Polyamines selectively inhibit a myosin phosphatase and increase
myosin lc20 phosphorylation and contraction in smooth muscle.
Sw[umlaut]ard, Karl, Mary D. Pato, Bengt-Olof Nilsson, Ina
Nordstr[diaeresis]om, and Per Hellstrand.
Department of Physiology and Biophysics, University of Lund, S-223
62 Lund, Sweden; and Department of Biochemistry, University of
Saskatchewan, Saskatoon, Saskatchewan, Canada, S7N 0W0
APStracts 2:0146C, 1995.
The increase in Ca2+-activated force caused by polyamines in [beta]
-escin permeabilized guinea pig ileum is shown to be associated with
increased myosin 20 kDa light chain (LC20) phosphorylation and
shortening velocity. Myosin LC20 dephosphorylation with arrested
kinase activity, was slower in the presence of 1 mM spermine. Smooth
muscle phosphatases (SMP-I, II, III and IV) isolated from turkey
gizzard are all active against phosphorylated LC20, but only SMP-III
and IV dephosphorylate heavy meromyosin (HMM). Spermine inhibited
SMP-III activity towards LC20 but stimulated HMM dephosphorylation
whereas SMP-IV was inhibited with both substrates. In contrast, SMP-I
and II were stimulated by spermine. The relative effects of different
polyamines correlated with increasing number of positive charges.
Spermine did not affect binding of SMP-IV to myosin and did not
dissociate any of the subunits of the enzyme. Incubation of
permeabilized strips with SMP-IV resulted in attenuated responses to
[Ca2+], an effect that was opposed by spermine and abolished by
microcystin-LR. We conclude that spermine selectively inhibits myosin
phosphatase activity and suggest that polyamines function as
endogenous myosin phosphatase inhibitors.
Received 24 August 1994; accepted in final form 9 February 1995.
APS Manuscript Number C510-4.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 28 March 1995.