Contribution of in vivo microvascular PO2 in the cat carotid body chemotransduction

A test of the interaction between central and peripheral respiratory chemoreflexes in humans

How central and peripheral chemoreceptor drives to breathe interact in humans remains contentious. We measured the peripheral chemoreflex sensitivity to hypoxia (PChS) at various isocapnic CO2 tensions (P C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) to determine the form of the relationship between PChS and centralP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ . Twenty participants (10F) completed three repetitions of modified rebreathing tests with end-tidalP O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ (P ET O 2 ${P_{{\mathrm{ET}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) clamped at 150, 70, 60 and 45 mmHg. End-tidalP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ (P ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ),P ET O 2 ${P_{{\mathrm{ET}}{{\mathrm{O}}_{\mathrm{2}}}}}$ , ventilation (V ̇ $\dot{V}$ E ) and calculated oxygen saturation (SC O2 ) were measured breath-by-breath by gas-analyser and pneumotach. TheV ̇ $\dot{V}$ E -P ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ relationship of repeat-trials were linear-interpolated, combined, averaged into 1 mmHg bins, and fitted with a double-linear function (V ̇ $\dot{V}$ E S, L min-1 mmHg-1 ). PChS was computed at intervals of 1 mmHg ofP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ as follows: the difference inV ̇ $\dot{V}$ E between the three hypoxic profiles and the hyperoxic profile (∆V ̇ $\dot{V}$ E ) was calculated; three ∆V ̇ $\dot{V}$ E values were plotted against corresponding SC O2 ; and linear regression determined PChS (Lmin-1 mmHg-1 %SC O2 -1 ). These processing steps were repeated at eachP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ to produce the PChS vs. isocapnicP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ relationship. These were fitted with linear and polynomial functions, and Akaike information criterion identified the best-fit model. One-way repeated measures analysis of variance assessed between-condition differences.V ̇ $\dot{V}$ E S increased (P < 0.0001) with isoxicP ET O 2 ${P_{{\mathrm{ET}}{{\mathrm{O}}_{\mathrm{2}}}}}$ from 3.7 ± 1.5 L min-1 mmHg-1 at 150 mmHg to 4.4 ± 1.8, 5.0 ± 1.6 and 6.0 ± 2.2 Lmin-1 mmHg-1 at 70, 60 and 45 mmHg, respectively. Mean SC O2 fell progressively (99.3 ± 0%, 93.7 ± 0.1%, 90.4 ± 0.1% and 80.5 ± 0.1%; P < 0.0001). In all individuals, PChS increased withP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ , and this relationship was best described by a linear model in 75%. Despite increasing central chemoreflex activation, PChS increased linearly withP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ indicative of an additive central-peripheral chemoreflex response. KEY POINTS: How central and peripheral chemoreceptor drives to breathe interact in humans remains contentious. We measured peripheral chemoreflex sensitivity to hypoxia (PChS) at various isocapnic carbon dioxide tensions (P C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) to determine the form of the relationship between PChS and centralP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ . Participants performed three repetitions of modified rebreathing with end-tidalP O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ fixed at 150, 70, 60 and 45 mmHg. PChS was computed at intervals of 1 mmHg of end-tidalP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ (P ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) as follows: the difference inV ̇ $\dot{V}$ E between the three hypoxic profiles and the hyperoxic profile (∆V ̇ $\dot{V}$ E ) was calculated; three ∆V ̇ $\dot{V}$ E values were plotted against corresponding calculated oxygen saturation (SC O2 ); and linear regression determined PChS (Lmin-1 mmHg-1 %SC O2 -1 ). In all individuals, PChS increased withP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ , and this relationship was best described by a linear (rather than polynomial) model in 15 of 20. Most participants did not exhibit a hypo- or hyper-additive effect of central chemoreceptors on the peripheral chemoreflex indicating that the interaction was additive.

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.

Measuring Peripheral Chemoreflex Hypersensitivity in Heart Failure

Heart failure with reduced ejection fraction (HFrEF) induces chronic sympathetic activation. This disturbance is a consequence of both compensatory reflex disinhibition in response to lower cardiac output and patient-specific activation of one or more excitatory stimuli. The result is the net adrenergic output that exceeds homeostatic need, which compromises cardiac, renal, and vascular function and foreshortens lifespan. One such sympatho-excitatory mechanism, evident in ~40-45% of those with HFrEF, is the augmentation of carotid (peripheral) chemoreflex ventilatory and sympathetic responsiveness to reductions in arterial oxygen tension and acidosis. Recognition of the contribution of increased chemoreflex gain to the pathophysiology of HFrEF and to patients' prognosis has focused attention on targeting the carotid body to attenuate sympathetic drive, alleviate heart failure symptoms, and prolong life. The current challenge is to identify those patients most likely to benefit from such interventions. Two assumptions underlying contemporary test protocols are that the ventilatory response to acute hypoxic exposure quantifies accurately peripheral chemoreflex sensitivity and that the unmeasured sympathetic response mirrors the determined ventilatory response. This Perspective questions both assumptions, illustrates the limitations of conventional transient hypoxic tests for assessing peripheral chemoreflex sensitivity and demonstrates how a modified rebreathing test capable of comprehensively quantifying both the ventilatory and sympathoneural efferent responses to peripheral chemoreflex perturbation, including their sensitivities and recruitment thresholds, can better identify individuals most likely to benefit from carotid body intervention.

Ca2+ oscillations in rat carotid body type 1 cells in normoxia and hypoxia

We studied the mechanisms by which carotid body glomus (type 1) cells produce spontaneous Ca²⁺ oscillations in normoxia and hypoxia. In cells perfused with normoxic solution at 37°C, we observed relatively uniform, low-frequency Ca²⁺ oscillations in >60% of cells, with each cell showing its own intrinsic frequency and amplitude. The mean frequency and amplitude of Ca²⁺ oscillations were 0.6 ± 0.1 Hz and 180 ± 42 nM, respectively. The duration of each Ca²⁺ oscillation ranged from 14 to 26 s (mean of ∼20 s). Inhibition of inositol (1,4,5)-trisphosphate receptor and store-operated Ca²⁺ entry (SOCE) using 2-APB abolished Ca²⁺ oscillations. Inhibition of endoplasmic reticulum Ca²⁺-ATPase (SERCA) using thapsigargin abolished Ca²⁺ oscillations. ML-9, an inhibitor of STIM1 translocation, also strongly reduced Ca²⁺ oscillations. Inhibitors of L- and T-type Ca²⁺ channels (Ca_v; verapamil>nifedipine>TTA-P2) markedly reduced the frequency of Ca²⁺ oscillations. Thus, Ca²⁺ oscillations observed in normoxia were caused by cyclical Ca²⁺ fluxes at the ER, which was supported by Ca²⁺ influx via Ca²⁺ channels. Hypoxia (2-5% O₂) increased the frequency and amplitude of Ca²⁺ oscillations, and Ca_v inhibitors (verapamil>nifedipine>>TTA-P2) reduced these effects of hypoxia. Our study shows that Ca²⁺ oscillations represent the basic Ca²⁺ signaling mechanism in normoxia and hypoxia in CB glomus cells.

