Acetazolamide prevents hypoxic pulmonary vasoconstriction in conscious dogs.

Acute hypoxia increases pulmonary arterial pressure and vascular resistance. Previous studies in isolated smooth muscle and perfused lungs have shown that carbonic anhydrase (CA) inhibition reduces the speed and magnitude of hypoxic pulmonary vasoconstriction (HPV). We studied whether CA inhibition by acetazolamide (Acz) is able to prevent HPV in the unanesthetized animal. Ten chronically tracheotomized, conscious dogs were investigated in three protocols. In all protocols, the dogs breathed 21% O(2) for the first hour and then 8 or 10% O(2) for the next 4 h spontaneously via a ventilator circuit. The protocols were as follows: protocol 1: controls given no Acz, inspired O(2) fraction (Fi(O(2))) = 0.10; protocol 2: Acz infused intravenously (250-mg bolus, followed by 167 microg.kg(-1).min(-1) continuously), Fi(O(2)) = 0.10; protocol 3: Acz given as above, but with Fi(O(2)) reduced to 0.08 to match the arterial Po(2) (Pa(O(2))) observed during hypoxia in controls. Pa(O(2)) was 37 Torr during hypoxia in controls, mean pulmonary arterial pressure increased from 17 +/- 1 to 23 +/- 1 mmHg, and pulmonary vascular resistance increased from 464 +/- 26 to 679 +/- 40 dyn.s(-1).cm(-5) (P < 0.05). In both Acz groups, mean pulmonary arterial pressure was 15 +/- 1 mmHg, and pulmonary vascular resistance ranged between 420 and 440 dyn.s(-1).cm(-5). These values did not change during hypoxia. In dogs given Acz at 10% O(2), the arterial Pa(O(2)) was 50 Torr owing to hyperventilation, whereas in those breathing 8% O(2) the Pa(O(2)) was 37 Torr, equivalent to controls. In conclusion, Acz prevents HPV in conscious spontaneously breathing dogs. The effect is not due to Acz-induced hyperventilation and higher alveolar Po(2), nor to changes in plasma endothelin-1, angiotensin-II, or potassium, and HPV suppression occurs despite the systemic acidosis with CA inhibition.     

Acute hypoxic pulmonary vasoconstriction in conscious dogs decreases renin and is unaffected by losartan.

Acute hypoxic pulmonary vasoconstriction (HPV) may be mediated by vasoactive peptides. We studied eight conscious, chronically tracheostomized dogs kept on a standardized dietary sodium intake. Normoxia (40 min) was followed by hypoxia (40 min, breathing 10% oxygen, arterial oxygen pressures 36 +/- 1 Torr) during both control (Con) and losartan experiments (Los; iv infusion of 100 microg. min-1. kg-1 losartan). During hypoxia, minute ventilation (by 0.9 l/min in Con, by 1.3 l/min in Los), cardiac output (by 0.36 l/min in Con, by 0.30 l/min in Los), heart rate (by 11 beats/min in Con, by 30 beats/min in Los), pulmonary artery pressure (by 9 mmHg in both protocols), and pulmonary vascular resistance (by 280 and 254 dyn. s. cm-5 in Con and Los, respectively) increased. Mean arterial pressure and systemic vascular resistance did not change. In Con, PRA decreased from 4.2 +/- 0.7 to 2.5 +/- 0.5 ng ANG I. ml-1. h-1, and plasma ANG II decreased from 11.9 +/- 3.0 to 8.2 +/- 2.1 pg/ml. The renin-angiotensin system is inhibited during acute hypoxia despite sympathetic activation. Under these conditions, ANG II AT1-receptor antagonism does not attenuate HPV.     

