Nitric oxide-dependent pulmonary vasodilation in polycythemic rats

Enhanced brain release of erythropoietin, cytokines and NO during carotid clamping

Although effective and safe, carotid endarterectomy (CEA) implies a reduced blood flow to the brain and likely an ischemia/reperfusion event. The high rate of uneventful outcomes associated with CEA suggests the activation of brain endogenous protection mechanisms aimed at limiting the possible ischemia/reperfusion damage. This study aims at assessing whether CEA triggers protective mechanisms such as brain release of erythropoietin and nitric oxide. CEA was performed in 12 patients; blood samples were withdrawn simultaneously from the surgically exposed ipsilateral jugular and leg veins before, during (2 and 40 min) and after clamp removal (2 min). Plasma antioxidant capacity, carbonylated proteins, erythropoietin, nitrates and nitrites (NOx) were determined. No changes in intraoperative EEG, peripheral and transcranial blood oxygen saturation were detectable, and no patients showed any neurologic sign after the intervention. Antioxidant capacity and protein carbonylation in plasma were unaffected. Differently, erythropoietin, VEGF, TNF-α and NOx increased during clamping in the jugular blood (2 and 40 min), while no changes were observed in the peripheral circulation. These results show that blood erythropoietin, VEGF, TNF-α, and NOx increased in the brain during uncomplicated CEA. This may represent an endogenous self-activated neuroprotective mechanism aimed at the prevention of ischemia/reperfusion damage.

Combination of erythropoietin and sildenafil can effectively attenuate hypoxia-induced pulmonary hypertension in mice

Pulmonary hypertension (PH) is an incurable disease that often leads to right ventricular hypertrophy and right heart failure. This study investigated single versus combined therapy with sildenafil and erythropoietin on hypoxia-induced pulmonary hypertension in mice. Mice were randomized into 5 groups and exposed to either hypoxia (10% oxygen) or normoxia for a total of 5 weeks. Hypoxic mice were treated with saline solution, erythropoietin (500 IU/kg 3 times weekly), sildenafil (10 mg/kg daily), or a combination of the two drugs for the last 2 weeks of hypoxic exposure. We measured right ventricular pressures using right heart catheterization, and the ventilatory response to hypoxia was recorded via whole-body plethysmography. Histological analyses were performed to elucidate changes in pulmonary morphology and appearance of right heart hypertrophy. Plasma levels of cardiotrophin-1 and atrial natriuretic peptide were quantified. Treatment with either erythropoietin or sildenafil alone lowered the hypoxia-induced increase of pulmonary pressure and reduced pulmonary edema formation, pulmonary vascular remodeling, and right ventricular hypertrophy. Notably, the combination of the two drugs had the most prominent effect. Changes in cardiotrophin-1 and atrial natriuretic protein levels confirmed these observations. The combination treatment with erythropoietin and sildenafil demonstrated an attenuation of the development of hypoxia-induced PH in mice that was superior to that observed for either drug when given alone.

Effects of lack of pulsatility on pulmonary endothelial function in the Fontan circulation

OBJECTIVES

Continuous flow in the Fontan circulation results in impairment of pulmonary artery endothelial function, increased pulmonary arterial resistance, and, potentially, late failure of Fontan circulation. We investigated the mechanisms of vascular remodeling and altered vascular reactivity associated with chronic privation of pulsatility on pulmonary vasculature.

METHODS

A total of 30 pigs were evenly distributed in 3 groups: 10 underwent a sham procedure (group I) and 20 underwent a cavopulmonary shunt between the superior vena cava and right pulmonary artery--10 with complete ligation of the proximal right pulmonary artery (group II, nonpulsatile) and 10 with partial ligation (group III, micropulsatile). At 3 months postoperatively, the in vivo hemodynamics, in vitro vasomotricity (concentration response curves on pulmonary artery isolated rings), and endothelial nitric oxide synthase protein level were assessed. A comparison between group and between the right and left lung in each group was performed.

RESULTS

Group II developed right pulmonary hypertension and increased right pulmonary resistance. Endothelial function was altered in group II, as reflected by a decrease in the vasodilation response to acetylcholine and ionophoric calcium but preservation of the nonendothelial-dependent response to sodium nitroprusside. Group III micropulsatility attenuated pulmonary hypertension but did not prevent impairment of the endothelial-dependant relaxation response. Right lung Western blotting revealed decreased endothelial nitric oxide synthase in group II (0.941 ± 0.149 vs sham 1.536 ± 0.222, P = .045) that was preserved in group III (1.275 ± 0.236, P = .39).

