A–B, Representative action potential recordings from female (A) and male (B) rabbit LV myocytes at a stimulation rate of 1 Hz at baseline, after the administration of 500nM isoproterenol (ISO) alone, or 500nM ISO plus 300nM β1-adrenergic receptor blocker CGP 20712A (ISO+CGP). C–D, Representative AP recordings from LV myocytes from female (C) and male (D) rabbits at baseline, after 500nM ISO, or 500nM ISO + 100nM ICI-118,551 (β2-adrenergic receptor blocker).