Benzolamide is not a membrane-impermeant carbonic anhydrase inhibitor

Benzolamide, an orphan drug belonging to the pharmacological class of sulfonamide carbonic anhydrase (CA, EC 4.2.1.1) inhibitors (CAIs) is widely used in many physiological and pharmacological studies, together with the clinically employed classical drugs, acetazolamide, methazolamide, ethoxzolamide or dichlorophenamide, it being frequently stated that benzolamide is a membrane-impermeant inhibitor. We prove here that this is false: in fact benzolamide is rather similar to acetazolamide from the point of view of penetrability through blood red cell membranes. Unlike these neutral drugs, the cationic, positively-charged CAIs incorporating either tetraalkyl ammonium or pyridinium moieties, due to their salt-like character are indeed membrane-impermeant, being the only type of low molecular weight compound possessing such properties. Selective inhibition of membrane-associated CA isozymes is relevant indeed in many physiological studies and also pharmacologically, since the tumor-associated isozymes (CA IX and XII) are both membrane-bound.     

Inhibition of hypoxia-induced calcium responses in pulmonary arterial smooth muscle by acetazolamide is independent of carbonic anhydrase inhibition

Hypoxic pulmonary vasoconstriction (HPV) occurs with ascent to high altitude and can contribute to development of high altitude pulmonary edema (HAPE). Vascular smooth muscle contains carbonic anhydrase (CA), and acetazolamide (AZ), a CA inhibitor, blunts HPV and might be useful in the prevention of HAPE. The mechanism by which AZ impairs HPV is uncertain. Originally developed as a diuretic, AZ also has direct effects on systemic vascular smooth muscle, including modulation of pH and membrane potential; however, the effect of AZ on pulmonary arterial smooth muscle cells (PASMCs) is unknown. Since HPV requires Ca2+ influx into PASMCs and can be modulated by pH, we hypothesized that AZ alters hypoxia-induced changes in PASMC intracellular pH (pH(i)) or Ca2+ concentration ([Ca2+](i)). Using fluorescent microscopy, we tested the effect of AZ as well as two other potent CA inhibitors, benzolamide and ethoxzolamide, which exhibit low and high membrane permeability, respectively, on hypoxia-induced responses in PASMCs. Hypoxia caused a significant increase in [Ca2+](i) but no change in pH(i). All three CA inhibitors slightly decreased basal pH(i), but only AZ caused a concentration-dependent decrease in the [Ca2+](i) response to hypoxia. AZ had no effect on the KCl-induced increase in [Ca2+](i) or membrane potential. N-methyl-AZ, a synthesized compound lacking the unsubstituted sulfonamide group required for CA inhibition, had no effect on pH(i) but inhibited hypoxia-induced Ca2+ responses. These results suggest that AZ attenuates HPV by selectively inhibiting hypoxia-induced Ca2+ responses via a mechanism independent of CA inhibition, changes in pH(i), or membrane potential.     

Lipoxygenase and cyclooxygenase blockade by BW 755C enhances pulmonary hypoxic vasoconstriction

Lipoxygenase products (leukotrienes) have been proposed as the mediators of pulmonary hypoxic vasoconstriction. However, the supporting data are inconclusive because the lipoxygenase and leukotriene receptor blockers that reduce hypoxic vasoconstriction (such as diethylcarbamazine and the FPL's) have confounding effects. We investigated BW 755C, a potent inhibitor of both lipoxygenase and cyclooxygenase, in eight intact anesthetized dogs with acute left lower lobe atelectasis. We examined two manifestations of hypoxic vasoconstriction: shunt fraction, as an inverse indicator of regional constriction in response to local hypoxia, and the pulmonary pressor response to global alveolar hypoxia, as an index of general hypoxic vasoconstriction. During normoxia, shunt fraction, measured using a sulfur hexafluoride infusion, was 32.0 +/- 7.0%. The pulmonary pressor response to hypoxia, defined as the increase in pulmonary end-diastolic gradient produced by 10% O2 inhalation, averaged 4.5 +/- 1.8 mmHg. Then, during normoxia, BW 755C was administered. Shunt fraction fell in all eight dogs from the previous mean of 32% to 25.5 +/- 6.1% (t = 6.5, P less than 0.0005). The hypoxic pressor response rose in all dogs, from the previous 4.5 mmHg to 9.0 +/- 3.5 mmHg (t = 4.5, P less than 0.005). BW 755C enhances hypoxic vasoconstriction, an effect consistent with its activity as a cyclooxygenase inhibitor. These data do not support a substantive role for the lipoxygenase pathway in hypoxic vasoconstriction.     

