Hypoxic contraction of pre-stretched human pulmonary artery

Retracted: Chest digital dynamic radiography to detect changes in human pulmonary perfusion in response to alveolar hypoxia

INTRODUCTION

Hypoxic pulmonary vasoconstriction optimises oxygenation in the lung by matching the local-blood perfusion to local-ventilation ratio upon exposure to alveolar hypoxia. It plays an important role in various pulmonary diseases, but few imaging evaluations of this phenomenon in humans. This study aimed to determine whether chest digital dynamic radiography could detect hypoxic pulmonary vasoconstriction as changes in pulmonary blood flow in healthy individuals.

METHODS

Five Asian men underwent chest digital dynamic radiography before and after 60 sec breath-holding at the maximal inspiratory level in upright and supine positions. Alveolar partial pressure of oxygen and atmospheric pressure were calculated using the blood gas test and digital dynamic radiography imaging, respectively. To evaluate the blood flow, the correlation rate of temporal change in each pixel value between the lung fields and left cardiac ventricles was analysed.

RESULTS

Sixty seconds of breath-holding caused a mean reduction of 26.7 ± 6.4 mmHg in alveolar partial pressure of oxygen. The mean correlation rate of blood flow in the whole lung was significantly lower after than before breath-holding (before, upright 51.5%, supine 52.2%; after, upright 45.5%, supine 46.1%; both P < 0.05). The correlation rate significantly differed before and after breath-holding in the lower lung fields (upright, 11.8% difference; supine, 10.7% difference; both P < 0.05). The mean radiation exposure of each scan was 0.98 ± 0.09 mGy. No complications occurred.

CONCLUSIONS

Chest digital dynamic radiography could detect the rapid decrease in pulmonary perfusion in response to alveolar hypoxia. It may suggest hypoxic pulmonary vasoconstriction in healthy individuals.

Hypoxic Pulmonary Vasoconstriction in Humans: Tale or Myth

Hypoxic Pulmonary vasoconstriction (HPV) describes the physiological adaptive process of lungs to preserves systemic oxygenation. It has clinical implications in the development of pulmonary hypertension which impacts on outcomes of patients undergoing cardiothoracic surgery. This review examines both acute and chronic hypoxic vasoconstriction focusing on the distinct clinical implications and highlights the role of calcium and mitochondria in acute versus the role of reactive oxygen species and Rho GTPases in chronic HPV. Furthermore it identifies gaps of knowledge and need for further research in humans to clearly define this phenomenon and the underlying mechanism.

Hypoxic pulmonary vasoconstriction in humans

Hypoxic pulmonary vasoconstriction is the elegant theory put forward more than six decades ago to explain regional variations in perfusion within the lung in certain animal species in response to localised restrictions in oxygenation. Although considerable progress has been made to describe the phenomenon at the macroscopic level and explain it at the microscopic level, we are far from a universal agreement about the process in humans. This review attempts to highlight some of the important evidence bases of hypoxic pulmonary vasoconstriction in humans and the significant gaps in our knowledge that would need bridging.

Cyclooxygenase-2 inhibition and thromboxane A(2) receptor antagonism attenuate hypoxic pulmonary vasoconstriction in a porcine model

AIM

Hypoxic pulmonary vasoconstriction (HPV) causes pulmonary hypertension that may lead to right heart failure. We hypothesized that the COX-2 inhibitor nimesulide and the thromboxane A(2) receptor antagonist daltroban would attenuate HPV.

METHODS

Haemodynamic measurements and blood sampling were performed in 18 anaesthetized, mechanically ventilated pigs, with mean ± SEM weights of 31.3 ± 0.6 kg, in normoxia (F(i)O(2)~0.21) and hypoxia (F(i)O(2)~0.10), before and 5, 15 and 45 min after initiation of right atrial infusion of nimesulide (n = 6) or daltroban (n = 6), respectively, and in six control pigs.

