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Department of Human Physiology, University of California, Davis, California 95616
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
ACKNOWLEDGEMENTS
FOOTNOTES
REFERENCES
ABSTRACT 
Albert, T. S. E., V. L. Tucker, and E. M. Renkin.
Atrial natriuretic peptide levels and plasma volume contraction in acute alveolar hypoxia. J. Appl.
Physiol. 82(1): 102-110, 1997.
Arterial oxygen
tensions (PaO2), atrial natriuretic
peptide (ANP) concentrations, and circulating plasma volumes (PV) were
measured in anesthetized rats ventilated with room air or 15, 10, or
8% O2
(n = 5-7). After 10 min of
ventilation, PaO2 values were 80 ± 3, 46 ± 1, 32 ± 1, and 35 ± 1 Torr
and plasma immunoreactive ANP (irANP) levels were 211 ± 29, 229 ± 28, 911 ± 205, and 4,374 ± 961 pg/ml, respectively. At
PaO2 
40 Torr, irANP responses were
more closely related to inspired
O2
(P = 0.014) than to
PaO2 (P = 0.168). PV was 36.3 ± 0.5 µl/g in controls but 8.5 and
9.9% lower (P 0.05) for
10 and 8% O2, respectively.
Proportional increases in hematocrit were observed in animals with
reduced PV; however, plasma protein concentrations were not different
from control. Between 10 and 50 min of hypoxia, small increases (+40%)
in irANP occurred in 15% O2;
however, there was no further change in PV, hematocrit, plasma protein,
or irANP levels in the lower O2
groups. Urine output tended to fall during hypoxia but was not
significantly different among groups. These findings are compatible
with a role for ANP in mediating PV contraction during acute alveolar
hypoxia.
fluid balance; cardiac output; arterial hypoxemia; blood gases; plasma protein concentration
INTRODUCTION THERE IS EVIDENCE that atrial natriuretic peptide (ANP)
plays a role in systemic and pulmonary responses to hypoxia. Hypoxia is
a known secretagogue for ANP (see Ref. 28 for review). Circulating levels of this cardiac peptide are rapidly increased in animals ventilated with hypoxic gas (2, 8, 16, 29) and are elevated in
hypoxemic states such as sleep apnea (21), chronic obstructive pulmonary disease (5), and altitude sickness (9). High-affinity binding
sites for ANP have been identified in the vasculature of numerous
tissues, with particularly large numbers being expressed in the lung
(see Ref. 6 for review). Hypoxia-induced ANP release is currently
believed to mitigate the development of hypoxic pulmonary hypertension. ANP has been shown to have a direct vasodilatory effect
on pulmonary arterial rings (15), and intravenous administration of ANP
blunts pulmonary vasoconstriction during acute hypoxia (2, 16).
In addition to its vasodilatory properties, exogenous ANP has been
shown to rapidly increase plasma protein extravasation (31, 34) and
induce extrarenal fluid shifts from blood to interstitium (4, 32).
Reductions in circulating volume subsequent to ANP-induced fluid shifts
could provide an additional mechanism for limiting the pressure load on
the right heart during hypoxia, thus minimizing right heart failure and
pulmonary edema. Diminution of plasma volume (PV) is a common
occurrence in hypoxia (11-13); however, a specific role for
endogenous ANP in mediating this response has not been adequately
tested.
The purpose of the present study was to assess the potential role of
ANP in mediating PV reductions during acute hypoxia. The magnitude and
time course of changes in circulating ANP concentrations and PV were
determined in anesthetized rats ventilated with graded levels of
O2 (8-21%) for 50 min. ANP
levels were measured by radioimmunoassay, and circulating PV were
assessed by using radiolabeled albumin. Blood pressures and arterial
blood gases, hematocrits (Hct), pH, lactate
(La METHODS
), and protein
concentrations were analyzed as well to further evaluate potential
mechanisms of volume regulation during acute hypoxia. Albumin transport
and fluid balance at the tissue level were also addressed in these
experiments; the data are presented in a companion paper (3).
measurements; and a
100-µl sample for blood gas and pH analysis. After blood sampling,
500 µl of replacement blood followed by 400-500 µl of LR were
infused. The bladder was then emptied by application of gentle pressure
to the abdomen. Ten additional minutes of stabilization were allowed
before the start of control (air) or hypoxic gas treatment.
