Changes in the biaxial viscoelastic response of the urinary bladder following spinal cord injury

In order to gain a deeper understanding of bladder function, it is necessary to study the time-dependent response of the bladder wall. The present study evaluated and compared the viscoelastic behaviors of normal and spinal cord injured (SCI) rat bladder wall tissue using an established rat model and planar biaxial stress relaxation tests. Bladders from normal and spinalized (3 weeks) rats were subjected to biaxial stress (either 25 or 100 kPa in each loading direction) rapidly (in 50 ms) and subsequently allowed to relax at the constant stretch levels in modified Kreb's solution (in the absence of calcium; with no smooth muscle tone) for 10,000 s. We observed slower and therefore less stress relaxation in the SCI group compared to the normal group, which varied with the stress-level. These experimental results were fitted (r(2)>0.98) to a reduced relaxation function. Furthermore, biochemical assays revealed that the collagen content of SCI rat bladders was significantly (p<0.05) lower by 43%, while the elastin content was significantly (p<0.001) higher by 260% than that of normal bladders. These results suggest that SCI and the associated urologic functional changes induce profound tissue remodeling, which, in turn, provided the structural basis for the alterations in the complex, time-dependent mechanical behavior of the urinary bladder wall observed in the present study.