1 Division of Pediatric
Critical Care, University of Colorado Health Sciences Center,
Denver 80262; and Departments of
2 Surgery and
3 Physiology, Tremendous changes
in pressure and flow occur in the pulmonary and systemic circulations
after birth, and these hemodynamic changes should markedly affect
endothelial cell replication. However, in vivo endothelial replication
rates in the neonatal period have not been reported. To label
replicating endothelial cells, we administered the thymidine analog
bromodeoxyuridine to calves ~1, 4, 7, 10, and 14 days old before they
were killed. Because we expected the ratio of replicating
to nonreplicating cells to vary with vascular segment, we examined the
main pulmonary artery, a large elastic artery, three sizes of
intrapulmonary arteries, the aorta, and the carotid artery. In normoxia
for arteries < 1,500 µm, ~27% of the endothelial cells were
labeled on day 1 but only ~2% on
day 14. In the main pulmonary artery,
only ~4% of the endothelial cells were labeled on
day 1 and ~2% on
day 14. In contrast, in the aorta,
~12% of the endothelial cells were labeled on day
1 and ~2% on day
14. In chronically hypoxic animals, only ~14% of the
endothelial cells were labeled on day
1 in small lung arteries and ~8% were still labeled
on day 14. We conclude that the
postnatal circulatory adaptation to extrauterine life includes
significant changes in endothelial cell proliferation that vary
dramatically with time and vascular location and that these changes are
altered in chronic hypoxia.
normoxia; hypoxia; bromodeoxyuridine; pulmonary arterial pressure; endothelial in vivo replication
AT BIRTH, when the fluid-filled fetal lung becomes
inflated with air, pressure and probably flow fall in the main
pulmonary artery, whereas flow increases in small lung arteries and the aortic arch (1). Concomitant with these hemodynamic changes are
significant structural changes, especially in small lung arteries where
the vascular walls thin and luminal diameters increase. Modification of
the endothelium appears to contribute in a major way to this arteriolar
structural adaptation. After birth, endothelial cells in lung
arterioles become flattened and stretched and increase their length
along the arteriolar axis and their width (8). In contrast, endothelial
cells in the main pulmonary artery (MPA) show little morphological
change, probably because they are not stretched by lung inflation and
because flow does not significantly increase. In addition to causing
morphological and structural changes in the endothelial cells,
mechanical stress from stretch alone or the combination of stretch plus
shear has been shown to increase endothelial replication in vitro (9,
15, 20, 22-24, 26, 27). It seemed likely, then, that significant
changes in endothelial replication rates might also be observed after birth and that important differences in replication rates would exist
between different arterial segments in the lung circulation. However,
although changes in endothelial cell replication likely contribute to
postnatal adaptation of the pulmonary circulation, we can find no
studies that have evaluated in vivo endothelial cell replication rates
in the immediate neonatal period.
Therefore, we performed quantitative estimates in vivo in newborn
animals (calves) of lung arterial endothelial cell replication rates in
terms of magnitude, arterial location, and time course as related to
hemodynamic parameters. Although we could not measure endothelial
stretch or shear, we could estimate directional changes with birth
based on reported fetal (18) and our measured postnatal hemodynamic
data. We considered that, after birth, the rate of endothelial cell
replication would be greatest in those lung arteries most likely to be
subjected to the greatest mechanical forces during the transition from
fetal to postnatal life.
Our approach was to measure endothelial replication rates sequentially
after birth in arteries along the pulmonary vascular tree to describe
the pattern of response. We administered the thymidine analog
bromodeoxyuridine (BrdU) to neonatal animals 17, 9, and 1 h before
death to label in vivo those cells synthesizing DNA. We utilized large
mammals (calves) because hemodynamic measurements could be conveniently
made and because pressures in the term fetus have been reported (18).
