Nitric oxide-endothelin-1 interactions after surgically induced acute increases in pulmonary blood flow in intact lambs

Several congenital heart defects require surgery that acutely increases pulmonary blood flow (PBF). This can lead to dynamic alterations in postoperative pulmonary vascular resistance (PVR) and can contribute to morbidity and mortality. Thus the objective of this study was to determine the role of nitric oxide (NO), endothelin (ET)-1, and their interactions in the alterations of PVR after surgically induced increases in PBF. Twenty lambs underwent placement of an aortopulmonary vascular graft. Lambs were instrumented to measure vascular pressures and PBF and studied for 4 h. Before and after shunt opening, lambs received an infusion of saline (n = 9), tezosentan, an ETA- and ETB -receptor antagonist (n = 6), or Nomega-nitro-L-arginine (L-NNA), a NO synthase (NOS) inhibitor (n = 5). In control lambs, shunt opening increased PBF by 117.8% and decreased PVR by 40.7% (P < 0.05) by 15 min, without further changes thereafter. Plasma ET-1 levels increased 17.6% (P < 0.05), and total NOS activity decreased 61.1% (P < 0.05) at 4 h. ET-receptor blockade (tezosentan) prevented the plateau of PBF and PVR, such that PBF was increased and PVR was decreased compared with controls at 3 and 4 h (P < 0.05). These changes were associated with an increase in total NOS activity (+61.4%; P < 0.05) at 4 h. NOS inhibition (L-NNA) after shunt placement prevented the sustained decrease in PVR seen in control lambs. In these lambs, PVR decreased by 15 min (P < 0.05) but returned to baseline by 2 h. Together, these data suggest that surgically induced increases in PBF are limited by vasoconstriction, at least in part by an ET-receptor-mediated decrease in lung NOS activity. Thus NO appears to be important in maintaining a reduction in PVR after acutely increased PBF.     

Nitric oxide-endothelin-1 interaction in humans

Ahlborg, Gunvor, and Jan M. Lundberg. Nitricoxide-endothelin-1 interaction in humans. J. Appl.Physiol. 82(5): 1593-1600, 1997.Healthy menreceived N^G-monomethyl-L-arginine(L-NMMA) intravenously to studycardiovascular and metabolic effects of nitric oxide synthase blockadeand whether this alters the response to endothelin-1 (ET-1) infusion.Controls only received ET-1.L-NMMA effects were that heartrate (17%), cardiac output (17%), and splanchnic and renal blood flow(both 33%) fell promptly (all P < 0.01). Mean arterial blood pressure (6%), and systemic (28%) andpulmonary (40%) vascular resistances increased(P < 0.05 to 0.001). Arterial ET-1levels (21%) increased due to a pulmonary net ET-1 release(P < 0.05 to 0.01). Splanchnic glucose output (SGO) fell (26%, P < 0.01). Arterial insulin and glucagon were unchanged. Subsequent ET-1infusion caused no change in mean arterial pressure, heart rate, orcardiac output, as found in the present controls, or in splanchnic andrenal blood flow or splanchnic glucose output as previously found withET-1 infusion (G. Ahlborg, E. Weitzberg, and J. M. Lundberg.J. Appl. Physiol. 79: 141-145,1995). In conclusion, L-NMMAlike ET-1, induces prolonged cardiovascular effects and suppresses SGO.L-NMMA causes pulmonary ET-1release and blocks responses to ET-1 infusion. The results indicatethat nitric oxide inhibits ET-1 production and thereby interacts withET-1 regarding increase in vascular tone and reduction of SGO inhumans.

     

Nitric oxide decreases lung liquid production in fetal lambs

Cummings, James J. Nitric oxide decreases lung liquidproduction in fetal lambs. J. Appl.Physiol. 83(5): 1538-1544, 1997.To examine theeffect of nitric oxide on fetal lung liquid production, I measured lungliquid production in fetal sheep at 130 ± 5 days gestation (range122-137 days) before and after intrapulmonary instillation ofnitric oxide. Thirty-one studies were done in which net lung luminalliquid production (Jv) was measured by plotting the change in lung luminal liquid concentration ofradiolabeled albumin, an impermeant tracer that was mixed into the lungliquid at the start of each study. To see whether changes inJvmight be associated with changes in pulmonary hemodynamics, pulmonary and systemic pressures were measured and left pulmonary arterial flowwas measured by an ultrasonic Doppler flow probe. Variables weremeasured during a 1- to 2-h control period and for 4 h after a smallbolus of isotonic saline saturated with nitric oxide gas (10 or 100%)was instilled into the lung liquid. Control (saline) instillations(n = 6) caused no change in anyvariable over 6 h. Nitric oxide instillation significantly decreasedJv and increased pulmonary blood flow;these effects were sustained for 1-2 h. There was also asignificant but transient decrease in pulmonary arterial pressure. Thusintrapulmonary nitric oxide causes a significant decrease in lungliquid and is associated with a decrease in pulmonary vascularresistance. In a separate series of experiments either amiloride orbenzamil, which blocks Na⁺transport, was mixed into the lung liquid before nitric oxide instillation; still, there was a similar reduction in lung liquid production. Thus the reduction in lung liquid secretion caused bynitric oxide does not appear to depend on apicalNa⁺ efflux.