Hyperbaric oxygen toxicity in brain: A case of hyperoxia induced hypoglycemic brain syndrome

Hyperbaric oxygen exposure is a recent hazzard for higher animals that originated as humans began underwater construction, exploration, and sports. Exposure can lead to abnormal brain EEG, convulsions, and death, the time to onset of each stage of pathology decreasing with increase in oxygen pressure. We provide evidence that hyperoxia, through oxidative phosphorylation, increases the energy state ([ATP]/[ADP][Pi]) of cells critical to providing glucose to cells behind the blood brain barrier (BBB). Brain cells without an absolute dependence on glucose metabolism; i.e. those having sufficient ATP synthesis using lactate and glutamate as oxidizable substrates, are not themselves very adversely affected by hyperoxia. The increased energy state and decrease in free [AMP], however, suppress glucose transport through the blood brain barrier (BBB) and into cells behind the BBB. Glucose has to pass in sequence through three steps of transport by facilitated diffusion and transporter activity for each step is regulated in part by AMP dependent protein kinase. The physiological role of this regulation is to increase glucose transport in response to hypoxia and/or systemic hypoglycemia. Hyperoxia, however, through unphysiological decrease in free [AMP] suppresses 1) glucose transport through the BBB (endothelial GLUT1 transporters) into cerebrospinal fluid (CSF); 2) glucose transport from CSF into cells behind the BBB (GLUT3 transporters) and (GLUT4 transporters). Cumulative suppression of glucose transport results in local regions of hypoglycemia and induces hypoglycemic failure. It is suggested that failure is initiated at axons and synapses with insufficient mitochondria to meet their energy requirements.

Functional Properties of Mitochondria in the Type-1 Cell and Their Role in Oxygen Sensing

The identity of the oxygen sensor in arterial chemoreceptors has been the subject of much speculation. One of the oldest hypotheses is that oxygen is sensed through oxidative phosphorylation. There is a wealth of data demonstrating that arterial chemoreceptors are excited by inhibitors of oxidative phosphorylation. These compounds mimic the effects of hypoxia inhibiting TASK1/3 potassium channels causing membrane depolarisation calcium influx and neurosecretion. The TASK channels of Type-I cells are also sensitive to cytosolic MgATP. The existence of a metabolic signalling pathway in Type-1 cells is thus established; the contentious issue is whether this pathway is also used for acute oxygen sensing. The main criticism is that because cytochrome oxidase has a high affinity for oxygen (P50 ≈ 0.2 mmHg) mitochondrial metabolism should be insensitive to physiological hypoxia. This argument is however predicated on the assumption that chemoreceptor mitochondria are analogous to those of other tissues. We have however obtained new evidence to support the hypothesis that type-1 cell mitochondria are not like those of other cells in that they have an unusually low affinity for oxygen (Mills E, Jobsis FF, J Neurophysiol 35(4):405-428, 1972; Duchen MR, Biscoe TJ, J Physiol 450:13-31, 1992a). Our data confirm that mitochondrial membrane potential, NADH, electron transport and cytochrome oxidase activity in the Type-1 cell are all highly sensitive to hypoxia. These observations not only provide exceptionally strong support for the metabolic hypothesis but also reveal an unknown side of mitochondrial behaviour.

Microvascular oxygen partial pressure during hyperbaric oxygen in diabetic rat skeletal muscle

Hyperbaric oxygen (HBO) is a major therapeutic treatment for ischemic ulcerations that perforate skin and underlying muscle in diabetic patients. These lesions do not heal effectively, in part, because of the hypoxic microvascular O2 partial pressures (PmvO2 ) resulting from diabetes-induced cardiovascular dysfunction, which alters the dynamic balance between O2 delivery (Q̇o2) and utilization (V̇o2) rates. We tested the hypothesis that HBO in diabetic muscle would exacerbate the hyperoxic PmvO2 dynamics due, in part, to a reduction or slowing of the cardiovascular, sympathetic nervous, and respiratory system responses to acute HBO exposure. Adult male Wistar rats were divided randomly into diabetic (DIA: streptozotocin ip) and healthy (control) groups. A small animal hyperbaric chamber was pressurized with oxygen (100% O2) to 3.0 atmospheres absolute (ATA) at 0.2 ATA/min. Phosphorescence quenching techniques were used to measure PmvO2 in tibialis anterior muscle of anesthetized rats during HBO. Lumbar sympathetic nerve activity (LSNA), heart rate (HR), and respiratory rate (RR) were measured electrophysiologically. During the normobaric hyperoxia and HBO, DIA tibialis anterior PmvO2 increased faster (mean response time, CONT 78 ± 8, DIA 55 ± 8 s, P < 0.05) than CONT. Subsequently, PmvO2 remained elevated at similar levels in CONT and DIA muscles until normobaric normoxic recovery where the DIA PmvO2 retained its hyperoxic level longer than CONT. Sympathetic nervous system and cardiac and respiratory responses to HBO were slower in DIA vs. CONT. Specifically the mean response times for RR (CONT: 6 ± 1 s, DIA: 29 ± 4 s, P < 0.05), HR (CONT: 16 ± 1 s, DIA: 45 ± 5 s, P < 0.05), and LSNA (CONT: 140 ± 16 s, DIA: 247 ± 34 s, P < 0.05) were greater following HBO onset in DIA than CONT. HBO treatment increases tibialis anterior muscle PmvO2 more rapidly and for a longer duration in DIA than CONT, but not to a greater level. Whereas respiratory, cardiovascular, and LSNA responses to HBO are profoundly slowed in DIA, only the cardiovascular arm (via HR) may contribute to the muscle vascular incompetence and these faster PmvO2 kinetics.

