Effects of thiopental on Ca²⁺ currents and intracellular Ca ²⁺ transient in single atrial cells from guinea pig

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 Ca ²⁺ 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.