Functional characterization of large conductance calcium-activated K+ channel openers in bladder and vascular smooth muscle

Calcium activated K⁺ channels (K_Ca channels) are found in a variety of smooth muscle tissues, the most characterized of which are the large conductance K_Ca channels (BK_Ca or maxi-K⁺ channels). Recent medicinal chemistry efforts have identified novel BK_Ca openers including 2-amino-5-(2-fluoro-phenyl)-4-methyl-1H-pyrrole-3-carbonitrile (NS-8), BMS-204352 and its analog 3-(5-chloro-2-hydroxy-phenyl)-3-hydroxy-6-trifluoromethyl-1,3-dihydro-indol-2-one (compound 1), and 5,7-dichloro-4-(5-chloro-2-hydroxy-phenyl)-3-hydroxy-1H-quinolin-2-one (compound 2). Although these compounds are effective BK_Ca openers as shown by electrophysiological methods, little is known about their effects on smooth muscle contractility. In this study, the responsiveness of structurally diverse BK_Ca openers—NS-8, compounds 1 and 2 and the well characterized nonselective NS-1619—was assessed using segments of endothelium denuded rat aorta, rat and guinea pig detrusor precontracted with extracellular K⁺, and Landrace pig detrusor stimulated by electrical field. In all preparations, the compounds tested inhibited or completely abolished contractions with similar potencies (–logIC₅₀ values: 3.8 to 5.1). In rat aorta, in the presence of 80 mM K⁺, the compounds significantly shifted the concentration-response curve to the right compared with those obtained in 30 mM K⁺. These data are consistent with K⁺ channel (BK_Ca channel) activation as the underlying mechanism of relaxation by compounds that share the electrophysiological property of BK_Ca current activation. The similar potencies at detrusor and vascular smooth muscle suggest that the achievement of smooth muscle selectivity in vitro with the representative compounds examined in this study may prove to be a challenge when targeting BK_Ca channels for smooth muscle indications such as overactive bladder.Abbreviations KCO Potassium channel opener - BK_Ca Large conductance calcium-activated K⁺ channel - IC₅₀ Molar concentration of test compound for 50% inhibition of response