Aortic body chemoreceptor responses to changes in PCO2 and PO2 in the cat

Effects of chemostimuli on [Ca2+]i responses of rat aortic body type I cells and endogenous local neurons: comparison with carotid body cells

Mammalian aortic bodies (ABs) are putative peripheral arterial chemoreceptors whose function remains controversial, partly because information on their cellular physiology is lacking. In this study, we used ratiometric Ca2+ imaging to investigate for the first time chemosensitivity in short-term cultures of dissociated cells of juvenile rat ABs, located near the junction of the left vagus and recurrent laryngeal nerves. Among the surviving cell population were glomus or type I cell clusters, endogenous local neurons and glia-like cells. A variety of chemostimuli, including hypoxia, isohydric or acidic hypercapnia, and isocapnic acidosis, caused a rise in intracellular [Ca2+] in AB type I cells. The [Ca2+]i responses were indistinguishable from those in carotid body (CB) type I cells grown in parallel cultures from the same animals, and responses to acidic hypercapnia were prevented by the non-specific voltage-gated Ca2+ channel antagonist, 2mM Ni2+. Furthermore, we identified a subpopulation (∼40%) of glia-like cells in AB cultures that resembled CB type II cells based on their approximately equal sensitivity to ATP and UTP, consistent with the expression of purinergic P2Y2 receptors. Finally, we showed that some local neurons, known to be uniquely associated with these AB paraganglia in situ, generated robust [Ca2+]i responses to these chemostimuli. Thus, these AB type I cells and associated putative type II cells resemble those from the well-studied CB. Unlike the CB, however, they also associate with a special group of endogenous neurons which we propose may subserve a sensory function in local cardiovascular reflexes.

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.

Development of ventilatory response to transient hypercapnia and hypercapnic hypoxia in term infants

Whereas peripheral chemoreceptor oxygen sensitivity increases markedly after birth, previous studies of ventilatory responses to CO(2) in term infants have shown no postnatal development. However, the hypercapnic challenges applied have usually been long-term, which meant that the effect of central chemoreceptors dominated. Oscillatory breathing, apneas, and sighs cause transient Pco(2) changes, probably primarily stimulating peripheral chemoreceptors. We wanted to assess whether the immediate ventilatory responses to step changes in inspired CO(2) and O(2) in term infants undergo postnatal developmental changes. Twenty-six healthy term infants were studied during natural sleep 2 d and 8 wk postnatally. Ventilatory responses to a randomized sequence of 15 s hypercapnia (3% CO(2)), hypoxia (15% O(2)), and hypercapnic hypoxia (3% CO(2) + 15% O(2)) were recorded breath-by-breath using a pneumotachometer. Response rate, stimulus-response time, and response magnitude were analyzed with ANOVA after coherent averaging. Response rate increased with age by 30% (hypercapnia), 318% (hypoxia), and 302% (hypercapnic hypoxia). Response rate during hypercapnic hypoxia exceeded rate during hypercapnia plus rate during hypoxia in wk 8, but not on d 2. Time to half-maximum response decreased by 3.4 s with age for the two hypercapnic stimuli but was unchanged for hypoxia. Response magnitude was unchanged for hypercapnia, but increased for the two hypoxic stimuli. In conclusion, an interaction between the effects of hypercapnia and hypoxia on ventilatory response rate emerged between postnatal d 2 and wk 8 in term infants. Concomitantly, stimulus-response time to hypercapnic stimuli declined markedly. The development of a prompt response to transient hypercapnia may be important for infant respiratory stability.

Aortic and carotid body chemoreception in prolonged hyperoxia in the cat

Carotid body chemosensory response to hypoxia is attenuated as a result of prolonged normobaric hyperoxia (NH) in the cat. The effect of NH is likely to be due to high cellular PO2 and O2-related free radicals. Accordingly, the effect would be less if O2 delivery to the chemoreceptor tissue could be compromised. The aortic bodies, which appear to have less of a circulatory O2 delivery, as suggested by their vigorous responses to a slight compromise of O2 flow compared with those of the carotid body, could provide a suitable testing material for the hypothesis. We tested the hypothesis by studying both aortic and carotid body chemoreceptors in the same cats (n = 6) which were exposed to nearly 100% O2 for about 60 h. These chemoreceptor organs were also studied in 6 control cats which were maintained in room air at sea-level. The cats were anesthetized and their carotid and aortic chemosensory fibers were identified by the usual procedure, and their responses to hypoxia and hypercapnia and to bolus injections (i.v.) of cyanide and nicotine were measured. In the NH cats, the carotid but not aortic chemosensory responses to hypoxia and cyanide were attenuated and to hypercapnia (both onset and steady state) augmented. The aortic chemoreceptors were stimulated by hypoxia, hypercapnia, cyanide and nicotine both in the NH and the control cats similarly. The results support the hypothesis that it is presumably a higher tissue blood flow and hence a higher concentration of O2-related free radicals which ultimately led to the specific attenuation of O2 chemoreception in the carotid body.

