Adaptation of fetal pulmonary blood flow to local infusion of tolazoline

Enhanced central and conduit pulmonary arterial reservoir function offsets reduced ductal systolic outflow during constriction of the fetal ductus arteriosus

Constriction of the fetal ductus arteriosus (DA) has disparate effects on mean and phasic hemodynamics, as mean DA blood flow is preserved until constriction is severe, but DA systolic and diastolic blood velocities change with only mild constriction. To determine the basis of this disparity and its physiological significance, seven anesthetized late-gestation fetal sheep were instrumented with pulmonary trunk (PT), DA, and left pulmonary artery (PA) micromanometer catheters and transit-time flow probes. Blood flow profile and wave intensity analyses were performed at baseline and during mild, moderate, and severe DA constriction (defined as pulmonary-aortic mean pressure differences of 4, 8, and 14 mmHg, respectively), produced with an adjustable snare. With DA constriction, mean DA flow was initially maintained but decreased with severe constriction (P < 0.05) in conjunction with a reduction (P < 0.05) in PT flow (i.e., right ventricular output). By contrast, DA systolic flow fell progressively during DA constriction (P < 0.001), due to decreased transmission of both early and midsystolic proximal flow-enhancing forward-running compression waves into the DA. However, DA constriction was also accompanied by greater systolic storage of blood in the PT and main PA (P < 0.025), and increased retrograde diastolic flow from compliant major branch PA (P < 0.001). Transductal discharge of these central and conduit PA blood reservoirs in diastole offset systolic DA flow reductions. These data suggest that, during DA constriction in the fetus, enhanced central and conduit PA reservoir function constitutes an important compensatory mechanism that contributes to preservation of mean DA flow via a systolic-to-diastolic redistribution of phasic DA flow.

Cinaciguat, a soluble guanylate cyclase activator, causes potent and sustained pulmonary vasodilation in the ovine fetus

Impaired nitric oxide-cGMP signaling contributes to severe pulmonary hypertension after birth, which may in part be due to decreased soluble guanylate cyclase (sGC) activity. Cinaciguat (BAY 58-2667) is a novel sGC activator that causes vasodilation, even in the presence of oxidized heme or heme-free sGC, but its hemodynamic effects have not been studied in the perinatal lung. We performed surgery on eight fetal (126 +/- 2 days gestation) lambs (full term = 147 days) and placed catheters in the main pulmonary artery, aorta, and left atrium to measure pressures. An ultrasonic flow transducer was placed on the left pulmonary artery to measure blood flow, and a catheter was placed in the left pulmonary artery for drug infusion. Cinaciguat (0.1-100 microg over 10 min) caused dose-related increases in pulmonary blood flow greater than fourfold above baseline and reduced pulmonary vascular resistance by 80%. Treatment with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), an sGC-oxidizing inhibitor, enhanced cinaciguat-induced pulmonary vasodilation by >120%. The pulmonary vasodilator effect of cinaciguat was prolonged, decreasing pulmonary vascular resistance for >1.5 h after brief infusion. In vitro stimulation of ovine fetal pulmonary artery smooth muscle cells with cinaciguat after ODQ treatment resulted in a 14-fold increase in cGMP compared with non-ODQ-treated cells. We conclude that cinaciguat causes potent and sustained fetal pulmonary vasodilation that is augmented in the presence of oxidized sGC and speculate that cinaciguat may have therapeutic potential for severe neonatal pulmonary hypertension.

