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1 Department of Respiratory Physiology, ABSTRACT Cremona, George, Tim Higenbottam, Motoshi Takao, Edward A. Bower, and Leslie W. Hall. Nature and site of action of endogenous
nitric oxide in vasculature of isolated pig lungs. J. Appl. Physiol. 82(1): 23-31, 1997.
arterial and venous occlusion technique; double occlusion
technique; NG-nitro-L-arginine; hypoxic pulmonary vasoconstriction
INTRODUCTION THE ABILITY TO ACCOMMODATE increases in flow with
little change in pressure and the constrictor response to hypoxia are
physiological characteristics of the pulmonary circulation. It has been
speculated that endogenous vasodilators may play a role in mediating
these phenomena (7). The importance of basal nitric oxide (NO) release in the regulation of pulmonary vascular tone has generated a
considerable amount of work. In some mammalian species (6, 34),
including humans (6), basal endothelial nitric oxide (eNO) release has been shown to be an important determinant of low normoxic pulmonary vascular tone, whereas in other species there is little evidence of eNO
release under normoxic conditions (6, 18, 31). In the systemic
circulation, eNO release is stimulated by longitudinal shear stress (4,
38) and contributes to the adaptation of the diameter of blood vessels
to changes in pressure and flow. A similar role for eNO may also exist
in the pulmonary circulation of certain species. Evidence of
flow-induced eNO release in the pulmonary circulation has been
demonstrated at birth (1), and inhibition of NO synthase causes
pulmonary hypertension in exercising sheep (21).
Contrasting data have been reported regarding the part played by eNO
during hypoxic vasoconstriction. A number of studies have shown that
pulmonary hypoxic vasoconstriction is enhanced when eNO release or
action is inhibited (2, 26, 39), suggesting that eNO may be released
under hypoxic conditions, thereby modulating the vasoconstrictor
response. Other studies have shown a decrease in eNO production in
pulmonary vessels during hypoxia, suggesting that eNO may be directly
mediating hypoxic vasoconstriction (20, 42).
Little is known of the regional distribution of eNO production in adult
animals. Studies on regional pulmonary vascular resistance have focused
mainly on the effects of exogenous NO on preconstricted lungs of
animals exhibiting no basal eNO release (24, 37). In the present study,
we have performed arterial (AO), venous (VO), and double occlusion (DO)
maneuvers on isolated pig lungs to compare the sites of action of the
NO synthase inhibitor
NG-nitro-L-arginine
(L-NNA), (19), of exogenous NO,
and of acute hypoxia. We have also investigated the effects of
L-NNA on the flow-related
effects in the different segments of the pulmonary vascular bed. Pig
lungs were chosen because they exhibit a vigorous constrictor response
to acute hypoxia (32, 40) and because NO secretion is an important
determinant of their basal pulmonary vascular tone (6).
METHODS Preparation of In Situ Perfused Lungs
The heart was then stopped by an intracoronary injection of potassium
chloride (10
The perfusion circuit involved collection of autologous blood from the
pulmonary veins draining passively into a jacketed reservoir that kept
the perfusate temperature at 38°C. From the reservoir the perfusate
was pumped into the pulmonary artery by means of a roller pump (Watson
Marlow model 5001R, UK). A 150-ml reservoir with a small cushion of air
was interposed between the pump and the arterial cannula. This
reservoir acted as a pulse damper as well as a bubble trap. Perfusate
temperature was monitored with a thermistor in the inflow cannula.
Pulmonary artery and left atrial pressures were measured by matched
transducers (model P50 Spectramed) connected to side ports placed near
the tips of the cannulas. Pressures were referenced to the top of the
lungs. Inflow and outflow were measured by Doppler flow probes placed
on the inflow and outflow cannulas (model T101D, Transonic Systems,
Ithaca, NY). Pressures and flow were recorded continuously on a chart
recorder (model 404, W&W Scientific Instruments, Basel, Switzerland).
