Reversal of reflex pulmonary vasoconstriction induced by main pulmonary arterial distension

Distension of the main pulmonary artery (MPA) induces pulmonary hypertension, most probably by neurogenic reflex pulmonary vasoconstriction, although constriction of the pulmonary vessels has not actually been demonstrated. In previous studies in dogs with increased pulmonary vascular resistance produced by airway hypoxia, exogenous arachidonic acid has led to the production of pulmonary vasodilator prostaglandins. Hence, in the present study, we investigated the effect of arachidonic acid in seven intact anesthetized dogs after pulmonary vascular resistance was increased by MPA distention. After steady-state pulmonary hypertension was established, arachidonic acid (1.0 mg/min) was infused into the right ventricle for 16 min; 15-20 min later a 16-mg bolus of arachidonic acid was injected. MPA distension was maintained throughout the study. Although the infusion of arachidonic acid significantly lowered the elevated pulmonary vascular resistance induced by MPA distension, the pulmonary vascular resistance returned to control levels only after the bolus injection of arachidonic acid. Notably, the bolus injection caused a biphasic response which first increased the pulmonary vascular resistance transiently before lowering it to control levels. In dogs with resting levels of pulmonary vascular resistance, administration of arachidonic acid in the same manner did not alter the pulmonary vascular resistance. It is concluded that MPA distension does indeed cause reflex pulmonary vasoconstriction which can be reversed by vasodilator metabolites of arachidonic acid. Even though this reflex may help maintain high pulmonary vascular resistance in the fetus, its function in the adult is obscure.     

Hypoxic pulmonary vasoconstriction does not affect hydrostatic pulmonary edema formation

We studied the effects of regional hypoxic pulmonary vasoconstriction (HPV) on lobar flow diversion in the presence of hydrostatic pulmonary edema. Ten anesthetized dogs with the left lower lobe (LLL) suspended in a net for continuous weighing were ventilated with a bronchial divider so the LLL could be ventilated with either 100% O2 or a hypoxic gas mixture (90% N2-5% CO2-5% O2). A balloon was inflated in the left atrium until hydrostatic pulmonary edema occurred, as evidenced by a continuous increase in LLL weight. Left lower lobe flow (QLLL) was measured by electromagnetic flow meter and cardiac output (QT) by thermal dilution. At a left atrial pressure of 30 +/- 5 mmHg, ventilation of the LLL with the hypoxic gas mixture caused QLLL/QT to decrease from 17 +/- 4 to 11 +/- 3% (P less than 0.05), pulmonary arterial pressure to increase from 35 +/- 5 to 37 +/- 6 mmHg (P less than 0.05), and no significant change in rate of LLL weight gain. Gravimetric confirmation of our results was provided by experiments in four animals where the LLL was ventilated with an hypoxic gas mixture for 2 h while the right lung was ventilated with 100% O2. In these animals there was no difference in bloodless lung water between the LLL and right lower lobe. We conclude that in the presence of left atrial pressures high enough to cause hydrostatic pulmonary edema, HPV causes significant flow diversion from an hypoxic lobe but the decrease in flow does not affect edema formation.     

Pressure-induced smooth muscle cell depolarization in pulmonary arteries from control and chronically hypoxic rats does not cause myogenic vasoconstriction.

