1 A hallmark for unstable bladder contractions is hyperexcitability and changes in the nature of spontaneous phasic activity of the bladder smooth muscle. In this study, we have characterized the spontaneous activity of the urinary bladder smooth muscle from the pig, a widely used model for studying human bladder function. 2 Our studies demonstrate that phasic activity of the pig detrusor is myogenic and is in¯uenced by the presence of urothelium. Denuded strips exhibit robust spontaneous activity measured as mean area under the contraction curve (AUC=188.9+15.63 mNs) compared to intact strips (AUC=7.3+1.94 mNs). 3 Spontaneous phasic activity, particularly the amplitude, is dependent on both calcium entry through voltage-dependent calcium channels and release from ryanodine receptors as shown by inhibition of spontaneous activity by nifedipine and ryanodine respectively. 4 Inhibition of BK Ca channels by iberiotoxin (100 nM) resulted in an increase in contraction amplitude (89.1+20.4%) and frequency (92.5+31.0%). The SK Ca channel blocker apamin (100 nM) also increased contraction amplitude (69.1+24.3%) and frequency (53.5+13.6%) demonstrating that these mechanisms are critical to the regulation of phasic spontaneous activity. 5 Inhibition of K ATP channels by glyburide (10 mM) did not signi®cantly alter myogenic contractions (AUC=18.5+12.3%). However, K ATP channel openers (KCOs) showed an exquisite sensitivity for suppression of spontaneous myogenic activity. KCOs were generally 15 fold more potent in suppressing spontaneous activity compared to contractions evoked by electrical ®eld-stimulation. These studies suggest that potassium channel modulation, particularly K ATP channels, may o er a unique mechanism for controlling spontaneous myogenic activity especially those associated with the hyperexcitability occurring in unstable bladders.

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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 (-logIC(50) 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.