Viscoelastic properties of cross bridges in cardiac muscle

Abstract

Tension responses of rat right ventricular trabeculae to fast length changes are measured with microsecond resolution to obtain information about elastic properties of ventricular myocardium. Responses of these isometrically mounted trabeculae at 22 degrees C to fast length changes completed within 30 microseconds at 22 degrees C to fast length changes completed within 30 microseconds were similar in shape to those of skeletal muscle fibers. Results of quantitative evaluation of responses are interpreted in terms of cross-bridge properties. An upper bound for the elastic range of cross bridges in trabeculae, derived from the maximal developed force during Ca2+ activation and from stiffness in rigor, has been estimated as 8.4 +/- 2.2 nm. Their working stroke, estimated from the tension loss in the rigor state due to a shortening and from tension remaining after (partial) recovery, was 20 +/- 4 nm. The estimated working stroke of cross bridges is about three times larger in trabeculae than in freeze-dried skeletal muscle fibers of the frog at 4 degrees C, which points to important differences between cross-bridge mechanisms of contraction in cardiac and skeletal muscle.

Initial mechanical efficiency of isolated cardiac muscle

“…However, other experimental evidence has been interpreted as indicating that the crossbridge stroke in cardiac muscle may be as great as 20-30·nm (see De Winkel et al, 1995 and references therein). For instance, De Winkel et al (1995) calculated cross-bridge working stroke from the dynamic stiffness of skinned rat trabelculae (22°C) and concluded that it was at least 20·nm. It has been suggested that these values are consistent with high efficiency values, such as those reported by Peterson and Alpert (1991).…”

High frequency characteristics of elasticity of skeletal muscle fibres kept in relaxed and rigor state

“…According to the formula, phase shift is almost zero at frequencies (a<< a) and rises in a transition range around o = ~1 to v n 2-l rad. The empirical formula yields a better description of Young's moduli of the fibre in rigor or in the relaxed state than the five parameter model (Jung et al, 1988) and, in these cases, is almost as good as the seven parameter model used by the same authors to describe the Young's modulus of activated fibres (de Winkel et al, 1993).…”

“…Tension changes following the length change of the muscle fibre are measured by a homemade force transducer with a resonance frequency of 70 kHz in air, a damping time constant of 500 ps and a noise of 2 PV (de Winkel et al, 1993). When the fibre is attached to the transducer the resonance frequency decreases by 5%.…”

“…This empirical formula describes the complex Young's modulus of a muscle fibre with three parameters: an elastic constant E, (N m-l), a reciprocal time constant a (rad s-r), and an exponent v. These parameters are determined by optimization of similarity of calculated and measured tension transient over 2 ms (de Winkel et al, 1993). At low frequencies (O << ~1) the complex Young's modulus tends to reduce to the elastic constant (E,), while the exponent (v) represents a linear increase with frequency in the log-log plot for frequencies o >> ~1.…”

“…Substitution of k by means of Equations (2) in (1) leads to an expression for the transfer function in terms of frequency and elastic impedance. By an iterative approximation, in which the parameters of the Young's modulus, found by simulation, are used as starting-points, the elastic impedance (complex Young's modulus) can be determined separately for each frequency from the measured recordings by insertion of T and E in left part of Equation (1) (de Winkel et al, 1993). The performance of the experimental apparatus was regularly checked by similar determination of the Young's modulus of a silk fibre, which is almost purely elastic (see de Winkel et al, 1993).…”