CONCLUSIONS

In a chronic model of unilateral cavopulmonary shunt, pulsatility loss resulted in an altered endothelial-dependant vasorelaxation response of the pulmonary arteries. Micropulsatility limited the effects of pulsatility loss. These results are of importance for potential therapies against pulmonary hypertension in the nonpulsatile Fontan circulation, by retaining accessory pulmonary flow or pharmaceutical modulation of nonendothelial-dependant pulmonary vasorelaxation.

Acute hemodynamic effects of erythropoietin do not mediate its cardioprotective properties

Activation of nitric oxide (NO) signaling is considered, at list partially, a mechanistic basis for EPO-induced cardioprotection. Surprisingly, hemodynamic response subsequent to NO activation after EPO administration has never been reported. The objectives of this study were to evaluate the acute hemodynamic and cardiovascular responses to EPO administration, to confirm their NO genesis, and to test the hypothesis that EPO-induced cardioprotection is mediated through cardiovascular changes related to NO activation. In Experiment 1, after 3000 U/kg of rhEPO was administered intravenously to Wistar rats, arterial blood pressure, monitored via indwelling catheter, progressively declined almost immediately until it leveled off 90 minutes after injection at 20% below control level. In Experiment 2 the 25% reduction of mean blood pressure, compared to control group, was observed 2 hours after intravenous injection of either 3000 or 150 U/kg of rhEPO. Detailed pressure–volume loop analyses of cardiac performance (Experiment 3) 2 hours after intravenous injection of human or rat recombinant EPO (3000 U/kg) revealed a significant reduction of systolic function (PRSW was 33% less than control). Reduction of arterial blood pressure and systolic cardiac function in response to rhEPO were blocked in rats pretreated with a non-selective inhibitor of nitric oxide synthase (L-NAME). In Experiment 4, 24 hours after a permanent ligation of a coronary artery, myocardial infarction (MI) measured 26±3.5% of left ventricle in untreated rats. MI in rats treated with 3000 U/kg of rhEPO immediately after coronary ligation was 56% smaller. Pretreatment with L-NAME did not attenuate the beneficial effect of rhEPO on MI size, while MI size in rats treated with L-NAME alone did not differ from control. Therefore, a single injection of rhEPO resulted in a significant, NO-mediated reduction of systemic blood pressure and corresponding reduction of cardiac systolic function. However, EPO-induced protection of myocardium from ischemic damage is not associated with NO activation or NO-mediated hemodynamic responses.

Erythropoietin and hypoxia increase erythropoietin receptor and nitric oxide levels in lung microvascular endothelial cells

Acute lung exposure to low oxygen results in pulmonary vasoconstriction and redistribution of blood flow. We used human microvascular endothelial cells from lung (HMVEC-L) to study the acute response to oxygen stress. We observed that hypoxia and erythropoietin (EPO) increased erythropoietin receptor (EPOR) gene expression and protein level in HMVEC-L. In addition, EPO dose- and time-dependently stimulated nitric oxide (NO) production. This NO stimulation was evident despite hypoxia induced reduction of endothelial NO synthase (eNOS) gene expression. Western blot of phospho-eNOS (serine1177) and eNOS and was significantly induced by hypoxia but not after EPO treatment. However, iNOS increased at hypoxia and with EPO stimulation compared to normal oxygen tension. In accordance with our previous results of NO induction by EPO at low oxygen tension in human umbilical vein endothelial cells and bone marrow endothelial cells, these results provide further evidence in HMVEC-L for EPO regulation of NO production to modify the effects of hypoxia and cause compensatory vasoconstriction.