Effect of endotoxemia on hypoxic pulmonary vasoconstriction in unanesthetized sheep

This study examined the effect of acute endotoxemia on hypoxic pulmonary vasoconstriction (HPV) in awake sheep. Thirteen sheep were chronically instrumented with Silastic catheters in the pulmonary artery, left atrium, jugular vein, and carotid artery; with a Swan-Ganz catheter in the main pulmonary artery; with a chronic lung lymph fistula; and with a tracheostomy. Base-line HPV was determined by measuring the change in pulmonary vascular resistance (PVR) while sheep breathed 12% O2 for 7 min. Concentrations of immunoreactive 6-keto-PGF1 alpha and thromboxane B2 (TXB2) were measured in lung lymph during the hypoxic challenge. Escherichia coli endotoxin (0.2-0.5 micrograms/kg) was infused intravenously. Four hours after endotoxemia, HPV was measured. In five sheep, meclofenamate was infused at 4.5 h after endotoxemia and HPV measured again. During the base-line hypoxic challenge, PVR increased by 36 +/- 9% (mean +/- SE). There was no significant change in lung lymph 6-keto-PGF1 alpha or TXB2 levels with hypoxia. Twelve of the 13 sheep showed a decrease in HPV 4 h after endotoxemia; the mean change in PVR with hypoxia was -8 +/- 5%, which was significantly (P less than 0.05) reduced compared with base-line HPV. The infusion of meclofenamate at 4.5 h after endotoxin did not restore HPV.     

The effect of repeated intermittent hypoxia on pulmonary vasoconstriction in the newborn

The effect of repeated intermittent hypoxia upon the basal pulmonary vascular tone in the newborn period is unknown. We therefore studied the central hemodynamic response to seven repeated intermittent hypoxic challenges in acutely prepared piglets under 2 weeks of age. Catheters were placed in the aorta, pulmonary artery, and atria, and an electromagnetic flow probe was positioned around the main pulmonary artery. Each hypoxic challenge (Fio2 = 0.14) lasted 5 min, and was separated by an equal duration of ventilation with air. Nine control animals were ventilated with air for 90 min, a period of time equivalent to the seven challenges in the experimental group, and subjected to one hypoxic challenge at the end. Hypoxia uniformly induced pulmonary vasoconstriction. Repeated intermittent hypoxic challenges produced a progressive increase in pulmonary artery pressure and vascular resistance, both during air ventilation and hypoxia. For each challenge, the vascular resistance value achieved during hypoxia was directly related to the immediately preceding air ventilation one, and the magnitude of hypoxic pulmonary vasoconstriction, defined as the incremental change in resistance from air to hypoxia, was not different from the first to the last challenge in the experimental group. In the control group the pulmonary vascular tone did not change during the 90 min of air ventilation, and the single hypoxic challenge induced an increase in pulmonary vascular pressure and resistance similar in magnitude to the first challenge in the experimental group. Indomethacin administration to five experimental animals, after the last challenge, reversed the increase in air ventilation pulmonary artery pressure and vascular resistance.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of CA V decreases glucose synthesis from pyruvate

