Partial outlet obstruction induces chronic distension and increased stiffness of rat urinary bladder

In order to identify the passive properties of the bladder during filling, we measured cystometrograms (CMGs) of rat urinary bladders that had been outlet obstructed for 6 weeks and age-matched controls in conscious, unrestrained animals and in fully relaxed whole bladders in an organ bath. In the organ bath, each bladder was allowed to empty passively at zero transmural pressure. The volume remaining was labelled zero pressure volume (ZPV) and was used as the reference volume to normalize contained volume, deriving wall stretch. Increased ZPV implies that the bladder contains more urine at low stresses and therefore is more distended. In awake animals, the obstructed bladder CMGs showed spontaneous contractions. The pressures between contractions were similar to those in CMGs performed in the organ bath, suggesting that passive properties determine the minimum pressures during filling in vivo. The ZPV of the obstructed and control bladders was 1.07 +/- 0.12 ml and 0.07 +/- 0.01 ml, respectively. The differences were significant (P < 0.01). The ZPV correlated with bladder weight and thus with degree of hypertrophy. Under conditions when weight cannot be determined, e.g., clinically, ZPV may provide a useful measure of the degree of chronic distension and bladder hypertrophy. The pressure-volume curves of the obstructed bladder CMGs in vitro varied between preparations. However, when pressure-volume was converted to stress-stretch using the law of Laplace, the obstructed bladders were all significantly stiffer than the controls. We confirmed this result by step-stretching relaxed bladder strips. The obstructed bladder strips again demonstrated stiffer stress-stretch curves than the controls.