At Physiological Temperatures the ATPase Rates of Shortening Soleus and Psoas Myofibrils Are Similar

We obtained the temperature dependences of the adenosine triphosphatase (ATPase) activities (calcium-activated and relaxed) of myofibrils from a slow muscle, which we compared with those from a fast muscle. We chose rabbit soleus and psoas because their myosin heavy chains are almost pure: isoforms I and IIX, respectively. The Arrhenius plots of the ATPases are linear (4–35°C) with energies of activation for soleus myofibrils 155 kJ mol−1 (activated) and 78 kJ mol−1 (relaxed). With psoas myofibrils, the energies of activation were 71 kJ mol−1 (activated) and 60 kJ mol−1 (relaxed). When extrapolated to 42°C the ATPase rates of the two types of myofibril were identical: 50 s−1 (activated) and 0.23 s−1 (relaxed). Whereas with psoas myofibrils the Km for adenosine triphosphate (activated ATPase) is relatively insensitive to temperature, that for soleus myofibrils increased from 0.3 μM at 4°C to 66.5 μM at 35°C. Our results illustrate the importance of temperature when comparing the mechanochemical coupling in different types of muscle. We discuss the problem of how to reconcile the similarity of the myofibrillar ATPase rates at physiological temperatures with their different mechanical properties.