Am J Physiol Cell Physiol AJP: Gastrointestinal and Liver Physiology
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Am J Physiol Cell Physiol 275: C684-C692, 1998;
0363-6143/98 $5.00
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Vol. 275, Issue 3, C684-C692, September 1998

Differences in contractile protein content and isoforms in phasic and tonic smooth muscles

Pawel T. Szymanski1, Thomas K. Chacko1, Arthur S. Rovner2, and Raj K. Goyal1

1 Center for Swallowing and Motility Disorders, Harvard Medical School, West Roxbury Veterans Affairs Medical Center, West Roxbury, Massachusetts 02132; and 2 Department of Molecular Physiology and Biophysics, College of Medicine, University of Vermont, Burlington, Vermont 05405-0068

The basis of tonic vs. phasic contractile phenotypes of visceral smooth muscles is poorly understood. We used gel electrophoresis and quantitative scanning densitometry to measure the content and isoform composition of contractile proteins in opossum lower esophageal sphincter (LES), to represent tonic muscle, and circular muscle of the esophageal body (EB), to represent phasic smooth muscle. The amount of protein in these two types of muscles is similar: ~27 mg/g of frozen tissue. There is no difference in the relative proportion of myosin, actin, calponin, and tropomyosin in the two muscle types. However, the EB contains ~2.4-times more caldesmon than the LES. The relative ratios of alpha - to gamma -contractile isoforms of actin are 0.9 in the LES and 0.3 in EB. The ratio between acidic (LC17a) and basic (LC17b) isoforms of the 17-kDa essential light chain of myosin is 0.7:1 in the LES, compared with 2.7:1 in the EB. There is no significant difference in the ratios of smooth muscle myosin SM1 and SM2 isoforms in the two muscle types. The level of the myosin heavy chain isoform, which contains the seven-amino acid insert in the myosin head, is about threefold higher in the EB compared with LES. In conclusion, the esophageal phasic muscle in contrast to the tonic LES contains proportionally more caldesmon, LC17a, and seven-amino acid-inserted myosin and proportionally less alpha -actin. These differences may provide a basis for functional differences between tonic and phasic smooth muscles.

lower esophageal sphincter; esophageal smooth muscle; visceral smooth muscle; actin; myosin; calponin; caldesmon; tropomyosin; opossum


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