Pulmonary vascular effects of nitric oxide-cGMP augmentation in a model of chronic pulmonary hypertension in fetal and neonatal sheep

Persistent pulmonary hypertension of the newborn (PPHN) is partly due to impaired nitric oxide (NO)-cGMP signaling. BAY 41-2272 is a novel direct activator of soluble guanylate cyclase, but whether this drug may be an effective therapy for PPHN is unknown. We hypothesized that BAY 41-2272 would cause pulmonary vasodilation in a model of severe PPHN. To test this hypothesis, we compared the hemodynamic response of BAY 41-2272 to acetylcholine, an endothelium-dependent vasodilator, and sildenafil, a selective inhibitor of PDE5 in chronically instrumented fetal lambs at 1 and 5 days after partial ligation of the ductus arteriosus. After 9 days, we delivered the animals by cesarean section to measure their hemodynamic responses to inhaled NO (iNO), sildenafil, and BAY 41-2272 alone or combined with iNO. BAY 41-2272 caused marked pulmonary vasodilation, as characterized by a twofold increase in blood flow and a nearly 60% fall in PVR at day 1. Effectiveness of BAY 41-2272-induced pulmonary vasodilation increased during the development of pulmonary hypertension. Despite a similar effect at day 1, the pulmonary vasodilator response to BAY 41-2272 was greater than sildenafil at day 5. At birth, BAY 41-2272 dramatically reduced PVR and augmented the pulmonary vasodilation induced by iNO. We concluded that BAY 41-2272 causes potent pulmonary vasodilation in fetal and neonatal sheep with severe pulmonary hypertension. We speculate that BAY 41-2272 may provide a novel treatment for severe PPHN, especially in newborns with partial response to iNO therapy.

Effects of BAY 41-2272, a soluble guanylate cyclase activator, on pulmonary vascular reactivity in the ovine fetus

Nitric oxide (NO)-cGMP signaling plays a critical role during the transition of the pulmonary circulation at birth. BAY 41-2272 is a novel NO-independent direct stimulator of soluble guanylate cyclase that causes vasodilation in systemic and local circulations. However, the hemodynamic effects of BAY 41-2272 have not been studied in the perinatal pulmonary circulation. We hypothesized that BAY 41-2272 causes potent and sustained fetal pulmonary vasodilation. We performed surgery on 14 fetal lambs (125-130 days gestation; term = 147 days) and placed catheters in the main pulmonary artery, aorta, and left atrium to measure pressures. An ultrasonic flow transducer was placed on the left pulmonary artery (LPA) to measure blood flow, and a catheter was placed in the LPA for drug infusion. Pulmonary vascular resistance (PVR) was calculated as pulmonary artery pressure minus left atrial pressure divided by LPA blood flow. BAY 41-2272 caused dose-related increases in pulmonary blood flow up to threefold above baseline and reduced PVR by 75% (P < 0.01). Prolonged infusion of BAY 41-2272 caused sustained pulmonary vasodilation throughout the 120-min infusion period. The pulmonary vasodilator effect of BAY 41-2272 was not attenuated by N(omega)-nitro-l-arginine, a NO synthase inhibitor. In addition, compared with sildenafil, a phosphodiesterase 5 inhibitor, the pulmonary vasodilator response to BAY 41-2272 was more prolonged. We conclude that BAY 41-2272 causes potent and sustained fetal pulmonary vasodilation independent of NO release. We speculate that BAY 41-2272 may have therapeutic potential for pulmonary hypertension associated with failure to circulatory adaptation at birth, especially in the setting of impaired NO production.