For comparison, we also measured endothelial replication rates and
hemodynamics in the aorta. We also performed similar studies in calves
in which normal postnatal transition had been interrupted by placing
them shortly after birth into a hypobaric chamber, thereby maintaining
the lung arteries in an hypoxic and high-pressure environment (4, 7,
21, 28). Because a previous study (2) demonstrated that
interruption of the normal postnatal pulmonary circulatory transition
caused endothelial cells to retain, for a time, their fetal shape, we expected, at least in small pulmonary arteries, interruption of replicative change. Such studies seemed important given the key role of
the endothelium in the fetal-neonatal transition (1, 8), the lability
of the lung circulation at this time of life (1), the lack of prior
reports on endothelial replication at birth, and the need for better
understanding of factors controlling normal endothelial replication in
vivo.
General. Thirty-two male Holstein
calves born near Fort Collins, CO, were studied. At ~24 h of age
(day 0), the calves were randomly
assigned to a normoxic or hypoxic group. After assignment, the 16 normoxic calves continued to live at Fort Collins altitude (1,500 m;
barometric pressure ~640 mmHg), where 1, 4, 7, 10, or 14 days later,
hemodynamic measurements were made and the calves were then killed,
with the number in each age group shown in Table 1. The other 16 calves followed the same
protocol except that, after assignment to the hypoxic group, they were
placed in a hypobaric chamber at 4,570-m simulated altitude (barometric
pressure 430 mmHg). We obtained pulmonary arterial and wedge pressures
under local anesthesia with a 7F Swan-Ganz catheter introduced
percutaneously via a jugular vein. We measured systemic arterial
pressure through a polyethylene catheter introduced via a saphenous
artery into the abdominal aorta. Arterial blood samples were taken for
blood gas analysis. Cardiac output was measured by injecting
cardiogreen dye into the pulmonary artery and sampling in the aorta.
[Ductal shunting is negligible in calves at these ages, but when
pulmonary arterial pressure greatly exceeds aortic pressure,
right-to-left shunting through the foramen ovale can give a falsely
high output measurement (21).] After the in vivo measurements,
the calves were then given heparin (100 mg iv) and killed by a rapid
intravenous injection of 800 mg of pentobarbital sodium (4). Calf
weights were not measured, but previous experience with this breed has shown birth weights of 40 ± 6 kg to be standard (7, 17, 21, 28).
Furthermore, after birth, body weight in calves does not change during
the first 2 wk of life (18).
ABSTRACT
Top
Abstract
Introduction
Methods
Results
Discussion
Appendix
References
INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
Appendix
References
METHODS
Top
Abstract
Introduction
Methods
Results
Discussion
Appendix
References
Table 1.
Hemodynamic and blood gas measurements in neonatal calves during
normoxia and hypoxia
To label dividing cells, we gave each calf a 500-mg intravenous injection of BrdU (6, 11, 17, 25) at 17, 9, and 1 h before death (4). BrdU is a thymidine analog that is incorporated into the nuclear DNA of cells traversing the S phase of the cell cycle during exposure to the analog. If it is assumed that cell cycle time, estimated at 8 h, does not change for the vascular endothelial cells studied, then the BrdU labeling indexes reflect the fraction of cells actively proliferating in the cell cycle versus those in the Go phase (4, 25) for three 8-h periods and thus provide indexes of active cell proliferation over ~24 h (6, 14, 17). BrdU endothelial staining for the MPA and a lung arteriole are shown in Fig. 1.
|
Immunohistochemistry. The right lung was quickly removed and perfused with the alcohol-based Omnifix (Xenetics Biomedical, Tustin, CA). The airways were distended with the fixative at 25 cmH2O. The pulmonary artery was perfusion fixed at the pressure measured during life. The airways and vessels were then ligated, and the lungs as well as sections of the aorta and common carotid artery were immersed in fixative. After fixation, four or more lung tissue blocks were taken along the longitudinal axis of the pulmonary artery from the proximal to the distal lung for study of the tunica media and tunica adventitia of these calves as reported previously (4). The blocks were embedded in paraffin and cut into 4-mm-thick sections. Immunohistochemistry was used to identify cells incorporating BrdU (4, 6, 14, 17, 25). As previously described from this laboratory (4, 17, 25), the paraffin was removed, and the sections were rehydrated by incubation in a graded series of xylene, acetone, and ethanol. Endogenous peroxidase activity was quenched by incubation with 0.3% H2O2 in absolute methanol. Sections were then treated with 0.1% protease in phosphate-buffered saline for 4 min, 20 N HCl for 20 min, and 1% diluted horse serum for 20 min (25). After washes in buffered saline, the sections were incubated sequentially with 1:200 monoclonal anti-BrdU in 0.5% Tween 20-1% bovine serum albumin in buffered saline, biotinylated horse anti-mouse immunoglobulin G, and avidin-biotin-horseradish peroxidase complex. The sections were then incubated for 7 min in diaminobenzidine tetrahydrochloride in Tris buffer (0.5 M, pH 7.6) with 3% hydrogen peroxide. The sections were counterstained with hematoxylin, dehydrated in a graded series of ethanol and xylene, covered with a coverslip, and examined by light microscopy.
