The presence of CO2/HCO3- is essential for hypoxic chemotransduction in the in vivo perfused carotid body

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.

Integration of Central and Peripheral Respiratory Chemoreflexes

A debate has raged since the discovery of central and peripheral respiratory chemoreceptors as to whether the reflexes they mediate combine in an additive (i.e., no interaction), hypoadditive or hyperadditive manner. Here we critically review pertinent literature related to O2 and CO2 sensing from the perspective of system integration and summarize many of the studies on which these seemingly opposing views are based. Despite the intensity and quality of this debate, we have yet to reach consensus, either within or between species. In reviewing this literature, we are struck by the merits of the approaches and preparations that have been brought to bear on this question. This suggests that either the nature of combination is not important to system responses, contrary to what has long been supposed, or that the nature of the combination is more malleable than previously assumed, changing depending on physiological state and/or respiratory requirement.

Effects of modulators of AMP-activated protein kinase on TASK-1/3 and intracellular Ca(2+) concentration in rat carotid body glomus cells

Acute hypoxia depolarizes carotid body chemoreceptor (glomus) cells and elevates intracellular Ca(2+) concentration ([Ca(2+)]i). Recent studies suggest that AMP-activated protein kinase (AMPK) mediates these effects of hypoxia by inhibiting the background K(+) channels such as TASK. Here we studied the effects of modulators of AMPK on TASK activity in cell-attached patches. Activators of AMPK (1mM AICAR and 0.1-0.5mM A769662) did not inhibit TASK activity or cause depolarization during acute (10min) or prolonged (2-3h) exposure. Hypoxia inhibited TASK activity by ∼70% in cells pretreated with AICAR or A769662. Both AICAR and A769662 (15-40min) failed to increase [Ca(2+)]i in glomus cells. Compound C (40μM), an inhibitor of AMPK, showed no effect on hypoxia-induced inhibition of TASK. AICAR and A769662 phosphorylated AMPKα in PC12 cells, and Compound C blocked the phosphorylation. Our results suggest that AMPK does not affect TASK activity and is not involved in hypoxia-induced elevation of intracellular [Ca(2+)] in isolated rat carotid body glomus cells.

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.

Developmental changes in the magnitude and activation characteristics of Na(+) currents of petrosal neurons projecting to the carotid body

Carotid bodies mediate hypoxia sensing for the respiratory system and increase their sensitivity in the post-natal period. The present study examined the characteristics and developmental change of fast Na(+) currents of chemoreceptor afferent neurons. Rat carotid bodies (P2-P19) were harvested intact with the petrosal ganglia and whole-cell recordings obtained from petrosal somas whose axons projected to the carotid body. The magnitude of Na(+) current increased in the post-natal period in parallel with increased conduction velocity and somal size. Voltage-dependence of activation significantly shifted towards negative potentials but no significant change occurred in the voltage dependence of inactivation or the slope factors for activation or inactivation. The leftward shift in activation increased slowly or non-inactivating currents around resting potential which increases afferent neuron excitability, a result confirmed in current clamp recordings. These results suggest that a developmental shift in Na(+) current activation plays a role in chemoreceptor maturation by enhancing excitability of the afferent neuron.

Hypoxia. 4. Hypoxia and ion channel function

The ability to sense and respond to oxygen deprivation is required for survival; thus, understanding the mechanisms by which changes in oxygen are linked to cell viability and function is of great importance. Ion channels play a critical role in regulating cell function in a wide variety of biological processes, including neuronal transmission, control of ventilation, cardiac contractility, and control of vasomotor tone. Since the 1988 discovery of oxygen-sensitive potassium channels in chemoreceptors, the effect of hypoxia on an assortment of ion channels has been studied in an array of cell types. In this review, we describe the effects of both acute and sustained hypoxia (continuous and intermittent) on mammalian ion channels in several tissues, the mode of action, and their contribution to diverse cellular processes.

Chemoreceptor and pulmonary stretch receptor interactions within amphibian respiratory control systems

The hypercapnic drive to breathe in amphibians is generally greater than hypoxic ventilatory drive and a variety of interdependent control systems function to regulate both the hypoxic and hypercapnic ventilatory responses. During exposure to hypercapnic conditions, breathing increases in response to input from central chemoreceptors (sensitive to CSF pH/CO(2) levels) and peripheral chemoreceptors (sensitive to arterial blood O(2) and CO(2)). On the other hand, olfactory CO(2) receptors in the nasal epithelium inhibit breathing during exposure to acute hypercapnia. Further complexity arises from the CO(2)-sensitive nature of the pulmonary stretch receptors (PSR) which provide both tonic (stimulates lung inflation at low lung volumes; deflation at higher volumes) and phasic (generally excitatory) feedback. This review focuses on interactions between the various populations of chemoreceptors and interactions between chemoreceptors and PSR. Differences between various levels of experimental reduction (i.e., in vitro; in situ; in vivo) are highlighted as are the effects of chronic respiratory challenges on acute hypoxic and hypercapnic chemoreflexes.