Reactive oxygen species and RhoA signaling in vascular smooth muscle: role in chronic hypoxia-induced pulmonary hypertension

Increases in myofilament Ca2+ sensitivity resulting from stimulation of RhoA and Rho kinase represent a primary mechanism of vasoconstriction and associated pulmonary hypertension resulting from chronic hypoxia (CH). This chapter summarizes recent advances in the understanding of RhoA/Rho kinase signaling mechanisms in pulmonary vascular smooth muscle (VSM) that increase the sensitivity of the contractile apparatus to Ca2+ and contribute to vasoconstriction in this setting. Such advances include the discovery of myogenic tone in small pulmonary arteries from CH rats that contributes to vasoconstriction through a mechanism inherent to the VSM, dependent on Rho kinase-induced Ca2+ sensitization but independent of L-type voltage-gated Ca2+ channels. Additional studies have revealed an important contribution of superoxide anion (O2-)-induced RhoA activation to both receptor-mediated and membrane depolarization-induced myofilament Ca2+ sensitization in hypertensive pulmonary arteries. Xanthine oxidase and NADPH oxidase isoforms are potential sources of O2- that mediate RhoA-dependent vasoconstriction and associated pulmonary hypertension.

Chronic hypoxia augments depolarization-induced Ca2+ sensitization in pulmonary vascular smooth muscle through superoxide-dependent stimulation of RhoA

Rho kinase (ROCK)-dependent vasoconstriction has been implicated as a major factor in chronic hypoxia (CH)-induced pulmonary hypertension. This component of pulmonary hypertension is associated with arterial myogenicity and increased vasoreactivity to receptor-mediated agonists and depolarizing stimuli resulting from ROCK-dependent myofilament Ca(2+) sensitization. On the basis of separate lines of evidence that CH increases pulmonary arterial superoxide (O(2)(-)) generation and that O(2)(-) stimulates RhoA/ROCK signaling in vascular smooth muscle (VSM), we hypothesized that depolarization-induced O(2)(-) generation mediates enhanced RhoA-dependent Ca(2+) sensitization in pulmonary VSM following CH. To test this hypothesis, we determined effects of the ROCK inhibitor HA-1077 and the O(2)(-)-specific spin trap tiron on vasoconstrictor reactivity to depolarizing concentrations of KCl in isolated lungs and Ca(2+)-permeabilized, pressurized small pulmonary arteries from control and CH (4 wk at 0.5 atm) rats. Using the same vessel preparation, we examined effects of CH on KCl-dependent VSM membrane depolarization and O(2)(-) generation using sharp electrodes and the fluorescent indicator dihydroethidium, respectively. Finally, using a RhoA-GTP pull-down assay, we investigated the contribution of O(2)(-) to depolarization-induced RhoA activation. We found that CH augmented KCl-dependent vasoconstriction through a Ca(2+) sensitization mechanism that was inhibited by HA-1077 and tiron. Furthermore, CH caused VSM membrane depolarization that persisted with increasing concentrations of KCl, enhanced KCl-induced O(2)(-) generation, and augmented depolarization-dependent RhoA activation in a O(2)(-)-dependent manner. These findings reveal a novel mechanistic link between VSM membrane depolarization, O(2)(-) generation, and RhoA activation that mediates enhanced myofilament Ca(2+) sensitization and pulmonary vasoconstriction following CH.

Erythropoietin: when liability becomes asset in neurovascular repair

Erythropoietin (Epo) leads to the proliferation and differentiation of erythroid precursors, but is also involved in diverse nonhematopoietic biological functions. In this issue of the JCI, Chen, Smith, and colleagues demonstrate that the temporal expression of Epo is critical for determining whether physiological or pathological repair occurs following neurovascular retinal injury in the oxygen-induced retinopathy neonatal mouse model (see the related article beginning on page 526). The pleiotrophic properties of Epo make it a likely novel therapy for treatment of neurovascular damage, but the timing of its use must be carefully considered to prevent untoward effects.

Intermittent hypoxia modulates nitric oxide-dependent vasodilation and capillary perfusion during ischemia-reperfusion-induced damage

