AJP - Lung Cellular and Molecular Physiology, Vol 268, Issue 5 739-L746, Copyright © 1995 by American Physiological Society
Increased Ca2+ and myosin phosphorylation, but not calmodulin activity in sensitized airway smooth muscles
H. Jiang, K. Rao, X. Liu, G. Liu and N. L. Stephens
Department of Physiology, Faculty of Medicine, University of Manitoba, Winnipeg, Canada.
The increased shortening velocity and capacity of airway smooth muscle
(ASM) from ragweed pollen-sensitized dogs, which may be responsible for its
in vivo airway hyperresponsiveness, have been shown to be associated with
higher actomyosin adenosinetriphosphatase activity and greater level of
phosphorylation of the 20-kDa myosin light-chain (MLC20) at rest and during
contraction. Current studies show that the elevated level of
phosphorylation may be the result of an increased myosin light-chain kinase
(MLCK) activity due to excessive quantity of MLCK. There were no
significant changes in total activity of calmodulin, a protein that binds
and activates MLCK, in sensitized dog ASM (SASM) compared with control ASM
(CASM). When normalized to the relative calmodulin content in the tissues,
the specific calmodulin activities (means +/- SE) in sensitized tracheal
smooth muscle (STSM) and sensitized bronchial smooth muscle (SBSM) and in
their controls were not different (STSM 0.359 +/- 0.117, CTSM 0.339 +/-
0.136. SBSM 0.201 +/- 0.098, and control bronchial smooth muscle 0.213 +/-
0.056 nmol Pi.calmodulin content-1.min-1, respectively). Intracellular Ca2+
levels indicated by fura 2 fluorescent dye remained unaltered in SASM. We
conclude that airway hyperresponsiveness may result from higher MLCK
content in SASM rather than from changes in Ca(2+)-calmodulin activities,
which is an example of alteration in Ca2- sensitivity of ASM.