The perfusate consisted of autologous blood mixed with Dextran 70 to
give a hematocrit of 19-25%. Perfusion was instituted at 10 ml · min The lungs were ventilated with 21%
O2-74%
N2-5%
CO2 at a tidal volume of 12 ml/kg
and a frequency of 8-12 breaths/min. An air-filled pressure
transducer attached to the side port of the endotracheal tube was used
to measure airway pressure. A positive end-expiratory tracheal pressure
of 2 mmHg was applied, and a deep inspiration was periodically
simulated to prevent atelectasis.
Occlusion Maneuvers
The site of
action of endogenous and exogenous nitric oxide (NO) in isolated pig
lungs was investigated by using arterial, double, and venous occlusion,
which allowed precapillary, postcapillary, and venous segments to be
partitioned into arterial, precapillary, postcapillary, and venous
segments. NG-nitro-L-arginine
(L-NNA;
10
5 M) increased resistance
in the arterial (35 ± 6.6%, P = 0.003), precapillary (39.3 ± 5.1%,
P = 0.001), and venous (18.3 ± 4.8%, P = 0.01) segments,
respectively. Sodium nitroprusside
(105 M) and NO (80 parts/million) reversed the effects of
L-NNA. Total pulmonary vascular
resistance fell with increasing flow, due to a fall in precapillary
resistance and dynamic resistance, and was significantly
lower than mean total resistance.
L-NNA increased the resistances
but did not alter the pattern of the pressure-flow relationships. It is
concluded that, in isolated pig lungs, the effect of endogenous NO
seems to be dependent on flow in the arterial segment and independent
of flow in the precapillary segment, but variation of its release does
not appear to be fundamental to accommodation to changes in steady
flow.
3 M), and a
stiff cannula (ID 13 mm) was placed in the main pulmonary artery.
Through an incision in the left ventricle, another cannula (ID 16 mm)
was retrogradely inserted into the left atrium and secured by heavy
ties that prevented ballooning of the atrial appendage. The cannulas
were then connected to an external perfusion system (Fig.
1). The time from cardiac arrest to the
start of perfusion was never more than 20 min.
Fig. 1.
Experimental setup for constant flow perfusion of pig lungs in situ.
Details are explained in text. A/D, analog-to-digital.
[View Larger Version of this Image (25K GIF file)]
1 · kg1
and slowly increased by 10 ml · min1 · kg1
steps over an hour until a flow rate of 100 ml · min1 · kg1
was reached. Oxygen and carbon dioxide tensions as well as pH were
checked periodically (Instrumentation Laboratory, Milan, Italy). The pH
was maintained between 7.3 and 7.4 by the addition of small volumes of
sodium bicarbonate (1 M).
Analysis of Occlusion Tracings
To minimize the effect of interference by the clamps and the roller pump on the pressure signals, the digitized pressure signals were filtered via software (Acqknowledge, Biopac) by using low-pass Bessel filters with a cut-off frequency of 50 Hz. Because the analysis of the occlusion experiments required comparisons between pressures measured before and after blood flow had been stopped, pulmonary arterial (Ppa) and venous (Ppv) pressures measured before occlusion were corrected to zero flow to correct for the kinetic energy component. The value of this component was very small (0.3-0.7 mmHg) but was included for accuracy by means of the formula
|
is
the flow in ml · min1 · kg1,
W is the weight of the animal in
kilograms, d is the vessel diameter in
millimeters, and 0.0075 converts pascals to millimeters of mercury.