Role of the alpha2-adrenoceptors on the pulmonary circulation in the ovine fetus

Recent in vitro studies reported that nitric oxide release and pulmonary vasorelaxation can be mediated by endothelial alpha2-adrenoceptor activation. As norepinephrine (alpha1-,alpha2-, and beta1-adrenoceptor agonist) was found to induce pulmonary vasodilation in the ovine fetus, we hypothesized that alpha2-adrenoceptors may modulate basal pulmonary vascular tone and mediate the vascular effect of norepinephrine during fetal life. To determine the role of alpha2-adrenoceptors and the mechanisms of norepinephrine-mediated vasodilation in the fetal pulmonary circulation, we tested, in chronically prepared late-gestation fetal lambs, the hemodynamic response to 1). yohimbine (alpha2 antagonist); 2). UK 14304 (alpha2 agonist) with and without l-nitro-arginine (nitric oxide synthase inhibitor); and 3). norepinephrine infusion with and without yohimbine. We found that yohimbine increased mean pulmonary artery pressure by 15% (p < 0.05), decreased pulmonary flow by 22% (p < 0.01), and increased pulmonary vascular resistance by 51% (p < 0.01). UK 14304 increased pulmonary flow by 145% (p < 0.01) and decreased pulmonary vascular resistance by 58% (p < 0.01). l-Nitro-arginine abolished the UK 14304-mediated pulmonary vasodilation. Norepinephrine (0.5 microg x kg(-1)x min(-1) increased both pulmonary flow by 61% (p < 0.01) and pulmonary arterial pressure by 13% (p < 0.01) and decreased pulmonary vascular resistance by 33% (p < 0.01). Yohimbine abolished the norepinephrine-induced pulmonary vasodilation. This study suggests that 1). a basal alpha2-adrenoceptor activation-induced pulmonary vasodilation exists during fetal life; 2). the pulmonary vascular effects of alpha2-adrenoceptor activation are related at least in part to nitric oxide production; and 3). the norepinephrine-mediated pulmonary vasodilation involves alpha2-adrenoceptor activation. As a surge of norepinephrine exists at birth, we speculate that norepinephrine and endothelial alpha2-adrenoceptor activation may play a significant role in pulmonary vasodilation at birth.

Vascular endothelial growth factor causes pulmonary vasodilation through activation of the phosphatidylinositol-3-kinase-nitric oxide pathway in the late-gestation ovine fetus

Vascular endothelial growth factor (VEGF) causes vasodilation in adult models of peripheral vascular disease and myocardial ischemia through the acute release of nitric oxide (NO). However, the hemodynamic effects of VEGF and its effects on NO production have not been studied in the developing lung circulation. We hypothesized that VEGF causes fetal pulmonary vasodilation, and that its actions are mediated through the release of endogenous NO. We performed surgery in 16 fetal lambs (125-135 d gestation; term = 147 d), and placed catheters in the main pulmonary artery, aorta, and left atrium to measure pressures. An ultrasonic flow transducer was placed on the left pulmonary artery (LPA) to measure blood flow, and a catheter was placed in the LPA for local drug infusion. Pulmonary vascular resistance in the left lung was calculated as pulmonary artery pressure minus left atrial pressure divided by LPA flow. Fetal lambs were treated with brief infusions of recombinant human VEGF (dose, 0.5-2.0 micro g) into the LPA. Recombinant human VEGF infusions acutely increased LPA flow by up to 3-fold (p < 0.02) and decreased pulmonary vascular resistance by 65% (p < 0.05) in a dose-related fashion, without affecting aortic pressure or heart rate. To determine the mechanism of VEGF-induced vasodilation, we studied the effects of nitro-L-arginine, an NO synthase inhibitor, and LY294002, a phosphatidylinositol-3-kinase inhibitor, on the response to VEGF. We found that pretreatment with either nitro-L-arginine or LY294002 completely inhibited the vasodilator response to recombinant human VEGF (p < 0.005). These findings suggest that recombinant human VEGF causes fetal pulmonary vasodilation, and that this response is likely mediated by the release of NO through activation of phosphatidylinositol-3-kinase.

Chronic hypertension impairs flow-induced vasodilation and augments the myogenic response in fetal lung

We hypothesized that altered vasoreactivity in perinatal pulmonary hypertension (PH) is characterized by abnormal responses to hemodynamic stress, including the loss of flow-induced vasodilation and an augmented myogenic response. Therefore, we studied the acute hemodynamic effects of brief compression of the ductus arteriosus (DA) in control fetal lambs and in lambs during exposure to chronic PH. In both groups, acute DA compression decreased pulmonary vascular resistance (PVR) by 20% at baseline (day 0). After 2 days of hypertension, acute DA compression paradoxically increased PVR by 50% in PH lambs, whereas PVR decreased by 25% in controls. During the 8-day study period, PVR increased during acute DA compression in PH lambs, whereas acute DA compression continued to cause vasodilation in controls. Brief treatment with the nitric oxide (NO) synthase inhibitor nitro-L-arginine (L-NA) increased basal PVR in control but not PH lambs, suggesting decreased NO production in PH lambs. Chronic hypertension increased the myogenic response after L-NA in PH lambs, whereas the myogenic response remained unchanged in controls. The myogenic response was inhibited by nifedipine in PH lambs, suggesting that the myogenic response is dependent upon the influx of extracellular calcium. We conclude that chronic PH impairs flow-induced vasodilation and increases the myogenic response in fetal lung. We speculate that decreased NO signaling and an augmented myogenic response contributes to abnormal vasoreactivity in PH.