The carbonic anhydrase inhibitor acetazolamide reduces citrulline synthesis by intact guinea pig liver mitochondria and also inhibits mitochondrial carbonic anhydrase (CA V) and the more lipophilic carbonic anhydrase inhibitor ethoxzolamide reduces urea synthesis by intact guinea pig hepatocytes in parallel with its inhibition of total hepatocytic carbonic anhydrase activity. Intact hepatocytes from 48-h starved male guinea pig livers were incubated at 37 degrees C in Krebs-Henseleit with 95% O2/5% CO2 at pH 7.1 with 5 mM pyruvate, 5 mM lactate, 3 mM ornithine, 10 mM NH4Cl, 1 mM oleate; with these inclusions both urea and glucose synthesis start with HCO3- -requiring enzymes, carbamyl phosphate synthetase I and pyruvate carboxylase, respectively. Urea and glucose synthesis were inhibited in parallel by increasing concentrations of ethoxzolamide, estimated Ki for each approximately 0.1 mM. In other experiments hepatocytes were incubated at 37 degrees C in Krebs-Henseleit with 95% O2/5% CO2 at pH 7.1 with 10 mM glutamine, 1 mM oleate; with these inclusions glucose synthesis no longer starts with a HCO3- -requiring enzyme. Urea synthesis was inhibited by ethoxzolamide with an estimated Ki of 0.1 mM, but glucose synthesis was unaffected. Intact mitochondria were prepared from 48-h starved male guinea pig livers. Pyruvate carboxylase activity of intact mitochondria was determined in isotonic KCl-Hepes buffer, pH 7.4, 25 degrees C, with 7.5 mM pyruvate, 3 mM ATP, and 10 mM NaHCO3. Inclusion of ethoxzolamide resulted in reduction in the rate of pyruvate carboxylation in intact mitochondria, but not in disrupted mitochondria. It is concluded that carbonic anhydrase is functionally important for gluconeogenesis in the male guinea pig liver when there is a requirement for bicarbonate as substrate.     

Atrial natriuretic peptide ameliorates hypoxic pulmonary vasoconstriction without influencing systemic circulation.

Hypoxic pulmonary vasoconstriction (HPV) is encountered during ascent to high altitude. Atrial natriuretic peptide (ANP) could be an option to treat HPV because of its natriuretic, diuretic, and vasodilatory properties. Data on effects of ANP on pulmonary and systemic circulation during HVP are conflicting, partly owing to anesthesia, surgical stress or uncontrolled dietary conditions. Therefore, ten conscious, chronically tracheotomized dogs were studied under standardized dietary conditions. The dogs were trained to breathe spontaneously at a ventilator circuit. Protocol: 30min of normoxia [inspiratory oxygen fraction (F(i)O(2))=0.21] were followed by 30min of hypoxia without ANP infusion (Hypoxia I, F(i)O(2)=0.1). While maintaining hypoxia an intravenous infusion of atrial natriuretic peptide was started with 50ng x kg body wt(-1) x min(-1) for 30min (Hypoxia+ANP1=low dose), followed by 1000ng x kg body wt(-1) x min(-1) for 30min (Hypoxia+ANP2=high dose). Thereafter, ANP infusion was stopped and hypoxia maintained for a final 30min (Hypoxia II). Compared to normoxia, mean pulmonary arterial pressure (MPAP) (16+/-0.7 vs. 26+/-1.3mmHg) and pulmonary vascular resistance (PVR) (448+/-28 vs. 764+/-89dyn x s(-1) x cm(-5)) increased during Hypoxia I and decreased during Hypoxia+ANP 1 (MPAP 20+/-1mmHg, PVR 542+/-55dyn x s(-1) x cm(-5)) (P<0.05). The higher dose of ANP did not further decrease MPAP or PVR, but started to have a tendency to decrease mean arterial pressure and cardiac output. We conclude that low dose ANP is able to reduce HPV without affecting systemic circulation during acute hypoxia.     

Carbonic anhydrase inhibitors. Inhibition of the transmembrane isozyme XII with sulfonamides—a new target for the design of antitumor and antiglaucoma drugs?

