It is unknown if nitric oxide (NO) modulates the relative levels of left (LV) and right (RV) ventricular output, fetal O2 consumption, or blood flow distribution between the body and placenta at midgestation. To address these questions, six fetal lambs were instrumented at 89–96 days gestation (term 147 days), and blood flows were measured with radioactive microspheres 3–4 days later at baseline and after inhibition of NO synthesis with 10 mg/kg (l-NNA10) and 25 mg/kg (l-NNA25)N ω-nitro-l-arginine. LV output fell by 74 ± 15 ml · min−1 · kg−1 atl-NNA10 (P < 0.005), whereas RV output decreased by 90 ± 18 ml · min−1 · kg−1 atl-NNA10 (P < 0.02) and by a further 80 ± 22 ml · min−1 · kg−1 atl-NNA25 (P < 0.05). As a result, RV output exceeded LV output at baseline (P = 0.03) and l-NNA10 (P < 0.02) but not at l-NNA25. Fetal body blood flow fell by 95 ± 25 ml · min−1 · kg−1 atl-NNA10 (P < 0.01), but because placental blood flow decreased by 70 ± 22 ml · min−1 · kg−1 atl-NNA10 (P < 0.01) and a further 71 ± 21 ml · min−1 · kg−1 atl-NNA25 (P < 0.01), the fetal body-to-placental blood flow ratio was near unity at baseline andl-NNA10 but rose to 1.5 ± 0.3 atl-NNA25 (P < 0.05). In association with these flow changes, fetal O2 consumption declined by 1.4 ± 0.3 ml · min−1 · kg−1 atl-NNA10 (P < 0.05) and by a further 1.5 ± 0.6 ml · min−1 · kg−1 atl-NNA25 (P < 0.02). These findings suggest that, in midgestation fetal lambs, NO supports an RV flow dominance, whole body O2 utilization, and the maintenance of a near-equal fetoplacental blood flow distribution.
- cardiac output
- blood flow
- oxygen consumption
- placental perfusion
findings from a number of laboratories have suggested that nitric oxide (NO), a labile compound produced from the amino acid l-arginine via the action of the ubiquitous enzyme NO synthase (21), acts as a modulator of both circulatory dynamics and metabolic processes in the mature fetus. Thus inhibition of NO synthesis in late-gestation fetal lambs increases arterial blood pressure (6, 7, 9, 22, 29,32) and reduces the combined left (LV) and right (RV) ventricular output (9, 29) without changing the degree of dominance of the RV output (29). Inhibition of NO synthesis also decreases fetal body (29) and placental blood flow (6, 7, 29). However, fetal body blood flow is reduced to a greater extent than placental blood flow, thereby redistributing systemic blood flow away from the fetus and toward the placenta (29). Moreover, the reduction in fetal body blood flow occurring with inhibition of NO synthesis is accompanied by an increase in fetal body O2 extraction that initially elevates fetal whole body O2 consumption and then maintains it at baseline levels in the face of falls in placental perfusion (29).
Inhibition of NO synthesis also increases arterial blood pressure and reduces the combined ventricular output in the immature, midgestation fetal lamb (10). However, several lines of evidence suggest that the physiological role of NO at midgestation may differ from late gestation in a number of other aspects. Specifically, in vitro data indicate that the potency of NO as a vasodilator in the umbilical-placental circulation is more pronounced at midgestation than at late gestation (18, 28). Because the placenta is mainly perfused by the right ventricle (24, 26, 30) and is central to fetomaternal gas exchange and fetal O2 delivery (25), it is therefore possible that, in the immature fetus, NO plays a pivotal role in maintaining not only the predominance of RV output within the “in parallel” fetal circulation (26) and the near equal distribution of the combined ventricular output between the fetus and placenta (16, 17) but also the level of fetal oxygenation. Accordingly, the aim of the present study was to test these hypotheses in chronically instrumented midgestation fetal lambs in which hemodynamic, blood flow, and blood gas measurements were performed after incremental inhibition of constitutive NO synthesis with the stereospecific and potent NO synthase inhibitorN ω-nitro-l-arginine (l-NNA; see Ref. 12).