In vivo Ca2+ buffering capacity and microvascular oxygen pressures following muscle contractions in diabetic rat skeletal muscles: fiber-type specific effects

In Type 1 diabetes, skeletal muscle resting intracellular Ca2+ concentration ([Ca2+]i) homeostasis is impaired following muscle contractions. It is unclear to what degree this behavior is contingent upon fiber type and muscle oxygenation conditions. We tested the hypotheses that: 1) the rise in resting [Ca2+]i evident in diabetic rat slow-twitch (type I) muscle would be exacerbated in fast-twitch (type II) muscle following contraction; and 2) these elevated [Ca2+]i levels would relate to derangement of microvascular partial pressure of oxygen (PmvO2) rather than sarcoplasmic reticulum dysfunction per se. Adult male Wistar rats were divided randomly into diabetic (DIA: streptozotocin ip) and healthy (CONT) groups. Four weeks later extensor digitorum longus (EDL, predominately type II fibers) and soleus (SOL, predominately type I fibers) muscle contractions were elicited by continuous electrical stimulation (120 s, 100 Hz). Ca2+ imaging was achieved using fura 2-AM in vivo (i.e., circulation intact). DIA increased fatigability in EDL ( P < 0.05) but not SOL. In recovery, SOL [Ca2+]i either returned to its resting baseline within 150 s (CONT 1.00 ± 0.02 at 600 s) or was not elevated in recovery at all (DIA 1.03 ± 0.02 at 600 s, P > 0.05). In recovery, EDL CONT [Ca2+]i also decreased to values not different from baseline (1.06 ± 0.01, P > 0.05) at 600 s. In marked contrast, EDL DIA [Ca2+]i remained elevated for the entire recovery period (i.e., 1.23 ± 0.03 at 600 s, P < 0.05). The inability of [Ca2+]i to return to baseline in EDL DIA was not associated with any reduction of SR Ca2+-ATPase (SERCA) 1 or SERCA2 protein levels (both increased 30–40%, P < 0.05). However, PmvO2 recovery kinetics were markedly slowed in EDL such that mean PmvO2 was substantially depressed (CONT 27.9 ± 2.0 vs. DIA 18.4 ± 2.0 Torr, P < 0.05), and this behavior was associated with the elevated [Ca2+]i. In contrast, this was not the case for SOL ( P > 0.05) in that neither [Ca2+]i nor PmvO2 were deranged in recovery with DIA. In conclusion, recovery of [Ca2+]i homeostasis is impaired in diabetic rat fast-twitch but not slow-twitch muscle in concert with reduced PmvO2 pressures.

The effects of PGC-1α on control of microvascular Po2 kinetics following onset of muscle contractions

During contractions, regulation of microvascular oxygen partial pressure (Pmvo2), which drives blood-myocyte O2 flux, is a function of skeletal muscle fiber type and oxidative capacity and can be altered by exercise training. The kinetics of Pmvo2 during contractions in predominantly fast-twitch muscles evinces a more rapid fall to far lower levels compared with slow-twitch counterparts. Peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) improves endurance performance, in part, due to mitochondrial biogenesis, a fiber-type switch to oxidative fibers, and angiogenesis in skeletal muscle. We tested the hypothesis that improvement of exercise capacity by genetic overexpression of PGC-1α would be associated with an altered Pmvo2 kinetics profile of the fast-twitch (white) gastrocnemius during contractions toward that seen in slow-twitch muscles (i.e., slowed response kinetics and elevated steady-state Pmvo2). Phosphorescence quenching techniques were used to measure Pmvo2 at rest and during separate bouts of twitch (1 Hz) and tetanic (100 Hz) contractions in gastrocnemius muscles of mice with overexpression of PGC-1α and wild-type littermates (WT) mice under isoflurane anesthesia. Muscles of PGC-1α mice exhibited less fatigue than WT ( P < 0.01). However, except for the Pmvo2 response immediately following onset of contractions, WT and PGC-1α mice demonstrated similar Pmvo2 kinetics. Specifically, the time delay of the Pmvo2 response was shortened in PGC-1α mice compared with WT (1 Hz: WT, 6.6 ± 2.4 s; PGC-1α, 2.9 ± 0.8 s; 100 Hz: WT, 3.3 ± 1.1 s, PGC-1α, 0.9 ± 0.3 s, both P < 0.05). The ratio of muscle force to Pmvo2 was higher for the duration of tetanic contractions in PGC-1α mice. Slower dynamics and maintenance of higher Pmvo2 following muscle contractions is not obligatory for improved fatigue resistance in fast-twitch muscle of PGC-1α mice. Moreover, overexpression of PGC-1α may accelerate O2 utilization kinetics to a greater extent than O2 delivery kinetics.

Measuring interstitial pH and pO2 in mouse tumors

This protocol outlines methods to measure two extravascular parameters, interstitial pH and partial pressure of oxygen (pO2), in mouse tumors. The method for measuring interstitial pH uses fluorescence ratio imaging microscopy (FRIM) of the pH-sensitive fluorescent dye 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF). The method for measuring interstitial pO2 is based on the oxygen-dependent quenching of the phosphorescence of albumin-bound palladium meso-tetra(4-carboxyphenyl)porphyrin, and can be used to measure microvascular as well as interstitial pO2. In addition, the two methods can be used sequentially to measure both pH and pO2 in the same tissues.

Oxygen sensitivity of mitochondrial function in rat arterial chemoreceptor cells

The mechanism of oxygen sensing in arterial chemoreceptors is unknown but has often been linked to mitochondrial function. A common criticism of this hypothesis is that mitochondrial function is insensitive to physiological levels of hypoxia. Here we investigate the effects of hypoxia (down to 0.5% O2) on mitochondrial function in neonatal rat type-1 cells. The oxygen sensitivity of mitochondrial [NADH] was assessed by monitoring autofluorescence and increased in hypoxia with a P50 of 15 mm Hg (1 mm Hg = 133.3 Pa) in normal Tyrode or 46 mm Hg in Ca(2+)-free Tyrode. Hypoxia also depolarised mitochondrial membrane potential (m, measured using rhodamine 123) with a P50 of 3.1, 3.3 and 2.8 mm Hg in normal Tyrode, Ca(2+)-free Tyrode and Tyrode containing the Ca(2+) channel antagonist Ni(2+), respectively. In the presence of oligomycin and low carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP; 75 nm) m is maintained by electron transport working against an artificial proton leak. Under these conditions hypoxia depolarised m/inhibited electron transport with a P50 of 5.4 mm Hg. The effects of hypoxia upon cytochrome oxidase activity were investigated using rotenone, myxothiazol, antimycin A, oligomycin, ascorbate and the electron donor tetramethyl-p-phenylenediamine. Under these conditions m is maintained by complex IV activity alone. Hypoxia inhibited cytochrome oxidase activity (depolarised m) with a P50 of 2.6 mm Hg. In contrast hypoxia had little or no effect upon NADH (P50 = 0.3 mm Hg), electron transport or cytochrome oxidase activity in sympathetic neurons. In summary, type-1 cell mitochondria display extraordinary oxygen sensitivity commensurate with a role in oxygen sensing. The reasons for this highly unusual behaviour are as yet unexplained.

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.

