The Hypoxic Signal in Different Tissues

Phylogenetically, the mammalian homeostatic O₂ sensing system (HOSS) has developed a multicomponent defense mechanism to hypoxic environments aimed to prevent cellular injuries, mitigate the decline in O₂ tension, and adjust the energetic metabolism. The HOSS implicates a synchronized transcriptional regulation of a variety of genes, and it is coordinated, in large part, by the hypoxic-inducible factor (HIF) and transcription factors such as nuclear factor-kB ­(NF-kB) and activated protein-1, which are “master regulators” for oxygen homeostasis and inflammatory processes.⁷ The HOSS entails a network of specialized cells such as VSMCs (pulmonary and placental arteries, ductus arteriosus), endothelial cells (ECs), glomic cells in the carotid body, renal myofibroblasts, adrenomedullary chromaffin cells, and neuroepithelial bodies in the airways. When exposed to hypoxia, these specialized tissues detect low O₂ tension in their local environments through mitochondrial redox-based or cytoplasmic HIF, and they orchestrate coordinated responses to optimize oxygen uptake and delivery (Fig. 15.1).⁸ In type I glomus cells from the carotid and aortic bodies, the low O₂ tension leads to inhibition of voltage-gated potassium channels (Kv), resulting in membrane depolarization and influx of extracellular Ca²⁺ via voltage-gated Ca²⁺ channels (VGCC), which triggers the release of neuromediators and a faster ventilatory drive. In the kidney, the low O₂ content (e.g., difference between O₂ delivery and O₂ utilization) leads to inhibition of prolyl-4-hydroxylase domain proteins, which activates HIF signaling to generate erythropoietin and in turn to enhance erythropoiesis. Hypoxia-mediated release of catecholamines, vascular endothelial growth factors, and erythropoietin all tend to enhance tissue O₂ delivery owing to increased cardiac output (CO), angiogenesis, and higher hemoglobin (Hb) concentrations.

The HPV response chiefly operates in arteriolar and venular pulmonary vessels (diameter < 900 μm), veins accounting for 20% of the total increase in PVR. By distorting the alveolar wall, contractile interstitial cells located within the alveolar septa slightly contribute to reduce capillary blood flow and to increase PVR.⁹ The ECs modulate the HPV through the local release of vasodilators (nitric oxide [NO], prostacyclin [PGI2]) and vasoconstrictors (endothelin [ET], epoxyeicosatrienoic acid [EET], thromboxane A2 [TXA2], and platelet activating factor), with pulmonary veins exhibiting greater sensitivity to vasoconstrictor stimuli than arteries.^10,11

From ex vivo ventilated/perfused lungs in which alveolar and blood O₂ tension were modified independently, Marshall et al. have demonstrated that acute elevation in PVR was predominantly triggered by sensing low P_AO₂and less by sensing deoxygenated blood in mixed venous blood (P_mvO₂).^12,13 The stimulus of HPV (PO₂S) has been modeled according to the following equation: PO₂ stimulus = P_AO₂ ^{0. 62} + P_mvO₂ ^{0. 38}.

When pulmonary VSMCs or isolated lungs are exposed to decremental levels of local PO₂, the HPV develops at a threshold of 85 to 90 mm Hg, to reach a maximum at 65 to 70 mm Hg (about halfway between P_AO₂ and P_mvO₂) and declining thereafter to stop around 5 mm Hg (Fig. 15.2).¹⁴

The O₂ sensing area is thought to extend from precapillary arterioles to upstream pulmonary arteries with different vasoconstrictive responses according to the size of the vessels.¹⁵ The small arteries (diameter < 0.5 mm) are closely exposed to P_AO₂ (from outside) and to P_mvO₂ (from inside), whereas larger arteries (diameter of 0.5–2 mm) that are supplied by systemic blood through the vasa vasorum are exposed to both P_mvO₂ (from inside) and to arterial O₂ pressure (P_aO₂ from outside). Hence, variable diffusion O₂ gradients are generated through the wall of pulmonary arteries that determine the true stimulus for HPV (PO₂S), being predominant in small arteries. In larger arteries, the HPV response is less profound and mainly activated when systemic hypoxemia develops (P_aO₂ < 40–50 mm Hg), as bronchial arteries uniformly dilate like systemic vessels.