[Experimental model of perinatal pulmonary circulation in lambs]

STUDY AIM

Mechanisms that modulate fetal pulmonary circulation and transitional circulation at birth are incompletely understood. The aim of this experimental study was to describe an animal model in order to study the perinatal pulmonary circulation.

MATERIAL AND METHODS

Pregnant ewes were operated on between 126 and 128 days gestation (term = 145 days). A skin incision was performed to the fetal lambs in utero and catheters were placed into the ascending aorta and the superior vena cava after insertion in the axillary artery and vein. Then, catheters were inserted into the left pulmonary artery (LPA), main pulmonary artery, and left atrium via a thoracotomy. Moreover, an ultrasonic flow transducer, and an inflatable vascular occluder were placed around the LPA and around the ductus arteriosus. During 10 days, studies were performed in utero (possibly continued when fetal lambs were delivered by caesarean section). This chronically prepared animal may be used to perform hemodynamic studies according to different protocols (drugs injection to the fetus or to the ewes, ductus arteriosus compression, oxygen test). The main pulmonary artery, aortic, left atrial and amniotic pressures, heart rhythm, and flow signal were continuously recorded.

RESULTS

Eighteen pregnant ewes were operated on and nine only could be used for experimentation. This ovine model permitted several studies, particularly about effects of catecholamines on the pulmonary circulation, and about effects of ductus arteriosus compression on the pulmonary circulation.

CONCLUSION

Chronically instrumented fetal lambs are an excellent model in order to study the perinatal pulmonary circulation.

Effects of catecholamines on the pulmonary circulation in the ovine fetus

High levels of circulating catecholamines are found in the fetus, and fetal stress and birth induce a marked surge in catecholamine secretion. Little is known about the role of catecholamines on the fetal pulmonary circulation. To determine the effects of catecholamines on the pulmonary vascular tone, we tested the hemodynamic response to norepinephrine and dopamine infusion in chronically prepared late-gestation fetal lambs. We found that norepinephrine infusion (0.5 microg. kg(-1). min(-1)) increased pulmonary artery pressure (PAP) by 10 +/- 1% (P < 0.01), left pulmonary artery blood flow by 73 +/- 14% (P < 0.01), and decreased pulmonary vascular resistance (PVR) by 33 +/- 6% (P < 0.01). The pulmonary vasodilator effect of norepinephrine was abolished after nitric oxide synthase inhibition. Dopamine infusion at 5 microg. kg(-1). min(-1) did not significantly change PVR. Conversely, dopamine infusion at 10 microg. kg(-1). min(-1) increased PAP (P < 0.01) and progressively increased PVR by 30 +/- 14% (P < 0.01). These results indicate that catecholamines may modulate basal pulmonary vascular tone in the ovine fetus. We speculate that catecholamines may play a significant role in the maintenance of the fetal pulmonary circulation and in mediating changes in the transitional pulmonary circulation.