RESULTS

Compared with normoxia, hypoxia (n = 18) increased mean pulmonary artery pressure by 15.8 ± 0.8 mmHg (P < 0.001), pulmonary vascular resistance (PVR) by 2.7 ± 0.3 WU (P < 0.05) and mean right atrial pressure by 2.3 ± 0.3 mmHg (P < 0.001). In the control pigs, mean pulmonary artery pressure, PVR and mean right atrial pressure remained stable (P = ns) throughout 45 min hypoxia, compared with hypoxia baseline. Nimesulide decreased mean pulmonary artery pressure by 3.7 ± 1.3 mmHg after 45 min (P < 0.013), as well as PVR by 0.8 ± 0.2 WU (P < 0.05), levelling off after 15 min. Daltroban transiently increased (P < 0.001) mean pulmonary artery pressure and mean right atrial pressure by 7.2 ± 1.2 and 2.7 ± 0.4 mmHg, respectively, but they returned to hypoxia baseline (P = ns) within 5 min. Daltroban then decreased mean pulmonary artery pressure to after 45 min be 4.2 ± 1.6 mmHg lower (P < 0.005) than at hypoxia baseline.

CONCLUSION

COX-2 inhibition and thromboxane A(2) receptor antagonism attenuate HPV by decreasing mean pulmonary artery pressure by approximately 10-11%, as measured 45 min after initiation of nimesulide or daltroban infusion respectively.

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.

Hypoxic pulmonary vasoconstriction: mechanisms and controversies

The pulmonary circulation differs from the systemic in several important aspects, the most important being that pulmonary arteries constrict to moderate physiological (20-60 mmHg PO2) hypoxia, whereas systemic arteries vasodilate. This phenomenon is called hypoxic pulmonary vasoconstriction (HPV), and is responsible for maintaining the ventilation-perfusion ratio during localized alveolar hypoxia. In disease, however, global hypoxia results in a detrimental increase in total pulmonary vascular resistance, and increased load on the right heart. Despite many years of study, the precise mechanisms underlying HPV remain unresolved. However, as we argue below, there is now overwhelming evidence that hypoxia can stimulate several pathways leading to a rise in the intracellular Ca2+ concentration ([Ca2+]i) in pulmonary artery smooth muscle cells (PASMC). This rise in [Ca2+]i is consistently found to be relatively small, and HPV seems also to require rho kinase-mediated Ca2+ sensitization. There is good evidence that HPV also has an as yet unexplained endothelium dependency. In this brief review, we highlight selected recent findings and ongoing controversies which continue to animate the study of this remarkable and unique response of the pulmonary vasculature to hypoxia.

Two temporal components within the human pulmonary vascular response to approximately 2 h of isocapnic hypoxia

The time course of the pulmonary vascular response to hypoxia in humans has not been fully defined. In this investigation, study A was designed to assess the form of the increase in pulmonary vascular tone at the onset of hypoxia and to determine whether a steady plateau ensues over the following approximately 20 min. Twelve volunteers were exposed twice to 5 min of isocapnic euoxia (end-tidal Po(2) = 100 Torr), 25 min of isocapnic hypoxia (end-tidal Po(2) = 50 Torr), and finally 5 min of isocapnic euoxia. Study B was designed to look for the onset of a slower pulmonary vascular response, and, if possible, to determine a latency for this process. Seven volunteers were exposed to 5 min of isocapnic euoxia, 105 min of isocapnic hypoxia, and finally 10 min of isocapnic euoxia. For both studies, control protocols consisting of isocapnic euoxia were undertaken. Doppler echocardiography was used to measure cardiac output and the maximum tricuspid pressure gradient during systole, and estimates of pulmonary vascular resistance were calculated. For study A, the initial response was well described by a monoexponential process with a time constant of 2.4 +/- 0.7 min (mean +/- SE). After this, there was a plateau phase lasting at least 20 min. In study B, a second slower phase was identified, with vascular tone beginning to rise again after a latency of 43 +/- 5 min. These findings demonstrate the presence of two distinct phases of hypoxic pulmonary vasoconstriction, which may result from two distinct underlying processes.

Human pulmonary vascular responses to hypoxia and hypercapnia

The ability of alveolar gas composition to influence pulmonary vascular tone has been appreciated for over 50 years. In particular, it has been proposed that both O2 and CO2 could play a role in the matching of perfusion to ventilation within the lung, improving the overall efficiency of gas exchange. A wide variety of experimental approaches has been used to investigate pulmonary vascular effects of the respiratory gases in a range of mammalian species. In this article, we review experiments performed in healthy humans, identify particular difficulties in the interpretation of such experiments, and discuss possible approaches to future study.

