The functional and structural response to distention of the rabbit whole bladder in vitro

PURPOSE: We investigated the effects of distention on intravesical pressure generation in response to stimuli in relation to its effects on bladder wall thickness and morphology. MATERIALS AND METHODS: A total of 25 New Zealand White rabbits were separated into 5 groups of 5 each. Each in vitro whole bladder preparation was filled with saline to 5% (undistended), 25%, 50%, 100% or 125% capacity (60 ml.). The effects of field stimulation, adenosine 5'-triphosphate, carbamylcholine chloride (carbachol) and KCl on intravesical pressure generation were determined. Additional bladders were similarly distended (3 at each percent of capacity), fixed in formalin and evaluated by semiquantitative morphometry. RESULTS: Pressure generation in response to field stimulation was maximal at 5% of capacity and it decreased progressively thereafter. Compared with the responses of undistended bladders, at 25% capacity the response to carbachol increased and then decreased progressively above 25%. Distention had no significant effect on the response to KCl. Interestingly pressure generation in response to adenosine 5'-triphosphate was increased more than 4-fold at 25% bladder capacity compared with 5% and it did not change significantly thereafter. Bladder wall thickness was decreased more than 50% at 25% capacity and by an additional 12% at 125% capacity. The greatest thinning was observed in the mucosa compared with the submucosa and muscularis. CONCLUSIONS: Bladder distention significantly reduced pressure generation in response to field stimulation only, indicating that distention reduced the effectiveness of synaptic transmission without altering muscarinic receptor function or membrane depolarization. The greatest change (decrease) in the response to field stimulation occurred between 5% (undistended) and 25% capacity, which was the change in distention that most affected bladder wall thinning.