Phosphorylation and thiophosphorylation of myosin light chains in vertebrate smooth muscle: Different relationships with tension

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Physiology and Biophysics

First Committee Member

W. G. L. Kerrick, Committee Chair


A chemically skinned preparation of chicken gizzard was used to examine the relationship between steady state isometric tension and myosin light chain phosphorylation. Tension was found to be steeply dependent on myosin light chain phosphorylation at 1.0 to 0.1 mM free Mg$\sp{2+}$. Phosphorylation of roughly twenty percent of the total number of myosin light chains led to the generation of maximum tension in agreement with earlier reports (Hoar et al. Science (204):503-506, 1979). The generation of high levels of tension by low levels of light chain phosphorylation in low (0.1 mM) free Mg$\sp{2+}$ solutions ruled out the possibility that the observed high tensions were due to the build up of MgADP$\sp{-}$ in these fibers. Changing the composition of the contracting solution increased the amount of light chain phosphorylation associated with maximum tension without changing the level of maximum tension developed. These results showed that phosphorylation of only twenty percent of the total number of light chains is sufficient to fully activate these skinned smooth muscles and that the total amount of phosphorylation observed at maximum tension was partially dependent upon the conditions under which the experiment was conducted. The relationship between steady state tension and light chain thiophosphorylation was different than the relationship that was seen between tension and light chain phosphorylation. A given amount of light chain thiophosphorylation generated less tension than an equivalent amount of light chain phosphorylation. This was true whether the fibers were thiophosphorylated slowly at low (0.1 mM), slowly at high (1.0 mM), or rapidly at high (1.0 mM) Mg$\sp{2+}$. The influence of changes in the relative rates of myosin light chain phosphorylation and dephosphorylation on the tension/light chain phosphorylation relationship is discussed and two mechanisms to account for generation of levels of tension by low levels of light chain phosphorylation are evaluated. One mechanism proposes that interaction between myosin molecules in the thick filament allows phosphorylation of the light chain of one myosin molecule to activate nonphosphorylated myosin molecules or to maintain previously phosphorylated myosin heads in an active conformation. The other mechanism discussed suggests that myosin must be phosphorylated to be active and that maximum tension is maintained by a low level of light chain phosphorylation because only a fraction of the total number of myosin heads are in a position to interact with actin, bind, and generate tension.


Biology, Animal Physiology

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