In anesthetized rats, increases in phrenic nerve amplitude and frequency during brief periods of hypoxia are followed by a reduction in phrenic nerve burst frequency [posthypoxia frequency decline (PHFD)]. We investigated the effects of chronic exposure to hypoxia on PHFD and on peripheral and central O2-sensing mechanisms. In Inactin-anesthetized (100 mg/kg) Sprague-Dawley rats, phrenic nerve discharge and arterial pressure responses to 10 s N2 inhalation were recorded after exposure to hypoxia (10 ± 0.5% O2) for 6-14 days. Compared with rats maintained at normoxia, PHFD was abolished in chronic hypoxic rats. Because of inhibition of PHFD, the increased phrenic burst frequency and amplitude after N2 inhalation persisted for 1.8-2.8 times longer in chronic hypoxic (70 s) compared with normoxic (25-40 s) rats (P < 0.05). After acute bilateral carotid body denervation, N2 inhalation produced a short depression of phrenic nerve discharge in both chronic hypoxic and normoxic rats. However, the degree and duration of depression of phrenic nerve discharge was smaller in chronic hypoxic compared with normoxic rats (P < 0.05). We conclude that after exposure to chronic hypoxia, a reduction in PHFD contributes to an increased duration of the acute hypoxic ventilatory response in anesthetized rats. Furthermore, after exposure to chronic hypoxia, the central network responsible for respiration is more resistant to the depressant effects of acute hypoxia in anesthetized rats.
- carotid body
- phrenic nerve discharge, acute hypoxia
in many respects, the ventilatory adaptation to chronic hypoxia is similar in humans and rats (14) and the rat has become a popular model for investigating the mechanisms that contribute to hypoxic adaptations (1-7, 9, 10, 13, 17, 18). However, the results of these studies are contradictory in that after exposure to chronic hypoxia various groups have reported either an attenuation (13, 18) or an enhancement (1, 7) of the respiratory response to acute hypoxia. Any number of factors could contribute to these discrepancies: the duration and degree of the hypoxic exposure, the nature of the acute hypoxic stimulus, the use of conscious or anesthetized preparations or the particular anesthetic being used, whether the hypoxia occurs under poikilocapnic or isocapnic conditions, blood gas levels at the moment of testing, and the specific rat strain being studied.
Studies in rats have shown that after an acute hypoxic stimulus, the initial increase in ventilation is followed by a pronounced and prolonged decrease in respiratory frequency termed posthypoxic frequency decline (PHFD; for reviews, see Refs. 6, 16). During PHFD, respiratory frequency actually falls below the prehypoxic control level. The mechanisms that underlie PHFD have yet to be fully resolved (2, 4, 6). It has been shown that PHFD is attenuated after repeated exposure to brief periods of hypoxia (2).
No previous studies have examined the effects of chronic hypoxia on the PHFD component of the response to acute hypoxia. Therefore, the present studies were undertaken to examine the effects of chronic hypoxia on the response to acute hypoxia in the presence and absence of carotid sinus nerve afferent inputs to the central nervous system. We hypothesized that PHFD would be attenuated after chronic exposure to hypoxia. Preliminary reports of this work have been published (11, 12).
Experiments were performed on 78 male Sprague-Dawley rats (Charles River) weighing 350-550 g. All animals were given at least 1 wk to acclimate in the vivarium before being placed in a study group. All procedures were approved by the Institutional Animal Care and Use Committee.
Chronic hypoxic exposure. Rats were divided into two groups: normoxic controls (n = 46) and rats maintained in a hypoxic environment (n = 32). Hypoxic rats were housed in their home cages within a normobaric hypoxia chamber at oxygen levels of 10 ± 0.5% for 6-14 days. The air within the chamber was recycled, and the percentage of oxygen within the chamber was investigator controlled by a computer-driven set of valves and pumps. An hypoxic environment was achieved by the addition of nitrogen gas, and a normoxic environment was maintained by addition of 100% oxygen or room air. Oxygen levels within the chamber were monitored with an electrochemical sensor, and this information completed the computer-driven feedback circuit so that deviations in the oxygen level from those desired were rapidly corrected by addition of the appropriate gas. Temperature and humidity were monitored, and the recycled air was passed through a desiccant and CO2 scrubber. Control rats were maintained in a second chamber at oxygen levels of 21 ± 0.5% or in the same room outside of the chambers. There were no differences in the respiratory responses to brief periods of anoxia between chamber-housed or non-chamber-housed normoxic rats, so the two were grouped together.
