POLYAMINES INHIBIT MYOSIN PHOSPHATASE AND INCREASE LC(20) PHOSPHORYLATION AND FORCE IN SMOOTH-MUSCLE

The increase in Ca2+-activated force caused by polyamines in beta-esci n-permeabilized guinea pig ileum is shown to be associated with increa sed myosin 20-kDa light chain (LC(20)) phosphorylation and shortening velocity. Myosin LC(20) dephosphorylation with arrested kinase activit y was slower in the presence of 1 mM spermine. Smooth muscle phosphata ses (SMP-I, -II, -III, and -IV) isolated from turkey gizzard are all a ctive against phosphorylated LC(20), but only SMP-III and -IV dephosph orylate heavy meromyosin (HMM). Spermine inhibited SMP-III activity to ward LC(20) but stimulated HMM dephosphorylation, whereas SMP-IV was i nhibited with both substrates. In contrast, SMP-I and -II were stimula ted by spermine. The relative effects of different polyamines correlat ed with an increasing number of positive charges. Spermine did not aff ect binding of SMP-IV to myosin and did not dissociate any of the subu nits of the enzyme. Incubation of permeabilized strips with SMP-IV res ulted in attenuated responses to Ca2+, an effect that was opposed by s permine and abolished by microcystin-LR. We conclude that spermine sel ectively inhibits myosin phosphatase activity and suggest that polyami nes function as endogenous myosin phosphatase inhibitors.