The following vessels were examined: 1) MPA, diameter ~2 × 104 mm; 2) the largest intrapulmonary pulmonary artery observed (large elastic artery), ~1 × 104 mm in diameter; 3) pulmonary arteries of external diameter 300-1,500 mm and accompanying airways > 500 mm (small elastic artery); 4) arteries of external diameter 100-150 mm associated with airways > 200 and < 500 mm (large muscular artery); and 5) arteries 30-150 mm in diameter with associated airways < 200 mm (small muscular artery) (3). The aorta and common carotid artery were also examined. The BrdU labeling index, which is the percentage of total nuclei labeled with BrdU, is reported. The total number of nuclei counted was >250 for each diameter class of artery in each animal studied. For the large muscular arteries, counts of nuclei were in a total of 8-12 vessels from histological sections from up to 4 different tissue blocks, and for the small muscular arteries, it was necessary to count nuclei in 20-32 vessels.
Statistics. For the hemodynamic data during normoxia or hypoxia (Table 1), correlation of each variable with postnatal age was examined for significance (P < 0.05). ANOVA was used to determine whether measurements for each variable during hypoxia differed from those during normoxia (P < 0.01).
The overall analyses of the percentages of endothelial cell nuclei labeled with BrdU during normoxia and hypoxia, shown in Table 2, are given in STATISTICAL APPENDIX. For a graphic presentation of the data in Table 2, percentages of the labeled nuclei plotted against postnatal age are shown along with regression coefficients (see Fig. 2) to emphasize the different findings for the three smallest diameter arteries (<1,500-mm diameter) in comparison to those of the two largest arteries (MPA and large elastic artery). For the aortic and carotid arterial endothelial cell labeling ratios during normoxia or hypoxia, we examined the correlation with postnatal age for significance (P < 0.05). We used ANOVA to determine whether measurements in the aorta and carotid artery during hypoxia differed from those during normoxia (P < 0.05).
|
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Hemodynamics and blood gases. Normoxic calves had lower pulmonary arterial pressures and higher arterial PO2 and PCO2 values than the hypoxic calves (Table 1). Wedge pressures were increased in the older hypoxic calves. Aortic pressures and cardiac output values were not different between the groups. There were no significant age-related changes in either group for the measured variables shown in Table 1.
Normoxia. A striking finding in normoxia on day 1 was that >25% of endothelial cells were labeled with BrdU in small (up to 1,500-mm-diameter) lung arteries, including small elastic, large muscular, and small muscular arteries (Table 2). In these three classes of small arteries, the percentages of labeled nuclei were sharply lower in calves of increasing postnatal age and were only 1-2% in calves age 14 days (Table 2, Fig. 2A). The percentages of labeled nuclei for the small arteries in each of the three calves on day 1 were all higher than the percentages observed for any of these sized arteries in the four calves on day 14 (Fig. 3A).
|
|
In contrast, on day 1, <6% of endothelial cells were labeled with BrdU in large lung arteries, the MPA, and the large elastic artery (Table 2, Fig. 2A). The percentages of labeled nuclei for these large arteries in each of the three calves on day 1 were not different from the percentages observed for these large-sized arteries in the four calves on day 14 (Fig. 3A). Thus, in normoxia, the labeling of lung endothelial cells with BrdU depended both on the size of the artery and on the age of the calf (see STATISTICAL APPENDIX).
