Blockade of dopamine-induced chemosensory inhibition by domperidone

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.

Breathing at high altitude

Acclimatization to long-term hypoxia takes place at high altitude and allows gradual improvement of the ability to tolerate the hypoxic environment. An important component of this process is the hypoxic ventilatory acclimatization (HVA) that develops over several days. HVA reveals profound cellular and neurochemical re-organization occurring both in the peripheral chemoreceptors and in the central nervous system (in brainstem respiratory groups). These changes lead to an enhanced activity of peripheral chemoreceptor and re-inforce the central translation of peripheral inputs to efficient respiratory motor activity under the steady low O(2) pressure. We will review the cellular processes underlying these changes with a particular emphasis on changes of neurotransmitter function and ion channel properties in peripheral chemoreceptors, and present evidence that low O(2) level acts directly on brainstem nuclei to induce cellular changes contributing to maintain a high tonic respiratory drive under chronic hypoxia.

Erythropoietin modulates the neural control of hypoxic ventilation

Numerous factors involved in general homeostasis are able to modulate ventilation. Classically, this comprises several kind of molecules, including neurotransmitters and steroids that are necessary for fine tuning ventilation under different conditions such as sleep, exercise, and acclimatization to high altitude. Recently, however, we have found that erythropoietin (Epo), the main regulator of red blood cell production, influences both central (brainstem) and peripheral (carotid bodies) respiratory centers when the organism is exposed to hypoxic conditions. Here, we summarize the effect of Epo on the respiratory control in mammals and highlight the potential implication of Epo in the ventilatory acclimatization to high altitude, as well as in the several respiratory sickness and syndromes occurring at low and high altitude.

Progesterone increases hypoxic ventilatory response and reduces apneas in newborn rats

We hypothesized that progesterone may enhance the hypoxic ventilatory response and reduce the occurrence of apneas in newborn male rats. We studied 10-day-old rats chronically exposed to progesterone (Prog) or vehicle through the milk of lactating mothers. Respiratory and metabolic recordings were performed using whole body plethysmography under normoxia and during hypoxic exposure (10% O(2)--30 min). While progesterone did not alter baseline breathing and metabolic rate, it increased hypoxic ventilatory response particularly by limiting the magnitude of the ventilatory roll-off during the second phase of the hypoxic ventilatory response (i.e. following 5 min of exposure). In parallel, progesterone lowered the number of spontaneous apneas and drastically reduced the occurrence of post-sigh apneas during hypoxic exposure by limiting the time of the post-sigh expiratory pause. Following domperidone injection (used to block peripheral D2 dopamine receptor), minute ventilation increased in Veh pups and the number of spontaneous apneas decreased. These responses were not observed in Prog pups, suggesting that progesterone reduces peripheral dopaminergic inhibition on breathing. We conclude that progesterone is a potent stimulant of hypoxic ventilatory response in newborn rats and effectively reduces the occurrence of apneas.

A murine model of hyperdopaminergic state displays altered respiratory control

The dopamine transporter (DAT) protein plays an important role in the termination of dopamine signaling. We addressed the hypothesis that loss of DAT function would result in a distinctive cardiorespiratory phenotype due to the significant role of dopamine in the control of breathing, especially with respect to chemical control, metabolism, and thermoregulation. The DAT knockout mouse (DAT-/-) displays a state of functional hyperdopaminergia characterized by marked novelty driven hyperactivity. Certain behavioral and drug responses in these mice are reminiscent of endophenotypes of individuals with attention deficit hyperactivity disorders (ADHD). We performed experiments on conscious, unrestrained DAT-/- mice (KO) and littermate DAT+/+ wild-type (WT) controls. Ventilation was measured by the barometric technique during normoxia, hypoxia, or hypercapnia. We measured core body temperature and CO2 production as an index of metabolism. DAT-/- mice displayed a significantly lower respiratory frequency than WT mice, reflecting a prolonged inspiratory time. DAT-/- mice exhibited a reduced ventilatory response to hypoxia characterized by an attenuation of both the respiratory frequency and tidal volume responses. Both groups showed similar metabolic responses to hypoxia. Circadian measurements of body temperature were significantly lower in DAT-/- mice than WT mice during inactive periods. We conclude that loss of the DAT protein in this murine model of altered dopaminergic neurotransmission results in a significant respiratory and thermal phenotype that has possible implications for understanding of conditions associated with altered dopamine regulation.