Acute intrauterine pulmonary hypertension impairs endothelium-dependent vasodilation in the ovine fetus

To determine whether acute pulmonary hypertension in utero alters fetal pulmonary vascular reactivity, we compared pulmonary vasodilation with an endothelium-dependent agonist, acetylcholine, with that of an endothelium-independent agonist, 8-bromo-guanosine 3',5'-cylic monophosphate. Acute pulmonary hypertension was produced in chronically prepared, late-gestation fetal lambs by 3 repeated 30-minute partial occlusions of the ductus arteriosus (DA). The first DA compression increased LPA blood flow from 80 +/- 10 to 180 +/- 21 mL/min (p < 0.01) and decreased pulmonary vascular resistance. In contrast, LPA blood flow did not change and pulmonary vascular resistance increased by 25% during the third period of DA compression. Pulmonary vasodilation during acetylcholine infusion after serial DA compressions was decreased in comparison with the acetylcholine-induced vasodilator response achieved during the baseline period (fall in pulmonary vascular resistance = -49 +/- 7% (baseline) versus -25 +/- 5% after repeated DA compressions; p < 0.05). In contrast, the vasodilator response to 8-bromo-guanosine 3',5'-cylic monophosphate remained intact. To determine whether decreased nitric oxide (NO) production may contribute to altered vasoreactivity after acute pulmonary hypertension, repeated DA compressions were performed after treatment with a nonspecific NO synthase inhibitor (nitro-L-arginine). NO synthase inhibition blocked the pulmonary vasodilation during the first DA compression period, and repeated DA compressions after NO synthase inhibition did not further alter the hemodynamic response to DA compression. These findings support the hypothesis that brief hypertension due to DA compression impairs endothelium-dependent pulmonary vasodilation in the fetus, and that this may be due to decreased NO production.

In vivo evidence for a myogenic response in the fetal pulmonary circulation

In vitro studies have suggested that pulmonary arteries can exhibit a myogenic response and that this myogenic response may be potent during the perinatal period. However, whether a myogenic response can be demonstrated to exist in vivo and the potential role of the myogenic response on the regulation of pulmonary blood flow during fetal life is unknown. We hypothesized that an acute increase in pulmonary artery pressure resulting from partial compression of the ductus arteriosus (DA) in the fetus may simultaneously activate two opposing responses: 1) blood flow-induced vasodilation (owing to shear stress); and 2) pressure-induced vasoconstriction (owing to the myogenic response). To test this hypothesis, we studied the hemodynamic response to partial DA compression with and without inhibition of shear stress-induced vasodilation by nitric oxide synthase blockade in chronically prepared late-gestation fetal lambs. Without inhibition of nitric oxide synthase, pulmonary vascular resistance progressively decreased by 39 +/- 5% during the DA compression period (p < 0.05). In contrast, DA compression after nitric oxide synthase inhibition caused left pulmonary artery blood flow to initially increase and then steadily decrease toward a plateau value, and caused pulmonary vascular resistance to progressively increase by 28 +/- 4% above baseline (p < 0.05). The plateau value of pulmonary vascular resistance was reached in less than 5 min after the onset of DA compression. Left pulmonary artery blood flow after 10 min of partial DA compression did not change with the rise in pulmonary artery pressure; plateau values of pulmonary vascular resistance increased linearly with the increase in pulmonary artery pressure. These results support the hypothesis that the perinatal lung circulation has a potent myogenic response, and that this response may be masked in vivo under physiologic conditions by nitric oxide synthase activity. We speculate that the myogenic response may become a predominant regulatory mechanism of pulmonary vascular resistance when endothelium-dependent vasoreactivity is impaired, such as in persistent pulmonary hypertension of the newborn.

K+-channel blockade inhibits shear stress-induced pulmonary vasodilation in the ovine fetus

