Effect of sub-skinning concentrations of saponin on intracellular Ca2+ and plasma membrane fluidity in cultured cardiac cells.
To determine the underlying mechanisms of the positive inotropic effect of sub-skinning concentrations of saponin, we studied changes in the intracellular Ca2+ ([Ca2+]i) and plasma membrane fluidity after exposure to digitonin (a representative saponin) in cultured cardiac cells. [Ca2+]i was measured by use of the fluorescent calcium indicator Calcium Green-1. The membrane fluidity was evaluated by measuring the diffusion coefficient using the method of fluorescence recovery after photobleaching. 1,1'-Dioctadecyl-3,3,3',3'-tetramethylindodicarbocyanine perchlorate was used as the fluorescent probe. Digitonin at a sub-skinning concentration (0.1 to 1 microM) produced an increase in cell motion and an augmentation of [Ca2+]i. Membrane fluidity which is evaluated by the diffusion coefficient (from 0.34.10(-8) to 0.28.10(-8) cm2/s; P < 0.05), decreased in the presence of 0.2 microM digitonin while the cell maintained an augmented motion and an increased [Ca2+]i. The skinning concentration of digitonin (5 microM) produced a rapid contracture with a marked increase in [Ca2+]i. The membrane fluidity was further reduced (diffusion coefficient: 0.24.10(-8) cm2/s; P < 0.001). These results suggest that saponin at the sub-skinning concentration also causes holes in the plasma membrane by interaction with cholesterol, as was shown with the skinning concentration, and it increases [Ca2+]i, which thereby induces a positive inotropic effect.
Ishida
H
H
Department of Physiology 2, School of Medicine, Tokai University, Kanagawa, Japan.
Hirota
Y
Y
Nakazawa
H
H
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects* / Q000378:metabolism; D002478:Cells, Cultured; D004072:Digitonin / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D006321:Heart / Q000187:drug effects* / Q000196:embryology; D008560:Membrane Fluidity / Q000187:drug effects*; D051379:Mice; D009200:Myocardial Contraction / Q000187:drug effects; D012503:Saponins / Q000494:pharmacology*
D012503:Saponins; D004072:Digitonin; D002118:Calcium