Hypoxic pulmonary vasoconstriction (HPV) is an essential mechanism of the lung to match blood perfusion to alveolar ventilation and optimize pulmonary gas exchange. Mitochondria have been suggested as possible O2 sensors of HPV. With regard to mitochondria the hypothesis of an increased or decreased production of reactive oxygen species (ROS) at complex I or III competes with the hypothesis of a localized small decrease of ATP production. Against this background, the present study investigated the O2 dependent regulation of mitochondrial cytochrome redox state and respiration in isolated lungs and pulmonary arterial smooth muscle cells (PASMC).Methodological limitations of the past were overcome by detection of mitochondrial cytochrome redox state under hypoxic and normoxic conditions using remission spectrophotometry in intact lungs and in PASMC. High-resolution respirometry was used to determine the respiration simultaneously in isolated cells. These alterations were compared to HPV and hypoxia-induced cellular responses, such as mitochondrial matrix superoxide release and mitochondrial membrane potential. Aortic and renal arterial smooth muscle cells (ASMC, RASMC) served as controls. The hypoxia-induced decrease of mitochondrial respiration paralleled the increase in the strength of HPV in isolated lungs. In PASMC, reduction of respiration and mitochondrial cytochrome c and aa3 (complex IV) matched an increase in matrix superoxide levels, as well as mitochondrial membrane hyperpolarization. Cytochrome bl/h (complex III) was relatively more oxidized during hypoxia compared to cytochrome c and aa3. In contrast to PASMC, RASMC displayed a smaller decrease in respiration and no rise in superoxide or mitochondrial membrane potential. Pharmacological inhibition of mitochondria revealed analogous kinetics of cytochrome redox state and the strength of HPV.The data of this thesis thus suggest inhibition of complex IV as an essential step in mitochondrial O2 sensing of HPV. Concomitantly, increased superoxide release from complex III and mitochondrial membrane hyperpolarization may initiate the pathway underlying HPV.In isolated lungs, HPV may be modulated by oxygenation state of hemoglobin, whereas there is no evidence for carbon monoxide playing a role via cytochrome P450.
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