Obstruction-induced changes in urinary bladder smooth muscle contractility: A role for Rho kinase

Detrusor smooth muscle (DSM) undergoes hypertrophy after partial bladder outlet obstruction (PBOO) in male rabbits, as it does in men with PBOO induced by benign prostatic hyperplasia. Despite detrusor hypertrophy, some bladders are severely dysfunctional (decompensated). In this study, the rabbit model for PBOO was used to determine the biochemical regulation of the contractile apparatus and force maintenance by the detrusor from decompensated bladders (DB). Bladders from sham-operated rabbits served as a control. On stimulation with 125 mM KCl, the DSM from sham-operated (SB) rabbits showed phasic contractions, whereas the detrusor from DB was tonic, exhibiting slow development of force, a longer duration of force maintenance, and slow relaxation. The Rho kinase (ROK) inhibitor Y-27632 enhanced the relaxation of precontracted DSM strips from DB. The enhancement of relaxation of the KCl-induced contraction of DB by Y-27632 was associated with dephosphorylation of myosin light chain (MLC₂₀). The DSM extract from DB showed low phosphatase activity compared with that from SB. The DB also showed more Ca ²⁺-independent MLC₂₀ phosphorylation, which was partially inhibited by Y-27632. RT-PCR and Western blotting revealed similar expression levels of MLC kinase and ROK-α in SB and DB, but ROK-β was overexpressed in DB. These results suggest that the ROK-mediated pathway is partly responsible for the high degree of force maintenance and slow relaxation in the detrusor from DB.