| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
1 The John Rankin Laboratory of Pulmonary Medicine, Department of Preventive Medicine, University of Wisconsin School of Medicine, Madison 53705-2368; 2 Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, Wisconsin 53706-1102; and 3 Division of Physiology, Department of Medicine, and White Mountain Research Station, University of California, San Diego, La Jolla, California 92093-0623
 |
ABSTRACT |
|---|
 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
We
tested the hypothesis that unanesthetized rats exhibit ventilatory
long-term facilitation (LTF) after intermittent, but not continuous,
hypoxia. Minute ventilation (
E) and carbon dioxide production (CO2) were measured in
unanesthetized, unrestrained male Sprague-Dawley rats via barometric
plethysmography before, during, and after exposure to continuous or
intermittent hypoxia. Hypoxia was either isocapnic [inspired
O2 fraction (FIO2) = 0.08-0.09 and inspired CO2 fraction
(FICO2) = 0.04] or poikilocapnic
(FIO2 = 0.11 and
FICO2 = 0.00). Sixty minutes after
intermittent hypoxia, E or
E/CO2 was
significantly greater than baseline in both isocapnic and poikilocapnic
conditions. In contrast, 60 min after continuous hypoxia,
E and
E/CO2 were not
significantly different from baseline values. These data demonstrate
ventilatory LTF after intermittent hypoxia in unanesthetized rats.
Ventilatory LTF appeared similar in its magnitude (after accounting for
CO2 feedback), time course, and dependence on intermittent
hypoxia to phrenic LTF previously observed in anesthetized,
vagotomized, paralyzed rats.
ventilation; plasticity; intermittent hypoxia; hypoxia; respiratory control
| |
INTRODUCTION |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
IN 1941 GESELL AND HAMILTON (14) electrically stimulated the carotid sinus (Hering's) nerve either continuously (for ~10 s) or intermittently (repeated short bursts during inspiration or expiration) of anesthetized, vagotomized dogs, and they followed spontaneous ventilation with a spirometer. In both instances, they observed a persistent hyperventilation, which they took as evidence for a "long enduring after-discharge." They did not emphasize the intermittent stimulation except to conclude that it had the capacity to establish new artificial ventilatory rhythms, and they did not follow the induced hyperpnea for more than a few minutes after stimulation. The unique importance of intermittent stimulation of peripheral chemoreceptors in causing a persistent poststimulus increase in respiratory motoneuron activity was not appreciated until after Millhorn et al. (22) described the effect of repeated carotid sinus nerve stimulation on phrenic nerve activity in anesthetized or decerebrate, vagotomized, paralyzed, artificially ventilated cats. Millhorn et al. described a long-duration (>1 h) facilitory effect on phrenic nerve activity after repeated carotid sinus nerve stimulation, an effect they originally termed "long-term potentiation" (22), and Eldridge and Millhorn subsequently referred to it as a "long-lasting facilitory memory" (9). It was further demonstrated that serotonin [5-hydroxytryptamine (5-HT)] is necessary for this prolonged stimulation of respiration (23). In recent years, this long-lasting respiratory memory has been referred to as long-term facilitation (LTF) (10, 15, 25).
Since the reports of Millhorn and colleagues (22, 23), LTF has been observed in a variety of species in a variety of conditions (3a, 11, 16a, 20, 28). LTF is more robust in some anesthetized animal preparations, including cats (10, 22) and rats (3, 12, 17), relative to some awake animal preparations, including goats (29) and ducks (24). However, a robust LTF was reported in awake dogs (7). McEvoy and colleagues (21) did not detect LTF in awake human subjects, although LTF occurs in some human subjects while asleep (2). In addition to potential effects of anesthesia and sleep-wakefulness state, sensory inputs may alter the expression of LTF. For example, LTF is minimized or absent in anesthetized spontaneously breathing cats with intact vagus nerves (19).
Most progress in understanding the mechanism of LTF has come from studies recording respiratory nerve activity in anesthetized, vagotomized, and artificially ventilated preparations. Given the importance of the anesthetized, vagotomized, paralyzed, and ventilated rat preparation to our understanding of the cellular and synaptic mechanisms underlying LTF (6, 11), and recent claims that LTF is lacking in spontaneously breathing anesthetized rats (16), we conducted experiments to determine whether ventilatory LTF can be observed in intact, unanesthetized, unrestrained rats. Furthermore, we investigated the dependence of ventilatory LTF on the degree of regulation of arterial CO2 during hypoxia (isocapnic vs. poikilocapnic hypoxia) and the pattern of hypoxia (intermittent vs. continuous).
| |
METHODS |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
The data reported were collected in two separate, collaborating laboratories, which will be designated as San Diego and Madison in instances when there are differences. Adult, male Sprague-Dawley rats were used (San Diego-Harlan, San Diego, CA; Madison-colony 236, Harlan Teklad, Madison, WI). Ventilatory and metabolic measurements were made using standard barometric plethysmography [San Diego (1); Madison (25)]. In selected rats, blood gases were measured in arterial blood obtained from chronically indwelling femoral artery catheters surgically placed at least 1 wk before sample withdrawal. Surgical, sampling, and analytic procedures for arterial blood-gas analysis have been described previously [San Diego (1); Madison (26, 27)].
Individual rats were placed within the plethysmograph chamber and
maintained in normoxia for at least 1 h. Baseline ventilatory and
metabolic measurements were then made. After the baseline measurements,
one of four hypoxia protocols was followed: 1) isocapnic intermittent hypoxia, 2) isocapnic continuous hypoxia,
3) poikilocapnic intermittent hypoxia, or 4)
poikilocapnic continuous hypoxia. Ventilatory measurements were made
continuously and metabolic rates [O2 uptake
(O2) and CO2 production
(CO2) were measured during baseline and
posthypoxia conditions].
