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The effects of thiopental on Ca2+ currents and intracellular Ca2+ transient in single atrial cells from guinea pigs were studied by means of a whole-cell voltage-clamp method and Ca2+-sensitive fluorescent dye. Thiopental inhibited L-type voltage-dependent Ca2+ currents in a concentration-dependent manner (IC50 = 2.8·10–5 mol/l). Moreover, the mode of Ca2+ current inhibition by thiopental showed no use dependency. Electrical stimulation-induced intracellular Ca2+ transient was significantly suppressed by 10–5 mol/l thiopental. However, the caffeine-sensitive Ca2+ releasing pathway from sarcoplasmic reticulum was not affected by thiopental. Our results indicate that thiopental inhibits L-type Ca2+ currents, but not release of Ca2+ from sarcoplasmic reticulum. These results suggest that the negative inotropic action of thiopental is mainly due to inhibition of L-type Ca2+ channels in guinea pig atrial myocytes. Copyright © 2007 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]