To determine whether K+-channel activation mediates shear stress-induced pulmonary vasodilation in the fetus, we studied the hemodynamic effects of K+-channel blockers on basal pulmonary vascular resistance and on the pulmonary vascular response to partial compression of the ductus arteriosus (DA) in chronically prepared late-gestation fetal lambs (128-132 days gestation). Study drugs included tetraethylammonium (TEA; Ca2+-dependent K+-channel blocker), glibenclamide (Glib; ATP-dependent K+-channel blocker), charybdotoxin (CTX; preferential high-conductance Ca2+-dependent K+-channel blocker), apamin (Apa; low-conductance Ca2+-dependent K+-channel blocker), and 4-aminopyridine (4-AP; voltage-dependent K+-channel blocker). Catheters were inserted in the left pulmonary artery (LPA) for selective drug infusion and in the main pulmonary artery, aorta, and left atrium to measure pressure. An inflatable vascular occluder was placed around the DA. LPA flow was measured with an ultrasonic flow transducer. Animals were treated with saline, high- or low-dose TEA, Glib, Apa, CTX, CTX plus Apa, or 4-AP injected into the LPA. DA compression caused a time-related decrease in pulmonary vascular resistance in the control, Glib, Apa, CTX, CTX plus Apa, and low-dose TEA groups but not in the high-dose TEA and 4-AP groups. These data suggest that pharmacological blockade of Ca2+- and voltage-dependent K+-channel activity but not of low-conductance Ca2+- and ATP-dependent K+-channel activity attenuates shear stress-induced fetal pulmonary vasodilation.

Dipyridamole potentiates pulmonary vasodilation induced by acetylcholine and nitric oxide in the ovine fetus

Nitric oxide (NO) modulates pulmonary vascular resistance (PVR) in the normal fetus by increasing the cyclic guanosine 3',5'-monophosphate (cGMP) content of pulmonary vascular smooth muscle cells. Although several vasodilator stimuli, including acetylcholine, decrease fetal PVR through the release of endogenous NO, fetal pulmonary vasodilation is often transient despite prolonged treatment. Because cGMP is hydrolyzed and inactivated by cGMP-specific (type 5) phosphodiesterases (PDE5), we hypothesized that PDE5 activity contributes to high fetal PVR and limits the capability of the fetal pulmonary circulation to dilate or sustain vasodilation in response to cGMP-dependent stimuli. To test this hypothesis, we studied the hemodynamic effects of dipyridamole in 19 late-gestation fetal lambs. To determine whether dipyridamole-induced vasodilation is dependent upon basal NO release, we measured the response to dipyridamole before and after pretreatment with the NO synthase antagonist nitro-L-arginine (L-NA) in five fetal lambs. L-NA completely blocked dipyridamole-induced pulmonary vasodilation. To evaluate the effect of dipyridamole on pulmonary vasodilation due to the stimulated release of NO, we studied effects of prolonged intrapulmonary acetylcholine infusions, with and without concomitant administration of low-dose dipyridamole, in six fetal lambs. During prolonged (2-h) infusions, acetylcholine and dipyridamole individually caused transient pulmonary vasodilation. When administered together, pulmonary vasodilation was of greater magnitude and was sustained for the entire study period. To determine the effects of dipyridamole on endothelium-independent pulmonary vasodilation, we investigated the hemodynamic effects of inhaled NO (5 and 20 ppm) alone and in combination with dipyridamole during mechanical ventilation with low FlO2. The combination of dipyridamole with inhaled NO resulted in a greater degree of pulmonary vasodilation than that achieved with inhaled NO alone. We conclude that dipyridamole-induced pulmonary vasodilation is dependent on endogenous (basal) NO production and that dipyridamole potentiates vasodilator responses to endothelium-dependent and -independent dilators in the ovine fetal pulmonary circulation. We speculate that PDES activity opposes vasodilation and maintains high PVR in the normal fetal lung.

Prolonged endothelin A receptor blockade attenuates chronic pulmonary hypertension in the ovine fetus

