Length Adaptation of the Passive-to-Active Tension Ratio in Rabbit Detrusor

The passive and active length–tension (L–Tp and L–Ta) relationships in airway, vascular, and detrusor smooth muscles can adapt with length changes and/or multiple contractions. The present objectives were to (1) determine whether short-term adaptation at one muscle length shifts the entire L–Ta curve in detrusor smooth muscle (DSM), (2) compare adaptation at shorter versus longer lengths, and (3) determine the effect of adaptation on the Tp/Ta ratio. Results showed that multiple KCl-induced contractions on the descending limb of the original L–Ta curve adapted DSM strips to that length and shifted the L–Ta curve rightward. Peak Ta at the new length was not different from the original peak Ta, and the L–Tp curve shifted rightward with the L–Ta curve. Multiple contractions on the ascending limb increased both Ta and Tp. In contrast, multiple contractions on the descending limb increased Ta but decreased Tp. The Tp/Ta ratio on the original descending limb adapted from 0.540 ± 0.084 to 0.223 ± 0.033 (mean ± SE, n = 7), such that it was not different from the ratio of 0.208 ± 0.033 at the original peak Ta length, suggesting a role of length adaptation may be to maintain a desirable Tp/Ta ratio as the bladder fills and voids over a broad DSM length range.