Endothelial labeling ratios were examined in the aorta to provide a comparison with those in the MPA and to determine whether the two great arteries would have a similar or dissimilar pattern of measurements. In contrast to the MPA, the aorta showed a high labeling ratio on day 1 and a subsequent decrease with postnatal age (Table 3). Endothelial labeling ratios were examined in the carotid artery to provide a comparison with those in a large elastic pulmonary artery. As in the large elastic pulmonary artery, the carotid artery labeling ratio showed no significant change with postnatal age, although the ratios in the carotid artery tended to be less than those in the large elastic pulmonary artery.
|
Chronic hypoxia. Chronic hypoxia in these neonatal calves maintained pulmonary arterial pressure at or above the values reported in the term fetus (81 ± 3 mmHg) (18), and, therefore, the hypoxia interrupted the normal transition from the high-resistance fetal lung circulation to the low-resistance circulation (Table 1). In hypoxia on day 1, averages of 12-17% of endothelial cells were labeled with BrdU in small (up to 1,500-mm-diameter) lung arteries, including small elastic, large muscular, and small muscular arteries (Table 2). These percentages of labeled nuclei were somewhat higher than those in calves of increasing postnatal age, where, at 14 days, the averages were 5-11% (Table 2, Fig. 2B). However, the percentages of labeled nuclei for these small arteries in each of the three calves on day 1 showed considerable overlap with the percentages observed for these sized arteries in the four calves on day 14 (Fig. 3B). Although there was a significant effect of postnatal age on the labeling index, the effect of age was relatively small (see STATISTICAL APPENDIX).
On day 1, averages of ~5 and 7% of endothelial cells were labeled with BrdU in the large elastic artery and MPA, respectively (Table 2, Fig. 2B). The percentages of labeled nuclei for these large arteries in each of the three calves on day 1 were not different from the percentages observed for the large-sized arteries in the four calves on day 14 (Fig. 3B).
Comparison of the percentages of endothelial cells labeled with BrdU for all sizes of pulmonary arteries at all ages studied indicated that the percentages for the hypoxic calves differed from those of the normoxic calves (see STATISTICAL APPENDIX). The detailed analysis indicated that in the hypoxic calves the percentages were higher than in the normoxic calves only for the smaller arteries in the older calves (Table 2).
On day 1, the hypoxic calves had similar labeling ratios in the aorta and the MPA (Tables 2 and 3). However, the ratio decreased in the aorta but not in the MPA with increasing postnatal age. Labeling of endothelial cells in the hypoxic carotid artery tended to be less than in the aorta; labeling did not change with postnatal age, and it did not differ from that in the large elastic pulmonary artery in the hypoxic calves.
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Normoxia. For the normoxic neonatal calves, the main finding was that the BrdU endothelial cell labeling index in the three groups of small lung arteries, i.e., those <1,500 mm in diameter, was high (>25%) during the first 2 days after birth and fell rapidly thereafter (to ~1% by day 14). Because BrdU is a thymidine analog that is incorporated into the nucleus during the ~8-h duration of the S phase of cell division and because we administered the BrdU label every 8 h before death, the percentages of labeled nuclei were considered indicators of cell replication per 24 h (4, 6, 14, 17, 25). We could not confirm these findings from prior reports because we did not find previous work describing developmentally related in vivo replication rates of endothelial cells. However, the endothelial replication rate of intra-acinar lung arteries < 100 mm in diameter in adult rats has been reported to be 1.4% (13), similar to the value we found in small arteries in 2-wk-old calves.
For the MPA and a large elastic lung artery in our calves, there was less labeling at postnatal age 1-2 days (~3-5% of nuclei), and the decrease in labeling by day 14 to ~2% was not significant, but the trend was similar to that in the small arteries. These replication rates are somewhat higher than the 0.54% replication rate reported (13) from the hilar pulmonary artery of the left lung in the adult rat. For the aorta in our calves, rates fell from 12 to 2% during the 14 days of study. Although the fall from 1.6 to 0.7% in the main carotid artery was not significant, the rate at 14 days considerably exceeded that of the 0.02% reported (19) for the carotid artery of the adult rat. Although the results suggested, for the arteries examined, that endothelial replication rates tended to be higher soon after birth than later in life, the striking decreases were in the small lung arteries and the aorta.
