Activation heat and latency relaxation in relation to calcium movement in skeletal and cardiac muscle

Measurements of activation heat, initial heat, twitch tension, and latency relaxation were made using thin-layer, vacuum-deposited thermopiles and isometric force transducers, respectively. Experiments were performed on frog skeletal muscle fiber bundles and on rabbit right ventricular papillary muscles at 0, 15, and 21 °C in normal and 1.75× to 2.5× mannitol hyperosmotic bathing solutions. In skeletal muscle, activation heat, obtained by stretching to zero overlap, was only slightly affected by 1.75× hyperosmotic solution and consisted of a fast and a slow component. Both components have a refractory period and a relatively refractory period which can be demonstrated by double pulse stimulation. The twitch potentiators Zn2+ and caffeine increase the total activation heat and the magnitude and rate of the fast component. The temporal relation between the latency relaxation and activation heat is demonstrated. The latency relaxation is independent of the number of sarcomeres in series in a muscle. Activation heat and latency relaxation records from heart muscle are obtained in 2.5× hyperosmotic bathing solution. A model of excitation–contraction coupling is presented which indicates that (1) the downstroke of the latency relaxation monitors the functioning of the Ca2+ -permeability or debinding mechanism in the terminal cisternae, (2) the fast component of activation heat monitors the amount of Ca2+ bound to troponin C, and (3) the total amplitude of activation heat is a measure of the total quantity of Ca2+ cycled in a twitch.