Surgical preparation. On the day of the experiment, the rat was brought into the laboratory and anesthetized with Inactin (thiobutabarbital sodium, Sigma, 100 mg/kg ip). The adequacy of anesthesia was monitored by the stability of blood pressure, heart rate, and respiration under control conditions and during a pinch of the hindpaw, and supplemental Inactin (10 mg iv) was administered as necessary.
A femoral artery and vein were cannulated for measurement of arterial pressure and injection of drugs, respectively. Mean arterial pressure and heart rate were derived from the pulsatile signal using a blood pressure processor (Coulbourn Instruments). The trachea was cannulated, the vagus nerves cut bilaterally, and the rat was paralyzed (gallamine triethiodide, 25 mg/kg iv, followed by 15-20 mg/kg as needed, Sigma) and artificially ventilated using room air. End-tidal CO2 in the expired gas was measured (Capstar-100, CWE), and respirator rate was set so that the Pco2 of the paralyzed animal was within 1-2 Torr of that measured before paralysis. This interim setting was maintained until the determination of apneic threshold (described below). Rectal temperature was monitored and maintained at 37 ± 1°C by a ventral heating pad. A phrenic nerve was isolated and cut and the central end was desheathed, placed on bipolar recording electrodes, and covered with a mixture of mineral oil and petroleum jelly. Phrenic nerve discharge was amplified, filtered (30 Hz-3 kHz) and passed through a moving averager (time constant = 100 ms; CWE).
In 8 normoxic and 11 hypoxic rats, the carotid sinus nerves (CSNs) were isolated bilaterally and freed from connective tissue. The CSNs were sectioned bilaterally, and the CS regions were treated with phenol. After CS denervation, a period of 60-90 min was allowed to elapse before further study.
Respiratory frequency was measured by counting spontaneous respiratory movements for 1 min during a resting period in the normoxic or hypoxic chambers before that rat was brought to the laboratory, then again after anesthesia and vagotomy. Apneic threshold was determined by increasing ventilator frequency until phrenic discharge was abolished. During the experiment, ventilator frequency was set to maintain end-tidal CO2 2-7 Torr above the apneic threshold. Steady-state changes in Pco2 were accomplished by changing ventilator frequency or by adding CO2 gas to the inspired gas.
A timer-controlled valve was used to switch the inspired gas from room air to 100% nitrogen. A 10-s duration of N2 inhalation was chosen after preliminary trials with periods ranging from 3 to 15 s because this duration of stimulation was sufficient to evoke a reproducible response but not so severe as to compromise the stability of the preparation.
Data analysis. The frequency of phrenic burst discharge (Pfr) was measured by counting the number of integrated bursts per unit time. Pfr was expressed as an absolute value and was also normalized as a percentage of the baseline, considered 100%. The peak amplitude of a burst of integrated phrenic nerve discharge (Pam) was calculated two ways, each being used in previous analyses of integrated phrenic discharge. In the first method, the level of phrenic discharge obtained at the apneic threshold was considered zero and the 100% phrenic discharge amplitude was measured during addition of CO2 to the inspired gas mixture. Pam during rest and N2 inhalation were then calculated as lying between the zero and 100% level (10). In the second method, the Pam recorded during the baseline period before N2 inhalation was considered 100% and increases in Pam during N2 inhalation were expressed as a percentage of baseline (2, 7). The product of Pfr and Pam was calculated as an index of neural “minute ventilation.” Both Pfr and Pam were averaged during the 15 s before N2 inhalation to obtain the baseline (time 0) value during 5-s periods beginning at 5, 10, 15, 20 s and during 10-s periods at 30, 40, 70, 100, 190, 240, and 615 s after the beginning of the 10-s period of N2 inhalation. N2 inhalation tests were separated by at least 10 min.