Animals were divided into a normoxic (room air) control group and three
hypoxic (15, 10, and 8%
O2-balance
N2) groups. Room air and gas
mixtures were administered from a Douglas bag attached to the inlet
port of the ventilator via Tygon tubing. To humidify the gas, 2 ml of
distilled water were added to the bag. Just before the start of the
treatment period, the Tygon tubing connecting the Douglas bag to the
ventilator was flushed with treatment gas. Arterial blood samples were
collected ~10 min into the treatment period, followed by intravenous
injection of 131I-labeled rat
serum albumin (131I-RSA; 6 µCi)
at time (t) = 15 min. Small blood
samples (100 µl) were drawn at 20 and 30 min, respectively, from the
start of O2 treatment for
measurement of plasma 131I
activity, Hct, TP, and La.
These samples were replaced with similar volumes of LR. At 45 min, a
bolus of 125I-labeled RSA (6 µCi) was injected for a final measurement of PV. Terminal blood
samples were drawn at 50 min, after which the rats were killed with
intravenous KCl.
All blood samples taken during the experiment were centrifuged, and Hct
was measured by the microtube method. Aliquots of plasma were saved for
determination of TP and La
analysis by colorimetric assay. Arterial pH and arterial
PO2 (PaO2) and
PCO2
(PaCO2) were immediately obtained from whole blood by using a Ciba-Corning pH/blood gas analyzer (model 238, Medfield, MA); corrections were made for animal body temperature. Urine
and abdominal cavity fluids were collected postmortem by syringe.
PV measurements.
Whole animal PV was obtained from the 5-min distribution volumes of
131I- and
125I-RSA
(t = 20 and 50 min, respectively). PV
was calculated by dividing total counts injected by counts per
milliliter of plasma after 5 min of circulation. The total counts
injected were determined by counting a standard aliquot of each tracer.
Plasma samples and tracer standards were counted in
131I and
125I channels of a gamma
scintillation counter (Searle Analytical, Des Plaines, IL). Over
100,000 counts/sample were collected and corrected for background,
crossover from 131I to
125I, and decay of
131I. Crossover of
131I to the
125I channel ranged from 10.9 to
11.3%.
Radioimmunoassay of plasma ANP.
Arterial blood was immediately placed in vials containing EDTA (1.6 mg/ml) and centrifuged at 4°C (12,000 g for 5 min). Plasma was stored at
70°C. Samples (200-500 µl) were extracted one day before assay by reverse-phase chromatography by using
C18 columns (Bond Elut, Varian,
Harbor City, CA). Recovery of 100, 200, and 500 pg of peptide added to
500 µl of rat plasma was 72 ± 7.5, 71 ± 10, and 72 ± 10%
(mean ± SD), respectively. Triplicate extracted samples and peptide standards were preincubated with antiserum for 24 h
at 0-4°C. After incubation,
125I-ANP was added (~14,000
counts/tube) and the tubes were incubated for an additional 24 h at
0-4°C. Bound tracer was immunoprecipitated by sequential
addition of pretitered goat anti-rabbit gamma globulin and rabbit
serum, followed by a 2-h incubation period at room temperature. Bound
tracer was measured in the gamma scintillation counter described above.
A Log-logit analysis was used to determine the amount of hormone in the
samples. Sensitivity (2 SD from 0) was 4.2 pg/tube. Values were
corrected for incomplete recovery.
Blood gas and cardiac output trials.
Animals in the main study group did not have blood gases analyzed after
15 min to avoid radioactive contamination of the blood gas machine. To
assess blood gases throughout the hypoxia period, a small series of
experiments were run by using a similar protocol without radioisotopes.
