Ca2+-ANTAGONISTIC EFFECTS OF FLURAZEPAM, A BENZODIAZEPINE DERIVATIVE, ON ISOLATED GUINEA PIG LEFT ATRIA

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Effects of flurazepam, a benzodiazepine derivative, on Ca2+-Induced cardiostimulant contractile activity in normal Tyrodes solution and Ca2+-mediated contraction in K+-rich (19-22 mM) Tyrodes solution were investigated in electrically driven left atrial preparations isolated from guinea-pigs. In normal Tyrodes solution, flurazepam (1×10-6, 1×10-5 and 1×10-4 M) noncompetitively shifted the dose-response curves for CaCI2 downwards. In K+ (19 mM)-rich Tyrodes solution, flurazepam (3×10-5 M) decreased contractile amplitude time-dependently; and after addition of CaCl2 (final: 8 mM), contractile amplitude was increased time-dependently. In K++ (19 mM)-depolarized preparations, flurazepam (3×10-5 M) competitively shifted the dose-response curve for CaCl2 rightwards. In the K+ (22 mM)-depolarized isoproterenol (3.8×10-6 M)-treated atrial preparation, flurazepam (3×10-5 M) consistently suppressed contraction. Flurazepam (9×10-5 M) suppressed atrial contraction in tetrodotoxin (TTX) (2×10-5 M)-added normal Tyrodes solution, and CaCl2 (final: 8 mM) partially restored the contraction. These results suggest that flurazepam inhibits transmembrane Ca2+-influx into the atrial muscle cell.
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