The microvascular function of nitric oxide (NO) during ischemia-reperfusion (I/R) in intermittent hypoxia (IH)-pretreated hamsters was analyzed using 20 mg/kg of the nonselective NO inhibitor N(omega)-nitro-l-arginine methyl ester (l-NAME) and 5 mg/kg of the preferential inducible NO inhibitor S-methylisothiourea sulphate (SMT) injected before I/R. Studies were made in the hamster cheek pouch microcirculation (intravital fluorescence microscopy). IH consisted of 6 min of 8% O(2) breathing followed by 6 min of 21% O(2) for every 8 h for 21 days. Normoxia controls (NCs) were exposed to room air for the same period. The effects were characterized in terms of systemic hemodynamics, diameter, flow, wall shear stress in arterioles, capillary perfusion, and the concentrations of thiobarbituric acid-reactive substances (TBARS) and plasma NO, assessed as nitrite/nitrate (NOx) levels. IH did not change arterial blood pressure and increased hematocrit and shear stress. IH increased NOx and TBARS levels and reduced arterial diameter, blood flow, and capillary perfusion versus the NC. Conversely, TBARS and NOx were lower during I/R in IH-pretreated hamsters, resulting in vasodilation and the increase of capillary perfusion and shear stress. After IH, capillary perfusion was reduced by 24% (2.3%) and enhanced by 115% (1.7%) after I/R (P < 0.05). Both modalities of NO blockade decreased NOx generation and increased TBARS versus IH. l-NAME and SMT induced a significant decrease in arteriolar diameter, blood flow, and capillary perfusion (P < 0.05). l-NAME enhanced TBARS more than SMT and aggravated I/R damage. In conclusion, we demonstrated that preconditioning with IH greatly reduces oxidative stress and stimulates NO-induced vasodilation during I/R injury, thus maintaining capillary perfusion.

Differential effects of chronic hypoxia and intermittent hypocapnic and eucapnic hypoxia on pulmonary vasoreactivity

Intermittent hypoxia (IH) resulting from sleep apnea can lead to pulmonary hypertension (PH) and right heart failure, similar to chronic sustained hypoxia (CH). Supplemental CO(2), however, attenuates hypoxic PH. We therefore hypothesized that, similar to CH, IH elicits PH and associated increases in arterial endothelial nitric oxide synthase (eNOS) expression, ionomycin-dependent vasodilation, and receptor-mediated pulmonary vasoconstriction. We further hypothesized that supplemental CO(2) inhibits these responses to IH. To test these hypotheses, we measured eNOS expression by Western blot in intrapulmonary arteries from CH (2 wk, 0.5 atm), hypocapnic IH (H-IH) (3 min cycles of 5% O(2)/air flush, 7 h/day, 2 wk), and eucapnic IH (E-IH) (3 min cycles of 5% O(2), 5% CO(2)/air flush, 7 h/day, 2 wk) rats and their respective controls. Furthermore, vasodilatory responses to the calcium ionophore ionomycin and vasoconstrictor responses to the thromboxane mimetic U-46619 were measured in isolated saline-perfused lungs from each group. Hematocrit, arterial wall thickness, and right ventricle-to-total ventricle weight ratios were additionally assessed as indexes of polycythemia, arterial remodeling, and PH, respectively. Consistent with our hypotheses, E-IH resulted in attenuated polycythemia, arterial remodeling, RV hypertrophy, and eNOS upregulation compared with H-IH. However, in contrast to CH, neither H-IH nor E-IH increased ionomycin-dependent vasodilation. Furthermore, H-IH and E-IH similarly augmented U-46619-induced pulmonary vasoconstriction but to a lesser degree than CH. We conclude that maintenance of eucapnia decreases IH-induced PH and upregulation of arterial eNOS. In contrast, increases in pulmonary vasoconstrictor reactivity following H-IH are unaltered by exposure to supplemental CO(2).

α1-Adrenoceptor-dependent vascular hypertrophy and remodeling in murine hypoxic pulmonary hypertension

