Zymosan activation of plasma reduces hypoxic pulmonary vasoconstriction

The role of redox changes in oxygen sensing

The specialized oxygen-sensing tissues include the carotid body and arterial smooth muscle cells in the pulmonary artery (PA) and ductus arteriosus (DA). We discuss the evidence that changes in oxygen tension are sensed through changes in redox status. "Redox" changes imply the giving or accepting of electrons. This might occur through the direct tunneling of electrons from mitochondria or redox couples to an effector protein (e.g. ion channel). Alternatively, the electron might be transferred through reactive oxygen species from mitochondria or an NADPH oxidase isoform. The PA's response to hypoxia and DA's response to normoxia result from reduction or oxidation, respectively. These opposing redox stimuli lead to K+ channel inhibition, membrane depolarization and an increase in cytosolic calcium and/or calcium sensitization that causes contraction. In the neuroendocrine cells (the type 1 cell of the carotid body, neuroepithelial body and adrenomedullary cells), the response is secretion. We examine the roles played by superoxide anion, hydrogen peroxide and the anti-oxidant enzymes in the signaling of oxygen tensions.

Hypoxic pulmonary vasoconstriction in the adult respiratory distress syndrome

Increased pulmonary vascular resistance (PVR) and microvascular hyperpermeability resulting in lung edema and arterial hypoxemia are mainstays in the development of adult respiratory distress syndrome (ARDS). The proposed pathophysiologic mechanisms include activation of complement and polymorphonuclear leukocytes secreting lysosomal enzymes, toxic oxygen metabolites (TOM) and eicosanoids. Platelets and coagulation factors are also involved, and in the most severe cases even monocytes are activated as reflected in release of thromboplastin. The latter may elicit disseminated intravascular coagulation (DIC). Under physiologic conditions lung blood flow is diverted from poorly to better oxygenated areas by way of hypoxic pulmonary vasoconstriction (HPV), thereby counteracting a decrease in arterial oxygenation. Many vasoactive substances have been proposed and again refuted as possible mediators of HPV. In this study we have focused on the following: histamine, catecholamines, arachidonates, calcium, phosphoinositides and TOM as well as endothelium-derived relaxing and constricting factors. Whether HPV is present in ARDS and whether it is advantageous or not seems to depend on the stage and extent of disease. We discuss possible interactions between HPV and ARDS mediators and between HPV and various vasoactive agents tested for therapeutic effects. Out of the abundance of mediators released, prostacyclin, prostaglandin E1, activated complement and platelet activating factor have been shown explicitly to inhibit HPV whereas others are suspected of doing so. In therapeutical use, prostacyclin has proved to reduce PVR and at the same time enhance cardiac output and oxygen delivery. In mild to moderate ARDS, improvement of arterial oxygenation has also been obtained employing almitrine bismesylate, a potentiator of HPV. Experimentally, adenosine effectively reduces increments in PVR and microvascular permeability with modest effects on systemic circulation. However, further investigations are warranted to decide whether adenosine or more specific blockers as, for instance, monoclonal antibodies against tumor necrosis factor should be integrated in ARDS therapy in the future.