Maturation of peripheral arterial chemoreceptors in relation to neonatal apnoea

Apnoea and periodic breathing are the hallmarks of breathing for the infant who is born prematurely. Sustained respiration is obtained through modulation of respiratory-related neurons with inputs from the periphery. The peripheral arterial chemoreceptors, uniquely and reflexly change ventilation in response to changes in oxygen tension. The chemoreflex in response to hypoxia is hyperventilation, bradycardia and vasoconstriction. The fast response time of the peripheral arterial chemoreceptors to changes in oxygen and carbon dioxide tension increases the risk of more periodicity in the breathing pattern. As a result of baseline hypoxaemia, peripheral arterial chemoreceptors contribute more to baseline breathing in premature than in term infants. While premature infants may have an augmented chemoreflex, infants who develop bronchopulmonary dysplasia have a blunted chemoreflex at term gestation. The development of chemosensitivity of the peripheral arterial chemoreceptors and environmental factors that might cause maldevelopment of chemosensitivity with continued maturation are reviewed in an attempt to help explain the physiology of apnoea of prematurity and the increased incidence of sudden infant death syndrome (SIDS) in infants born prematurely and those who are exposed to tobacco smoke.

Dopaminergic excitation of the goat carotid body is mediated by the serotonin type 3 receptor subtype

The purpose of the present study was to use chemoafferent recordings from the goat carotid body (CB) to pharmacologically identify the putative low affinity excitatory receptor for dopamine (DA). Close arterial injections of DA (1-50 microg kg(-1)) induced a dose-dependent excitatory burst followed by inhibition of the CB chemoafferent activity. The inhibition is likely DA D(2) receptor-mediated as it was blocked by domperidone (0.5-1.0 mg kg(-1) iv). The initial high frequency burst of CB chemoafferent activity could not be attenuated by selective antagonists for the DA D(1-4) receptors but could be blocked by D-tubocurarine or the selective serotonin(3) (5-HT(3)) receptor antagonists, tropisetron and MDL72222. The selective nicotinic antagonists, hexamethonium and vecuronium, were without effect. Selective blockade of the 5-HT(3)-receptor subtype using tropisetron significantly reduced both normoxic and hypoxic unitary CB discharge. These results suggest that DA-mediated excitation of the goat CB chemosensitive afferents occurs via the 5-HT(3)-receptor subtype and that the 5-HT(3)-receptor may exert an excitatory modulation of CB output during normal physiological responses in the goat.

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.

Effect of domperidone on ventilation and polycythemia after 5 weeks of chronic hypoxia in rats

Chronically hypoxic humans and some mammals have attenuated ventilatory responses, which have been associated with high dopamine level in carotid bodies. Alveolar hypoventilation and blunted ventilatory response have been recognized to be at the basis of Chronic Mountain Sickness by generating arterial hypoxemia and polycythemia. To investigate whether dopamine antagonism could decrease the hemoglobin concentration by stimulating resting ventilation (VE) and/or hypoxic ventilatory response, 18 chronically hypoxic rats (5 weeks, PB=433 Torr) were studied with and without domperidone treatment (a peripheral dopamine antagonist). Acute and prolonged treatment significantly increased poikilocapnic ventilatory response to hypoxia (RVE ml/min/kg=VE at 0.1 FI(O(2))-VE at 0.21 FI(O(2))), from 506+/-36 to 697+/-48; and from 394+/-37 to 660+/-81, respectively. In addition, Domperidone treatment decreased hemoglobin concentration from 21.6+/-0.29 to 18.9+/-0.19 (P<0.01) in rats chronically exposed to hypobaric hypoxia. Our study suggests that the stimulant effect of D(2)-R blockade on ventilatory response to hypoxia seems to compensate the low hypoxic peripheral chemosensitivity after chronic exposure and the latter in turn decrease hemoglobin concentration.

