Abstract Body (Do not enter title and authors here): Background: Isoproterenol activates apamin-sensitive small conductance Ca²⁺-activated K⁺ currents (I_KAS) more in female than male rabbit ventricles. Isoproterenol activates all β adrenoceptors (ARs), which couple to stimulatory G-protein, but β2-AR and β3-AR may also couple to inhibitory G protein.

Objective: To test whether β1-AR, but not β2- or β3-AR, mediates sex differences in I_KAS activation in rabbit ventricles.

Methods: Langendorff-perfused rabbit hearts were used with sequential drug administration and optical mapping to measure the action potential duration at 80% repolarization (APD₈₀) during atrial pacing at 200 ms pacing cycle length (PCL). Study I (6 females and 6 males): baseline-denopamine (specific β1 AR agonist) 1 µM-apamin 100 nM. Study II-a (2 females and 3 males): baseline-pirbuterol (specific β2 AR agonist) 1 µM-apamin 100 nM. Study II-b (3 females): baseline-pirbuterol 1 µM-pirbuterol 10 µM-apamin 100 nM. Study III (3 females): baseline-mirabegron (specific β3 AR agonist) 1 µM-apamin 100 nM.

Results: Figure 1 shows apamin prolonged APD₈₀ with the presence of denopamine in female but not in male ventricles. A represents typical membrane potential (V_m) traces and APD₈₀ maps in Study I. Denopamine shortened APD₈₀. Apamin significantly increased APD₈₀ in female (from 117±9 to 124±7 ms, p=0.007) but not in male hearts (from 118±9 to 120±7 ms, p=0.184) (B, C). Females showed a larger delta APD₈₀ (apamin-denopamine) (p=0.021). D showed the measurement method of Ca_i decay and Ca_i peak. Ca_i decay decreased in both male and female rabbits after adding denopamine, while Ca_i peak decreased only in female rabbits (E, F). Figure 2 shows apamin did not affect APD₈₀ with the presence of pirbuterol. A represents typical V_m traces and APD₈₀ maps in Study II-a. In both male and female rabbits, 1 µM pirbuterol accelerated rabbits’ heart rate but did not consistently shorten APD₈₀ (B, C). Adding apamin did not increase APD₈₀ in the presence of 1 µM pirbuterol in female or male rabbits (D). E and F indicated 10 µM pirbuterol shortened APD₈₀ in female hearts, and apamin did not affect APD₈₀ either, with the presence of 10 µM pirbuterol (from 117±8 to 119±8 ms, p=0.293). Figure 3 shows apamin did not affect APD₈₀ with the presence of mirabegron in female rabbits (from 131±9 to 132±10 ms, p=0.231).

Conclusion: At 200ms PCL, β1 but not β2 or β3 AR activation accelerates Ca²⁺ release and I_KAS activation in female, but not male rabbit ventricles.