The inhibition of a newly cloned human carbonic anhydrase (CA, EC 4.2.1.1), isozyme XII (hCA XII), has been investigated with a series of sulfonamides, including some clinically used derivatives (acetazolamide, methazolamide, ethoxzolamide, dichlorophenamide, dorzolamide, brinzolamide, benzolamide, and sulpiride, or indisulam, a compound in clinical development as antitumor drug), as well as the sulfamate antiepileptic drug topiramate. Some simple amino-/hydrazine-/hydroxy-substituted aromatic/heterocyclic sulfonamides have also been included in the study. All types of activity have been detected, with several medium potency inhibitors (K(I)s in the range of 34-220 nM), whereas ethoxzolamide and several halogenated sulfanilamides showed stronger potency, with K(I)s in the range of 11-22 nM. The antiglaucoma sulfonamides used clinically, except dichlorophenamide, which is a moderate inhibitor (K(I) of 50 nM), as well as topiramate, indisulam, and sulpiride behave as very potent hCA XII inhibitors, with K(I)s in the range of 3.0-5.7 nM. Several subnanomolar inhibitors (K(I)s in the range of 0.30-0.85 nM) have also been detected. Compounds with excellent selectivity against hCA XII over hCA II have been found, showing selectivity ratios in the range of 177.7-566.7. Apparently, hCA XII is a target of the antiglaucoma sulfonamides, and potent hCA XII inhibitors may be developed/used for the management of hypoxic tumors, together with inhibitors of the other tumor-associated isozyme, CA IX.     

Simultaneous measurement of O2 radicals and pulmonary vascular reactivity in rat lung

The role of endogenous radicals in the regulation of pulmonary vascular tone was evaluated by simultaneous measurement of pulmonary artery pressure and lung radical levels during exposure of isolated rat lungs to varying inspired O2 concentrations (0-95%) and angiotensin II. Lung radical levels, measured "on-line" using luminol and lucigenin-enhanced chemiluminescence, decreased in proportion to the degree of alveolar hypoxia. Radical levels fell during hypoxia before the onset of pulmonary vasoconstriction and promptly returned to basal levels with restoration of normoxic ventilation. Mild alveolar hypoxia (10% O2), which failed to decrease chemiluminescence, did not trigger pulmonary vasoconstriction. Although chemiluminescence tended to decrease more as the hypoxic response strengthened, there was not a simple correlation between the magnitude of the change in chemiluminescence induced by hypoxia and the strength of the hypoxic pressor response. Normoxic chemiluminescence was largely inhibited by superoxide dismutase but not catalase. Superoxide dismutase also increased normoxic pulmonary vascular tone and the strength of the pressor response to hypoxia and angiotensin II. Thus the predominant activated O2 species in the lung, during normoxia, was the superoxide anion or a closely related substance. Alteration of endogenous radical levels can result in changes in vascular tone. It remains uncertain whether the decrease in lung radical production during hypoxia caused pulmonary vasoconstriction or was merely associated with hypoxic ventilation.     

Metalloproteinase inhibition by Batimastat attenuates pulmonary hypertension in chronically hypoxic rats

Chronic hypoxia induces lung vascular remodeling, which results in pulmonary hypertension. We hypothesized that a previously found increase in collagenolytic activity of matrix metalloproteinases during hypoxia promotes pulmonary vascular remodeling and hypertension. To test this hypothesis, we exposed rats to hypoxia (fraction of inspired oxygen = 0.1, 3 wk) and treated them with a metalloproteinase inhibitor, Batimastat (30 mg/kg body wt, daily ip injection). Hypoxia-induced increases in concentration of collagen breakdown products and in collagenolytic activity in pulmonary vessels were inhibited by Batimastat, attesting to the effectiveness of Batimastat administration. Batimastat markedly reduced hypoxic pulmonary hypertension: pulmonary arterial blood pressure was 32 +/- 3 mmHg in hypoxic controls, 24 +/- 1 mmHg in Batimastat-treated hypoxic rats, and 16 +/- 1 mmHg in normoxic controls. Right ventricular hypertrophy and muscularization of peripheral lung vessels were also diminished. Batimastat had no influence on systemic arterial pressure or cardiac output and was without any effect in rats kept in normoxia. We conclude that stimulation of collagenolytic activity in chronic hypoxia is a substantial causative factor in the pathogenesis of pulmonary vascular remodeling and hypertension.     

