Ca (2+) -insensitive sustained contraction of skinned smooth muscle after acidic ADP treatment
Morimoto, S.; Ogawa, Y.
American Journal of Physiology 268(1 Pt 1): C21-C29
1995
ISSN/ISBN: 0002-9513 PMID: 7840149 Document Number: 453752
After an acidic treatment in the presence of ADP, Triton X-100-skinned rabbit aortic smooth muscle strips were found to develop a large sustained, Ca-2+-insensitive tension when returned to a relaxing solution with neutral pH. The presence of ADP during treatment was essential for the manifestation of the Ca-2+-insensitive contraction. This contraction was reversibly eliminated by withdrawal of MgATP or addition of vanadate and was found to be accompanied by an extraordinarily high level of 20-kDa myosin light-chain (MLC-20) phosphorylation. The rate constant for dephosphorylation of MLC-20 in treated strips was about one-twenty-fifth that in untreated control, when determined after removal of Ca-2+, Mg-2+, and ATP. Two-dimensional phosphopeptide mapping of tryptic digests of MLC-20 showed that most incorporated phosphate was in the peptides which would be phosphorylated by myosin light-chain kinase. These results provide strong evidence that ADP inactivates myosin light-chain phosphatase under acidic conditions.