Gene expression in peripheral arterial chemoreceptors

The peripheral arterial chemoreceptors of the carotid body participate in the ventilatory responses to hypoxia and hypercapnia, the arousal responses to asphyxial apnea, and the acclimatization to high altitude. In response to an excitatory stimuli, glomus cells in the carotid body depolarize, their intracellular calcium levels rise, and neurotransmitters are released from them. Neurotransmitters then bind to autoreceptors on glomus cells and postsynaptic receptors on chemoafferents of the carotid sinus nerve. Binding to inhibitory or excitatory receptors on chemoafferents control the electrical activity of the carotid sinus nerve, which provides the input to respiratory-related brainstem nuclei. We and others have used gene expression in the carotid body as a tool to determine what neurotransmitters mediate the response of peripheral arterial chemoreceptors to excitatory stimuli, specifically hypoxia. Data from physiological studies support the involvement of numerous putative neurotransmitters in hypoxic chemosensitivity. This article reviews how in situ hybridization histochemistry and other cellular localization techniques confirm, refute, or expand what is known about the role of dopamine, norepinephrine, substance P, acetylcholine, adenosine, and ATP in chemotransmission. In spite of some species differences, review of the available data support that 1). dopamine and norepinephrine are synthesized and released from glomus cells in all species and play an inhibitory role in hypoxic chemosensitivity; 2). substance P and acetylcholine are not synthesized in glomus cells of most species but may be made and released from nerve fibers innervating the carotid body in essentially all species; 3). adenosine and ATP are ubiquitous molecules that most likely play an excitatory role in hypoxic chemosensitivity.

Dopaminergic metabolism in carotid bodies and high-altitude acclimatization in female rats

We tested the hypothesis that ovarian steroids stimulate breathing through a dopaminergic mechanism in the carotid bodies. In ovariectomized female rats raised at sea level, domperidone, a peripheral D2-receptor antagonist, increased ventilation in normoxia (minute ventilation = +55%) and acute hypoxia (+32%). This effect disappeared after 10 daily injections of ovarian steroids (progesterone + estradiol). At high altitude (3,600 m, Bolivian Institute for High-Altitude Biology-IBBA, La Paz, Bolivia), neutered females had higher carotid body tyrosine hydroxylase activity (the rate-limiting enzyme for catecholamine synthesis: +129%) and dopamine utilization (+150%), lower minute ventilation (-30%) and hypoxic ventilatory response (-57%), and higher hematocrit (+18%) and Hb concentration (+21%) than intact female rats. Consistent signs of arterial pulmonary hypertension (right ventricular hypertrophy) also appeared in ovariectomized females. None of these parameters was affected by gonadectomy in males. Our results show that ovarian steroids stimulate breathing by lowering a peripheral dopaminergic inhibitory drive. This process may partially explain the deacclimatization of postmenopausal women at high altitude.

Dopaminergic modulation of ventilation in obese Zucker rats

To investigate the hypothesis that the impaired respiratory drive noted in morbid obesity was attributable to altered dopaminergic mechanisms acting on peripheral and/or central chemoreflex sensitivity, seven obese and seven lean Zucker rats were studied at 11 wk of age. Ventilation (VE) was measured by the barometric technique during hyperoxic (100% O(2)), normoxic (21% O(2)), hypoxic (10% O(2)), and hypercapnic (7% CO(2)) exposures after the administration of vehicle (control), haloperidol [Hal, 1 mg/kg, a central and peripheral dopamine (Da) receptor antagonist], or domperidone (Dom, 0.5 mg/kg, a peripheral Da receptor antagonist). In both lean and obese rats, Hal increased tidal volume and decreased respiratory frequency during hyperoxia or normoxia, resulting in an unchanged VE. In contrast, Dom did not affect tidal volume, frequency, or VE during hyperoxia or normoxia. During hypoxia, however, VE significantly increased from 1,132 +/- 136 to 1,348 +/- 98 ml. kg(-1). min(-1) (P < 0.01) after the administration of Dom in obese rats, whereas no change was observed in lean rats. Hal significantly decreased VE during hypoxia compared with control in lean but not obese rats. In both lean and obese rats, Hal decreased VE in response to hypercapnia, whereas Dom had no effect. Our major findings suggest that peripheral chemosensitivity to hypoxia in obese Zucker rats is reduced as a result of an increased dopaminergic receptor modulation in the carotid body.

