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Conference: Bucharest University Faculty of Physics 2005 Meeting

Section: Electricity and Biophysics

Title:

ATPase Rates of Shortening Soleus and Psoas Myofibrils are Similar at Physiological Temperatures

Authors:

Bogdan Iorga*, Corinne Lionne***, Robin Candau**, Tom Barman*** and Franck Travers***

Affiliation:

* Department of Vegetative Physiology, Faculty of Medicine, University of Cologne, Robert-Koch Str. 39, 50931 Köln, GERMANY (present address); Department of Physics, Faculty of Chemistry, University of Bucharest, Regina Elisabeta 4-12, 70346 Bucharest, ROMANIA (permanent address).

** EA 3759, Institut de Biologie, Université Montpellier I, 4 bd Henri IV (CS 89508), 34960 Montpellier cedex 2, FRANCE

*** UMR 5121, CNRS, Université Montpellier I, Institut de Biologie, 4 bd Henri IV (CS 89508), 34960 Montpellier cedex 2, FRANCE

E-mail

bogdan.iorga@uni-koeln.de

Keywords:

myofibrils, skeletal muscle, kinetics, Rapid-Flow-Quench

Abstract:

A key problem in muscle contraction is to relate the mechanical events to the actin-activated myosin ATPase pathway. Here, we have studied chemical kinetics of the skeletal myofibrils. We have chosen rabbit soleus and psoas myofibrils, because they are almost pure in terms of MHC isoforms composition: 97% of MHC-I (slow isoform) and 92% of MHC-IIX (fast isoform), respectively. The rates of myofibrillar ATPases are studied under activating (+Ca2+) and relaxing (-Ca2+) conditions using a Rapid-Flow-Quench apparatus. The steady state kinetics of the soleus and psoas myofibril ATPases (activated, kF, and relaxed, kss) were studied in the temperature range 4 – 35°C. At 4°C, ATPase rates with psoas myofibrils are larger than those with soleus: by a factor of about 2.7 in the relaxed, and about 80 in the Ca2+-activated conditions. Arrhenius plots reveal that kF and kss with soleus myofibrils are more sensitive to the temperature than those with psoas myofibrils: the energies of activation of kF with soleus and psoas myofibrils are 155 and 71 kJ/mol, respectively, and of kss are 78 and 60 kJ/mol, respectively. At higher temperatures, the difference between the myofibrillar ATPase rates of the two types of myofibrils decreases dramatically. When extrapolated to the physiological temperature of the rabbit (~39°C), the ATPase rates with soleus and psoas myofibrils are similar, whether relaxed or activated. When extrapolated to 42°C (characteristic to muscles during intense and prolonged muscular activity) the rates are identical. The identity of the steady state rates at 42°C does not mean that the relative amounts of the different intermediates on the reaction ATPase pathways are identical with the soleus and psoas myofibrils. These results are presented in terms of mechanochemical coupling and an ATPase kinetic model is proposed in order to accommodate both myofibrillar mechanical and chemical data. Acknowledgements: This work was supported by INSERM and European Union (HPRN-CT-2000-00091).