Chronic obstructive pulmonary diseases, as well as prolonged residence at high altitude, can result in generalized airway hypoxia, eliciting an increase in pulmonary vascular resistance. We hypothesized that a portion of the elevated pulmonary vascular resistance following chronic hypoxia (CH) is due to the development of myogenic tone. Isolated, pressurized small pulmonary arteries from control (barometric pressure congruent with 630 Torr) and CH (4 wk, barometric pressure = 380 Torr) rats were loaded with fura 2-AM and perfused with warm (37 degrees C), aerated (21% O(2)-6% CO(2)-balance N(2)) physiological saline solution. Vascular smooth muscle (VSM) intracellular Ca(2+) concentration ([Ca(2+)](i)) and diameter responses to increasing intraluminal pressure were determined. Diameter and VSM cell [Ca(2+)](i) responses to KCl were also determined. In a separate set of experiments, VSM cell membrane potential responses to increasing luminal pressure were determined in arteries from control and CH rats. VSM cell membrane potential in arteries from CH animals was depolarized relative to control at each pressure step. VSM cells from both groups exhibited a further depolarization in response to step increases in intraluminal pressure. However, arteries from both control and CH rats distended passively to increasing intraluminal pressure, and VSM cell [Ca(2+)](i) was not affected. KCl elicited a dose-dependent vasoconstriction that was nearly identical between control and CH groups. Whereas KCl administration resulted in a dose-dependent increase in VSM cell [Ca(2+)](i) in arteries taken from control animals, this stimulus elicited only a slight increase in VSM cell [Ca(2+)](i) in arteries from CH animals. We conclude that the pulmonary circulation of the rat does not demonstrate pressure-induced vasoconstriction.     

Endothelin-3-dependent pulmonary vasoconstriction in monocrotaline-induced pulmonary arterial hypertension

Blockade of the endothelin (ET) system is beneficial in pulmonary arterial hypertension (PAH). The contribution of ET-3 and its interactions with ET receptors have never been evaluated in the monocrotaline (MCT)-induced model of PAH. Vasoreactivity of pulmonary arteries was investigated; ET-3 localization was determined by confocal imaging and gene expression of prepro-ET-3 quantified using RT-PCR. ET-3 plasma levels tended to increase in PAH. ET-3 localized in the media of pulmonary arteries, where gene expression of prepro-ET-3 was reduced in PAH. ET-3 induced similar pulmonary vasoconstrictions in sham and PAH rats. In sham rats, the ET(A) antagonist A-147627 (10nmol/l) significantly reduced the maximal response to ET-3 (E(max) 77+/-1 to 46+/-2%, mean+/-S.E.M., P<0.001), while the ET(B) antagonist A-192621 (1mumol/l) reduced the sensitivity (EC(50) 21+/-7 to 59+/-16nmol/l, P<0.05) without affecting E(max). The combination of both antagonists completely abolished ET-3-induced pulmonary vasoconstriction. In PAH, the ET(A) antagonist further reduced the maximal response to ET-3 and shifted the EC(50) (E(max) 23+/-2%, P<0.001, EC(50) 104+/-24nmol/l, P<0.05), while the ET(B) antagonist only shifted the EC(50) (123+/-36nmol/l, P<0.05) without affecting the E(max). In PAH, dual ET receptor inhibition did not further reduce constriction compared to selective ET(A) inhibition. ET-3 significantly contributes to pulmonary vasoconstriction by activating the ET(B) at low concentration, and the ET(A) at high concentration. The increased inhibitory effect of the ET(A) antagonist in PAH suggests that the contribution of ET(B) to ET-3-induced vasoconstriction is reduced. Although ET-3 is a potent pulmonary vasoconstrictor in PAH, its potential pathophysiologic contribution remains uncertain.     

Inhaled carbon monoxide does not cause pulmonary vasodilation in the late-gestation fetal lamb

As observed with nitric oxide (NO), carbon monoxide (CO) binds and may activate soluble guanylate cyclase and increase cGMP levels in smooth muscle cells in vitro. Because inhaled NO (I(NO)) causes potent and sustained pulmonary vasodilation, we hypothesized that inhaled CO (I(CO)) may have similar effects on the perinatal lung. To determine whether I(CO) can lower pulmonary vascular resistance (PVR) during the perinatal period, we studied the effects of I(CO) on late-gestation fetal lambs. Catheters were placed in the main pulmonary artery, left pulmonary artery (LPA), aorta, and left atrium to measure pressure. An ultrasonic flow transducer was placed on the LPA to measure blood flow to the left lung. After baseline measurements, fetal lambs were mechanically ventilated with a hypoxic gas mixture (inspired O(2) fraction < 0.10) to maintain a constant fetal arterial PO(2). After 60 min (baseline), the lambs were treated with I(CO) [5-2,500 parts/million (ppm)]. Comparisons were made with I(NO) (5 and 20 ppm) and combined I(NO) (5 ppm) and I(CO) (100 and 2,500 ppm). We found that I(CO) did not alter left lung blood flow or PVR at any of the study doses. In contrast, low-dose I(NO) decreased PVR by 47% (P < 0.005). The combination of I(NO) and I(CO) did not enhance the vasodilator response to I(NO). To determine whether endogenous CO contributes to vascular tone in the fetal lung, zinc protoporphyrin IX, an inhibitor of heme oxygenase, was infused into the LPA in three lambs. Zinc protoporphyrin IX had no effect on baseline PVR, aortic pressure, or the pressure gradient across the ductus arteriosus. We conclude that I(CO) does not cause vasodilation in the near-term ovine transitional circulation, and endogenous CO does not contribute significantly to baseline pulmonary vascular tone or ductus arteriosus tone in the late-gestation ovine fetus.     

