The Rising Phase of the Active State in Single Skeletal Muscle Fibres of the Frog

Abstract

An approach of analyzing the initial phase of the active state in single skeletal muscle fibres of the frog has been designed. The fibre was stimulated to generate a series of four incompletely fused isometric twitches with the object of producing distinct peaks and throughs in the myo‐gram. The peaks and troughs of the records both represent the tension output when the contractile unit was stationary. The tension recorded at these instances therefore provided an adequate measure of the intensity of the active state at respective times after the stimulus. By releasing the fibre at different moments during the third cycle it was possible to define 1. the decay phase of the active state in the third cycle and 2. a substantial portion of the rising phase of the active state in the fourth cycle. It was found that the active state underwent a very rapid increase starting at 12 msec after the stimulus (at 1–2° C). Only 3–4 msec were required for the active state to rise from 25 to 65 per cent of its peak value. The rate of rise of the active state was independent of the sarcomere length (between 2.8 μ and 2.0 μ) and of the degree of activity existing in the fibre when the fourth stimulus was applied. The implications of the results with respect to the kinetics of the calcium‐activator mechanism in the excitation‐contraction coupling are discussed.