Stimulation of β3-adrenoceptors relaxes rat urinary bladder smooth muscle via activation of the large-conductance Ca²⁺-activated K⁺ channels

Abstract

We investigated the role of large-conductance Ca(2+)-activated K(+) (BK) channels in beta3-adrenoceptor (beta3-AR)-induced relaxation in rat urinary bladder smooth muscle (UBSM). BRL 37344, a specific beta3-AR agonist, inhibits spontaneous contractions of isolated UBSM strips. SR59230A, a specific beta3-AR antagonist, and H89, a PKA inhibitor, reduced the inhibitory effect of BRL 37344. Iberiotoxin, a specific BK channel inhibitor, shifts the BRL 37344 concentration response curves for contraction amplitude, net muscle force, and tone to the right. Freshly dispersed UBSM cells and the perforated mode of the patch-clamp technique were used to determine further the role of beta3-AR stimulation by BRL 37344 on BK channel activity. BRL 37344 increased spontaneous, transient, outward BK current (STOC) frequency by 46.0 +/- 20.1%. In whole cell mode at a holding potential of V(h) = 0 mV, the single BK channel amplitude was 5.17 +/- 0.28 pA, whereas in the presence of BRL 37344, it was 5.55 +/- 0.41 pA. The BK channel open probability was also unchanged. In the presence of ryanodine and nifedipine, the current-voltage relationship in response to depolarization steps in the presence and absence of BRL 37344 was identical. In current-clamp mode, BRL 37344 caused membrane potential hyperpolarization from -26.1 +/- 2.1 mV (control) to -29.0 +/- 2.2 mV. The BRL 37344-induced hyperpolarization was eliminated by application of iberiotoxin, tetraethylammonium or ryanodine. The data indicate that stimulation of beta3-AR relaxes rat UBSM by increasing the BK channel STOC frequency, which causes membrane hyperpolarization and thus relaxation.

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).…”

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

“…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).Two types of VDCCs (L-type and T-type) have been identified in DSM cells similar to other smooth muscle cell types (1, 13, 48). The lower threshold of activation for the T-type VDCCs determines that they are active at the resting membrane potential of approximately Ϫ40 mV and could regulate DSM excitability.…”

“…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).…”

Suppression of human detrusor smooth muscle excitability and contractility via pharmacological activation of large conductance Ca2+-activated K+channels

“…We used the amphotericinperforated whole cell patch-clamp technique on freshly isolated human DSM cells, live-cell Ca Our data suggest that pharmacological activation of BK channels could represent a novel treatment option to control bladder dysfunction in humans. BK channels; iberiotoxin; NS-1619; patch clamp; urinary bladder THE LARGE CONDUCTANCE voltage-and Ca 2ϩ -activated K ϩ (BK) channels play a central role in controlling excitability and contractility in detrusor smooth muscle (DSM) (15,16,20,21,34,35). The BK channel is the only member of the K ϩ channel family that is activated by both Ca 2ϩ and voltage and serves as a unique signal integrator in the modulation of membrane excitability (32).…”

“…THE LARGE CONDUCTANCE voltage-and Ca 2ϩ -activated K ϩ (BK) channels play a central role in controlling excitability and contractility in detrusor smooth muscle (DSM) (15,16,20,21,34,35). The BK channel is the only member of the K ϩ channel family that is activated by both Ca 2ϩ and voltage and serves as a unique signal integrator in the modulation of membrane excitability (32).…”