One wonders whether the changes in endothelial replication were related to the transition that occurs at birth. By the time proliferation was assessed at the youngest age, the calves were already 1-2 days old (experimental day 1), and most of the dramatic postnatal pressure, flow, and blood gas changes known to occur in the pulmonary circulation had already taken place. Thus the high labeling index on day 1 in the small pulmonary arteries occurred in the setting of dramatic hemodynamic and PO2 changes from the fetal state. Compared with the arterioles in the fetus, those in the 1- to 2-day-old newborn must have dilated as the alveolar PO2 increased (21) and must have stretched longitudinally as the lung expanded (8). Vascular dilation and stretch both would be expected to place mechanical stress on the endothelial cells of the small pulmonary arteries (8). Of interest, the likely increases in pressure and flow soon after birth in the central aorta would cause it to dilate (28) and place hemodynamic stress on its endothelium. Little or no dilation in the MPA would be expected with normal lung inflation, nor has endothelial stretch been found (8).
Although effects of PO2 per se on lung endothelial cells in vivo is not clear, mechanical stretch, shear stress, and both together alter endothelial cell structure and function and may within 1 or 2 days lead to increased replication (3, 5, 15, 22-24, 27). A primary endothelial function is to provide a lining for the arterial lumen. Shear stress or arterial wall stretch during the first 2 postnatal days could stimulate endothelial replication, which would act to maintain integrity of the lining. One wonders whether the relative stabilization of pressures and flows after day 2 allowed the high endothelial replication rates to diminish thereafter.
Endothelium is merely one component of the arterial wall, and a more complete picture of developmental changes within the vessel wall would include at least a comparison with the other two major components, the media and the adventitia. In these calves, as previously reported (4), BrdU-labeled nuclei were counted, and labeling indexes were also computed in the media and adventitia. Comparison of the endothelium, media, and adventitia for all three arterial segments of diameter < 1,500 mm showed that the replication rates in the endothelium greatly exceeded those in the media and adventitia on day 1 but that, by day 14, the replication rates were similar in all three layers (Fig. 4, A-C). The large elastic pulmonary artery and the carotid artery did not show this pattern (data not shown), and adventitial replication rates were not measured in the aorta and MPA. One possibility not explored in the present study might be that endothelial replication is uniquely active in the small pulmonary arteries in fetal life and rapid replication continued for the first days after birth. More likely, perhaps, is that endothelial cells are regulated differently from cells of the media and adventitia, and thus they respond differently to the transition at birth.
|
Hypoxia. Hypoxia, which prevented the normal postnatal transition to a low pulmonary vascular resistance, altered endothelial cell replication rates compared with those in the normoxic calves. In hypoxia in the small lung arteries, the postnatal decreases in replication rates were smaller because the rates on day 1 tended to be less than those in normoxia, whereas, by day 14, they were clearly higher than those in normoxia. Thus hypoxia and/or pulmonary hypertension blunted the normal fall in endothelial replication rates in the small arteries. In the MPA and the large elastic artery, the trend toward higher labeling in hypoxia versus normoxia was not significant for the few calves studied. Thus our main finding during hypoxia was that the exposure over 2 wk acted to maintain high postnatal endothelial replication rates in the smaller pulmonary arteries.
Blunting of the age-related postnatal decrease in small artery endothelial replication rates in hypoxia occurred in the setting of several severe abnormalities: 1) hypoxemia of a magnitude similar to that known to exist in the near-term bovine fetus (18), 2) pulmonary arterial pressure at or above fetal levels (18), 3) pulmonary vascular resistance (data not shown) threefold higher than in normoxic calves (7, 21), and 4) a significantly thickened media and adventitia and a reduced luminal diameter of the arterioles (2, 21).
For the MPA, the tendency for higher endothelial replication rates under hypoxic versus normoxic conditions was accompanied by severe pulmonary hypertension, known to cause circumferential stretch of the artery (28). For the aorta, higher pressures in the day 1-2 neonate than in the fetus also implied circumferential stretch. Whether such mechanical factors contributed to endothelial replication in the hypoxic great arteries requires further study.