Time Course of Hypoxic Pulmonary Vasoconstriction

Hypoxic exposure of pulmonary VSMCs results in a biphasic vasoconstrictive response. The early phase starts immediately, peaking at 5 to 20 minutes and is associated with transient elevations of intracellular calcium concentrations. Phase 2 develops more gradually to reach a second peak at 60 to 120 minutes, as a result of myofibrillar calcium-sensitization coupled with enhanced release of vasoconstrictive compounds (ET-1 and EET) and decreased production of NO from the ECs (Fig. 15.3).¹⁴ A sustained vasoconstrictive response may not persist in case of severe hypoxia. If normoxia is restored after or during the early hypoxic phase (<20 minutes), the HPV stops allowing normalization of PVR and pulmonary blood flow with improved V/Q ratios. In contrast, after a prolonged period of hypoxia, the HPV disappears over several hours despite full reoxygenation, resulting in greater ventilation-perfusion (V/Q) mismatch and consequently a larger alveolar-to-arterial O₂ pressure gradient (A-aPO₂). A second hypoxic challenge (in ipsilateral lung) will result in an enhanced HPV response.

Both atelectasis formation and persistent HPV are incriminated in causing hypoxemia in the early hours after thoracic anesthesia. Likewise, in bilateral thoracic interventions and after starting the second stage, a V/Q mismatch may develop in the dependent ventilated lung, which is attributed to the residual HPV response that lags behind the time of reoxygenation.

The Oxygen Sensing and Effector Mechanisms of Hypoxic Pulmonary Vasoconstriction

In eukaryotic cells, mitochondria are acting as powerhouses using mainly O₂ to convert nutrients into chemical energy in the form of adenosine triphosphate (ATP) that is used in processes including muscle contraction, ion transport, nerve impulse propagation, and chemical synthesis. A series of oxidation-reduction reactions involves the transfer of electrons from nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2) across the electron transport chain (ETC). Because mitochondrial respiration continues even at very low O₂ levels (∼7 mm Hg) to generate ATP, it is unlikely that variations in ATP concentrations trigger the hypoxic response.¹⁶ Ultimately, in the process of oxidative phosphorylation, O₂ serves as the terminal electron acceptor of the ETC to form H₂O, while, at multiple sites along the ETC, some leaking electrons react with O₂, forming reactive oxygen species (ROSs), such as superoxide and hydrogen peroxide.

In pulmonary VSMCs, the most plausible mechanism todetect hypoxia is the mitochondrial PO₂-sensitive ability tovary the production of diffusible ROSs and/or to alter the cytosolic redox state. Although controversies surround the issue on whether ROSs generated by the ETC decrease or increase in response to hypoxia, there is strong evidence that HPV is triggered by mitochondrial signals that launch a coordinated response involving membranar and sarcolemnal potassium and calcium channel receptors to increase cytoplasmic calcium concentrations.¹⁷ As proposed by Smith and Schumacker,¹⁷ mitochondrial ROSs (see Fig. 15.4) move to the cytosol, where they activate several pathways leading to intracellular calcium mobilization: (1) inhibition of Kv channels causing membrane depolarization, which promotes the opening of VGCC; (2) activation of phospholipase C (PLC) to generate diacyl glycerol (DAG) and inositol 1,4,5-triphosphate (IP3) that stimulate calcium release, respectively, from membranal receptor-operating channels (ROCs) and sarcolemnal IP3 receptors; and (3) activation of ryanodine receptors (RyR) by oxidation of cysteine residues. In addition, by increasing the ratio of adenosine monophosphate (AMP) to ATP, low O₂ tension stimulates AMP-kinase (AMPK), resulting in activation of the RyRs and further Ca²⁺ release from the sarcoplasmic reticulum (SR). Together, the elevation of intracellular Ca²⁺ concentrations induces calcium binding to calmodulin, subsequent activation of myosin light chain (MLC) kinase, myosin phosphorylation, and ultimately contractions of VSMCs.

After activation of Rho kinase and MLC phosphatase, myofibrillar proteins become more sensitized to the effects of calcium sensitization, resulting in sustained contractions of pulmonary VSMCs. Besides acting directly on calcium channels, ROSs influence the cytoplasmic redox state, namely, the ratio of oxidized to reduced proteins, glutathione (GSSG/GSH), and nicotinamide adenine dinucleotide (NAD/NADH). According to the redox hypothesis, hypoxic conditions are associated with lesser release of ROSs from the mitochondria that promotes a shift from an oxidized to reduced cellular redox state in the cytoplasm, which in turn triggers Ca²⁺ mobilization through inhibition of Kv channels and opening of VGCCs.