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.

CO2/H(+) sensing: peripheral and central chemoreception

H(+) is maintained constant in the internal environment at a given body temperature independent of external environment according to Bernard's principle of "milieu interieur". But CO2 relates to ventilation and H(+) to kidney. Hence, the title of the chapter. In order to do this, sensors for H(+) in the internal environment are needed. The sensor-receptor is CO2/H(+) sensing. The sensor-receptor is coupled to integrate and to maintain the body's chemical environment at equilibrium. This chapter dwells on this theme of constancy of H(+) of the blood and of the other internal environments. [H(+)] is regulated jointly by respiratory and renal systems. The respiratory response to [H(+)] originates from the activities of two groups of chemoreceptors in two separate body fluid compartments: (A) carotid and aortic bodies which sense arterial P(O2) and H(+); and (B) the medullary H(+) receptors on the ventrolateral medulla of the central nervous system (CNS). The arterial chemoreceptors function to maintain arterial P(O2) and H(+) constant, and medullary H(+) receptors to maintain H(+) of the brain fluid constant. Any acute change of H(+) in these compartments is taken care of almost instantly by pulmonary ventilation, and slowly by the kidney. This general theme is considered in Section 1. The general principles involving cellular CO2 reactions mediated by carbonic anhydrase (CA), transport of CO2 and H(+) are described in Section 2. Since the rest of the chapter is dependent on these key mechanisms, they are given in detail, including the role of Jacobs-Stewart Cycle and its interaction with carbonic anhydrase. Also, this section deals briefly with the mechanisms of membrane depolarization of the chemoreceptor cells because this is one mechanism on which the responses depend. The metabolic impact of endogenous CO2 appears in the section with a historical twist, in the context of acclimatization to high altitude (Section 3). Because low P(O2) at high altitude stimulates the peripheral chemoreceptors (PC) increasing ventilation, the endogenous CO2 is blown off, making the internal milieu alkaline. With acclimatization however ventilation increases. This alkalinity is compensated in the course of time by the kidney and the acidity tends to be restored, but the acidification is not great enough to increase ventilation further. The question is what drives ventilation during acclimatization when the central pH is alkaline? The peripheral chemoreceptor came to the rescue. Its sensitivity to P(O2) is increased which continues to drive ventilation further during acclimatization at high altitude even when pH is alkaline. This link of CO2 through the O2 chemoreceptor is described in Section 4 which led to hypoxia-inducible factor (HIF-1). HIF-1 is stabilized during hypoxia, including the carotid body (CB) and brain cells, the seat of CO2 chemoreception. The cells are always hypoxic even at sea level. But how CO2 can affect the HIF-1 in the brain is considered in this section. CO2 sensing in the central chemoreceptors (CC) is given in Section 5. CO(2)/H(+) is sensed by the various structures in the central nervous system but its respiratory and cardiovascular responses are restricted only to some areas. How the membranes are depolarized by CO2 or how it works through Na(+)/Ca(2+) exchange are discussed in this section. It is obvious, however, that CO2 is not maintained constant, decreasing with altitude as alveolar P(O2) decreases and ventilation increases. Rather, it is the [H(+)] that the organism strives to maintain at the expense of CO2. But then again, [H(+)] where? Perhaps it is in the intracellular environment. Gap junctions in the carotid body and in the brain are ubiquitous. What functions they perform have been considered in Section 6. CO2 changes take place in lung alveoli where inspired air mixes with the CO2 from the returning venous blood. It is the interface between the inspired and expired air in the lungs where CO2 change is most dramatic. As a result, various investigators have looked for CO2 receptors in the lung, but none have been found in the mammals. Instead, CO2/H(+) receptors were found in birds and amphibians. However, they are inhibited by increasing CO2/H(+), instead of stimulated. But the afferent impulses transmitted to the brain produced stimulation in the efferents. This reversal of afferent-efferent inputs is a curious situation in nature, and this is considered in Section 7. The NO and CO effects on CO2 sensing are interesting and have been briefly mentioned in Section 8. A model for CO2/H(+) sensing by cells, neurons and bare nerve endings are also considered. These NO effects, models for CO2/H(+) and O2-sensitive cells in the CNS have been considered in the perspectives. Finally, in conclusion, the general theme of constancy of internal environment for CO2/H(+) is reiterated, and for that CO2/H(+) sensors-receptors systems are essential. Since CO2/H(+) sensing as such has not been reviewed before, the recent findings in addition to defining basic CO2/H(+) reactions in the cells have been briefly summarized.

