Ca2+ sensitization of smooth muscle contractility induced by ruthenium red

The effects of ruthenium red (RuR) on contractility were examined in skinne d fibers of guinea pig smooth muscles, where sarcoplasmic reticulum functio n was destroyed by treatment with A-23187. Contractions of skinned fibers o f the urinary bladder were enhanced by RuR in a concentration-dependent man ner (EC50 = 60 mu M at pCa 6.0). The magnitude of contraction at pCa 6.0 wa s increased to 320% of control by 100 mu M RuR. Qualitatively, the same res ults were obtained in skinned fibers prepared from the ileal longitudinal s mooth muscle layer and mesenteric artery. The maximal contraction induced b y pCa 4.5 was not affected significantly by RuR. The enhanced contraction b y 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 co ntraction of skinned smooth muscle at low Ca2+ concentration ([Ca2+]; pCa > 8.0). RuR had a dual effect on the microcystin-induced contraction-to-enha ncement ratio at low concentrations and suppression at high concentrations. The relaxation following the decrease in [Ca2+] from pCa 5.0 to >8.0 was s ignificantly slowed down by an addition of RuR. Phosphorylation of the myos in light chain at pCa 6.3 was significantly increased by RuR in skinned fib ers of the guinea pig ileum. These results indicate that RuR markedly incre ases the Ca2+ sensitivity of the contractile system, at least in part via i nhibition of myosin light chain phosphatase.