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1 Centre d'Étude des Rythmes Biologiques, Université Libre de Bruxelles-Hôpital Erasme, B-1070 Brussels, Belgium; 2 Physiologisches Institut, Universität Zürich-Irchel, CH-8057 Zurich, Switzerland; 3 Instituto Boliviano de Biología de Altura, Embajada de Francia, La Paz, Bolivia; and 4 Laboratoire de Physiologie des Régulations Energétiques, Cellulaires et Moléculaires, Centre National de la Recherche Scientifique et Faculté de Médecine, Université Claude Bernard, Unité Mixte de Recherche 5123, F-69373 Lyon, France
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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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.
hypoxia; ovarian steroids; chronic mountain sickness
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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MONGE'S DISEASE, or chronic mountain sickness (CMS), has been recognized for many years as a major risk for high-altitude natives. Its major features are elevated hematocrit (Hct) and hemoglobin (Hb) concentration, chronic alveolar hypoventilation associated with low arterial O2 saturation, and low hypoxic ventilatory responsiveness (15, 22). Pulmonary arterial hypertension and right heart hypertrophy are also classically associated with CMS and may ultimately lead to right heart failure. Most of the patients are young to old adult men who have been living for years above 3,000 m of altitude, whereas women are usually protected until they reach menopause. After menopause, women living at high altitude are submitted to a deacclimatization process that includes an increase of blood Hb concentration and Hct and a decrease of arterial O2 saturation (16, 17).
Most of the human and animal studies carried out to better understand the pathophysiological factors of CMS have focused on the implication of gonadal steroids (17, 20): progesterone associated with estrogens has been found to be effective in the treatment of CMS by stimulating minute ventilation (14). More recently, some studies carried out on cats or rats pointed out the carotid body to be one of the major sites for gender differentiation of ventilatory control under acute or chronic hypoxic stimulation (13, 24), but the underlying mechanisms of this hormonal stimulation remain poorly understood.
Among the various neuromodulators synthesized by glomic cells in the carotid bodies and released under hypoxemic challenge, dopamine (DA) is found at high concentration and has been recognized as a potent inhibitory neuromodulator of carotid body chemotransduction (7, 8, 10, 29).
Our hypothesis is that ovarian steroids can modulate breathing by reducing a dopaminergic drive in the carotid bodies. To test this hypothesis, we performed two complementary studies. First we studied, at sea level, the effect of domperidone in ovariectomized female rats before and after repeated hormonal injections (progesterone + estradiol). Because domperidone is a highly specific peripheral D2-dopaminergic receptor antagonist and potent ventilatory stimulant drug acting directly on carotid sinus nerve discharge (10, 25, 28, 29), we hypothesized that before hormonal treatment domperidone should stimulate resting minute ventilation and/or hypoxic ventilatory response (HVR) and that this influence should be reduced after repeated hormonal injections.
The second part of this study was performed in rats permanently living at high altitude (Bolivian Institute for High-Altitude Biology-IBBA, La Paz, Bolivia; 3,600 m). Under such conditions, there is a marked hyperventilation and stimulation of the catecholaminergic activity in the carotid bodies with a notable gender dimorphism for both variables (13, 24). We hypothesized that endogenous ovarian steroids specifically modulate the dopaminergic metabolism in carotid bodies. To test this hypothesis, we measured the activity of tyrosine hydroxylase (TH), as well as the utilization rate (% of the content used per hour) of norepinephrine (NE) and DA, in the carotid body of intact or neutered female rats. Resting minute ventilation, HVR, Hct, Hb concentration, and right ventricular (RV) hypertrophy, a direct reflection of pulmonary arterial hypertension (21), which are all relevant outcomes for health at high altitude, were also determined in these animals. Male rats were also added in this study to test the gender specificity of the response to gonadectomy.
