Dynamic Stiffness of Cat Heart Muscle in Ba-Induced Contracting

We analyzed mechanical properties of kitten papillary muscles both at rest and in Bi + contractore by the frequency response method. Hie mosde length was perturbed sinusoidal)}, with an amplitude less than 0.3% of Lou, over a frequency range from 0.1 to 60 Hz to determine the dynamic stiffness, F(a>)/L(a>), in which F(a>) = amplitude of the force response wave, L(to) = amplitude of sinusoidal length wave, and a> = frequency, and the phase shift of F(<u) relative to L(<u). In resting muscles, the dynamic stiffness increased minutely with increasing frequency and the phase relation showed a small lead over the entire frequency range. In muscles in contracture at low temperature (22-24°C), the stiffness first decreased with increasing frequency from about 0.2 to 1 Hz, then increased with a slope of 10-fold/decade, and finally plateaued over the range above 8 Hz. The phase relation snowed a small lag between 0.3 and 0.5 Hz, but a clear lead of up to 60° between 0.8 and 16 Hz. With an increase in temperature to 36°C, the peculiar decrease in stiffness and the phase lag in the low frequency region decreased in size and shifted to a higher frequency region (about 4 Hz). These findings led us to two alternative, approximate analogues, which are similar to but simpler than that previoudy proposed for a twitching papillary muscle.