Hypoxia stimulates prostacyclin generation by dog lung in vitro

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Iloprost inhalation redistributes pulmonary perfusion and decreases arterial oxygenation in healthy volunteers

BACKGROUND

Previous studies have shown that ventilation-perfusion matching is improved in the prone as compared with that in the supine position. Regional differences in the regulation of vascular tone may explain this. We have recently demonstrated higher production of nitric oxide in dorsal compared with ventral human lung tissue. The purpose of the present study was to investigate regional differences in actions by another vasoactive mediator, namely prostacyclin. The effects on gas exchange and regional pulmonary perfusion in different body positions were investigated at increased prostacyclin levels by inhalation of a synthetic prostacyclin analogue and decreased prostacyclin levels by unselective cyclooxygenase (COX) inhibition.

METHODS

In 19 volunteers, regional pulmonary perfusion in the prone and supine position was assessed by single photon emission computed tomography using (99m)Tc macro-aggregated albumin before and after inhalation of iloprost, a stable prostacyclin analogue, or an intravenous infusion of a non-selective COX inhibitor, diclofenac. In addition, gas distribution was assessed in seven subjects using (99m)Tc-labelled ultra-fine carbon particles before and after iloprost inhalation in the supine position.

RESULTS

Iloprost inhalation decreased arterial PaO(2) in both prone (from 14.2+/-0.5 to 11.7+/-1.7 kPa, P<0.01) and supine (from 13.7+/-1.4 to 10.9+/-2.1 kPa, P<0.01) positions. Iloprost inhalation redistributed lung perfusion from non-dependent to dependent lung regions in both prone and supine positions, while ventilation in the supine position was distributed in the opposite direction. No significant effects of non-selective COX inhibition were found in this study.

CONCLUSIONS

Iloprost inhalation decreases arterial oxygenation and results in a more gravity-dependent pulmonary perfusion in both supine and prone positions in healthy humans.

Regulation of vasodilator synthesis during lung development

The endothelium-derived vasodilator molecules prostaglandin I2 (PGI2) and nitric oxide (NO) are critically involved in the dramatic increase in pulmonary blood flow that occurs during cardiopulmonary transition at birth. Studies in animal and cell culture models have revealed that there is increased PGI2 and NO production in the pulmonary circulation of the late fetus in direct response to increased oxygenation, and that this response is unique to the pulmonary endothelium. Additional work has demonstrated that there is normally marked upregulation in the expression of the key synthetic enzymes cyclooxygenase type I and endothelial NO synthase in the lung during late gestation, thereby maximizing the capacity for vasodilator production at the time of birth. Furthermore, studies in animal models of neonatal pulmonary hypertension indicate that attenuated expression of these genes may frequently contribute to the pathogenesis of the disorder. A greater understanding of the mechanisms regulating PGI2 and NO synthesis in the developing lung will potentially lead to novel therapies for neonatal pulmonary hypertension aimed at optimizing endogenous vasodilator production.

Hypoxia stimulates prostacyclin synthesis in newborn pulmonary artery endothelium by increasing cyclooxygenase-1 protein

In newborn lambs, pulmonary prostacyclin (PGI2) production increases acutely in response to low oxygen. We tested the hypothesis that decreased oxygenation directly stimulates PGI2 synthesis in arterial segments and cultured endothelial cells from newborn lamb intrapulmonary arteries. In segments studied at PO2 of 680 mm Hg, the synthesis of PGI2 exceeded prostaglandin E2 (PGE2) by 73%. Endothelium removal lowered PGI2 by 77% and PGE2 by 66%. At low oxygen tension (PO2, 40 mm Hg), PGI2 and PGE2 synthesis rose by 96% and 102%, respectively. Similarly, in endothelial cells studied at PO2 of 680 mm Hg, the synthesis of PGI2 exceeded PGE2 by 50%, and at low oxygen tension both PGI2 and PGE2 increased (89% and 64%, respectively). Endothelial cell PGI2 synthesis maximally stimulated by bradykinin, A23187, or arachidonic acid was also increased at low PO2 by 50%, 66%, and 48%, respectively. PGE2 synthesis was similarly altered, increasing by 33%, 37%, and 41%, respectively. In contrast, lowering oxygen had minimal effect on PGI2 and PGE2 synthesis with exogenous PGH2, which is the product of cyclooxygenase. Immunoblot analyses revealed that there was a 2.6-fold greater abundance of cyclooxygenase-1 protein at PO2 of 40 versus 680 mm Hg, and the increase at lower oxygen tension was inhibited by cycloheximide. The cyclooxygenase-2 isoform was not detected. Thus, attenuated oxygenation directly stimulates PGI2 and PGE2 synthesis in intrapulmonary arterial segments and endothelial cells from newborn lambs. This process is due to enhanced cyclooxygenase activity related to increased abundance of the cyclooxygenase-1 protein, and this effect may be due to increased synthesis of the enzyme protein.

