Effect of Ca ion concentration on cross-bridge kinetics in rabbit psoas fibers. Evidence for the presence of two Ca-activated states of thin filament

Abstract

The effect of Ca ion concentration on cross-bridge kinetics in a small bundle (one to three fibers) of chemically skinned rabbit psoas muscle is studied. The length of the muscle is oscillated in small amplitude sine waves (0.2% L0 peak-to-peak) at varying frequencies (0.125 -- 167 Hz), and the resulting amplitude and phase shift in tension are measured. The frequency response function (complex stiffness) thus obtained can be divided into three parts, which we name process (A) (centered at 1 Hz), process (B) (3--17 Hz), and process (C) (50 Hz). Process (B), which represents oscillatory work, further splits into two processes (B' and B) at partial Ca activation (less than 50% P0), where the phase-frequency plot appears W-shaped. The slower of the two processes (B') disappears by full activation, at which time the plot appears V-shaped. The characteristic frequencies associated with the minima of the plot do not shift in a graded way with Ca concentration, indicating that there is no change in apparent rate constants. Apparent rate constants of processes (A) and (C) are minimally affected by Ca. The above results are not altered when ionic strength is changed between 128 and 265 mM. We propose that activated thin filaments can have two "on" states and that Ca concentration controls the distribution of these two states. This mechanism generally supports the "switch" hypothesis of Ca regulation.

The role of calcium ions in the activation of rabbit psoas muscle

“…The form of these curves is in agreement with those of Kawai's group (e.g. Kawai & Brandt, 1980;Kawai et al, 1981) who applied sinusoidal length changes at discrete frequencies.…”

“…The other approach involved studies in the frequency domain, where sinusoidal vibrations at a number of frequencies have been applied to the muscle (e.g. Machin & Pringle, i960;Jewell & Ruegg, 1966;Thorson & White, 1969Abbott, 1973a, b;Rossmanith et al, 1980;Cuminetti & Rossmanith, 1980;Kawai & Brandt, 1980;Kawai et al, 1981;Wilson & White, 1983;Loxdale & Tregear, 1985), in order to obtain spectra of stiffness and phase, permitting the deduction of parameters such as fw, the frequency for maximum oscillatory work per cycle. Fig.…”

“…Because this mechanical rate parameter mirrors the rate at which chemical energy can be converted into osciUatory mechanical work (Steiger & Ruegg, 1969), this finding suggests that the kinetics underlying such energy conversions is not affected by the level of [Ca ++] over this range of activations. Kawai et al (1981) have also studied the effects of varying [Ca ++] on muscle dynamics brought about by small-amplitude length oscillations of glycero!-treated rabbit psoas fibres. Their methodology differs from ours in two ways.…”

“…This quantity was selected for comparison with the rate pararneters measured by other workers from srnall-amplitude sinusoidal perturbations (e.g. Kawai et al, 1981), and frorn transient force responses resulting frorn step changes in length (e.g. Abbott & Steiger, 1977).…”

Effect of cross-bridge kinetics on apparent Ca2+ sensitivity.

“…An ionic strength decrease at least to 0.128, however, is not associated with a change in this frequency (Kawai et al, 1981), although the magnitude of oscillatory work increases. V. of shortening (Edman and Hwang, 1977) and maximum tension (April and Brandt, 1973) have been reported to increase in intact fibers bathed in hypotonic media.…”