Effect of obstruction on ureteral circumferential force-length relations

After 2 wk of obstruction, rabbit ureteral length increases by 24%, outer diameter by 100%, and cross-sectional muscle area by 248%. In addition to undergoing muscle hypertrophy, obstructed ureters developed greater circumferential contractile forces than control ureters. The peak active force of obstructed ureteral segments was approximately twice that of control ureters. The increase in force was associated with a corresponding doubling of active circumferential stress (force/unit area), suggesting that the force increase resulted from 1) an increase in contractility rather than being a reflection of the muscle hypertrophy that occurred with obstruction, and 2) muscle fiber reorientation. Thus, the ureter, dilated secondary to 2 wk of obstruction, is not mechanically decompensated bur rather undergoes changes that result in an increase in contractility, Despite muscle hypertrophy and increased contractility, the obstructed ureter's ability to develop the intraluminal pressures required for urine transport decreases. The decrease in intraluminal pressure despite an increase in contractility results from the increased ureteral diameter following obstruction according to the Laplace relationship.