Our results are consistent with the hypothesis that ovarian steroids stimulate breathing, at least partially, by lowering a peripheral dopaminergic inhibitory drive that is likely to originate from the carotid bodies. The implication of this finding for high-altitude dwellers is discussed.
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MATERIALS AND METHODS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Sea-Level Studies
General experimental design.
Ovariectomized Sprague-Dawley female rats were purchased from
IFFA-CREDO (l'Arbresle, France) and installed under standardized conditions in an animal room for 1 mo (Lyon, France, altitude 150 m). The animal room was climatized at 24 ± 1°C with a 12:12-h light-dark cycle, and animals had free access to standard chow and
water. Basal ventilatory measurements (see below) and HVR (10%
O2; 20 min) measurements were made in ovariectomized
females after intraperitoneal saline injection. The same animals were used 48 h later to assess the effect of domperidone on resting minute ventilation and HVR (1 mg/kg ip provided by Janssen-Cilag; n = 9; domperidone was dissolved in saline solution
with 1 equivalent of tartaric acid). Every measurement was
taken between 1 and 2 h after the corresponding injection. The
females were then treated for 10 consecutive days with progesterone (1 mg/day; Sigma) and 17
-estradiol (0.05 mg/day; Sigma; subcutaneous
injection of hormones in sesame oil), and the same procedure was
repeated to measure HVR after saline and domperidone injections
(n = 8). All experiments were carried out according to
the ethical principles laid down by the French (Ministry of
Agriculture) and European Union Council Directives for care of
laboratory animals.
Basal ventilation and HVR.
Ventilation was measured in awake unrestrained rats using a barometric
plethysmograph chamber. Once the animal was quiet, the inlet and outlet
tubes of the animal chamber were closed, and pressure fluctuations
related to breathing were recorded with a differential pressure
transducer (Celesco, CA). The pressure signal was calibrated by 10 consecutive injections of an adequate volume of air (1 ml) into the
animal box and by recording the related changes in pressure. Tidal
volume (Vt; ml), respiratory frequency
(Fr; breaths/min), minute ventilation corrected
for body weight (
E100;
ml · min
1 · 100 g1), and Vt-to-body weight (BW) ratio
(Vt/BW; ml/100 g) were calculated from breath-by-breath
computer analysis of the spirogram over 30-50 consecutive breaths
using standard methods (1). The temperature inside the
chamber was set around 24-25°C and permanently checked on a
digital thermometer (±0.1°C) during measurements. Atmospheric pressure for correction of the plethysmographic pressure signal was
read at the beginning of each measurement. On the basis of the previous
demonstration that changes in body temperature related to gender or
hormonal status do not affect HVR or minute ventilation in rats
(19), body temperature was considered to be 37°C
(1) for all groups. Time periods for collecting the
ventilatory data lasted for 30 s to 1 min, and during this time
the temperature and CO2 level within the box remained
unaltered. Measurements were performed at least in triplicate, and no
significant differences were found between the first and the last
measurement. The mean of these values was defined as the basal level of ventilation.
High-Altitude Studies
Animals: general housing conditions and operative procedures. Sprague-Dawley rats were reared in La Paz, Bolivia, at the altitude of 3,600 m [mean barometric pressure (Pb) = 495-500 mmHg]. The high-altitude native rats are descendants of a lineage implanted in the Bolivian Institute for High-Altitude Biology-IBBA since 1992 (originally purchased from IFFA-CREDO, l'Arbresle, France). The animal room was climatized at 24 ± 1°C with a 12:12-h light-dark cycle, and animals had free access to standard chow and water.
