Hypoxia-induced skeletal muscle fiber dysfunction: role for reactive nitrogen species

Abstract

Hypoxia impairs skeletal muscle function, but the precise mechanisms are incompletely understood. In hypoxic rat diaphragm muscle, generation of peroxynitrite is elevated. Peroxynitrite and other reactive nitrogen species have been shown to impair contractility of skinned muscle fibers, reflecting contractile protein dysfunction. We hypothesized that hypoxia induces contractile protein dysfunction and that reactive nitrogen species are involved. In addition, we hypothesized that muscle reoxygenation reverses contractile protein dysfunction. In vitro contractility of rat soleus muscle bundles was studied after 30 min of hyperoxia (Po2 approximately 90 kPa), hypoxia (Po2 approximately 5 kPa), hypoxia + 30 microM N(G)-monomethyl-L-arginine (L-NMMA, a nitric oxide synthase inhibitor), hyperoxia + 30 microM L-NMMA, and hypoxia (30 min) + reoxygenation (15 min). One part of the muscle bundle was used for single fiber contractile measurements and the other part for nitrotyrosine detection. In skinned single fibers, maximal Ca2+-activated specific force (Fmax), fraction of strongly attached cross bridges (alphafs), rate constant of force redevelopment (ktr), and myofibrillar Ca2+ sensitivity were determined. Thirty minutes of hypoxia reduced muscle bundle contractility. In the hypoxic group, single fiber Fmax, alphafs, and ktr were significantly reduced compared with hyperoxic, L-NMMA, and reoxygenation groups. Myofibrillar Ca2+ sensitivity was not different between groups. Nitrotyrosine levels were increased in hypoxia compared with all other groups. We concluded that acute hypoxia induces dysfunction of skinned muscle fibers, reflecting contractile protein dysfunction. In addition, our data indicate that reactive nitrogen species play a role in hypoxia-induced contractile protein dysfunction. Reoxygenation of the muscle bundle partially restores bundle contractility but completely reverses contractile protein dysfunction.

Maternal creatine supplementation from mid-pregnancy protects the newborn spiny mouse diaphragm from intrapartum hypoxia-induced damage

“…Intrapartum hypoxia caused contractile dysfunction in the neonatal diaphragm, significantly reducing the force-generating capacity and sensitivity to Ca 2ϩ . This finding is in agreement with studies in the adult diaphragm (9,10). Supplementation of the maternal diet with creatine completely prevented contractile dysfunction in offspring from hypoxic births.…”

“…Excessive production of reactive oxygen species occurs and promotes myosin and actin degradation (7), leading to damage of myofibrillar proteins (8) and reduced functional capacity of skeletal muscles (9). These changes occur at the single fiber level of adult diaphragm (10). Hypoxia can also result in muscle fiber atrophy in skeletal muscle (11).…”

Acute hypoxia limits endurance but does not affect muscle contractile properties

“…single fibers are altered during hypoxia. 20 The decline in maximal and submaximal force was at least partly related to protein modifications, 2 such as increased nitrotyrosine formation. 20 Indeed, hemoglobin, a scavenger of nitric oxide (NO), in the incubation medium alleviated the effects of an NO donor on in vitro diaphragm contractility during hypoxia.…”

“…It is also unlikely that the duration of exposure to hypoxia plays a role as a decline in force was already detected after 2 min of hypoxia, 6 whereas others found no change in MVC even after a 40-day simulated climb. 10 Whereas in vivo force-generating capacity of the muscle tissue itself seems to be maintained during hypoxia, in vitro studies generally show a decline in force-generating capacity 2,17,20,26 and a downward shift of the force-frequency relation. 18 We did not find such a depression of force or shift in the forcefrequency relation in the in vivo situation.…”

“…Indeed, hypoxia is thought to be a contributing factor to skeletal muscle dysfunction and remodeling during chronic heart failure and chronic obstructive pulmonary disease. 3,13,25 Moreover, the accelerated production of reactive oxygen species during acute hypoxia affects the contractile properties of the diaphragm in vitro, 14,18,20,26 but little is known about the effects of acute hypoxia on the in vivo contractile properties of skeletal muscle.Most human studies on in vivo skeletal muscle function during hypoxia are performed with voluntary contractions. 8,9 Skeletal muscle function, howAbbreviations: ATP, adenosine 5Ј-triphosphate; FIO 2 , fraction of inspired air consisting of oxygen; MVC, maximal voluntary contraction; NO, nitric oxide; P/Po, for the fatigue test, force at a certain time point as a percentage of the force at the onset of the fatigue test; P/Po, for the force-frequency relation, force at a given stimulation frequency as a percentage of the force generated during 100-HZ stimulation; PaO 2 , arterial oxygen partial pressure; ROS, reactive oxygen species; SaO 2 , oxygen saturation of hemoglobin…”