Vasoreactions to acute hypoxia, whole lungs and isolated vessels compared: modulation by NO

We aimed to explain diverse pulmonary vascular responses to hypoxia in different preparations and their modulation by NO. In rats we compared isolated perfused lungs (IPL), small vessels in vitro (PRVs) and in vivo preparations. In IPL and in vivo, acute and chronic nitric oxide synthase (NOS) blockade with L-NAME left normoxic pulmonary artery pressure unchanged but enhanced hypoxic vasoconstriction, hypoxia-induced pulmonary vasoconstriction (HPV). PRVs in vitro, precontracted with PGF(2alpha), showed four tension changes in acute hypoxia: dilatation, contraction, dilatation, contraction. Acute and chronic NOS blockade reduced the first two phases. In non-precontracted PRVs (from other laboratories), NOS inhibition enhanced HPV as in vivo and IPL; attenuation of HPV seems associated with precontraction. Thus reduced NOS activity does not cause pulmonary hypertension but exaggerates HPV. In IPL, prolonged severe hypoxia caused biphasic vasoconstriction separated by dilatation; the time course resembled that seen in PRVs. We suggest that the sequence of events during hypoxia in PRVs can be detected in whole lung preparations.

Multiple sites of oxygen sensing and their contributions to hypoxic pulmonary vasoconstriction

Oxygen sensing by the pulmonary vasculature is important for the regulation of vessel tone and the matching of lung perfusion to ventilation. Airways hypoxia is a major stimulus for vasoconstriction, which diverts blood from hypoxic alveoli to better ventilated areas of the lung. Several hypotheses have emerged to explain how pulmonary arteries sense a decrease in oxygen and mediate hypoxic pulmonary vasoconstriction (HPV). They differ mainly in where they place the main site of HPV: in the endothelial or smooth muscle cells of the artery wall. HPV probably results from synergistic actions on both cell types, but it can proceed in the absence of endothelium, suggesting that the primary oxygen sensor is the smooth muscle cell and endothelium-derived agents modulate the muscle response. Several oxygen-sensing targets have been identified in smooth muscle, including potassium channels, Ca(2+) stores in the sarcoplasmic reticulum (SR) and the Ca(2+) sensitivity of the contractile proteins. The evidence for different oxygen-sensing mechanisms in pulmonary vessels is discussed.

5-hydroxytryptamine induces vasoconstriction of the human umbilical artery: effects of hypoxia and nicorandil

Human umbilical arteries are known to be modulated by oxygen partial pressure. To further understand the underlying mechanisms, rings were suspended in organ chambers for the measurement of isometric force. The effects of 5-hydroxytryptamine (10(-9) to 10(-5) M) were first investigated before and after hypoxic conditions (5% O(2)-5% CO(2) in N(2)). Then after pretreatment, we tested indomethacin (10(-5) M), N-nitro-L-arginine (L-NNA, 10(-5) M) and nicorandil (10(-5) M) each separately, then each of the three substances together with hypoxia. In separate experiments the contractions to 5-hydroxytryptamine (10(-9) to 10(-5) M) were effectuated in a glucose-free medium, and mitochondrial respiration was inhibited by cyanide (2 mM). Hypoxic conditions significantly reduced the contractive response to 5-hydroxytryptamine. Contractions were enhanced after indomethacin, but remained unchanged after L-NNA. Pretreatment with nicorandil decreased the contraction. Furthermore, hypoxia and nicorandil dramatically decreased the contraction to 5-hydroxytryptamine. In glucose-free medium under normoxia or in hypoxic conditions, 5-hydroxytryptamine did not induce any contraction. Moreover, cyanide (2 mM) remained without effect on the contraction obtained by 5-hydroxytryptamine. These results suggest that hypoxia and nicorandil attenuate vasoconstrictor responses to 5-hydroxytryptamine in human umbilical arteries. Furthermore, these findings suggest that prostacyclin acts as a functional antagonist to vasoconstriction whereas nitric oxide does not. Finally, glycolysis seems to be involved rather than mitochondrial metabolism.