MATERIALS AND METHODS
Experiments were approved by the Monash University Animal Experimentation Committee and were conducted in accord with guidelines established by the National Health and Medical Research Council of Australia.
Six fetuses with known breeding dates were chronically instrumented under aseptic conditions at 89–96 days' gestation (term = 147 days). Border-Leicester cross ewes were anesthetized with intravenous propofol (5 mg/kg), intubated, and then mechanically ventilated with 1–3% halothane and a 2:1 nitrous oxide-oxygen mixture. The pregnant horn of the uterus was exposed through a midline laparotomy. After delivery of the fetal hindlimbs through a hysterotomy, polyvinyl catheters were inserted in the femoral artery bilaterally in the groin and in a femoral vein and advanced to the descending aorta and inferior vena cava, respectively. The fetus was returned to the uterus, and the fetal head, left forelimb, and upper thorax were delivered through a second hysterotomy. A fetal thoracotomy was performed in the third left interspace, and the fourth rib was removed to increase exposure of the heart and great vessels. After incision of the pericardium, a 6- or 8-mm ultrasonic flow probe (Transonic Systems, Ithaca, NY) was placed around the pulmonary trunk. A 22-guage Teflon cannula was inserted in the pulmonary trunk through an adventitial purse-string suture distal to the flow probe and was connected to a polyvinyl catheter. After insertion of a polyvinyl catheter in the left atrial cavity through a purse-string suture in the appendage, the pericardium was loosely closed, and overlying muscle layers were repaired. The left axillary artery and vein were then exposed, and catheters were passed via these vessels into the ascending aorta or brachiocephalic trunk and the superior vena cava, respectively. A polyvinyl catheter was inserted in a cotyledonary vein, and the tip was advanced to a major umbilical vein. All catheters were exteriorized, and, after suturing of a wide-bore catheter to the skin of the anterior chest wall for measurement of amniotic fluid pressure, the fetal skin and maternal uterine incisions were closed.
Vascular catheters were filled with sodium heparin solution (1,000 IU/ml) and sealed. The catheters were tunneled subcutaneously to the ewe's right flank and secured with elastic netting. Postoperatively, vascular catheters were flushed daily and refilled with concentrated sodium heparin. Antibiotics (500 mg procaine penicillin and 500 mg dihydrostreptomycin sulfate) were instilled in the amniotic cavity at the time of surgery and on each subsequent postoperative day.
Ewes were placed in a mobile laboratory cart 3–4 days after surgery and were allowed free access to feed and water. Fetal ascending aortic and pulmonary trunk blood pressures, heart rate, and pulmonary trunk flow were recorded, and 0.4-ml blood samples were collected anerobically from the ascending aorta, pulmonary trunk, abdominal aorta, and the umbilical vein for Hb and blood gas analysis. Fetal ventricular outputs and blood flows to the fetal body and placenta were then measured with radioactive microspheres using the reference sample method (15). After baseline measurements, NO synthesis was inhibited with l-NNA (Sigma Chemical), which was dissolved in normal saline to a concentration of 5 mg/ml and infused continuously through the femoral venous catheter at a rate of 0.68 ml/min to achieve cumulative doses of 10 mg/kg (l-NNA10) and 25 mg/kg (l-NNA25) of estimated fetal body weight. At each l-NNA dose, hemodynamics were allowed to stabilize over a 5-min period, and blood pressure, heart rate, blood flow, and blood gas measurements were then repeated.
Ascending aortic and pulmonary arterial blood pressures were referenced to amniotic fluid pressure. Both pressures were monitored with silicon-chip pressure transducers (model CDX-111; Cobe Laboratories, Lakewood, CO), which were calibrated against a manometer before each experiment. Pulmonary trunk blood flow (i.e., RV output) was measured continuously with an ultrasonic flowmeter (model T208; Transonic Systems). The outputs from the pressure transducers and flowmeter were amplified using a programmable signal conditioner (Cyberamp model 380; Axon Instruments, Foster City, CA), and signals were displayed continuously on a paper recorder (model 800Z; Neomedix Systems, Sydney, New South Wales, Australia). At baseline and the two l-NNA doses, a 30-s segment of hemodynamic data was also digitized at a sampling rate of 200 Hz and stored on computer hard disk for subsequent off-line analysis using customized interactive software.
