Activation of muscarinic M3 receptors inhibits large-conductance voltage- and Ca2+-activated K+ channels in rat urinary bladder smooth muscle cells

Large conductance voltage- and Ca2+-activated K+ (BK) channels are key regulators of detrusor smooth muscle (DSM) contraction and relaxation during urine voiding and storage. Here, we explored whether BK channels are regulated by muscarinic receptors (M-Rs) in native freshly isolated rat DSM cells under physiological conditions using the perforated whole cell patch-clamp technique and pharmacological inhibitors. M-R activation with carbachol (1 mu M) initially evoked large transient outward BK currents, followed by inhibition of the spontaneous transient outward BK currents (STBKCs) in DSM cells. Carbachol (1 mu M) also inhibited the amplitude and frequency of spontaneous transient hyperpolarizations (STHs) and depolarized the DSM cell membrane potential. Selective inhibition of the muscarinic M-3 receptors (M-3-Rs) with 4-diphenylacetoxy-N-methylpiperidine (4-DAMP; 0.1 mu M), but not muscarinic M-2 receptors with methoctramine (1 mu M), blocked the carbachol inhibitory effects on STBKCs. Furthermore, blocking the inositol 1,4,5-triphosphate (IP3) receptors with xestospongin-C (1 mu M) inhibited the carbachol-induced large transient outward BK currents without affecting carbachol inhibitory effects on STBKCs. Upon pharmacological inhibition of all known cellular sources of Ca2+ for BK channel activation, carbachol (1 mu M) did not affect the voltage-step-induced steady-state BK currents, suggesting that the muscarinic effects in DSM cells are mediated by mobilization of intracellular Ca2+. In conclusion, our findings provide strong evidence that activation of M-3-Rs leads to inhibition of the STBKCs, STHs, and depolarization of DSM cells. Collectively, the data suggest the existence of functional interactions between BK channels and M-3-Rs at a cellular level in DSM.