We have followed the method described by Hakim et al. (12) to analyze AO and VO tracings. After AO, the arterial pressure trace shows an initial rapid drop, followed by a more gradual fall (Fig. 2, A and B). The mean Ppa between 8 and 10 s after the occlusion (Ppa00) was determined, and this value used as the asymptote in the equation describing the fall in Ppa after AO because there was little variation in pressure at this time. The first 0.3 s after the occlusion was discarded because of noise caused by the maneuver, and a monoexponential relationship was fitted to the following 1.5 s of data by using a standard software package (Igor, Wavemetrics, CA).
|
). The fitted curve was
extrapolated to the instant of occlusion, defined as the time when
inflow reached zero. The instant when flow reached zero coincided with
the point at which the extrapolated line intercepted the rapid phase in pressure change. The intercept was taken as the pressure at the distal
end of the arterial segment (Pa
). Subtraction of Pa from Ppa yielded the pressure drop across the relatively noncompliant arterial segment (
Pa). The choice of these intervals was based on
previous work by Hakim et al., who showed that in this time interval,
selection of the begining and end of the data for extrapolation had
little effect on the slope of the exponential and the extrapolated occlusion pressure.
5 M) and of pulmonary
venous pressure during venous occlusion (VO) maneuvers before
(C) and after
(D)
L-NNA
(105 M). From these
tracings, changes in pressure across arterial and venous segments were
determined, as described in text. Pa, pressure at the distal end
of arterial segment; Pv, pressure at proximal end of downstream
venous segment.
VO tracings showed a rapid rise immediately after occlusion, followed
by a more gradual rise (Fig.2, C and
D). As in AO, the first 0.2 s was
discarded because of noise and a linear relationship was used to fit
the first 1.5 s of the gradual rise in Ppv. This was extrapolated back
to the time of occlusion, and the intercept was taken as the pressure
at the proximal end of the downstream segment (Pv). The pressure
gradient across the downstream veins was calculated as Pv = Ppv 
Pv. From DO tracings, the mean pressure for 2 s after
equilibration was measured and taken as capillary pressure (Pc) (8).
From this measurement, another two gradients, namely, precapillary
(Pa = Pa Pc) and postcapillary (Pv = Pc Pv) venous gradients, were obtained. In
this way the pressure drop across the pulmonary vasculature was
partitioned into four functional segments, defined for convenience
as arterial, precapillary, postcapillary, and venous segments (11).
Experimental Protocols
Dose-dependent effects of exogenous NO and sodium nitroprusside. The dose-dependent effects of NO and sodium nitroprusside on total pulmonary vascular resistance (PVR) were studied in four pig lungs perfused at 100 ml · min1 · kg1
and with Ppv set at 6-8 mmHg by adjustment of the height of the venous reservoir. These were first preconstricted by addition of
L-NNA (Sigma Chemical) to the
venous reservoir (10 mg in 2 ml saline to give a final concentration in
the perfusate of 105 M). NO
was subsequently added to ventilation mixture such that concentrations
of 10, 40, 80, and 160 parts/million (ppm) NO measured by
chemiluminescence (model 42, Thermoelectron, Warrington, UK) were
delivered to the lungs for 10 min. This was followed by cumulative doses of sodium nitroprusside (from 100 µg to 100 mg in 2 ml vehicle, final concentrations = 107
to 104 M).
Inhibition of NO synthesis and the effects of exogenous NO.
The effect of inhibition of NO synthesis on segmental pressure
gradients was studied in five lungs. After 1 h of perfusion, flow was
set at 100 ml · min1 · kg1
and Ppv at 6-8 mmHg. After the pressures had stabilized, AO, VO,
and DO were performed three times. The order of the maneuvers was
interchanged in consecutive measurements. After control measurements were taken, L-NNA was added to
the venous reservoir (10 mg in 2 ml saline to give a final
concentration in the perfusate of 105 M) and, after a stable
Ppa tracing was achieved, usually ~5 min, AO, DO, and VO maneuvers
were repeated. NO was subsequently added to the ventilation mixture
such that a final concentration of 80 ppm NO measured with a
chemiluminescent NO analyzer (model 42, Thermoelectron) was delivered
to the lungs. AO, DO, and VO were performed after 10-min ventilation
with NO, after 10 min without NO, and after addition of sodium
nitroprusside (10 mg in 2 ml vehicle, final concentration = 105 M). The dose of
L-NNA selected was one that had
given maximal effects in previous experiments (6).