Based on past studies of an experimental model of severe intrauterine pulmonary hypertension, we hypothesized that endothelin-1 (ET-1) contributes to high pulmonary vascular resistance (PVR), hypertensive lung structural changes, and right ventricular hypertrophy (RVH) caused by prolonged closure of the ductus arteriosus. To test this hypothesis, we studied the effects of BQ 123, a selective ET(A) receptor antagonist, after ligation of the ductus arteriosus in utero. In 19 late gestation fetal lambs (126+/-3 d; 147 d, term) we ligated the ductus arteriosus at surgery, and treated animals with either BQ 123 (1 mg/d) or vehicle (0.1% DMSO, HTN) in the pulmonary artery for 8 d. Chronic BQ 123 treatment attenuated the rise in mean pulmonary artery pressure (PAP) 8 d after ductus arteriosus ligation (78+/-2, HTN vs. 70+/-4 mmHg, BQ 123, P < 0.05). To study the effects of ET(A) blockade at birth, 15 animals were delivered by cesarean section and ventilated with 10% oxygen (O2), 100% O2 and inhaled nitric oxide (NO). Lambs treated with BQ 123 had lower PVR after delivery during ventilation with 10% O2, 100% O2, and inhaled NO (HTN vs. BQ 123, P < 0.05 for each intervention). Acute BQ 123 treatment (2 mg/30 min) lowered PVR in three HTN animals ventilated with 100% O2 and inhaled NO (P < 0.05). Chronic BQ 123 treatment prevented the development of RVH as determined by the ratio of the right ventricle/left ventricle + septum (0.79+/-0.03, HTN vs. 0.57+/-0.06, BQ 123, P < 0.05) and attenuated the increase in wall thickness of small pulmonary arteries (61+/-2, HTN vs. 50+/-2%, BQ 123, P < 0.05). In summary, chronic intrauterine ET(A) receptor blockade decreased PAP in utero, decreased RVH and distal muscularization of small pulmonary arteries, and increased the fall in PVR at delivery. We conclude that ET(A) receptor stimulation contributes to the pathogenesis and pathophysiology of experimental perinatal pulmonary hypertension.

Endothelin blockade augments pulmonary vasodilation in the ovine fetus

The physiological role of endothelin-1 (ET-1) in regulation of vascular tone in the perinatal lung is controversial. Recent studies suggest that ET-1 contributes to high basal pulmonary vascular resistance in the normal fetus, but its role in the modulation of pulmonary vascular tone remains uncertain. We hypothesized that high ET-1 activity opposes the vasodilator response to some physiological stimuli such as increased pressure. To test the hypothesis that ET-1 modulates fetal pulmonary vascular responses to acute and prolonged physiological stimuli, we performed a series of experiments in the late-gestation ovine fetus. We studied the hemodynamic effects of two ET-1 antagonists, BQ-123 (a selective ETA-receptor antagonist) and phosphoramidon (a nonselective ET-1-converting enzyme inhibitor) during mechanical increases in pressure due to partial ductus arteriosus compression in chronically prepared late-gestation fetal lambs. In control studies, partial ductus arteriosus compression decreased the ratio of pulmonary arterial pressure to pulmonary artery flow in the left lung 34 +/- 6% from baseline. Intrapulmonary infusions of BQ-123 (0.5 microgram/min for 10 min; 0.025 microgram/min for 2 h) or phosphoramidon (1.0 mg/min for 10 min) augmented the peak vasodilator response during ductus arteriosus compression (52 +/- 3 and 49 +/- 6% from baseline, respectively, P < 0.05 vs. control). In addition, unlike the transient vasodilator response to ductus arteriosus compression in control studies, ET-1 blockade with BQ-123 or phosphoramidon prolonged the increase in flow caused by ductus arteriosus compression. In summary, ETA-receptor blockade and ET-1-converting enzyme inhibition augment and prolong fetal pulmonary vasodilation during partial compression of the ductus arteriosus. We conclude that ET-1 activity modulates acute and prolonged responses of the fetal pulmonary circulation to changes in vascular pressure. We speculate that ET-1 contributes to regulation and maintenance of high pulmonary vascular resistance in the normal ovine fetal lung.