All parameters (blood pressure, raw and integrated phrenic nerve discharge, end-tidal CO2) were digitized and stored on a personal computer using an analog-to-digital system (Cambridge Electronics Design) and associated software (Spike2). Two- and one-way ANOVA were used to test the significance of differences between and within normoxic, hypoxic, and CSN-denervated groups. Tests of percentile data were performed after logarithmic transformation of the data. Post hoc analyses were performed with a Student-Newman-Keuls test, and differences were considered significant at the P < 0.05 level. Data are presented as means ± SE.
Rats in the hypoxia chamber lost 7-10% of their body weight during the first 3-4 days of hypoxic exposure. There was a gradual recovery so that by the day of the experiment (6-14 days) body weight was 2-8% less (n = 24) or slightly more (n = 8) than the prehypoxic weight.
When measured after 6-14 days in the chambers, hypoxic rats had a higher respiratory frequency than normoxic rats (162 ± 6 vs. 96 ± 2 breaths/min, respectively, P < 0.05). Once transferred to the laboratory, anesthetized, vagotomized, and spontaneously breathing room air, the Pfr was less in hypoxic than in normoxic rats (50 ± 2 vs. 65 ± 2 bursts/min, respectively, P < 0.05). Responses to acute hypoxia were obtained 3-9 h after removing the rats from the chambers, and no significant difference in the response to acute hypoxia at the beginning of the experiment compared with the response obtained at the end of the experiment was observed in chronic hypoxic or normoxic rats.
Apneic threshold was 23.3 ± 0.8 and 18.0 ± 0.7 Torr Pco2 for normoxic and hypoxic rats, respectively (P < 0.05). The Pam was very dependent on the level of Pco2, but did not vary much between rats within the range of 2-7 Torr above the apneic threshold. The Pfr response to N2 inhalation normalized to baseline in normoxic rats at 2-7 (46 tests) and 8-20 (11 tests) Torr above apneic threshold was not different (P < 0.05).
Carotid sinus nerve-intact rats. In our preparation, the respiratory response to 10 s N2 inhalation began after a 3- to 4-s latency. Figure 1 illustrates responses in normoxic rats (A and B) and in hypoxic rats (C and D). The response onset latency was the same in hypoxic and normoxic animals. At the onset of the response, Pfr and Pam increased rapidly in both normoxic and hypoxic rats. Occasionally (n = 15 rats), the duration of the first 1-2 bursts of the phrenic nerve response was increased, but this was not observed in all tests in the same animal. The initial period of excitation after N2 inhalation was occasionally interrupted by one or two short periods where phrenic nerve discharge was absent (Figs. 1, A and B and 6A1, arrows), but this was not observed in all tests in the same animal.
In normoxic rats, the initial increase in Pfr after N2 inhalation was followed by a dramatic decrease in Pfr to well below the preinhalation level of Pfr (Fig. 2). The maximal increase in Pfr measured between 10 and 15 s after the onset of N2 inhalation was 133 ± 4%, followed by a reduction in Pfr to 67 ± 4% between 20 and 25 s. PHFD was evident in all normoxic rats. After N2 inhalation, in normoxic rats Pfr was significantly different from the baseline value at t = 0 from 5 to 240 s after the onset of N2 inhalation (Fig. 2A).
In chronic hypoxic rats, the increase in Pfr after N2 inhalation was not followed by PHFD (Fig. 2A). In chronic hypoxic rats, Pfr was significantly different from the baseline value at time 0 from 5 to 15 s after the onset of N2 inhalation. Resting Pfr at time 0 was less in chronic hypoxic than in normoxic rats (Fig. 2A, P < 0.05); therefore changes in Pfr were also expressed as percentages relative to baseline. The percentage increase in Pfr after N2 inhalation was greater in the hypoxic compared with normoxic rats and was not followed by PHFD (Fig. 2B). In hypoxic rats, the maximal increase in Pfr measured 10-15 s after the onset of N2 inhalation was 145 ± 4%, and Pfr was 111 ± 3% at 20-25 s, when PHFD was most marked in the normoxic rats. The difference in Pfr between normoxic and hypoxic rats was significant at 0, 5, 10, 20, and 100-615 s in the raw data (Fig. 2A) and from 10 to 615 s in the normalized data (Fig. 2B).