Hypoxic pulmonary vasoconstriction is not endothelium dependent.

Feline intrapulmonary arteries (mean diameter, 0.9 mm) were equilibrated in Earle's solution at constant tension in a chamber bubbled with an hyperoxic gas mixture (30% oxygen, 5% carbon dioxide, balance nitrogen). The endothelium was removed from half the vessels by gentle rubbing. The isometric response to the addition of acetylcholine (1*10(-6) M) was dilator in the vessels with endothelium and constrictor in those without endothelium. Intermittent exposure to a hypoxic gas mixture (0% oxygen, 5% carbon dioxide, balance nitrogen) for 20 min with five repetitions demonstrated sustained constrictor responses in the presence or absence of endothelium. Endothelial cells are, therefore, not required for the mediation of hypoxic pulmonary vasoconstriction.     

Effects of spinal anesthesia on response to main pulmonary arterial distension

Nonocclusive main pulmonary arterial distension produces peripheral pulmonary hypertension. The mechanism of this response is unknown. The effects of total spinal anesthesia on the response were studied in halothane-anesthetized dogs. Before total spinal anesthesia, main pulmonary arterial balloon inflation increased pulmonary arterial pressure and resistance without affecting systemic hemodynamic variables. Both right and left pulmonary arterial pressures were monitored to exclude unilateral obstruction with main pulmonary arterial balloon inflation. Total spinal anesthesia decreased cardiac output and systemic arterial pressures. After total spinal anesthesia, main pulmonary arterial distension still increased pulmonary arterial pressure and resistance. Right atrial pacing, discontinuation of halothane anesthesia, and norepinephrine infusion during total spinal anesthesia partially reversed the hemodynamic changes caused by total spinal anesthesia. The percent increase in pulmonary vascular resistance due to main pulmonary arterial distension was similar before total spinal anesthesia and during all experimental conditions during total spinal anesthesia. The pulmonary hypertensive response is therefore not dependent on central synaptic connections.     

Pulmonary vasoconstriction in a canine model of neurogenic pulmonary edema

The intracisternal administration of veratrine to the chloralose-anesthetized dog produces pulmonary hypertension (PH) and neurogenic pulmonary edema (NPE). To determine whether pulmonary vasoconstriction, mediated by a circulating agent, contributes to the PH, the left lower lung lobe (LLL) perfusion of seven splenectomized (to keep hematocrit and blood viscosity constant) dogs was isolated so the LLL could be perfused at constant flow and outflow pressure with blood pumped from the pulmonary artery. The LLL was denervated by removing it from the dog. Veratrine (40-160 micrograms/kg) increased LLL arterial pressure by 39.2% and produced large increases in plasma catecholamine concentrations. The double-occlusion technique indicated that 74% of the increase in the LLL arteriovenous pressure gradient was due to an increase in venous tone. This pattern of vasoconstriction was similar to that previously observed during the infusion of exogenous catecholamines and suggested that catecholamines mediated the LLL response. The more severe degree of PH observed in the intact animal in NPE, however, suggests that passive rather than active changes in pulmonary hemodynamics are predominantly responsible for the development of PH in this disorder.     