Blood gas tensions and pH were measured at 40°C with a blood analyzer (model ABL 500; Radiometer, Copenhagen, Denmark). Blood Hb concentration and Hb O2 saturation were measured in duplicate with a hemoximeter (model OSM2; Radiometer).
Radioactive microsphere technique.
Radioactive microspheres, 15 μm in diameter and labeled with one of five gamma emitting isotopes (141Ce, 113Sn,85Sr, 95Nb, or 46Sc; New England Nuclear, Boston, MA), were ultrasonicated for 10–15 min before injection and injected over 30–45 s. At baseline and atl-NNA25, two different microsphere labels were injected simultaneously, one in the left atrium to measure LV output and the other in the superior vena cava to measure RV output (31), whereas reference samples were drawn simultaneously from the ascending aorta, pulmonary trunk, and descending aorta for determination of fetal body and umbilical-placental flows. Atl-NNA10, LV output was obtained with a single microsphere label injected in the left atrium, whereas reference samples were withdrawn from the ascending and descending aorta to obtain fetal body and placental flows. Approximately 0.5–1 × 106 microspheres were injected per radiolabel, and reference samples were withdrawn at a rate of 2.05 ml/min with a mechanical pump (model 901A; Harvard Apparatus, South Natick, MA). Reference sample collection was commenced 5–10 s before injection of microspheres and was continued for an additional 75 s after the end of injection. Blood withdrawn in the reference samples was replaced simultaneously with maternal blood mixed with a plasma substitute (Hemaccel; Behring, Marburg, Germany).
At the end of the experiment, the ewe was killed with an intravenous overdose of pentobarbital sodium, the fetus was weighed, and the position of catheters was checked carefully at autopsy. The placenta was removed from the uterus by gentle traction, weighed, placed in 10% formalin fixative for 7–10 days, and then carbonized at a temperature of 280°C in a vented box furnace. The carbonized tissue was subsequently ground into a coarse powder, which was packed in plastic counting vials to a height of ≤2 cm. The radioactivity of the blood reference samples and tissue vials was counted in a gamma counter (model 1282 CompuGamma; LKB-Wallac, Turku, Finland) at the appropriate window settings, and the photopeaks of individual isotopes were separated by an on-line computer program.
Calculation of fetal ventricular outputs and fetal body and placental flows.
Radioactive microsphere blood flow measurements were calculated using the general relation QTissue = (QReference × RTissue)/RReference, where Q is flow (ml/min) and R is radioactivity (counts/min). LV and RV outputs and systemic flows were obtained using an adaptation of this general relation for the fetal circulation (31). Thus fetal LV output (QLV) was equal to (QReference × RLA)/RLA→AA, where RLA is the radioactivity of the label injected into the left atrial cavity and RLA→AA is the radioactivity of the same label collected in the ascending aortic reference sample. Fetal RV output (QRV) was equivalent to (QReference × RRV)/RV→PT, where RRV is the radioactivity of venous label passing into the right ventricle, calculated as the injected radioactivity of this label minus that portion crossing the foramen ovale to appear in the LV output, and RV→PT is the radioactivity of the venous label in the pulmonary reference sample (31). RV output atl-NNA10 was obtained by interpolation from the measured pulmonary trunk flow, using baseline andl-NNA25 ultrasonic flow probe measurements of pulmonary trunk flow and radioactive microsphere determinations of RV output (9, 10, 29).
