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The contractile properties of slow and fast skeletal muscles from protease activated receptor-1 null mice

K. Sitparan,1 G.J. Pinniger,1 C.N. Pagel,2 E.J. Mackie2 and A.J. Bakker,1 1School of Biomedical, Biomolecular and Chemical sciences, University of Westrn Australia, Crawley, WA 6009, Australia. and 2School of Veterinary Science, The University of Melbourne, Parkville, VIC 3052, Australia..

Protease activated receptors (PARs) are G-protein-coupled receptors that are activated by proteolytic cleavage. During early skeletal muscle development, muscle fibres receive multiple motor-innervation. These multiple neural connections are eventually lost during the postnatal period through a process known as polyneuronal synapse elimination. PAR-1 receptors are thought to play a role in this process, and therefore, PAR-1 activation could affect motor unit distribution within a muscle and alter the contractile properties. The aim of this study was to compare the contractile properties of slow and fast skeletal muscles from PAR-1 null (PAR-1 KO) and control mice.

Experiments were conducted on 12-14 weeks old PAR-1 null (KO) (n=5) and wildtype (WT) littermate mice (n=4). Approval for this study was obtained from the animal ethics committee of the University of Western Australia. Mice were anaesthetized by intraperitoneal injection of pentobarbitone (40mg/kg body weight). The soleus (mainly slow twitch) and extensor digitorum longus (EDL) (fast twitch) muscles of left hind limb were surgically removed and connected to a force transducer system. The muscles were maintained in Krebs mammalian Ringer solution (pH 7.3) bubbled with Carbogen (95% O2 and 5% CO2) at 25°C. Twitch force characteristics, the force frequency relationship, maximum specific force (force normalized to muscle size) were determined, and the rate of fatigue and post-fatigue recovery were assessed.

The results showed that the absence of PAR-1 receptors in soleus muscles resulted in a significantly greater mean peak twitch force (KO: 6.2 ± 0.231, WT: 4.6 ± 0.392 N.cm2, P<0.05) and mean twitch time to peak values (KO: 57.0 ± 3.0 ms, WT: 41.0 ± 1.0 ms, P<0.05) and produced a significant shift of the force-frequency curve to the left. In addition, the soleus muscles from the PAR-1 null mice fatigued significantly more slowly (P<0.05) and exhibited a significantly faster post fatigue recovery (P<0.05) than muscles from control mice. There was no difference in maximum specific force between soleus muscles from PAR-1 KO and WT mice. EDL muscles from PAR-1 KO mice exhibited a small but significant decrease in the rate of fatigue compared to EDL muscles from control mice.

The results of this study indicate that the absence of PAR-1 receptors significantly alters the contractile properties of skeletal muscle, especially in muscles with a predominance of slow twitch fibres. The effects of the absence of PAR-1 receptors is consistent with a fibre type transition to a slower phenotype.