Calcium Ions Modulate Regulation of Smooth Muscle Contraction Mediated by Phosphorylation of Myosin Regulatory Light Chains

S. V. Avrova¹, Yu. S. Borovikov^1*, N. N. Efimova¹, K. Y. Horiuchi², and S. Chacko²

¹Institute of Cytology, Russian Academy of Sciences, Tikhoretskii pr. 4, St. Petersburg, 194064 Russia; fax: (812) 247-0341; E-mail: boroviko@mail.cytspb.rssi.ru

²Department of Pathobiology, University of Pennsylvania, 3800 Spruce Street, Philadelphia, PA 19104, USA; fax: (215) 898-0719

^* To whom correspondence should be addressed.

Received March 25, 1998; Revision received September 14, 1998

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The effect of calcium ions on conformational changes of F-actin initiated by decoration of thin filaments with phosphorylated and dephosphorylated heavy meromyosin from smooth muscles was studied by fluorescence polarization spectroscopy. It is shown that heavy meromyosin with phosphorylated regulatory light chains (pHMM) promotes structural changes of F-actin which are typical for the "strong" binding of actin to the myosin heads. Heavy meromyosin with dephosphorylated regulatory light chains (dpHMM) causes conformational changes of F-actin which are typical for the "weak" binding of actin to the myosin heads. The presence of calcium enhances the pHMM effect and attenuates the dpHMM effect. We propose that a Ca²⁺-dependent mechanism exists in smooth muscles which modulates the regulation of actin--myosin interaction occurring via phosphorylation of myosin regulatory light chains.

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KEY WORDS: smooth muscle, regulation by calcium ions, actin conformation, "weak" binding, "strong" binding, fluorescence polarization

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