     

Experimental cryptosporidiosis in fetal lambs

Fetal lambs were infected in utero with purified sporulated oocysts of Cryptosporidium parvum in order to study pathogenesis and host cellular response to the enteropathogen. Ileal loops (IL) of fetuses, 124-130 days of gestation, were inoculated with 1-4 x 10(6) oocysts usually via cannulae in the abdominal wall of the ewe. Oocysts, both free and phagocytosed, were observed in the IL content as early as day 1 post-inoculation (PI). The percentage of oocysts phagocytosed by the host's polymorphonuclear neutrophils (PMN's) and mononuclear cells remained high up to day 13, the last day of examination. Numerous parasites were observed at days 6, 7, and 12 PI in the microvilli of the ileum with hypercellularity of the lamina propria, which consisted of a mixed infiltration of PMN's, mononuclear cells, including lymphoid cells, and a few eosinophils. Cytolysis and extrusion of epithelial cells, often heavily parasitized by various stages of the parasite, as well as inflammatory cells, were prominent in luminal contents. Germinal centers were prominent in mesenteric lymph nodes draining the infected loops by day 12 PI. Depletion of lymphoid cells was already present in Peyer's patches by day 4 PI.     

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.     

Postnatal lung function and protein permeability after fetal or maternal corticosteroids in preterm lambs

Rebello, Celso M., Machiko Ikegami, M. Gore Ervin, Daniel H. Polk, and Alan H. Jobe. Postnatal lung function and protein permeability after fetal or maternal corticosteroids in preterm lambs.J. Appl. Physiol. 83(1): 213-218, 1997.We evaluated postnatal lung function andintravascular albumin loss to tissues of 123-days-gestation pretermsurfactant-treated and ventilated lambs 15 h after direct fetal(n = 8) or maternal(n = 9) betamethasone treatment orsaline placebo (n = 9). Thebetamethasone-treated groups had similar increases in dynamiccompliances, ventilatory efficiency indexes, and lung volumes relativeto controls (P < 0.05). The lossesof ¹²⁵I-labeled albumin fromblood, a marker of intravascular integrity, and the recoveries of¹²⁵I-albumin in muscle and brainwere similar for control and betamethasone-exposed lambs.Betamethasone-treated lambs had lower recoveries of¹²⁵I-albumin in lung tissues andin alveolar washes than did controls (P < 0.01). Although blood pressureswere higher for the treated groups (P < 0.05), all groups had similar blood volumes, cardiac outputs, andorgan blood flows. Maternal or fetal treatment with betamethasone 15 hbefore preterm delivery equivalently improved postnatal lung function,reduced albumin recoveries in lungs, and increased blood pressures.However, prenatal betamethasone had no effects on the systemicintravascular losses of albumin or did not change blood volumes.

     

Pulmonary vascular response to prostacyclin in fetal lambs

Eighteen prostacyclin injections (19.4±1.5 μg/kg) were performed in five chronically instrumented, intact fetal lambs in order to study the effects on pulmonary blood flow. These resulted in a brief period of bradycardia followed by a more prolonged period of increased pulmonary blood flow. In this latter phase, pulmonary blood flow increased from a baseline value of 49±4 ml/(kg min) to 122±10 ml/(kg min). Systolic/diastolic pulmonary arterial pressure simultaneously fell from to mm Hg. Flow through the ductus arteriosus was unchanged and right ventricular output increased to account for the increased pulmonary blood flow. Thus, prostacyclin causes pulmonary vasodilation in intact fetal lambs and may participate in the control of fetal pulmonary blood flow and the circulatory adjustments to extra-uterine life.     

Nitric oxide signaling in plant-pathogen interactions

Nitric oxide (NO), first characterized as an endothelium-derived relaxation factor, is involved in diverse cellular processes including neuronal signaling, blood pressure homeostasis, and immune response. Recent studies have also revealed a role for NO as a signaling molecule in plants. As a developmental regulator, NO promotes germination, leaf extension and root growth, and delays leaf senescence and fruit maturation. Moreover, NO acts as a key signal in plant resistance to incompatible pathogens by triggering resistance-associated hypersensitive cell death. In addition, NO activates the expression of several defense genes (e.g. pathogenesis-related genes, phenylalanine ammonialyase, chalcone synthase) and could play a role in pathways leading to systemic acquired resistance.     

