Potentiation of Potassium Currents by Beta-Adrenoceptor Agonists in Human Urinary Bladder Smooth Muscle Cells: A Possible Electrical Mechanism of Relaxation

Abstract

We examined the effects of β-adrenoceptor agonists on the membrane currents of smooth muscle cells from the human urinary bladder using a whole-cell patch clamp to investigate the involvement of Ca²⁺-activated K⁺ (K_Ca) channels in relaxation by β-adrenergic agonists. With 0.05 mmol/l EGTA in the patch pipette, depolarizing pulses evoked outward rectifying currents. Isoproterenol (1 µmol/l) significantly increased the membrane currents by 75% at +80 mV with 0.05 mmol/l EGTA pipette solution. BRL 37344 (1 µmol/l) significantly increased the membrane currents by 44% at +80 mV. Iberiotoxin (100 nmol/l) significantly decreased the membrane currents by 60% at +80 mV. In the presence of iberiotoxin, the potentiation of the outward currents by isoproterenol was greatly suppressed and, in the presence of iberiotoxin and apamin (1 µmol/l), the potentiation by isoproterenol was totally abolished. On the other hand, with 5 mmol/l EGTA pipette solution, depolarizing pulses evoked smaller outward currents. Isoproterenol (1 µmol/l) did not change the membrane currents with 5 mmol/l EGTA pipette solution. The real-time PCR analysis revealed the expression of β₂-adrenoceptors in the cells. These results suggest that Ca²⁺-activated and iberiotoxin- and apamin-sensitive currents via both large-conductance and small-conductance K_Ca channels could be increased by stimulation of β₂-adrenoceptors.

Central role of the BK channel in urinary bladder smooth muscle physiology and pathophysiology

“…These essential findings have also been confirmed independently by a follow-up study in which a smooth musclespecific BK channel ␣-subunit KO mouse has been used (133). ␤-AR agonists can also modulate K ϩ conductance in human UBSM (137). As ␤-ARs, ␤3-ARs, in particular, and BK channels are functionally coupled at the PKA/RyR level to mediate relaxation of UBSM (Fig.…”

“…Specifically, it has been shown that in guinea pig UBSM cells, BK channels have an extremely high density of ϳ21 BK channels per square micrometer (110). The initial experimental evidence regarding BK channel physiological roles was based exclusively on animal studies, and until recently, our knowledge about the expression and function of the BK channel in human UBSM was very limited to several nonsystematic studies (35,44,130,137). In the past 2 or 3 years, a series of multidisciplinary studies, including combined molecular approaches, patch-clamp electrophysiology, live-cell Ca 2ϩ imag- ing, and confocal microscopy on freshly isolated human UBSM cells, as well as functional studies on UBSM contractility of tissues, have provided a methodological identification and characterization of the BK channel functional and regulatory roles in native human UBSM (5,56,58,61,89,155).…”

“…In rat UBSM, which also highly expresses ␤3-ARs ␤3-ARs and BK channels are functionally coupled to promote UBSM relaxation by a complex Ca 2ϩ -dependent mechanism, which involves an increase in TBKC frequency leading to UBSM cell membrane hyperpolarization and inhibition of UBSM spontaneous phasic contractions and tone (60). In rat and human UBSM, ␤3-AR agonists effectively inhibit both the spontaneous and nerve-evoked UBSM contractions, an inhibitory effect which is significantly reduced by iberiotoxin, suggesting that functional BK channels play a critical role in this process (4,5,60,137). In contrast, although ␤3-ARs are expressed at the mRNA level in guinea pig UBSM, they serve a negligible role, if any, in UBSM spontaneous and nerve-evoked contractions without affecting cell excitability (2).…”

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

“…In rat DSM, the ␤3-adrenoceptor subtype and the BK channel are functionally coupled at the RyRs level to mediate DSM relaxation (21). It has been suggested that ␤-adrenoceptor agonists can modulate K ϩ conductance in human DSM (40). However, future studies are needed to define the role of the BK channel in ␤-adrenergic relaxation of human DSM and its clinical implications.…”

Mirabegron in the treatment of overactive bladder

“…Bladder relaxation is obtained by the activation of b 3 -AR and the subsequent activation of adenylyl cyclase, which brings to cAMP formation that is thought to be crucial for achieving the therapeutic effect in OAB. Furthermore, recent studies showed a possible role of potassium (K+) channels, and particularly, big potassium channels, in the b-AR-mediated bladder relaxation independently of cAMP [40][41][42][43].…”