Prostacyclin production and mediation of adenylate cyclase activity in the pulmonary artery. Alterations after prolonged hypoxia in the rat

Prostacyclin is a critical mediator of structure and function in the pulmonary circulation, causing both the inhibition of vascular smooth muscle growth and vasodilation via the stimulation of adenylate cyclase. To examine the potential role of alterations in prostacyclin production or mechanism of action in chronic hypoxic pulmonary hypertension, we determined the effects of prolonged (7 d) in vivo hypoxia on in vitro prostacyclin synthesis and mediation of adenylate cyclase activity in rat main pulmonary arteries. In control arteries prostacyclin production exceeded that of prostaglandin (PG) E2 by 25-fold, with 42% originating from the endothelium. Studies utilizing indomethacin revealed that endogenous prostaglandins mediate at least 69% of basal adenylate cyclase activity. Prostacyclin-stimulated enzyme activity was enhanced by exogenous GTP, indicating that this is a receptor-mediated process involving G protein amplification. Comparable dose-related responses to prostacyclin and PGE2 suggest that these agents may activate a common receptor. After 7 d of in vivo hypoxia there was a 2.7-fold increase in in vitro prostacyclin production, with equivalent increases in synthesis in the endothelium and vascular smooth muscle. However, despite this increase there was no change in basal adenylate cyclase activity, and this was associated with attenuated sensitivity of the enzyme to prostacyclin stimulation. Concomitant diminution of the response to beta-adrenergic stimulation, with previously-demonstrated beta receptor downregulation and unaltered postreceptor-mediated activity, suggests that the blunted response to prostacyclin is due to receptor downregulation. Parallel studies of the thoracic aorta indicated that these changes are specific to the pulmonary artery. It is postulated that attenuation of the response of adenylate cyclase to prostacyclin may contribute to the structural changes and hypertension observed in the pulmonary vasculature of the rat with chronic hypoxia.

Differences in prostaglandin metabolism in cultured aortic and pulmonary arterial endothelial cells exposed to acute and chronic hypoxia

In vivo, a marked difference in blood oxygen tension exists between the pulmonary artery and the aorta. Responses of vascular endothelial cells from these vessels to changes in ambient oxygen might be influenced by the oxygen tension to which they are continuously exposed in vivo or by their anatomic site. To explore this hypothesis, we initially studied the production of the cyclooxygenase metabolites prostacyclin and thromboxane in bovine aortic and main pulmonary arterial endothelial cells grown in 21% O2 and exposed to different degrees of acute hypoxia over a wide range of times. We found that short-term hypoxia (3% or 0% O2) rapidly and transiently activates the cyclooxygenase pathway in both cell types, with a more rapid response in bovine aortic endothelial cells. To determine whether culture in an oxygen tension similar to that to which main pulmonary arterial endothelial cells are exposed in vivo alters this response, we evaluated these cyclooxygenase metabolites in bovine aortic and main pulmonary arterial endothelial cells cultured long-term in 3% O2, both at baseline and after exposure to acute anoxia (0% O2). In both cell types, we found a decrease in prostacyclin and thromboxane synthesis at baseline and evidence of an increase in the Vmax of thromboxane synthetase following stimulation with exogenous arachidonic acid. In chronically hypoxic cells exposed to acute anoxia, there were marked differences in enzyme activity compared with that in endothelial cells maintained in 21% O2 with differences depending on the origin of the endothelial cells. In bovine aortic endothelial cells, production of neither cyclooxygenase metabolite increased; in bovine main pulmonary arterial endothelial cells, only thromboxane production increased, suggesting isolated activation of the cyclooxygenase-thromboxane synthetase pathway. These studies demonstrate that acute and chronic hypoxia have profound effects on endothelial cell cyclooxygenase metabolism and that these effects depend on the duration and degree of the hypoxic exposure and the vascular bed from which the endothelial cells are derived.

