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

The passive and active length-tension (L-T p and L-T a) 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-T a curve in detrusor smooth muscle (DSM), (2) compare adaptation at shorter versus longer lengths, and (3) determine the effect of adaptation on the T p/T a ratio. Results showed that multiple KCl-induced contractions on the descending limb of the original L-T a curve adapted DSM strips to that length and shifted the L-T a curve rightward. Peak T a at the new length was not different from the original peak T a, and the L-T p curve shifted rightward with the L-T a curve. Multiple contractions on the ascending limb increased both T a and T p. In contrast, multiple contractions on the descending limb increased T a but decreased T p. The T p/T a 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 T a length, suggesting a role of length adaptation may be to maintain a desirable T p/T a ratio as the bladder fills and voids over a broad DSM length range.