Excessive proliferation of vascular wall cells underlies the development of elevated vascular resistance in hypoxic pulmonary hypertension (PH), but the responsible mechanisms remain unclear. Growth-promoting effects of catecholamines may contribute. Hypoxemia causes sympathoexcitation, and prolonged stimulation of α1-adrenoceptors (α1-ARs) induces hypertrophy and hyperplasia of arterial smooth muscle cells and adventitial fibroblasts. Catecholamine trophic actions in arteries are enhanced when other conditions favoring growth or remodeling are present, e.g., injury or altered shear stress, in isolated pulmonary arteries from rats with hypoxic PH. The present study examined the hypothesis that catecholamines contribute to pulmonary vascular remodeling in vivo in hypoxic PH. Mice genetically deficient in norepinephrine and epinephrine production [dopamine β-hydroxylase−/− (DBH−/−)] or α1-ARs were examined for alterations in PH, cardiac hypertrophy, and vascular remodeling after 21 days exposure to normobaric 0.1 inspired oxygen fraction (FiO2). A decrease in the lumen area and an increase in the wall thickness of arteries were strongly inhibited in knockout mice (order of extent of inhibition: DBH−/− = α1D-AR−/− > α1B-AR−/−). Distal muscularization of small arterioles was also reduced (DBH−/− > α1D-AR−/− > α1B-AR−/− mice). Despite these reductions, increases in right ventricular pressure and hypertrophy were not attenuated in DBH−/− and α1B-AR−/− mice. However, hematocrit increased more in these mice, possibly as a consequence of impaired cardiovascular activation that occurs during reduction of FiO2. In contrast, in α1D-AR−/− mice, where hematocrit increased the same as in wild-type mice, right ventricular pressure was reduced. These data suggest that catecholamine stimulation of α1B- and α1D-ARs contributes significantly to vascular remodeling in hypoxic PH.

Divergent contractile and structural responses of the murine PKC-epsilon null pulmonary circulation to chronic hypoxia

Loss of PKC-epsilon limits the magnitude of acute hypoxic pulmonary vasoconstriction (HPV) in the mouse. Therefore, we hypothesized that loss of PKC-epsilon would decrease the contractile and/or structural response of the murine pulmonary circulation to chronic hypoxia (Hx). However, the pattern of lung vascular responses to chronic Hx may or may not be predicted by the acute HPV response. Adult PKC-epsilon wild-type (PKC-epsilon(+/+)), heterozygous null, and homozygous null (PKC-epsilon(-/-)) mice were exposed to normoxia or Hx for 5 wk. PKC-epsilon(-/-) mice actually had a greater increase in right ventricular (RV) systolic pressure, RV mass, and hematocrit in response to chronic Hx than PKC-epsilon(+/+) mice. In contrast to the augmented PA pressure and RV hypertrophy, pulmonary vascular remodeling was increased less than expected (i.e., equal to PKC-epsilon(+/+) mice) in both the proximal and distal PKC-epsilon(-/-) pulmonary vasculature. The contribution of increased vascular tone to this pulmonary hypertension (PHTN) was assessed by measuring the acute vasodilator response to nitric oxide (NO). Acute inhalation of NO reversed the increased PA pressure in hypoxic PKC-epsilon(-/-) mice, implying that the exaggerated PHTN may be due to a relative deficiency in nitric oxide synthase (NOS). Despite the higher PA pressure, chronic Hx stimulated less of an increase in lung endothelial (e) and inducible (i) NOS expression in PKC-epsilon(-/-) than PKC-epsilon(+/+) mice. In contrast, expression of nNOS in PKC-epsilon(+/+) mice decreased in response to chronic Hx, while lung levels in PKC-epsilon(-/-) mice remained unchanged. In summary, loss of PKC-epsilon results in increased vascular tone, but not pulmonary vascular remodeling in response to chronic Hx. Blunting of Hx-induced eNOS and iNOS expression may contribute to the increased vascular tone. PKC-epsilon appears to be an important signaling intermediate in the hypoxic regulation of each NOS isoform.

Erythropoietin mimics ischemic preconditioning

Ischemic preconditioning is a powerful endogenous phenomenon in which brief periods of a sub-toxic ischemic insult induce robust protection against future, lengthy, lethal ischemia. The cardioprotective effects of ischemic preconditioning are manifest in all species studied so far, including humans. The ability to reproduce the cardioprotective effects of ischemic preconditioning with pharmacological agents raises the possibility that a drug may ultimately be introduced into clinical practice to treat human hearts undergoing ischemia/reperfusion. This chapter focuses on erythropoietin (Epo), a drug that has already been approved for humans and is in current use for the treatment of anemia associated with chronic renal failure, HIV infection, cancer patients on chemotherapy, and to reduce allogenic blood transfusion in surgery patients. Several recent studies have suggested that this cytokine possesses properties far beyond its capacity to produce red blood cells such as the ability to protect tissues including brain, kidney and heart against injury caused by ischemia/reperfusion. Cardioprotection conferred by Epo has been shown to be equal in magnitude to that conferred by ischemic preconditioning. However, the underlying mechanisms by which Epo protects the heart against injury caused by ischemia remain unknown.