Immunohistochemical localization of carbonic anhydrase isozymes in the rat carotid body

The rat carotid body was immunohistochemically stained for carbonic anhydrase I, II and III (CA-I, CA-II and CA-III). Immunoreactivity for CA-I was distributed in type I cells, type II cells and nerve bundles. Smooth muscle cells and endothelial cells of blood vessels were also strongly stained for CA-I. CA-II immunoreactivity was distinctly positive in type I cells and nerve bundles. Vascular smooth muscle cells were weakly positive, and type II cells were negative for CA-II. CA-III immunoreactivity was identified in type I cells and vascular smooth muscle cells. Our results suggest that carbonic anhydrase isozymes in type I cells play an important role in chemoreception for hypercapnia. Immunoreactivities for CA-I and CA-II in the nerve fibres may participate in the synergic action of carotid sinus nerve between hypoxic and hypercapnic stimuli.

Atrial natriuretic peptide stimulates cat carotid body chemoreceptors in vivo

It is known that atrial natriuretic peptide (ANP) is released from cardiac myocyte and other stores during hypoxia and is involved in pulmonary-cardiovascular reflexes and in natriuresis and diuresis. Since the carotid body initiates hypoxic chemoreflexes, we hypothesized that ANP could potentiate the hypoxic stimulation of the carotid body chemoreceptor in vivo. We studied the effect of close intra-arterial injection of ANP on carotid chemoreceptor activity in anesthetized male cats which were paralyzed and artificially ventilated. Graded doses of ANP (0-10 nmoles) were administered by intra-arterial injections and they produced an excitatory response. Single dose of ANP (6.5 nmoles) at four steady-state levels of arterial PO(2), at constant PCO(2), produced increases of chemoreceptor activity. This increase of chemoreceptor activity with ANP in the presence of CO(2)-HCO(3)(-) in vitro could make a difference from those without CO(2)-HCO(3)(-) in vivo.

Release of substance P by low oxygen in the rabbit carotid body: evidence for the involvement of calcium channels

Carotid bodies from diverse species contain substance P (SP), an 11-residue peptide that belongs to the tachykinin peptide family. Previous studies indicated that SP is excitatory to the carotid body and is associated with sensory response to hypoxia. However, release of SP from the carotid body during hypoxia has not been documented. In the present study, we determined whether hypoxia releases SP from the carotid body and further characterized the mechanism(s) associated with SP release by low oxygen. The release of SP from superfused rabbit carotid body was determined by an enzyme immunoassay (EIA). SP-like immunoreactivity was localized to many glomus cells and nerve fibers and the concentration of SP in the rabbit carotid body was 1.5+/-0.1 ng/mg protein. For release studies, carotid bodies (n=56) were superfused with a modified Tyrode medium containing Hepes buffer, pH 7.4, saturated with either room air (normoxia) or hypoxic gas mixtures. The basal release of SP during normoxia was 51.0+/-1.5 fmol/min per mg protein. Hypoxia increased SP release from the carotid body and the magnitude of release is dependent on the severity of hypoxic stimulus. Moderate hypoxia (pO2, 79+/-4 mmHg) stimulated SP release by approximately 50%, whereas SP release during severe hypoxia (pO2, 11+/-6 mmHg) was 2-fold higher than the normoxic control. A similar pattern of SP release was also observed when superfusion medium containing CO2-HCO3 buffer, pH 7.4, was used for release studies. To examine the mechanism(s) associated with hypoxia-induced SP release from the carotid body, moderate level of hypoxia (12% O2+N2) was used. Omission of calcium in the superfusion medium markedly attenuated hypoxia-induced SP release (>95%), whereas the basal release of SP was unaffected. Cd2+ (100 microM), a voltage-dependent Ca2+ channel blocker, abolished hypoxia-induced SP release. About 85% of SP release by hypoxia was inhibited by omega-conotoxin GVIA (1 microM), an N-type Ca2+ channel blocker, whereas nitrendipine (1.5 microM), an inhibitor of L-type Ca2+ channel partially attenuated ( approximately 65%) hypoxia-induced SP release. By contrast, omega-agatoxin TK (50 nM), a P/Q-type Ca2+ channel inhibitor, had no significant effect (P>0.05, n=6). These results suggest that SP is released from the rabbit carotid body by hypoxia that depends on the severity of the hypoxic stimulus. Further, SP release by hypoxia is a calcium-dependent process and is primarily mediated by N- and L-type Ca2+ channels.