When averaged for the whole populations, baseline Pfr was lower in the hypoxic compared with the normoxic group, as previously discussed. Attempts to normalize Pfr by adding CO2 to the room air inspired by the hypoxic rats did not increase Pfr to the level observed in normoxic rats. Therefore, to determine if the absence of PHFD in chronic hypoxic rats was the result of differences in baseline Pfr, chronic hypoxic rats with baseline Pfr larger than the mean baseline Pfr (“high-frequency” hypoxic subgroup) were compared with normoxic rats with a baseline Pfr lower than the mean value in the group (“low-frequency” normoxic subgroup). The baseline Pfr in the low-frequency normoxic subgroup (56 ± 2 bursts/min, n = 18) was not different from the baseline Pfr in the high-frequency hypoxic subgroup (58 ± 1 bursts/min, n = 18; P > 0.05). In the normoxic low-frequency subgroup, PHFD was similar in degree and duration to that observed in the whole normoxic population, and PHFD was not observed in the hypoxic high-frequency subgroup (Fig. 3). Pfr in the hypoxic high-frequency subgroup was greater than that in the normoxic low-frequency subgroup between 15 and 240 s when expressed as an absolute number (Fig. 3A) and at 5 and 15-240 s when expressed as a percentage increase from baseline (Fig. 3B).
When expressed as a percentage increase relative to baseline, the increase in Pam after N2 inhalation was less in chronic hypoxic rats compared with normoxic rats between 5 and 25 s after N2 inhalation (Fig. 4A). However, expressing Pam as a scaled value between zero (apneic threshold) and 100% (CO2 inhalation) revealed a different picture. At rest, the baseline level of Pam at time 0 in chronic hypoxic rats was closer to 100% than normoxic rats (Fig. 4B; 61 ± 3 vs. 47 ± 2%, P < 0.05). After N2 inhalation, both normoxic and hypoxic rats increased Pam to 100%, suggesting that the apparently reduced response in Fig. 4A is a ceiling effect. Pam, expressed as a scaled value, was significantly different between normoxic and hypoxic rats at 0, 5, and 30-615 s after N2 inhalation.
After N2 inhalation, Pam, expressed as a percentage of baseline, was significantly elevated above the baseline value at time 0 at all time points up to and including 615 s in both the normoxic and hypoxic rats (Fig. 4A). After N2 inhalation, Pam, expressed as a scaled value, was significantly elevated above the baseline value at time 0 from 5 to 240 s after N2 inhalation in normoxic rats and from 10 to 100 s in hypoxic rats (Fig. 4B).
The product of Pfr and Pam provides an index of neural minute ventilation. As a result of the absence of PHFD in the hypoxic rats, this product was greater in hypoxic compared with normoxic rats between 15 and 70 s after N2 inhalation, when Pam was expressed as a scaled value and Pfr was expressed as an absolute number (Fig. 4C). The product was greater than the baseline value at time 0 from 5 to 25 s in normoxic rats and from 5 to 70 s in hypoxic rats. Therefore, the duration-increased product was 2.8 times longer in hypoxic compared with normoxic rats. When Pam was expressed as a scaled value and Pfr was expressed as a percentage of baseline, the product was greater in hypoxic compared with normoxic rats at all time points up to and including 615 s after N2 inhalation (Fig. 4D). Calculated in this manner, the product was greater than the baseline value at time 0 from 5 to 40 s in normoxic rats and from 5 to 70 s in hypoxic rats. Therefore, the duration-increased product was 1.8 times longer in hypoxic compared with normoxic rats.
