Inhibition of hypoxic pulmonary vasoconstriction of rats by carbon monoxide

Coronary and cerebral metabolism-blood flow coupling and pulmonary alveolar ventilation-blood flow coupling may be disabled during acute carbon monoxide poisoning

Current evidence indicates that the toxicity of carbon monoxide (CO) poisoning results from increases in reactive oxygen species (ROS) generation plus tissue hypoxia resulting from decreases in capillary Po₂ evoked by effects of increases in blood [carboxyhemoglobin] on the oxyhemoglobin dissociation curve. There has not been consideration of how increases in Pco could influence metabolism-blood flow coupling, a physiological mechanism that regulates the uniformity of tissue Po₂, and alveolar ventilation-blood flow coupling, a mechanism that increases the efficiency of pulmonary O₂ uptake. Using published data, I consider hypotheses that these coupling mechanisms, triggered by O₂ and CO sensors located in arterial and arteriolar vessels in the coronary and cerebral circulations and in lung intralobar arteries, are disrupted during acute CO poisoning. These hypotheses are supported by calculations that show that the Pco in these vessels can reach levels during CO poisoning that would exert effects on signal transduction molecules involved in these coupling mechanisms.NEW & NOTEWORTHY This article introduces and supports a postulate that the tissue hypoxia component of carbon monoxide poisoning results in part from impairment of physiological adaptation mechanisms whereby tissues can match regional blood flow to O₂ uptake, and the lung can match regional blood flow to alveolar ventilation.

The endothelium in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) in combination with hypercapnic pulmonary vasoconstriction redistributes pulmonary blood flow from poorly aerated to better ventilated lung regions by an active process of local vasoconstriction. Impairment of HPV results in ventilation-perfusion mismatch and is commonly associated with various lung diseases including pneumonia, sepsis, or cystic fibrosis. Although several regulatory pathways have been identified, considerable knowledge gaps persist, and a unifying concept of the signaling pathways that underlie HPV and their impairment in lung diseases has not yet emerged. In the past, conceptual models of HPV have focused on pulmonary arterial smooth muscle cells (PASMC) acting as sensor and effector of hypoxia in the pulmonary vasculature. In contrast, the endothelium was considered a modulating bystander in this scenario. For an ideal design, however, the oxygen sensor in HPV should be located in the region of gas exchange, i.e., in the alveolar capillary network. This concept requires the retrograde propagation of the hypoxic signal along the endothelial layer of the vascular wall and subsequent contraction of PASMC in upstream arterioles that is elicited via temporospatially tightly controlled endothelial-smooth muscle cell crosstalk. The present review summarizes recent work that provides proof-of-principle for the existence and functional relevance of such signaling pathway in HPV that involves important roles for connexin 40, epoxyeicosatrienoic acids, sphingolipids, and cystic fibrosis transmembrane conductance regulator. Of translational relevance, implication of these molecules provides for novel mechanistic explanations for impaired ventilation/perfusion matching in patients with pneumonia, sepsis, cystic fibrosis, and presumably various other lung diseases.

Biological signaling by carbon monoxide and carbon monoxide-releasing molecules

Carbon monoxide (CO) is continuously produced in mammalian cells during the degradation of heme. It is a stable gaseous molecule that reacts selectively with transition metals in a specific redox state, and these characteristics restrict the interaction of CO with defined biological targets that transduce its signaling activity. Because of the high affinity of CO for ferrous heme, these targets can be grouped into heme-containing proteins, representing a large variety of sensors and enzymes with a series of diverse function in the cell and the organism. Despite this notion, progress in identifying which of these targets are selective for CO has been slow and even the significance of elevated carbonmonoxy hemoglobin, a classical marker used to diagnose CO poisoning, is not well understood. This is also due to the lack of technologies capable of assessing in a comprehensive fashion the distribution and local levels of CO between the blood circulation, the tissue, and the mitochondria, one of the cellular compartments where CO exerts its signaling or detrimental effects. Nevertheless, the use of CO gas and CO-releasing molecules as pharmacological approaches in models of disease has provided new important information about the signaling properties of CO. In this review we will analyze the most salient effects of CO in biology and discuss how the binding of CO with key ferrous hemoproteins serves as a posttranslational modification that regulates important processes as diverse as aerobic metabolism, oxidative stress, and mitochondrial bioenergetics.