Control of microvascular PO₂ kinetics following onset of muscle contractions: role for AMPK

The microvascular partial pressure of oxygen (Pmv(o(2))) kinetics following the onset of exercise reflects the relationship between muscle O(2) delivery and uptake (Vo(2)). Although AMP-activated protein kinase (AMPK) is known as a regulator of mitochondria and nitric oxide metabolism, it is unclear whether the dynamic balance of O(2) delivery and Vo(2) at exercise onset is dependent on AMPK activation level. We used transgenic mice with muscle-specific AMPK dominant-negative (AMPK-DN) to investigate a role for skeletal muscle AMPK on Pmv(o(2)) kinetics following onset of muscle contractions. Phosphorescence quenching techniques were used to measure Pmv(o(2)) at rest and across the transition to twitch (1 Hz) and tetanic (100 Hz, 3-5 V, 4-ms pulse duration, stimulus duration of 100 ms every 1 s for 1 min) contractions in gastrocnemius muscles (each group n = 6) of AMPK-DN mice and wild-type littermates (WT) under isoflurane anesthesia with 100% inspired O(2) to avoid hypoxemia. Baseline Pmv(o(2)) before contractions was not different between groups (P > 0.05). Both muscle contraction conditions exhibited a delay followed by an exponential decrease in Pmv(o(2)). However, compared with WT, AMPK-DN demonstrated 1) prolongation of the time delay before Pmv(o(2)) began to decline (1 Hz: WT, 3.2 ± 0.5 s; AMPK-DN, 6.5 ± 0.4 s; 100 Hz: WT, 4.4 ± 1.0 s; AMPK-DN, 6.5 ± 1.4 s; P < 0.05), 2) a faster response time (i.e., time constant; 1 Hz: WT, 19.4 ± 3.9 s; AMPK-DN, 12.4 ± 2.6 s; 100 Hz: WT, 15.1 ± 2.2 s; AMPK-DN, 9.0 ± 1.7 s; P < 0.05). These findings are consistent with the presence of substantial mitochondrial and microvascular dysfunction in AMPK-DN mice, which likely slows O(2) consumption kinetics (i.e., oxidative phosphorylation response) and impairs the hyperemic response at the onset of contractions thereby sowing the seeds for exercise intolerance.

Serotonin nerve terminals in the dorsomedial medulla facilitate sympathetic and ventilatory responses to hemorrhage and peripheral chemoreflex activation

Serotonin neurons of the caudal raphe facilitate ventilatory and sympathetic responses that develop following blood loss in conscious rats. Here, we tested whether serotonin projections to the caudal portion of the dorsomedial brain stem (including regions of the nucleus tractus solitarius that receive cardiovascular and chemosensory afferents) contribute to cardiorespiratory compensation following hemorrhage. Injections of the serotonin neurotoxin 5,7-dihydroxytryptamine produced >90% depletion of serotonin nerve terminals in the region of injection. Withdrawal of ∼21% of blood volume over 10 min produced a characteristic three-phase response that included 1) a normotensive compensatory phase, 2) rapid sympathetic withdrawal and hypotension, and 3) rapid blood pressure recovery accompanied by slower recovery of heart rate and sympathetic activity. A gradual tachypnea developed throughout hemorrhage, which quickly reversed with the advent of sympathetic withdrawal. Subsequently, breathing frequency and neural minute volume (determined by diaphragmatic electromyography) declined below baseline following termination of hemorrhage but gradually recovered over time. Lesioned rats showed attenuated sympathetic and ventilatory responses during early compensation and later recovery from hemorrhage. Both ventilatory and sympathetic responses to chemoreceptor activation with potassium cyanide injection were attenuated by the lesion. In contrast, the gain of sympathetic and heart rate baroreflex responses was greater, and low-frequency oscillations in blood pressure were reduced after lesion. Together, the data are consistent with the view that serotonin innervation of the caudal dorsomedial brain stem contributes to sympathetic compensation during hypovolemia, possibly through facilitation of peripheral chemoreflex responses.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

AMP-activated protein kinase and chemotransduction in the carotid body

AMP-activated protein kinase (AMPK) is a key component of a kinase cascade that regulates energy balance at the cellular level. Our recent research has raised the possibility that AMPK may also function to couple hypoxic inhibition of mitochondrial oxidative phosphorylation to O(2)-sensitive K(+) channel inhibition and hence underpin carotid body type I cell excitation. Thus, in addition to maintaining the cellular energy state AMPK may act as the primary metabolic sensor and effector of hypoxic chemotransduction in type I cells. These findings provide a unifying link between two previously separate theories pertaining to O(2)-sensing in the carotid body, namely the 'membrane hypothesis' and the 'mitochondrial hypothesis'. Furthermore, our data suggest that in addition to its effects at the cellular level the AMPK signalling cascade can mediate vital physiological mechanisms essential for meeting the metabolic needs of the whole organism.

TASK-like potassium channels and oxygen sensing in the carotid body

Chemosensing by type-1 cells of the carotid body involves a series of events which culminate in the calcium-dependent secretion of neurotransmitter substances which then excite afferent nerves. This response is mediated via membrane depolarisation and voltage-gated calcium entry. Studies utilising isolated cells indicates that the membrane depolarisation in response to hypoxia, and acidosis, appears to be primarily mediated via the inhibition of a background K(+)-current. The pharmacological and biophysical characteristics of these channels suggest that they are probably closely related to the TASK subfamily of tandem-P-domain K(+)-channels. Indeed they show greatest similarity to TASK-1 and -3. In addition to being sensitive to hypoxia and acidosis, the background K(+)-channels of the type-1 cell are also remarkably sensitive to inhibition of mitochondrial energy metabolism. Metabolic poisons are known potent stimulants of the carotid body and cause membrane depolarisation of type-1 cells. In the presence of metabolic inhibitors hypoxic sensitivity is lost suggesting that oxygen sensing may itself be mediated via depression of mitochondrial energy production. Thus these TASK-like background channels play a central role in mediating the chemotransduction of several different stimuli within the type-1 cell. The mechanisms by which metabolic/oxygen sensitivity might be conferred upon these channels are briefly discussed.

Purines, the carotid body and respiration

The carotid body is essential to detecting levels of oxygen in the blood and initiating the compensatory response. Increasing evidence suggests that the purines ATP and adenosine make a key contribution to this signaling by the carotid body. The glomus cells release ATP in response to hypoxia. This released ATP can stimulate P2X receptors on the carotid body to elevate intracellular Ca(2+) and to produce an excitatory response. This released ATP can be dephosphorylated to adenosine by a series of extracellular enzymes, which in turn can stimulate A(1), A(2A) and A(2B) adenosine receptors. Levels of extracellular adenosine can also be altered by membrane transporters. Endogenous adenosine stimulates these receptors to increase the ventilation rate and may modulate the catecholamine release from the carotid sinus nerve. Prolonged hypoxic challenge can alter the expression of purinergic receptors, suggesting a role in the adaptation. This review discusses evidence for a key role of ATP and adenosine in the hypoxic response of the carotid body, and emphasizes areas of new contributions likely to be important in the future.

Effects of altered nitric oxide availability on rat muscle microvascular oxygenation during contractions

AIM

To explore the role of nitric oxide (NO) in controlling microvascular O2 pressure (P(O2)mv) at rest and during contractions (1 Hz). We hypothesized that at the onset of contractions sodium nitroprusside (SNP) would raise P(O2)mv and slow the kinetics of P(O2)mv change whereas l-nitro arginine methyl ester (L-NAME) would decrease P(O2)mv and speed its kinetics.