Effects of hypoxia.
The effects of hypoxia on the segmental pressure gradients was
investigated in five lungs. After 1 h of perfusion, flow was set at 100 ml · min1 · kg1
and Ppv at 5-6 mmHg, and AO, VO, and DO were performed during normoxic conditions. Hypoxic vasoconstriction was induced by
ventilation with a gas mixture containing 5%
O2-5%
CO2-90%
N2 for 5-10 min (Table
1).
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1 · kg1)
and at a constant outflow pressure of 5-6 mmHg. The flow rates were chosen because of the linear relationship between flow and pressure in this range (41). The lungs were subsequently treated with
L-NNA
(105 M), and the occlusion
maneuvers were repeated at the same three flow rates.
Statistical Analysis
PVR in the different segments was calculated from the pressure gradients divided by the flow. The changes in PVR during inhibition of NO synthesis and administration of exogenous NO were analyzed by one-way analysis of variance by using Scheffé's correction for multiple comparisons. To compare the effects of L-NNA and hypoxia on segmental PVR, the data were normalized by expressing the change in PVR in each segment as a percentage of the change in total PVR across the pulmonary vascular bed
|
In the experiments at varying flow, linear-regression analyses of
pressure gradients as functions of flow were performed on the data for
each experimental protocol. From the regression data, dynamic
resistance was calculated as the slope of the relationship between
pressure gradient and flow (dP/d) and was compared
with the mean of the PVR values at different rates of flow
(
).
This was carried out to avoid extrapolation beyond the range of the data. A significantly greater value of
indicated that the regression line would extrapolate to a positive
intercept on the pressure axis. The differences between
dP/d and
and between PVR values at maximum and minimum flows were analyzed by
Student's t-test for paired data.
Results are presented as means ± SE.
P values < 0.05 were considered
significant.
RESULTS
Action of L-NNA, Inhaled NO, and Perfused Nitroprusside
The mean arterial and venous pressures at a flow rate of 100 ml · min1 · kg1
as well as the perfusate gas tensions for each experimental condition are shown in Table 1. The mean PVR across the total pulmonary vascular
bed under control conditions was 0.243 ± 0.014 mmHg · ml1 · min · kg.
The resistance was distributed as follows: arterial 31 ± 1.7%,
precapillary 20 ± 2.1%, postcapillary 14 ± 1.8%, and venous
35 ± 1.6%. Inhibition of NO synthesis with
L-NNA
(105 M) increased total PVR
by 0.19 ± 0.02 mmHg · ml1 · min · kg
(P < 0.05). Both NO and sodium
nitroprusside showed dose-dependent decreases in total PVR with little
change at higher concentrations (Fig. 3).
The increase was mainly observed in the arterial and precapillary
segments, which increased by 35 ± 6.6% (P = 0.003) and 39.3 ± 5.1%
(P = 0.001), respectively (Fig.
4). A smaller but significant increase was
also observed in the venous segment (18.3 ± 4.8%,
P = 0.01), but changes in the
postcapillary segment were not consistent. No change in pH was observed
after addition of L-NNA (Table
1).
, NO; 
, SNP. Dose-response relationships of PVR to NO and SNP after preconstriction with L-NNA
(105 M) are shown. Values
are means ± SE. ppm, Parts per million. Brackets denote
concentration. * Significantly different from constricted level,
P < 0.05 (Scheffé's test).
5 M) and subsequent
inhaled NO (80 ppm) and SNP on segmental PVR measured by occlusion
technique. After termination of ventilation with NO (off NO),
resistance returned to elevated levels caused by
L-NNA, and this was taken as a
2nd baseline. Values are means ± SE. * Significantly
different from control, P < 0.05 (Scheffé's
test).