Blood gases from both arterial and central venous samples were measured
in three groups subjected to different O2 gas treatments: room air
(n = 3), 15%
O2
(n = 2), and 8%
O2 (n = 3). Cardiac outputs were measured
at approximately the same times by using a Columbus Instruments
thermodilution machine (Cardiomax-II, model 85, Columbus, OH). Surgical
preparation was the same as in the main study except for the insertion
of a thermistor into the right carotid artery. Vital signs were
measured as previously described. Blood gas and cardiac output
measurements were taken at baseline (t = 10), after the first 10 min of
O2 treatment (t = 10), after 30 min of
O2 treatment
(t = 30), and at the end of the
experimental period (t = 50 min). Both
arterial and venous blood samples were taken simultaneously. The
volumes of fluids withdrawn and infused during the course of the
experiment were identical to those in the main study. Saline (100 µl
at room temperature) was used as the injectate for thermodilution
measurement of cardiac output. The saline was injected through a right
jugular vein cannula with the tip placed either at the entrance to the
right atrium or inside the right atrium; the accompanying blood
temperature changes were measured by a thermistor located just proximal
to the aortic valve. The locations of both of these were verified by
postmortem visualization. To minimize error in the cardiac output
calculation due to warming of the saline in the jugular line, the line
was flushed before each measurement. Duplicate measurements of cardiac
output were taken at each point, and the values were averaged.
Materials.
The 131I and
125I radionuclides were purchased
from New England Nuclear-DuPont (Wilmington, DE). BSA (5% solution)
and RSA fraction V powder were purchased from Sigma Chemical (St.
Louis, MO). ANP antibodies were obtained from Research and Diagnostic
Antibodies (Berkeley, CA). ANP (rat, 28 amino acids) standard was from
Bachem (Torrance, CA);
(3-[125I]iodotyrosyl28)
ANP was from Amersham (Arlington Heights, IL). Goat anti-rabbit gamma
globulin and rabbit serum were purchased from Peninsula Laboratories
(Belmont, CA). The La assay
kit was from Sigma Chemical.
RSA was iodinated by using chloramine-T and purified by anion exchange
and repeated concentration-dilution with Minicon filters (Amicon,
Danvers, MA). Labeled RSA (specific activity = 10-50 µCi/mg) was
repurified on the day of use to reduce free
I levels to 1%.
Statistics.
Data are expressed as means ± SE unless otherwise indicated.
Differences among treatment means were tested by using one-way analysis
of variance followed by Duncan's new multiple-range test for post hoc
comparisons among individual means. Within-animal effects (paired
comparisons) and relationships among variables were tested by using
analysis of variance repeated measures and regression models,
respectively. For the ANP radioimmunoassay results, a logarithmic
transformation of the data was performed before analysis to reduce
variance heterogeneity among treatments. Differences are reported as
statistically significant when P 0.05.
RESULTS
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50%) reductions in
MAP immediately after the start of treatment, with no apparent
relationship to the percentage of inspired
O2 (Fig.
1A). The depression in MAP was maintained throughout the experimental period. The room air group demonstrated a much smaller, yet
significant, reduction in MAP from its baseline over the course of the
experiment. Unlike the consistent reduction in MAP, HR changes were
more variable and clearly related to the percentage of inspired
O2 (Fig.
1B). Within 5 min of the start of
15% O2 ventilation, HR fell
significantly to 84% of its baseline. HR remained depressed for at
least 20 min and then tended to rise, reaching control levels by 50 min of hypoxia. Exposure to 10% O2
resulted in a smaller initial fall in HR followed by a
gradual rise to average values slightly higher than, but not different
from, controls. The 8% O2 group
did not exhibit an early drop in HR, but it rose above
control and 15% O2 after 10 min
and remained significantly elevated for the remainder of the treatment
period. Control HR tended to fall slightly during the course of the
treatment period, but the changes were not statistically significant.
Central venous pressures decreased to levels below paired baseline
values in all groups (Fig. 1C).
Although there were no significant differences between group means at
any time point, repeated-measures analysis indicated a greater decline in central venous pressure with time for both 8 and 10%
O2 groups compared with either
room air or 15% O2 treatment.
0.05.