Female rats were mated with adult males, left undisturbed for 2 wk, and then checked daily for pregnancy and separated in individual cages a few days before delivery. Gonadectomy was performed under cold anesthesia (induced by total immersion in ice during 4-5 min, one of the best choices for anesthesia of rat pups; Ref. 5a) in 1-wk-old rats. In females, ovaries were removed by a bilateral dorsal incision, and in males testes were removed from the abdominal cavity by a slight incision at the bottom of the abdomen. Postoperative care included a progressive return to ambient temperature that allowed complete recovery within 10-15 min after surgery. Rat pups were left with their mothers until weaning (on postnatal day 21), when they were separated according to their status and gender (4-6 animals/cage). Control animals were undisturbed until weaning. All measurements were carried out during the 12th wk of life. Different groups of animals, born to different mothers, were used for noninvasive (ventilatory measurements) and invasive (hematological status, cardiac weight, and neurochemical measurements) studies to avoid possible interactions between the hypoxic challenge done to measure HVR and catecholaminergic metabolism in the carotid bodies. All measures including control and gonadectomized animals were carried out within a period of 2 mo.Basal ventilation and HVR. Resting minute ventilation was measured as described above. After basal ventilation measurement, the inlet tube of the plethysmograph was derived from the hypoxic gas mixture. The O2 percentage within the animal chamber was gradually decreased and set at the desired level (12% O2 in N2). Ventilatory measurements were taken 10 min after the onset of hypoxic exposure.
Hematological status and cardiac weight. Rats were anesthetized by an intraperitoneal injection of pentobarbital sodium (60 mg/kg body wt; Sanofi Santé Animale). Blood samples were drawn by cardiac puncture into a heparinized tube, Hct (%) was measured by a microtechnique method, and the Hb concentration was determined by using the Hemocue (AB Ängelholm, Sweden) field spectrophotometer. After blood sampling, the chest was opened and the heart was dissected out. The whole heart was rapidly washed in a saline solution and weighed. The atria were separated from the ventricles, the RV was cut off from the left ventricle (LV), and the septum was left as a part of the LV. The RV and the LV + septum were immediately weighed.
Estimation of in vivo catecholaminergic activity in the carotid bodies. The rate of catecholamine biosynthesis was assessed from in vivo TH activity, estimated by measuring L-DOPA accumulation after the inhibition of L-amino acid decarboxylase by 3-hydroxybenzylhydrazine dihydrochloride (NSD-1015; Sigma; Ref. 5). NSD-1015 was injected intraperitoneally (75 mg/kg in saline solution) 20 min before the animal was killed as already described (12, 13).
After cardiac puncture for hematological determinations and cardiac dissection, the carotid bodies were rapidly removed in a solution of perchloric acid (0.1 M) and disodium EDTA (1 mg/ml), frozen in liquid nitrogen, and stored at80°C. Further analyses were done in
France (Lyon). Samples from Bolivia were shipped in dry ice and arrived
in the French laboratory (and stored at 80°C) in 24 h.
L-DOPA, DA, and NE were assayed by high-performance liquid chromatography coupled with electrochemical detection. The
mobile phase consisted of 0.1 M potassium phosphate buffer, pH 3.0, containing 0.15 mM disodium EDTA. The flow rate was 0.8 ml/min. L-DOPA was measured at +0.65 V. The
detection limit, calculated by doubling the noise ratio and expressed
in terms of picomoles of injected amounts, was <0.03 pmol, and the
intra-assay coefficient was 0.2%.
The TH activity was expressed as picomoles of L-DOPA
formed per 20 min per carotid body. This assay has been
recognized as a valuable tool to assess catecholaminergic activity in
the carotid body because the minute size of this organ does not allow
one to measure total protein content. An index of total catecholamine synthesis to total, or specific, catecholamine content of the structure
was calculated by expressing catecholamine synthesis per hour (TH
activity/20 min × 3) and by dividing it by the total amount of
catecholamine present in the structure (DA + NE) or by the amount
of the specific catecholamine (DA or NE). Results are expressed as the
percentage of the amine content renewed per hour. In a steady state of
catecholamine utilization rate, this corresponds to the percentage of
total content of amine used per hour in the structure.