Effects of cigarette smoking, hypoxia and vasoactive mediators on the production of PGI2 and TXA2 in cultured pulmonary artery endothelial cells

Effects of cigarette smoke extract (CSE) and some vasoactive mediators on the production of PGI2 and TXA2 in normoxic and hypoxic pulmonary artery endothelial cells (PAECs) in culture were studied. The production of PGI2 in PAECs was inhibited by hypoxia or verapamil, but promoted by angiotensin II (A II), noradrenaline (NE) or platelet activating factor (PAF), while that of TXA2 slightly increased except when treated with PAF. The effect of AII, NE, PAF and verapamil, however, was not influenced by hypoxia. CSE inhibited the production of PGI2 in normoxic PAECs but did not further reduce 6-keto-PGF1 alpha in hypoxic PAECs medium. The results suggest that a) the production of PGI2 during hypoxia might be stimulated by vasoactive mediators produced during hypoxia, not by hypoxia directly; b) the production and release of PGI2 were related to intracellular calcium; c) the augmented production of PGI2 might be one of the mechanisms in the pulmonary vasodilating role of PAF; and d) prostaglandin production might be associated with the alteration of hypoxic pulmonary vasoreactivity after cigarette smoking.

Hypoxic contraction of cultured pulmonary vascular smooth muscle cells

The cellular events involved in generating the hypoxic pulmonary vasoconstriction response are not clearly understood, in part because of the multitude of factors that alter pulmonary vascular tone. The goal of the present studies was to determine if a cell culture preparation containing vascular smooth muscle (VSM) cells could be made to contract when exposed to a hypoxic atmosphere. Cultures containing only fetal bovine pulmonary artery VSM cells were assessed for contractile responses to hypoxic stimuli by two methods. In the first, tension forces generated by cells grown on a flexible growth surface (polymerized polydimethyl siloxane) were manifested as wrinkles and distortions of the surface under the cells. Wrinkling of the surface was noted to progressively increase with time as the culture medium bathing the cells was made hypoxic (PO2 approximately 25 mmHg). The changes were sometimes reversible upon return to normoxic conditions and appeared to be enhanced in cells already exhibiting evidence of some baseline tone. Repeated passage in culture did not diminish the hypoxic response. Evidence for contractile responses to hypoxia was also obtained from measurements of myosin light chain (MLC) phosphorylation. Conversion of MLC to the phosphorylated species is an early step in the activation of smooth muscle contraction. Lowering the PO2 in the culture medium to 59 mmHg caused a 45% increase in the proportion of MLC in the phosphorylated form as determined by two-dimensional gel electrophoresis. Similarly, cultures preincubated for 4 h with 32P and then exposed to normoxia or hypoxia for a 5-min experimental period showed more than twice as much of the label in MLCs of the hypoxic cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of prostacyclin in the mesenteric traction syndrome during anesthesia for abdominal aortic reconstructive surgery

Mesenteric traction syndrome consists of sudden tachycardia, hypotension, and cutaneous hyperemia, and frequently occurs during mesenteric traction in patients undergoing abdominal aortic aneurysm (AAA) reconstructive surgery. The etiology and clinical impact of this phenomenon are unknown, but the symptoms suggest a release of vasoactive materials from the mesenteric vascular bed. Thirty-one patients who underwent AAA surgery were studied. Mesenteric traction was accompanied by a decrease in systolic (p = 0.005) and diastolic (p less than 0.05) blood pressures, and in systemic vascular resistance (p less than 0.005), and was accompanied by an increase in heart rate (HR) (p less than 0.005), and cardiac output (p = 0.01). These hemodynamic changes coincided with an increase (p less than 0.001) in plasma concentrations of 6-keto-prostaglandin F1 (6-K-PGF1). No apparent change was found in prostaglandin E2, thromboxane B2, and histamine concentrations. The concentration of 6-K-PGF1 was correlated with diastolic blood pressure (r = -0.52, p less than 0.005) and HR (r = 0.65, p less than 0.001). Cutaneous hyperemia was observed in 58% of the patients. In an additional six patients, who had taken aspirin daily before AAA surgery, no significant changes were observed in the hemodynamic measurements or 6-K-PGF1 concentrations. These data suggest that mesenteric traction syndrome may be mediated at least in part by a selective release of prostacyclin.

