Components of activation heat in skeletal muscle

Activation heat (qA) production by muscle is the thermal accompaniment of the release of Ca2+ from the sarcoplasmic reticulum (SR) into the cytoplasm, its interactions with regulatory proteins and other cytoplasmic Ca2+ buffers and its return to the SR. The contribution of different Ca2+-related reactions to qA is difficult to determine empirically and therefore, for this study, a mathematical model was developed to describe Ca2+ movements and accompanying thermal changes in muscle fibres in response to stimulation. The major sources of heat within a few milliseconds of the initiation of Ca2+ release are Ca2+ binding to Tn and Pv. Ca2+ binding to ATP produces a relatively small amount of heat. Ca2+ dissociation from ATP and Tn, with heat absorption, are of similar time course to the decline of force. In muscle lacking Pv (e.g. mouse soleus), Ca2+ is then rapidly pumped into the SR. In muscles with Pv, Ca2+ that dissociates from Tn and ATP binds to Pv and then dissociates slowly (over 10 s of seconds) and is then pumped into the SR; the net effect of these two processes is heat absorption. It is proposed that this underlies Hill’s “negative delayed heat”. After all the Ca2+ is returned to the SR, qA is proportional to the amount of Ca2+ released into the cytoplasm. In muscles with Pv this is 20–60 s after Ca2+ release; in muscles without Pv, all Ca2+ is returned to the SR soon after the end of force relaxation.