Statistical Analyses
All analyses were done with Statview software (Abacus Concepts, Berkeley, CA). For simple measurements, data were analyzed by one-way ANOVA followed by a post hoc Fisher's protected least significant difference (PLSD) test. For HVR, data were analyzed by two-way ANOVA for repeated measures. If a global effect or a significant interaction appeared between groups on the general ANOVA, data were analyzed for factorial measures by a PLSD post-ANOVA test. The results of the PLSD are indicated by symbols placed on the corresponding point in Figs. 1-6 (P < 0.05). All data are presented as means ± SE. The level of significance for all analyses was set at 0.05.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Sea-Level Studies
Effects of domperidone on minute ventilation and HVR in
ovariectomized females.
In this study we used ovariectomized females as controls before
hormonal treatment. In ovariectomized females, domperidone injection
resulted in an increase of resting
E100 (+39%; 59.4 ± 2.6 ml · min1 · 100 g1 after
domperidone injection vs. 42.6 ± 5.1 ml · min1 · 100 g1 after
saline injection; P = 0.01) and Vt/BW
(+46%; 0.70 ± 0.03 vs. 0.48 ± 0.07 ml/100 g,
P < 0.05). During hypoxic exposure, E100, and Vt/BW were also
elevated after domperidone injection compared with saline
injection (+32 and +39%, respectively; Fig. 1).
E100 after domperidone injection and
E100 after saline injection) for each
animal (see Fig. 4A). Similar correlations were found for Vt/BW (r2 = 0.85;
P < 0.001) and Fr
(r2 = 0.95; P < 0.0001) under normoxia (see Fig. 4A).
Effects of hormonal treatment on minute ventilation and HVR.
After 10 days of daily hormonal injection, females had a lower BW
(9.5%; 297 ± 6 vs. 328 ± 6 g, P < 0.001), higher resting E100 (+74%;
74.2 ± 4.0 vs. 42.6 ± 5.1 ml · min1 · 100 g1;
P < 0.001), and higher Vt (+55%;
2.42 ± 0.15 vs. 1.56 ± 0.25 ml, P = 0.01)
and Vt/BW (+68%; 0.81 ± 0.04 vs. 0.48 ± 0.07 ml/100 g, P < 0.01; Fig.
2). Fr was
unaffected by repeated hormonal injections [92.1 ± 5.0 vs.
92.3 ± 4.0 breaths/min; not significant (NS)]. When exposed to
hypoxia, the gain of the ventilatory response, defined as the
difference between the values of respiratory parameters after 20 min of
hypoxia and the normoxic values, was more elevated in treated females,
for E100 (+51%; 72.3 ± 5.9 vs.
47.8 ± 11.4 ml · min1 · 100 g1, P < 0.05) but not for
Fr (52.3 ± 7.9 vs. 35.1 ± 6.3 breaths/min, P = 0.07), Vt
(0.73 ± 0.23 vs. 0.72 ± 0.21 ml; NS), or Vt/BW
(0.24 ± 0.08 vs. 0.22 ± 0.07 ml/100 g; NS).
Effects of domperidone on minute ventilation and HVR after hormonal
treatment.
After domperidone injection in treated females, there was a slight
decrease of E100 (17%; 61.7 ± 3.3 vs. 74.2 ± 4.2 ml · min1 · 100 g1, P < 0.05) and
Fr (13%; 80.1 ± 1.3 vs. 92.1 ± 5.0 breaths/min, P < 0.05). Vt and
Vt/BW were not affected by domperidone [2.33 ± 0.14 vs. 2.42 ± 0.15 ml (NS) and 0.77 ± 0.05 vs. 0.81 ± 0.04 ml/100 g (NS)], and domperidone had no effect on the HVR
after hormonal treatment (Fig. 3). A
negative correlation between the values in normoxia after saline
injection and the effect of domperidone was also apparent after
hormonal treatment for E100
(r2 = 0.62, P < 0.05),
Vt/BW (r2 = 0.72;
P < 0.01), and Fr
(r2 = 0.99, P < 0.0001;
Fig. 4B).
