mRNA and TH protein are enhanced by exposure to hypoxia. However, it is not known whether CO(2) affects the expression of TH in the CB. We examined the expression of TH mRNA and the immunoreactivity for TH in the CB of rats exposed to hypoxia (10% O(2)), hypercapnia (10% CO(2)) and hypercapnic hypoxia (10% O(2) and 10% CO(2)) for 2-24 h. The expression of TH mRNA in the CB was markedly enhanced in rats exposed to hypoxia for 4 h (6.6-fold), 6 h (6.0-fold) and 8 h (7.8-fold), and in rats exposed to hypercapnic hypoxia for 12 h (4.8-fold). The most intense TH immunoreactivity was observed in the CB from rats exposed to hypoxia for 12 and 24 h and to hypercapnic hypoxia for 24 h. The expressions of TH mRNA and the immunoreactivity for TH were not altered in the CB of rats exposed to hypercapnia. It is suggested that CO(2) does not affect TH expression in the CB, and that it inhibits hypoxia-enhanced TH expression.

Rat adrenal chromaffin cells are neonatal CO2 sensors

We studied the participation of adrenal medulla (AM) chromaffin cells in hypercapnic chemotransduction. Using amperometric recordings, we measured catecholamine (CAT) secretion from cells in AM slices of neonatal and adult rats perfused with solutions bubbled with different concentrations of CO2. The secretory activity augmented from 1.74 +/- 0.19 pC/min at 5% CO2 to 6.36 +/- 0.77 pC/min at 10% CO2. This response to CO2 was dose dependent and appeared without changes in extracellular pH, although it was paralleled by a drop in intracellular pH. Responsiveness to hypercapnia was higher in neonatal than in adult slices. The secretory response to hypercapnia required extracellular Ca2+ influx. Both the CO2-induced internal pH drop and increase in CAT secretion were markedly diminished by methazolamide (2 microm), a membrane-permeant carbonic anhydrase (CA) inhibitor. We detected the presence of two CA isoforms (CAI and CAII) in neonatal AM slices by in situ hybridization and real-time PCR. The expression of these enzymes decreased in adult AM together with the disappearance of responsiveness to CO2. In patch-clamped chromaffin cells, hypercapnia elicited a depolarizing receptor potential, which led to action potential firing, extracellular Ca2+ influx, and CAT secretion. This receptor potential (inhibited by methazolamide) was primarily attributable to activation of a resting cationic conductance. In addition, voltage-gated K+ current amplitude was also decreased by high CO2. The CO2-sensing properties of chromaffin cells may be of physiologic relevance, particularly for the adaptation of neonates to extrauterine life, before complete maturation of peripheral and central chemoreceptors.

The impact of PCO2 and H+ on the release of acetylcholine from the cat carotid body

The carotid body (CB) is a sensor of oxygen, carbon dioxide, hydrogen ion, and glucose in the arterial blood. Many studies of the CB's responses to low oxygen (hypoxia) have been reported. Recently attention has been increasingly focused on its responses to elevated CO2 (hypercapnia). An increase in ventilation or carotid body neural output (CBNO) can result from stimulating the CB with blood or perfusion fluids having an elevated CO2 or H+. The increase in ventilation seen with a hypoxic stimulus is accompanied with an increase in CBNO and an increased release of both acetylcholine (ACh) and ATP from the CB. The present in vitro study using both CBs harvested from six cats was undertaken to determine if hypercapnia also provoked an increased release of ACh from the incubated CBs. The anesthetizing, handling, and euthanizing of the animals were according to the guidelines of the Johns Hopkins Animal Care and Use Committee which are totally consonant with those of the NIH. CBs, once harvested and prepared for the experimental protocol, were subjected to the following steps each lasting 10 min: (1) control; (2) stress; (3) recovery. The stresses were respiratory acidosis (RAC; acidic hypercapnia), compensated respiratory acidosis (CRAC; isohydric hypercapnia), and metabolic acidosis (MtAC). The first and last forms of acidosis generated small but significant increases in the release of ACh from the CBs; the second generated a very small and insignificant increase in ACh release. Since it is generally accepted that ACh is a key excitatory neurotransmitter in the CB along with ATP, these data are consistent with other studies measuring the increase in ventilation in response to a small increase in CO2 and those studies recording CBNO in response to hypercapnia. In five of the six animals the responses to RAC and MtAC were compared to the responses to hypoxia. The latter were statistically indistinguishable from the former two.

