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Am J Physiol Cell Physiol 269: C563-C571, 1995;
0363-6143/95 $5.00
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AJP - Cell Physiology, Vol 269, Issue 3 C563-C571, Copyright © 1995 by American Physiological Society

ARTICLES

Polyamines inhibit myosin phosphatase and increase LC20 phosphorylation and force in smooth muscle

K. Sward, M. D. Pato, B. O. Nilsson, I. Nordstrom and P. Hellstrand
Department of Physiology and Biophysics, University of Lund, Sweden.

The increase in Ca(2+)-activated force caused by polyamines in beta-escin-permeabilized guinda 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 toward 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 an 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.


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