Bradykinin receptor blockade does not affect oxygen-mediated pulmonary vasodilation in fetal lambs

Pulmonary hemodynamic responses to in utero ventilation in very immature fetal sheep

BACKGROUND

The onset of ventilation at birth decreases pulmonary vascular resistance (PVR) resulting in a large increase in pulmonary blood flow (PBF). As the large cross sectional area of the pulmonary vascular bed develops late in gestation, we have investigated whether the ventilation-induced increase in PBF is reduced in immature lungs.

METHODS

Surgery was performed in fetal sheep at 105 d GA (n = 7; term ~147 d) to insert an endotracheal tube, which was connected to a neonatal ventilation circuit, and a transonic flow probe was placed around the left pulmonary artery. At 110 d GA, fetuses (n = 7) were ventilated in utero (IUV) for 12 hrs while continuous measurements of PBF were made, fetuses were allowed to develop in utero for a further 7 days following ventilation.

RESULTS

PBF changes were highly variable between animals, increasing from 12.2 ± 6.6 mL/min to a maximum of 78.1 ± 23.1 mL/min in four fetuses after 10 minutes of ventilation. In the remaining three fetuses, little change in PBF was measured in response to IUV. The increases in PBF measured in responding fetuses were not sustained throughout the ventilation period and by 2 hrs of IUV had returned to pre-IUV control values.

DISCUSSION AND CONCLUSION

Ventilation of very immature fetal sheep in utero increased PBF in 57% of fetuses but this increase was not sustained for more than 2 hrs, despite continuing ventilation. Immature lungs can increase PBF during ventilation, however, the present studies show these changes are transient and highly variable.

Developmental expression of vasoactive and growth factors in human lung. Role in pulmonary vascular resistance adaptation at birth

The factors that mediate the postnatal fall in pulmonary vascular resistance, which is crucial for normal gas exchange, are not fully understood. The endothelium has been implicated in this phenomenon, through the release of vasorelaxant factors such as nitric oxide (NO). Human pulmonary expression of endothelial NO synthase increases up to 31 wk of gestation, together with vascular endothelial growth factor (VEGF), and both factors potently mediate pulmonary angiogenesis and vasorelaxation. During the perinatal period, when pulmonary vasodilatation is maximal, endothelial NO synthase and VEGF are weakly expressed. This raises the involvement of vasorelaxant factors other than NO at birth. One candidate is endothelial-derived hyperpolarizing factor, which induces smooth muscle cell hyperpolarization by activating K(ATP) channels. The marked vasorelaxation induced by activation of these channels in newborn animals, and their strong perinatal expression in the human lung, suggest their involvement during this phase. Another candidate is endothelin (ET)-1, together with its receptors ET-A and ET-B. ET-A receptors are located exclusively on smooth muscle cells and mediate vasoconstriction, whereas ET-B receptors mediate vasoconstriction when located on smooth muscle cells and vasodilatation when located on endothelial cells. ET-B receptors, which are strongly expressed in the human fetal lung both at the end of gestation and after birth, may be involved in perinatal pulmonary vasodilatation. Thus, in human fetal lung, K(ATP) channels and ET-B receptors could be important in mediating the perinatal pulmonary vasodilatation crucial for adapting the pulmonary circulation to extrauterine life.

Mechanisms of bradykinin-mediated dilation in newborn piglet pulmonary conducting and resistance vessels

Bradykinin (BK) is a potent dilator of the perinatal pulmonary circulation. We investigated segmental differences in BK-induced dilation in newborn pig large conducting pulmonary artery and vein rings and in pressurized pulmonary resistance arteries (PRA). In conducting pulmonary arteries and veins, BK-induced relaxation is abolished by endothelial disruption and by inhibition of nitric oxide (NO) synthase with nitro-L-arginine (L-NA). In PRA, two-thirds of the dilation response is L-NA insensitive. Charybdotoxin plus apamin and depolarization with KCl abolish the L-NA-insensitive dilations, findings that implicate the release of endothelium-derived hyperpolarizing factor (EDHF). However, endothelium-disrupted PRA retain the ability to dilate to BK but not to ACh or A-23187. In endothelium-disrupted PRA, dilation was inhibited by charybdotoxin. Thus in PRA, BK elicits dilation by multiple and duplicative signaling pathways. Release of NO and EDHF contributes to the response in endothelium-intact PRA; in endothelium-disrupted PRA, dilation occurs by direct activation of vascular smooth muscle calcium-dependent potassium channels. Redundant signaling pathways mediating pulmonary dilation to BK may be required to assure a smooth transition to extrauterine life.

