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Articles in PresS, published online ahead of print September 5, 2001
Am J Physiol Cell Physiol, 10.1152/ajpcell.00257.2001
Submitted on June 8, 2001
Accepted on August 24, 2001
1 Center for Swallowing and Motility Disorders, VA Medical Center, West Roxbury, MA, USA
2 Department of Biology, Boston College, Boston, MA, USA
* To whom correspondence should be addressed. E-mail: raj_goyal{at}hms.harvard.edu.
In order to determine if densities of calmodulin (CaM) and CaM binding proteins are related to phasic and tonic behavior of smooth muscles, studies were performed to quantify these proteins in the opossum esophageal body (EB) and lower esophageal sphincter (LES), that represent phasic and tonic smooth muscles, respectively. Combination of gel electrophoresis, immunoprecipitation, Western blot, hemagglutinin epitope-tagged calmodulin (HA-CaM) overlay assay together with quantitative scanning densitometry and phosphorylation measurements were employed. Total protein contents in the two smooth muscles was similar (~30mg protein/g frozen tissue). Total tissue concentration of CaM was significantly (25%) higher in the EB than in LES (p<0.05). HA-CaM binding proteins were qualitatively similar in LES and EB extracts. Contents of myosin, myristoylated alanine-rich C kinase substrate (MARCKS) protein, Ca2+/CaM kinase II and calponin (CaP) were also similar in the two muscles. However, content and total activity of myosin light chain kinase (MLCK) and content of caldesmon (CaD) were 3-4 fold higher in EB than LES. Increased contents of CaM and MLCK may allow for a wide range of contractile force varying from complete relaxation in the basal state to a large amplitude, high velocity contraction in phasic muscle of EB. Increased content of CaD, that provides a breaking mechanism on contraction, may further contribute to the phasic contractile behavior. In contrast, low content of CaM, MLCK and CaD may be responsible for a small range of contractile force that is seen in tonic muscle of LES.
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