Neurotransmission in the carotid body: transmitters and modulators between glomus cells and petrosal ganglion nerve terminals

The carotid body (CB) is the main arterial chemoreceptor. The most accepted model of arterial chemoreception postulates that carotid body glomus (type I) cells are the primary receptors, which are synaptically connected to the nerve terminals of petrosal ganglion (PG) neurons. In response to natural stimuli, glomus cells are expected to release one (or more) transmitter(s) which, acting on the peripheral nerve terminals of processes from chemosensory petrosal neurons, increases the sensory discharge. Among several molecules present in glomus cells, acetylcholine and adenosine nucleotides and dopamine are considered as excitatory transmitter candidates. In this review, we will examine recent evidence supporting the notion that acetylcholine and adenosine 5'-triphosphate are the main excitatory transmitters in the cat and rat carotid bodies. On the other hand, dopamine may act as a modulator of the chemoreception process in the cat, but as an excitatory transmitter in the rabbit carotid body.

ACh and ATP mediate excitatory transmission in cat carotid identified chemoreceptor units in vitro

Several molecules have been proposed as excitatory transmitters between glomus (type 1) cells and nerve terminals of petrosal ganglion (PG) neurons in the carotid body (CB). We tested whether ACh and ATP have a role to play as excitatory transmitters in the cat CB by recording intracellularly from identified PG neurons functionally connected to the CB in vitro. PG neurons projecting to the CB were classified according to their intracellular responses as: (a) neurons with humped action potentials (hAP neurons) responding phasically to long-lasting depolarizing pulses (53/67), and (b) neurons with smooth action potentials (non-hAP neurons) that fire tonically during long-lasting depolarizations (14/67). CB stimulation by stop flow and/or acidosis induced activity in 28 of 39 hAP-type neurons, being classified as chemosensory, but in none of the non-hAP neurons. Hexamethonium (10 microM) and suramin (100 microM) reversibly abolished the increased discharges evoked in chemosensory neurons (8/9) by stop flow or acidosis. Moreover, 24 of 27 chemosensory neurons responded to ganglionar application of ACh and ATP, while two neurons responded only to ACh and one to ATP. Mechanical deformation of the carotid sinus induced firing activity in 10 of 13 non-hAP neurons, but in none of the hAP neurons tested. Interestingly, 4/10 non-hAP neurons, which responded to carotid sinus mechanical stimulation also responded to ganglionar application of ATP, but were insensitive to ACh. Present results favor the hypothesis that ACh and ATP are excitatory transmitters in the cat CB, acting-at least-on the PG neuron terminals in the CB.

Dual effects of nitric oxide on cat carotid body chemoreception

We studied the effects of nitric oxide (NO) released by NO donors on cat carotid body (CB) chemosensory activity during normoxia and hypoxia. CBs excised from pentobarbital sodium-anaesthetized cats were perfused with Tyrode at 38 degrees C and pH 7.40. The frequency of chemosensory discharges (f(x)) was recorded from the carotid sinus nerve, and changes of NO concentration were measured by a chronoamperometric technique, with NO-selective carbon-fiber microelectrodes inserted in the CB. During steady chemosensory excitation induced by hypoxia, bolus injections of NO (DeltaNO = 0. 5-12 microM), released by S-nitroso-N-acetylpenicillamine (SNAP) and 6-(2-hydroxy-1-methyl-nitrosohydrazino)-N-methyl-1-hexanamine++ + (NOC-9), transiently reduced f(x) in a dose-dependent manner. However, during normoxia, the same concentration of NO (DeltaNO = 0. 5-13 microM) released by the NO donors increased f(x) in a dose-dependent manner. The present results show a dual effect of NO on CB chemoreception that is dependent on the PO(2) levels. During hypoxia, NO is predominantly an inhibitor of chemoreception, whereas, in normoxia, NO increased f(x). The mechanisms by which NO produces chemosensory excitation during normoxia remain to be determined.

