We previously synthesized a novel library of anilinoanthraquinone derivatives termed the GoSlo-SR family, that shifted the half maximal activation voltage (V1/2) of BK channels by ~ -100 mV with an EC50 of ~ 2 μM (Roy et al., 2012). In the current study we hypothesised that the replacement of the phenylamino ring of the anthraquinone moiety by hydrophobic naphthylamino could enhance their potency and efficacy on BK channels. All experiments were approved by DKIT Animal Care and Use Committee. Rabbits were humanely killed with pentobarbitone (I.V.), their bladders removed and smooth muscle strips cut into 1 mm 3 pieces. Rabbit Bladder SMC (RBSMC) were isolated by enzymatic digestion and plated in 35 mm Petri dishes. Experiments were performed at 37oC using excised inside/out patch configuration with symmetrical K+ solutions (pH 7.2) containing 140 mM KCl, 10 mM Glucose, 10 mM HEPES and either 1 mM EGTA (for [Ca2+] 100 nM) or 1 mM HEDTA (for [Ca2+] > 300 nM). Excised patches of RBSMC were held at -60 mV and BK channel currents were evoked using voltage ramps (100 mV sec-1). The single channels had a large conductance (~330 pS), reversed at the K+ equilibrium potential (0 mV), were blocked in the presence of penitrem A (100 nM, n=6) and were sensitive to [Ca2+] such that a 10 fold increase in [Ca2+] to 1 μM shifted the activation V1/2 from 142±4 mV to 13±3 mV (ΔV1/2 = -129±5 mV, n=18). The compounds shown in Figure 1 were synthesized by microwave irradiation using copper catalyzed Ullmann coupling reaction (Baqi & Müller, 2007). Their structures were assigned by 1H NMR and HRMS and purity was determined by HPLC. The addition of naphthylamino or tetrahydro-naphthylamino rings to anthraquinone moiety (Fig. 1) produced variable effects. For example,10 μM GoSlo-SR-5-103 (Fig. 1B) was significantly less effective than GoSlo-SR-5-6 and only shifted the the ΔV1/2 by -44±4 mV (n=18). Application of 10 μM GoSlo-SR-5-65 (Fig. 1C) produced effects very similar to those of GoSlo-SR-5-6 and the ΔV1/2 was -116±10 mV (n=6). We then synthesized the compounds shown in Figures 1D & 1E to examine if the addition of 2-naphthylamino or tetrahydro-2-naphthylamino ring altered the potency. We found the application of 1 μM GoSlo-SR-5-95 (Fig. 1E) produced a ΔV1/2 of -94±9 mV (n=6), suggesting that it was more efficacious than GoSlo-SR-5-6 at this concentration. Similarly, application of 1 μM GoSlo-SR-5-69 (Fig. 1D) produced a ΔV1/2 of -113±10 mV (n=10) and the mean EC50 was 189 nM (confidence intervals 65 nM to 546 nM, n=3-8 patches). In conclusion, the results of this study suggest that the addition of a tetrahydro-2-naphthylamino ring to the anthraquinone moiety significantly enhances its ability to activate the BK channels in the rabbit bladder.