When exposed to hypoxia, both pulmonary and systemic VSMCs share the same effector mechanisms of calcium-mediated changes in vascular tone, whereas pulmonary VSMCs present the unique property to detect low levels of O₂ tension and transmit signals to mobilize Ca²⁺ close to myofibrillar proteins. Mitochondria are located closer to the plasmalemmal membrane in pulmonary VSMCs compared with systemic VSMCs.¹⁸ Therefore a greater structural and functional coupling between mitochondria and Kv channels in pulmonary VSMCs could explain the specific O₂ sensitivity and HPV response.

Neurohumoral Modulation of Hypoxic Pulmonary Vasoconstriction

Exposures to intermittent or chronic hypoxia (e.g., pulmonary diseases, altitude) represent stressful challenges that, by stimulating the sympathetic nervous and renin-angiotensin-aldosterone systems (SNS and RAAS), result in elevation of systemic and pulmonary arterial pressures (PAPs).

Airways and pulmonary vessels are innervated by autonomic and sensory nerve fibers, the highest density of noradrenergic and cholinergic neural endings being found around extrapulmonary and hilar arteries and decreasing toward the periphery (artery size > 60–80 mcm).¹⁹ The vagal nerve of the parasympathetic pathway originates from the nucleus ambiguus in the brain stem, whereas sympathetic nerves emerge from the middle and inferior cervical and the first five thoracic ganglia. Vagal stimulation causes mild vasorelaxation mediated by the endothelial release of NO and the vasoactive intestinal factor (VIP). Stimulation of the sympathetic nervous system produces the release of norepinephrine (NE) from sympathetic neural endings close to the arterial wall and of epinephrine from the adrenal medulla in the blood stream. Stimulation of alpha-1–adrenergic-receptors (AAR-1) with NE causes vasoconstriction, owing to the inhibition of adenylate cyclase and the combined activation of Gq/11-proteins and PLC, that produce IP3 and DAG, which in turn enhance the internal/external release of Ca²⁺. In contrast, epinephrine causes potent vasodilatory effects by stimulation of beta-1 and beta-2 adrenergic receptors (BAR-1 and BAR-2) and consequent activation of the cAMP-dependent protein kinase pathway and the release of NO from ECs. Under normoxic and unstressed physiologic conditions, the sympathovagal systems play a negligible role in pulmonary vasomotor tone because PVR remains largely unchanged in animals subjected to a chemical, pharmacologic, or anesthetic sympathectomy. However, under hypoxic conditions, pulmonary BAR-mediated vasodilatation has been shown to predominate over AAR-1 vasoconstriction, partially blunting the potent acute HPV.²⁰ For instance, in patients with sleep apnea, BAR-mediated relaxation of pulmonary arteries offsets the HPV, mitigates the elevation of PVR which in turn alleviates the workload of the right ventricle. Lending support to these clinical findings, experimental data have demonstrated that the HPV response is attenuated with alpha-adrenergic blockers and enhanced with beta-adrenergic blockers.²¹

Hypoxic challenge has been shown to increase the activity of renal adrenergic receptors (AAR-1 and BAR-1) in juxtaglomerular cells, resulting in increased renin secretion and release of angiotensin II (Ang II) by the action of an angiotensin converting enzyme (ACE) largely present in the endothelium of pulmonary capillaries and in other tissues (e.g., brain, heart). In VSMCs, Ang II exerts dual effects on angiotensin receptors type 1 and 2 (AT-1 and AT-2): (1) vasoconstriction mediated by stimulation of the G-protein of AT-1, leading to Ca²⁺ mobilization, owing to activation of PLC and subsequent formation of IP3 and DAG; and (2) vasodilation mediated by stimulation of AT-2, leading to increased cyclic guanosine monophosphate (cGMP) and subsequent release of NO.²² The attenuation of HPV in patients chronically treated with ACE inhibitors suggests the predominant constrictive effect of Ang II.²³