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.

Effects of dopamine and domperidone on ventilatory sensitivity to hypoxia after 8 h of isocapnic hypoxia

Acclimatization to altitude involves an increase in the acute hypoxic ventilatory response (AHVR). Because low-dose dopamine decreases AHVR and domperidone increases AHVR, the increase in AHVR at altitude may be generated by a decrease in peripheral dopaminergic activity. The AHVR of nine subjects was determined with and without a prior period of 8 h of isocapnic hypoxia under each of three pharmacological conditions: 1) control, with no drug administered; 2) dopamine (3 microg. min-1. kg-1); and 3) domperidone (Motilin, 40 mg). AHVR increased after hypoxia (P </= 0. 001). Dopamine decreased (P </= 0.01), and domperidone increased (P </= 0.005) AHVR. The effect of both drugs on AHVR appeared larger after hypoxia, an observation supported by a significant interaction between prior hypoxia and drug in the analysis of variance (P </= 0. 05). Although the increased effect of domperidone after hypoxia of 0. 40 l. min-1. %saturation-1 [95% confidence interval (CI) -0.11 to 0. 92 l. min-1. %-1] did not reach significance, the lower limit for this confidence interval suggests that little of the increase in AHVR after sustained hypoxia was brought about by a decrease in peripheral dopaminergic inhibition.

Dopaminergic modulation of respiratory motor output in peripherally chemodenervated goats

We examined the ventilatory effects of exogenous dopamine (DA) and norepinephrine (NE) administration in chloralose-anesthetized, paralyzed, artificially ventilated adult goats before and after carotid body denervation (CBD). Intravenous (iv) DA bolus injections and slow iv infusions caused dose-dependent inhibition of phrenic nerve activity (PNA) in carotid body (CB)-intact animals during normoxia and hyperoxia but not during hypercapnia. NE administration in CB-intact goats caused dose-dependent inhibition of PNA of similar magnitude to DA trials. The DA D2-receptor agonists quinelorane and quinpirole inhibited PNA, whereas the DA D1-receptor agonist SKF-81297 had no effect. After CBD, the ventilatory depressant effects of DA persisted, but responses were significantly attenuated compared with CB-intact trials. CBD abolished the inhibitory effect of low-dose NE administration but did not alter ventilatory responses to high-dose NE injection. The peripheral DA D2-receptor antagonist domperidone substantially attenuated the inhibitory effects of DA bolus injections and infusions and reversed the inhibitory ventilatory effect of high-dose DA administration to excitation in some animals. The alpha-adrenoceptor antagonist phentolamine had no effect on DA-induced ventilatory depression. Beta-Adrenoceptor stimulation with isoproterenol produced similar hemodynamic effects to DA administration but had no effect on PNA. We conclude that DA and NE exert both CB-mediated and non-CB-mediated inhibitory effects on respiratory motor output in anesthetized goats. The ventilatory depressant effects that persist in peripherally chemodenervated animals are DA D2-receptor mediated, but their exact location remains speculative.

Intracarotid dopamine infusion does not prevent acclimatization to hypoxia

Ventilatory acclimatization to hypoxia (VAH) is the time-dependent increase in ventilation that occurs during sustained exposure to hypoxia. The mechanism for VAH remains elusive. We sought to determine whether a deficiency in the availability of carotid body dopamine is the mechanism of increased ventilatory responsiveness to hypoxia during VAH in awake goats. This was based on the evidence that dopamine (DA) is primarily an inhibitory neuromodulator of carotid body (CB) function. The hypothesis was tested by intracarotid infusion of DA (5.0 micrograms kg-1 min-1) throughout VAH. VAH was not prevented by DA infusion, failing to support the hypothesis. We conclude that a deficiency in the availability of inhibitory DA release within the CB is probably not responsible for VAH. However, increased ventilatory responses to acute hypoxia after either prolonged DA infusion or hypoxia may have similar CB mechanisms.