Acute and chronic hypoxic pulmonary hypertension in guinea pigs

To determine whether the strength of acute hypoxic vasoconstriction predicts the magnitude of chronic hypoxic pulmonary hypertension, we performed serial studies on guinea pigs. Unanesthetized, chronically catheterized guinea pigs increased mean pulmonary arterial pressure (PAP) from 11 +/- 0.5 to 13 +/- 0.7 Torr in acute hypoxia (10% O2 for 65 min). The response was maximal at 5 min, remained stable for 1 h, and was reversible on return to room air. Cardiac index did not change with acute hypoxia or recovery. Guinea pigs exposed to chronic hypoxia increased PAP, measured in room air 1 h after removal from the hypoxic chamber, to 18 +/- 1 Torr by 5 days with little further increase in PAP to 20 +/- 1 Torr after 21 days. Cardiac index fell from 273 +/- 12 to 206 +/- 7 ml.kg-1.min-1 (P less than 0.05) after 21 days of hypoxia. Medial thickness of pulmonary arteries adjacent to terminal bronchioles and alveolar ducts increased significantly by 10 days. The magnitude of the pulmonary vasoconstriction to acute hypoxia persisted and was unabated during the development and apparent stabilization of chronic hypoxic pulmonary hypertension, suggesting that if vasoconstriction is the stimulus for remodeling, then the importance of the stimulus lessens with duration of hypoxia. In individual animals followed serially, we found no correlation between the magnitude of the acute vasoconstrictor response before chronic hypoxia and the severity of chronic pulmonary hypertension that subsequently developed either because the initial response was small and variable or because vasoconstriction may not be the sole stimulus for vascular remodeling in the guinea pig.     

Polycythemia and vascular remodeling in chronic hypoxic pulmonary hypertension in guinea pigs.

Chronic hypoxia increases pulmonary arterial pressure (PAP) as a result of vasoconstriction, polycythemia, and vascular remodeling with medial thickening. To determine whether preventing the polycythemia with repeated bleeding would diminish the pulmonary hypertension and remodeling, we compared hemodynamic and histological profiles in hypoxic bled (HB, n = 6) and hypoxic polycythemic guinea pigs (H, n = 6). After 10 days in hypoxia (10% O2), PAP was increased from 10 +/- 1 (SE) mmHg in room air controls (RA, n = 5) to 20 +/- 1 mmHg in H (P less than 0.05) but was lower in HB (15 +/- 1 mmHg, P less than 0.05 vs. H). Cardiac output and pulmonary artery vasoreactivity did not differ among groups. Total pulmonary vascular resistance increased from 0.072 +/- 0.011 mmHg.ml-1.min in RA to 0.131 mmHg.ml-1.min in H but was significantly lower in HB (0.109 +/- 0.006 mmHg.ml-1.min). Hematocrit increased with hypoxia (57 +/- 3% in H vs. 42 +/- 1% in RA, P less than 0.05), and bleeding prevented the increase (46 +/- 4% in HB, P less than 0.05 vs. H only). The proportion of thick-walled peripheral pulmonary vessels (53.2 +/- 2.9% in HB and 50.6 +/- 4.8% in H vs. 31.6 +/- 2.6% in RA, P less than 0.05) and the percent medial thickness of pulmonary arteries adjacent to alveolar ducts (7.2 +/- 0.6% in HB and 7.0 +/- 0.4% in H vs. 5.2 +/- 0.4% in RA, P less than 0.05) increased to a similar degree in both hypoxic groups. A similar tendency was present in larger bronchiolar vessels.(ABSTRACT TRUNCATED AT 250 WORDS)     

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 x 10(-5) M) attenuated hypoxic vasoconstriction as well as that due to angiotensin II and KCl. Chronic treatment with Y-27632 (30 mg x kg(-1) x 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.     