METHODS

We superfused the spinotrapezius muscle of female Sprague-Dawley rats (n = 7, body mass = 298 +/- 10 g) with SNP (300 microM) and L-NAME (1.5 mm) and measured P(O2)mv (phosphorescence quenching) during contractions.

RESULTS

SNP decreased mean arterial pressure (92 +/- 5 mmHg) below that of control (CON, 124 +/- 4 mmHg) and L-NAME (120 +/- 4 mmHg) conditions. SNP did not raise P(O2)mv at rest but it did elevate the P(O2)mv-to-MAP ratio (50% increase, P < 0.05) and slow the kinetics by lengthening the time-delay (TD, 14.0 +/- 5.0 s) and time constant (tau, 24.0 +/- 10.0 s) of the response compared with CON (TD, 8.4 +/- 3.3 s; tau, 16.0 +/- 4.5 s, P < 0.05 vs. SNP). L-NAME decreased P(O2)mv at rest and tended to speed tau (10.1 +/- 3.8 s, P = 0.1), while TD (8.1 +/- 1.0 s) was not significantly different. L-NAME also caused P(O2)mv to fall transiently below steady-state contracting values.

CONCLUSIONS

These results indicate that NO availability can significantly affect P(O2)mv at rest and during contractions and suggests that P(O2)mv derangements in ageing and chronic disease conditions may potentially result from impairments in NO availability.

Effects of eccentric exercise on microcirculation and microvascular oxygen pressures in rat spinotrapezius muscle

A single bout of eccentric exercise results in muscle damage, but it is not known whether this is correlated with microcirculatory dysfunction. We tested the following hypotheses in the spinotrapezius muscle of rats either 1 (DH-1; n = 6) or 3 (DH-3; n = 6) days after a downhill run to exhaustion (90-120 min; -14 degrees grade): 1) in resting muscle, capillary hemodynamics would be impaired, and 2) at the onset of subsequent acute concentric contractions, the decrease of microvascular O(2) pressure (Pmv(o(2))), which reflects the dynamic balance between O(2) delivery and O(2) utilization, would be accelerated compared with control (Con, n = 6) rats. In contrast to Con muscles, intravital microscopy observations revealed the presence of sarcomere disruptions in DH-1 and DH-3 and increased capillary diameter in DH-3 (Con: 5.2 +/- 0.1; DH-1: 5.1 +/- 0.1; DH-3: 5.6 +/- 0.1 mum; both P < 0.05 vs. DH-3). At rest, there was a significant reduction in the percentage of capillaries that sustained continuous red blood cell (RBC) flux in both DH running groups (Con: 90.0 +/- 2.1; DH-1: 66.4 +/- 5.2; DH-3: 72.9 +/- 4.1%, both P < 0.05 vs. Con). Capillary tube hematocrit was elevated in DH-1 but reduced in DH-3 (Con: 22 +/- 2; DH-1: 28 +/- 1; DH-3: 16 +/- 1%; all P < 0.05). Although capillary RBC flux did not differ between groups (P > 0.05), RBC velocity was lower in DH-1 compared with Con (Con: 324 +/- 43; DH-1: 212 +/- 30; DH-3: 266 +/- 45 mum/s; P < 0.05 DH-1 vs. Con). Baseline Pmv(O(2)) before contractions was not different between groups (P > 0.05), but the time constant of the exponential fall to contracting Pmv(O(2)) values was accelerated in the DH running groups (Con: 14.7 +/- 1.4; DH-1: 8.9 +/- 1.4; DH-3: 8.7 +/- 1.4 s, both P < 0.05 vs. Con). These findings are consistent with the presence of substantial microvascular dysfunction after downhill eccentric running, which slows the exercise hyperemic response at the onset of contractions and reduces the Pmv(O(2)) available to drive blood-muscle O(2) delivery.

Immediate and long-term responses of the carotid body to high altitude

High altitude and the decreased environmental oxygen pressure have both immediate and chronic effects on the carotid body. An immediate effect is to limit the oxygen available for mitochondrial oxidative phosphorylation, and this leads to increased activity on the afferent nerves leading to the brain. In the isolated carotid body preparation, the afferent nerve activity depends on the ratio of carbon monoxide (CO), an inhibitor of respiratory chain function, to oxygen. The CO-induced increase in afferent neural activity is reversed by light, and the wavelength dependence of this reversal shows that the site of CO (and therefore oxygen) interaction is cytochrome a3 of the mitochondrial respiratory chain. Thus, primary sensing of ambient oxygen pressure is through the oxygen dependence of mitochondrial oxidative phosphorylation. The conductance of ion channels in the cellular membranes may also be sensitive to oxygen pressure and, through this, modulate the sensitivity to oxygen pressure. Longer-term exposure to high altitude results in progressive changes in the carotid body that involve several mechanisms, including cellular energy metabolism and hypoxia inducible factor-1alpha (HIF-1alpha). These changes begin within minutes of exposure, but progress such that chronic exposure results in morphological and biochemical alterations in the carotid body, including enlarged cells, increased catecholamine levels, altered cellular appearance, and others. In the chronically adapted carotid body, responses to acute changes in oxygen pressure are enhanced. The adaptive changes due to chronic hypoxia are largely reversed upon return to lower altitudes.

Red blood cell velocity and oxygen tension measurement in cerebral microvessels by double-wavelength photoexcitation

Because the regulation of microcirculation in the cerebral cortex cannot be analyzed without measuring the blood flow dynamics and oxygen concentration in cerebral microvessels, we developed a fluorescence and phosphorescence system for estimating red blood cell velocity and oxygen tension in cerebral microcirculation noninvasively and continuously with high spatial resolution. Using red blood cells labeled with fluorescent isothiocyanate to visualize red cell distribution and using the oxygen quenching of Pd-meso-tetra-(4-carboxyphenyl)-porphyrin phosphorescence to measure oxygen tension enabled simultaneous measurement of blood velocity and oxygen tension. We examined how the measurement accuracy was affected by the spatial resolution and by the excitation laser light passing through the targeted microvessel and exciting the oxygen probe dye in the tissue beneath it. Focusing the excitation light into the microvessel stabilized the phosphorescence lifetime at each spatial resolution; moreover, it greatly reduced phosphorescence from the brain tissue. Animal experiments involving acute hemorrhagic shock demonstrated the feasibility of our system by showing that the changes in venular velocity and oxygen tension are synchronized to the change in mean arterial pressure. Our system measures the red cell velocity and oxygen concentration in the cerebral microcirculation by using the differences in luminescence and wavelength between fluorescence and phosphorescence, making it possible to easily acquire information about cerebral microcirculatory distribution and oxygen tension simultaneously.