The cardiovascular response to N2 inhalation was much more variable than the respiratory response. In most normoxic rats (n = 29), arterial pressure increased sharply (Figs. 1A, 5A, 6A, and 8, A and B) and, in many cases, exhibited an oscillatory behavior (Fig. 5B). In 10 normoxic rats, a fall in arterial pressure was observed. Regardless of whether arterial pressure rose or fell, the duration of the change in pressure was much shorter than the duration of the changes in Pfr and Pam. However, in seven normoxic rats after the initial peak increase in arterial pressure, pressure remained elevated for 50-200 s, followed by a gradual or sharp decline to the control level. Coincident with the steplike decline in arterial pressure was an increase in Pfr from 50 to 60 bursts/min in the example illustrated in Fig. 5A. After 10 s N2 inhalation, occasionally there were short periods where phrenic nerve discharge was absent (Figs. 1, A and B, and 6A1). These short “breaks” in phrenic nerve discharge were followed by rapid increases in arterial pressure. In hypoxic rats, the patterns of arterial pressure response to N2 inhalation were similar to those observed in normoxic rats, although the peak changes were smaller in the hypoxic group (Figs. 1B and 6B1) and no long-lasting or oscillatory responses were observed.
Carotid sinus nerve-sectioned rats. Baseline Pfr was reduced by bilateral CSN section and denervation of the CS region. In normoxic rats (n = 8) before CSN denervation, Pfr was 68 ± 3, whereas 60-90 min after the denervation Pfr was 56 ± 4 (P = 0.02). In hypoxic rats (n = 11) Pfr before CSN denervation was 45 ± 3, whereas after denervation Pfr was 37 ± 3 (P = 0.06).
After bilateral CSN denervation, the response to N2 inhalation was dramatically altered in both normoxic (n = 8) and hypoxic (n = 11) rats. After CSN denervation, N2 inhalation did not evoke an increase in Pfr or Pam, rather there was a reduction in both variables (Fig. 6). The response to N2 inhalation was significantly different comparing CSN-intact to CSN-denervated rats from 5 to 20 and from 40 to 240 s in normoxic rats (Fig. 7A) and from 5 to 30 s in hypoxic rats (Fig. 7B).
After CSN denervation, Pfr was significantly reduced between 15 and 30 s after the onset of the N2 inhalation in normoxic rats (Fig. 7A) and between 10 and 30 s in hypoxic rats (Fig. 7B). The maximal reduction in Pfr was observed between 20 and 25 s after the onset of the N2 inhalation in both normoxic and hypoxic rats and was less in hypoxic rats compared with normoxic rats. Between 20 and 25 s, Pfr was reduced by 74 ± 11% in normoxic rats and by 26 ± 9% in hypoxic rats (P < 0.05).
The results of these studies further support the concept that during chronic exposure to hypoxia adaptations occur in the ventilatory response to acute hypoxia. Six to fourteen days in an hypoxic environment abolished the PHFD normally observed after acute hypoxia. This resulted in a two- to threefold increase in the duration of the increase in phrenic nerve discharge (product of Pfr and Pam) evoked by acute hypoxia. Furthermore, the depression of respiration during acute hypoxia in chemoreceptor-denervated rats was reduced after chronic exposure to hypoxia, suggesting that adaptations render the relevant central nervous system network(s) more resistant to the depressant effects of hypoxia.
Comparison of our responses to acute hypoxia in anesthetized rats with those obtained in conscious rats (5) indicates that the responses are qualitatively similar. In the conscious rat, inhalation of 8% O2 for 45 s evoked a 47% increase in Pfr, followed by a 32% reduction relative to baseline during PHFD. In our anesthetized preparation, the absolute value of Pfr was reduced, nonetheless inhalation of 100% N2 for 10 s evoked a 33% increase in Pfr and a 33% reduction relative to baseline during PHFD.
Consistent with our results, chronic hypoxia has been shown to lower the Pco2 level at which apnea occurs, the “apneic threshold” (7). Therefore, we performed experiments from a set level above the apneic threshold in normoxic and hypoxic rats. With this approach, we observed a significant increase in the peak frequency response of the phrenic nerve to acute hypoxia after chronic hypoxia when Pfr is expressed as a percentage change from baseline to normalize differences in baseline levels (Fig. 2B). The peak increase in Pam evoked by N2 inhalation, expressed as a percentage change from baseline, was reduced after chronic hypoxia (Fig. 4A). However, when scaled to a value between the maximum and minimum determined in each rat, the peak increase in Pam evoked by N2 inhalation was not altered after chronic hypoxia. Rather, both normoxic and hypoxic rats reached their respective 100% level (Fig. 4B). If Pam is roughly analogous to tidal volume, the finding that conscious, chronic hypoxic rats exhibit an elevated tidal volume at rest (1) suggests that expressing Pam as a scaled value provides a result more consistent with data obtained from conscious rats than that obtained when Pam is expressed as a percentage of baseline.