Effects of carbon monoxide-releasing molecules on pulmonary vasoreactivity in isolated perfused lungs

In addition to its renowned poisonous effects, carbon monoxide (CO) is being recognized for its beneficial actions on inflammatory and vasoregulatory pathways, particularly when applied at low concentrations via CO-releasing molecules (CO-RMs). In the lung, CO gas and CO-RMs are suggested to decrease pulmonary vascular tone and hypoxic pulmonary vasoconstriction (HPV). However, the direct effect of CO-RMs on the pulmonary vasoreactivity in isolated lungs has not yet been investigated. We assessed the effect of CORM-2 and CORM-3 on the pulmonary vasculature during normoxia and acute hypoxia (1% oxygen for 10 min) in isolated ventilated and perfused mouse lungs. The effects were compared with those of inhaled CO gas (10%). The interaction of CORM-2 or CO with cytochrome P-450 (CYP) was measured simultaneously by tissue spectrophotometry. Inhaled CO decreased HPV and vasoconstriction induced by the thromboxane mimetic U-46619 but did not alter KCl-induced vasoconstriction. In contrast, concentrations of CORM-2 and CORM-3 used to elicit beneficial effects on the systemic circulation did not affect pulmonary vascular tone. High concentration of CO-RMs or long-term application induced a continuous increase in normoxic pressure. Inhaled CO showed spectral alterations correlating with the inhibition of CYP. In contrast, during application of CORM-2 spectrophotometric signs of interaction with CYP could not be detected. Application of CO-RMs in therapeutic doses in isolated lungs neither decreases pulmonary vascular tone and HPV nor does it induce spectral alterations that are characteristic of CO-inhibited CYP. High doses, however, may cause pulmonary vasoconstriction.

Integrative understanding of hypoxic pulmonary vasoconstriction using in vitro models: from ventilated/perfused lung to single arterial myocyte

Contractile response of a pulmonary artery (PA) to hypoxia (hypoxic pulmonary vasoconstriction; HPV) is a unique physiological reaction. HPV is beneficial for the optimal distribution of blood flow to differentially ventilated alveolar regions in the lung, thereby preventing systemic hypoxemia. Numerous in vitro studies have been conducted to elucidate the mechanisms underlying HPV. These studies indicate that PA smooth muscle cells (PASMCs) sense lowers the oxygen partial pressure (PO₂) and contract under hypoxia. As for the PO₂-sensing molecules, a variety of ion channels in PASMCs had been suggested. Nonetheless, the modulator(s) of the ion channels alone cannot mimic HPV in the experiments using PA segments and/or isolated organs. We compared the hypoxic responses of PASMCs, PAs, lung slices, and total lungs using a variety of methods (e.g., patch-clamp technique, isometric contraction measurement, video analysis of precision-cut lung slices, and PA pressure measurement in ventilated/perfused lungs). In this review, the relevant results are compared to provide a comprehensive understanding of HPV. Integration of the influences from surrounding tissues including blood cells as well as the hypoxic regulation of ion channels in PASMCs are indispensable for insights into HPV and other related clinical conditions.

Optimization of isolated perfused/ventilated mouse lung to study hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is a compensatory physiological mechanism in the lung that optimizes the matching of ventilation to perfusion and thereby maximizes gas exchange. Historically, HPV has been primarily studied in isolated perfused/ventilated lungs; however, the results of these studies have varied greatly due to different experimental conditions and species. Therefore, in the present study, we utilized the mouse isolated perfused/ventilated lung model for investigation of the role of extracellular Ca²⁺ and caveolin-1 and endothelial nitric oxide synthase expression on HPV. We also compared HPV using different perfusate solutions: Physiological salt solution (PSS) with albumin, Ficoll, rat blood, fetal bovine serum (FBS), or Dulbecco's Modified Eagle Medium (DMEM). After stabilization of the pulmonary arterial pressure (PAP), hypoxic (1% O₂) and normoxic (21% O₂) gases were applied via a ventilator in five-minute intervals to measure HPV. The addition of albumin or Ficoll with PSS did not induce persistent and strong HPV with or without a pretone agent. DMEM with the inclusion of FBS in the perfusate induced strong HPV in the first hypoxic challenge, but the HPV was neither persistent nor repetitive. PSS with rat blood only induced a small increase in HPV amplitude. Persistent and repetitive HPV occurred with PSS with 20% FBS as perfusate. HPV was significantly decreased by the removal of extracellular Ca²⁺ along with addition of 1 mM EGTA to chelate residual Ca²⁺ and voltage-dependent Ca²⁺ channel blocker (nifedipine 1 μM). PAP was also reactive to contractile stimulation by high K⁺ depolarization and U46619 (a stable analogue of thromboxane A₂). In summary, optimal conditions for measuring HPV were established in the isolated perfused/ventilated mouse lung. Using this method, we further confirmed that HPV is dependent on Ca²⁺ influx.