Mikrozirkulatorische und metabolische Veränderungen des Skelettmuskels bei obstruktiver Schlafapnoe - Untersuchungen am Mausmodell
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Philipps-Universität Marburg
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Abstract
The obstructive sleep apnea syndrome is characterized by a repetitive collapse of the upper airway during sleep and the resulting chronic intermittent hypoxia. Over the last two decades, the prevalence of OSAS has more than doubled, so that currently about 50 % of men and 23 % of women aged between 40 and 85 years suffer from a moderate to severe OSA.
While regular exercise lowers the incidence of sleep-related breathing disorders, OSAS represents a major risk factor for limited physical performance. The few studies concentrating on the skeletal muscle indicate that patients suffering from OSAS experience load-dependent myalgia and morphologic alterations. As an established independent risk factor for the development of insulin resistance, which in skeletal muscle manifests itself in a decreased insulin dependent glucose uptake, OSAS interferes with the muscular metabolism.
The aim of the present study was to investigate OSAS-related microcirculatory and metabolic alterations which are able to cause functional disorders both in glycolytic (gastrocnemius, plantaris, vastus muscle) and oxidative (soleus muscle) skeletal muscles. To mimic the OSAS, we used an established murine CIH model. Male C57BL/6 mice (wildtype) and iNOS-/- mice were exposed to 6 weeks of alternating inspiratory O2 fractions between 7% and 21% at cycles of 120s (AHI = 30h-1) for 8h/d during the daytime.
Using BSI-B4-Lectin staining, we detected a 27.2% higher capillary density and approximately 10% increase in capillary contacts per muscle fiber in the soleus muscle of CIH-exposed WT mice, which was accompanied by an upregulation of proangiogenetic markers (VEGFa, VEGFR2) in qRT-PCR. Transmission electron microscopy of capillary profiles showed signs of endothelial activation and proliferation in soleus muscle after exposure to CIH. Furthermore, the presence of angiogenesis most likely via splitting was confirmed by the significantly increased percentage of capillaries exhibiting luminal sprouts in CIH compared to normoxia.
In soleus muscle of WT, atrophic alterations consisting of a more than threefold increased count of small fibers (< 800µm²) as well as a 13.5% decreased fiber cross sectional area of type IIa fibers were observed with CIH compared to normoxia. In view of the unaffected expression of the ubiquitin ligase MuRF1 both at the protein and the mRNA level, these changes were seemingly independent of the ubiquitin-proteasome system. A simultaneous shift of fiber types towards the oxidative type I fibers might indicate denervation re-innervation processes under CIH exposure. Moreover, the percentage of center-nucleated fibers was increased by a factor of 7.5 in comparison to normoxia.
Using bungarotoxin staining, the investigation of the motor endplates revealed a significantly lower relative endplate area and furthermore a decreased length in relation to the myofiber perimeter comparing CIH-exposed soleus muscle to normoxic controls. Such morphologic remodeling has been attributed to denervation due to old age and impaired neuromuscular recovery. Moreover, using TEM, an increase of 80% of destructed mitochondria was observed in CIH exposed soleus muscle compared to normoxia. Statistical analysis further revealed an inverse correlation between the percentage of damaged mitochondria and the endplate area.
Both gene expression and protein density of various inflammatory markers (IL1β, IL6, COX2, CD68) demonstrated either unchanged or lower expressions in the CIH-exposed WT muscles compared to the normoxic controls.
Remarkably, the aforementioned CIH and iNOS-/- related alterations showed a muscle specificity and were restricted to the oxidative soleus muscle. Therefore, muscle specificity should also be considered in future studies.
However, iNOS deficiency displayed a protective effect on the CIH induced atrophy in soleus muscle. When comparing the soleus muscle of normoxic iNOS mice with WT mice the capillary density of the former was significantly increased by 30.4%. Thereby, no additional increase of the capillarization was observed under CIH exposure.
Rather than acting protectively on the integrity of the neuromuscular junction, the normoxic iNOS-/- mice exhibited a diminished area and length of muscle endplates and a significantly higher amount of fragmented post-synaptic sites compared to the wildtype controls. CIH exposure further aggravated the decrease of the NMJ in area and length.
Moreover, in comparison to WT, the removal of iNOS led to a destruction of the mitochondrial ultrastructure together with mitochondrial swelling.
Nevertheless, there was no evidence of muscular iNOS expression in either the normoxic or the CIH-exposed WT animals. The aforementioned alternations must therefore be caused by an extra-muscular iNOS deficiency which demonstrates the need of further research.
Finally, CIH exposure resulted neither in WT nor in iNOS-/- mice in metabolic alterations except for a significant reduction in the weight-gain. Especially, CIH caused no reduced insulin sensitivity which was determined by analyzing fructosamine. Therefore, we assume that the coincidence between OSAS and insulin resistance is due to factors, which are not covered by the CIH model used here.
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Created: 2024Issued: 2024-08-20Updated: 2024-08-20
Faculty
Medizin
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Philipps-Universität Marburg
Language
ger
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DoctoralThesis
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610
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Bannow, Laura Isabel (0000-0002-5770-5963): Mikrozirkulatorische und metabolische Veränderungen des Skelettmuskels bei obstruktiver Schlafapnoe - Untersuchungen am Mausmodell. : Philipps-Universität Marburg 2024-08-20. DOI: https://doi.org/10.17192/z2024.0289.
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This item has been published with the following license: In Copyright