Blood gases and cardiac output trials. Because blood gases were not measured during the last 35 min of the experimental period during which radioisotopes were given, eight additional rats were subjected to the same experimental protocol except for omission of the radioisotopes. A control group (room air) and hypoxic groups at 15 and 8% O2 were studied. Arterial and venous blood samples were taken for PO2, PCO2, and pH measurements 10 min before gas treatment (baseline) and at 10, 30, and 50 min, respectively, after the onset of gas treatment. Cardiac output was also measured at approximately the same times by using the thermodilution technique. These data are summarized in Table 2. Animals in this series had similar baseline values and experienced comparable changes in arterial pressure, HR, and Hct as rats in the main study. Additional observations include 1) graded reduction in mixed venous PO2 with decreasing fraction of inspired O2, 2) continued decline of PaCO2 in the face of constant minute ventilation, 3) increase in PaO2 (compared with paired baseline) during the later phase of 8% O2 treatment, and 4) decrease in both arterial and venous pH after 50 min of 8% O2 ventilation.
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1 · kg1.1
In the room air and 15% O2
groups, cardiac output did not change significantly over the
experimental period. In the 8% O2
animals, cardiac output at 10 min was unchanged but subsequently fell
to 50% of the baseline value at 50 min.
Fluid balance.
Figure 2 illustrates absolute changes in
Hct (
Hct) from baseline over the course of the gas treatment period.
Control Hct (41 ± 0.8%) fell slightly but not significantly with
time. After 10 min of hypoxia, rats ventilated with 15%
O2 exhibited a significant decrease in Hct from 40.9 ± 0.7 to 38.3 ± 0.8%, and the level remained depressed (paralleling the controls) for the rest of the
treatment period. The 10 and 8%
O2 groups (baseline Hct = 42.9 ± 1.2 and 41.7 ± 0.65%, respectively) showed little or no early decline; however, Hct had increased significantly by 20 min of
hypoxia, with the greatest increase occurring in the group given 8%
O2. For all groups, Hct levels
were sustained after 20 min of gas treatment.
Hct) from individual baseline
values. Arrow, onset of treatment (room air or hypoxia).
* Significantly different from room air control,
P 0.05. 
Significantly different from 15% O2,
P 0.05.
Table 3 shows values of PV, TP, and Hct obtained at 20 and 50 min after start of gas treatment and calculated values of total protein mass (TPM; TPM = PV × TP) at these times. Compared with controls, PV values were significantly lower in 10 and 8% groups after 20 min of hypoxia. These results indicate a loss of fluid from the circulation early in the treatment period. This is further supported by concomitant increases in arterial Hct that were linearly and inversely related to changes in PV (P = 0.004 by regression analysis, n = 24 points). There were no subsequent changes in either PV or Hct between 20 and 50 min of gas treatment.
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0.05).
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40 Torr. Among these
"responders," there was no correlation of irANP with
PaO2; inspired
O2 level was the best predictor of
irANP response magnitude (P = 0.0001, n = 24) .
Table 4 also shows baseline, 20-, and 50-min plasma La
concentrations. Baseline
La did not differ among
groups and remained unchanged over the treatment period in the control
group. However, there was a progressive increase in plasma
La with hypoxia, reaching
significance compared with controls and increasing with time in both
the 8 and 10% O2 groups.
DISCUSSION
concentrations in the
8% O2 animals were significantly
higher than in 10% O2 animals,
even though their PaO2 were similar.
Decreased tissue perfusion due to progressive vasoconstriction over the course of the hypoxic period could be responsible for elevated levels
of tissue hypoxia as well as the observed decrement in venous
PO2 in the 8% group (see Table
2). Given the large elevations in
plasma irANP concentrations, one might suspect the vasodilatory actions
of this peptide to modify vasoconstrictor responses. This possibility
is difficult to assess in the present study because ANP measurements
were not obtained in animals where CO was measured. If such a mechanism
was operative, it apparently was unable to overcome systemic
vasoconstriction during ventilation with 8%
O2.