High-Altitude Studies
Catecholaminergic activity in the carotid bodies.
In intact animals, all indexes of catecholaminergic activity (TH
activity, amine content, and utilization rate index) were lower
(between 30 and 60%) in females than in males (Table
1). Ovariectomized females had higher TH
activity (+129%, P < 0.01) and NE content (+55%,
P < 0.01) than intact ones, whereas DA content was
unaffected by ovariectomy (14%, NS, Table 1). The calculated index
of total catecholamine utilization was higher (+88%, P < 0.01) in ovariectomized females than in intact ones. The index of
individual catecholamine utilization revealed that DA utilization was
enhanced (+150%, P < 0.001), while NE utilization was
left unchanged (+20%, NS, Table 1). In males, the only effect observed after orchidectomy was a lower DA content (31%, P < 0.0001) vs. intact males. As a result of this differential effect of
gonadectomy in males and females, no difference in the metabolism of
catecholamine was observed in the carotid body between neutered males
and females (Table 1).
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Resting minute ventilation.
Ovariectomized females weighed more (+19%) than intact females
(209 ± 6 vs. 176 ± 6 g, P < 0.001)
but had a lower (16%) Vt (1.46 ± 0.08 ml) than
control females (1.72 ± 0.10 ml, P < 0.05), which resulted in a reduction (28%) of Vt/BW (0.70 ± 0.04 vs. 0.98 ± 0.10 ml/100 g, P < 0.001;
Fig. 5) and
E100 (29%; 101 ± 10 vs.
143 ± 11 ml · min1 · 100 g1, P < 0.01; Fig. 5) compared
with intact females. Fr was not different between intact (148 ± 9 breaths/min) and ovariectomized females (142 ± 8 breaths/min). In males, the effect of gonadectomy was limited to Vt, castrated males having lower (11%)
Vt than intact ones (1.71 ± 0.04 vs. 1.93 ± 0.06 ml, P < 0.05), but the relation of Vt
to BW was maintained (0.71 ± 0.03 vs. 0.73 ± 0.02 ml/100 g, respectively; NS; Fig. 5). Therefore, the changes in
Vt seem to be directly related to changes in BW in neutered
males compared with control (10%; 240 ± 5 vs. 264 ± 8 g, P = 0.001; Fig. 5).
HVR.
Ovariectomized females had a lower ability to increase the absolute
value of E100 under acute hypoxic
exposure (150 ± 22 ml · min1 · 100 g1) compared with all other groups (intact
females, 259 ± 15; intact males, 234 ± 20; neutered males,
221 ± 20 ml · min1 · 100 g1, P = 0.01; Fig.
6). The gain of the response (difference
between normoxia and hypoxia) in ovariectomized females (50 ± 16 ml · min1 · 100 g1) was
also decreased compared with other groups (intact females, 116 ± 16; intact males, 125 ± 14; neutered males, 121 ± 13 ml · min1 · 100 g1,
P < 0.01). The decrease of HVR was mediated by a
reduced ability to increase Vt under acute hypoxic exposure
in ovariectomized females (Fig. 6).
Hematological parameters. Hct and Hb concentration were not different between control (51.2 ± 0.9% and 18.1 ± 0.2 g/dl) and neutered males (53.5 ± 1.8% and 18.8 ± 0.7 g/dl; NS for both values). However, ovariectomized females had higher Hct (53.1 ± 1.9%) and Hb concentration (18.9 ± 0.6 g/dl) than control females (44.8 ± 1.0% and 15.6 ± 0.3 g/dl, respectively; P < 0.001; Fig. 5).
Cardiac hypertrophy.