The effect of mitochondrial inhibitors on membrane currents in isolated neonatal rat carotid body type I cells

Inhibitors of mitochondrial energy metabolism have long been known to be potent stimulants of the carotid body, yet their mechanism of action remains obscure. We have therefore investigated the effects of rotenone, myxothiazol, antimycin A, cyanide (CN(-)) and oligomycin on isolated carotid body type I cells. All five compounds caused a rapid rise in intracellular Ca(2+), which was inhibited on removal of extracellular Ca(2+). Under current clamp conditions rotenone and CN(-) caused a rapid membrane depolarization and elevation of [Ca(2+)](i). Voltage clamping cells to -70 mV substantially attenuated this rise in [Ca(2+)](i). Rotenone, cyanide, myxothiazol and oligomycin significantly inhibited resting background K(+) currents. Thus rotenone, myxothiazol, cyanide and oligomycin mimic the effects of hypoxia in that they all inhibit background K(+) current leading to membrane depolarization and voltage-gated calcium entry. Hypoxia, however, failed to have any additional effect upon membrane currents in the presence of CN(-) or rotenone or the mitochondrial uncoupler p-trifluoromethoxyphenyl hydrazone (FCCP). Thus not only do mitochondrial inhibitors mimic the effects of hypoxia, but they also abolish oxygen sensitivity. These observations suggest that there is a close link between oxygen sensing and mitochondrial function in type I cells. Mechanisms that could account for this link and the actions of mitochondrial inhibitors are discussed.

Inhibitory effects of NO on carotid body: contribution of neural and endothelial nitric oxide synthase isoforms

We tested the hypothesis that nitric oxide (NO) produced within the carotid body is a tonic inhibitor of chemoreception and determined the contribution of neuronal and endothelial nitric oxide synthase (eNOS) isoforms to the inhibitory NO effect. Accordingly, we studied the effect of NO generated from S-nitroso-N-acetylpenicillamide (SNAP) and compared the effects of the nonselective inhibitor N(omega)-nitro-l-arginine methyl ester (l-NAME) and the selective nNOS inhibitor 1-(2-trifluoromethylphenyl)-imidazole (TRIM) on chemosensory dose-response curves induced by nicotine and NaCN and responses to hypoxia (Po(2) approximately 30 Torr). CBs excised from pentobarbitone-anesthetized cats were perfused in vitro with Tyrode at 38 degrees C and pH 7.40, and chemosensory discharges were recorded from the carotid sinus nerve. SNAP (100 microM) reduced the responses to nicotine and NaCN. l-NAME (1 mM) enhanced the responses to nicotine and NaCN by increasing their duration, but TRIM (100 microM) only enhanced the responses to high doses of NaCN. The amplitude of the response to hypoxia was enhanced by l-NAME but not by TRIM. Our results suggest that both isoforms contribute to the NO action, but eNOS being the main source for NO in the cat CB and exerting a tonic effect upon chemoreceptor activity.

Effects of prior contractions on muscle microvascular oxygen pressure at onset of subsequent contractions

In humans, pulmonary oxygen uptake (.V(O2)) kinetics may be speeded by prior exercise in the heavy domain. This "speeding" arises potentially as the result of an increased muscle O(2) delivery (.Q(O2)) and/or a more rapid elevation of oxidative phosphorylation. We adapted phosphorescence quenching techniques to determine the.Q(O2)-to-O(2) utilization (.Q(O2)/.V(O2)) characteristics via microvascular O(2) pressure (P(O2,m)) measurements across sequential bouts of contractions in rat spinotrapezius muscle. Spinotrapezius muscles from female Sprague-Dawley rats (n = 6) were electrically stimulated (1 Hz twitch, 3-5 V) for two 3 min bouts (ST(1) and ST(2)) separated by 10 min rest. P(O2,m) responses were analysed using an exponential + time delay (TD) model. There was no significant difference in baseline and DeltaP(O2,m) between ST(1) and ST(2) (28.5 +/- 2.6 vs. 27.9 +/- 2.4 mmHg, and 13.9 +/- 1.8 vs. 14.1 +/- 1.3 mmHg, respectively). The TD was reduced significantly in the second contraction bout (ST(1), 12.2 +/- 1.9; ST(2), 5.7 +/- 2.2 s, P < 0.05), whereas the time constant of the exponential P(O2,m) decrease was unchanged (ST(1), 16.3 +/- 2.6; ST(2), 17.6 +/- 2.7 s, P > 0.1). The shortened TD found in ST(2) led to a reduced time to reach 63 % of the final response of ST(2) compared to ST(1) (ST(1), 28.3 +/- 3.0; ST(2), 20.2 +/- 1.8 s, P < 0.05). The speeding of the overall response in the absence of an elevated P(O2,m) baseline (which had it occurred would indicate an elevated.Q(O2)/.V(O2) or muscle blood flow suggests that some intracellular process(es) (e.g. more rapid increase in oxidative phosphorylation) may be responsible for the increased speed of P(O2,m) kinetics after prior contractions under these conditions.

Altered structure and function of the carotid body at high altitude and associated chemoreflexes

The ventilatory response to hypoxia is complex. First contact with hypoxia causes an increase in ventilation within seconds that reaches full intensity within minutes because of an increase in carotid sinus nerve (CSN) input to the brain stem. With continued exposure, ventilation increases further over days (ventilatory acclimatization). Initially, it was hypothesized that ventilatory acclimatization arose from a central nervous system (CNS) mechanism. Compensation for alkalosis in the brain and restoration of pH in the vicinity of central chemoreceptors was believed to cause the secondary increase in ventilation. However, when this hypothesis could not be substantiated, attention was turned to the peripheral chemoreceptors. With the lowering of arterial PO2 at high altitude, there is an immediate increase in firing of afferents from chemoreceptors in the carotid body. After peaking over the next few minutes, the firing rate of afferents begins to rise again within hours until a steady state is reached. This secondary increase occurs along with increase in neurotransmitter synthesis and release and altered gene expression followed by hypertrophy of carotid body glomus cells. Further exposure to hypoxia eventually leads to blunting of the CSN output and ventilatory response in some species. This mini review is about the altered structure and function of the carotid body at high altitude and the associated blunting of the chemoreceptor and ventilatory responses observed in some species.

P(O(2))-P(CO(2)) stimulus interaction in [Ca(2+)](i) and CSN activity in the adult rat carotid body

Since glomus cell intracellular calcium ([Ca(2+)](i)) plays a key role in generating carotid sinus nerve (CSN) discharge, we hypothesized that glomus cell [Ca(2+)](i) would correspond to CSN discharge rates during P(O(2))-P(CO(2)) stimulus interaction in adult rat carotid body (CB). Accordingly, we measured steady state P(O(2))-P(CO(2)) interaction in CSN discharge rates during hypocapnia (P(CO(2))=8-10 Torr), normocapnia (P(CO(2))=33-35 Torr) and hypercapnia (P(CO(2))=68-70 Torr) in normoxia (P(O(2)) approximately 130 Torr) and hypoxia (P(O(2)) approximately 36 Torr). The results showed P(O(2))-P(CO(2)) stimulus interaction in CSN responses. [Ca(2+)](i) levels were measured in isolated type I cells (2-3 cells/field), using Ca(2+) sensitive fluoroprobe indo-1AM. The [Ca(2+)](i) responses increased with increasing P(CO(2)) in normoxia. In hypoxia, [Ca(2+)](i) did not increase during hypocapnia but increased during normocapnia, showing P(O(2))-P(CO(2)) interaction. However, CSN response during hypoxia was far greater than that for [Ca(2+)](i) response, particularly during hypocapnic hypoxia. Thus, the [Ca(2+)](i) interaction cannot account for the whole CSN interaction. The origin of this CSN P(O(2)-)P(CO(2)) interaction must have occurred in part beyond cellular [Ca(2+)](i) interaction. Interactions at both sites (glomus cell membrane and sinus nerve endings) are reminiscent of reversible O(2)-heme protein reaction with a Bohr effect.