Oxygen-induced vasodilation in pulmonary arterioles from fetal rats

BACKGROUND

Oxygen is a potent stimulus for pulmonary vasodilation in the perinatal period. Little information is available regarding mediators of oxygen-induced pulmonary vasodilation in fetal rats. We have investigated the effect of blocking several proposed mediators of oxygen-induced vasodilation on the responses of isolated, third-generation pulmonary arterioles from term fetal rats to an increase in oxygen tension.

MATERIALS AND METHODS

Third-generation pulmonary arterioles were isolated from fetal rats at term. Arterioles were preconstricted by suffusion with "hypoxic" (pO(2) 25-40 mm Hg) solution containing 40 mM KCl. The vasodilation induced by suffusion with "normoxic" (pO(2) 90-150 mm Hg) 40 mM KCl solution was recorded for control pulmonary arterioles and for arterioles pretreated with inhibitors of nitric oxide synthase and cyclooxygenase, and blockers of bradykinin receptors and purinergic receptors. Responses to oxygen suffusion were also recorded for pulmonary arterioles denuded of endothelium.

RESULTS

Control arterioles dilated 113 +/- 28% of the potassium-induced preconstriction after 60 min of normoxic suffusion. Pretreatment with indomethacin completely blocked oxygen-induced vasodilation. Inhibitors of nitric oxide synthase, blockers of bradykinin and purinergic receptors, and removal of the endothelium did not significantly change normoxic vasodilation.

CONCLUSIONS

Our results are most consistent with a vasodilator product of cyclooxygenase metabolism as a primary stimulus for oxygen-induced vasodilation in isolated, third-generation pulmonary arterioles from fetal rats.

Control of the pulmonary circulation in the fetus and during the transitional period to air breathing

During fetal life and the transition to extra-uterine air breathing, pulmonary vascular tone is regulated by a complex, interactive group of mechanisms. Arachidonic acid metabolites play an important role in this regulation. Although prostaglandins may not be central to regulation of the resting fetal pulmonary circulation, PGI2 acts to modulate tone and thereby maintain pulmonary vascular resistance relatively constant. PGI2 also may play an important role as one of the components involved in the major changes that occur with the onset of air breathing. Leukotrienes, also metabolites of arachidonic acid and potent smooth muscle constrictors, may play an active role in maintaining the normally high fetal pulmonary vascular resistance, because leukotriene receptor blockade or synthesis inhibition increases pulmonary blood flow about eight-fold; the presence of leukotrienes in fetal tracheal fluid further supports this. In addition to PGI2, vascular endothelial cells produce other vasoactive factors. These include potent vasodilators, such as endothelium-derived relaxing factor (EDRF). EDRF, known to be nitric oxide (NO) and often called endothelium-derived nitric oxide (EDNO), is produced by endothelial cells in response to varied stimuli, generally involving specific receptors and the activation of endothelial NO synthetase (eNOS); subsequent smooth muscle relaxation is produced by a NO/guanylyl cyclase/cGMP-mediated mechanism. NO clearly is involved in regulation of vascular tone in the fetal pulmonary circulation, although it plays a far more important role in the postnatal transition to air breathing. Superfused fetal sheep pulmonary arteries release NO when stimulated with bradykinin. In fetal lambs the vasodilating effects of bradykinin are attenuated by methylene blue and resting tone falls with N(omega)-nitro-L-arginine, an inhibitor of NO synthesis, suggesting that a NO/cGMP-dependent mechanism continuously modulates or offsets the increased tone of the resting fetal pulmonary circulation. Inhibition of NO synthesis blocks the pulmonary vasodilation with oxygenation of fetal lungs in utero. Shear stress-induced NO production as well as the relationship of oxygenation to NO production further support the important function of NO in the transition. Although endothelin-1 (ET-1) has potent vasoactivity as well as ontogenetic differences in effect on pulmonary vascular resistance, its exact physiological role has not been defined. Adrenomedullin and calcitonin gene-related peptide (CGRP), two additional vasoactive substances, have profound, and prolonged, vasodilating effects in the fetal pulmonary circulation. Their physiological roles have not yet been established.

