Dynamic stiffness measured in central segment of excised rabbit papillary muscles during barium contracture.

Abstract

The dynamic mechanical behavior of excised rabbit papillary muscles that had been tonically activated by replacing bathing Ca2+ with Ba2+ was studied. Steady activation was used to visualize the dynamic behavior of cardiac myofilaments more clearly than is possible during twitches, which are complicated by the kinetics of excitation-contraction coupling. To avoid artifacts due to damaged ends of the muscle, the length of a central segment, which was defined by 2 tungsten pins inserted through the muscle, was measured. To test the mechanical behavior of the contractured muscles (at 24 degrees C), the central segment length was sinusoidally oscillated (amplitude 1%) at 15 different frequencies (0.05-30 Hz). The dynamic stiffness of the central segment was calculated from the ratio of force response amplitude to length perturbation amplitude. At low frequencies (below 0.4 Hz), stiffness was approximately constant and reflected the force-length relation. However, in a localized range near 1 Hz, there was a distinct drop in the magnitude of dynamic stiffness to approximately half its low-frequency baseline. This range may reflect the dynamics of attachment and detachment of force generators. The frequency of minimum stiffness was consistent among all muscles (1.3 +/- 0.3 Hz). Moreover, no significant change in this frequency was found over the examined range of lengths (90-100% of the segment length that produced maximal developed force) and activation levels (Ba2+ concentration 0.3-1.0 mM). From 2 to 8 Hz, dynamic stiffness appeared to reflect force-velocity properties, but at higher frequencies, another elastic property emerged. At 30 Hz, stiffness was proportional to force, with an apparent series elasticity less than 1.8%. Even though the muscles had only moderate longitudinal inhomogeneity, quantitatively significant (35%) errors would have been introduced had the study relied on total muscle length instead of central segment length.

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

“…The dip frequency was found at 10-13 Hz in frog sartorius muscle [23,27], around 10 Hz in rabbit psoas muscle [6,10], around 1 Hz in rabbit soleus muscle [8] and at 1-2 Hz in cardiac muscle [24][25][26]29]. 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.…”

“…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].…”

Ca-dependence of isometric force kinetics in single skinned ventricular cardiomyocytes from rats

“…Hancock et al [11,21] used ferret and rat intact cardiac muscle tetanized to mimick steady activation as well as a moderate step release (2-3% of maximal length)• While a significant difference in crossbridge behaviour of intact and skinned preparations cannot be involved [30], the moderate length release might be relevant. Indeed, as in the study using frequency analysis of stiffness [12], the low-amplitude length variations might not disrupt the force-generating crossbriges. This might be due to the fact that cardiac more than skeletal tissues contains a large amount of elastic elements [31] that will dampen length variations of strongly bound crossbridges.…”

“…In contrast, k~ was reported to be insensitive to Ca ions in intact cardiac muscle whose different levels of force were achieved by tetanization [11]. Similarly, frequency analysis of stiffness showed no effect of activation level on crossbridge kinetics [ 12,13]. More recently, a significant sensitivity of tension redevelopment to Ca ions in cardiac muscle was reported [14,15].…”

Dynamic myocardial contractile parameters from left ventricular pressure-volume measurements

“…Dynamic stiffness focuses on frequencydependent force-length relations during small length changes and is profoundly sensitive to myofilament kinetic processes (3,4,30,35,46,50,51,57,62,63,66). Importantly, the frequency-domain expression of dynamic stiffness may be easily converted into an equivalent time-domain expression that allows prediction of the transient time course of muscle force in response to muscle length perturbations.…”