Altogether, there is strong evidence supporting that hypoxia-induced stimulation of the SNS and RAAS results in systemic cardiovascular and renal changes (e.g., tachycardia, hypertension, oliguria), a blunted HPV response, owing to the predominant pulmonary BAR-2 vasodilatory effects, which partially offset the constricting effects on AAR-1 and AT-1 receptors.²⁴ Interestingly, a preserved HPV response has been reported in patients after bilateral lung transplantation, despite the loss of the autonomic neural innervation.²⁵

Modulation of Hypoxic Pulmonary Vasoconstriction by the Endothelium

Depending on experimental settings, denuding the endothelium from the pulmonary arteries may result in either suppression or enhancement of the HPV response, and the reasons for these opposing effects still remain unclear.²⁶ Nevertheless, by sensing low O₂ tension and releasing active vasodilatory and vasoconstrictor compounds, ECs contribute to regulate vascular tone.

Real-time fluorescence imaging in intact lungs demonstrated that hypoxia induces membrane depolarization in capillary ECs that propagates retrogradely to the feeding arterioles via gap junctions and connexin proteins (C×40). In parallel to this endothelial signal, a similar response is propagated upstream through precapillary VSMCs via other connexin proteins (C×43).²⁷ A growing body of evidence indicates that the release of endothelial vasoactive compounds enhances the second phase of acute HPV, owing to decreased NO levels, increased ET-1 and EETs production, whereas the increased PG12-mediated relaxation is offset by the enhanced TXA2-mediated constriction of VSMCs.

During prolonged hypoxia, proliferation of ECs is associated with the release of mitogenic factors for VSMCs, and generation of ET-1 dose-dependently mediates the overexpression of Kv and transient receptor potential channels, thereby contributing to the development of pulmonary hypertension (PH).²⁸

Physiologic Variations in Hypoxic Pulmonary Vasoconstriction Responses

In healthy individuals breathing air at sea level, pulmonary vessels are almost fully dilated, and inhalation of NO—a selective pulmonary vasodilator—does not improve oxygenation, suggesting that the HPV response is nonactivated under normoxic conditions.

Pulmonary Circulation

Compared with the systemic circulation, pulmonary vessels offer greater distensibility and recruitability to accommodate higher blood flow and volemia while blunting the elevation in pulmonary blood pressure. In contrast with the systemic circulation, the pressure-flow relationship in the pulmonary circulation is not linear because of the greater distensibility and potential recruitability of its vasculature. Therefore the pulmonary circulation may adapt to changing blood flow conditions, while minimizing the changes in PAP.²⁹ For instance, a reduction in CO (e.g., head-up position, blood loss) leads to a larger drop in pulmonary vascular driving pressure, even though the vasomotor tone remains per se unchanged.³⁰ Conversely, as pressure along the entire circuit increases, the pressure-flow curve becomes more linear because the fully distended vessels are by definition no longer distensible. Hence, when left atrial pressure increases with fluid loading or head-down position, the ability of hypoxic constriction to either redirect blood flow or reduce conductance is curtailed.

Lung volume also influences PVR, as described by a parabolic U-shaped curve. The PVR reaches a nadir at the functional residual capacity (FRC), and it increases at higher inflating pressures as alveolar capillaries are “squeezed” by elevated alveolar pressures, or when breathing operates below the FRC, as the extraalveolar vessels become distorted and partially obstructed, offering greater resistance to blood flow. Complete lung collapse, as occurs during OLV in thoracic surgery, results in mechanical vascular obstruction that contributes to blood flow diversion to the ventilated dependent lung.

Besides the P_AO₂ level, the strength of HPV also depends on the basal vascular tone and the volume of the lung exposed to hypoxia. A more potent HPV response is elicited when PVR is low (near FRC), and it decreases when the lung is overdistended or totally collapsed. From animal studies, we know that if exposure to hypoxia exceeds 40% of the whole lung, the redistribution of blood flow to nonhypoxic pulmonary areas is decreased, suggesting that the increased PAP tends to overcome the increased local arteriolar resistance generated by HPV.³¹

Physical Factors: Calcium, CO₂, pH, Temperature

Pulmonary arteries are minimally sensitive to changes in plasma calcium concentrations.³² In the isolated blood perfused canine lung, severe hypocalcemia is required to induce vasodilation and fails to influence the HPV response. Conversely, acute hypercalcemia fails to modify the pressure-flow curves and PVR, suggesting that calcium chloride can be safely administered intraoperatively through a right-sided venous catheter.