Contribution of endothelin to pulmonary vascular tone under normoxic and hypoxic conditions

The contribution of endothelin to resting pulmonary vascular tone and hypoxic pulmonary vasoconstriction in humans is unknown. We studied the hemodynamic effects of BQ-123, an endothelin type A receptor antagonist, on healthy volunteers exposed to normoxia and hypoxia. Hemodynamics were measured at room air and after 15 min of exposure to hypoxia (arterial PO(2) 99.8 +/- 1.8 and 49.4 +/- 0.4 mmHg, respectively). Measurements were then repeated in the presence of BQ-123. BQ-123 decreased pulmonary vascular resistance (PVR) 26% and systemic vascular resistance (SVR) 21%, whereas it increased cardiac output (CO) 22% (all P < 0.05). Hypoxia raised CO 28% and PVR 95%, whereas it reduced SVR 23% (all P < 0.01). During BQ-123 infusion, hypoxia increased CO 29% and PVR 97% and decreased SVR 22% (all P < 0.01). The pulmonary vasoconstrictive response to hypoxia was similar in the absence and presence of BQ-123 [P = not significant (NS)]. In vehicle-treated control subjects, hypoxic pulmonary vasoconstriction did not change with repeated exposure to hypoxia (P = NS). Endothelin contributes to basal pulmonary and systemic vascular tone during normoxia, but does not mediate the additional pulmonary vasoconstriction induced by acute hypoxia.     

Pulmonary vascular tone is increased by a voltage-dependent calcium channel potentiator

The mechanism of hypoxia-induced pulmonary vasoconstriction remains unknown. To explore the possible dependence of the hypoxic response on voltage-activated calcium (Ca2+) channels, the effects of BAY K 8644 (BAY), a voltage-dependent Ca2+ channel potentiator, were observed on the pulmonary vascular response to hypoxia of both the intact anesthetized dog and the perfused isolated rat lung. In six rat lungs given BAY (1 X 10(-6)M), hypoxia increased mean pulmonary arterial pressure (Ppa) to 30.5 +/- 1.7 (SEM) Torr compared with 14.8 +/- 1.2 Torr for six untreated rat lungs (P less than 0.01). After nifedipine, the maximum Ppa during hypoxia fell 14.1 +/- 2.4 Torr from the previous hypoxic challenge in the BAY-stimulated rats (P less than 0.01). BAY (1.2 X 10(-7) mol/kg) given during normoxia in seven dogs increased pulmonary vascular resistance 2.5 +/- 0.3 to 5.0 +/- 1.2 Torr X 1(-1) X min (P less than 0.05), and systemic vascular resistance 55 +/- 4.9 to 126 +/- 20.7 Torr X 1(-1) X min (P less than 0.05). Systemic mean arterial pressure rose 68 Torr, whereas Ppa remained unchanged. Administration of BAY during hypoxia produced an increase in Ppa: 28 +/- 1.5 to 33 +/- 1.9 Torr (P less than 0.05). Thus BAY, a Ca2+ channel potentiator, enhances the hypoxic pulmonary response in vitro and in vivo. This, together with the effect of nifedipine on BAY potentiation, suggests that increased Ca2+ channel activity may be important in the mechanism of hypoxic pulmonary vasoconstriction.     