Thermal effects on ventilation in cats: participation of carotid body chemoreceptors

In pentobarbitone anesthetized cats, raising body temperature from 37 to 40 degrees C by external heat increased respiratory frequency, tidal volume, frequency of spontaneous gasps and mean inspiratory flow. It reduced end-tidal CO2 pressure, together with inspiratory and expiratory durations. After bilateral section of the carotid nerves, raising body temperature still induced hyperventilation, but the increase in gasp frequency was less pronounced and no significant change in tidal volume was observed. In comparison to steady ventilatory values in the intact condition, significant reductions in tidal volume at 38 degrees C and in gasp frequency at 37, 39 and 40 degrees C were observed after bilateral carotid neurotomy. Brief hyperoxic tests induced transient decreases in tidal volume and increases in end-tidal CO2 pressure which were significantly larger at 40 degrees C than at 37 degrees C. These changes disappeared after bilateral carotid neurotomy. Anesthetic block of both carotid nerves produced transient reductions in tidal volume at any given temperature. We conclude that carotid body afferents contribute to the hyperventilation evoked by hyperthermia. After their interruption, such contribution is replaceable from other thermal afferents.

The influence of the sympathetic outflow on aortic chemoreceptors of the cat during hypoxia and hypercapnia

1. An attempt has been made to reconcile differing observations, made by different groups of investigators, on the responses of aortic chemoreceptors of cats during normoxia, hypoxia and hypercapnia. 2. In cats anaesthetized with sodium pentobarbitone it was observed that during hypoxic stimulation of twelve chemoreceptors, an intravenous injection of about 20 mg sodium pentobarbitone produced hypotension which was accompanied by an initial fall in chemoreceptor activity instead of the expected increase that invariably occurred in all the receptors when hypotension was produced mechanically by distending a balloon in the right atrium (twenty-six during normoxia, eleven during hypoxia and eight during hypercapnia). 3. In twelve receptors a reflex fall in blood pressure produced by injecting 8-25 micrograms veratridine (Bezold-Jarisch reflex) yielded results qualitatively similar to those following injection of sodium pentobarbitone. 4. In sixteen out of twenty-five chemoreceptors it was observed that ventilating the cat with 5.6-6.7% CO2 produced either no or little increase in activity; in nine receptors there was a clear increase in activity, which fell initially or was abolished after injecting a single dose of 20 mg sodium pentobarbitone. 5. In all seven chemoreceptors tested in seven deeply anaesthetized cats it was found that a larger dose (about 50-60 mg) of sodium pentobarbitone had no direct depressant effect on aortic chemoreceptor activity. It followed that the initial depressant effect of the much smaller doses of sodium pentobarbitone observed during hypoxic and hypercapnic stimulation (see above) must be due to reduction in the sympathetic outflow to the aortic bodies. This conclusion was supported by the results following injections of veratridine. 6. By comparing the present results with those reported previously it was concluded that the variations in the responses of aortic chemoreceptors during hypoxia and hypercapnia reported by different investigators could be partly due to variations in the level of sympathetic activity prevailing under different experimental conditions.

Influence of peripheral O2 tension on the ventilatory response to CO2 in cats

The effects of peripheral hypoxia on ventilation were investigated in 18 cats anaesthetized with chloralose urethane. The ponto-medullary region of the brain was artificially perfused via a cannulated vertebral artery, using an extracorporeal circuit fed from a femoral artery. In this way the carbon dioxide tension (PacCO2) and the oxygen tension in the blood supplying the brainstem could be imposed independently from the peripheral PCO2 (PapCO2) and PO2 (PapO2) in the systemic circulation. In all experiments the brainstem was kept hyperoxic. The steady-state ventilation VE could be described by (formula; see text) where Sp and Sc represent the peripheral and central sensitivity to carbon dioxide and K is a constant. Sc and K were independent of the PapO2. In general, peripheral hypoxia increased and peripheral hyperoxia decreased Sp, compared to normoxia. It is concluded that: (1) there is no interaction in the ventilatory response between peripheral O2-CO2 and central CO2 stimuli; and (2) a positive interaction in the ventilatory response between peripheral hypoxia and CO2 originates from the arterial chemoreceptors.

Sympathectomy does not alter the response of carotid chemoreceptors to hypoxemia during carboxyhemoglobinemia or anemia

We tested the effects of section of the sympathetic innervation to the carotid body on the responses of single carotid chemoreceptor fibers to carboxyhemoglobinemia (40-50%) and anemia (15-20% Hct) at various levels of paO2 (range: 20-450 Torr). Sympathectomy had virtually no effect on the steady-state response curves of carotid chemoreceptors to O2, either in normal cats or in those with reduced oxygen transport due to anemia or carboxyhemoglobinemia.