In obesity, the skeletal muscle capillary network regresses and the insulin-mediated capillary recruitment is impaired. However, it has been shown that in the early stage of advanced obesity, an increased functional vascular response can partially compensate for other mechanisms of insulin resistance. The present study aimed to investigate the changes in the capillary network around individual muscle fibres during the early stage of obesity and insulin resistance in mice using 3D analysis. Capillaries and muscle fibres of the gluteus maximus muscles of seven high-fat-diet-induced obese and insulin-resistant mice and seven age-matched lean healthy mice were immunofluorescently labelled in thick transverse muscle sections. Stacks of images were acquired using confocal microscope. Capillary network characteristics were estimated by methods of quantitative image analysis. Muscle fibre typing was performed by histochemical analysis of myosin heavy chain isoforms on thin serial sections of skeletal muscle. Capillary length per muscle fibre length and capillary length per muscle fibre surface were increased by 27% and 23%, respectively, around small muscle fibres in obese mice, while there were no significant comparative differences around large fibres of obese and lean mice. Furthermore, the capillarization was larger around small compared to large fibres and there was a shift toward fast type myosin heavy chain isoforms, with no significant changes in muscle fibre diameters, tortuosity and anisotropy in obese mice. Overall, the results show that obese insulin-resistant mice have selective increase in capillarization around small predominantly intermediate muscle fibres, which is most likely related to the impaired glucose metabolism characteristic of type 2 diabetes.

Department of Animal Science Chair for Genetics Biotechnology and Immunology Biotechnical Faculty University of Ljubljana Groblje 3 1230 Domžale Slovenia

Department of Biomathematics Institute of Physiology Czech Academy of Science Vídeňská 1083 14220 Prague 4 Czech Republic

Department of Physiology Anthropology and Ethology Biotechnical Faculty University of Ljubljana Večna pot 111 1000 Ljubljana Slovenia

Faculty of Medicine Institute of Anatomy University of Ljubljana Korytkova 2 1000 Ljubljana Slovenia

Faculty of Medicine Institute of Anatomy University of Ljubljana Korytkova 2 1000 Ljubljana Slovenia Department of Anaesthesiology and Surgical Intensive Therapy University Medical Centre Ljubljana Zaloška cesta 7 1000 Ljubljana Slovenia

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