BRL37344, a β3-adrenergic receptor agonist, decreases nerve-evoked contractions in human detrusor smooth muscle isolated strips: role of BK channels

Abstract

Objective: To investigate the mechanism by which BRL37344, a β3-adrenergic receptor (β3-ARs) agonist, facilitates the inhibition of nerve-evoked contractions in human detrusor smooth muscle (DSM) isolated strips and to identify the role of large-conductance Ca(2+)-activated K(+) (BK) channels in this process.

Methods: Human DSM specimens were obtained from open bladder surgeries on patients without preoperative history of overactive bladder symptoms. Isometric DSM tension recordings were conducted using force-displacement transducers and thermostatically controlled tissue baths. Nerve-evoked contractions were generated by electrical field stimulation (EFS).

Results: BRL37344, a β3-AR agonist, significantly decreased the amplitude, muscle force, and duration of the DSM contractions induced by 20 Hz EFS, in a concentration-dependent manner. This BRL37344-mediated inhibition of the amplitude and muscle force of the nerve-evoked DSM contraction was significantly reduced by iberiotoxin, a highly selective inhibitor of the BK channel, revealing a role for BK channels in the β3-AR-induced inhibition of human DSM nerve-evoked contractions. We further used atropine, α,β-methylene-ATP, and suramin to separate the cholinergic and purinergic components of human DSM nerve-evoked contractions. We found that the β3-AR agonist, BRL37344, inhibited both components of the EFS-induced (0.5-50 Hz) DSM contractions.

Conclusion: This study supports the concept that β3-AR agonists inhibit nerve-evoked contractions in human DSM. We have further revealed that BK channels play a critical role in BRL37344-mediated relaxation of nerve-evoked contractions in human DSM. The study suggests that in addition to β3-ARs, BK channels may also represent promising pharmacologic targets in the treatment of urinary bladder dysfunction.

Figure 1. Iberiotoxin, a selective BK channel blocker, significantly reduced the BRL37344 inhibitory effects on 20 Hz EFS-induced contractions in human DSM isolated strips

A) An original DSM tension recording illustrating BRL37344 (10 nM-100 μM) inhibitory effects on 20 Hz EFS-induced contractions in a human DSM isolated strip. B) An original DSM tension recording illustrating that iberiotoxin (200 nM) increased the 20 Hz EFS-induced contractions in human DSM and that the BRL37344 (10 nM-100 μM) inhibitory effects on the EFS-induced contraction amplitude, force, and duration were significantly attenuated in the presence of iberiotoxin (200 nM). C–E) Cumulative concentration-response curves illustrating iberiotoxin (200 nM) antagonistic action on BRL37344 inhibitory effects on EFS-induced contraction amplitude (C), force (D), and duration (E), respectively (n=4, N=3; ^#P<0.05, ^##P<0.01, ^###P<0.005 vs. control; *P<0.05, **P<0.01, ***P<0.005 vs. iberiotoxin).

Figure 2. BRL37344 decreases the amplitude of the EFS-induced contractions in human DSM isolated strips in a wide range of stimulation frequencies

A) An original DSM tension recording illustrating the inhibitory effects of 10 μM BRL37344 on 0.5–50 Hz EFS-induced contractions of human DSM. B) Frequency-response curves showing 0.5–50 Hz EFS-induced contractions in response to 10 μM BRL37344 (n=8, N=4; **P<0.01, ***P<0.005).

Figure 3. BRL37344 significantly inhibited the purinergic component of the 0.5–50 Hz EFS-induced contractions in human DSM isolated strips

A) An original DSM tension recording illustrating BRL37344 (10 μM) inhibitory effects on EFS-induced contractions of human DSM isolated strips in the presence of atropine (1 μM). B) EFS frequency-response curves showing 10 μM BRL37344 inhibitory effects on nerve-evoked contractions of human DSM isolated strips (n=4, N=3; *P<0.05, **P<0.01).

Figure 4. BRL37344 reduced the cholinergic component of the 0.5–50 Hz EFS-induced contractions of the human DSM

A) An original DSM tension recording illustrating BRL37344 (10 μM) inhibitory effects on human DSM EFS-induced contractions in the presence of suramin (10 μM) and α,β-methylene-ATP (10 μM). B) Frequency-response curves illustrating BRL37344 (10 μM) inhibitory effects on EFS-induced contraction amplitude of human DSM isolated strips in the presence of suramin and α,β-methylene-ATP (n=8, N=5; *P<0.05, **P<0.01, ***P<0.005).