Protocols
Isocapnic intermittent hypoxia. In San Diego, seven rats were exposed to the isocapnic intermittent hypoxia protocol, which consisted of five 3-min exposures to inspired O2 fraction (FIO2) = 0.08-0.09 and inspired CO2 fraction (FICO2) = 0.04, interspersed with four 2-min exposures to FIO2 = 0.50. Arterial blood drawn from five of the rats during the final hypoxic exposure had a PO2 = 38 ± 1 Torr (mean ± 95% confidence limits) and a PCO2= 38 ± 2 Torr. In Madison, seven rats underwent a slightly different isocapnic intermittent hypoxia protocol, which was five 5-min exposures to FIO2 = 0.09 and FICO2 = 0.04, interspersed with four 5-min exposures to FIO2 = 0.21. In San Diego, five of the seven rats exposed to isocapnic intermittent hypoxia were administered supplemental CO2 (FICO2 = 0.012-0.026) to reestablish isocapnia 60 min after intermittent hypoxia. Arterial blood drawn at this time from these five rats had a PO2 = 97 ± 7 Torr and a PCO2 = 41 ± 1 Torr.
Isocapnic continuous hypoxia. In San Diego, six rats were exposed to 25 min of continuous FIO2 = 0.09-0.10 and FICO2 = 0.04. In Madison, eight rats were exposed to 120-300 min of FIO2 = 0.10 and FICO2 = 0.04; the cumulative blood-gas experience for rats under these conditions in Madison is arterial PO2 (PaO2) = 50 ± 1 Torr and arterial PCO2 (PaCO2) = 39.9 ± 0.6 Torr (32 rats).
Poikilocapnic intermittent hypoxia. Twenty-four rats underwent five 5-min exposures to FIO2 = 0.11, interspersed with 5-min exposures to FIO2 = 0.21.
Poikilocapnic continuous hypoxia. Nine rats were exposed to 50-min of continuous FIO2 = 0.11.
In each of the four protocols, ventilatory and metabolic measurements were made continuously for 1 h after the end of hypoxia while the plethysmograph was normoxic.Determination of CO2
O2 and
CO2 using the difference in the ongoing
measurement of O2 and CO2 flowing into and out
of the chamber (25). In San Diego,
O2 and
CO2 were measured by sealing the
plethysmograph for 3-6 min and measuring the decrease in
FIO2 and increase in
FICO2 (~0.003) during this period by use of a mass spectrometer (1).
Nine rats were studied in both poikilocapnic continuous hypoxia and poikilocapnic intermittent hypoxia, and seven of these were later studied in isocapnic intermittent hypoxia. In these cases, at least 4 days were allowed between poikilocapnic continuous hypoxia and poikilocapnic intermittent hypoxia and at least 28 days between the poikilocapnic intermittent hypoxia and isocapnic intermittent hypoxia protocols. Sixteen rats were studied repeatedly (2-4 repetitions) using the same protocols: 12 rats in poikilocapnic intermittent hypoxia (some with 1 day recovery between sessions), and 4 other rats in isocapnic continuous hypoxia (at least 16 days recovery between sessions). Data for an individual rat were averaged when repetitive observations were made using the same protocol.
Data Analysis
Statistical analyses were performed using commercially available software (SigmaStat, Jandel Scientific, San Rafael, CA; EXCEL 2000, Microsoft). With one exception, two-way repeated-measures analysis of variance revealed no differences between the two laboratories in any ventilatory measurement. There was a significant difference between San Diego and Madison in minute ventilation (E)
[although not in tidal volume (VT) or frequency]
determined during isocapnic continuous hypoxia (P = 0.046). This was most likely due to the somewhat lower
FIO2 used in San Diego during hypoxia.
However, this difference during hypoxia did not result in any
differences between laboratories during the baseline period or during
the 1-h period of normoxia after hypoxia. Therefore, all data have been
pooled and are presented as average data.
Because each individual rat was studied during baseline conditions before hypoxia and during the hour after hypoxia, we present ventilatory responses after hypoxia as the percent change from baseline. Differences from baseline measurements were determined using one-way repeated-measures analysis of variance. Individual comparisons were made using the Bonferroni t-test (SigmaStat, Jandel Scientific). Although most data were normally distributed, a few instances where the normality test failed are noted.
Data are presented as mean ± 95% confidence limits (with
n indicating no. of rats) throughout. In cases where
arterial blood-gas analysis was performed, PaCO2 was
used to calculate alveolar ventilation (A).
The equation used was
A/CO2
= (870 Torr)/PaCO2.
| |
RESULTS |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
Blood-Gas Values
Ventilatory and metabolic rate measurements made in San Diego and Madison were equivalent. Therefore, all data have been pooled and are presented as average data. Blood-gas values were not obtained from the rats studied in Madison. In San Diego, arterial blood was obtained from seven rats during normoxic baseline conditions and in normoxia 60 min after isocapnic intermittent hypoxia. Because PaCO2 significantly decreased from normoxic baseline conditions (41.4 ± 1.4 Torr) to normoxia 60 min after isocapnic intermittent hypoxia (39.4 ± 1.1 Torr) (P < 0.005), intermittent hypoxia initiated a persistent hyperventilation, suggesting LTF. Five of these seven rats exposed to isocapnic intermittent hypoxia were administered supplemental CO2 60 min after intermittent hypoxia to bring their PaCO2 within 0.1 Torr below their original baseline PaCO2 (40.7 ± 1.5 Torr at baseline, n = 5, vs. 40.6 ± 1.0 Torr 60 min posthypoxia with supplemental CO2, n = 5). Four of the five rats had substantial increases inE after supplemental CO2, with an
overall average 29 ± 21% increase over the 60-min
poikilocapnic E (n = 5).
