Intracarotid dopamine infusion does not prevent acclimatization to hypoxia

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.

Dopaminergic mechanisms of neural plasticity in respiratory control: transgenic approaches

Data supporting the hypothesis that dopamine-2 receptors (D(2)-R) contribute to time-dependent changes in the hypoxic ventilatory response (HVR) during acclimatization to hypoxia are briefly reviewed. Previous experiments with transgenic animals (D(2)-R 'knockout' mice) support this hypothesis (J. Appl. Physiol. 89 (2000) 1142). However, those experiments could not determine (1) if D(2)-R in the carotid body, the CNS, or both were involved, or (2) if D(2)-R were necessary during the acclimatization to hypoxia versus some time prior to chronic hypoxia, e.g. during a critical period of development. Additional experiments on C57BL/6J mice support the idea that D(2)-R are critical during the period of exposure to hypoxia for normal ventilatory acclimatization. D(2)-R in carotid body chemoreceptors predominate under control conditions to inhibit normoxic ventilation, but excitatory effects of D(2)-R, presumably in the CNS, predominate after acclimatization to hypoxia. The inhibitory effects of D(2)-R in the carotid body are reset to operate primarily under hypoxic conditions in acclimatized rats, thereby optimizing O(2)-sensitivity.

Role of endothelin and endothelin A-type receptor in adaptation of the carotid body to chronic hypoxia

Chronic exposure in a low-PO(2) environment (i.e., chronic hypoxia, CH) elicits an elevated hypoxic ventilatory response and increased hypoxic chemosensitivity in arterial chemoreceptors in the carotid body. In the present study, we examine the hypothesis that changes in chemosensitivity are mediated by endothelin (ET), a 21-amino-acid peptide, and ET(A) receptors, both of which are normally expressed by O(2)-sensitive type I cells. Immunocytochemical staining showed incremental increases in ET and ET(A) expression in type I cells after 3, 7, and 14 days of CH (380 Torr). Peptide and receptor upregulation was confirmed in quantitative RT-PCR assays conducted after 14 days of CH. In vitro recordings of carotid sinus nerve activity after in vivo exposure to CH for 1-16 days demonstrated a time-dependent increase in chemoreceptor activity evoked by acute hypoxia. In normal carotid body, the specific ET(A) antagonist BQ-123 (5 microM) inhibited 11% of the nerve discharge elicited by hypoxia, and after 3 days of CH the drug diminished the hypoxia-evoked discharge by 20% (P < 0.01). This inhibitory effect progressed to 45% at day 9 of CH and to nearly 50% after 12, 14, and 16 days of CH. Furthermore, in the presence of BQ-123, the magnitude of the activity evoked by hypoxia did not differ in normal vs. CH preparations, indicating that the increased activity was the result of endogenous ET acting on an increasing number of ET(A). Collectively, our data suggest that ET and ET(A) autoreceptors on O(2)-sensitive type I cells play a critical role in CH-induced increased chemosensitivity in the rat carotid body.

PDGF-beta receptor expression and ventilatory acclimatization to hypoxia in the rat

Activation of platelet-derived growth factor-beta (PDGF-beta) receptors in the nucleus of the solitary tract (nTS) modulates the late phase of the acute hypoxic ventilatory response (HVR) in the rat. We hypothesized that temporal changes in PDGF-beta receptor expression could underlie the ventilatory acclimatization to hypoxia (VAH). Normoxic ventilation was examined in adult Sprague-Dawley rats chronically exposed to 10% O(2), and at 0, 1, 2, 7, and 14 days, Northern and Western blots of the dorsocaudal brain stem were performed for assessment of PDGF-beta receptor expression. Although no significant changes in PDGF-beta receptor mRNA occurred over time, marked attenuation of PDGF-beta receptor protein became apparent after day 7 of hypoxic exposure. Such changes were significantly correlated with concomitant increases in normoxic ventilation, i.e., with VAH (r: -0.56, P < 0.005). In addition, long-term administration of PDGF-BB in the nTS via osmotic pumps loaded with either PDGF-BB (n = 8) or vehicle (Veh; n = 8) showed that although no significant changes in the magnitude of acute HVR occurred in Veh over time, the typical attenuation of HVR by PDGF-BB decreased over time. Furthermore, PDGF-BB microinjections did not attenuate HVR in acclimatized rats at 7 and 14 days of hypoxia (n = 10). We conclude that decreased expression of PDGF-beta receptors in the dorsocaudal brain stem correlates with the magnitude of VAH. We speculate that the decreased expression of PDGF-beta receptors is mediated via internalization and degradation of the receptor rather than by transcriptional regulation.

Carotid body mechanisms in acclimatization to hypoxia

Most studies oriented toward examining mechanisms increasing carotid body (CB) sensitivity to hypoxia during ventilatory acclimatization (VAH) have focussed on the role of known neuromodulators of CB function. Two general categories of the neuromodulatory agents studied most extensively could be considered: those thought to be primarily inhibitory to CB function: dopamine, norepinephrine, nitric oxide and those thought to be primarily excitatory: substance P, endothelin. There is evidence that these putative inhibitory agents are up-regulated in the first weeks of chronic hypoxia and that substance P is down-regulated. All these changes would favor a decrease in CB sensitivity to hypoxia. There are data suggesting that CB endothelin activity is up-regulated in rats subjected to chronic hypoxia, a direction suggesting increased CB sensitivity to hypoxia. Dopamine may have an excitatory as well as an inhibitory role on the CB, but there is not yet evidence to indicate that an excitatory role for DA exists in chronic hypoxia. Ion channel studies of type I CB cells suggest increased excitability after prolonged hypoxia. The role of excitatory CB nicotinic receptors and putative serotonin type 3 receptors should be examined further for their potential role in VAH. It is suggested that a balance of excitatory and inhibitory modulation is responsible for increased CB sensitivity to hypoxia during VAH.