PaO2 in animals ventilated with 8%
O2 for 10 min were equal to or
greater than those in 10% O2
animals. Because ventilation rate and volume were maintained constant,
this cannot be due to increased stimulation of respiration by the lower
inspired O2, nor can it be due to
decreased O2 extraction by the
tissues because venous
PO2 was lower at 8%
O2. Control group
PaO2 values were on the
low side of normal relative to their
PaCO2 (which would indicate alveolar
PO2 values >100 Torr), suggesting that there was substantial venous-to-arterial shunting in the lungs
and/or considerable ventilation-perfusion inequality. It is
possible that increased sympathetic nerve activity to bronchioles or
pulmonary arterioles in the 8% O2
group might have decreased shunting, thus preventing a further
fall in PaO2. Alternatively, the
higher levels of ANP observed during initial treatment with 8%
O2 could have antagonized the
direct action of alveolar hypoxia on the lungs, reducing pulmonary
vasoconstriction (16) (see below), thus diminishing
ventilation-perfusion inequality and improving blood oxygenation. The
continued rise in PaO2 during the later
phase of 8% O2 ventilation could
be attributed to an overall increase in ventilation-to-perfusion ratio
because cardiac output decreased progressively in these animals in the
face of a constant ventilation.
Hypoxia-induced changes in Hct and PV.
LaForte et al. (22) have provided evidence that reductions in
circulating volume or pressure result in a shift of low Hct blood from
the microcirculation into the large blood vessels. This could explain
the decrease in arterial Hct observed in the 15%
O2 group in the face of a
negligible increase in PV, as well as the early tendency for Hct to
fall in the lower O2 groups. By 20 min of gas treatment, animals ventilated with 10 and 8% O2 exhibited reciprocal changes in
Hct and PV, indicating that the rise in Hct was primarily due to the
loss of fluid from the circulation rather than an increase in
erythrocyte volume. Additional support is provided by quantitative
analysis of the relative changes in PV and Hct compared with the
control group. Such analysis assumes that initial PV values were
identical among groups, which is supported by similar weight, age,
plasma protein concentrations, and starting Hct. For example, because
10 and 8% O2 groups had
respective Hct values that were 3.5 and 5.2 Hct percentage points
greater than control (at 20 min; Table 3), the calculated PV would be
~8.1 and 11.6% lower than control, respectively. These figures are close to the observed 8.5 and 9.9% reductions in
125I-RSA distribution volume
(i.e., PV). Because urine flow was not increased during hypoxia in
these anesthetized rats, plasma fluid must have entered extravascular
compartments within the body.
Role of ANP in hypoxia-induced PV reduction.
What are the mechanisms responsible for hypoxia-induced reductions of
PV? Fluid movement across capillary walls is a function of the
prevailing hydrostatic and colloid osmotic pressure gradients, the
permeability of the capillary walls to fluid and protein, and the
exchange surface area. Given the early fall in arterial pressure and
subsequent reductions in cardiac output, it is unlikely that increases
in capillary hydrostatic pressures or surface area could have accounted
for the elevated filtration. Furthermore, changes in MAP were similar
for all levels of hypoxia, whereas significant reductions in PV were
observed only in rats ventilated with 10 and 8%
O2. The elevated plasma
La concentrations suggest
the possibility of a direct effect of tissue hypoxia on capillary
permeability. Increases in capillary hydraulic conductivity and protein
permeability in response to hypoxia have been observed (30, 33);
however, there is presently no general consensus regarding this
response. Alternatively, the rapid and large elevations in irANP could
have contributed to the early reductions in PV. It is well established
that intravenous administration of ANP enhances diuresis and
natriuresis; however, ANP also reduces PV and increases Hct in
nephrectomized rats (4, 32). The present experiments are analogous to
the nephrectomized-rat model in the respect that plasma ANP levels were
elevated in the absence of enhanced diuresis.
In conscious humans and animals, acute hypoxic exposures usually result
in natriuresis and diuresis (10, 13). However, these conscious models
did not exhibit the significant hypotension and cardiac output
reduction observed in our anesthetized rats ventilated with 10 or 8%
O2. For example, Colice et. al
(10) found arterial pressure to be only slightly depressed
(10%) in chronically instrumented rats exposed to a chamber
filled with 10.5% O2. Thus one
explanation for a lack of diuresis in the present study is a reduction
in renal perfusion pressure secondary to systemic hypotension. Along
this same line, Karim and Al-Obaidi (18) observed that direct
stimulation of carotid chemoreceptors with mixed venous blood resulted
in diuresis and natriuresis in anesthetized dogs only when carotid
sinus pressure was maintained at high levels. At low sinus pressure (72 ± 1.3 mmHg), chemoreceptor stimulation resulted in a reflex
decrease in renal blood flow and urine output. These findings are also
consistent with a reduction in renal perfusion as a mechanism of
antidiuresis in the present study.