Other possibilities are that severe hypoxemia and elevated intraluminal pressure altered in vivo endothelial replication rates (10, 16, 24) and/or that interaction between the endothelium and cells in the other layers of the arteriolar wall affected replication in the endothelium. The latter possibility, in particular, deserves consideration because hypoxic pulmonary hypertension substantially augmented replication rates in the cells of the media and adventitia of the small arteries (4) (Fig. 4, D-F). Remarkably, replication rates of adventitial cells in the arteriolar wall of the younger calves were greater than those of the medial cells (4) and approached or exceeded rates in the endothelium (Figs. 4, D-F). A potential role for adventitial cells in stimulating vascular wall remodeling during development of hypoxic pulmonary hypertension has been proposed (13, 21). Whether or not the other vascular layers stimulated the persistent endothelial replication in our hypoxic calves, it is possible that the chronic endothelial activation contributed to the vascular remodeling. The findings of the present in vivo study emphasize that hypoxia and/or pulmonary hypertension alters factors normally regulating developmental changes in endothelial cell replication, and the altered endothelium likely contributes to thickening of the wall.
In conclusion, immediately after birth, there are abrupt changes in blood pressure and flow and oxygenation, which vary among arterial segments depending on their location. These local changes determine, to a major degree, arterial wall remodeling (11). In newborn lambs, for example, a marked fall in flow in the distal abdominal aorta causes a near arrest of wall tissue accumulation, whereas the increased pressure in the central aorta may be the stimulus for rapid matrix accumulation (11). The present study, which has focused on lung arteries, has indicated that major changes at birth also normally occur in endothelial replication rates, that the magnitude of the changes vary with the arterial segment examined, and that vascular stretch- and flow-induced shear forces unique to the various vascular segments could be important in these processes. The most remarkable findings among the various vascular segments studied were the high endothelial replication rates observed in the small lung arteries shortly after birth. Furthermore, for these small arteries, hypoxia and the consequent pulmonary hypertension not only altered the replication rates of the endothelial cells but also changed the replication rates of other cells in the wall in relation to the endothelium, suggesting the importance of future research into in vivo communication among the different cell types in the lung arterial wall. Changes in intercellular communication may thus be an important determinant for normal adaptation of vascular structure and function after birth as well as a critical factor in maladaptation of the vasculature in various pathophysiological states in the neonatal period.
![]() |
APPENDIX. STATISTICAL APPENDIX |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The analysis of the endothelial labeling ratios in the pulmonary arterial bed was complex because there were labeling ratios from five pulmonary arterial vessel sizes at five postnatal ages (1, 4, 7, 10 and 14 days) as shown in Table 2. The data in normoxia were analyzed as one data set and those in hypoxia as another set. Multiple regression analysis [vessel size in mm (x-axis); postnatal age in days (y-axis); and labeling ratio in positive nuclei per 1,000 nuclei counted (the dependent variable; z-axis)] was used and took into account repeated measures for the labeling ratios in each calf. The SAS mixed-model procedure (12) was used to determine the equation
![]() |
(A1) |
For the normoxic calves, the resulting equation was
![]() |
![]() |
(A2) |
For the calves reared in hypobaric hypoxia, the multiple regression equation for the best plane that could be passed through the data was
![]() |
![]() |
(A3) |
One issue was whether the labeling ratio in chronic hypoxia differed from that in normoxia. The multiple regression analysis indicated that the plane that passed through all the data from hypoxic calves differed from that in normoxic calves with regard both to age (P < 0.001) and to arterial diameter (P < 0.05). On testing (SAS multiple regression, mixed-model procedure) between normoxic and hypoxic calves for each age-diameter combination, significant differences (i.e., higher labeling ratios in hypoxia) were found for the smaller arteries in the older calves (P < 0.05; Table 2). Although the older, hypoxic calves tended to have higher labeling ratios in the MPA than the older, normoxic calves, the differences were not significant.
![]() |
ACKNOWLEDGEMENTS |
---|
We gratefully acknowledge the help of Drs. Mary Weiser for project consultation and Gary Zerbe for statistical analysis.