Strength of hypoxic vasoconstriction determines shunt fraction in dogs with atelectasis

We investigated the degree to which strength of pulmonary hypoxic vasoconstriction affects perfusion of pulmonary shunt pathways in acute atelectasis. In 17 intact supine dogs (anesthetized, paralyzed, and ventilated) we produced left lower lobe atelectasis by occluding the lobar bronchus during oxygen inhalation. Subsequently, shunt fraction (reflecting perfusion of that lobe) was measured using an SF6 infusion while the dogs breathed room air; the mean was 26% (range 14-40%). Pulmonary pressor response to hypoxia was assessed in 13 dogs using the increase in pulmonary end-diastolic gradient (PDG) produced by inhalation of 10% oxygen. Those animals with the largest increase in pulmonary diastolic gradient had the smallest shunt fraction while breathing room air, whereas those with the smallest response had the largest shunt fraction. The contribution of local hypoxia to vasoconstriction in the shunt pathway was assessed in 13 dogs breathing room air by measuring the increase in shunt fraction produced by infusing prostaglandin E1 (PGE1). Those with the largest increase in shunt fraction had the smallest pre-PGE1 shunt fraction. Thus the strength of pulmonary vascular reactivity to hypoxia markedly influences the degree of vasoconstriction in shunt pathways and is a major determinant of shunt pathway perfusion.     

The role of peripheral chemoreceptors in the rapid response of the pulmonary vasculature of the late gestation sheep fetus to changes in PaO2.

The effect of removing the input from the peripheral arterial chemoreceptors on pulmonary vascular responses to changes in PaO2 was examined in late gestation fetal sheep. Blood flow in the left pulmonary artery and driving pressure across the pulmonary vascular bed were monitored in chronically prepared fetal sheep at 126-129 days gestation. Five fetuses had carotid sinus and vagus nerves sectioned bilaterally and four were left intact. In normoxia (PaO2 ca. 23 mmHg) pulmonary vascular resistance was slightly greater and pulmonary blood flow reduced in the denervated group relative to the intact group but these differences were not significant. When made hypoxic (PaO2 ca. 14 mmHg), pulmonary blood flow fell and pulmonary vascular resistance increased in all fetuses. However, in the intact fetuses these changes were significantly more rapid. In all fetuses the vasoconstriction was prolonged after their return to normoxia. When made hyperoxic (PaO2 ca. 27 mmHg), pulmonary blood flow increased by a similar amount in all fetuses. We conclude that in the term fetus the peripheral chemoreceptors play no appreciable role in the maintenance of the high pulmonary vascular resistance in normoxia, or the fall in resistance produced by a rise in PaO2. The chemoreceptors do however initiate the rapid phase of pulmonary vasoconstriction in hypoxia.     

Reduction of chronic hypoxic pulmonary hypertension in the rat by beta-aminopropionitrile

We administered antifibrotic agent beta-aminopropionitrile (BAPN) to rats exposed to 10% O2-90% N2 for 3 wk to prevent excess vascular collagen accumulation. Groups of Sprague-Dawley rats studied were air breathing, hypoxic, and hypoxic treated with BAPN, 150 mg/kg twice daily intraperitoneally. After the 3-wk period, we measured mean right ventricular pressure (RVP), the ratio of weight of right ventricle to left ventricle plus septum (RV/LV + S), and hydroxyproline content of the main pulmonary artery (PA) trunk. Hypoxia increased RVP from 14 to 29 mmHg; RVP was 21 mmHg in hypoxic BAPN-treated animals. Hypoxia increased the RV/LV + S ratio from 0.28 to 0.41; the ratio was 0.32 in hypoxic BAPN-treated animals. Hypoxia increased PA hydroxyproline from 20 to 239 micrograms/artery; hydroxyproline was 179 micrograms/artery in hypoxic BAPN-treated animals. Thus BAPN prevented pulmonary hypertension, right ventricular hypertrophy, and excess vascular collagen produced by hypoxia. We conclude that vascular collagen contributes to the maintenance of chronic hypoxic pulmonary hypertension.