Augmentation of calcium current by hypoxia in carotid body glomus cells

Several lines of evidence indicate that transduction of the hypoxic stimulus at the carotid body involves an increase in cytosolic Ca2+ ([Ca2+]i) via activation of voltage-gated Ca2+ channels in the glomus cells. However, reported responses to hypoxia include either no effect on or inhibition of Ca2+ current in glomus cells. The apparent discrepancy between the effects of hypoxia on [Ca2+]i and Ca2+ channel activity prompted us to re-examine the effects of low oxygen on Ca2+ currents in glomus cells. Experiments were performed on freshly dissociated glomus cells from rabbit carotid bodies. Ca2+ channel activity was monitored using the whole-cell configuration of the patch clamp technique with Ba2+ as the charge carrier. Hypoxia (pO2 = 40 mmHg) augmented the Ca2+ current by 24% (at 0 mV). This augmentation was seen in a CO2/HCO3- but not in a HEPES buffered extracellular solution. However, when the extracellular pH (pHo) of a HEPES buffered solution is lowered from 7.4 to 7.0, then the Ca2+ current in glomus cells is augmented by hypoxia by 20%. Nisoldipine, an L-type Ca2+ channel blocker (2 microM), prevented augmentation of the Ca2+ current by hypoxia. On the other hand, an N- and P-type Ca2+ channel blocker (2 microM omega-conotoxin MVIIC) did not prevent the augmentation of the Ca2+ current by hypoxia. Protein kinase C (PKC) inhibitors, staurosporine (100 nM) and bisindolylmaleimide (2 microM), prevented augmentation by hypoxia. Okadaic acid (100 nM), an inhibitor of serine/threonine phosphatases also prevented augmentation of Ca2+ current by hypoxia; whereas, norokadaone, an inactive analog of okadaic acid, had no effect. These results suggest that hypoxia augments Ca2+ current through L-type Ca2+ channels via a PKC and/or phosphatase-sensitive pathways.

Augmentation of L-type calcium current by hypoxia in rabbit carotid body glomus cells: evidence for a PKC-sensitive pathway

Previous studies have suggested that voltage-gated Ca(2+) influx in glomus cells plays a critical role in sensory transduction at the carotid body chemoreceptors. The purpose of the present study was to determine the effects of hypoxia on the Ca(2+) current in glomus cells and to elucidate the underlying mechanism(s). Experiments were performed on freshly dissociated glomus cells from rabbit carotid bodies. Ca(2+) current was monitored using the whole cell configuration of the patch-clamp technique, with Ba(2+) as the charge carrier. Hypoxia (pO(2) = 40 mmHg) augmented the Ca(2+) current by 24 +/- 3% (n = 42, at 0 mV) in a voltage-independent manner. This effect was seen in a CO(2)/HCO(3)(-)-, but not in a HEPES-buffered extracellular solution at pH 7.4 (n = 6). When the pH of a HEPES-buffered extracellular solution was lowered from 7.4 to 7. 0, hypoxia augmented the Ca(2+) current by 20 +/- 5% (n = 4, at 0 mV). Nisoldipine, an L-type Ca(2+) channel blocker (2 microM, n = 6), prevented, whereas, omega-conotoxin MVIIC (2 microM, n = 6), an inhibitor of N and P/Q type Ca(2+) channels, did not prevent augmentation of the Ca(2+) current by hypoxia, implying that low oxygen affects L-type Ca(2+) channels in glomus cells. Protein kinase C (PKC) inhibitors, staurosporine (100 nM, n = 6) and bisindolylmaleimide (2 microM, n = 8, at 0 mV), prevented, whereas, a protein kinase A inhibitor (4 nM PKAi, n = 10) did not prevent the hypoxia-induced increase of the Ca(2+) current. Phorbol 12-myristate 13-acetate (PMA, 100 nM), a PKC activator, augmented the Ca(2+) current by 20 +/- 3% (n = 8, at 0 mV). In glomus cells treated with PMA overnight (100 nM), hypoxia did not augment the Ca(2+) current (-3 + 4%, n = 5, at 0 mV). Immunocytochemical analysis revealed PKCdelta-like immunoreactivity in the cytosol of the glomus cells. Following hypoxia (6% O(2) for 5 min), PKCdelta-like immunoreactivity translocated to the plasma membrane in 87 +/- 3% of the cells, indicating PKC activation. These results demonstrate that hypoxia augments Ca(2+) current through L-type Ca(2+) channels via a PKC-sensitive mechanism.