Another similarity between the action of severe hypoxia and exogenous
ANP administration is that changes in TP concentrations were negligible
compared with changes in Hct. ANP has been shown to elevate
microvascular permeability to plasma proteins, increasing their
extravasation (31, 34). The resulting decreases in transcapillary colloid osmotic gradient could have favored filtration into tissues. The role of ANP in hypoxia-induced protein extravasation is discussed in more detail in a companion study (3).
Mechanisms of increased irANP during hypoxia.
Elevation in the plasma levels of ANP under conditions of acute hypoxia
has been reported in humans (19) and other mammals (2, 29) and is
attributed to increased secretion from the heart rather than decreased
metabolism of circulating ANP (8). In the present study, irANP
increased dramatically after only 10 min of hypoxic treatment with the
two lowest concentrations of O2.
Such immediate ANP responses to acute hypoxia have also been described
in conscious lambs (8) and in rats (29) and anesthetized pigs (2).
Although the response time is similar between species, the magnitude of
elevation is very different. The irANP response to hypoxia in our study
was 11-fold greater than that measured in either lambs or pigs (2, 8).
However, the 24-fold increase in irANP that we observed after 10 min of 8% O2 ventilation corresponds
closely to the ~22-fold increase seen by Shirakami et al. (29) in
conscious rats breathing 5% O2.
This suggests that even though arterial pressures were depressed during
acute hypoxia, the ANP response was not blunted in our pentobarbital
sodium-anesthetized rats. Sustained elevations of plasma irANP in rats
ventilated with 8 or 10% O2 have
also been reported in rats subjected to 90 min of hypoxia (16) and in chronically hypoxic rats (26, 35).
A number of mechanisms appear to be responsible for release of ANP in
hypoxic animals. ANP is released directly from isolated hearts perfused
with substantially lower PO2s than
those in our experiments (24). One additional effect of low inspired O2 in vivo is pulmonary
vasoconstriction. Correlation of pulmonary arterial pressure and plasma
ANP levels (1) and the presence of elevated plasma ANP in diseases in
which pulmonary hypertension is present (5, 21) have led others to the
conclusion that pulmonary hypertension is a major cause of ANP release.
Jin et al. (16) demonstrated an association between hypoxic pulmonary arterial hypertension and elevated plasma irANP in rats after 90 min of
exposure to 10% O2; however, the
observed increases in plasma ANP were small (2-fold) compared with
the present study. Moreover, these investigators have found that
observed increases in pulmonary arterial pressure were unaffected by
pretreatment with an ANP monoclonal antibody during the first 6 h of
hypoxia, whereas pressures were significantly reduced after 7 h of
hypoxia. Their results suggest that antihypertensive actions of ANP may be more important in the subacute rather than acute phase of hypoxia. It is not clear how this finding relates to the present study, in which
larger increases in irANP were observed.
Whether ANP was released immediately after the development of acute
hypoxic pulmonary hypertension in the present study is not known; the
first measurements of irANP were obtained after 10 min of hypoxia. A
pulmonary neural reflex active during hypoxic pulmonary hypertension
has been shown to account for ~50% of the ANP response to hypoxia in
the pig (7). One way that a pulmonary neural reflex could act on the
heart to release ANP is through the release of sympathetic agonists. A
study by Lew and Baertschi (24) supports such a role for sympathetic
control of hypoxia-induced ANP release, although the release mechanisms
they considered were intrinsic to the heart itself. They demonstrated
that about one-half of the hypoxia-induced ANP released from the
isolated rat heart is mediated by adrenergic receptors.
The present finding confirms previous reports that hypoxia-induced ANP
release is related to the level of alveolar
O2 rather than to the degree of
arterial hypoxemia (8). This points to an airway or pulmonary origin
for the difference in ANP release between the 10 and 8%
O2 groups. This could be due to a
higher sustained level of pulmonary arterial pressure in the 8%
O2 group relative to the 10%
O2 group or to a greater rate of
development of pressure to the same level. There is experimental
support that ANP release is more strongly correlated with the rate of
increase in pulmonary arterial pressure rather than the average
pressure (7). Another possibility is that airway or pulmonary
chemoreceptors sensitive to the level of inspired
O2 are involved in a reflex leading to ANP release. The presence of hypoxia-sensitive airway receptors called pulmonary neuroepithelial bodies has been described in
the lungs of rabbits (23), and recent work has identified the presence
of an O2-sensing mechanism on the
plasma membranes of these cells (36).