PHFD. PHFD has been described in anesthetized (2, 4, 10) and conscious (5) rats. The mechanism responsible for this phenomenon remains under investigation and pontine centers have been implicated (4, 6). It has been suggested that PHFD and the posthypoxic increase in phrenic burst amplitude, termed short-term potentiation (16), are mediated by neural rather than metabolic mechanisms (6). Our data, as well as that from other studies (2, 4, 10) in artificially ventilated rats under isocapnic conditions, demonstrate that PHFD is not the result of a hyperventilation-induced fall in Pco2. PHFD and short-term potentiation have been proposed to stabilize ventilation after brief periods of hypoxia. Brief repetitive bouts of hypoxia (2 or 3 5-min episodes of isocapnic hypoxia, separated by 30 min) reduce PHFD (2), indicating some degree of plasticity in this component of the response to acute hypoxia. We found that chronic hypoxia essentially abolished PHFD.
Our measures of resting Pfr when the rats were in the chambers indicate that, as expected, chronic exposure to hypoxia results in tachypnea. However, when brought up to the laboratory anesthetized and vagotomized, the resting Pfr of the hypoxic rats was less than that of the normoxic rats. This could reflect the fact that once in the laboratory, the animals were in a hyperoxic environment compared with their previous time in the hypoxia chamber. Alternatively, the reduced Pfr observed in the hypoxic rats when anesthetized and in the laboratory could be the result of PHFD on return to normoxia. Comparison of subgroups of hypoxic and normoxic rats with similar resting Pfrs found that PHFD was still absent in the hypoxic group (Fig. 3). This suggests that the absence of PHFD in the hypoxic rats was not related to baseline Pfr. It also suggests that PHFD does not play a significant role in determining resting Pfr.
Chronic hypoxia enhanced the increase in Pfr after a brief (10 s) N2 inhalation and markedly attenuated or abolished PHFD compared with normoxic rats. When quantified as a variable percentage for each animal, the baseline level of Pam in chronic hypoxic rats was much closer to the 100% maximal level of phrenic nerve discharge achieved by CO2 inhalation. Although the caveats of quantitative comparisons of whole nerve discharge between groups of animals should be considered, this is consistent with previous reports of increased tidal volume after chronic hypoxia in conscious rats (1, 14).
The response to acute hypoxia appears to consist of an initial excitatory phase where Pfr and Pam increase, followed by PHFD. In hypoxic rats the increase in Pfr induced by N2 inhalation is increased for a longer time than in normoxic rats. This suggests that there is temporal overlap between the excitatory phase and PHFD. Our data cannot discern if chronic hypoxia directly alters the mechanisms underlying the initial excitatory phase or if the increase in the peak Pfr (Fig. 2B) and the duration of increased Pfr is due to the reduction in PHFD.
When expressed as a scaled value, Pam was increased at rest in hypoxic compared with normoxic rats. During the initial excitatory phase (5-20 s), Pam was not different between the two groups. However, Pam in hypoxic rats was elevated compared with normoxic rats at all other time points after the initial excitatory phase. Due to the absence of PHFD and the increased Pam, the product of Pfr and Pam after N2 inhalation remained elevated for a significantly longer period in chronic hypoxic rats than in normoxic rats. This is consistent with previous studies demonstrating that under isocapnic conditions the enhanced hypoxic ventilatory responses of hypoxic rats (1) and goats (8) are the result of alterations in both respiratory frequency and tidal volume.