Interactions between hypoxia and hypercapnic acidosis on calcium signaling in carotid body type I cells

The effects of hypercapnic acidosis and hypoxia on intracellular Ca(2+) concentration ([Ca(2+)](i)) were determined with Indo 1 in enzymatically isolated single type I cells from neonatal rat carotid bodies. Type I cells responded to graded hypoxic stimuli with graded [Ca(2+)](i) rises. The percentage of cells responding was also dependent on the severity of the hypoxic stimulus. Raising CO(2) from 5 to 10 or 20% elicited a significant increase in [Ca(2+)](i) in the same cells as those that responded to hypoxia. Thus both stimuli can be sensed by each individual cell. When combinations of hypoxic and acidic stimuli were given simultaneously, the responses were invariably greater than the response to either stimulus given alone. Indeed, in most cases, the response to hypercapnia was slightly potentiated by hypoxia. These data provide the first evidence that the classic synergy between hypoxic and hypercapnic stimuli observed in the intact carotid body may, in part, be an inherent property of the type I cell.

Suppression of glomus cell K+ conductance by 4-aminopyridine is not related to [Ca2+]i, dopamine release and chemosensory discharge from carotid body

The hypothesis that suppression of O2-sensitive K+ current is the initial event in hypoxic chemotransduction in the carotid body glomus cells was tested by using 4-aminopyridine (4-AP), a known suppressant of K+ current, on intracellular [Ca2+]i, dopamine secretion and chemosensory discharge in cat carotid body (CB). In vitro experiments were performed with superfused-perfused cat CBs, measuring chemosensory discharge, monitoring dopamine release by microsensors without and with 4-AP (0.2, 1.0 and 2.0 mM in CO2-HCO3- buffer) and recording [Ca2+]i by ratio fluorometry in isolated cat and rat glomus cells. 4-AP decreased the chemosensory activities in normoxia but remained the same in hypoxia and in flow interruption. It decreased the tissue dopamine release in normoxia, and showed an additional inhibition with hypoxia. Also, 4-AP did not evoke any rise in [Ca2+]i in glomus cells either during normoxia and hypoxia, although hypoxia stimulated it. Thus, the lack of stimulatory effect on chemosensory discharge, inhibition of dopamine release and unaltered [Ca2+]i by 4-AP are not consistent with the implied meaning of the suppressant effect on K+ current of glomus cells.

Ventilatory effects of specific carotid body hypocapnia and hypoxia in awake dogs

Specific carotid body (CB) hypocapnia in the-10-Torr (less than eupneic) range reduced ventilation in the awake and sleeping dog to the same degree as did CB hyperoxia [CB PO2 (PCBO2); > 500 Torr; C.A. Smith, K.W. Saupe, K. S. Henderson, and J. A. Dempsey. J. Appl. Physiol. 79:689-699, 1995], suggesting a powerful inhibitory effect of hypocapnia at the carotid chemosensor over a range of PCO2 encountered commonly in physiological hyperpneas. The primary purpose of this study was to assess the ventilatory effect of CB hypocapnia on the ventilatory response to concomitant CB hypoxia. The secondary purpose was to assess the relative gains of the CB and central chemoreceptors to hypocapnia. In eight awake female dogs the vascularly isolated CB was perfused with hypoxic blood (mild, PCBO2 approximately equal to 50 Torr or severe, PCBO2 approximately equal to 36 Torr) in a background of normocapnia or hypocapnia (10 Torr less than eupneic arterial PCO2) in the perfusate. The systemic (and brain) circulation was normoxic throughout, and arterial PCO2 was not controlled (poikilocapnia). With CB hypocapnia, the peak ventilation (range 19-27 s) in response to hypoxic CB perfusion increased 48% (mild) and 77% (severe) due to increased tidal volume. When CB hypocapnia was present, these increases in ventilation were reduced to 21 and 27%, respectively. With systemic hypocapnia, with the isolated CB maintained normocapnic and hypoxic for > 70 s, the steady-state poikilocapnic ventilatory response (i.e., to systemic hypocapnia alone) decreased 15% (mild CB hypoxia) and 27% (severe CB hypoxia) from the peak response, respectively. We conclude that carotid body hypocapnia can be a major source of inhibitory feedback to respiratory motor output during the hyperventilatory response to hypoxic carotid body stimulation.

A novel oxygen-sensitive potassium current in rat carotid body type I cells

1. Hypoxic stimuli depolarize carotid body type I cells causing voltage-gated calcium influx. This study investigates the cause of this membrane depolarization. Isolated type I cells from neonatal (11-16 day) rat carotid bodies were used in the experiments. 2. Tetraethylammonium (TEA; 10 mM), 1 and 5 mM 4-aminopyridine (4-AP) and 20 nM charybdotoxin all failed to evoke a significant rise in [Ca2+]i. Similarly, in perforated patch whole-cell recordings, a combination of 10 mM TEA and 5 mM 4-AP failed to depolarize type I cells. 3. In type I cells voltage clamped at -70 mV, anoxia evoked a small inward current under control conditions, but had no effect in the absence of pipette and extracellular K+. 4. Anoxia decreased resting membrane conductance from 322 to 131 pS. The anoxia-sensitive current (measured using voltage ramps from -100 to -40 mV) had a reversal potential of -89 mV in 4.5 mM Ko+ and -66 mV in 20 mM Ko+, indicating that this current was carried principally by potassium ions. In contrast, 10 mM TEA + 5 mM 4-AP had little effect on the current-voltage relationship of the cells over the same range. 5. This O2-sensitive K+ conductance showed only mild outward rectification over the range -90 to +30 mV, which could be approximated by the Goldman-Hodgkin-Katz current equation. In addition, there was no time-dependent activation or inactivation of membrane currents elicited by voltage steps in the range -100 to -30 mV. 6. The O2-sensitive K+ conductance was inhibited by graded reductions in PO2 to 40 Torr and below, with a K1/2 of about 12 Torr. 7. The data suggest that hypoxia depolarizes type I cells principally through the inhibition of a small voltage-insensitive resting (or background) K+ conductance, and not through the inhibition of voltage-gated TEA and 4-AP-sensitive K+ channels (e.g. maxi-K or KO2 channels), as has been previously suggested.

