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Force maintenance in smooth muscle: analysis using sinusoidal perturbations.

Albert Y Rhee1, Frank V Brozovich

  • 1Department of Physiology and Biophysics, Case Western Reserve University School of Medicine, 10900 Euclid Avenue, Cleveland, OH 44106-4970, USA.

Archives of Biochemistry and Biophysics
|February 1, 2003
PubMed
Summary

This study explored how smooth muscle maintains force without continuous energy input, a phenomenon known as the latch state. Researchers used rabbit portal vein and aorta strips and applied two activation methods: KCl depolarization and phenylephrine stimulation. They measured stiffness by oscillating muscle length and analyzing the force response at different frequencies. KCl caused a shift in stiffness toward lower frequencies, suggesting slower cross-bridge cycling during force maintenance. Phenylephrine did not show a significant shift, indicating no major change in cycling rate. The results suggest that the latch state may depend on the activation method used, with KCl-induced force maintenance involving slower cycling rates.

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Area of Science:

  • Smooth muscle physiology
  • Muscle mechanics
  • Pharmacological effects on muscle

Background:

Smooth muscle can maintain force without continuous energy input, a phenomenon known as the latch state. Prior research has shown that this state may involve dephosphorylated cross-bridges or slower cycling rates. However, the exact mechanism remains unclear. No prior work had resolved whether latch state is due to cross-bridge cycling or non-cross-bridge factors. This uncertainty drove the need for a study using dynamic mechanical testing. Researchers wanted to distinguish between different hypotheses about how force is maintained. They focused on stiffness changes during oscillatory stimulation. The study aimed to clarify whether latch state involves altered cross-bridge cycling rates. By analyzing stiffness profiles, the authors sought to determine the role of cycling rate in force maintenance.

Purpose Of The Study:

The study aimed to investigate the mechanism of force maintenance in smooth muscle. Specifically, it sought to determine whether latch state arises from slower cross-bridge cycling or non-cross-bridge contributions. The researchers used sinusoidal perturbations to measure stiffness changes in smooth muscle strips. They tested two activation methods: KCl depolarization and phenylephrine stimulation. The goal was to compare how each method affected stiffness distribution profiles. By analyzing frequency-dependent stiffness, the authors aimed to identify cycling rate changes. The study focused on rabbit portal vein and aorta as model systems. The results were intended to clarify the role of cross-bridge cycling in latch state.

Keywords:
Smooth muscle mechanicsSinusoidal perturbationForce maintenanceMuscle stiffness analysis

Frequently Asked Questions

The study found that KCl depolarization caused a shift in stiffness toward lower frequencies, suggesting slower cross-bridge cycling during force maintenance.

Stiffness was calculated from the force response to small sine-wave oscillations in muscle length at frequencies between 1 and 100 Hz.

The activation method influenced stiffness changes: KCl caused a frequency shift, while phenylephrine did not, suggesting different mechanisms for force maintenance.

A shift toward lower frequencies suggests a general slowing of cross-bridge cycling during force maintenance, as observed with KCl depolarization.

Related Experiment Videos

Main Methods:

The researchers used intact rabbit portal vein and aorta strips for their experiments. Muscle strips were activated using KCl depolarization or phenylephrine stimulation. KCl caused sustained MLC(20) phosphorylation, while phenylephrine led to transient phosphorylation. Stiffness was measured using small sine-wave length oscillations (1-100 Hz). The force response was recorded to calculate stiffness at each frequency. A 3D plot of stiffness versus frequency and time was generated. The stiffness distribution profile was analyzed for shifts in frequency. The study compared stiffness changes between the two activation methods.

Main Results:

KCl depolarization caused a shift in the stiffness distribution profile toward lower frequencies. This suggests a general slowing of cross-bridge cycling during force maintenance. The stiffness values decreased at higher frequencies during KCl activation. Phenylephrine stimulation did not produce a significant shift in the stiffness profile. Stiffness remained relatively unchanged across frequencies during phenylephrine activation. The results indicate that KCl affects cross-bridge cycling rates. Phenylephrine appears to maintain force without altering cycling rates. The 3D stiffness plots revealed distinct patterns for each activation method. These findings support the hypothesis that latch state involves slower cycling rates.

Conclusions:

The study suggests that force maintenance in smooth muscle may involve a slower cross-bridge cycling rate. KCl depolarization led to a stiffness shift toward lower frequencies, consistent with this hypothesis. Phenylephrine stimulation did not show a similar shift, indicating no significant cycling rate change. The authors propose that latch state may depend on the activation method used. KCl-induced force maintenance may rely on slower cycling, while phenylephrine does not. These findings support the idea that latch state is not a universal phenomenon. The study highlights the importance of activation method in determining stiffness changes. The results suggest that cross-bridge cycling rate is a key factor in force maintenance.

The 3D plot showed stiffness changes over time and frequency, revealing distinct patterns for KCl and phenylephrine activation methods.

The authors propose that latch state may depend on activation method, with KCl-induced force maintenance involving slower cross-bridge cycling.