The effect of the nitric oxide synthase inhibitor N-gamma-nitro-L-argine methyl ester on hypoxic pulmonary vasoconstriction

We studied the role of nitric oxide in the regulation of pulmonary arterial tone and hypoxic pulmonary vasoconstriction. Rat pulmonary arteries (n=65, diameter=440+/-12 microm) were loaded to 17.5 mm Hg in a wire myograph and incubated with the nitric oxide synthase inhibitor N-gamma-nitro-L-argine methyl ester (L-NAME; 1, 10 or 100 microM) or distilled water (50 microl) prior to preconstriction with either 100 microM prostaglandin F(2 alpha) followed by acetylcholine (0.1-100 microM) or 5 microM prostaglandin F(2 alpha) followed by hypoxia. Concentrations of L-NAME (10 and 100 microM) which attenuated acetylcholine dilatation, elevated basal tone from 0. 2+/-0.5% to 9.4+/-2.1% (P<0.01) and 18.3+/-3.2% (P<0.001), respectively, potentiated contraction to 5 microM prostaglandin F(2 alpha) from 35.9+/-3.1% to 56.2+/-6.8% (P<0.05) and 66.4+/-5.8% (P<0.001), respectively, but had no significant effect on hypoxic pulmonary vasoconstriction. This suggests basal pulmonary nitric oxide release occurs, as well as in response to agonist-induced contraction, but not hypoxic pulmonary vasoconstriction.

Endothelin, but not angiotensin II, contributes to the hypoxic contractile response of large isolated pulmonary arteries in the rat

The aims of this study were to investigate whether angiotensin II and/or endothelin could contribute to the hypoxic contractile response of isolated rat pulmonary artery. Experiments were performed for 1 h on noradrenaline precontracted arterial rings in hypoxic conditions (95% N2 and 5% CO2). Nicardipine, lisinopril, losartan, phosphoramidon, FR139317 and bosentan were used to block Ca2+ channels, angiotensin I-converting enzyme, AT1 receptors, endothelin-converting enzyme, ETA receptors, and ETA/ETB receptors, respectively. The profile of the hypoxic contractile response was biphasic, displaying, after a short relaxation, a weak and transient contraction (from 2-4 min) and then, before complete relaxation, a slowly developed but sustained contraction (from 14-60 min). Endothelium removal abolished the transient contraction and reduced (-59%) the sustained contraction. Nicardipine did not modify the transient contraction, but concentration-dependently decreased (from -35% to -100%) the sustained contraction (P = 0.024). Lisinopril and losartan did not affect the response (P = 0.418 and P = 0.973, respectively). Bosentan did not modify the transient contraction, but concentration-dependently decreased (from -14% to -71%) the sustained contraction (P = 0.016), whereas phosphoramidon and FR139317 did not affect the response (P = 0.830 and P = 0.806, respectively).

CONCLUSIONS

In rat, (i) both phases of the hypoxic contractile response are endothelium-dependent and independent of angiotensin II; (ii) the transient contraction does not depend on endothelin; (iii) the sustained contraction, which involves calcium influx, appears partly dependent on mature endothelin released from storage granules by stimulating ETB receptors.

Role of wall tension in hypoxic responses of isolated rat pulmonary arteries

The changes in force developed during 40-min exposures to hypoxia (37 +/- 1 mmHg) were recorded in large (0.84 +/- 0.02-mm-diameter) and small (0.39 +/- 0.01-mm-diameter) intrapulmonary arteries during combinations of mechanical wall stretch tensions (passive + active myogenic components), equivalent to transmural vascular pressures of 5, 15, 30, 50, and 100 mmHg, and active (vasoconstriction) tensions, stimulated by PGF2alpha in doses of 0, 25, 50, and 75% effective concentrations. Constriction was observed in all arteries during the first minute; however, at any active tension, the pattern of the subsequent response was a function of the stretch tension. At 5, 15, and 30 mmHg, the constriction decreased slightly at 5 min and then increased again to remain constrictor throughout. At 50 and 100 mmHg, the initial constriction was followed by persistent dilation. Hypoxic constrictor responses, most resembling those observed in lungs in vivo and in vitro, were observed when the mechanical stretch wall tension was equivalent to 15 or 30 mmHg and the dose of PGF2alpha was 25 or 50% effective concentration. These observations reconcile many apparently contradictory results reported previously.

