Hypoxically Induced Nitric Oxide: Potential Role as a Vasodilator in Mytilus edulis Gills

Abstract

Intertidal Mytilus edulis experience rapid transgression to hypoxia when they close theirvalves during low tide. This induces a physiological stress response aiming to stabilizetissue perfusion against declining oxygen partial pressure in shell water.We hypothesizedthat nitric oxide (NO) accumulation supports blood vessel opening in hypoxia andused live imaging techniques to measure NO and superoxide anion (O?−2 ) formationin hypoxia-exposed gill filaments. Thirty minutes of moderate (7 kPa pO2) and severehypoxia (1 kPa pO2) caused 1.6- and 2.4-fold increase, respectively, of NO accumulationin the endothelial muscle cells of the hemolymphatic vessels of the gill filaments. This ledto a dilatation of blood vessel diameter by 43% (7 kPa) and 56% (1 kPa), which facilitatesblood flow. Experiments in which we applied the chemical NO-donor Spermine NONOate(concentrations ranging from 1 to 6mM) under normoxic conditions corroborate thedilatational effect of NO on the blood vessel. The formation of O?−2 within the filamentepithelial cells increased 1.5 (7 kPa) and 2-fold (1 kPa) upon treatment. Biochemicalanalysis of mitochondrial electron transport complexes in hypoxia-exposed gill tissueindicates decreased activity of complexes I and III in both hypoxic conditions; whereascomplex IV (cytochrome-c oxidase) activity increased at 7 kPa and decreased at 1kPa compared to normoxic exposure conditions. This corresponds to the pattern ofpO2-dependent gill respiration rates recorded in ex-vivo experiments. Severe hypoxia(1 kPa) appears to have a stabilizing effect on NO accumulation in gill cells, sinceless O2 is available for NO oxidation to nitrite/nitrate. Hypoxia thus supports theNO dependent inhibition of complex IV activity, a mechanism that could fine tunemitochondrial respiration to the local O2 availability in a tissue. Our study highlights abasal function of NO in improving perfusion of hypoxic invertebrate tissues, which couldbe a key mechanism of tolerance toward environmental O2 variations.Fil: González, Paula Mariela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Bioquímica y Medicina Molecular. Universidad de Buenos Aires. Facultad Medicina. Instituto de Bioquímica y Medicina Molecular; ArgentinaFil: Rocchetta, Iara. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; ArgentinaFil: Abele, Doris. Alfred Wegener Institute For Polar And Marine Research; AlemaniaFil: Rivera Ingraham, Georgina A.. Alfred Wegener Institute For Polar And Marine Research; Alemani