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

Abstract

This study tested the effects of free Ca++ on both the small-amplitude mechanical behaviour (dynamic stiffness and phase between 1 and 500 Hz) and the large-scale filament-sliding behaviour (Vmax) of single fibres of chemically-activated glycerol-extracted rabbit psoas muscle. Small-amplitude vibrations (0.1% peak-to-peak of initial length L0) were used to elicit fw, the frequency for maximum oscillatory work-production per cycle. The unloaded contraction velocity Vmax was measured during the same contractions, using the slack-test method with shortening length steps of up to 10% L0. These produced nonlinear length-time plots demonstrating that the unloaded contraction velocity was not constant as contraction progressed, but fell with time. This behaviour was approximated by two velocities, V1 the velocity observed for about the first 15 ms and V2 the velocity after this time. V1 and V2 were found to have different sensitivities to Ca++. The value of V2 fell as the level of [Ca++] was reduced, and was linearly proportional to the active tension over the range 0.2 Pmax to Pmax (where Pmax is the isometric tension for saturating amounts of Ca++). In contrast to this V1 remained insensitive to changes in [Ca++] for levels of activation corresponding to active tensions ranging from Pmax to 0.6 Pmax, and then fell as the level of activation was further reduced. It was found that the level of [Ca++] did not affect the magnitude of fw over the range of concentrations yielding active tensions from 0.2 Pmax to Pmax. These results are discussed in terms of the kinetic processes underlying transient readjustments to perturbations from isometric equilibrium.

Strong binding of myosin increases shortening velocity of rabbit skinned skeletal muscle fibres at low levels of Ca2+

“…Moss, 1986) and other laboratories (e.g. Farrow et al 1988). As reported previously, slack-test plots obtained at submaximal concentrations of Ca 2+ were biphasic (see control data in Fig.…”

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In mammalian skinned muscle fibres, several investigators have found that unloaded shortening velocity (V o ) decreases when Ca 2+ concentration is reduced to levels that induce submaximal isometric tensions (Julian, 1971;Julian et al 1986;Moss, 1986;Farrow et al 1988;Martyn et al 1994;Metzger, 1996). At saturating [Ca 2+ ], V o is maximal and invariant throughout the time course of shortening.

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The stiffness and phase frequency response of taenia coli smooth muscle: Comparison of the step and sinusoidal perturbation analysis

“…As it is seen, the dip frequency for skeletal or cardiac muscle is at least three orders of magnitude higher than what we obtained for the taenia coli. Since a negative phase in the spectrum indicates that the cross-bridges are generating energy and negative phase reaches maximum around the dip frequency, this frequency is considered to be related to the cross-bridge cycle [9,28,32]. It is well-known that smooth muscles have lower V max [33,34].…”

“…This method is also used to find the differences in the contraction mechanisms of fast and slow muscles [8,18,22]. In skeletal and cardiac muscles, the frequency responses are obtained by changing the length of the muscle in the form of sinusoidal function [8][9][10][23][24][25][26] or pseudo-random binary noise (PRBN) [27][28][29]. Since analysis by means of the sine waves takes longer time, Brozovich and his colleagues developed a length perturbation sequence that consisted of multiple sine waves with increasing frequency and they applied this length perturbation to smooth muscle [16,18].…”

Contractile properties of skeletal muscle fibers in relation to myofibrillar protein isoforms

“…When a muscle is shortening at Vmax, the proportions of cross-bridges with strains greater than 1.0 and those with cross-bridge strains less than 1.0 would be equal, and external force developed by the muscle would be zero. More recent work (Moss 1986;Farrow et al 1988) clearly shows that [Ca 2+] has a profound effect on Vmax and also provides an explanation for the earlier discrepancies. At levels of Ca 2+ that were saturating with respect to isometric tension, shortening velocity is high and is constant during the period of shortening (Fig.…”