Hypoxic contractile response in isolated rat thoracic aorta: role of endothelium, extracellular calcium and endothelin

The effects of hypoxia on isolated arteries remain controversial, depending on the species, vascular beds and protocols. The aims of the study were to characterize the response of rat thoracic aorta to hypoxia and to examine the roles of endothelium, extracellular calcium and endothelin in this response. Hypoxia was induced by bubbling Krebs solution with 95% N2 and 5% CO2 instead of 95% O2 and 5% CO2. Experiments were performed during 1 h in norepinephrine (0.01 microM) precontracted rings. Hypoxia produced a biphasic response consisting of an initial transient partial relaxation (67% at 14 min) followed by a slow but sustained contraction (27% from 40 to 60 min). After endothelium removal, relaxation appeared faster with increased magnitude (82% at 12 min) and was followed by a weak transient contraction (16% at 25 min). In endothelium-intact rings, Ca2+ free medium (EGTA, 0.1 mM) and Ca2+ channel blockers, verapamil (0.05, 0.5 and 5 microM) or nicardipine (0.1, 1 and 10 microM), had no effect on relaxation but inhibited the contraction, the effects of both calcium antagonists being concentration-dependent. Similarly, the ETA/ETB receptor antagonist, bosentan (0.1, 10 and 1,000 nM), induced a concentration-dependent decrease in the contraction. We conclude that 1) the response of rat thoracic aorta during 1 h of hypoxia is biphasic (relaxation followed by contraction); 2) the endothelium is involved in the contraction whereas its role in the relaxation remains to be elucidated; 3) extracellular calcium is involved in the contraction; and 4) endothelin may play a role in the contraction.

Endothelin-1 does not mediate the endothelium-dependent hypoxic contractions of small pulmonary arteries in rats

Various pulmonary artery preparations in vitro demonstrate sustained endothelium-dependent contractions upon hypoxia. To determine whether endothelin-1 could mediate this phenomenon, we examined the effect of bosentan, a new antagonist of both the ETA and ETB subtypes of the endothelin receptor. Small (300 pm) pulmonary arteries from rats were mounted on a myograph, precontracted with prostaglandin F2 alpha and exposed to hypoxia (PO2, 10 to 15 mm Hg, measured on-line) for 45 min. Endothelium-intact control rings exhibited a biphasic response, with a transient initial vasoconstriction (phase 1) followed by a second slowly developing sustained contraction (phase 2). Expressed in percent of the maximal response to 80 mmol/L KCl, the amplitudes of phase 1 (peak tension) and 2 (tension after 45 min of hypoxia) averaged 37 +/- 12% and 17 +/- 14%, respectively (n = 11). In endothelium-denuded rings, phase 1 persisted while the amplitude of phase 2 was reduced to 2 +/- 12% (p < 0.05, n = 8), showing the endothelium dependence of this contraction. Neither phase was significantly decreased in rings treated with 10(-5) mmol/L bosentan (38 +/- 15% and 17 +/- 12%, respectively, n = 6). The PO2 threshold for onset of hypoxic contraction was not significantly different among these three groups and averaged 32 +/- 24 mm Hg. In a separate experiment, we assessed the inhibitory effect of 10(-5) mol/L bosentan on the response to 10(-8) mol/L endothelin-I. Rings treated for 45 min with 10(-8) mol/L endothelin-1 alone exhibited a maximal contraction of 75 +/- 27% (n = 6). This was reduced to 4 +/- 17% (p < 0.01, n = 6) in rings treated with both 10(-8) mol/L endothelin-1 and 10(-5) mol/L bosentan. We conclude that complete blockade of all endothelin receptor subtypes has no effect on either endothelium-dependent or -independent hypoxic contractions in this preparation. This suggests that endothelial factors other than endothelin-I mediate the acute hypoxic contractions of small pulmonary arteries in the rat.

Potentiation of the hypoxic contraction of guinea-pig isolated pulmonary arteries by two inhibitors of superoxide dismutase

1. Isolated proximal and distal extralobar branches of the pulmonary artery of the guinea-pig develop slow and well-sustained contractions in response to hypoxia (PO2 11-15 mm Hg) without prior stimulation with an agonist. These contractions are readily reversible by readministration of oxygen. 2. Incubation of these preparations with diethyldithiocarbamic acid (DETCA, 5 mM for 30 min), an inhibitor of superoxide dismutase, significantly increased the hypoxic contractions whether DETCA was added before the challenge with hypoxia or after the hypoxic contraction had reached a plateau. This treatment also reduced the oxygen-induced relaxation. 3. Similarly, incubation with triethylenetetramine (TETA, 5 mM for 30 min), another inhibitor of superoxide dismutase, produced larger potentiation of the hypoxic contraction in the two preparations and reduced the oxygen-induced relaxation. 4. Furthermore, addition of H2O2 (10(-5) M-3 x 10(-4) M) caused concentration-dependent relaxation of the hypoxic contraction while larger concentrations (10(-3) M and 3 x 10(-3) M) caused contraction that did not respond to readministration of oxygen. 5. These observations suggest that during hypoxic stress, the accumulation of superoxide anions may participate in the hypoxia-induced contraction and that the metabolism of these radicals into H2O2 by superoxide dismutase maintains the relaxed state during normoxia.

