Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCB378

Poster Communications

Influence of muscle mass on capillarity and fibre composition in fast muscles of inbred and wild type rats

M. H. Nazir1, S. Egginton1

1. University of Leeds, Leeds, United Kingdom.

Skeletal muscle composition is usually heterogeneous containing different fibre types with specific contractile and metabolic properties. Typically, Type IIa and IIb fibres are used for phasic activity and Type I to maintain posture. Genetic background may contribute to variability in composition of individuals, e.g. aerobic capacity is thought to be higher in inbred compared to wild strains. This study compared fibre type composition and capillarity of fast extensor digitorum longus (EDL) and tibialis anterior (TA) muscles between inbred (Wistar, n=6, body mass 207±3g) and wild type (Brown, n=6, body mass 209±2g) strains. Rats were sacrificed and cryosections (10µm at -20°C) stained to identify fibre type (monoclonal anti-MHC antibodies; BA-D5 for Type I fibre and SC-71 for Type IIa fibre, DSHB, University of Iowa) and capillaries (Griffonia simplicifolia lectin I, Vector) and visualised (20x and 16x magnification) under fluorescent illumination (Nikon E600), with systematic-random sampling. Capillary to fibre ratio (C:F), capillary density (CD, mm-2), and mean fibre area (MFA, µm2) were determined using in-house morphometric analysis programmes. Both EDL and TA muscle mass were higher in Brown rat (0.1021 vs. 0.0851g, P<0.05 and 0.400 vs. 0.355g, P<0.05) despite similar body masses. In EDL, global capillarity (C:F) and MFA were greater in Brown than Wistar rats (1.29±0.04 vs. 1.14±0.03 and 1388±16 vs. 1108±93 µm2, respectively, both P<0.05) with similar CD (984±35 and 1001±45 mm-2, n.s.). In the TA, C:F was not significantly different in either core or cortex but CD of Wistar rat was greater in cortex (556 vs. 445 mm-2, P=0.022) while MFA was slightly higher in Brown rat (n.s.). Both EDL and TA of Brown rat showed higher Type I numerical density (NN) than Wistar (0.058±0.003 vs. 0.038±0.003, P<0.05) and (0.060±0.005 vs. 0.038±0.004, P<0.05), respectively. Type IIb NN in EDL was lower (0.67±0.01 vs. 0.75±0.02, P<0.05) than Wistar rat but was similar in TA. Areal density (AA) of TI fibres of EDL and TA were higher in Brown than Wistar rat (0.038±0.001 vs. 0.023±0.003, P<0.01) and (0.050±0.01 vs. 0.028±0.004, P<0.01), respectively. Local capillary to fibre ratio (LCFR) of Brown EDL was higher than Wistar rat (1.55±0.05 vs. 1.08±0.03) while it was similar in TA. Accounting for differences in fibre size, local capillary density (LCD) of EDL and TA were similar (1174±71 vs.1250±66 mm-2) and (1501±93 vs. 1267±51 mm-2), respectively. These data suggest that muscle fibre composition and capillarity of the two strains are different, and indicate that fast muscles of Brown rats are more aerobic than those of Wistar rats, potentially increasing exercise tolerance.

Where applicable, experiments conform with Society ethical requirements