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Differences in contractile protein content and isoforms in phasic and tonic smooth muscles
P T Szymanski1, T K Chacko, A S Rovner
1Center for Swallowing and Motility Disorders, Harvard Medical School, West Roxbury Veterans Affairs Medical Center, West Roxbury, Massachusetts 02132, USA.
The American Journal of Physiology
|September 9, 1998
Summary
Visceral smooth muscle differences in tonic and phasic contractile proteins were investigated. Phasic esophageal body muscle has more caldesmon and specific myosin variants than tonic lower esophageal sphincter muscle, explaining functional variations.
Area of Science:
- Gastroenterology
- Muscle Physiology
- Biochemistry
Background:
- Tonic and phasic contractile phenotypes in visceral smooth muscles are not well understood.
- Contractile protein differences may underlie functional variations between muscle types.
Purpose of the Study:
- To compare contractile protein content and isoform composition between tonic (opossum lower esophageal sphincter) and phasic (esophageal body circular muscle) smooth muscles.
- To identify molecular distinctions that may explain the different contractile behaviors.
Main Methods:
- Gel electrophoresis and quantitative scanning densitometry were employed.
- Contractile proteins were measured in opossum lower esophageal sphincter (LES) and esophageal body (EB) circular muscle tissues.
Main Results:
- Esophageal body (phasic) muscle exhibits higher caldesmon content (2.4-fold) compared to the lower esophageal sphincter (tonic).
- Phasic muscle has a lower ratio of alpha- to gamma-actin and a higher ratio of acidic (LC17a) to basic (LC17b) myosin light chain isoforms.
- Phasic muscle shows a threefold higher level of a specific myosin heavy chain isoform with a seven-amino acid insert.
Conclusions:
- Significant differences in caldesmon, actin isoforms, myosin light chain isoforms, and myosin heavy chain isoforms exist between tonic and phasic visceral smooth muscles.
- These molecular distinctions provide a potential basis for the observed functional differences in contractility between the lower esophageal sphincter and esophageal body muscle.