For the 42 rats studied, overall baseline measurements were breathing
frequency = 75 ± 3 breaths/min, VT = 0.52 ± 0.04 ml/100 g, E = 38 ± 3 ml · min
1 · 100 g1,
CO2 = 1.4 ± 0.1 ml · min1 · 100 g1, and
E/CO2 = 29 ± 2.
Isocapnic Intermittent Hypoxia
The stimulation ofE, frequency, and
VT during isocapnic intermittent hypoxia and for the hour
after isocapnic intermittent hypoxia is shown in Fig.
1. There is a significant, approximately fourfold increase in E during each of the five
intermittent periods of isocapnic hypoxia. The E
remained significantly elevated during the intervening periods with
either hyperoxia (San Diego) or normoxia (Madison). There are
equivalent contributions of breathing frequency and VT to
the increase in E during hypoxia, although breathing
frequency usually returned to levels that did not significantly differ
from baseline during the periods between hypoxic exposures. E remained elevated 10 and 60 min after the final
isocapnic episode (P < 0.05). At 60 min, the
E elevation was primarily due to an elevation in
frequency (P < 0.05), with no significant difference
in VT.
|
Isocapnic Continuous Hypoxia
Isocapnic continuous hypoxia (Fig. 2) also produced an approximately fourfold increase inE, which is sustained for as
long as the isocapnic hypoxia continues. Both breathing frequency and VT are significantly elevated during this period, with
breathing frequency making a somewhat greater contribution to
E during the initial phase. After exposure to
continuous isocapnic hypoxia, ventilatory measurements remained
significantly elevated from baseline for ~20 min but were not
significantly different from baseline levels for the remainder of the
60 min in normoxia.
|
Poikilocapnic Hypoxia
In contrast to isocapnic hypoxia, poikilocapnic hypoxia elicited a smaller increase in ventilation. There was a two- to threefold increase inE during exposure to poikilocapnic
hypoxia (Figs. 3 and
4).
|
|
Poikilocapnic Intermittent Hypoxia
During the poikilocapnic intermittent hypoxia,E was significantly elevated over baseline during
the four periods of normoxia separating the five hypoxic exposures.
E fell toward baseline 10 min after the exposure to
poikilocapnic intermittent hypoxia but was significantly elevated over
baseline for the remainder of the 60-min period after poikilocapnic
intermittent hypoxia. There appears to be comparable contributions of
breathing frequency and VT on the E time
course before, during, and after poikilocapnic intermittent hypoxia.
(Fig. 3).
Poikilocapnic Continuous Hypoxia
Figure 4 illustrates the ventilatory effects of poikilocapnic continuous hypoxia. As seen during poikilocapnic intermittent hypoxia (Fig. 3), there is a two- to threefold increase inE during continuous hypoxia. E fell to levels that
were not significantly elevated over baseline by 30 min in normoxia and
for the remainder of the 60-min period of normoxia. Although
ventilatory frequency was significantly elevated at 50 min posthypoxia
and VT was significantly elevated at 60 min posthypoxia,
there were no consistent effects of poikilocapnic continuous hypoxia on
posthypoxic frequency or VT.
Ventilatory Dead Space
Arterial blood gases were only obtained in the isocapnic intermittent hypoxia experiments during baseline conditions and at 60 min posthypoxia. On the basis of these measurements in conjunction withE and CO2
measurements, the dead space-to-tidal volume ratio
(VDS/VT) = (E/CO2 
A/CO2)/(E
and CO2) (25) was
calculated. Baseline VDS/VT was 0.3 ± 0.2 , and 1 h after isocapnic intermittent hypoxia,
VDS/VT was 0.4 ± 0.4 (n = 7). There was no consistent significant change in
VDS/VT before and after isocapnic intermittent hypoxia.
Ventilation Relative to CO2
CO2 during intermittent hypoxia.
However, we measured CO2 during baseline and during the 1-h period in normoxia after hypoxic exposure. Figures
5 and 6
contrast intermittent vs. continuous hypoxia effects on both
E and
E/CO2 during
the 60 min after either isocapnic (Fig. 5) or poikilocapnic (Fig. 6)
hypoxia, expressed as a percent change from baseline.
|
|
The data presented in Fig. 5 show that E was
significantly elevated over baseline 20 min after exposure to either
isocapnic continuous hypoxia or isocapnic intermittent hypoxia.
However, at 40 and 60 min after continuous hypoxia,
E was no longer significantly elevated over the
baseline level (11 ± 24% at 60 min). In contrast, E was significantly elevated (22 ± 13%) from
baseline 60 min after intermittent hypoxia. Similarly,
E/CO2 was
elevated significantly from baseline 20 min after continuous hypoxia
but was no longer significantly elevated over baseline at 40 and 60 min
(12 ± 16% at 60 min), whereas
E/CO2 was not
significantly elevated over baseline 20 min after intermittent hypoxia
but was significantly elevated at 40 and 60 min (27 ± 17% at 60 min; P < 0.05). Hence, intermittent hypoxia was more
effective than continuous hypoxia in inducing ventilatory LTF in
unanesthetized rats when PaCO2 was not allowed to
decrease during the hypoxic exposure(s).
Responses after poikilocapnic hypoxia are summarized in Fig. 6. The
E measurements after poikilocapnic continuous
hypoxia failed the test for normality (P = 0.003).