![]() |
FOOTNOTES |
---|
This work was supported in part by National Heart, Lung, and Blood Institute Specialized Center of Research Grant HL-57144 and Program Project Grant HL-14985.
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. §1734 solely to indicate this fact.
Address for reprint requests: K. R. Stenmark, Division of Pediatric Critical Care, B-131, Developmental Lung Biology Laboratory, Univ. of Colorado Health Sciences Center, Denver, CO 80262.
Received 17 February 1998; accepted in final form 4 May 1998.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Abman, S. H.
Mechanisms of abnormal vasoreactivity in persistent pulmonary hypertension of the newborn infant.
J. Perinatol.
16:
516-523,
1996.
2.
Allen, K. M.,
and
S. G. Haworth.
Impaired adaptation of pulmonary circulation to extrauterine life in newborn pigs exposed to hypoxia: an ultrastructural study.
J. Pathol.
150:
205-212,
1986[Medline].
3.
Banes, A. J.,
J. Tsuzaki,
J. Yamamoto,
T. Fischer,
B. Brigman,
T. Brown,
and
L. Miller.
Mechanoreception at the cellular level: the detection, interpretation, and diversity of responses to mechanical signals.
Biochem. Cell Biol.
73:
349-365,
1995[Medline].
4.
Belknap, J. K.,
E. C. Orton,
B. Ensley,
A. Tucker,
and
K. R. Stenmark.
Hypoxia increases bromodeoxyuridine labeling indices in bovine neonatal pulmonary arteries.
Am. J. Respir. Cell Mol. Biol.
16:
366-371,
1997[Abstract].
5.
Bishop, J. E.,
R. P. Butt,
and
R. B. Low.
The effect of mechanical forces on cell function: implications for pulmonary vascular remodelling in pulmonary hypertension.
In: Pulmonary Vascular Remodelling, edited by J. E. Bishop,
J. T. Reeves,
and L. G. Laurent. London: Portland, 1995, p. 213-239.
6.
DeFazio, A.,
J. A. Leary,
D. W. Hedley,
and
M. H. N. Tattersall.
Immunohistochemical detection of proliferating cells in vivo.
J. Histochem. Cytochem.
35:
571-577,
1987[Abstract].
7.
Durmowicz, A. G.,
E. C. Orton,
and
K. R. Stenmark.
Progressive loss of vasodilator responsive component of pulmonary hypertension in neonatal calves exposed to 4,570 m.
Am. J. Physiol.
265 (Heart Circ. Physiol. 34):
H2175-H2183,
1993
8.
Hall, S. M.,
and
S. G. Haworth.
Normal adaptation of pulmonary arterial intima to extrauterine life in the pig: ultrastructural studies.
J. Pathol.
149:
55-66,
1986[Medline].
9.
Iba, T.,
S. Maitz,
T. Furbert,
O. Rosales,
M. D. Widmann,
B. Spillane,
T. Shin,
T. Sonoda,
and
B. E. Sumpio.
Effect of cyclic stretch on endothelial cells from different vascular beds.
Circ. Shock
35:
193-198,
1991[Medline].
10.
Kourembanas, S.,
T. Morita,
Y. Lui,
and
H. Christou.
Mechanisms by which oxygen regulates gene expression and cell-cell interaction in the vasculature.
Kidney Int.
51:
438-443,
1997[Medline].
11.
Langille, B. L. Remodeling of developing and
mature arteries: endothelium, smooth muscle, and matrix.
J. Cardiovasc. Pharmacol. 1, Suppl. 1: S11-S17, 1993.
12.
Littell, R. C.,
G. A. Milliken,
W. W. Stroup,
and
R. D. Wolfinger.
SAS System for Mixed Models. Cary, NC: SAS Institute, 1996.
13.
Meyrick, B.,
and
L. Reid.
Hypoxia and incorporation of 3H-thymidine by cells of the rat pulmonary arteries and alveolar wall.
Am. J. Pathol.
96:
51-70,
1979[Abstract].
14.
Morstyn, G.,
K. Pike,
J. Gardner,
R. Ashcroft,
A. deFazio,
and
P. Bathal.
Immunohistochemical identification of proliferating cells in organ culture using bromodeoxyuridine and a monoclonal antibody.
