J. Biol. Chem., Vol. 260, Issue 6, 3469-3476, Mar, 1985
Adenosine and 5'-chloro-5'-deoxyadenosine inhibit the phosphorylation of phosphatidylinositol and myosin light chain in calf aorta smooth muscle
SR Doctrow and JM Lowenstein
Smooth muscle from calf aorta is homogenized and centrifuged. The insoluble
material is subjected to sucrose density gradient centrifugation. When the
heaviest fraction so obtained is incubated with radioactive ATP, two
components incorporate most of the acid- insoluble radioactivity. One is a
phosphoprotein with a molecular weight of 21,000. It has been identified as
myosin light chain by its molecular weight, isoelectric point, and
precipitation by antibody to calf aorta myosin. Its phosphorylation is
strongly inhibited by EGTA, in agreement with published reports that myosin
light chain kinase of smooth muscle is Ca2+ dependent. The other product is
of low molecular weight, is extracted into acidic chloroform-methanol, and
has been identified as phosphatidylinositol 4-phosphate. Adenosine and
5'-chloro- 5'-deoxyadenosine, which are vasodilators, inhibit the
phosphorylation of both substrates. Phosphorylation of phosphatidylinositol
is inhibited at lower concentrations of the nucleosides than is the
phosphorylation of myosin light chain. The inhibitory effects of the two
nucleosides are not associated with changes in the concentration of cyclic
AMP. The precise function of phosphatidylinositol phosphorylation in smooth
muscle is not known, but correlations between smooth muscle contraction and
increased turnover of phosphatidylinositol and its mono- and diphosphates
have been reported. Myosin light chain is phosphorylated under conditions
which favor smooth muscle contraction. We conclude that the inhibitory
effects of adenosine described here are consistent with their physiological
action as vasodilators.
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