Therefore, significance was assessed using one-way repeated-measures
analysis of variance on ranks (SigmaStat, Jandel Scientific). With the
use of this conservative approach, E was
significantly elevated over baseline 20 min after poikilocapnic
continuous hypoxia but did not significantly differ from baseline at 40 and 60 min (31 ± 28% at 60 min). In contrast, all
E measurements made during the 60 min after
poikilocapnic intermittent hypoxia were significantly elevated over
baseline (17 ± 9% at 60 min). All
E/CO2
measurements made during the 60 min after poikilocapnic intermittent
hypoxia are significantly elevated over baseline (20 ± 9% at 60 min), although not significantly greater than those with poikilocapnic
continuous hypoxia (12 ± 11% at 60 min, Fig. 6).
In summary, E decreased between 20 and 60 min after
continuous hypoxia, whereas E increased or stayed
the same between 20 and 60 min after intermittent hypoxia (Figs. 5 and
6). E/CO2 also
decreased between 20 and 60 min after continuous hypoxia, whereas it
increased over the same time period after intermittent hypoxia. Thus
the data demonstrate ventilatory LTF after intermittent hypoxia in
unanesthetized rats. The results also suggest that intermittent hypoxia
(independent of the CO2 level during hypoxia) is more
effective than continuous hypoxia in triggering LTF of at least 60-min
duration. However, the shorter period of elevated ventilation after
continuous hypoxia (i.e., <40 min; Figs. 5 and 6) may represent a form
of LTF.
| |
DISCUSSION |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
These studies are the first demonstration of ventilatory LTF in unanesthetized rats. In several important respects (magnitude, duration, pattern), ventilatory LTF in unanesthetized rats is similar to phrenic LTF in anesthetized, vagotomized, and ventilated rats (3, 4, 12, 17) but is unlike the response of anesthetized, spontaneously breathing rats (16).
There are at least two explanations for the discrepancies between the
present paper and the Janssen and Fregosi (16) study reporting that LTF cannot be evoked in spontaneously breathing rats.
First, in contrast to the present study, Janssen and Fregosi studied
anesthetized rats. As pointed out by these authors, anesthetized rats
may need to be near the PaCO2 apneic threshold in
order for LTF to be observed (16). Second, Janssen and
Fregosi did not measure
E/CO2 or
phrenic nerve activity but instead monitored tongue and intercostal
muscle EMG activities as well as esophageal pressure (16).
Interestingly, the esophageal pressures, reported in their study, show
a nonsignificant elevation over baseline between 60 and 90 min after
intermittent hypoxia (see Fig. 3 in Ref. 16). The
ventilatory LTF we observed in unanesthetized rats could have occurred
without any changes in the tongue or intercostal muscle activity.
Most studies examining the cellular and synaptic mechanisms of LTF have used anesthetized, vagotomized, paralyzed, artificially ventilated rats as an experimental model. In this model, intermittent but not continuous hypoxia elicits phrenic LTF (4). Phrenic LTF requires 5-HT-receptor activation (3, 17) during but not after intermittent hypoxia (13). The relevant 5-HT receptors are located in the spinal cord (6). Furthermore, phrenic LTF requires spinal protein synthesis (5), although the identity of the relevant spinal protein is not yet clear. Given the rapid pace of progress in understanding phrenic LTF in anesthetized rats, and reports that awake animals (21, 24, 29) or even anesthetized, spontaneously breathing rats (16) exhibit relatively small or short manifestations of LTF, it seemed important to determine whether rats, in fact, exhibit ventilatory LTF under more physiological conditions.
Figures 1-4 show that intermittent and continuous hypoxia have
dramatically different effects on the time-course patterns of E, VT, and breathing frequency. The
absolute values of E, VT, and frequency
depend on the parallel metabolic activity of the rats and are therefore
inherently more variable than ventilation expressed relative to an
index of metabolic rate (for example, E/CO2 as shown
in Figs. 5 and 6). It is not clear why some groups of rats have a
higher baseline metabolic activity than others, but this variability
explains the somewhat different levels of baseline E
occasionally observed (compare Figs. 1 and 2). For the most part, the
presentation of the time courses of VT and frequency in
Figs. 1B-4B demonstrates a balance between these two ventilatory parameters in generating the overall changes in E.
Expressing the posthypoxia ventilatory data as a percent change from
baseline (see Figs. 5 and 6) is analogous to previously published
phrenic LTF data from anesthetized, paralyzed, vagotomized rats
(3, 4, 12, 17), allowing a more direct comparison. With
this normalization procedure, ventilatory LTF in unanesthetized rats
has some of the same properties as phrenic LTF in anesthetized rats and
has some differences. Specifically, ventilatory LTF in unanesthetized
rats appears to have at least partial dependence on the pattern of
hypoxia (intermittent, but not continuous) (4). The same
pattern dependence is observed in unanesthetized ducks (24) and goats (8, 29). The time course of
the ventilatory LTF that we observed in unanesthetized rats, a constant
or slightly increasing
E/CO2 during the 60 min after intermittent hypoxia (see Figs. 5 and 6), is similar to the
progressively increasing time course of phrenic LTF after intermittent
hypoxia (3, 4, 12, 17) but is notably longer than phrenic
LTF after carotid sinus nerve stimulation in the same preparation
(15, 18) or the minimal diaphragmatic LTF in anesthetized
but spontaneously breathing rats after intermittent hypoxia
(16).