J. Histochem. Cytochem.
34:
697-701,
1986[Abstract].
15.
Murata, K.,
I. Mills,
and
B. E. Sumpio.
Protein phosphatase 2A in stretch-induced endothelial cell proliferation.
J. Cell. Biochem.
63:
311-319,
1996[Medline].
16.
Nomura, M.,
S. Yamagishi,
S. Harada,
Y. Hayashi,
T. Yamashima,
J. Yamashita,
and
H. Yamamoto.
Possible participation of autocrine and paracrine vascular endothelial growth factors in hypoxia-induced proliferation of endothelial cells and pericytes.
J. Biol. Chem.
270:
28316-28324,
1995
17.
Orton, E. C.,
S. M. LaRue,
B. Ensley,
and
K. R. Stenmark.
Bromodeoxyuridine labeling and DNA content of pulmonary arterial medial cells from hypoxia-exposed and nonexposed healthy calves.
Am. J. Vet. Res.
53:
1925-1930,
1992[Medline].
18.
Reeves, J. T.,
F. S. Daoud,
and
M. Gentry.
Growth of the fetal calf and its arterial pressure, blood gases, and hematologic data.
J. Appl. Physiol.
23:
240-244,
1972.
19.
Reidy, M. A.,
A. W. Clowes,
and
S. M. Schwartz.
Endothelial regeneration. V. Inhibition of endothelial regrowth in arteries of rat and rabbit.
Lab. Invest.
49:
569-575,
1983[Medline].
20.
Skalak, T. C.,
and
R. J. Price.
The role of mechanical stresses in microvascular remodeling.
Microcirculation
3:
143-165,
1996[Medline].
21.
Stenmark, K. R.,
J. Fasules,
D. M. Hyde,
N. F. Voelkel,
J. Henson,
A. Tucker,
H. Wilson,
and
J. T. Reeves.
Severe pulmonary hypertension and arterial adventitial changes in newborn calves at 4,300 m.
J. Appl. Physiol.
62:
821-830,
1987
22.
Sumpio, B. E.
Hemodynamic forces and the biology of the endothelium: signal transduction pathways in endothelial cells subjected to physical forces.
J. Vasc. Surg.
13:
744-746,
1991[Medline].
23.
Sumpio, B. E.,
A. J. Banes,
L. G Levin,
and
G. Johnson, Jr.
Mechanical stress stimulates aortic endothelial cells to proliferate.
J. Vasc. Surg.
6:
252-256,
1987[Medline].
24.
Sumpio, B. E.,
M. D. Widmann,
J. Ricotta,
M. A. Awolesi,
and
M. Watase.
Increased ambient pressure stimulates proliferation and morphologic changes in cultured endothelial cells.
J. Cell. Physiol.
158:
133-139,
1994[Medline].
25.
Weiser, M. C. M.,
R. A. Majack,
A. Tucker,
and
E. C. Orton.
Static tension is associated with increased smooth muscle cell DNA synthesis in rat pulmonary arteries.
Am. J. Physiol.
268 (Heart Circ. Physiol. 37):
H1133-H1138,
1995
26.
Ziegler, T.,
and
R. M. Nerem.
Tissue engineering a blood vessel: regulation of vascular biology by mechanical stress.
J. Cell. Biochem.
56:
204-209,
1994[Medline].
27.
Zhao, S.,
A. Suciu,
T. Ziegler,
J. E. Moore,
E. Burki,
J. J. Meister,
and
H. R. Brunner.
Synergistic effects of fluid shear stress and cyclic circumferential stretch on vascular endothelial cell morphology and cytoskeleton.
Arterioscler. Thromb. Vasc. Biol.
15:
1781-1786,
1995
28.
Zuckerman, B. D.,
E. C. Orton,
K. R. Stenmark,
J. A. Trapp,
J. R. Murphy,
P. R. Coffeen,
and
J. T. Reeves.
Alteration of the pulsatile load in the high-altitude calf model of pulmonary hypertension.
J. Appl. Physiol.
70:
859-868,
1991
HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Visit Other APS Journals Online |