Effects of hypoxia and dithionite on catecholamine release from isolated type I cells of the rat carotid body

1. Amperometric recordings were conducted to investigate the ability of hypoxia and anoxia to evoke quantal catecholamine secretion from isolated type I cells of the rat carotid body. 2. Hypoxia (PO2 8-14 mmHg) consistently failed to evoke catecholamine secretion from type I cells, when cells were perfused either at room temperature (21-24 C) or at 35-37 C, and regardless of whether Hepes- or HCO3-/CO2-buffered solutions were used. 3. Elevating extracellular [K+] caused concentration-dependent secretion from individual type I cells, with a threshold concentration of approximately 25 mM. In the presence of this level of extracellular K+, hypoxia (PO2 8-14 mmHg) caused a marked enhancement of secretion which was fully blocked by 200 microM Cd2+, a non-specific blocker of voltage-gated Ca2+ channels. 4. Anoxia (N2-equilibrated solution containing 0.5 mM dithionite) evoked exocytosis from type I cells when extracellular [K+] was 5 mM. This secretion was completely inhibited by removal of extracellular Ca2+, but was not significantly affected by Cd2+ (200 microM), Ni2+ (2 mM), Zn2+ (1 mM) or nifedipine (2 microM). Secretion was also observed when 0.5 mM dithionite was added to air-equilibrated solutions. 5. Anoxia also evoked secretion from chemoreceptive phaeochromocytoma (PC12) cells, which was wholly Ca2+ dependent, but unaffected by Cd2+ (200 microM). 6. Our results suggest that hypoxia can evoke catecholamine secretion from isolated type I cells, but only in the presence of elevated extracellular [K+]. This may be due to the cells being relatively hyperpolarized following dissociation. In addition, we have shown that dithionite evokes catecholamine release regardless of PO2 levels, and this release is due mainly to an artefactual Ca2+ influx pathway activated in the presence of dithionite.

Chemotransduction by carotid body chemoreceptors is dependent on bicarbonate currents

Previous studies have demonstrated that bicarbonate enhances the speed and magnitude of the carotid body chemoreceptor response to hypoxia. We hypothesized that this enhancement is associated with enhanced hypoxia-induced catecholamine (CAT) secretion from glomus cells. Single-fiber nerve activity and free tissue catecholamine (carbon fiber microvoltammetry) were measured in rat carotid body, in vitro. The peak CAT and nerve responses during 1 min anoxia were larger in the presence of bicarbonate than in its absence (peak CAT: 16.7 +/- 2.7 vs. 5.1 +/- 1.1 microM; peak nerve: 28.2 +/- 1.6 vs. 16.7 +/- 1.4 Hz). Bicarbonate particularly enhanced the responses to moderate hypoxia (PO2 approximately 80 Torr) which caused no secretion or increased nerve activity in the absence of bicarbonate, but caused significant stimulation in the presence of bicarbonate (peak nerve = 15.2 Hz; peak CAT = 8.6 microM). The bicarbonate effect was not due to alterations in intracellular pH since it was not blocked by exchanger blockers (DIDS) or mimicked by acidification of the medium. However, anion channel blockade by 9-AC or DPC reduced anoxia-induced CAT secretion in the presence of bicarbonate. We conclude that bicarbonate greatly enhances stimulus/secretion coupling in glomus cells, probably through modulation of an anion current carried by bicarbonate.

Anion exchanger and chloride channel in cat carotid body chemotransduction

In order to test the hypothesis that carotid body (CB) chemoreception depends on the functions of anion channels and HCO3-/Cl- exchangers, we studied the effects of the anion channel blocker anthracene-9-carboxylic acid (9-ANC), the carbonic anhydrase inhibitor methazolamide, and the HCO3-/Cl- exchanger blocker 4,4 diisothiocyanatostilbene-2-2'disulfonic acid (DIDS) on the chemosensory discharges of cat CB, perfused-superfused in vitro at 36.5 +/- 0.5 degrees C, with a modified Tyrode solution. The chemosensory responses to hypoxia (PO2 approximately 50 Torr), hypercapnia (PCO2 approximately 60 Torr, pH = 7.10), nicotine (2-4 nmol) and NaCN (20-40 nmol) were recorded. 9-ANC (2 microM) and DIDS (10 microM) decreased the chemosensory baseline activity, and eliminated the initial peak responses to hypercapnia and hypoxia and increased the time to achieve it. Methazolamide (0.13 mM) did not alter the effect of 9-ANC. The steady state responses to hypoxia and hypercapnia were not diminished after 9-ANC but DIDS lowered the responses. Responses to NaCN effects were all diminished but those to nicotine were not affected. The results suggest that the functions of anion channels and HCO3-/Cl- exchangers are important for the resting dischargers and for the fast responses to hypoxia and hypercapnia.