Carotid body dopaminergic mechanisms are functional after acclimatization to hypoxia in goats

Ventilatory acclimatization to sustained hypoxia (VASH) is the time-dependent increase in ventilation that occurs during prolonged exposure to hypoxia. We tested the hypothesis that carotid body (CB) dopaminergic mechanisms are down-regulated during VASH, which would allow CB afferent discharge and ventilation to increase beyond the initial response to hypoxia. Domperidone (DOM; 1.0 mg.kg-1) was administered intravenously to block CB dopamine (DA) receptors after VASH was complete in awake goats. DOM caused a significant augmentation of the ventilatory response to hypoxia in acclimatized goats, failing to support the hypothesis. We conclude that inhibitory CB dopaminergic function is not significantly reduced following prolonged hypoxia, and that down-regulation of CB dopaminergic mechanisms may not be involved in VASH in the goat.

Localization of dopamine D2 receptor mRNA in glomus cells of the rabbit carotid body by in situ hybridization

The localization of mRNA coding for the dopamine D2 receptor was studied in the rabbit carotid body using in situ hybridization with synthetic 35S-labelled oligodeoxynucleotides. Using autoradiography on cryostat or semi-thin sections, labelling was observed over the cytoplasm of glomus cells, but not over sustentacular cells. A quantitative study showed that labelling intensity (silver grain density) was increased by haloperidol treatment. These results suggest that glomus cells express the dopamine D2 receptor gene and that this expression is regulated.

Effects of dopaminergic blockade upon carotid chemosensory activity and its hypoxia-induced excitation

The effects of domperidone, antagonist of D2 receptors, on arterial chemoreceptor activity were studied in spontaneously breathing and pentobarbitone anesthetized cats, in which recordings of chemosensory impulse activity were obtained simultaneously from both cut carotid (sinus) nerves. Intravenous injections of domperidone 50 micrograms/kg produced a maintained increase in the basal frequency of chemosensory discharges, after which hyperoxic tests (breathing 100% O2 for 30 s) evoked larger falls in the rate of chemosensory impulses. Chemosensory responses evoked by hypoxic hypoxia (100% N2 tests) and by cytotoxic hypoxia (i.v. injections of NaCN) reached higher impulse rates after domperidone treatment. The effects of domperidone reveal that a resting release of dopamine from glomus cells maintains a low level of basal chemosensory activity under normoxic conditions. Domperidone turns off such restraining dopaminergic control and enhances the transient chemosensory responses to hypoxic stimuli. Present data support a modulatory role for dopamine within the chemoreceptor process, but not its participation as excitatory transmitter between glomus cells and sensory nerve endings.

Effects of dopamine and domperidone on ventilation during isocapnic hypoxia in humans

In order to investigate the role of dopamine in the ventilatory response to sustained, isocapnic hypoxia six subjects were studied three times in each of three pharmacological conditions: (1) in the absence of any drug administration, (2) during i.v. infusion of dopamine (3 micrograms.kg-1.min-1), and (3) after pretreatment with domperidone. Otherwise the experimental protocol was identical on each day and consisted of holding the subjects' end-tidal PO2 at 100 Torr for 10 min, then 50 Torr for 20 min and finally at 100 Torr again for 5 min. End-tidal PCO2 was held constant 2-3 Torr above normal throughout the experiment. Domperidone increased, and dopamine decreased the magnitudes of both the fast on- and off-responses, but neither drug affected the magnitude of the hypoxic ventilatory decline (HVD). The results of this study suggests: (1) that a peripheral dopaminergic mechanism is not involved in the genesis of HVD, and (2) the peripheral chemoreflex may be modulated peripherally to produce HVD.

