Am J Physiol Cell Physiol AJP: Renal Physiology
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Am J Physiol Cell Physiol 276: C566-C575, 1999;
0363-6143/99 $5.00
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Vol. 276, Issue 3, C566-C575, March 1999

Ca2+ sensitization of smooth muscle contractility induced by ruthenium red

Aki Yamada1, Susumu Ohya2, Masaru Hirano2, Minoru Watanabe2, Michael P. Walsh3, and Yuji Imaizumi1

1 Department of Pharmacology and Therapeutics and 2 Department of Chemical Pharmacology, Faculty of Pharmaceutical Sciences, Nagoya City University, Nagoya 467-8603, Japan; and 3 Department of Biochemistry and Molecular Biology, Faculty of Medicine, University of Calgary, Calgary, Alberta T2N 4N1, Canada

The effects of ruthenium red (RuR) on contractility were examined in skinned fibers of guinea pig smooth muscles, where sarcoplasmic reticulum function was destroyed by treatment with A-23187. Contractions of skinned fibers of the urinary bladder were enhanced by RuR in a concentration-dependent manner (EC50 = 60 µM at pCa 6.0). The magnitude of contraction at pCa 6.0 was increased to 320% of control by 100 µM RuR. Qualitatively, the same results were obtained in skinned fibers prepared from the ileal longitudinal smooth muscle layer and mesenteric artery. The maximal contraction induced by pCa 4.5 was not affected significantly by RuR. The enhanced contraction by RuR was not reversed by the addition of guanosine 5'-O-(2-thiodiphosphate) or a peptide inhibitor of protein kinase C [PKC-(19---31)]. The application of microcystin, a potent protein phosphatase inhibitor, induced a tonic contraction of skinned smooth muscle at low Ca2+ concentration ([Ca2+]; pCa > 8.0). RuR had a dual effect on the microcystin-induced contraction-to- enhancement ratio at low concentrations and suppression at high concentrations. The relaxation following the decrease in [Ca2+] from pCa 5.0 to >8.0 was significantly slowed down by an addition of RuR. Phosphorylation of the myosin light chain at pCa 6.3 was significantly increased by RuR in skinned fibers of the guinea pig ileum. These results indicate that RuR markedly increases the Ca2+ sensitivity of the contractile system, at least in part via inhibition of myosin light chain phosphatase.

phosphatase, skinned fiber, myosin light chain


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