Kinin B2 receptor-mediated contraction of tail arteries from normal or streptozotocin-induced diabetic rats

1. The vasoactive effects of bradykinin (BK) are mediated by different subtypes of kinin receptors, of which the expression varies among different tissues. In rat tail artery tissues, BK elicited a concentration-dependent vasoconstriction (EC50, 25.9+/-2.4nM; Emax, 0.39+/-0.01 g; n=16). This effect of BK was endothelium independent and indomethacin insensitive. The BK-induced contraction of tail artery tissues, however, depended on both membrane potential-sensitive extracellular Ca2+ entry and thapsigargin-sensitive intracellular Ca2+ release. 2. Kinin B1 receptor antagonist or agonist did not affect the basal tension or the BK-induced contraction of tail artery tissues in the absence or presence of endothelium (P>0.05). In contrast, the BK-induced vasoconstriction was inhibited by kinin B2 receptor antagonists. Pretreatment of vascular tissues with Hoe 140 (1 nM) significantly changed EC50 of the BK-induced vasoconstriction from 25.5+/-7.4 nM to 82.6+/-16.8nM (n=8, P<0.01) and Emax from 0.43+/-0.03g to 0.16+/-0.01 g (n=8, P<0.01). 3. In the tail artery tissues from streptozotocin-induced diabetic rats, the BK-elicited vasoconstriction was significantly reduced (EC50, 67.8+/-11 nM: Emax, 0.19+/-0.01 g) compared to their counterparts from normal rats. The decreased vasoconstrictive effects of BK on diabetic arteries were endothelium independent and indomethacin insensitive. 4. Our study demonstrated that the contraction of rat tail arteries induced by BK was mediated by B2 receptors located on vascular smooth muscles. The altered B2 receptor-mediated vascular activity may play an important role in the vascular complications of diabetes.

Effect of endothelium removal on prostaglandin and nitric oxide function in pulmonary resistance arteries in the lamb

We have recently shown that isolated pulmonary resistance arteries of the fetal lamb have prostaglandin (PG) I2 based and nitric oxide (NO) based relaxing mechanisms, which are activated by oxygen (at neonatal levels) and bradykinin. The present study was carried out to ascertain whether these mechanisms remain operational after removal of the endothelium. Endothelium-denuded vessels pre-equilibrated at a neonatal PO2 were not affected by indomethacin (2.8 µM), while they contracted weakly to NG-nitro-L-arginine methyl ester (L-NAME, 100 µM). However, the latter response did not reach significance and resembled that of intact vessels at fetal PO2. Bradykinin (0.1-100 nM) dose dependently (from 1-3 nM upwards) relaxed endothelium-denuded arteries that had been precontracted with a thromboxane (TX) A2 analog (ONO-11113, 0.1 µM) or excess potassium (5 mM Ca2+ in K+-Krebs) at a neonatal PO2. The response was the same under the two conditions, but it was smaller than that of intact arteries. Bradykinin relaxation of ONO-11113-contracted arteries was completely or nearly completely inhibited by indomethacin and L-NAME. We conclude that endothelium-denuded, pulmonary resistance arteries maintain PG (conceivably PGI2) mediated and NO-mediated relaxing mechanisms. These extra-endothelial mechanisms are activated by bradykinin but not by oxygen.Key words: perinatal pulmonary circulation, endothelium, nitric oxide, prostaglandin, oxygen.