We have synthesized novel anthraquinone derivatives called the GoSlo-SR family, which open large conductance Ca2+ activated K+ (BK) channels. All experiments were approved by DKIT Animal Care and Use Committee. Rabbits were humanely killed with pentobarbitone (125 mg/kg, I.V.), their bladder removed and smooth muscle strips cut into 1 mm3 pieces. Single bladder smooth muscle cells (UBSMC) were isolated as previously described (Hollywood et al., 1998) and plated in 35mm Petri dishes prior to study. RBSMC were isolated as previously described (Hollywood et al., 1998) and plated in 35mm 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 1μM EGTA (for [Ca2+] 100 nM) or 1 μM HEDTA (for [Ca2+] > 300 nM). Free [Ca2+] was determined using Chelator software (Schoenmaker et al., 1992). Patches were held at -60 mV and currents evoked by voltage ramps (100 mV sec-1). Single channel conductance was 335±5 pS, (mean±SEM, n=9). Currents reversed at K+ equilibrium potential, were abolished by the BK channel blocker penitrem A (100 nM, n=5) and a 10 fold increase in [Ca2+]i to 1 μM shifted the activation V1/2 from 120±6 mV to 5±9 mV (p<0.01, paired t test; ΔV1/2= -115±5 mV, n=9). Compounds (structure shown Fig 1) were synthesized by microwave irradiation using the copper-catalyzed Ullmann coupling reaction (Baqi & Müller, 2007), their structures assigned by 1H NMR and high resolution mass spectra and purity determined by HPLC. All compounds were screened at 10 μM. When aliphatic D ring size was changed from cyclopropane to cycloheptane, ΔV1/2 changed from 8±6 mV (n=5) to -61±6 mV (n=6). Substitutions of the cyclohexane ring with a benzene ring did not alter ΔV1/2 further (-51±10 mV, n=4) but replacement with bromine slightly inhibited the channels (ΔV1/2 +14±9 mV, n=6). In subsequent syntheses, aniline derivatives were used to produce a library of compounds in order to study the structure activity relationships (SAR) further. Addition of electron withdrawing or hydrophilic groups in the meta position of ring D significantly reduced efficacy (ΔV1/2 by <-25 mV), whereas hydrophobic meta-substituents, such as ethyl, isopropyl, and tert-butyl increased efficacy and shifted V1/2 by >-90 mV. Enhancing phenyl ring lipophilicity with meta-CF3 and para-methyl groups produced the most efficacious member of this family, GoSlo-SR-5-44, which shifted V1/2 by -142 ± 8 mV (n=12). This was more effective than the meta-CF3 substituent, GoSlo-SR-5-6 (-107±7 mV, n=12, p<0.05). However, these compounds were equipotent (EC50 of 2.3μM and 2.4 μM respectively). The effects of GoSlo-SR-5-6 were abolished in the presence of penitrem A (100 nM, n=6). The GoSlo-SR family are novel BK channel openers and increasing hydrophobicity of ring D enhances efficacy.