Myosin light chain phosphatase activities and the effects of phosphatase inhibitors in tonic and phasic smooth muscle.
Phosphatase inhibitors microcystin-LR, tautomycin, and okadaic acid caused contraction and increased 20-kDa myosin light chain (MLC20) phosphorylation in Ca(2+)-free solutions in both phasic and tonic smooth muscle permeabilized with beta-escin, and inhibited the heavy meromyosin (HMM) phosphatase activity of smooth muscle homogenates with the same potency sequence: microcystin-LR greater than tautomycin greater than okadaic acid. The sensitivity to all three inhibitors was significantly higher, the half-times of relaxation and dephosphorylation were 4-6 times longer, and the HMM phosphatase and MLC20 kinase activity/smooth muscle cell wet weight was 2.0- and 1.9-fold lower in the tonic, femoral artery, than in the phasic, ileum or portal vein, smooth muscle. Preincubation with 0.2 microM inhibitor-2 decreased the HMM phosphatase activity by 35% in the ileum and by 60% in the femoral artery. The results suggest that the HMM phosphatases of smooth muscle have properties common to type 1 protein phosphatases, but are inhibited only partially by high concentrations of inhibitor-2, and that the lower HMM phosphatase activity of tonic smooth muscle may contribute to its greater sensitivity to phosphatase inhibitors and its slower rate of relaxation.
Gong
M C
MC
Department of Physiology, University of Virginia School of Medicine, Charlottesville 22908.
Cohen
P
P
Kitazawa
T
T
Ikebe
M
M
Masuo
M
M
Somlyo
A P
AP
Somlyo
A V
AV
D000818:Animals; D000935:Antifungal Agents / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002413:Cations, Divalent; D004928:Escin / Q000494:pharmacology; D004988:Ethers, Cyclic / Q000494:pharmacology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008387:Marine Toxins; D052998:Microcystins; D008854:Microscopy, Electron; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects / Q000201:enzymology* / Q000502:physiology / Q000648:ultrastructure; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000201:enzymology* / Q000502:physiology / Q000648:ultrastructure; D043263:Myosin-Light-Chain Phosphatase; D019319:Okadaic Acid; D010456:Peptides, Cyclic / Q000494:pharmacology; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010766:Phosphorylation; D011714:Pyrans; D011817:Rabbits; D013141:Spiro Compounds
D000935:Antifungal Agents; D002413:Cations, Divalent; D004988:Ethers, Cyclic; D008387:Marine Toxins; D052998:Microcystins; D010456:Peptides, Cyclic; D011714:Pyrans; D013141:Spiro Compounds; C053079:tautomycin; D019319:Okadaic Acid; D004928:Escin; D010749:Phosphoprotein Phosphatases; D043263:Myosin-Light-Chain Phosphatase; C057862:cyanoginosin LR; D002118:Calcium