APA, action potential amplitude; RMP, resting membrane potential; dV /dt _max, maximal action potential upstroke velocity; APD₁₀, action potential duration at 10% repolarization; APD₅₀, action potential duration at 50% repolarization; APD₉₀, action potential duration at 90% repolarization. E2, β-Estradiol; ISO, isoproterenol. Control, 0.01 nmol/L E2, 1 nmol/L E2, 100 nmol/L E2, ISO, 0.01 nmol/L E2+ISO, 1 nmol/L E2+ISO and 100 nmol/L E2+ISO group originated in 4, 5, 4, 6, 4, 5, 4, 4 Petri dishes respectively.