Partial bladder outlet obstruction induces urethral smooth muscle hypertrophy and decreased force generation

Purpose: PBOO leads to increased urinary frequency, decreased void volume, hypertrophy of the detrusor SM, and alterations in contractile and regulatory proteins. This study was done to determine whether PBOO induced increases in urinary frequency and detrusor SM hypertrophy are associated with an alteration in the contractility and expression of myosin isoforms in urethral SM. Materials and Methods: PBOO was surgically induced in male New Zealand White rabbits, and sham operated rabbits served as controls. After surgery, rabbits were kept 12 days, and prior to sacrifice, urine output and voiding frequency were monitored by keeping the animals in metabolic cages for 24 hours. Animals with increased urinary frequency (mean ± SEM 43 ± 12 voids per 24 hours) and sham operated rabbits (6 ± 3 voids per 24 hours) were used for this study. Morphology of the urethra was studied using light and immunofluorescence microscopy. The expression of myosin isoforms was analyzed at the mRNA and protein levels by RT-PCR and Western blotting. Results: The urethral wall and SM of PBOO rabbits showed hypertrophy. The force produced by the longitudinal muscle strips of PBOO animals in response to phenylephrine, KCl, or electrical field stimulation was decreased 50%, 37% and 40%, respectively. Immunofluorescence microscopy revealed a decrease in nerve density. RT-PCR and Western blotting showed a decrease in the expression of myosin isoform SM-B with a concomitant increase in SM-A at the mRNA and protein levels. Conclusions: Our data show hypertrophy of the urethral wall and SM, and alterations in contraction, innervation, and myosin isoforms in PBOO induced detrusor hypertrophy.