Effect of the SK/IK channel modulator 4,5‐dichloro‐1,3‐diethyl‐1,3‐dihydro‐benzoimidazol‐2‐one (NS4591) on contractile force in rat, pig and human detrusor smooth muscle
Authors: Nielsen Jens Steen , Rode Frederik , Rahbek Mette , Andersson Karl‐Erik , Rønn Lars Christian , Bouchelouche Kirsten , Nordling Jorgen , Bouchelouche Pierre 
Published: January 2011

What’s known on the subject? and What does the study add?

Increased urinary bladder detrusor smooth muscle phasic contractility has been suggested to be associated with idiopathic bladder overactivity. Small conductance Ca2+‐activated K+(SK 1–3) channels have attracted considerable interest as putative target for new therapeutic strategy for treating overactive bladder. These channels play an important role in regulating the function and activity of urinary bladder smooth muscle (UBSM), and the loss of SK channel function has been shown to increase UBSM excitability and contractility. However, it is not known whether activation of SK channels has the converse effect of reducing UBSM excitability and contractility. In this paper, we investigated this possibility in the rat, pig and human UBSM by using the novel SK channel opener NS4591. These studies demonstrate that the SK channel modulator NS4591 has a potential role as a pharmacological tool to study the involvement of SK (and IK) channels in mammals and rodents urinary bladder. NS4591 may have therapeutic potential for treatment of detrusor overactivity.

OBJECTIVE

To investigate the importance of small (SK)‐ and intermediate (IK)‐conductance Ca2+‐activated K+channels on bladder function, by studying the effects of 4,5‐dichloro‐1,3‐diethyl‐1,3‐dihydro‐benzoimidazol‐2‐one (NS4591), a new modulator of SK/IK channels, on contractions induced by electrical field stimulation (EFS) and carbachol in rat, pig and human detrusor.

PATIENTS AND METHODS

Detrusor biopsies were obtained from rats, pigs and male patients undergoing cystectomy because of bladder cancer.

Force was recorded using myographs.

Intracellular free Ca2+was measured in myocytes using microfluorimetry.

RESULTS

In rat bladder rings subjected to EFS, cumulative addition of NS4591 (0.1–30 µM) decreased force by 82 ± 2.9% (n = 6).This effect was reduced by 64 ± 5.2% in the presence of 0.3 µM apamin, a specific inhibitor of SK channels. Apamin increased the force evoked by EFS significantly: force was increased by 14.2 ± 3.4% (n = 5) and 10.1 ± 2.6% (n = 7) in pig and human detrusor strips, respectively (P = 0.04 andP = 0.02).

The cumulative addition of NS4591 (0.3–30 µM) significantly reduced the amplitude of carbachol‐induced rhythmic oscillations by 62.0 ± 12.0% (n = 12) and the minimum force between oscillations by 30 ± 5% (n = 9) in pig detrusor strips (P < 0.005). In the presence of 10 µM NS4591, carbachol (1 µM) induced rhythmic contractions with an amplitude and normalized mean power frequency (nmeanPF) of 8.4 ± 5.1% and 0.11 ± 0.06 mN root mean square (rms) Hz (n = 12), respectively, vs. 21 ± 3.4% and 0.17 ± 0.04 mN rms Hz in control strips (n = 13). Apamin induced 6‐ and 11‐fold increases in amplitude and nmeanPF vs. 1.3‐ and 2‐fold increases in control strips.

In human detrusor strips (n = 15), the cumulative addition of NS4591 (1–30 µM) significantly reduced the amplitude by 69 ± 11%, the nmeanPF by 78 ± 6% and the minimum force between carbachol‐induced oscillations by 59 ± 5% (P < 0.008). The addition of apamin (0.3 µM) before application of 1 µM carbachol abolished the effects of NS4591 on amplitude and partially abolished its effect on nmeanPF by 41 ± 7%, vs. a 78 ± 6% reduction in the absence of apamin (n = 8).

In spontaneously active detrusor preparations, NS4591 reduced or abolished contractions.

Furthermore, NS4591 (10 µM) decreased the carbachol‐induced increase in the fura‐2 ratio by 43 ± 3% compared with control (n = 12) (P < 0.03).

CONCLUSIONS

The SK/IK channel modulator NS4591 inhibits EFS‐ and carbachol‐induced contractions in rat, pig and human detrusor muscle.

NS4591 may have therapeutic potential for treatment of detrusor overactivity.