No difference in cardiac weight was observed between control males and
females (Table 2). In males gonadectomy
resulted in an increased heart weight-to-BW ratio (+27%;
P < 0.05) and (LV + S)-to-BW ratio (+23%;
P < 0.05, Table 2), but no RV hypertrophy appeared in
neutered males compared with control (Table 2). In females ovariectomy
resulted in a specific hypertrophy of the RV: RV-to-(LV + S) ratio
was 40% higher in ovariectomized than in control females
(P = 0.0001, Table 2).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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The main finding of this study is that in ovariectomized females raised at sea level, a peripheral dopaminergic drive inhibits ventilation under normoxia or acute hypoxia and that this inhibitory drive disappears after repeated injections of ovarian steroids for 10 days. This mechanism appears to be physiologically relevant for female rats at high altitude: DA utilization index in carotid bodies was 2.5 times higher in ovariectomized females compared with control. This was associated with a deacclimatization process in ovariectomized females living at high altitude, i.e., lower minute ventilation, higher Hct, higher Hb concentration, and right heart hypertrophy. This finding is strikingly similar to the clinical observations associated with menopause in high-altitude women (16, 17).
Methodological Considerations
At sea level, we used ovariectomized females as control animals to study the effects of hormonal injections on peripheral dopaminergic influence on breathing and HVR. The dose of domperidone that we used (1 mg/kg) has already been shown to be effective (9) and corresponded to the recommendations made by the provider of the drug. The doses for hormonal treatment were also chosen with regard to previous works and recommendations. The corresponding effects on BW, resting minute ventilation, and HVR were as expected.The high-altitude study was carried out in a strain of rats that have been bred at high altitude for at least 15-20 generations (Bolivian Institute for High-Altitude Biology, La Paz, 3,600 m, mean Pb = 495-500 mmHg). A complete study concerning these animals and some physiological and neurochemical changes that characterized them has been previously published (13). In the present study, we used a chronic model of hormonal deficiency; orchidectomy and ovariectomy were performed in 1-wk-old rats, and the effect of the surgery was assessed in 3-mo-old animals. No hormonal titration was done after death; however, various points indicated that the surgery resulted in a chronic hormonal deficiency in those animals. Males were checked during growth to verify the absence of external genital organs, and females were checked during dissection to verify the absence of the internal genital tract. Furthermore, lower BW in neutered males vs. controls and higher BW of ovariectomized females vs. controls underlined chronic hormonal deficiency. A confounding point of the design of this study is that control females were not checked for period of the estrous cycle; therefore, the exact hormonal status of these animals cannot be identified. Nevertheless, the estrous cycle in rats is of short duration (4 days) compared with the typical time course of the hormonal ventilatory effects, which is usually around 7-10 days (23). According to this discrepancy, the control females may be considered as being in a state of chronic hormonal stimulation, and possible short-term effects are likely to be minor compared with the long-term hormonal effects.
Ovarian Steroids Modulate an Inhibitory Dopaminergic Tone in the Carotid Bodies to Stimulate Breathing
DA, which is synthesized and stored in carotid body type I cells, is recognized as the most abundant and potent neuromodulator influencing global nerve ending response under hypoxemic challenge. When released from carotid body type I cells under hypoxemic stimulation, DA acts through low-affinity excitatory postsynaptic and high-affinity inhibitory presynaptic D2 receptors (7). Despite this discrepancy, in physiological conditions DA is recognized as a potent inhibitor of resting ventilation and HVR, as increase of catecholaminergic activity in carotid body type I cells has been related to an inhibition of HVR, both under acute and chronic hypoxic exposure (7, 12, 27).In the present study, we used TH activity as a direct reflection of catecholamine biosynthesis rate, as has been previously done (12, 13). Nevertheless, the carotid body is also a noradrenergic structure, and NE released by glomic cells and by sympathetic nerve endings under hypoxemic stimulation is active on hypoxic chemosensitivity (6, 7, 18). Therefore, a complete understanding of the catecholaminergic modifications after gonadectomy needed a better discrimination between dopaminergic and noradrenergic metabolisms. For this reason, we calculated distinct indexes of utilization for DA and NE in the carotid body. They are expressed as percentage of specific catecholamine synthesized per hour. If we assume that we are in steady conditions, this specific index of synthesis is similar to the utilization of the amine in the structure, i.e., the turnover rate.