Ventilation with NO (80 ppm) decreased the resistance in the arterial
and precapillary segments to control levels but had no effect on the
venous segment. After cessation of ventilation with NO, resistances in
all segments returned to the high level observed after addition of
L-NNA. Addition of sodium
nitroprusside (105 M)
decreased the resistances in the arterial, precapillary, and venous
segments to levels not significantly different from control.
Effects of Hypoxia
The mean inflow and outflow vascular pressures as well as the gas tensions and pH are given in Table 1. In the five lungs studied, the mean total PVR across the pulmonary vascular bed under normoxic conditions was 0.243 ± 0.024 mmHg · ml1 · min · kg
at a flow rate of 100 ml · min1 · kg1.
The PVR was distributed as follows: arterial 31 ± 2.7%,
precapillary 20 ± 2%, postcapillary 15 ± 2.4%, and venous 32 ± 1.8%. Ventilation with a hypoxic mixture of 5%
O2 caused an increase of 0.108 ± 0.04 mmHg · ml1 · min · kg
in total PVR (P = 0.01; Fig.
5). This increase was concentrated in the
precapillary (75 ± 10.6%, P = 0.001) and venous (26.4 ± 1%,
P = 0.01) segments, but changes in
other segments were not significant.
Effects of Changing Flow Rate at Constant Outflow Pressure
Under normoxic conditions, there was a linear relationship between the total pressure gradient and flow in the range of flow explored (r = 0.99, P = 0.005; Fig. 6). The dynamic resistance (measured as dP/d) was significantly lower than
(0.147 ± 0.02 and 0.318 ± 0.04 mmHg · ml1 · min · kg,
respectively, P = 0.03) indicating
that there was a positive intercept pressure, and the total PVR was
smaller at a flow rate of 100 ml/min than at 50 ml/min as expected
(Table 2). The decrease in total PVR was
mainly due to the precapillary segment (Table 2.)
,
Before L-NNA; , after
L-NNA (105 M). Results are means ± SE. See text and Table 2 for significant effects.
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L-NNA
(105 M) increased both
dP/d (0.40 ± 0.04 vs. 0.32 ± 0.04 mmHg · ml1 · min · kg)
and
(0.548 ± 0.06 mmHg · ml1 · min · kg,
but
was still significantly higher than dP/d (P = 0.03). After
L-NNA, total PVR was lower at
maximum than at minimum flow (Table 2), due to a significant fall in
precapillary segmental resistance
(Ra). The
decrease in Ra with flow was
similar before and after L-NNA
(42 vs. 43%); however, the absolute change was greater after
L-NNA (0.08 vs. 0.04 mmHg · ml1 · min · kg,
P < 0.05). Furthermore,
L-NNA increased the slope of the
dPa/ line (0.084 ± 0.01 vs. 0.268 ± 0.01 mmHg · ml1 · min · kg,
P < 0.05) and caused an
upward shift in the dPa/ line
(
from 0.066 ± 0.01 to 0.139 ± 0.02 mmHg · ml1 ·
min · kg, P < 0.05), with little or no change in dPv/ and dPv/ lines (Fig.
7).
Pa), precapillary (Pa), postcapillary (Pv), and
venous (Pv) segments] measured by arterial, venous, and double
occlusion in isolated pig lungs (n = 5). , Before L-NNA; ,
after L-NNA
(105 M). Results are means ± SE. See text and Table 2 for significant effects.
DISCUSSION
The main findings of this study are that inhibition of basal eNO
release increased resistance in the arterial, precapillary, and venous
segments and that L-NNA caused
an increase in dPpa/ due to arterial segmental
resistance and an upward shift of the pressure-flow line
due to Ra. Hypoxia caused a
significant increase in Ra and
a small increase in venous segmental resistance.
