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1 Cardiovascular Division, Department of Internal Medicine, University of Virginia, Charlottesville, VA, USA; Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, VA, USA
2 Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, VA, USA
3 Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, VA, USA; Cardiovascular Division, Department of Internal Medicine, University of Virginia, Charlottesville, VA, USA
* To whom correspondence should be addressed. E-mail: crembold{at}virginia.edu.
Serine19 phosphorylation of the myosin regulatory light chain (MRLC) appears to be the primary determinant of smooth muscle force development. The relationship between MRLC phosphorylation and force is non-linear showing that phosphorylation is not a simple switch regulating the number of cycling crossbridges. We reexamined the MRLC phosphorylation-force relationship in slow, tonic swine carotid media; fast, phasic rabbit urinary bladder detrussor; and very fast, tonic rat anococcygeus. We found a sigmoidal dependence of force on MRLC phosphorylation in all three tissues with a threshold for force development of ~ 0.15 mol Pi/mol MRLC. This behavior suggests that force is regulated in a highly cooperative manner. We then determined if a model that employs both the latchbridge hypothesis and cooperative activation could reproduce the relation between ser19-MRLC phosphorylation and force without the need for a second regulatory system. We based this model on skeletal muscle in which attached crossbridges cooperatively activate thin filaments to facilitate crossbridge attachment. We find that such a model describes both the steady-state and time course relation between ser19-MRLC phosphorylation and force. The model required both cooperative activation and latchbridge formation to predict force. The best fit of the model occurred when binding of a crossbridge cooperatively activated 7 myosin binding sites on the thin filament. This result suggests cooperative mechanisms analogous to skeletal muscle that will require testing.
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