Effects of the dopamine antagonists haloperidol and domperidone on the normoxic ventilatory response to CO2 in cats

We investigated the effects of the dopamine antagonists haloperidol and domperidone on the ventilatory response following square-wave changes in end-tidal CO2 during normoxia in chloralose-urethane anaesthetized cats. In 7 cats these responses were measured before (control, 28 runs) and after the administration of 1 mg/kg haloperidol i.v. (26 runs) and in 8 other cats before (39 runs) and after 0.5 mg/kg domperidone i.v. (34 runs). Each response was separated into a slow central and a fast peripheral part by fitting two exponential functions to the measured ventilation. These functions have as parameters a CO2 sensitivity, a time constant, a time delay and an apnoeic threshold B (extrapolated PETCO2 of the steady-state response curve at zero ventilation). Haloperidol significantly diminished the peripheral (Sp) and the central (Sc) ventilatory sensitivity to CO2 and the B-value (P less than 0.001). The ratio Sp/Sc, the time constants and the time delays were not significantly changed. Domperidone only diminished the B-value significantly (P less than 0.001). Since domperidone does not readily cross the blood-brain barrier, its effect was a CO2 independent increase of the ventilation mediated by the peripheral chemoreceptors. Haloperidol exhibited, besides the peripheral stimulatory effect a depressant central effect due to an action on the central integrative structures, resulting in a proportional decrease of Sp and Sc.

Dopamine blockade alters ventilatory acclimatization to hypoxia in goats

Dopamine (DA) is generally accepted to be an inhibitory neurotransmitter in the carotid body (CB). It is released and depleted from the CB by acute hypoxia. From this background we made the hypothesis that hypoxic depletion of CB DA could be responsible for a time-dependent increase in CB afferent output and the early phase of ventilatory acclimatization to hypoxia (VAH) in goats. We reasoned, then, that blockade of DA receptors in the CB would accelerate the time course of VAH in the goat, i.e. produce a greater acute response to hypoxia (first 15 min) followed by a reduced rate of change of the subsequent time-dependent hyperventilation. We tested this hypothesis by exposing 7 adult female goats to up to 28 h of hypobaric hypoxia (PB = 380 Torr) on 3 different occasions separated by at least 2 months. The first was as control. During the second and third exposures different doses of the DA antagonist, domperidone, were administered prior to and during the hypoxic exposure (0.5 mg/kg followed by 0.25 mg/kg every 3 h and in the second study 1.0 mg/kg followed by 0.5 mg/kg every 2 h). The time course of acclimatization was assessed by measurement of arterial blood gases and pH in the awake goats. The data obtained in the first 4-5 h of hypoxia in domperidone treated animals appeared to support the hypothesis. Domperidone treated animals had a significantly greater acute ventilatory response to hypoxia followed by a lower rate of progressive hyperventilation in this period. However, variation in control values, greater respiratory alkalosis and a secondary significant hyperventilation after 6-7 h of hypoxia in the domperidone treated animals prevents a clear conclusion as to the precise role of CB dopaminergic mechanisms in acclimatization to hypoxia. Nevertheless, peripheral DA receptor blockade with domperidone does alter the time course and magnitude of hyperventilation during the first 7 h of hypobaric hypoxia in goats.

Domperidone-induced potentiation of ventilatory responses in awake goats

Dopamine has been implicated in maintaining tonic inhibition of carotid body activity. We tested this hypothesis by assessing the ventilatory effects of a peripheral dopamine antagonist, domperidone. The effects of this agent on the ventilatory responses to hypoxia and hypercapnia were also examined. The study was performed in awake carotid body intact and carotid body denervated goats. Resting minute ventilation increased while PaCO2 decreased (4 Torr) following domperidone administration (0.5 mg/kg, I.V.) in carotid body intact goats. This response did not occur in carotid body denervated goats supporting the hypothesis that endogenous dopamine provides tonic inhibition in the carotid body. Hypoxic and hypercapnic ventilatory responses were significantly augmented following domperidone administration in the carotid body intact goats. This supports the concept of dopaminergic modulation of the response of the carotid body to stimuli. Domperidone allows study of carotid chemoreceptor dopaminergic activity in awake animals because of its high affinity for carotid body D2 dopamine receptors and its lack of CNS effects.