Programme Contents

Increased fatigue resistance in EDL muscle of the obese mouse is associated with an increase in the proportion of hybrid IIB+IID fibres

Fatigue resistance is an important indicator of the functional status of a muscle. Current data on the fatigue characteristics of the extensor digitorum longus (EDL) muscle from the genetically obese (ob/ob) mouse, a commonly used animal model of type 2 diabetes, are limited and inconsistent. Of the two studies carried out to date on this muscle, one shows an increased fatigue resistance in the obese animal (Warmington et al., 2000) while the other shows no difference between the obese animal and its lean control (Bruton et al., 2002). Therefore, in the present study we re-examined the fatigue characteristics of EDL muscles from ob/ob and lean mice. We also determined, using a single fibre approach, the fibre type composition of the two muscles as this parameter is closely related to muscle fatigability.

Male ob/ob and lean mice (18-22 weeks, C57BL strain) were killed by halothane overdose in accordance with Victoria University AEEC procedures, and muscle dissection was carried out as described in Bortolotto et al. (2000). Isometric contractions in EDL muscle were elicited at optimal length via supramaximal pulses (13 V cm^-1; 0.2 ms duration) in carbogen bubbled Krebs solution (Pedersen et al., 2003) with 10 mmol l^-1 glucose and 10 μmol l^-1 tubocurarin, at 25 ± 1°C. Force-frequency responses were determined using stimulation trains of 500 ms and train frequencies of 1-110 Hz, with a 3 min rest period between stimuli. Fatigue resistance was evaluated using a fatigue protocol similar to that described in Chin & Allen (1997), and consisted of repeated maximum tetanic stimulation (110 Hz, 350 ms train duration) at decreasing time intervals (4 s, 3 s, 2.5 s; each for total 2 min) until the force declined to 30% of the initial force (P₀). This protocol was repeated following a 60 min rest period. Contralateral EDL muscles were employed for electrophoretic analyses of myosin heavy chain isoform (MHC_i) composition in whole muscle homogenates and single muscle fibres using a modified version of the Talmadge & Roy (1993) SDS-PAGE protocol.

In comparison to EDL muscle from lean mice (n=8), EDL muscle from ob/ob mice (n=8) displayed an increased resistance to the first fatigue bout (time to 30% P₀: 164.4 ± 6.2 s vs 146.1 ± 2.8 s; P<0.05) and greater recovery of peak force between fatigue bouts. Type IIB was the predominant fibre type in randomly dissected single fibres from EDL muscle of ob/ob (78.9%, n=57) and lean (95.1%, n=61) mice. However, the fibre population from ob/ob mice contained a greater proportion of hybrid fibres (21.1% vs 4.9%) co-expressing MHCIIb and MHCIId isoforms (i.e. hybrid IIB+IID fibres). Consistent with this result, EDL muscle (n=6) from ob/ob mice contained a smaller proportion of MHCIIb (52.4% vs 65.7%) and larger proportions of MHCIId (31.9% vs 25.7%) and MHCIIa (15.7% vs 8.6%) isoforms. This shift in the MHC_i composition of EDL muscle from ob/ob mice towards a slower profile was also reflected in the force-frequency relationship at suboptimal frequencies (greater % force relative to maximum force at 30 Hz and 50 Hz in obese muscle) and a prolonged twitch half-relaxation rate (72.4 ± 6.0 ms in obese vs 49.2 ± 3.4 ms in lean; P<0.05).

The shift towards slower fibre types and the increased fatigue resistance observed in the present study for EDL muscle from the ob/ob mouse may be part of an adaptive response to the obese/diabetic condition, whereby the physiological role of the EDL muscle changes from a muscle enabling rapid movement to a muscle enabling better maintenance of posture under conditions of increased body weight.

Bortolotto, S.K., Cellini, M., Stephenson, D.G. & Stephenson, G.M.M. (2000) American Journal of Physiology, 279, C1564-C1577.

Bruton, J.D., Katz, A., Lännergren, J., Abbate, F. & Westerblad, H. (2002) Pflügers Archiv, 444, 692-699.

Chin, E.R. & Allen, D.G. (1997) Journal of Physiology, 498, 17-29.

Pedersen, T.H., Clausen, T. & Nielsen, O.B. (2003) Journal of Physiology, 551, 277-286.

Warmington, S.A., Tolan, R. & McBennett, S. (2000) International Journal of Obesity, 24, 1040-1050.

Talmadge, R.J. & Roy, R.R. (1993) Journal of Applied Physiology, 75, 2337-2340.