Biochemical and Mechanical Properties of Murine Extraocular Muscles

Abstract

Extraocular muscles (EOMs) are among the fastest and most fatigue resistant skeletal muscles. The distinct origin and innervation of EOMs are probably responsible for their different gene and protein expression. Indeed, western blot analysis revealed that the expression of the protein isoforms forming the Excitation-Contraction Coupling Macromolecular Complex (ECC-MC) of human EOM is substantially different from human quadriceps muscles. Human EOM muscles express, in addition to the skeletal muscle isoforms of proteins involved in ECC, Cav1.2, CASQ2 and RYR3. Because of the use of genetically modified mouse models to study human diseases we thought it important to investigate the biochemical and mechanical properties of murine EOM. We found that the expression level of the ECC-MC proteins in mouse EOM is different from that of mouse hind limb muscles and from that of human EOM. We also studied the mechanical properties of mouse EOM. In the presence of 1.8 mM CaCl2 in the extracellular medium the force induced by a single action potential and by a train of action potentials delivered at 100 Hz was 40,4±35,7 µN (n=8), 359±213 µN (n=8 ), respectively . The half time to peak, the time to peak and the half relaxation times of force development evoked by a single action potential were 2.53±0.69 ms (n=8), 10.1±0.3 ms (n=8), 42.3±27.8 ms (n= 8), respectively. Addition of 100 µM La3+ to the extrcellular medium caused a mean decrease of 75 % of the peak tetanic tension. The effect of La3+ was reversed by washing the muscle with Tyrode's solution containing 1.8 mM CaCl2 This study defines the biochemical and mechanical properties of preparations of mouse EOM under normal conditions and will be used as reference to investigate the phenotype of EOM from mice carrying modifications of genes encoding key proteins encompassing the ECC-MC.