1. Phospholamban (PLB) is an inhibitor of the sarcoplasmic reticulum (SR) 
Ca2+-ATPase (SERCA). Its presence and/or functional significance in 
contractility of bladder, a smooth muscle tissue particularly dependent on SR 
function, is unknown. We investigated this by measuring the effects of carbachol 
(CCh) on force and [Ca2+]i in bladder from mice in which the PLB gene was 
ablated (PLB-KO mice). In the PLB-KO bladder, the maximum increases in [Ca2+]i 
and force were significantly decreased (41.5 and 47.4 % of WT), and the EC50 
values increased. 2. Inhibition of SERCA with cyclopiazonic acid (CPA) abolished 
these differences between WT and PLB-KO bladder, localizing the effects to the 
SR. 3. To determine whether these effects were specific to PLB, we generated 
mice with smooth-muscle-specific expression of PLB (PLB-SMOE mice), using the 
SMP8 alpha-actin promoter. Western blot analysis of PLB-SMOE mice showed 
approximately an eightfold overexpression of PLB while SERCA was downregulated 
12-fold. 4. In PLB-SMOE bladders, in contrast, the response of [Ca2+]i and force 
to CCh was significantly increased and the EC50 values were decreased. CPA had 
little affect on the CCh-induced increases in [Ca2+]i and force in PLB-SMOE 
bladder. 5. These results show that alteration of the PLB:SERCA ratio can 
significantly modulate smooth muscle [Ca2+]i. Importantly, our data show that 
PLB can play a major role in modulation of bladder contractility.