Metabolic aspects of urinary bladder filling

Urinary bladder compliance allows the bladder to fill to near capacity without a large increase in intravesical pressure. Bladder compliance is composed of two interrelated factors: passive characteristics of the connective tissue elements of the bladder, and active properties of the smooth muscle elements. The tension generated by the smooth muscle elements can significantly affect bladder compliance. This study utilized an in-vitro whole-bladder model in rabbits to determine the effects of hypoxia, alterations in calcium concentrations, and muscarinic agonists and antagonists on bladder capacity and compliance. The urinary bladder was excised together with a short segment of proximal urethra. A catheter was inserted into the bladder via the dome and the urethra was canulated with a saline-filled tube. The bladder was mounted in an isolated bath containing Tyrode's buffer. The catheter was connected to a pressure transducer to monitor the intravesical pressure and connected to a saline-filled i.v. bag. The weight of the saline bag was continually monitored. The height of the saline bag was set to 80 mmH2O above the baseline intravesical pressure. Bladder filling was started by opening the bladder to the saline reservoir. Intravesical pressure, rate of pressure increase, rate of volume increase, and maximal volume were digitally recorded. The bladder filling was repeated while the whole bladder was subjected to hypoxia, high calcium concentration, the presence of EGTA, carbachol, atropine and tetrodotoxin, respectively. Results are summarized as follows: (a) control bladder filling was biphasic, there was an initial rapid rise in intravesical pressure followed by a slower linear rise to the pre-set pressure; (b) hypoxia significantly decreased the initial rate of the rise in intravesical pressure, increased the rate of bladder filling, and significantly increased final bladder volume; (c) incubation of the bladder in the presence of EGTA also significantly decreased the initial rate of intravesical pressure rise, increased the rate of filling and also significantly increased the final bladder volume; (d) high concentrations of calcium increased the initial rate of rise in intravesical pressure; (e) carbachol significantly increased the rate of intravesical pressure rise, decreased the rate of bladder filling, and significantly decreased final bladder volume; (f) atropine and tetrodotoxin (TTX) had no effects on bladder filling. In summary, alterations in muscle tone had significant effects on bladder capacity and compliance.