The pressure gradient across the pulmonary vasculature perfused at 100 ml · min1 · kg1
in our study was more than twice that reported in dogs (16), although
very similar to that reported in isolated pig lungs (36, 40, 41). In
the study by Rock et al. (36) on isolated pig lungs by using AO and VO,
the arterial segment accounted for 26%, the middle compartment for
39%, and the venous 35% of the total pressure gradient. This is in
agreement with our results, although by using AO, VO, and DO we were
able to further subdivide the pressure gradient across the pulmonary
vascular bed into four functional segments. In this study, the middle
compartment (Pa + Pv) accounted for a similar
proportion of the total pressure drop (34%), of which 20 ± 2.1%
was due to the precapillary segment. In dogs studied by AO, VO, and DO
where occlusion curves were analyzed in the same way, the middle
segment accounted for 26% of the pressure drop, but only 9% of this
was due to the precapillary segment (11). Pa and Pv are
thought to represent pressures in vessels between 900 and 50 µm (15); whether the same applies in pig lungs is not
known, but it is probable. The difference in longitudinal distribution
of resistance may be related to the structural differences of the
pulmonary vasculature between the two species because in the pulmonary
arteries of the pig, unlike in those in the dog, muscularization is
normally present down to diameters of about 50 µm (22).
The increase in total PVR with L-NNA observed in this study was similar to that reported previously with other specific inhibitors of NO synthase, such as NG-nitro-L-arginine methyl ester and NG-monomethyl-L-arginine (6). Resistance was increased mainly in the arterial and precapillary segments. A small but significant increase was also seen in the venous segment. Endogenous basal production of NO therefore acts principally on the arterial side of the porcine pulmonary vascular bed. In neonatal pig lungs, NG-nitro-L-arginine methyl ester increased both the resistance upstream and downstream of the double occlusion pressure (29), presumably due to the greater muscularization of small arteries after the first 2 wk of life (35). Similar differences have been observed in lambs; during normoxia, L-NNA increased arterial but not venous segmental resistance at 1 mo of life, whereas at 2 days an increase in both arterial and venous segments was observed (9). Administration of 80 ppm NO by inhalation reversed the effects of L-NNA on the arterial and precapillary segments in agreement with the notion that NO synthesis maintains tone in the pulmonary arterial tree. NO inhalation did not reverse the effects of L-NNA on the venous segment, perhaps due to the greater diffusion distance between airways and the vessels or due to the low tone that was present in the veins. The addition of sodium nitroprusside reversed the effects in all the segments. Very few studies have examined the effects of eNO synthesis inhibition on basal segmental resistance. In the dog lobe, PVR and its distribution change little after L-NNA (13). A number of studies have examined the effects of NO inhalation in preparations at elevated tone. For example, in lungs constricted with endothelin-1, inhaled NO dilated small arteries and veins but had no effect on larger vessels (37). In lungs perfused with Krebs solution, both inhaled NO and sodium nitroprusside affected large vein capacitance. On the other hand, in isolated rabbit lungs perfused with Krebs solution or blood and constricted with the thromboxane analogue U-46619, inhaled NO affected all the resistance segments equally (24). The discrepancy among these findings may be attributed to the difference in animal models used and to the difference in the agents used to constrict the vasculature. In contrast to the pig, the rat and rabbit pulmonary vascular beds show little or no basal eNO release, and inhibition of NO synthesis had little or no effect under normoxic conditions. It may be that NO-induced dilatation may be dependent on the existing tone present in the particular vascular segment. In the pig, veins and arteries follow the branching pattern of the airways and it is likely that the distance across which NO would have to diffuse to reach the arteries and veins is similar.