Time-dependent changes in dopamine D(2)-receptor mRNA in the arterial chemoreflex pathway with chronic hypoxia

The hypoxic ventilatory response (HVR) can be modulated by dopamine D(2)-receptors (D(2)-R) in both the carotid body arterial chemoreceptors and the nucleus tractus solitarius (NTS), the primary synapse site of carotid body afferents. We hypothesized that chronic hypoxia alters D(2)-R gene expression to initiate changes in D(2)-R modulation of the HVR and enhance ventilatory acclimatization to hypoxia. Thus, we used a competitive reverse transcription-polymerase chain reaction (RT-PCR) method to quantify changes in D(2)-R mRNA levels in the rat carotid body and NTS after 0, 6, 12, 24, 48, or 168 h of hypobaric hypoxia (P(IO(2))=80 Torr). In the rostral NTS, hypoxia significantly increased D(2)-R mRNA at all time points. In the caudal NTS, D(2)-R mRNA levels initially increased in response to hypoxia and then significantly decreased to 71+/-5% and 71+/-6% of control after 48 and 168 h of hypoxia, respectively. In the carotid body, D(2)-R mRNA levels significantly decreased to 59+/-2% of control after 48 h of hypoxia; however, they significantly increased to 274+/-22% of control after 168 h. These results suggest that changes in D(2)-R mRNA in the arterial chemoreflex pathway and corresponding changes at the protein and signaling levels may contribute to the time-dependent changes in ventilation observed with chronic hypoxia. Specifically, decreased carotid body inhibition by D(2)-R could increase the HVR after 2 days of hypoxia.

Human hypoxic ventilatory response with blood dopamine content under intermittent hypoxic training

Adaptation to intermittent hypoxia can enhance a hypoxic ventilatory response (HVR) in healthy humans. Naturally occurring oscillations in blood dopamine (DA) level may modulate these responses. We have measured ventilatory response to hypoxia relative to blood DA concentration and its precursor DOPA before and after a 2-week course of intermittent hypoxic training (IHT). Eighteen healthy male subjects (mean 22.8 ± 2.1 years old) participated in the study. HVRs to isocapnic, progressive, hypoxic rebreathing were recorded and analyzed using piecewise linear approximation. Rebreathing lasted for 5-6 min until inspired O2 reached 8 to 7%. IHT consisted of three identical daily rebreathing sessions separated by 5-min breaks for 14 consecutive days. Before and after the 2-week course of IHT, blood was sampled from the antecubital vein to measure DA and DOPA content. The investigation associated pretraining high blood DA and DOPA values with low HVR (r = -0.66 and -0.75, respectively), elevated tidal volume (r = 0.58 and 0.37) and vital capacity (r = 0.69 and 0.58), and reduced respiratory frequency (r = -0.89 and -0.82). IHT produced no significant change in ventilatory responses to mild hypoxic challenge (PetO2 from 110 to 70-80 mmHg; 1 mmHg = 133.3 Pa) but elicited a 96% increase in ventilatory response to severe hypoxia (from 70-80 to 45 mmHg). Changes in HVRs were not accompanied by statistically significant shifts in blood DA content (24% change), although a twofold increase in DOPA concentration was observed. Individual subject's changes in DA and DOPA content were not correlated with HVR changes when these two parameters were evaluated in relation to the IHT. We hypothesize that DA flowing to the carotid body through the blood may provoke DA autoreceptor-mediated inhibition of endogenous DA synthesis-release, as shown in our baseline data.Key words: hypoxic ventilatory response, dopamine, intermittent hypoxia.

Methysergide augments the acute, but not the sustained, hypoxic ventilatory response in goats

Ventilatory acclimatization to hypoxia (VAH) is the time-dependent increase in ventilation that occurs during sustained hypoxia. As serotonin (5-HT) has been reported to be an important modulator of respiratory output, 5-HT may also play a role in VAH. Methysergide (a broad-spectrum 5-HT antagonist), was given to awake goats (1 mg kg(-1) i.v.) 30 min prior to being exposed to 4 h of isocapnic hypoxia. Although methysergide slightly decreased arterial pH, presumably due to a non-significant increase in arterial P(CO2), it did not alter normoxic ventilation. Following methysergide, the expired minute ventilation (VE) was significantly elevated above the control (saline) response after 30 min of hypoxia, but methysergide did not otherwise alter VAH. We repeated the study in the same goats using ketanserin, a specific 5-HT2A/2C receptor antagonist (1.2 mg kg(-1) i.v.). Ketanserin had no effect on the acute hypoxic ventilatory response, or on VAH. We conclude that while 5-HT modulates the acute hypoxic ventilatory response in goats, it does not appear to act through the 5-HT2A/2C receptor subtypes.