Dibutyryl cyclic AMP inhibits acute hypoxic pulmonary vasoconstriction in conscious sheep

We examined the effects of cell-permeable dibutyryl cyclic AMP (DBcAMP) on acute hypoxic pulmonary vasoconstriction (HPV) in conscious sheep. Mean left and right atrial, pulmonary, and systemic pressures (Pla, Pra, Ppa, and Psa, mm Hg), cardiac output (CO, L/min), and heart rate were measured continuously. Systemic (SVR) and pulmonary vascular resistances (PVR) were calculated by (Psa-Pra)/CO and (Ppa-Pla)/CO, respectively. Five groups of experiments were performed using the same sheep (n = 6). After a 30-min baseline period, sheep inhaled a hypoxic gas mixture (O2:N2 = 1:9) for 40 min. Pretreatment with DBcAMP (200 micrograms/kg/min) inhibited HPV (Ppa, 12.0 +/- 2.3 to 20.0 +/- 2.3 versus 13.2 +/- 2.5 to 14.3 +/- 1.4 mm Hg, p less than 0.01; PVR, 2.61 +/- 0.81 to 4.15 +/- 1.14 versus 2.30 +/- 0.87 to 2.52 +/- 0.59 mm Hg/L/min, p less than 0.01). DBcAMP treatment (200 micrograms/kg/min) after induction of HPV also significantly attenuated hypoxic pulmonary response (Ppa, 19.0 +/- 1.7 to 14.2 +/- 2.3 mm Hg, p less than 0.01; PVR, 3.92 +/- 0.39 to 2.34 +/- 0.34 mm Hg/L/min, p less than 0.01) without significant decreases in Psa and SVR. Pretreatment with DBcAMP (200 micrograms/kg/min) did not significantly alter pulmonary pressor responses to bolus injections of prostaglandin F2 alpha (PGF2 alpha) (10 micrograms/kg) and norepinephrine (4 micrograms/kg). These results may suggest that intracellular augmentation of cyclic AMP plays a crucial role in modulating HPV.

Pressor responses to arachidonic acid in pump-perfused sheep lungs

The reported actions of arachidonic acid in the adult pulmonary circulation are controversial. Some authors reported that arachidonic acid causes only pulmonary vasoconstriction; others have found decreases in pulmonary vascular resistance with low-dose infusions. We have previously reported that arachidonic acid causes only pulmonary vasoconstriction in perinatal lambs during both normoxia and hypoxia. The effects of arachidonic acid on pulmonary vascular resistance were determined in adult sheep using an in situ pump-perfused left lower lung preparation. Arachidonic acid infusions (10.5-31.9 micrograms/kg . min) resulted in pulmonary vasoconstriction. The pulmonary vascular response to arachidonic acid was not altered by hypoxia or by infusion of PGF2 alpha. However, the pulmonary pressor response to hypoxia was increased by concomitant arachidonate infusions. Infusions of arachidonic acid during hypoxia resulted in systemic hypotension. Thus, pulmonary arachidonate metabolism appears to be unaffected by hypoxia or PGF2 alpha; however, hypoxia may enhance the formation of dilator PG's from the lung.

Effect of hypoxia on prostacyclin production in cultured pulmonary artery endothelium

Exposure of cultured bovine pulmonary artery endothelial cells to varying levels of hypoxia (10% or 0% O2) for 4 hours resulted in a significant dose-dependent inhibition in endothelial prostacyclin synthesis (51% and 98%, at the 10% and 0% O2 levels respectively, p less than 0.05, compared to 21% O2 exposure values). Release of 3H-arachidonic acid from cellular pools was not altered by hypoxia. Some of the cells were incubated with arachidonic acid (20 microM for 5 min) or PGH2 (4 microM for 2 min) immediately after exposure. Endothelium exposed to 0% O2, but not to 10% O2, produced significantly less prostacyclin after addition of either arachidonic acid (25 +/- 5% of 21% O2 exposure values, n = 6, p less than 0.01) or PGH2 (31 +/- 3% of 21% O2 exposure values, n = 6, p less than 0.05). These results suggest that hypoxia inhibits cyclooxygenase at the 10% O2 level and both cyclooxygenase and prostacyclin synthetase enzymes at the 0% O2 exposure levels. Exposure of aortic endothelial cells resulted in a 44% inhibition of prostacyclin at the 0% exposure level. No significant alteration in prostacyclin production was found in pulmonary vascular smooth muscle cells exposed to hypoxia. These data suggest that the increased prostacyclin production reported in lungs exposed to hypoxia is not due to a direct effect of hypoxia on the main prostacyclin producing cells of the pulmonary circulation.

