Force maintenance in smooth muscle: Analysis using sinusoidal perturbations

The "latch state" or force maintenance may be due to the emergence of a distinct set of dephosphorylated, slowly cycling "latch" cross-bridges, slowing of the overall cross-bridge cycling rate, or a non-cross-bridge contribution. This was investigated by sinusoidally oscillating strips of intact rabbit portal vein or aorta. Tissue strips were activated with KCl depolarization, resulting in a sustained increase of MLC₂₀ phosphorylation or 10μM phenylephrine, resulting in a transient increase in MLC₂₀ phosphorylation. Stiffness was calculated from the force response to a small, sine-wave oscillation in muscle length (1-100Hz). The results produced a 3-dimensional plot of stiffness versus the frequency of oscillation (Hz) versus time (s), or stiffness distribution profile. During KCl depolarization, the stiffness distribution profile displayed a shift toward lower frequencies, suggesting a general slowing in the overall cross-bridge cycling rate during force maintenance. On the other hand, phenylephrine stimulation did not display a significant change in the stiffness distribution profile, suggesting that the overall cross-bridge cycling rate did not significantly change during force maintenance.