Contractile protein expression in bladder smooth muscle is a marker of phenotypic modulation after outlet obstruction in the rabbit model

Purpose: We determined changes in contractile protein expression before and after the relief of partial bladder outlet obstruction in the rabbit model and assessed their potential role as predictors of recovery. Materials and Methods: We examined the ratio of the smooth muscle myosin heavy chain isoforms SM2-to-SM1, caldesmon isoform expression and bladder function in obstructed and unobstructed adult rabbit bladders. Cystometry, sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analysis were done to determine changes in bladder function and contractile protein expression. Results: Overall we observed significant correlation of bladder weight with the SM2-to-SM1 ratio (p <0.05). Regardless of the duration of obstruction (up to 10 weeks) the ratio appeared to stabilize around a value comparable to that in fetal rabbit smooth muscle cells, suggesting a reversal of SM2 and SM1 expression to a level similar to that at the fetal stage. The pattern of h and l-caldesmon isoform expression showed an increase in l-caldesmon expression in obstructed bladders. Except for decreased leak point pressure in the obstructed group we noted no statistically significant urodynamic changes in bladder capacity or compliance. Conclusions: There is significant correlation of bladder weight, which is the best known marker of obstruction, with the SM2-to-SM1 ratio. The myosin heavy chain isoform expression ratio appears to be an indicator of phenotypic modulation in bladder smooth muscle before and after the relief of bladder outlet obstruction. Thus, it may be useful as a marker of bladder dysfunction and predictor of functional recovery. Regression to a fetal pattern of protein expression may suggest irreversible damage to smooth muscle cells, possibly limiting recovery.