Release of ANP due to hypoxic damage to atrial tissue cannot be ruled
out, but the likelihood of this being the primary cause is remote.
Elevations of irANP in the 8% O2
group occurred immediately after the start of the hypoxia treatment,
before any evidence of possible heart damage or failure was present
(e.g., decreased CO). Studies on isolated perfused rat hearts have
shown elevations in ANP release during hypoxia to be reversible,
repeatable, and independent of tissue damage (24). HR changes were not
associated with ANP release immediately after the start of hypoxia;
however, a significant association was found between HR and ANP levels toward the end of the hypoxic period. This is consistent with the idea
that increased HR may be involved in the maintenance of ANP elevation
during hypoxia, as Lew and Baertschi (24) observed in isolated rat
hearts.
Summary.
Ventilation of anesthetized rats with 10 and 8%
O2 resulted in
PaO2s <40 Torr and large increases in
plasma irANP within 10 min. In rats ventilated with 15%
O2
(PaO2 ranging from 40 to 50 Torr), there
was no initial increase of irANP, but after 50 min of hypoxia, irANP
was slightly elevated. Although
PaO2s at 10 and 8%
O2 did not differ significantly,
irANP concentrations were substantially and significantly higher at 8%
O2. Thus the main stimulus for ANP
release in these animals is more closely related to inspired or
alveolar PO2 than to
PaO2 and may have arisen from receptors
in the airways or lungs. Proportional increases in Hct and decreases in
PV were observed in irANP-responsive rats. Plasma TP did not increase
in proportion to Hct; thus plasma protein was lost from the circulation
as well as fluid. Similar reductions in PV have been produced by
infusion of exogenous ANP in normoxic anesthetized rats (4, 32, 34).
Furthermore, our findings reveal that hypoxia-induced PV contraction
can occur even under conditions in which vascular pressures and renal
excretory function are suppressed. In a similar fashion, transvascular
fluid and protein shifts induced by exogenous ANP occur in the absence
of diuresis (4, 32) and independently of changes in capillary hydrostatic pressure (31). The results of the present study are
consistent with the hypothesis that ANP contributes to PV contraction
during acute hypoxia by favoring a shift of fluid and protein from the
circulation into extravascular spaces.
ACKNOWLEDGEMENTS
We thank E. Bravo, T. Myers, and G. Johnson for their technical assistance.
FOOTNOTES
1 · kg1
is almost exactly the same as that reported in the study by Kissling et
al., care must be used in interpreting our CO data. In general, thermal
exchange would result in underestimation of the fall in CO produced by
our treatments. Qualitative assessments of CO changes should be correct
in our study; however, because the direction and relative magnitude of
errors can be estimated. Briefly, this can be done by reliance on the
results from the Kissling et al. study, which show that heat diffusion
is directly related to the time that it takes for the bolus of sub-body
temperature fluid to flow through the lungs and be registered by the
thermocouple (i.e., passage time). In other words, an increased passage
time for the injectate leads to greater overestimation of CO. Because passage times did not change among any groups over the first 10 min, we
can assume errors to be similar among groups. Therefore, the
observation that CO was not different among groups over the first 10 min of treatment is not in error. The potential meaning of the
insignificantly lower CO for the 8%
O2 group at 30 min is
difficult to interpret because passage time did show a tendency to
increase above that for the other groups. In a consideration of the
later significant drop in CO, it appears likely that CO was decreasing
in the 8% O2 group at this time.
The measurement of a significantly lower CO in the 8%
O2 group at 50 min is
accurate, even though injectate passage time increased, because the
value is overestimated to a greater extent than that for the control.
Address for reprint requests: V. L. Tucker, Dept. Human Physiology, Univ. of California, Davis, Davis, CA 95616-.
Received 1 February 1996; accepted in final form 4 September 1996.
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3 SD from the group mean (at 50 min; see Fig.