The contractile properties of slow and fast skeletal muscle from protease activated receptor-2 null mice

J. Lam,¹ G.J. Pinniger,¹ C.N. Pagel,² E.J. Mackie² and A.J. Bakker,¹ ¹School of Anatomy, Physiology and Human Biology, University of Western Australia, Crawley, WA 6009, Australia and ²School of Veterinary Science, University of Melbourne, Parkville, VIC 3052, Australia.

Protease activated receptor-2 (PAR-2) is a member of a family of G-protein coupled receptors that are activated by proteolytic cleavage. PAR-2 activation results in the initiation of multiple intracellular signalling cascades that regulate a number of cellular processes, including the response to injury and cell survival (Ossovskaya and Bunnett, 2004). As PAR-2 is highly expressed in developing myoblasts in culture, and is reported to influence muscle proliferation and differentiation (Chinni et al., 2000; Jenkins et al., 2000), PAR-2 has been proposed to play a role in skeletal muscle development and regeneration. However, the effects of PAR-2 on adult skeletal muscle function and contractile phenotype are unknown. The aim of this study was to compare the contractile properties of slow and fast skeletal muscle from PAR-2 null and control, wild-type mice.

Experiments were performed on 17-18 week old female PAR-2 null (n = 6) and wild-type littermate control mice (PAR-2 Wt, n = 6). Mice were anaesthetised (sodium pentobarbitone, 40 mg/kg, i.p) and the soleus (mainly composed of slow-twitch fibres) and extensor digitorum longus (EDL, mainly composed of fast-twitch fibres) muscles were surgically removed and attached to a force transducer system. Muscles were maintained in mammalian Krebs Ringer solution (pH 7.3) bubbled with Carbogen (5% CO₂ in O₂) at 25°C. Twitch force characteristics, the force-frequency relationship, maximum specific force and rate of fatigue and post-fatigue recovery were compared in soleus and EDL muscles from PAR-2 null and PAR-2 Wt mice. All values are expressed as means ± SEM.

The absence of PAR-2 in EDL muscles resulted in a 44% reduction in the time to fatigue compared to Wt controls (time taken to reach ∼20% of pre-fatigue force; null: 475.0 ± 63.18 s; Wt: 269.0 ± 37.02 s, P < 0.05). However, the twitch time to peak, maximum rate of force development and half relaxation times, were not significantly different in the EDL muscles of PAR-2 null and Wt control, as were the force-frequency relationship, maximum specific force output and post-fatigue recovery rate. No significant differences in the contractile properties of soleus muscles were found between the PAR-2 null and Wt mice.

These findings indicate that the absence of PAR-2 receptors significantly alters the contractile function of fast-twitch skeletal muscle. The marked increase in fatigue resistance, in the absence of changes in contractile and relaxation times in the PAR-2 null EDL muscles, suggests that the absence of PAR-2 receptors may alter fatigue resistance through alteration of cellular metabolic components rather than a fibre type transition.

Chinni C, de Niese MR, Jenkins AL, Pike RN, Bottomley SP & Mackie EJ. (2000) Journal of Cell Science 113, 4427-33.

Jenkins AL, Chinni C, De Niese MR, Blackhart B & Mackie EJ. (2000) Developmental Dynamics 218, 465-71.

Ossovskaya VS & Bunnett NW. (2004) Physiological Reviews 84, 579-621.