Functional link between muscarinic receptors and large-conductance Ca2+-activated K+ channels in freshly isolated human detrusor smooth muscle cells

Activation of muscarinic acetylcholine receptors (mAChRs) constitutes the primary mechanism for enhancing excitability and contractility of human detrusor smooth muscle (DSM). Since the large-conductance Ca2+-activated K+ (K(Ca)1.1) channels are key regulators of human DSM function, we investigated whether mAChR activation increases human DSM excitability by inhibiting K(Ca)1.1 channels. We used the mAChR agonist, carbachol, to determine the changes in K(Ca)1.1 channel activity upon mAChR activation in freshly isolated human DSM cells obtained from open bladder surgeries using the perforated whole cell and single K(Ca)1.1 channel patch-clamp recordings. Human DSM cells were collected from 29 patients (23 males and 6 females, average age of 65.9 +/- 1.5 years). Carbachol inhibited the amplitude and frequency of K(Ca)1.1 channel-mediated spontaneous transient outward currents and spontaneous transient hyperpolarizations, which are triggered by the release of Ca2+ from ryanodine receptors. Carbachol also caused membrane potential depolarization, which was not observed in the presence of iberiotoxin, a K(Ca)1.1 channel inhibitor, indicating the critical role of the K(Ca)1.1 channels. The potential direct carbachol effects on K(Ca)1.1 channels were examined under conditions of removing the major cellular Ca2+ sources for K(Ca)1.1 channel activation with pharmacological inhibitors (thapsigargin, ryanodine, and nifedipine). In the presence of these inhibitors, carbachol did not affect the single K(Ca)1.1 channel open probability and mean K(Ca)1.1 channel conductance (cell-attached configuration) or depolarization-induced whole cell steady-state K(Ca)1.1 currents. The data support the concept that mAChR activation triggers indirect functional K(Ca)1.1 channel inhibition mediated by intracellular Ca2+, thus increasing the excitability in human DSM cells.