Physiologic characterization of endothelin A and B receptor activity in the ovine fetal pulmonary circulation

To determine the potential contribution of endothelin (ET) to modulation of high pulmonary vascular resistance in the normal fetus, we studied the effects of BQ 123, a selective ET-A receptor antagonist, and sarafoxotoxin S6c (SFX), a selective ET-B receptor agonist, in 31 chronically prepared late gestation fetal lambs. Brief intrapulmonary infusions of BQ 123 (0.1-1.0 mcg/min for 10 min) caused sustained increases in left pulmonary artery flow (Qp) without changing main pulmonary artery (MPA) and aortic (Ao) pressures. In contrast, BQ 123 did not change vascular resistance in a regional systemic circulation (the fetal hindlimb). To determine whether big-endothelin-1 (big-ET-1)-induced pulmonary vasoconstriction is mediated by ET-A receptor stimulation, we studied the effects of big-ET-1 with or without pretreatment with BQ 123. BQ 123 (0.5 mcg/min for 10 min) blocked the rise in total pulmonary resistance caused by big-ET-1. CGS 27830 (100 mcg/min for 10 min), an ET-A and -B receptor antagonist, did not change basal tone but blocked big-ET-1-induced pulmonary vasoconstriction. Brief and prolonged intrapulmonary infusion of SFX (0.1 mcg/min for 10 min) increased Qp twofold without changing MPA or Ao pressures. Nitro-L-arginine (L-NA), a selective endothelium-derived nitric oxide (EDNO) antagonist, blocked vasodilation caused by BQ 123 and SFX. We conclude that: (a) BQ 123 causes sustained fetal pulmonary vasodilation, but did not change vascular resistance in the fetal hindlimb; (b) Big-ET-1-induced pulmonary vasoconstriction may be mediated through ET-A receptor stimulation; and (c) ET-B receptor stimulation causes pulmonary vasodilation through EDNO release. These findings support the hypothesis that endothelin may play a role in modulation of high basal pulmonary vascular resistance in the normal fetus.

Sodium and potassium in red blood cells of premature infants during the first few days: risk of hyperkalaemia

Erythrocyte sodium and potassium were studied in 64 newborn infants including 21 very low birthweight infants (birthweight less than 1500 g) during the first three days after birth. Erythrocyte sodium showed a positive correlation with gestational age (r = 0.63, p less than 0.01) and birthweight (r = 0.66, p less than 0.01). Erythrocyte potassium was negatively correlated with birthweight (r = -0.33, p less than 0.05). The Na/K ratio in red blood cells showed a positive correlation with gestational age (r = 0.60, p less than 0.01) and birthweight (r = 0.65, p less than 0.01). In VLBW infants plasma potassium rose significantly (p less than 0.01) from 0-6 h to 12-30 h and decreased from 12-30 h to 30-60 h (p less than 0.05). Erythrocyte potassium decreased slightly from 0-6 h to 12-30 h, but not significantly. A new finding that "more immature infants have higher potassium and lower sodium concentration in RBC" may suggest a potential risk of hyperkalaemia in tiny infants.

Failure of postnatal adaptation of the pulmonary circulation after chronic intrauterine pulmonary hypertension in fetal lambs

To determine the effects of chronic intrauterine pulmonary hypertension on the perinatal pulmonary circulation, we induced chronic elevations of pulmonary artery pressure in 24 late-gestation fetal lambs by maintaining partial compression of the ductus arteriosus with an inflatable vascular occluder. Pulmonary artery pressure was increased from 44 +/- 1 to 62 +/- 3 mmHg for 3-14 d. Although left pulmonary artery blood flow initially increased during acute partial ductus compression, the increase in flow was not sustained during chronic ductus compression despite persistent elevations of pulmonary artery pressure (P less than 0.01). Chronic hypertension decreased the slope of the pressure-flow relationship from 3.4 +/- 0.3 (initial) to 0.9 +/- 0.1 ml/min per mmHg, and blunted the fetal pulmonary vascular response to small increases in PO2 (P less than 0.0001). Pulmonary hypertension for greater than 8 d increased the wall thickness of small pulmonary arteries (P less than 0.001). Compared with controls, hypertensive animals had higher pulmonary artery pressure, lower pulmonary blood flow, and predominant right-to-left ductus shunting after cesarean-section delivery (P less than 0.0001). We conclude that chronic pulmonary hypertension in utero, in the absence of hypoxemia or sustained increases in blood flow, causes abnormal fetal pulmonary vasoreactivity, structural remodeling, and the failure to achieve the normal decline in pulmonary resistance at birth.