Placental blood flow (QP) was calculated as (QReference × RP)/RDA, where RP is the radioactivity of the placenta and RDAis the radioactivity of the microsphere label in the descending aortic reference sample. Placental blood flow at baseline andl-NNA25 comprised the average of the flow values obtained from the different microsphere labels injected in the left atrium and superior vena cava, while only the single label injected into the left atrium was used atl-NNA10. Blood flow to the fetal body at baseline and after l-NNA was calculated as QLV + QRV − QP.
Blood gas calculations.
The O2 content of arterial or venous blood (ml O2/dl blood) was calculated as (1.36 × HbS × Hb/100) + (0.003 × Po 2) where HbS is Hb O2 saturation (%), Hb is Hb level (g/dl), and Po 2 is O2 tension (mmHg).
Systemic O2 delivery to the fetal body (FBQO2) was calculated as (QLV × CAAO2) + (QRV × CPTO2) − (QP × CDAO2), where CAAO2, CDAO2, and CPTO2 are the O2 contents in ascending aorta, descending aorta, and pulmonary trunk, respectively (29). Whole body O2 consumption (FBMVO2) at baseline and afterl-NNA was calculated according to the Fick principle as QP × (CUVO2 − CDAO2) where CUVO2 is umbilical venous O2 content (25, 29, 30). Average arterial O2 content of the fetal body (FBAO2) was computed as [(QLV × CAAO2) + (QRV × CPTO2) − (QP × CDAO2)]/(QLV + QRV − QP); the fetal arteriovenous O2 content difference (FBA-VO2) was calculated as FBMVO2/QFB; fetal mixed venous O2 content was calculated as FBAO2 − FBA-VO2; and the fetal O2 extraction coefficient was calculated as FBA-VO2/FBAO2(29, 30).
Changes in hemodynamics, blood flows, and blood gas variables were analyzed with repeated-measures one-way ANOVA using the Statistical Package for Social Sciences version 9.0.1 (SPSS). The sums of squares were partitioned into individual degrees of freedom, and the significance of changes was evaluated using the Bonferroni procedure, as appropriate, for multiple tests (34). Results are reported as means ± SE, and P < 0.05 was considered significant.
On the day of study, gestational age was 97 ± 1 days, fetal body weight 1.04 ± 0.09 kg, and placental weight 0.75 ± 0.06 kg.
Hemodynamics and blood gases.
At baseline, mean ascending aortic blood pressure was 1.4 ± 0.3 mmHg lower than pulmonary arterial blood pressure (P < 0.005). Ascending aortic blood pressure increased by 13.5 ± 1.5 mmHg and pulmonary arterial blood pressure by 14.3 ± 1.6 mmHg atl-NNA10 (both P < 0.005), but neither was altered further at l-NNA25. With the exception of an increase in Hb concentration (P < 0.005), blood gas variables at l-NNA10 were unchanged from baseline. However, l-NNA25 was accompanied by a further rise in Hb concentration (P < 0.025), as well as falls in pH (P < 0.025) and Hb O2 saturation (P < 0.005). Neither heart rate, Po 2, Pco 2, nor O2 content changed significantly with l-NNA (Table 1).
The baseline combined ventricular output (539 ± 24 ml · min−1 · kg−1) fell by 164 ± 15 ml · min−1 · kg−1 atl-NNA10 (P < 0.005) and by a further 93 ± 31 ml · min−1 · kg−1 atl-NNA25 (P < 0.05, Fig.1 A). Comparing ventricles, baseline LV output (222 ± 20 ml · min−1 · kg−1) decreased by 74 ± 15 ml · min−1 · kg−1 atl-NNA10 (P < 0.005) and was then unchanged at l-NNA25, whereas resting RV output (317 ± 31 ml · min−1 · kg−1) fell by 90 ± 18 ml · min−1 · kg−1 atl-NNA10 (P < 0.02) and by a further 80 ± 22 ml · min−1 · kg−1 atl-NNA25 (P < 0.05). Thus, whereas RV output exceeded LV output at baseline (P = 0.03) and l-NNA10 (P < 0.02), these outputs were not different at l-NNA25(P > 0.7; Fig. 1 B).
Fetal and placental blood flows.