Ozone inhibits prostacyclin synthesis in pulmonary endothelium

The effects of ozone on lung arachidonate metabolism in-vitro were studied in cultured bovine pulmonary endothelial cells exposed for 2 hours to ozone in concentrations up to 1.0 ppm. A concentration-dependent decrease in prostacyclin synthesis was found (90% decrease at the highest ozone level of 1.0 ppm). The inhibition of prostacyclin synthesis was not due to a decreased release of arachidonic acid from membrane lipids. We also examined the hypoxic pulmonary vasoconstrictive response to 10% oxygen inhalation in anesthetized dogs in-vivo after exposure to 1.0 ppm ozone for 1 hour. Pulmonary vascular resistance was significantly increased after ozone exposure, similar to the findings in dogs given indomethacin (15 mg/kg). The percentage change in the hypoxic pulmonary pressor response was similar between the ozone exposure and indomethacin-treated groups, although due to the variance of the pulmonary vascular resistance values during hypoxia the results did not reach statistical significance. These results suggest that ozone inhalation affects pulmonary endothelial arachidonate metabolism in-vivo as well as in-vitro.

Effects of high frequency oscillatory ventilation compared to conventional ventilation upon pulmonary vascular prostanoid production in neonatal piglets

In order to investigate the possibility that high-frequency oscillatory ventilation (HFO) might preferentially stimulate intrapulmonary prostacyclin (PGI2) synthesis thereby decreasing pulmonary vascular smooth muscle tone, we determined pulmonary prostacyclin and thromboxane production in neonatal piglets ventilated by conventional means and by HFO (8 Hz). There was no detectable release of prostacyclin or thromboxane into blood passing through the lungs (i.e., pulmonary arterial concentrations were greater than aortic concentrations) during ventilation by conventional means or during HFO. Furthermore, there were no differences between the two modes of ventilation in cardiac output, systemic or pulmonary vascular resistance, or pulmonary vascular response to hypoxia/hypercapnia. We conclude that HFO does not stimulate pulmonary prostacyclin production and does not affect pulmonary vascular resistance or the pulmonary vasoconstriction associated with alveolar hypoxia/hypercapnia when compared to conventional ventilation in anesthetized newborn piglets.

Cardiovascular effects of leukotrienes in neonatal piglets. Role in hypoxic pulmonary vasoconstriction?

We investigated the effects of exogenous leukotriene D4, synthesis inhibitors, and a leukotriene receptor antagonist upon chloralose anesthetized, mechanically ventilated, neonatal piglets with constant left pulmonary blood flow and upon piglets with uncontrolled pulmonary blood flow. Leukotriene D4 (100-10,000 ng, intravenously) caused dose-dependent increases in peak tracheal pressure, pulmonary vascular resistance, and systemic arterial pressure. In a limited number of experiments, cardiovascular responses to exogenous leukotriene C4 were qualitatively similar but quantitatively less than those to leukotriene D4. Neither treatment with diethylcarbamazine or the lipoxygenase inhibitor nordihydroguaiaretic acid, nor with the leukotriene receptor antagonist, FPL55712, altered any baseline cardiovascular parameter measured, suggesting the absence of any influence of leukotrienes on resting hemodynamics. Hypoxia or hypoxia combined with mild hypercapnia caused pulmonary vasoconstriction. Neither treatment with diethylcarbamazine or the lipoxygenase inhibitor nordihydroguaiaretic acid, nor with the leukotriene receptor antagonist FPL55712, altered the pulmonary vasoconstrictor response to hypoxia or combined hypoxia/hypercapnia. We conclude that endogenous leukotrienes do not appear to have an influence on resting cardiovascular function, neither do they appear to be necessary for hypoxic pulmonary vasoconstriction in the neonatal piglet, although exogenous leukotrienes are capable of producing cardiovascular effects.

Kinetic analyses of the effects of hyperoxia and hypoxia on vascular cyclooxygenase activity in vitro

Kinetic analyses were performed to understand the mechanism of hyperoxic induced inhibition of prostacyclin synthesis by human umbilical arteries. Brief exposure of arterial segments to oxygen resulted in over 30% decrease in Vmax of cyclooxygenase in treated vessels. In contrast, cyclooxygenase from hypoxic arterial segments showed approximately a 49% increase in Vmax. There were no significant differences in apparent Km values. These studies suggest that the decreased production of prostacyclin by hyperoxic tissue is due to cyclooxygenase inactivation.