Postnatal maturation of carotid body and type I cell chemoreception in the rat

The site of postnatal maturation of carotid body chemoreception is unclear. To test the hypothesis that maturation occurs synchronously in type I cells and the whole carotid body, the development of changes in the intracellular Ca2+ concentration responses to hypoxia, CO2, and combined challenges was studied with fluorescence microscopy in type I cells and compared with the development of carotid sinus nerve (CSN) responses recorded in vitro from term fetal to 3-wk animals. Type I cell responses to all challenges increased between 1 and 8 days and then remained constant, with no multiplicative O2-CO2 interaction at any age. The CSN response to hypoxia also matured by 8 days, but CSN responses to CO2 did not change significantly with age. Multiplicative O2-CO2 interaction occurred in the CSN response at 2-3 wk but not in younger groups. We conclude that type I cell maturation underlies maturation of the CSN response to hypoxia. However, because development of responses to CO2 and combined hypoxia-CO2 challenges differed between type I cells and the CSN, responses to these stimuli must mature at other, unidentified sites within the developing carotid body.

Dopamine, sensory discharge, and stimulus interaction with CO2 and O2 in cat carotid body

It is hypothesized that carotid body chemosensory activity is coupled to neurosecretion. The purpose of this study was to examine whether there was a correspondence between carotid body tissue dopamine (DA) levels and neuronal discharge (ND) measured from the carotid sinus nerve of perfused cat carotid bodies and to characterize interaction between CO2 and O2 in these responses. ND and tissue DA were measured after changing from normoxic, normocapnic control bicarbonate buffer (PO2 >120 Torr, PCO2 25-30 Torr, pH approximately 7.4) to normoxic hypercapnia (PCO2 55-57 Torr, pH 7.1-7.2) or to hypoxic solutions (PO2 30-35 Torr) with normocapnia (PCO2 25-30 Torr, pH approximately 7.4) or hypocapnia (PCO2 10-15 Torr, pH 7.6-7.8). Similar temporal changes for ND and tissue DA were found for all of the stimuli, although there was a much different proportional relationship for normoxic hypercapnia. Both ND and DA increased above baseline values during flow interruption and normocapnic hypoxia, and both decreased below baseline values during hypoxic hypocapnia. In contrast, normoxic hypercapnia caused an initial increase in ND, from a baseline of 175 +/- 12 (SE) to a peak of 593 +/- 20 impulses/s within 4.6 +/- 0.9 s, followed by adaptation, whereas ND declined to 423 +/- 20 impulses/s after 1 min. Tissue DA initially increased from a baseline of 17.9 +/- 1.2 microM to a peak of 23.2 +/- 1.2 microM within 3.0 +/- 0.7 s, then declined to 2.6 +/- 1.0 microM. The substantial decrease in tissue DA during normoxic hypercapnia was not consistent with the parallel changes in DA with ND that were observed for hypoxic stimuli.

Inhibition of dopamine release with simultaneous chemosensory excitation by hypercapnia with and without [Ca2+]0 in the cat carotid body

The hypothesis that dopamine (DA) overflow corresponds to carotid sinus nerve (CSN) discharge during hypercapnia and is dependent on [Ca2+]0 was tested. We simultaneously measured the time course of DA overflow and CSN discharge of the cat carotid body, perfused/superfused in vitro at 37 degrees C at decreasing [Ca2+]0, during transition from normocapnia (PCO2 approximately 30-35 Torr) to hypercapnia (PCO2 approximately 60-65 Torr). In the presence of normal [Ca2+]0, hypercapnia instantaneously increased nerve discharge to peak levels followed by a decrease to steady states which were above the basal rate of activity. CSN discharge rate did not differ at decreasing [Ca2+]0 between 2.2 and 1.0 mM, and it began to decline at 0.1 mM [Ca2+]0, culminating to zero level in most cases, at zero [Ca2+]0. DA overflow increased slightly during hypercapnic peak CSN activity. Thereafter it declined to steady state levels below those of normocapnic conditions. Decreases in steady state DA levels were significantly less at 0 mM [Ca2+]0 compared to the higher calcium concentrations (0.1, 1.0 and 2.2 mM). Overall, steady state CSN activity and DA overflow were inversely related. Thus, DA release cannot have excitatory implications for carotid chemoreceptors during hypercapnia in the cat.