There are some apparent differences between ventilatory LTF and phrenic LTF. The increase in phrenic amplitude 60 min after intermittent hypoxia in anesthetized rats has been reported as 63% (3), 37% (17), 78% (4), and 57% (13). In contrast, we observed an average increase of 22% in ventilation 60 min after intermittent hypoxia in unanesthetized rats. These apparent quantitative differences in LTF between anesthetized, vagotomized rats vs. unanesthetized rats could reflect an inhibitory feedback in intact, spontaneously breathing rats, or could be related to differences inherent in our direct measurement of ventilation vs. assessment of phrenic nerve activity. We suggest that the major cause of quantitatively smaller ventilatory LTF in unanesthetized rats is the hypocapnic condition allowed posthypoxia. Specifically, with no inspired CO2, PaCO2 would decrease due to ventilatory LTF, thereby reducing CO2 chemoreceptor feedback and constraining ventilation. In support of this hypothesis, five of the seven rats exposed to isocapnic intermittent hypoxia (San Diego) were administered supplemental CO2 60 min after intermittent hypoxia to bring their PaCO2 within 0.1 Torr below their original baseline PaCO2. Four of the five rats had substantial increases in ventilation, with an overall 29% increase over the 60-min poikilocapnic ventilation, indicating that our measurements of ventilatory LTF were low. A 29% increase, superimposed on the preexisting 22% increase, suggests an overall increase in ventilation of 57% above baseline under isocapnic conditions. A 57% ventilatory LTF is well within the range of values reported for phrenic LTF (3, 4, 13, 17).
There was an approximately fourfold increase in E
during isocapnic hypoxia (Figs. 1 and 2) vs. an
approximately two- to threefold increase during poikilocapnic hypoxia
(see Figs. 3 and 4). Nevertheless, ventilatory LTF after intermittent
exposure to either isocapnic (Fig. 5) or poikilocapnic (Fig. 6) hypoxia was equivalent. Therefore, the magnitude of the ventilatory increase during hypoxia does not appear to be a critical determinant of ventilatory LTF. This conclusion is similar to the relative
PaO2 independence of phrenic LTF in anesthetized rats
(11) but contrasts with the correlation between the
hypoxic phrenic response and phrenic LTF magnitude (11).
We are not aware of other laboratories having systematically compared
ventilatory LTF after exposure to isocapnic or poikilocapnic
intermittent hypoxia.
The evidence demonstrating ventilatory LTF in unanesthetized rats is a
significant, prolonged increase in E and
E/CO2 for at
least 60 min after intermittent hypoxia (see Figs. 5 and 6). Although
E and
E/CO2 were
always significantly elevated over baseline at 60 min after
intermittent hypoxia, E and
E/CO2 at this
time were seldom significantly elevated over their respective values 60 min after continuous hypoxia. Thus it is difficult to clearly
differentiate between these patterns of hypoxia. However, the time
course of ventilation after hypoxia differs. Ventilation after
continuous hypoxia is highest immediately after the hypoxia ends and
then decreases progressively (Figs. 5 and 6). In contrast, immediately
after intermittent hypoxia, ventilation is at its lowest (although
still above baseline) and progressively increases with time. The peak
ventilation after intermittent hypoxia occurs at least 30 min
posthypoxia. This ventilatory pattern after intermittent hypoxia is a
hallmark of phrenic LTF in the rat (4).
Previous studies have shown that there is an acute increase in
VDS/VT during continuous hypoxia
(25). Nevertheless, VDS/VT calculations presented in RESULTS, as well as previous
experience, support the premise that an acute increase in physiological
VDS during hypoxia will have returned to normal within
60 min after return to normoxia (E. B. Olson, Jr.,
unpublished observation). Therefore, differences seen in
E/CO2 values
were not considered to be distorted by VDS changes caused
by hypoxia.
Ventilatory LTF after intermittent (usually isocapnic) hypoxia has been reported in several unanesthetized models including awake dogs (7), goats (28), and ducks (24) and in some sleeping humans (2). There have been equivocal findings in humans (21), and the existence of LTF in spontaneously breathing animals is not universally accepted (16). Our study is the first demonstration of ventilatory LTF in an unanesthetized animal that can be directly compared with anesthetized results from the same species and strain (3, 4, 12, 17; M. R. Dwinell and F. L. Powell, unpublished observation). Furthermore, we demonstrated that LTF can be elicited even when the vagi are intact. The magnitude of LTF in unanesthetized rats is comparable to anesthetized rats once the effects of decreased CO2 are accounted for. Hence, the rat may be a good model to study both the mechanisms of LTF (in anesthetized preparations) and its physiological significance (in unanesthetized preparations).
| |
ACKNOWLEDGEMENTS |
|---|
These experiments were supported by National Heart, Lung, and Blood Institute Grants HL-53319, HL-68383, HL-17731, and HL-07212.
| |
FOOTNOTES |
|---|
Address for reprint requests and other correspondence: E. B. Olson, Jr., The John Rankin Laboratory of Pulmonary Medicine, Dept. of Preventive Medicine, Univ. of Wisconsin School of Medicine, 504 N. Walnut St., Madison, WI 53705-2368 (E-mail: ebolson{at}facstaff.wisc.edu).
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.
Received 20 February 2001; accepted in final form 13 April 2001.
| |
REFERENCES |
|---|
|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
1.
Aaron, EA,
and
Powell FL.
Effect of chronic hypoxia on hypoxic ventilatory response in awake rats.
J Appl Physiol
74:
1635-1640,
1993
2.
Babcock, MA,
and
Badr MS.
Long-term facilitation of ventilation in humans during NREM sleep.
Sleep
21:
709-716,
1998[Web of Science][Medline].
3.
Bach, KB,
and
Mitchell GS.
Hypoxia-induced long-term facilitation of respiratory activity is serotonin dependent.
