Vascular reactivity of pulmonary arteries from premature lambs subjected to liquid ventilation

The role of nitric oxide synthase-derived reactive oxygen species in the altered relaxation of pulmonary arteries from lambs with increased pulmonary blood flow

Congenital cardiac defects associated with increased pulmonary blood flow (Q(p)) produce pulmonary hypertension. We have previously reported attenuated endothelium-dependent relaxations in pulmonary arteries (PA) isolated from lambs with increased Q(p) and pulmonary hypertension. To better characterize the vascular alterations in the nitric oxide-superoxide system, 12 fetal lambs underwent in utero placement of an aortopulmonary vascular graft (shunt). Twin lambs served as controls. PA were isolated from these lambs at 4-6 wk of age. Electron paramagnetic resonance spectroscopy on fourth-generation PA showed significantly increased superoxide anion generation in shunt PA that were decreased to control levels following inhibition of nitric oxide synthase (NOS) with 2-ethyl-2-thiopseudourea. Preconstricted fifth-generation PA rings were relaxed with a NOS agonist (A-23187), a nitric oxide donor [S-nitrosyl amino penicillamine (SNAP)], polyethylene glycol-conjugated superoxide dismutase (PEG-SOD), or H(2)O(2). A-23187-, PEG-SOD-, and H(2)O(2)-mediated relaxations were impaired in shunt PA compared with controls. Pretreatment with PEG-SOD significantly enhanced the relaxation response to A-23187 and SNAP in shunt but not control PA. Inhibition of NOS with nitro-L-arginine or scavenging superoxide anions with tiron enhanced relaxation to SNAP and inhibited relaxation to PEG-SOD in shunt PA. Pretreatment with catalase inhibited relaxation of shunt PA to A-23187, SOD, and H(2)O(2). We conclude that NOS catalyzes the production of superoxide anions in shunt PA. PEG-SOD relaxes shunt PA by converting these anions to H(2)O(2), a pulmonary vasodilator. The redox environment, influenced by the balance between production and scavenging of ROS, may have important consequences on pulmonary vascular reactivity in the setting of increased Q(p).

Pulmonary arterial contractility in neonatal lambs increases with 100% oxygen resuscitation

The optimal Fi(O2) during neonatal resuscitation is a subject of controversy. The effect of exposure to high levels of inspired oxygen on pulmonary arterial (PA) contractility is not known. We studied differences in PA vasoreactivity in term lambs initially ventilated with 21% or 100% oxygen, followed by continued ventilation using oxygen as needed for 24 h, or ventilated with 100% oxygen for 24 h and room air breathing 1-d-old lambs. Term lambs were delivered by cesarean section, intubated, and ventilated with 21% (21%Res) or 100% oxygen (100%Res) for the first 30 min of life. Subsequently, the ventilator Fi(O2) was adjusted to maintain a Pa(O2) between 45 and 65 mm Hg for 24 h. Five lambs were ventilated continuously with 100% oxygen (100%24h). Six spontaneously breathing newborn lambs (RA Spont) were studied for comparison. Lambs were killed at 24 h of life and PA rings were isolated and contracted with norepinephrine (NE) and KCl and some were relaxed with A23187 and SNAP in tissue baths. NE and KCl induced contractions were highest in PA isolated from 100%24h lambs, and were significantly higher in 100%Res lambs than PA from 21%Res lambs. Contraction responses in PA from RA Spont lambs were similar to 21%Res lambs. Relaxations to A23187 and SNAP were similar among all ventilated groups. PA contractility to NE and KCl is increased following both brief (30 min) and prolonged (24 h) exposure to 100% oxygen during mechanical ventilation. In contrast, normoxic resuscitation and ventilation do not increase PA contractility.

Cardiopulmonary effects of chronic administration of the NO synthase inhibitor L-NAME in the chick embryo

BACKGROUND

Experimental observations in mammalian models suggest that endothelial nitric oxide (NO) synthase (NOS) content and activity are decreased in persistent pulmonary hypertension of the newborn.

OBJECTIVES

To test the hypothesis that disruption of NO signaling in the developing chick embryo lung may contribute to pulmonary hypertension.

METHODS

We analyzed pulmonary arterial reactivity and structure and heart morphology of 19-day chick embryos (incubation time 21 days) that received a daily injection of the NOS inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME, 20 mug per gram egg) or vehicle from day 12 until day 18.

RESULTS

Exposure to L-NAME did not affect embryonic survival or body mass of the embryos. The contractile responses to KCl, endothelin-1, the thromboxane A2 mimetic U46619, noradrenaline, and electrical-field stimulation were not affected by exposure to L-NAME. In contrast, in ovo L-NAME exposure reduced the sensitivity of pulmonary arteries to acetylcholine (pD2: 6.53 +/- 0.14 vs. 6.96 +/- 0.13; p < 0.05) and this effect was reversed by the NOS substrate L-arginine. Relaxations induced by sodium nitroprusside or forskolin were not altered by chronic L-NAME. Pulmonary vessel density was not different, but the percentage medial wall area of small pulmonary arteries (external diameter 10-50 microm) was slightly but significantly increased in the embryos exposed to L-NAME. In addition, hearts of L-NAME-exposed embryos showed an increase in right and left ventricular wall area.

CONCLUSIONS

Chronic inhibition of NOS produced, in the chick embryo, impairment of endothelium-dependent relaxation, structural remodeling of the pulmonary vascular bed and biventricular cardiac enlargement.