Domperidone, a highly selective peripheral D2-dopaminergic antagonist, has been reported as a tool to assess the dopaminergic tone on carotid sinus nerve activity under normoxia or hypoxia (10, 28, 29) and has also been used to study in vivo the peripheral dopaminergic tone on the control of breathing (9, 25, 26). In the present study, the inhibitory effect of repeated ovarian steroid injections in ovariectomized females on domperidone-induced hyperventilation demonstrates that ovarian steroids are able to abolish the dopaminergic inhibitory influence on breathing. At high altitude, the neurochemical results are consistent with a specific control of dopaminergic metabolism in the carotid bodies by ovarian steroids.
Evidence for central mechanisms in the control of breathing after
chronic hormonal treatment has also been reported
(2-4), and some of our results after the hormonal
treatment at sea level may also be explained by a central effect and/or
by changes affecting other neurochemical processes in the carotid
bodies (for example, the enhanced hypoxic response of
E100 after hormonal treatment was not
affected by domperidone). Nevertheless, the elimination of the
peripheral dopaminergic inhibitory drive on breathing after hormonal
treatment clearly shows that ovarian steroids are able to act on
peripheral dopaminergic metabolism to stimulate breathing.
Perspectives
In the present study, the hematological and ventilatory data obtained in neutered females are strikingly similar to the findings of F. León-Velarde and coworkers (16, 17) in high-altitude postmenopausal women (increased Hct, impaired ventilation, and lower O2 arterial saturation). According to these results, it may be hypothesized that the sequence leading to CMS in postmenopausal women at high altitude includes a gradual increase in carotid body TH activity and DA utilization after menopause. This increased dopaminergic activity in the carotid bodies limits the efficiency of the chemosensitive hypoxic stimulation on ventilation. As a result, a more severe hypoxemia appears, the Hct rises, and pulmonary hypertension and right heart hypertrophy are enhanced. The conditions for development of CMS in susceptible women are no longer masked by the protective effects of ovarian steroids, and the occurrence of the disease increases in high-altitude postmenopausal women (16, 17).This mechanism may also explain the alveolar hypoventilation (15) and the low hypoxic peripheral chemosensitivity (22) associated with CMS in men, and it can be hypothesized that these ventilatory disturbances are linked to an elevated dopaminergic metabolism in the carotid body. Accordingly, a possible strategy for therapy of CMS in high-altitude residents may aim at limiting the dopaminergic neuromodulation of carotid body chemosensitivity as has already been done in animals (9, 25) and men (11) with domperidone.
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ACKNOWLEDGEMENTS |
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This work was supported by grants from Centre National de la Recherche Scientifique, Ministère des Affaires Etrangères, French Embassy in Bolivia (Projet International de Coopèration Scientifique no. 492 Bolivia), and from Région Rhône-Alpes (souffrance fetale et maturation neuronale). Janssen-Cilag provided pharmacological compounds (domperidone). V. Joseph successively held a fellowship from the French Ministère de l'Enseignement Supérieur et de la Recherche and from Janssen Research Foundation.
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FOOTNOTES |
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Address for reprint requests and other correspondence: Jean Marc Pequignot, Laboratoire de Physiologie des Régulations Energétiques, Cellulaires et Moléculaires, CNRS, Université Claude Bernard, UMR 5123, F-69373 Lyon, France (E-mail: pequigno{at}rockefeller.univ-lyon1.fr).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
10.1152/ajpregu.00398.2001
Received 10 June 2001; accepted in final form 18 October 2001.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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P < 0.05, hypoxia vs.
normoxia.