Respir Physiol
104:
251-260,
1996[Web of Science][Medline].
3a.
Baker TL, Fuller DD, Zabka AG, and Mitchell GS. Respiratory
plasticity: different effects of continuous and episodic hypoxia and
hypercapnia. Respir Physiol In press.
4.
Baker, TL,
and
Mitchell GS.
Episodic but not continuous hypoxia elicits long-term facilitation of phrenic motor output in rats.
J Physiol (Lond)
529:
215-219,
2000
5.
Baker, TL,
and
Mitchell GS.
Spinal protein synthesis inhibition attenuates serotonin dependent long term facilitation of phrenic motor output (Abstract).
Soc Neurosci Abstr
26:
1458,
2000.
6.
Baker, TL,
and
Mitchell GS.
Spinal serotonin receptor activation is necessary for phrenic long term facilitation following episodic hypoxia.
FASEB J
15:
A422,
2001.
7.
Cao, KY,
Zwillich CW,
Berthon-Jones M,
and
Sullivan CE.
Increased normoxic ventilation induced by repetitive hypoxia in conscious dogs.
J Appl Physiol
73:
2083-2088,
1992
8.
Dwinell, MR,
Janssen PL,
and
Bisgard GE.
Lack of long-term facilitation of ventilation after exposure to hypoxia in goats.
Respir Physiol
108:
1-9,
1997[Web of Science][Medline].
9.
Eldridge, FL,
and
Millhorn DE.
Oscillation, gating, and memory in the respiratory control system.
In: Handbook of Physiology. The Respiratory System. Control of Breathing. Bethesda, MD: Am. Physiol. Soc, 1986, sect. 3, vol. II, pt. 1, chapt. 3, p. 93-114.
10.
Fregosi, RF,
and
Mitchell GS.
Long-term facilitation of inspiratory intercostal nerve activity following carotid sinus nerve stimulation in cats.
J Physiol (Lond)
477:
469-479,
1994
11.
Fuller, DD,
Bach KB,
Baker TL,
Kinkead R,
and
Mitchell GS.
Long term facilitation of phrenic motor output.
Respir Physiol
121:
135-146,
2000[Web of Science][Medline].
12.
Fuller, DD,
Baker TL,
Behan M,
and
Mitchell GS.
Expression of hypoglossal long term facilitation differs between sub-strains of Sprague-Dawley rats.
Physiol Genomics
4:
175-181,
2001
13.
Fuller, DD,
Zabka AG,
Baker TL,
and
Mitchell GS.
Phrenic long-term facilitation requires 5-HT receptor activation during but not following episodic hypoxia.
J Appl Physiol
90:
2001-2006,
2001
14.
Gesell, R,
and
Hamilton MA.
Reflexogenic components of breathing.
Am J Physiol
133:
694-719,
1941.
15.
Hayashi, F,
Coles SK,
Bach KB,
Mitchell GS,
and
McCrimmon DR.
Time-dependent phrenic nerve responses to carotid afferent activation: intact vs. decerebellate rats.
Am J Physiol Regulatory Integrative Comp Physiol
265:
R811-R819,
1993
16.
Janssen, PL,
and
Fregosi RF.
No evidence for long-term facilitation after episodic hypoxia in spontaneously breathing, anesthetized rats.
J Appl Physiol
89:
1345-1351,
2000
16a.
Kinkead R, Bach KB, Johnson SM, Hodgeman BA, and Mitchell GS.
Plasticity in respiratory motor control: intermittent hypoxia and
hypercapnia activate opposing serotonergic and noradrenergic systems.
Comp Biochem Physiol In press.
17.
Kinkead, R,
and
Mitchell GS.
Time-dependent hypoxic ventilatory responses in rats: effects of ketanserin and 5-carboxamidotryptamine.
Am J Physiol Regulatory Integrative Comp Physiol
277:
R658-R666,
1999
18.
Ling, L,
Olson EB,
Vidruk EH,
and
Mitchell GS.
Integrated phrenic responses to carotid afferent stimulation in adult rats following perinatal hyperoxia.
J Physiol (Lond)
500:
787-796,
1997
19.
Mateika, JH,
and
Fregosi RF.
Long-term facilitation of upper airway muscle activities in vagotomized and vagally intact cats.
J Appl Physiol
82:
419-425,
1997
20.
McCrimmon, DR,
Dekin MS,
and
Mitchell GS.
Glutamate, GABA and serotonin in ventilatory control.
In: Regulation of Breathing, edited by Dempsey JA,
and Pack AI, Jr.. New York: Dekker, 1995, p. 151-218.
21.
McEvoy, RD,
Popovic RM,
Saunders NA,
and
White DP.
Effects of sustained and repetitive isocapnic hypoxia on ventilation and genioglossal and diaphragmatic EMGs.
J Appl Physiol
81:
866-875,
1996
22.
Millhorn, DE,
Eldridge FL,
and
Waldrop TG.
Prolonged stimulation of respiration by a new central neural mechanism.
Respir Physiol
41:
87-103,
1980[Web of Science][Medline].
23.
Millhorn, DE,
Eldridge FL,
and
Waldrop TG.
Prolonged stimulation of respiration by endogenous central serotonin.
Respir Physiol
42:
171-188,
1980[Web of Science][Medline].
24.
Mitchell GS, Powell FL, Hopkins SR, and Milsom WK. Time domains of
the hypoxic ventilatory response in awake ducks: episodic and
continuous hypoxia. Respir Physiol. In press.
25.
Olson, EB, Jr.
Physiological dead space increases during initial hours of chronic hypoxemia with or without hypocapnia.
J Appl Physiol
77:
1526-1531,
1994
26.