Overexpression of human bone morphogenetic protein receptor 2 does not ameliorate monocrotaline pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is associated with mutations of bone morphogenetic protein receptor 2 (BMPR2), and BMPR2 expression decreases with the development of experimental PAH. Decreased BMPR2 expression and impaired intracellular BMP signaling in pulmonary artery (PA) smooth muscle cells (PASMC) suppresses apoptosis and promotes proliferation, thereby contributing to the pathogenesis of PAH. We hypothesized that overexpression of BMPR2 in resistance PAs would ameliorate established monocrotaline PAH. Human BMPR2 was inserted into a serotype 5 adenovirus with a green fluorescent protein (GFP) reporter. Dose-dependent transgene expression was confirmed in PASMC using fluorescence microscopy, quantitative RT-PCR, and immunoblots. PAH was induced by injecting Sprague-Dawley rats with monocrotaline (60 mg/kg ip) or saline. On day 14, post-monocrotaline (MCT) rats received 5 x 10(9) plaque-forming units of either Ad-human BMPR2 (Ad-hBMPR2) or Ad-GFP. Transgene expression was confirmed by fluorescence microscopy, quantitative RT-PCR of whole lung samples, and laser-capture microdissected resistance PAs. Invasive hemodynamic and echocardiographic end points of pulmonary hypertension were assessed on day 24. Endogenous BMPR2 mRNA levels were greatest in resistance PAs, and expression declined with MCT PAH. Despite robust hBMPR2 expression in all lung lobes and within resistance PAs of treated rats, hBMPR2 did not lower mean PA pressure, pulmonary vascular resistance index, right ventricular hypertrophy, or remodeling of resistance PAs. Nebulized intratracheal adenoviral gene therapy with hBMPR2 reliably distributed hBMPR2 to resistance PAs but did not ameliorate PAH. Depressed BMPR2 expression may be a marker of PAH but is not central to the pathogenesis of this model of PAH.     

Pulmonary microembolism: Attenuated pulmonary vasoconstriction with prostaglandin inhibitors and antihistamines

The mechanism(s) involved in the pulmonary vascular and airway responses to pulmonary microembolism have not been clearly defined. Therefore, we determined the effects of specific prostaglandin and histamine blockade on the hemodynamic and arterial blood gas tension responses to particulate microembolism (200 μ glass beads) in intact anesthetized dogs. The marked increases in pulmonary arterial pressure and pulmonary vascular resistance observed in the untreated dogs were attenuated, but not abolished, following both prostaglandin blockade (with either meclofenamate or polyphloretin phosphate) and histamine blockade (with chlorpheniramine and metiamide) at 5 minutes, and were still attenuated 30 minutes post embolization. Combined prostaglandin and histamine blockade further attenuated, but again did not abolish, the pulmonary vascular responses. Cardiac outputs and systemic arterial pressures were unchanged from control by embolism. The alveolar hypoventilation (decreased arterial oxygen tension and increased carbon dioxide tension) observed in the untreated embolized dogs was prevented only with the prostaglandin inhibitors. Pulmonary microembolism in intact dogs, therefore, appears to induce vasoconstriction mediated partially by prostaglandin and histamine action, and alveolar hypoventilation mediated by prostaglandin, but not histamine, action.     

Leukotriene inhibitors do not block hypoxic pulmonary vasoconstriction in dogs

We studied whether intravenously administered inhibitors of leukotriene synthesis (diethylcarbamazine, DEC) or end-organ effect (FPL-55712) would change the distribution of regional pulmonary blood flow (rPBF) caused by left lower lobe (LLL) alveolar hypoxia in dogs. Both drugs failed to alter rPBF. In addition, the pressor response to whole-lung hypoxia was not blocked by an FPL-55712 infusion. On the other hand, nitroprusside, as a nonspecific vasodilator also administered intravenously, was able to partially reverse the effects of LLL hypoxia on rPBF. Thus our data do not support a role for leukotriene mediation of hypoxic pulmonary vasoconstriction in dogs.