The cardiovascular response to N2 inhalation can best be described as variable. This is likely a reflection of sympathoexcitatory drive to vascular smooth muscle and direct depressant effects of hypoxia on the vasculature. It was noted that after 10 s N2 inhalation there were short periods where phrenic nerve discharge was absent (Figs. 1A and 6A1). These short “breaks” in phrenic nerve discharge were followed by rapid increases in arterial pressure. We observed a similar phenomenon during electrical stimulation of the CSN (O. Ilyinsky and S. Mifflin, unpublished observations), suggesting that the breaks and increases in blood pressure were not due to central nervous system hypoxia.
Responses after carotid denervation. In agreement with Cardenes and Zapata (3), we found that acute bilateral CS denervation combined with vagotomy completely blocked the excitatory component of the phrenic nerve response to N2 inhalation. However, these authors did not observe the decrease in Pfr during N2 inhalation in denervated rats seen in the present study. Our observations also confirm a recent report (5) that after bilateral CS denervation, acute hypoxia resulted in a reduction in Pfr. These authors considered this inhibition the same as PHFD; however, they did acknowledge that it is difficult to determine if PHFD and the inhibition of Pfr observed after removal of peripheral arterial chemoreceptors are mediated by similar mechanisms. These authors also reported that CSN denervation reduced baseline ventilation, an effect observed in the present study.
The inhibition of Pam and Pfr after N2 inhalation in bilateral CS-denervated rats was significantly smaller in chronically hypoxic rats compared with normoxic rats. The smaller inhibition observed after N2 inhalation in CS-denervated chronic hypoxic rats suggests that during chronic hypoxia, adaptations occur making the respiratory network more resistant to the depressant effects of hypoxia (e.g., increases in brain vascularization, increased metabolic capacity, and/or oxygen transport in neurons).
In contrast to the variable nature of the cardiovascular response to N2 inhalation in CS-intact rats, in CS-denervated rats N2 inhalation invariably resulted in dramatic falls in blood pressure. Presumably, CS denervation eliminated the sympathoexcitatory component of the response leaving the direct depressant effects of hypoxia on the vasculature unopposed.
Integrated response. The phrenic nerve response to brief N2 inhalation consists of an initial excitatory response characterized by an increase in Pfr and Pam, followed by PHFD consisting of a reduction in Pfr, while Pam remains increased with Pfr and Pam returning to baseline levels within several minutes. It is not known if the brief but strong inhibition of phrenic nerve discharge revealed when peripheral chemoreceptors are eliminated by CSN section contributes to PHFD or is only manifested in the absence of arterial chemoreceptors. Chronic hypoxia slightly increased the initial excitatory component of the phrenic nerve response and dramatically reduced both PHFD and the inhibition observed after peripheral chemodenervation. This resulted in a much longer excitation of phrenic nerve discharge in response to acute hypoxia in the chronic hypoxic rats.
Our experimental approach provided precise temporal control of the stimulus and temporal resolution of the phrenic nerve response. Although our approach is artificial, it may provide some insights into the responses to acute and chronic hypoxia in the conscious state. Studies of hypoxic ventilatory responses in the conscious state typically measure ventilation in the steady state and average measured parameters over several minutes (1, 8). During acute and chronic exposures to hypoxia, the excitatory drive to the respiratory network and the mechanisms that mediate PHFD may overlap temporally so that inhibition modulates the degree and duration of excitation. Whether the direct depressant effects of hypoxia, as revealed in CS-denervated animals, also contribute to PHFD in CS-intact animals and modulate the excitatory phase is unknown. The abolition of PHFD in chronically hypoxic rats could be an important factor in mediating the enhanced hypoxic ventilatory response after chronic hypoxia. It has been suggested that the enhanced hypoxic ventilatory response after chronic hypoxia is due to an increase in excitatory synaptic mechanisms and/or a reduction in inhibitory synaptic mechanisms (15). More insight into the physiology and pharmacology of PHFD is necessary before we can determine if the abolition of PHFD in chronic hypoxic rats is due to alterations in excitatory and/or inhibitory synaptic mechanisms.
This work was supported by National Heart, Lung, and Blood Institute Grant HL-41894.
The authors gratefully acknowledge the technical assistance of M. Herrera-Rosales, O. Tolstykh, and M. Vitela. The authors also acknowledge the assistance of Dr. W. Morgan with the statistical analyses.
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.
- Copyright © 2003 the American Physiological Society