Force generation simplified. Insights from laser temperature-jump experiments on contracting muscle fibers

Raising the temperature of a maximally Ca(2+)-activated muscle fiber causes a sigmoidal increase in tension. The kinetics that govern this process can be explored by step-heating the fiber a few degrees with a laser temperature-jump. A biexponential increase in tension results; a third exponential phase that opposes this biphasic rise in tension is only observed when phosphate, a reaction product normally at low concentration, is added to the fiber. This chapter explains how the temperature dependencies of isometric tension and the temperature jump kinetics interrelate, and how these insights have modified and simplified our understanding of current mechanisms of force generation. The fast kinetic phase of the tension rise appears associated with single-step force generation or a power stroke, a process largely isolated from adjacent steps in the crossbridge cycle. The amplitude of the slow phase of the tension rise exhibits a remarkable approximately 1:1 ratio to the amplitude of the fast, tension generating phase above 10 degrees C. The similarity of these two amplitudes, that combine to give the complete rise in isometric tension with temperature, appear to fit a model in which one of a pair of myosin heads generates force while the second head is poised to function after the power stroke of the first has occurred. The phase with the negative amplitude seen with added phosphate points to a mechanism in which phosphate release is indirectly linked to the tension generation by forward flow through the crossbridge cycle to tension generation.