Nitric oxide-oxygen radicals interactions in atherosclerosis

Atherosclerosis is one of the most common diseases and the principal cause of death in western civilization. The pathogenesis of this disease can be explained on the basis of the 'oxidative-modification hypothesis,' which proposes that low-density lipoprotein (LDL) oxidation represents a key early event. Nitric oxide (*NO) regulates critical lipid membrane and lipoprotein oxidation events by a) contributing to the formation of more potent secondary oxidants from superoxide (i.e.: peroxynitrite), and b) its antioxidant properties through termination reactions with lipid radicals to possibly less reactive secondary nitrogen-containing products (LONO, LOONO). Relative rates of production and steady state concentrations of superoxide and *NO and cellular sites of production will profoundly influence the expression of differential oxidant injury-enhancing and protective effects of *NO. Full understanding of the physiological roles of *NO, coupled with detailed insight into *NO regulation of oxygen radical-dependent reactions, will yield a more rational basis for intervention strategies directed toward oxidant-dependent atherogenic processes.     

Effect of tracheostomy on lung development in fetal lambs

The effects of fetal tracheostomy on lung development in lambs have been investigated. Seven ewes, all with twin fetuses, were given a general anesthetic. One fetus in each set of twins was tracheostomized and the other fetus was sham operated (117-122 days gestation). The fetuses were delivered by cesarean section between 137 and 140 days gestation. Fetal tracheostomy decreased lung weight and volume, altered DNA concentration and the structural development of the acinus, and decreased lung distensibility. However, tissue and airway saturated phosphatidylcholine and lung stability during deflation were not significantly affected by fetal tracheostomy. It seems that tracheostomy in fetal lambs alters lung growth but does not affect the formation or release of surfactant. These data support the hypothesis that lung volume is actively maintained and lung growth promoted by the secretion of lung fluid against the resistance of the upper airway in fetal lambs.     

Pulmonary vascular response to prostacyclin in fetal lambs.

Eighteen prostacyclin injections (19.4 +/- 1.5 micrograms/kg) were performed in five chronically instrumented, intact fetal lambs in order to study the effects on pulmonary blood flow. These resulted in a brief period of bradycardia followed by a more prolonged period of increased pulmonary blood flow. In this latter phase, pulmonary blood flow increased from a baseline value of 49 +/- 4 ml/(kg min) to 122 +/- 10 ml/(kg min). Systolic/diastolic pulmonary arterial pressure simultaneously fell from 73 +/- 2/48 +/- 1 to 68 +/- 2/42 +/- 1 mm Hg. Flow through the ductus arteriosus was unchanged and right ventricular output increased to account for the increased pulmonary blood flow. Thus, prostacyclin causes pulmonary vasodilation in intact fetal lambs and may participate in the control of fetal pulmonary blood flow and the circulatory adjustments to extra-uterine life.     

Endothelin-1-induced pulmonary arterial dilation is reduced by N omega-nitro-L-arginine in fetal lambs.

Endothelin-1 (ET-1) is a pulmonary vasodilator in the unventilated fetal lamb. The site and mechanism of this vasodilator response were investigated in isolated blood-perfused lungs from nine fetal lambs delivered at 127-140 days gestation. The vascular occlusion technique was used to partition the total pulmonary pressure gradient into pressure gradients across large and small arteries (delta PLA and delta PSA, respectively) and veins (delta PV). Injection of ET-1 (74 ng/kg) into the pulmonary artery significantly decreased delta PLA from 12.4 +/- 2.1 to 5.2 +/- 1.1 mmHg and delta PSA from 49.2 +/- 2.7 to 31.3 +/- 4.9 mmHg. The pressure measured by double occlusion, an estimate of pulmonary capillary pressure, was not altered by ET-1 (15.5 +/- 1.0 vs. 14.8 +/- 1.0 mmHg), indicating that ET-1 had no effect on pulmonary veins. Addition of N omega-nitro-L-arginine (estimated perfusate concentration 2-6 mM), an analogue of L-arginine that inhibits the production of endothelium-derived relaxing factor (EDRF), significantly attenuated the dilator responses to acetylcholine (10 micrograms) and ET-1 (74 ng/kg) by 35 and 56%, respectively. These results in unventilated fetal lungs indicate that 1) ET-1 dilates both large and small pulmonary arteries with no effect on pulmonary veins, and 2) this effect is mediated in part through the action of the EDRF pathway.     

Nitric oxide signalling functions in plant-pathogen interactions

Nitric oxide (NO) is a highly reactive molecule that rapidly diffuses and permeates cell membranes. During the last few years NO has been detected in several plant species, and the increasing number of reports on its function in plants have implicated NO as a key molecular signal that participates in the regulation of several physiological processes; in particular, it has a significant role in plant resistance to pathogens by triggering resistance-associated cell death and by contributing to the local and systemic induction of defence genes. NO stimulates signal transduction pathways through protein kinases, cytosolic Ca2+ mobilization and protein modification (i.e. nitrosylation and nitration). In this review we will examine the synthesis of NO, its effects, functions and signalling giving rise to the hypersensitive response and systemic acquired resistance during plant-pathogen interactions.