Cellular mechanisms of hypoxia-induced contraction in human and rat pulmonary arteries

The effect of hypoxia was investigated in human (HPA) and rat (RPA) pulmonary arteries. Hypoxia-induced contraction was 95 +/- 8.7% and 9.3 +/- 4.8% of the control response to K(+)-rich (80 mM) solution in HPA and RPA, respectively (n = 10). When RPA strips were precontracted with phorbol 12,13 dibutyrate (0.2 microM), hypoxia elicited a larger contraction (105 +/- 13.4% of the control response, n = 8). In both types of artery, hypoxia-induced contraction was dependent on the extracellular calcium concentration (66 +/- 8.4% and 40 +/- 14.4%, reduction for 1.25 mM Ca2+ in HPA and RPA, respectively, n = 6) and was inhibited by verapamil (0.05-10 microM) and nifedipine (0.05-1 microM). Glibenclamide (5-10 microM) increased the amplitude of hypoxia-induced contraction (+42 +/- 5.3%, n = 5). Hypoxia-induced contraction was blocked by cromakalim (1 microM) and this effect was reversed by glibenclamide (5 microM). This contraction was also inhibited by iodoacetic acid (250 microM). In beta-escin skinned pulmonary arterial strips, hypoxia had no effect on the calcium concentration-tension relationship. These results suggest that the O2 sensor in the pulmonary artery is located on the vascular smooth muscle plasmalemma. Hypoxia-induced contraction is dependent on calcium influx through voltage sensitive calcium channels. Its amplitude is modulated by the functioning of potassium channels.

Effects of hypoxia, mechanical and chemical endothelium denudation on guinea-pig isolated pulmonary arteries

1. The isolated unstimulated main trunk, extralobar and intralobar branches of the pulmonary artery of the guinea-pig developed well-sustained contractions upon exposure to hypoxia (95% N2-5% CO2 gas mixture; PO2 11-15 mm Hg). The contractions were readily reversible by reoxygenation (95% O2-5% CO2). 2. Mechanical removal of the endothelium did not significantly affect the magnitude of the hypoxia-induced contractions in rings obtained from the main trunk of the pulmonary artery but reduced those of rings obtained from the proximal and distal extralobar branches. 3. Mechanical removal of the endothelium also did not affect the magnitude of contractions induced by BaCl2 in the main but significantly reduced contractions induced by the same agent in the proximal and distal extralobar branches of the pulmonary artery, suggesting that the reduction of hypoxia-induced contractions in the endothelium-denuded rings is due to impairment of vascular reactivity. 4. Pretreatment with L-N-nitro arginine, an inhibitor of the synthesis of the endothelium-derived relaxing factor, did not significantly affect the hypoxia-induced contractions but increased the magnitude of BaCl2-induced contractions in the main and the extralobar branches. 5. These observations demonstrate that isolated pulmonary artery rings of the guinea-pig develop slow contractions in response to hypoxia without prior contraction with an agonist, and that the endothelium plays little role in the hypoxia-induced contractions of guinea-pig isolated large pulmonary arteries. 6. Furthermore, these observations suggest that the effect of mechanical endothelium denudation or pharmacological manipulation, such as EDRF inhibition, on vascular reactivity should be considered when the effect of hypoxia is studied in isolated pulmonary arteries.

Vasoconstrictor and vasodilator effects of hypoxia

Hypoxia has marked effects on artery calibre, which reflects important physiological control mechanisms that are altered in disease states. Hypoxia modifies the release of mediators, especially from the endothelium, and influences smooth muscle membrane potential and Ca2+ regulation. In this review, Roger Wadsworth evaluates the vasoconstrictor and vasodilator effects of hypoxia studied in vitro. In the future, drugs developed to act on the mediators or smooth muscle may be beneficial in the therapy of, for example, pulmonary hypertension or coronary vasospasm.