Large-conductance voltage- and Ca²⁺-activated K⁺ channels regulate human detrusor smooth muscle function

Abstract

The large-conductance voltage- and Ca(2+)-activated K(+) (BK) channel is expressed in many smooth muscle types, but its role in human detrusor smooth muscle (DSM) is unclear. With a multidisciplinary approach spanning channel molecules, single-channel activity, freshly isolated human DSM cells, intact DSM preparations, and the BK channel specific inhibitor iberiotoxin, we elucidated human DSM BK channel function and regulation. Native human DSM tissues were obtained during open surgeries from patients with no preoperative history of overactive bladder. RT-PCR experiments on single human DSM cells showed mRNA expression of BK channel α-, β(1)-, and β(4)-subunits. Western blot and immunocytochemistry confirmed BK channel α, β(1), and β(4) protein expression. Native human BK channel properties were described using the perforated whole cell configuration of the patch-clamp technique. In freshly isolated human DSM cells, BK channel blockade with iberiotoxin inhibited a significant portion of the total voltage step-induced whole cell K(+) current. From single BK channel recordings, human BK channel conductance was calculated to be 136 pS. Voltage-dependent iberiotoxin- and ryanodine-sensitive transient BK currents were identified in human DSM cells. In current-clamp mode, iberiotoxin inhibited the hyperpolarizing membrane potential transients and depolarized the cell resting membrane potential. Isometric DSM tension recordings revealed that BK channels principally control the contractions of isolated human DSM strips. Collectively, our results indicate that BK channels are fundamental regulators of DSM excitability and contractility and may represent new targets for pharmacological or genetic control of urinary bladder function in humans.

Ethanol-mediated relaxation of guinea pig urinary bladder smooth muscle: involvement of BK and L-type Ca2+ channels

“…Paxilline inhibited currents at 0 mV and above (P Ͻ 0.05, Fig. 1B) consistent with the previous observations in DSM cells showing contribution of BK channels at voltages of ϳ0 to ϩ10 mV and above (19,22). For instance, at ϩ40 and ϩ110 mV the mean current density value significantly decreased to 2.18 Ϯ 1.28 pA/pF and to 4.18 Ϯ 5.69 pA/pF (n ϭ 6, N ϭ 6, P Ͻ 0.05), respectively.…”

“…For example, the BK channel blockade with either iberiotoxin or charybdotoxin prolonged the action potentials correlating with enhanced contractility (16,18), whereas the BK channel opener NS11021 reduced action potential generation and DSM contractility in guinea pigs (29). In human DSM, the selective BK channel inhibitor iberiotoxin and activator NS1619 increased and decreased, respectively, the contractility of tissue strips and excitability of freshly isolated DSM cells confirming the important regulatory role of this K ϩ channel subtype (22,25). Key characteristics of the BK channels are 1) their large conductance whereby the opening or closure of few channels has pronounced effects on cellular excitability; 2) a dual sensitivity to metabolic factors (e.g., Ca 2ϩ and cAMP-pathways) and membrane voltage; and 3) a close functional interrelationship with VDCCs, which are the primary regulators of contractility mediating the influx of Ca 2ϩ to initiate DSM contractions (3,24,44,46,54).…”

“…Among them, the large-conductance voltage-and Ca 2ϩ -activated K ϩ channels (also known as BK, Slo, or K Ca 1.1 channels) and L-type voltage-dependent Ca 2ϩ channels (VDCCs) are recognized as key regulators of excitability and contractility of DSM (44). Supporting data, provided for DSM studies using rat, guinea pig, mouse, and importantly human tissues and cells (5,17,18,22,24,25,46), demonstrate the role of BK channels in the regulation of the resting membrane potential, modulation of the repolarization phase of DSM action potentials, and generation of spontaneous transient BK currents (TBKCs), also known as spontaneous transient outward currents. For example, the BK channel blockade with either iberiotoxin or charybdotoxin prolonged the action potentials correlating with enhanced contractility (16, 18), whereas the BK channel opener NS11021 reduced action potential generation and DSM contractility in guinea pigs (29).…”

Control of Urinary Drainage and Voiding

“…BSM expresses a rich variety of K channels, including large-conductance calcium-activated K (BK or maxi K), ATP-sensitive K (K ATP ), small-conductance K (SK), inwardly rectifying K (K ir ), and voltage-activated K (K v ) channels. As noted above, the effects of b-AR stimulation appear to be at least partially mediated by activation of BK channels in rat, guinea pig, and human bladder (62)(63)(64). Overall, it appears that pharmacologic activation of virtually all K channels leads to smooth muscle relaxation as a result of K efflux hyperpolarizing the membrane potential (65-68).…”

BK channel regulation by phosphodiesterase type 1: a novel signaling pathway controlling human detrusor smooth muscle function

“…BK channel activity is regulated by localized intracellular Ca 2ϩ release events from the sarcoplasmic reticulum (SR) ryanodine receptors (RyRs), known as "Ca 2ϩ sparks." Ca 2ϩ sparks transiently activate BK channels generating transient BK currents (TBKCs) (8,12,16,27,28). TBKCs correspond to transient hyperpolarizations in DSM cells (19).…”