<i>In vitro</i> and <i>in vivo</i> relaxation of urinary bladder smooth muscle by the selective myosin II inhibitor, blebbistatin

Abstract

What’s known on the subject? and What does the study add? Various pathological conditions of the bladder, including overactive bladder syndrome (OAB), are associated with unregulated increases in bladder smooth muscle (SM) contraction. Although a number of new pre‐clinical pharmacological agents for OAB have been identified, they predominantly target at the neural or detrusor cell membrane level rather than the SM contractile apparatus itself. The current study provides the first demonstration that blebbistatin, a novel small cell permeable molecule with demonstrated high affinity and selectivity toward the myosin II contractile molecule, potently and efficiently relaxes both rat and human bladder SM in vitro and also significantly alters urodynamic parameters in vivo to values reflective of decreased bladder overactivity. OBJECTIVE To investigate the in vitro and in vivo effects of blebbistatin (a small cell‐permeable molecule with high affinity and selectivity toward the myosin II contractile molecule) on bladder smooth muscle (SM) contractility, as antimuscarinic therapy is only 65–75% effective in treating overactive bladder (OAB) and is associated with considerable side‐effects, with a &lt;25% continuation rate at 1 year. MATERIALS AND METHODS Bladder and aortic strips from adult male rats, and human bladder strips obtained from open prostatectomy, were used for organ‐bath studies of blebbistatin. Awake cystometry was also used in rats in both the presence and absence of intravesically delivered blebbistatin. RESULTS Blebbistatin dose‐dependently and completely relaxed both KCl‐ and carbachol‐induced rat detrusor and endothelin‐1‐induced human bladder contraction. Pre‐incubation with 10 µ m blebbistatin attenuated carbachol responsiveness by ≈65% while blocking electrical field stimulation‐induced bladder contraction reaching 50% inhibition at 32 Hz. The basal tone and amplitude of spontaneous contraction were also significantly diminished. Urodynamic variables were obviously altered by intravesical infusion with blebbistatin. CONCLUSION Our novel data show that blebbistatin strongly relaxes both rat and human bladder contraction induced by various physiological stimuli. Coupled with our in vivo data showing that nanomole doses of blebbistatin significantly alter urodynamic variables to produce a less active bladder, our results suggest the possibility of intravesically administered blebbistatin binding at myosin II being developed as a therapeutic treatment for OAB via a novel targeting of the SM contractile apparatus.