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

Using a chronoamperometric technique with carbon-fiber microelectrodes and neural recordings, we simultaneously measured the effects of the following procedures on catecholamine efflux (delta CA) and frequency of chemosensory discharges (fx) from superfused cat carotid body: 1) the addition of CO2-HCO3- to Tyrode solution previously buffered with N-2-hydroxyethylpiperazine-N'-2-ethane-sulfonic acid, maintaining pH at 7.40; 2) hypercapnia (10% CO2, pH 7.10); 3) hypoxia (PO2 h approximately 40 Torr) with and without CO2-HCO3-; and 4) the impact of several boluses of dopamine (DA; 10-100 micrograms) on hypoxic and hypercapnic challenges. With CO2-HCO3-, hypoxia increased fx which preceded delta CA increases, whereas hypercapnia raised fx but did not consistently increase delta CA. Repeated stimuli induced similar fx increases, but attenuated delta CA. After DA, hypoxia produced larger delta CA, which preceded chemosensory responses. Without CO2-HCO3-, hypoxia produced a similar pattern of delta CA and fx responses. Switching to Tyrode solution with CO2-HCO3- at pH 7.40 raised fx but did not increase delta CA. With CO2-HCO3- and after DA, hypoxic-induced delta CAs were larger than in its absence. Results suggest that DA release is not essential for chemosensory excitation.

Synapse formation and hypoxic signalling in co-cultures of rat petrosal neurones and carotid body type 1 cells

1. To investigate synaptic mechanisms mediating chemosensory signalling in the carotid body, we developed co-cultures of chemoreceptor type 1 cell clusters and dissociated petrosal neurones (PNs) from 7- to 14-day-old rat pups and tested for functional connectivity in CO2-HCO3(-)-or Hepes-buffered medium at approximately 35 degrees C. 2. When cultured without type 1 cells, PNs were almost always quiescent (n = 104) and unresponsive to hypoxia (Po2 = 5-25 mmHg) during perforated patch, whole-cell recordings of membrane potential or voltage-activated currents; in contrast, many PNs (77 out of 170) that were juxtaposed to type 1 cell clusters in co-culture displayed spontaneous activity, comprising spikes and subthreshold potentials (SSPs) that resembled synaptic potentials. 3. Additional tests suggested that de novo chemical synapses developed between PNs and type 1 cell clusters in vitro. For example: (i) the spontaneous activity was reversibly suppressed by substituting low calcium-high magnesium in the bath; (ii) SSPs had variable amplitudes and persisted following action potential blockade with TTX (1 microM); (iii) the interval distribution between successive spontaneous events appeared random; and (iv) the frequency of spontaneous potentials was diminished (reversibly) by the nicotinic antagonist hexamethonium (100 microM), suggesting contributions from the spontaneous release of ACh. 4. Many complexes of 'juxtaposed' PNs and type 1 clusters were physiologically functional, since exposure to hypoxia caused a reversible depolarization and/or increased spike discharge in approximately 30% of such neurones (n = 140). The hypoxia-induced spike discharge persisted in the presence of the dopamine D2 receptor blocker spiperone (10-50 microM; n = 5); however, this discharge was reversibly inhibited by 100-200 microM hexamethonium, suggesting that it was mediated, at least in part, by ACh acting through nicotinic receptors. 5. The hypoxia-induced spike discharge and frequency of spontaneous potentials in co-cultured PNs were reversibly suppressed when the buffer was switched from CO2-HCO3- to Hepes (10 mM) at pH 7.4; further, 'functional' PNs that displayed spontaneous activity and/or hypoxia-induced responses in co-culture were encountered more frequently in CO2-HCO3- (> or = 40%) than in Hepes (< or = 26%) buffer. 6. We conclude that functional chemical synapses can develop de novo in cultures of carotid body type 1 cells and PNs and that ACh is probably an important excitatory neurotransmitter secreted from type 1 cells during hypoxic chemotransduction in the rat carotid body.

CO interact with intracellular [H+] with and without CO2-HCO3- in the cat carotid chemosensory discharge

To test the hypothesis whether CO2-HCO3- buffer is essential for the expression of chemoreception and to distinguish between pHi and pHo interaction with pCO in the carotid chemosensory response, we superfused-perfused in vitro cat carotid bodies using HEPES-Tyrode's solution with and without CO2-HCO3-, and compared the responses at the same pHo in the absence and presence of light. In the absence of light, pCO (> 138 Torr) stimulated the carotid body chemoreceptors in CO2-HCO3- buffer at pHo of 7.40, whereas pCO (69-550 Torr) did not stimulate the neural discharge in HEPES buffer at the pHo of 7.4-7.1 but did so below pHo 7.1. In the presence of light, all the responses were diminished proportionately.