Oxygen sensing by ion channels and chemotransduction in single glomus cells

We have monitored cytosolic [Ca2+] and dopamine release in intact fura-2-loaded glomus cells with microfluoroimetry and a polarized carbon fiber electrode. Exposure to low PO2 produced a rise of cytosolic [Ca2+] with two distinguishable phases: an initial period (with PO2 values between 150 and approximately 70 mm Hg) during which the increase of [Ca2+] is very small and never exceeds 150-200 nM, and a second phase (with PO2 below approximately 70 mm Hg) characterized by a sharp rise of cytosolic [Ca2+]. Secretion occurs once cytosolic [Ca2+] reaches a threshold value of 180 +/- 43 nM. The results demonstrate a characteristic relationship between PO2 and transmitter secretion at the cellular level that is comparable with the relation described for the input (O2 tension)output (afferent neural discharges) variables in the carotid body. Thus, the properties of single glomus cells can explain the sensory functions of the entire organ. In whole-cell, patch-clamped cells, we have found that in addition to O2-sensitive K+ channels, there are Ca2+ channels whose activity is also regulated by PO2. Ca2+ channel activity is inhibited by hpoxia, although in a strongly voltage-dependent manner. The average hypoxic inhibition of the calcium current in 30% +/- 10% at -20 mV but only 2% +/- 2% at +30 mV. The differential inhibition of K+ and Ca2+ channels by hypoxia helps to explain why the secretory response of the cells is displaced toward PO2 values (below approximately 70 mm Hg) within the range of those normally existing in arterial blood. These data provide a conceptual framework for understanding the cellular mechanisms of O2 chemotransduction in the carotid body.

Reciprocal photolabile O2 consumption and chemoreceptor excitation by carbon monoxide in the cat carotid body: evidence for cytochrome a3 as the primary O2 sensor

High carbon monoxide (CO) gas tensions (> 500 Torr) at normoxic PO2 (125-140 Torr) stimulates carotid chemosensory discharge in the perfused carotid body (CB) in the absence but not in the presence of light. According to a metabolic hypothesis of O2 chemoreception, the increased chemosensory discharge should correspond to a photoreversible decrease of O2 consumption, unlike a non-respiratory hypothesis. We tested the respiratory vs. non-respiratory hypotheses of O2 chemoreception in the cat CB by measuring the effect of high CO. Experiments were conducted using CBs perfused and superfused in vitro with high CO in normoxic, normocapnic cell-free CO2-HCO3- buffer solution at 37 degrees C. Simultaneous measurements of the rate of O2 disappearance with recessed PO2 microelectrodes and chemosensory discharge were made after flow interruption with and without CO in the perfusate. The control O2 disappearance rate without CO was -3.66 +/- 0.43 (S.E.) Torr/s (100 measurements in 12 cat CBs). In the dark, high CO reduced the O2 disappearance rate to -2.35 +/- 0.33 Torr/s, or 64.2 +/- 9.0% of control (P < 0.005, 34 measurements). High CO was excitatory in the dark, with an increase in baseline neural discharge from 129.2 +/- 47.0 to 399.3 +/- 49.1 impulses per s (P < 0.0001), and maximum discharge rate of 659 +/- 76 impulses/s (N.S. compared to control) during flow interruption. During perfusion with high CO in the light, there were no significant differences in baseline neural discharge or in the maximum neural discharge after flow interruption, and little effect on O2 metabolism (88.8 +/- 11.5% of control, N.S., 29 measurements).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of chronic hypoxaemia from birth upon chemosensitivity in the adult rat carotid body in vitro

1. The effect of chronic hypoxaemia upon in vitro carotid body chemosensitivity was observed in eight rats > 5 weeks of age born and reared in 12% oxygen. Comparisons were made with eight age-matched normoxic rats. 2. Single exponential functions with offset were fitted to the normalized (percentage of maximum) discharge responses to ramp decreases in PO2 at three steady levels of PCO2. CO2 sensitivity was derived from these functions. 3. Increasing hypercapnia increased the horizontal asymptote of the exponential functions in the normoxic (0.15 +/- 0.03% discharge per mmHg PCO2; P < 0.001) and chronically hypoxic (0.13 +/- 0.04% discharge per mmHg PCO2; P < 0.005) animals but was without effect upon the rate constants in both groups (-0.04 +/- 0.18 mmHg PO2 per mmHg PCO2, P > 0.50 and 0.63 +/- 0.48 mmHg PO2 per mmHg PCO2, P > 0.20, respectively). Rate constants were greater in the chronically hypoxic animals (P < 0.05) compared with the normoxic animals. 4. CO2 chemosensitivity increased with decreasing PO2 in normoxic (P < 0.05) but not in chronically hypoxic (P > 0.50) rats. 5. Our results show that chronic hypoxaemia from birth attenuates the maturation of CO2-O2 interaction at the carotid body.

The primary oxygen sensor of the cat carotid body is cytochrome a3 of the mitochondrial respiratory chain

Carbon monoxide was shown to be competitive with O2 in oxygen sensing by perfused carotid bodies isolated from cats, afferent electrical activity increasing with either decreasing O2 or increasing CO. The CO-induced increase in afferent activity was fully reversed by bright light. At submaximal light intensities the extent of reversal, after correcting to equal light intensity of light quanta at each wavelength, was maximal for light of 432 +/- 2 and 590 +/- 2 nm, with a ratio (432/590) of approximately 6. This spectrum is characteristic of the CO compound of mitochondrial cytochrome a3. The photo-reversible inhibition of oxygen sensing activity by CO accounts for at least 80% of the oxygen chemosensory activity of the carotid body.

CO reveals dual mechanisms of O2 chemoreception in the cat carotid body

The hypothesis that CO-binding pigments in the carotid body participate in O2 chemoreception was tested. The chemosensory nerve discharges of cat carotid body perfused and superfused in vitro at 36-37 degrees C with cell-free solution containing CO2-HCO3- (pH approximately equal to 7.39) were recorded to monitor O2 chemoreception. Several levels of PCO (60-550 Torr) at two levels of PO2 (50 Torr-140 Torr) were used. With high PCO of 500-550 Torr at any PO2 the discharge rate peaked promptly but the effect was significantly less than that to hypoxia. At any stage of the CO effect, exposure to light promptly attenuated or eliminated the response, as if the stimulatory effect of hypoxia was absent. Lower PCO of 60-70 Torr attenuated the response to hypoxia which was not suppressed by light. PCO of 140 Torr also attenuated the response to hypoxia and made the activity partially photolabile. During high PCO exposure the excitatory response to cyanide but not to nicotine was attenuated, consistent with the idea that the effects of nicotine are downstream from those of CO. Both inhibitory and excitatory effects of CO were promptly reversible. The results indicate that two types of CO-binding chromophores participate in O2 chemoreception in the carotid body.