With the development of whole body hypothermia (from 37°C –27°C), both systemic and PVR are increased mainly as a result of cardiac depression, lower CO, and enhanced vasomotor tone,³³ whereas the HPV decreases linearly with the drop in central temperature.³⁴ Blunting of HPV is also observed with hypocapnia and metabolic alkalosis. Conversely, hypercapnia has been associated with decreased exhaled NO and enhanced HPV response in isolated ventilated-perfused lungs.³⁵

Senescence

With aging, oxidative stress and proinflammatory processes are the culprit mechanisms involved in atherogenesis, thickening and stiffening of systemic arteries that manifest clinically by hypertension, coronary artery disease, and heart failure. Likewise, the low pressure pulmonary circulation presents age-related reductions in arterial and venous distensibility, along with decreased left ventricle compliance leading to increased PAP. The reduction in antioxidant capacity with increased oxidative stress is incriminated in endothelial dysfunction and vascular extramatrix deposition of collagen.³⁶ Accordingly, PAP increases disproportionately more during exercise in older than in younger healthy humans, given lesser vasodilating mediators released from the ECs, stiffening of the vascular wall, and left ventricular diastolic dysfunction. In the elderly, the lesser increase in CO is associated with higher O₂ extraction in skeletal muscles, together with a greater fall in P_mvO2 that may act as a stimulus for enhanced HPV and further elevation in PAP during intense aerobic exercise.

Middle-aged subjects exposed to moderate alveolar hypoxia (P_AO₂ ∼50 mm Hg) over 20 minutes exhibit a greater increase in mean systolic PAP compared with younger individuals (+9 mm Hg) and similar changes in heart rate and in CO. These age-related enhanced HPV responses could be explained by the greater sensitivity of the O₂ sensing apparatus, greater propensity for Ca²⁺ cell entry, and greater activity of the calcium-calmodulin-dependent MLC kinase that contribute to sustained contractile response of pulmonary VSMCs.³⁷

Altitude

In contrast with regional HPV acting in case of atelectasis or pneumonia, generalized HPV in response to hypobaric hypoxia causes various degrees of PH that may be associated with interstitial and alveolar edema in susceptible patients.³⁸ The so-called “high-altitude pulmonary edema” (HAPE) occurs in nonacclimatized healthy individuals within 1 to 5 days after arrival at altitudes higher than 2500 m but also in well-acclimatized high-landers returning from low altitudes.³⁹ Lung imaging in symptomatic HAPE shows a patchy peripheral and nodular distribution of edema, whereas bronchoalveolar lavage reveals a protein-rich exudate with mild hemorrhage, neutrophilia, and proinflammatory cytokines consistent with the heterogeneous distribution of HPV and the involvement of inflammation secondary to alveolar-capillary barrier disruption. In addition, the alveolar fluid clearance mechanisms are impaired by hypoxia, owing to the reduced expression and activity of epithelial sodium channels and sodium/potassium (Na+/K+) ATPase proteins linked to (BAR-2) signaling.

Genetics

Epidemiologic studies have shown an association between early hypoxic exposure and later development of hyperreactivity in pulmonary vascular response.⁴⁰ For instance, transient perinatal hypoxia was found to predispose to exaggerated HPV when low-altitude dwellers were exposed to acute hypobaric hypoxic conditions. Likewise, the occurrence of preeclampsia in Bolivian high-altitude dwellers was shown to predispose their offspring to greater sensitivity of HPV response, owing to augmented oxidative stress and impaired NO bioavailability.

Different adaptive phenotypes have been identified in populations living in high altitudes. Compared with Han Chinese low-landers, Tibetans high-landers show hyporesponsiveness of the HIF transcriptional system characterized by a blunted HPV response associated with high exhaled NO concentrations, few hypertrophic changes in pulmonary arteries, and minimal increase in Hb concentrations.⁴¹ Andean highlanders exhibit other physiologic characteristics, namely, a blunted hypoxic ventilatory response, polycythemia, larger lung volumes, and better pulmonary diffusion compared with Tibetans. These variable adaptive solutions reflect different evolutionary pathways selected to survive in hypoxic environments.⁴²