Dynamic stiffness of cat heart muscle in Ba2+-induced contracture.

We analyzed mechanical properties of kitten papillary muscles both at rest and in Ba2+ contracture by the frequency response method. The muscle length was perturbed sinusoidally, with an amplitude less than 0.3% of Lmax over a frequency range from 0.1 to 60 Hz to determine the dynamic stiffness, F(omega)/L(omega), in which F(omega) = amplitude of the force response wave, L(omega) = amplitude of sinusoidal length wave, and omega = frequency, and the phase shift of F(omega) relative to L(omega). 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 degrees 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 showed a small lag between 0.3 and 0.5 Hz, but a clear lead of up to 60 degrees between 0.8 and 16 Hz. With an increase in temperature to 36 degrees 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 previously proposed for a twitching papillary muscle.