Attenuation of acute hypoxic pulmonary vasoconstriction and hypoxic pulmonary hypertension in mice by inhibition of Rho-kinase

RhoA GTPase mediates a variety of cellular responses, including activation of the contractile apparatus, growth, and gene expression. Acute hypoxia activates RhoA and, in turn, its downstream effector, Rho-kinase, and previous studies in rats have suggested a role for Rho/Rho-kinase signaling in both acute and chronically hypoxic pulmonary vasoconstriction. We therefore hypothesized that activation of Rho/Rho-kinase in the pulmonary circulation of mice contributes to acute hypoxic pulmonary vasoconstriction and chronic hypoxia-induced pulmonary hypertension and vascular remodeling. In isolated, salt solution-perfused mouse lungs, acute administration of the Rho-kinase inhibitor Y-27632 (1 × 10−5 M) attenuated hypoxic vasoconstriction as well as that due to angiotensin II and KCl. Chronic treatment with Y-27632 (30 mg·kg−1·day−1) via subcutaneous osmotic pump decreased right ventricular systolic pressure, right ventricular hypertrophy, and neomuscularization of the distal pulmonary vasculature in mice exposed to hypobaric hypoxia for 14 days. Analysis of a small number of proximal pulmonary arteries suggested that Y-27632 treatment reduced the level of phospho-CPI-17, a Rho-kinase target, in hypoxic lungs. We also found that endothelial nitric oxide synthase protein in hypoxic lungs was augmented by Y-27632, suggesting that enhanced nitric oxide production might have played a role in the Y-27632-induced attenuation of chronically hypoxic pulmonary hypertension. In conclusion, Rho/Rho-kinase activation is important in the effects of both acute and chronic hypoxia on the pulmonary circulation of mice, possibly by contributing to both vasoconstriction and vascular remodeling.

Pulmonary Hypertension in Transgenic Mice Expressing a Dominant-Negative BMPRII Gene in Smooth Muscle

Bone morphogenetic peptides (BMPs), a family of cytokines critical to normal development, were recently implicated in the pathogenesis of familial pulmonary arterial hypertension. The type-II receptor (BMPRII) is required for recognition of all BMPs, and targeted deletion of BMPRII in mice results in fetal lethality before gastrulation. To overcome this limitation and study the role of BMP signaling in postnatal vascular disease, we constructed a smooth muscle–specific transgenic mouse expressing a dominant-negative BMPRII under control of the tetracycline gene switch (SM22-tet-BMPRII delx4+ mice). When the mutation was activated after birth, mice developed increased pulmonary artery pressure, RV/LV+S ratio, and pulmonary arterial muscularization with no increase in systemic arterial pressure. Studies with SM22-tet-BMPRII delx4+ mice support the hypothesis that loss of BMPRII signaling in smooth muscle is sufficient to produce the pulmonary hypertensive phenotype.

Altered Pulmonary Vascular Reactivity in Mice with Excessive Erythrocytosis

Pulmonary vascular remodeling during chronic hypoxia may be the result of either oxygen deprivation or erythrocytosis. To separate experimentally the effects of hypoxia and erythrocytosis, we analyzed transgenic mice that constitutively overexpress the human erythropoietin gene in an oxygen-independent manner. These mice are characterized by polycythemia but have normal blood pressure, heart rate, and cardiac output. In transgenic mice, pulmonary artery pressure (PAP) was increased in vivo but was reduced in blood-free perfused lungs. The thromboxane receptor agonist U46619 caused a smaller rise in PAP in isolated transgenic lungs than in lungs from wild-type mice. The transgenic pulmonary vasculature was characterized by elevated prostacyclin production, stronger endothelial nitric oxide synthase expression, and reduced pulmonary vascular smooth muscle thickness. The fact that transgenic polycythemic mice have marked pulmonary hypertension in vivo but not in vitro suggests that their pulmonary hypertension is due to the increased blood viscosity, thus supporting an independent role of polycythemia in the development of pulmonary hypertension. In addition, our findings indicate that the lungs of transgenic animals adapt to the high PAP by elevated synthesis of vasodilators and reduced vascular smooth muscle thickness that tend to reduce vascular tone and vascular responsiveness.