Baseline fetal body blood flow (275 ± 24 ml · min−1 · kg−1) decreased by 95 ± 25 ml · min−1 · kg−1 atl-NNA10 (P < 0.01) and was unaltered at l-NNA25 (Fig.2 A). By contrast, resting placental blood flow (264 ± 28 ml · min−1 · kg−1) fell by 70 ± 22 ml · min−1 · kg−1 atl-NNA10 (P < 0.01) and by a further 71 ± 21 ml · min−1 · kg−1at l-NNA25 (P < 0.01; Fig.2 B). Consequently, although the fetal body-to-placental blood flow ratio was similar at baseline (1.1 ± 0.2) andl-NNA10 (1.0 ± 0.2), it rose to 1.5 ± 0.3 at l-NNA25 (P < 0.05; Fig. 2 C).
Fetal O2 content, extraction, delivery, and consumption.
Fetal average arterial O2 content, venous O2content, O2 arteriovenous content difference, and O2 extraction ratio did not change significantly with inhibition of NO synthesis (Table 2). However, in association with the blood flow changes, average fetal body O2 delivery declined progressively from 15.4 ± 1.5 ml · min−1 · kg−1 at baseline to 10.0 ± 2.0 ml · min−1 · kg−1 atl-NNA25 (P < 0.01; Fig.3 A), whereas resting whole body O2 consumption (7.2 ± 0.6 ml · min−1 · kg−1) decreased by 1.4 ± 0.3 ml · min−1 · kg−1 atl-NNA10 (P < 0.05) and by a further 1.5 ± 0.6 ml · min−1 · kg−1 atl-NNA25 (P < 0.02; Fig.3 B).
The main findings of this study were that incremental inhibition of NO synthesis in midgestation fetal lambs initially lowered combined ventricular output without affecting the degree of dominance of the RV output and reduced systemic perfusion while maintaining the fetoplacental blood flow ratio near unity. With a greater degree of inhibition of NO synthesis, however, a further reduction in placental perfusion was not only accompanied by a preferential decrease in RV output that equalized the univentricular outputs but also a redistribution of systemic flow away from the placenta and toward the fetal body. Moreover, because fetal arterial O2 content and systemic O2 extraction were unaltered after inhibition of NO synthesis, these flow changes were accompanied by reductions in both fetal O2 delivery and O2 consumption. These findings suggest that, in the midgestation fetal lamb, NO plays a major role in supporting a dominant RV output, a maintenance of whole body O2 utilization, and a near-equal distribution of systemic blood flow between the fetus and placenta.
Resting values for arterial blood pressure (2, 16, 17,20), combined ventricular output (16, 17), the ratio of fetal body and placental flows (16), Hb concentration (16, 17, 20), aortic O2saturation (17, 20), and whole body O2consumption (17) in the present study were similar to those previously reported in chronically instrumented, midgestation fetal lambs. The baseline heart rate (188 beats/min) was, however, slightly lower than the reported range of 196–224 beats/min (2, 16, 17, 20), possibly related to a variation among different sheep breeds.
As noted in a prior report (10), aortic blood pressure increased after NO synthase inhibition in midgestation fetuses. Moreover, as in late-gestation fetuses (22, 32), aortic and pulmonary arterial blood pressures rose by a similar amount at midgestation so that the pattern of a higher pulmonary arterial blood pressure evident at baseline was also present after inhibition of NO synthesis. However, as pointed out previously (10), the lack of any significant change in heart rate with inhibition of NO synthase in midgestation fetal lambs contrasted with the reduction in heart rate observed in late-gestation fetuses (6, 7, 29), presumably reflecting an incomplete development of baroreceptor responses due to an immaturity of parasympathetic chronotropic mechanisms (33).
