[Ruthenium red inhibits energy-dependent and passive Ca2+ transport in permeabilized smooth muscle cells]

The effects of ruthenium red, pH, and sodium ions on active and passive Ca2+ transport were studied in digitonin-treated (0.1 mg/ml) suspension of myometrial cells using radiolabeled 45Ca2+. The inhibitor of mitochondrial Ca2+ accumulation ruthenium red (10 microM) suppressed active accumulation of the cation in permeabilized myocytes by 78-85%. Ruthenium red-sensitive Ca2+ accumulation significantly (10-fold, mean) exceeds ruthenium red-resistant Mg2+, ATP-dependent Ca2+ accumulation assayed in the absence of oxalate and inhibited by 50 nM tapsigargin (blocker of endo/sarcoplasmic reticulum calcium pump). Acidification of incubation medium (pH 8.0-6.0, Tris-malcate/KOH buffer) inhibits ruthenium red-sensitive active Ca2+ accumulation in permeabilized myocytes and passive release of this cation in the dilution medium after blockage of mitochondrial Ca2+ accumulation by ruthenium red. Partial isotonic substitution of KCl (135-95 mM) with NaCl (15-55 mM) in incubation medium did not affect the initial rate of active ruthenium red-sensitive Ca2+ accumulation and the kinetics of passive Ca2+ release from permeabilized cells after blockade of active Ca2+ accumulation by ruthenium red. Mechanism of Ca2+ exchange in myometrial mitochondria is discussed.