Catheterization of pulmonary artery in rats with an ultraminiature catheter pressure transducer

Utilizing new materials and miniaturization techniques, an ultraminiature catheter pressure transducer for catheterization of the pulmonary artery (PA) has been developed and applied in intact, spontaneously breathing, anesthetized rats. The catheter arrangement consists of three components: 1) an SPR-671 ultraminiature pressure transducer (measuring catheter), 2) a plastic introducer (sheath) that is slipped over the measuring catheter, and 3) an external wire mounted on the outside of the introducer for bending its tip. The measuring catheter is first inserted through the right jugular vein into the right ventricle. The introducer is then slipped over it. The tip of the introducer is bent so that there is an angle of approximately 90 degrees or less to the shaft. The measuring catheter is advanced across the pulmonary valve into the PA. Measurements of pulmonary arterial pressure were made in five male Long Evans (364 +/- 7 g body wt) and five female Sprague-Dawley (244 +/- 7 g body wt) rats under control conditions. The effects of infusion of norepinephrine (0.1 mg.kg(-1).h(-1) iv for 20-min duration) were tested in Long Evans rats. Pulmonary arterial systolic pressure measurements were 34.0 +/- 0.8 and 29.5 +/- 0.4 mmHg, and diastolic pressure values were 23.6 +/- 0.8 and 18.1 +/- 0.6 mmHg in male Long Evans and female Sprague-Dawley rats, respectively. Norepinephrine induced an increase in pulmonary arterial systolic (40.8 +/- 0.1 mmHg) and diastolic (28.6 +/- 0.4 mmHg) pressures and an elevation in pulmonary vascular resistance from a control value of 0.093 +/- 0.003 to 0.103 +/- 0.004 mmHg.kg.min.ml(-1).

Erythropoietin Administration In Vivo Increases Vascular Nitric Oxide Synthase Expression

This study was designed to determine whether recombinant human erythropoietin (rHuEpo) administration increases vascular nitric oxide (NO) production in healthy rats. We hypothesized that rHuEpo hypertension is associated with increased endothelial expression of nitric oxide synthase and augmented NO-dependent vasodilation. Male rats were instrumented with pulsed Doppler flow probes around their ascending aorta and with arterial and femoral catheters. Rats were treated for 14 days with rHuEpo (2 U/d) or vehicle. rHuEpo elevated hematocrit and increased mean arterial pressure (142 +/- 3 versus 116 +/- 4 mm Hg). Thoracic aorta segments from rHuEpo rats had a modest increase in NO-dependent relaxation assessed by acetylcholine (10(-10) to 10(-5) mol/L) relaxation of phenylephrine (PE) (10(-6) mol/L) contracted arteries. Relaxation to NO-donor, s-nitrosyl acetylpenicillamine, and PE contraction were not different from control arteries. The NO synthase inhibitor, N-omega-nitro-L-arginine, increased blood pressure and total peripheral resistance more in rHuEpo rats at both 10 and 30 mg/kg. NOS expression in rHuEpo aorta and plasma NOx concentrations were increased compared with control. Thus, it appears that vascular eNOS expression is increased and causes basal vasodilation in rHuEpo hypertensive rats.

17Beta-estradiol decreases hypoxic induction of erythropoietin gene expression

Exposure to chronic hypoxia induces erythropoietin (EPO) production to facilitate oxygen delivery to hypoxic tissues. Previous studies from our laboratory found that ovariectomy (OVX) exacerbates the polycythemic response to hypoxia and treatment with 17beta-estradiol (E2-beta) inhibits this effect. We hypothesized that E2-beta decreases EPO gene expression during hypoxia. Because E2-beta can induce nitric oxide (NO) production and NO can attenuate EPO synthesis, we further hypothesized that E2-beta inhibition of EPO gene expression is mediated by NO. These hypotheses were tested in OVX catheterized rats treated with E2-beta (20 microg/day) or vehicle for 14 days and exposed to 8 or 12 h of hypoxia (12% O(2)) or normoxia. We found that E2-beta treatment significantly decreased EPO synthesis and gene expression during hypoxia. E2-beta treatment did not induce endothelial NO synthase (eNOS) expression in the kidney but potentiated hypoxia-induced increases in plasma nitrates. We conclude that E2-beta decreases hypoxic induction of EPO. However, this effect does not appear to be related to changes in renal eNOS expression.