The main effect of hypoxia was observed in the precapillary segment, although a small but significant increase was also observed in the venous segment. These findings are in agreement with those reported in adult pig lungs (36), canine lungs (11, 17), and neonatal pig lungs (30). The relationship among hypoxia, NO synthesis, and hypoxic pulmonary vasoconstriction is complex. Direct measurement of NO in cultured cells (42) or in expired air (5, 10) has been shown to decrease during acute hypoxia. However, inhibitors of NO synthesis appear to enhance hypoxic pulmonary vasoconstriction (2, 30, 34) and during unilateral alveolar hypoxia cause a reduction in flow to the hypoxic lung (39). Furthermore, in both adult and neonatal pig lungs, hypoxic vasoconstriction still occurred after inhibition of eNO synthesis (6, 29), suggesting that reduced eNO release is not the mechanism directly responsible for hypoxic pulmonary vasoconstriction. In the present study, hypoxia resembled L-NNA in acting predominantly on the precapillary segment. The lack of effect of hypoxia on the proximal arterial segment may reflect a qualitative difference in the action of L-NNA. L-NNA abolishes NO production, whereas hypoxia may merely reduce it.
The pressure-flow relationship in the pig pulmonary vascular bed was
linear over the range of flows investigated, in agreement with
previously reported studies in this species (28, 41). In this study, it
was possible to examine only three flow rates, which precluded a
precise extrapolation to zero flow. However, the observed fall in
resistance with increasing flow is in accordance with previous reports
that attribute it to mechanical expansion of partially collapsed
vessels (3, 27, 33). In the present study, resistance was constant with
increasing flow in arterial, venous, and postcapillary segments but
fell significantly in the precapillary segment. The concentration of
sensitivity to flow in the precapillary segment is consistent with the
presence of a Starling resistance in the precapillary segment that
increased after L-NNA. Similar
effects of flow on segmental resistance have been reported in AO and VO
studies carried out in pigs (36) and other species (14). Although the
results of these studies have been interpreted according to the classic
Ohmic-Starling resistor model, alternative models have been proposed
that could similarly explain the results (25). The current work does
not present data to support any particular model, and for this reason we have purposely chosen to refer to purely descriptive terms such as
dynamic resistance and
to interpret our results.
In the range of flow rates studied, the localization and persistence of the effects of flow on segmental resistance after L-NNA treatment do not support variation of eNO release by shear stress as a fundamental mechanism of accommodation to flow, although it may be a contributing factor. In sheep (23), NO synthase inhibition raised pulmonary vascular tone equally at rest and during exercise, suggesting that NO release is not enhanced by exercise. In canine femoral arteries, Rubanyi et al. (38) showed that doubling flow rate produced significant relaxation of the perfused vessels even in the presence of indomethacin. The low tone and rapidly branching structure of the pulmonary vascular bed may account for the lack of evidence of shear stress release of NO in our study. This may also explain the results reported by Hakim (13), who found that the decrease in precapillary resistance with flow was attenuated following L-NNA only during hypoxia and with the use of pulsatile flow, suggesting that the increments in shear stress necessary to demonstrate flow-induced NO release in the pulmonary vascular bed must be much larger than those in the systemic circulation.
In conclusion, there appear to be important regional differences in basal eNO production in the segments of the pulmonary circulation. The arterial and precapillary eNO release in pig lungs contribute more than the venous segment to low PVR. NO release does not appear to be fundamental to the mechanism underlying accommodation to increasing steady flow because flow-dependent changes in resistance occurred before and after L-NNA. Because there are important differences among animal species in basal release of eNO in the lung, it is likely that differences among species also occur in terms of regional eNO production.
ACKNOWLEDGEMENTS
We are indebted to Dr. Tawfic Hakim, Department of Surgery, State University of New York Health Science Center, for considerable help and discussion on the occlusion technique. We thank Mr Richard Eastwood of the School of Clinical Veterinary Medicine for technical support.
FOOTNOTES
Address for reprint requests: T. Higenbottam, Dept. of Respiratory Medicine, Medical School, Floor F, Univ. of Sheffield, Beach Hill Road, Sheffield S10 2RX, UK.
Received 8 July 1994; accepted in final form 12 August 1996.
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