Similar to late-gestation lambs (7, 29), Hb concentration rose at midgestation after inhibition of NO synthase. Because no significant release of erythrocytes occurs from storage sites such as the spleen in fetal sheep (3), the most plausible explanation for this rise in Hb concentration was a reduction in plasma volume associated with a fluid shift from intravascular to extravascular compartments. However, reductions in plasma volume appeared to be more pronounced at midgestation because Hb concentration increased by 2.4 g/dl from a baseline value of 6.6 g/dl, whereas the corresponding rise was only 1.6 g/dl from a baseline value of 9.6 g/dl in late-gestation fetuses (29). Indeed, substitution of these Hb values into the formula devised by Brace (3) indicated that the reduction in blood volume during inhibition of NO synthesis at midgestation (36%) was more than double that in late-gestation fetuses (15%). The basis of this difference is not entirely clear at present but could relate to factors such as a higher permeability of fetal capillaries at midgestation, leading to a greater degree of fluid extravasation with rises in hydrostatic pressure (4) or a greater role for NO in the maintenance of normal vascular permeability (19) within the immature fetal circulation.
Combined ventricular output fell stepwise with incremental inhibition of NO synthase in midgestation fetuses, with the magnitude of the reductions being similar to that occurring at late gestation (29). As at late gestation, it is likely that increased arterial blood pressure (14, 23), hemoconcentration (11), and reduced circulating blood volume (13) contributed to this decline in the combined ventricular output. However, within the similarity in the response of the combined ventricular output, an important gestation-dependent difference was apparent in the effect of inhibition of NO synthase on the univentricular outputs. Thus, in accord with data from a range of laboratories (1, 26, 30), RV output exceeded LV output at baseline in late-gestation fetuses; furthermore, the relative contribution of these outputs to the combined ventricular output (≈40 and 60%, respectively) was unaffected by inhibition of NO synthesis (29). At midgestation, RV output also constituted ≈60% of the combined ventricular output at baseline andl-NNA10. However, because RV output fell further at l-NNA25 while LV output was similar to that at l-NNA10, the univentricular outputs were not different at the higher dose of NO synthase inhibitor, suggesting that NO played an important role in supporting a dominance of the RV output in midgestation fetal lambs.
Two main factors could have underpinned the preferential reduction in RV output observed at l-NNA25 in the present study. The first relates to the finding that rises in arterial pressure in utero produce greater reductions in RV output than in LV output (23). However, this is unlikely to be the explanation for our observation because, while RV output exceeded LV output atl-NNA10 but notl-NNA25, ascending aortic and pulmonary arterial pressures were unchanged between these l-NNA doses (Table 1). The second factor, which stems from the notion that the dominance of the RV output in the fetal circulation is due to the right ventricle being the major source of blood to the low-resistance placental circulation (24), is that the preferential reduction in RV output observed at l-NNA25 was primarily related to the accompanying marked fall in placental perfusion. In support of this explanation, the reduction in placental perfusion between l-NNA10 andl-NNA25 (71 ml · min−1 · kg−1) accounted for ≈90% of the associated decrease in RV output (80 ml · min−1 · kg−1; Figs. 1 and2).
The pattern of change in fetal body blood flow occurring with inhibition of NO synthesis at midgestation, namely a reduction in blood flow at l-NNA10 of ≈30% and no further change at l-NNA25, was similar to that observed at late gestation (29). By contrast, three main differences in the placental blood flow pattern were apparent on comparison of present findings with late-gestation fetal lamb data from our laboratory (29). First, resting placental flow at midgestation was ≈50% higher than the value of 175 ml · min−1 · kg−1 measured at late gestation. Second, in midgestation fetuses, placental flow fell by 23% at l-NNA10 and by a further 30% from baseline at l-NNA25 (Fig. 2), whereas, in late-gestation fetuses, placental flow was unchanged between baseline and l-NNA10 and only fell by 21% atl-NNA25. Third, atl-NNA25, placental blood flow at midgestation (123 ml · min−1 · kg−1) was not too dissimilar from the value of 140 ml · min−1 · kg−1 measured at late gestation. Taken together, these findings imply that an NO-related mechanism not only underpinned a higher resting level of placental perfusion at midgestation compared with late gestation but also contributed to ≈50% of resting placental perfusion at midgestation compared with ≈20% at late gestation. These conclusions are in accord with in vitro human data that indicate that NO is a potent vasodilator within the umbilical-placental vascular bed (18) and that this vasodilator action diminishes with advancing gestation (18, 28).