Developmental differences in pulmonary eNOS expression in response to chronic hypoxia in the rat

Chronic hypoxia (CH) increases pulmonary endothelial nitric oxide synthase (eNOS) protein levels in adult rats but decreases eNOS protein levels in neonatal pigs. We hypothesized that this differing response to CH is due to developmental rather than species differences. Adult and neonatal rats were placed in either hypobaric hypoxia or normoxia for 2 wk. At that time, body weight, hematocrit, plasma nitrite/nitrate (NOx(-)), and right ventricular and total ventricular heart weights were measured. Percent pulmonary arterial wall area of 20-50 and 51-100 microm arteries were also determined. Total lung protein extracts were assayed for eNOS levels by using immunoblot analysis. Compared with their respective normoxic controls, both adult and neonatal hypoxic groups demonstrated significantly decreased body weight, elevated hematocrit, and elevated right ventricular-to-total ventricular weight ratios. Both adult and neonatal hypoxic groups also demonstrated significantly larger percent pulmonary arterial wall area compared with their respective normoxic controls. Hypoxic adult pulmonary eNOS protein and plasma NOx(-) were significantly greater than levels found in normoxic adults. In contrast, hypoxic neonatal pulmonary eNOS protein and plasma NOx(-) were significantly less compared with normoxic neonates. We conclude that there is a developmental difference in eNOS expression and nitric oxide production in response to CH.

Effects of hematocrit changes on flow-mediated and metabolic vasodilation in humans

Endothelial function is noninvasively assessed by measuring nitric oxide-dependent increase in radial artery diameter accompanying the elevation in shear stress induced by increasing blood flow through a short-lasting ischemia of the hand. However, shear stress also depends on blood viscosity, whose changes might thus affect nitric oxide increase in a manner that is not properly reflected by blood flow changes. In 12 subjects with hemochromatosis, we measured ultrasonographically radial artery diameter and blood flow responses to a 4-minute ischemia of the hand. This was done also after removing 500 mL of blood (and concomitantly infusing 500 mL of saline), which significantly (P<0.01) reduced hemoglobin concentration and hematocrit. The increase in blood flow induced by the 4-minute ischemia was similar before and after blood removal (+76% and +80%), which, in contrast, markedly attenuated the accompanying increase in radial artery diameter (+25% versus +13%, P<0.01). Thus, in humans, blood viscosity is involved in the endothelial response to an increase in shear stress. This implies that this response may not be accurately assessed and compared by quantifying the stimulus only through an increase in blood flow.

Hypoxia-induced pulmonary endothelin-1 expression is unaltered by nitric oxide

Nitric oxide (NO) attenuates hypoxia-induced endothelin (ET)-1 expression in cultured umbilical vein endothelial cells. We hypothesized that NO similarly attenuates hypoxia-induced increases in ET-1 expression in the lungs of intact animals and reasoned that potentially reduced ET-1 levels may contribute to the protective effects of NO against the development of pulmonary hypertension during chronic hypoxia. As expected, hypoxic exposure (24 h, 10% O(2)) increased rat lung ET-1 peptide and prepro-ET-1 mRNA levels. Contrary to our hypothesis, inhaled NO (iNO) did not attenuate hypoxia-induced increases in pulmonary ET-1 peptide or prepro-ET-1 mRNA levels. Because of this surprising finding, we also examined the effects of NO on hypoxia-induced increases in ET peptide levels in cultured cell experiments. Consistent with the results of iNO experiments, administration of the NO donor S-nitroso-N-acetyl-penicillamine to cultured bovine pulmonary endothelial cells did not attenuate increases in ET peptide levels resulting from hypoxic (24 h, 3% O(2)) exposure. In additional experiments, we examined the effects of NO on the activity of a cloned ET-1 promoter fragment containing a functional hypoxia inducible factor-1 binding site in reporter gene experiments. Whereas moderate hypoxia (24 h, 3% O(2)) had no effect on ET-1 promoter activity, activity was increased by severe hypoxic (24 h, 0.5% O(2)) exposure. ET-1 promoter activity after S-nitroso-N-acetyl-penicillamine administration during severe hypoxia was greater than that in normoxic controls, although activity was reduced compared with that in hypoxic controls. These findings suggest that hypoxia-induced pulmonary ET-1 expression is unaffected by NO.