Hypoxic dilatation of porcine small coronary arteries: role of endothelium and KATP-channels

1. The aim of the present study was to determine the cellular mechanims and potential mediators involved in hypoxic dilatation of porcine small coronary arteries. 2. Small coronary arteries were isolated from a branch of the left anterior descending artery of porcine hearts, cannulated with glass micropipettes and studied in a perfusion myograph system. At a transmural pressure of 40 mmHg, the arteries had an internal diameter of 167.8 +/- 6.6 microns (n = 37). 3. In arteries contracted with acetylcholine (ACh), hypoxia (0% O2, 30 min) caused dilatation (86.9 +/- 6.7% relaxation, n = 6) in vessels with endothelium but constriction in endothelium-denuded vessels. 4. Hypoxic vasodilatation occurring in arteries with endothelium was abolished by the KATP channel inhibitor, glibenclamide (0.44 microM), but was not affected by inhibition of nitric oxide synthase (L-NAME, 44 microM) or cyclo-oxygenase (indomethacin, 4.4 microM). 5. Bradykinin evoked endothelium-dependent relaxation that was inhibited by L-NAME (44 microM) but not glibenclamide 0.44 microM). Cromakalim (0.1-0.3 microM), a KATP channel opener, caused relaxation that was inhibited by glibenclamide, but was not affected by L-NAME (44 microM) and/or indomethacin (4.4 microM). 6. Endothelium-removal inhibited vasodilatation evoked by cromakalim, but increased vasodilator responses to the NO donor, SIN-1 (10(-8) to 10(-5) M). 7. These results indicate that hypoxia acted directly on vascular smooth muscle of small coronary arteries to cause contraction. However, this effect was overwhelmed by endothelium-dependent relaxation in response to hypoxia. This relaxation was most likely mediated by release of an endothelium-derived factor, distinct from nitric oxide or prostacyclin, that activated smooth muscle KATP-channels.

Ionic basis of the membrane potential responses of rat dorsal vagal motoneurones to HEPES buffer

The effects of 10 mM HEPES (N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid) buffered artificial cerebrospinal fluid (aCSF) on membrane potential and the action potential were studied in 93 dorsal vagal motoneurones (DVMs) using an in vitro slice preparation of the rat medulla. Changing from bicarbonate/CO2 aCSF to HEPES aCSF resulted in a depolarisation of 6.0 +/- 0.6 mV and an increase in input resistance (RIn; n = 61). In the presence of 5 mM 4-AP, HEPES either had little effect (n = 9) or hyperpolarised the membrane (n = 10). Mn2+ (3 mM) or Ni2+ (200 microM) abolished the hyperpolarisation and its associated increase in RIn. In voltage-clamp studies 5 mM 4-AP eliminated a transient outward current and Ni2+ blocked an inactivating inward current. It is concluded that HEPES buffer reduces the contribution of the A current to resting membrane potential and also reduces a Ni(2+)-sensitive transient ICa.

Modulation of glomus cell membrane currents of intact rat carotid body

1. In order to understand better the relationship between sinus nerve chemoreceptor activity and changes in glomus cell membrane current, both were measured simultaneously in rat carotid bodies in vitro. Mean membrane resistance of intact glomus cells was 1327 +/- 140 M omega (n = 104, mean +/- S.E.M.) and membrane capacitance was 7.9 +/- 0.8 pF (n = 28). 2. Over the course of 15 min following the start of whole-cell recording, outward current increased by 169 +/- 48% (n = 19), but there was no significant change in holding current or membrane resistance. Reversal potential of the tail current was not changed over this time period. Current run-up was not affected by addition of ATP, Ca2+, okadaic acid or H-7 to the pipette fluid. 3. Brief hypoxia (30-45 s duration, 0 mmHg at nadir) caused a rapid increase in nerve activity, but, on average, no significant change in cell holding current, or resistance. Outward current slightly decreased during hypoxia but failed to recover in the post-hypoxia period. 4. Tetraethylammonium (20 mM), and 4-aminopyridine (1 mM) reduced the outward current to 54 +/- 7 and 66 +/- 3% of control, respectively, but basal nerve activity was unchanged and the nerve response to hypoxia remained intact. 5. These results suggest that hypoxia modulation of glomus cell K+ current is not the primary initiating factor in the nerve response to brief periods of hypoxia in the rat carotid body.