Olson, EB, Jr,
and
Dempsey JA.
Rat as a model for humanlike ventilatory adaptation to chronic hypoxia.
J Appl Physiol
44:
763-769,
1978
27.
Olson, EB, Jr,
Dempsey JA,
and
McCrimmon DR.
Serotonin and the control of ventilation in awake rats.
J Clin Invest
64:
689-693,
1979.
28.
Powell, FL,
Milsom WK,
and
Mitchell GS.
Time domains of the hypoxic ventilatory response.
Respir Physiol
112:
123-134,
1998[Web of Science][Medline].
29.
Turner, DL,
and
Mitchell GS.
Long-term facilitation of ventilation following repeated hypoxic episodes in awake goats.
J Physiol (Lond)
499:
543-550,
1997
This article has been cited by other articles:
|
|
 |
|
  S. Ryan and P. Nolan Long-term facilitation of upper airway muscle activity induced by episodic upper airway negative pressure and hypoxia in spontaneously breathing anaesthetized rats J. Physiol., July 1, 2009; 587(13): 3343 - 3353. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Ryan and P. Nolan Episodic hypoxia induces long-term facilitation of upper airway muscle activity in spontaneously breathing anaesthetized rats J. Physiol., July 1, 2009; 587(13): 3329 - 3342. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. Kervern, C. Dubois, M. Naassila, M. Daoust, and O. Pierrefiche Perinatal Alcohol Exposure in Rat Induces Long-Term Depression of Respiration after Episodic Hypoxia Am. J. Respir. Crit. Care Med., April 1, 2009; 179(7): 608 - 614. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. H. Mateika and G. Narwani Intermittent hypoxia and respiratory plasticity in humans and other animals: does exposure to intermittent hypoxia promote or mitigate sleep apnoea? Exp Physiol, March 1, 2009; 94(3): 279 - 296. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. McGuire, J. L. Tartar, Y. Cao, R. W. McCarley, D. P. White, R. E. Strecker, and L. Ling Sleep fragmentation impairs ventilatory long-term facilitation via adenosine A1 receptors J. Physiol., November 1, 2008; 586(21): 5215 - 5229. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. McGuire, C. Liu, Y. Cao, and L. Ling Formation and maintenance of ventilatory long-term facilitation require NMDA but not non-NMDA receptors in awake rats J Appl Physiol, September 1, 2008; 105(3): 942 - 950. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Mahamed and G. S. Mitchell Simulated apnoeas induce serotonin-dependent respiratory long-term facilitation in rats J. Physiol., April 15, 2008; 586(8): 2171 - 2181. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 F. J. Golder, L. Ranganathan, I. Satriotomo, M. Hoffman, M. R. Lovett-Barr, J. J. Watters, T. L. Baker-Herman, and G. S. Mitchell Spinal Adenosine A2a Receptor Activation Elicits Long-Lasting Phrenic Motor Facilitation J. Neurosci., February 27, 2008; 28(9): 2033 - 2042. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. Terada, A. Nakamura, W. Zhang, M. Yanagisawa, T. Kuriyama, Y. Fukuda, and T. Kuwaki Ventilatory long-term facilitation in mice can be observed during both sleep and wake periods and depends on orexin J Appl Physiol, February 1, 2008; 104(2): 499 - 507. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. Katayama, C. A. Smith, K. S. Henderson, and J. A. Dempsey Chronic intermittent hypoxia increases the CO2 reserve in sleeping dogs J Appl Physiol, December 1, 2007; 103(6): 1942 - 1949. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. Tadjalli, J. Duffin, Y. M. Li, H. Hong, and J. Peever Inspiratory activation is not required for episodic hypoxia-induced respiratory long-term facilitation in postnatal rats J. Physiol., December 1, 2007; 585(2): 593 - 606. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. Berner, Y. Shvarev, H. Lagercrantz, A. Bilkei-Gorzo, T. Hokfelt, and R. Wickstrom Altered respiratory pattern and hypoxic response in transgenic newborn mice lacking the tachykinin-1 gene J Appl Physiol, August 1, 2007; 103(2): 552 - 559. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. A. Day and R. J. A. Wilson Brainstem PCO2 modulates phrenic responses to specific carotid body hypoxia in an in situ dual perfused rat preparation J. Physiol., February 1, 2007; 578(3): 843 - 857. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Mahamed and G. S. Mitchell Sleep Apnoea & Hypertension: Physiological bases for a causal relation: Is there a link between intermittent hypoxia-induced respiratory plasticity and obstructive sleep apnoea? Exp Physiol, January 1, 2007; 92(1): 27 - 37. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 D. P. Harris, A. Balasubramaniam, M. S. Badr, and J. H. Mateika Long-term facilitation of ventilation and genioglossus muscle activity is evident in the presence of elevated levels of carbon dioxide in awake humans Am J Physiol Regulatory Integrative Comp Physiol, October 1, 2006; 291(4): R1111 - R1119. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. J. Lai, C. C. H. Yang, Y. Y. Hsu, Y. N. Lin, and T. B. J. Kuo Enhanced sympathetic outflow and decreased baroreflex sensitivity are associated with intermittent hypoxia-induced systemic hypertension in conscious rats J Appl Physiol, June 1, 2006; 100(6): 1974 - 1982. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. McGuire, Y. Zhang, D. P. White, and L. Ling Phrenic long-term facilitation requires NMDA receptors in the phrenic motonucleus in rats J. Physiol., September 1, 2005; 567(2): 599 - 611. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. A. Day and R. J. A. Wilson Specific carotid body chemostimulation is sufficient to elicit phrenic poststimulus frequency decline in a novel in situ dual-perfused rat preparation Am J Physiol Regulatory Integrative Comp Physiol, August 1, 2005; 289(2): R532 - R544. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. J. Cummings and R. J. A. Wilson Time-dependent modulation of carotid body afferent activity during and after intermittent hypoxia Am J Physiol Regulatory Integrative Comp Physiol, June 1, 2005; 288(6): R1571 - R1580. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. D. Fuller Episodic hypoxia induces long-term facilitation of neural drive to tongue protrudor and retractor muscles J Appl Physiol, May 1, 2005; 98(5): 1761 - 1767. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Morelli, M. S. Badr, and J. H. Mateika Ventilatory responses to carbon dioxide at low and high levels of oxygen are elevated after episodic hypoxia in men compared with women J Appl Physiol, November 1, 2004; 97(5): 1673 - 1680. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. R. Reeves and D. Gozal Platelet-activating factor receptor modulates respiratory adaptation to long-term intermittent hypoxia in mice Am J Physiol Regulatory Integrative Comp Physiol, August 1, 2004; 287(2): R369 - R374. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. C. McKay, W. A. Janczewski, and J. L. Feldman Episodic hypoxia evokes long-term facilitation of genioglossus muscle activity in neonatal rats J. Physiol., May 15, 2004; 557(1): 13 - 18. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. H. Mateika, C. Mendello, D. Obeid, and M. S. Badr Peripheral chemoreflex responsiveness is increased at elevated levels of carbon dioxide after episodic hypoxia in awake humans J Appl Physiol, March 1, 2004; 96(3): 1197 - 1205. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. McGuire, Y. Zhang, D. P. White, and L. Ling Serotonin receptor subtypes required for ventilatory long-term facilitation and its enhancement after chronic intermittent hypoxia in awake rats Am J Physiol Regulatory Integrative Comp Physiol, February 1, 2004; 286(2): R334 - R341. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. A. Neubauer and J. Sunderram Oxygen-sensing neurons in the central nervous system J Appl Physiol, January 1, 2004; 96(1): 367 - 374. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. G. Zabka, G. S. Mitchell, E. B. Olson Jr, and M. Behan Selected Contribution: Chronic intermittent hypoxia enhances respiratory long-term facilitation in geriatric female rats J Appl Physiol, December 1, 2003; 95(6): 2614 - 2623. [Abstract] [Full Text]
|
|
|
|
|
|
M. McGuire, Y. Zhang, D. P. White, and L. Ling Chronic intermittent hypoxia enhances ventilatory long-term facilitation in awake rats J Appl Physiol, October 1, 2003; 95(4): 1499 - 1508. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Y. Zhang, M. McGuire, D. P White, and L. Ling Episodic phrenic-inhibitory vagus nerve stimulation paradoxically induces phrenic long-term facilitation in rats J. Physiol., September 15, 2003; 551(3): 981 - 991. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Y.-J. Peng and N. R. Prabhakar Reactive oxygen species in the plasticity of respiratory behavior elicited by chronic intermittent hypoxia J Appl Physiol, June 1, 2003; 94(6): 2342 - 2349. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. B. Mantilla and G. C. Sieck Plasticity in Respiratory Motor Control: Invited Review: Mechanisms underlying motor unit plasticity in the respiratory system J Appl Physiol, March 1, 2003; 94(3): 1230 - 1241. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Babcock, M. Shkoukani, S. E. Aboubakr, and M. S. Badr Determinants of long-term facilitation in humans during NREM sleep J Appl Physiol, January 1, 2003; 94(1): 53 - 59. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
R. W. Bavis and G. S. Mitchell Plasticity in Respiratory Motor Control: Selected Contribution: Intermittent hypoxia induces phrenic long-term facilitation in carotid-denervated rats J Appl Physiol, January 1, 2003; 94(1): 399 - 409. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. McGuire, Y. Zhang, D. P. White, and L. Ling Effect of hypoxic episode number and severity on ventilatory long-term facilitation in awake rats J Appl Physiol, December 1, 2002; 93(6): 2155 - 2161. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Shkoukani, M. A. Babcock, and M. S. Badr Effect of episodic hypoxia on upper airway mechanics in humans during NREM sleep J Appl Physiol, June 1, 2002; 92(6): 2565 - 2570. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. D Kline, J. L Overholt, and N. R Prabhakar Mutant mice deficient in NOS-1 exhibit attenuated long-term facilitation and short-term potentiation in breathing J. Physiol., February 15, 2002; 539(1): 309 - 315. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. G. Zabka, M. Behan, and G. S. Mitchell Genome and Hormones: Gender Differences in Physiology: Selected Contribution: Time-dependent hypoxic respiratory responses in female rats are influenced by age and by the estrus cycle J Appl Physiol, December 1, 2001; 91(6): 2831 - 2838. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
and
). B: tidal volume (VT;
and 
) and breathing frequency
(
and 
) expressed relative to baseline.
Data are mean values ± 95% confidence limits determined in 7 rats from San Diego and 8 rats from Madison. The initial data point
(
) represents the average baseline measurement (BL) in poikilocapnic
normoxia. The shaded areas along the x-axis indicate 5 time
periods during which the rats were exposed to isocapnic hypoxia,
interspersed with poikilocapnic normoxia. After isocapnic intermittent
hypoxia, the rats were observed for 1 h in poikilocapnic normoxia.
Data points indicated by solid symbols differ significantly from
baseline (P < 0.05).
) in 
and 
; 6 rats from San Diego and 8 rats from Madison). Data are mean values ± 95% confidence
limits. Data points indicated by solid symbols differ significantly
from baseline (P < 0.05).
