Diabetes-induced alternations in biomechanical properties of urinary bladder wall in rats.

OBJECTIVES: To determine whether diabetes mellitus and the associated changes in bladder function will trigger bladder wall tissue remodeling and concomitant alterations in the mechanical properties. We investigated the time course of changes in function and mechanical properties of diabetic and diuretic rat bladders using both in vivo and in vitro techniques METHODS: Cystometry was performed at 2, 4, and 8 weeks on female Sprague-Dawley rats that had received either a single injection of streptozotocin (65 mg/kg intraperitoneally) or 5% sucrose in drinking water for the duration of the experiments. At each point, the biaxial mechanical properties of 10 x 10-mm tissue specimens obtained from the posterior part of bladder wall were quantified. The changes in overall tissue compliance and mechanical anisotropy as a function of time were examined RESULTS: Both diabetic and diuretic conditions led to increases in bladder weight, bladder capacity, and in vivo compliance compared with the controls at all points tested. Under biaxial loading, all bladder wall tissues exhibited a nonlinear stress-strain relationship and mechanical anisotropy, with greater tissue compliance in the circumferential direction than in the longitudinal direction. Although the compliance of the bladder wall increased progressively and synchronously in both diabetic and diuretic bladders for < or = 4 weeks, only the diabetic bladders continued to increase the compliance for < or = 8 weeks (diabetic 0.64 +/- 0.04 vs diuretic 0.48 +/- 0.05, P = .03) CONCLUSIONS: The results of our study have shown that diuresis mainly contributes to the "early" changes of mechanical properties of the bladder, with diabetes inducing additional "late" changes of mechanical properties of the rat bladders after 4 weeks.

Quantification of bladder smooth muscle orientation in normal and spinal cord injured rats.

Spinal cord injuries (SCI) often lead to severe bladder dysfunctions. Our previous studies have demonstrated that following SCI, rat bladder wall tissue became hypertrophied, significantly more compliant, and changed its mechanical behavior from orthotropic to isotropic. In order to elucidate the link between the tissue microstructure and mechanical properties of the wall, we have developed a novel semi-automated image analysis method to quantify smooth muscle bundle orientation and mass fraction in the bladder wall tissues from normal and 10 day-post-SCI rats. Results of the present study revealed that there were significant (p < 0.05) increases in smooth muscle area fractions as well as significantly (p < 0.001) fewer cell nuclei per muscle area in the SCI groups compared to the normal groups. Furthermore, while the normal rat bladders exhibited predominant smooth muscle orientation only in the longitudinal direction, the SCI rat bladders exhibited smooth muscles oriented in both the circumferential and longitudinal directions. These results provide first evidence that bladder smooth muscle cells exhibit hypertrophy rather than hyperplasia and developed a second, orthogonal orientation of smooth muscle bundles following SCI. The results of the present study corroborate our previous mechanical anisotropy data and provide the basis for development of structure-based constitutive models for urinary bladder wall tissue.