Developmental changes in hypoxia-induced catecholamine release from rat carotid body, in vitro

1. Developmental changes in free tissue catecholamine levels were studied using Nafion-coated, carbon fibre electrodes placed in rat carotid bodies, in vitro. Simultaneously, single fibre chemoreceptor afferent activity was recorded from the sinus nerve. Five age groups were examined: 1, 2, 6, 10 and 20-30 days of age. 2. Using fast-scan voltammetry, similar current peaks were observed during exposure to exogenous dopamine and during superfusion with hypoxic saline. This suggests that changes in carbon fibre electrode current are due to an increase in free tissue catecholamines. 3. Baseline catecholamine levels were significantly less in the 1-6 day age groups compared to 10 day and 20-30 day rats. 4. During 1 min of hypoxia the peak concentration of tissue catecholamine was significantly less in the 1 day compared to the 2 day age groups, and these were less than in 10 day and 20-30 day rats. 5. Peak nerve response during hypoxia increased with age from 4.5 +/- 0.6 Hz in the 1 day to 10.5 +/- 1.6 Hz in the 6 day and to 15.5 +/- 2.2 Hz in the 20-30 day rats. 6. We conclude that (1) resting free tissue catecholamine levels increase with age in the newborn period, (2) hypoxia causes enhanced catecholamine release, and (3) the magnitude of the release increases in the postnatal period as does the nerve activity.

Response to cyanide of two types of glomoid cells in mature rat carotid body

Cells belonging to glomoids of mature rat carotid bodies were studied using the whole-cell patch clamp technique following acute dissociation. The recorded population encompassed two subtypes: one type (n = 202), termed G(out), was characterized by a small voltage-dependent inward current (43 +/- 9 pA, mean +/- S.E.M.), large outward current (671 +/- 31 pA @ +40 mV), high membrane resistance (1910 +/- 110 M omega) and low capacitance (5.1 +/- 0.1 pF). A second subtype (n = 56), termed G(in), had significantly lower membrane resistance (177 +/- 35 M omega), higher membrane capacitance (15.0 +/- 1.0 pF) and little voltage-dependent current. Neither subtype supported generation of multiple action potentials during depolarization in the current clamp mode. Intracellular staining of the recorded cells by Lucifer yellow showed co-localization of both subtypes to clusters of cells which stained positively for catecholamines. Somal diameter was slightly, but significantly, larger for G(in) cells (8.7 +/- 0.4 microM, n = 7) compared to G(out) cells (7.8 +/- 0.2 microM, n = 31) and all cells had fine cytoplasmic processes extending around neighboring cells. During recordings using the perforated patch technique, histotoxic hypoxia significantly decreased a voltage-dependent outward current in G(out) cells by 113 +/- 60 pA (n = 13), and decreased the holding current by 10 +/- 4 pA (n = 13) from a control value of -32 +/- 6 pA. In G(in) cells, cyanide significant decreased membrane resistance and decreased holding current by 55 +/- 28 pA from a control value of +120 +/- 42 pA (n = 7), but caused no significant change in outward current. These results show that glomoids of mature rat carotid bodies contain at least two types of cells which differ in their morphologic and electrophysiologic characteristics. The subtypes rapidly respond to histotoxic hypoxia and thus may mediate separate roles in the organ response to chemostimuli.

Carotid body chemoreception in the absence and presence of CO2-HCO3-

Carotid body (CB) chemosensory responses to natural and pharmacological stimuli were studied in vitro in the presence and nominal absence of CO2-HCO3- in the perfusion-superfusion media. The CBs obtained from cats (n = 10), anesthetized with sodium pentobarbitone, were simultaneously perfused and superfused with a modified Tyrode solution at 36.5 +/- 0.5 degrees C, equilibrated respectively with PO2 of 120 and less than 20 Torr. The Tyrode, nominally free of CO2-HCO3- (HEPES-NaOH, pH 7.38, 310 mOsm), was used first. Subsequently the Tyrode containing HEPES-HCO3-, equilibrated with PCO2 of 36.8 Torr (pH 7.38) was used. Chemosensory discharges were recorded from the carotid sinus nerve. Both hypoxia (PO2 = 20-25 Torr) and ischemic hypoxia stimulated the discharge in the absence and presence of CO2-HCO3-. However, the presence of CO2-HCO3- significantly raised the baseline activity, augmented the speed, sensitivity and the maximal responses to both types of hypoxia. Hypercapnic perfusate (PCO2 = 65 Torr at pH 7.17) produced a peak response equally promptly in the absence and presence of CO2-HCO3- in the ongoing perfusate but generated a larger and more sustained response. Presence of CO2-HCO3- strongly potentiated the responses to cyanide (10(-10)-10(-7) mol) but less strikingly the responses to nicotine (10(-11)-10(-8) mol). Thus, the extracellular CO2-HCO3- significantly improved the response to hypoxia but was not essential for O2 chemoreception. The underlying mechanisms of the effect of CO2-HCO3- is likely to be mediated by the Cl(-)-HCO3- anion exchanger in the pH regulation of glomus cells.