Effect of vesical outlet obstruction on detrusor contractility and passive properties in rabbits

Outlet obstruction was induced in 16 New Zealand rabbits by implanting a polyethylene tube (20 F) for a period of three months. The tube was slit longitudinally and placed around the bladder neck, between the ureters and vasa deferentia. It was left open to induce moderate obstruction and closed by a suture to induce severe obstruction. The animals were studied by cutting consecutive rings from each bladder, which were subjected to the following studies: morphology, contractility and mechanical properties. Morphology. Histology sections of the rings, studied by Hematoxylin and Eosin and Masson-Trichrome stains, demonstrated smooth muscle hypertrophy in moderate obstruction, while hyperplasia was the predominant response in severe obstruction. The average nuclear count per square millimeter of smooth muscle was 251 in controls, 87 in moderate obstruction and 705 in severe obstruction. Bladder wall thickness was significantly increased after both moderate and severe obstruction, as compared to controls. Contractility and mechanical properties. Each ring was tested in a muscle chamber under conditions of maximal electrical stimulation (100 V, 2 ms, 40 Hz). For each ring, full force-length relationships (active and passive) were obtained by stretching the rings in successive increments until length of maximal active force development was reached. Force was normalized per unit cross-sectional area (stress), and length according to a "reference" underformed state (per cent of unloaded length). The maximum active stress of the rings was taken as a measure of bladder contractility, and the rate of increase in passive force as a measure of detrusor stiffness. In all groups, the body of the detrusor exerted better contractility, as compared to a rigid, less contractile base. In the obstructed groups, detrusor contractility was significantly decreased at the body level, with increased stiffness, as compared to controls. The length at which maximum contractility was exerted, however, increased in moderate obstruction, and decreased in severe obstruction.