An important consequence of the pronounced placental flow reduction occurring with inhibition of NO synthase at midgestation was a redistribution of systemic blood flow between the fetal and placental compartments. Thus, although blood flow was distributed near equally between these compartments at baseline andl-NNA10, it favored the fetal body atl-NNA25. This redistribution was the opposite of that observed in late-gestation fetuses, where the resting fetal-to-placental blood flow ratio fell from 1.9 to 1.4 after inhibition of NO synthesis (29). Interestingly, these divergent fetoplacental blood flow responses bear a striking resemblance to the differing systemic flow redistribution patterns observed at mid (16)- and late gestation (8) during acute fetal hypoxemia, which further supports the prior suggestion that a reduction in NO production may contribute to the physiological manifestations of acute in utero hypoxemia (29).
As at late gestation (29), arterial O2 content was unchanged during the reduction in fetal body blood flow occurring with inhibition of NO synthase at midgestation; hence, fetal O2 delivery was correspondingly decreased. However, in contrast to the reduction in mixed venous O2 content and a rise in systemic O2 extraction evident in late-gestation fetuses (29), no significant changes in mixed venous O2 content, systemic O2 extraction, or the O2 extraction coefficient occurred after inhibition of NO synthesis at midgestation (Table 2). Consequently, although fetal whole body O2 consumption in late-gestation fetuses was increased initially by ≈15% and then remained at near-baseline levels after a reduction in placental perfusion (29), inhibition of NO synthesis at midgestation was associated with a progressive fall in fetal whole body O2 consumption to a level that was only ≈60% of baseline at the highest dose of NO synthase inhibitor (Fig.3).
A possible explanation for the marked difference in the response of whole body O2 consumption to inhibition of NO synthesis between mid- and late-gestation fetal lambs relates to the modulatory effects of NO on oxidative metabolism. Thus NO is known to be a potent inhibitor of enzymes within the mitochondrial respiratory chain, particularly cytochrome c oxidase (5). This action of NO appears to impose a substantial brake on oxidative metabolism in the adult because, although inhibition of NO synthesis reduces cardiac output, it results in an even greater increase in O2 extraction, thereby raising whole body O2consumption (27). A similar modulatory action is apparent in late-gestation fetal lambs, with inhibition of NO synthesis resulting in an increased O2 extraction that maintains whole body O2 consumption at or above the baseline level despite falls in fetal perfusion (29). However, the absence of an increase in O2 extraction in the face of reduced tissue perfusion in the present study suggests that this modulatory action was not present in midgestation fetuses, so that fetal whole body O2 consumption exhibited a flow dependence after inhibition of NO synthesis.
The association of a pronounced fall in umbilical-placental perfusion after inhibition of NO synthesis with the loss of an RV output dominance, a fall in whole body O2 consumption, and a redistribution of systemic blood flow toward the fetal body emphasizes the important homeostatic role of NO in the immature fetus. These effects are strikingly different to late-gestation fetuses (29) where the dominance of the RV output was preserved, whole body O2 consumption was maintained at or above baseline levels, and systemic blood flow was distributed away from the fetal body and toward the placenta after inhibition of NO synthesis. These findings suggest not only that the modulatory role of NO undergoes substantial alteration during fetal maturation but also that impairment of NO production at midgestation may be associated with more profound circulatory consequences than at late gestation.
The valuable technical assistance of Jennene Wild, Karyn Forster, Ann Oates, and Kellie Eede is acknowledged, as is the support and assistance of Dr. Adrian Walker.
This work was funded by a Project Grant from the National Health and Medical Research Council of Australia.
Address for reprint requests and other correspondence: J. J. Smolich, Cardiology Unit, Monash Medical Centre, 246 Clayton Rd., Clayton, Victoria, Australia 3168 (E-mail:).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
- Copyright © 2001 the American Physiological Society