The length-tension (L-T) relationships in airway and vascular smooth muscles have been shown to adapt with length changes over time. Our prior studies have shown that the active and passive L-T relationships in rabbit detrusor smooth muscle (DSM) can adapt and that DSM exhibits adjustable passive stiffness (APS) characterized by a passive L-T curve that is a function of strain and activation history. The present study demonstrates that passive tension due to APS can represent a substantial fraction of total tension over a broad length range. Our previous studies have shown that maximal KCl-induced contractions at short muscle lengths generate APS that is revealed by increased pseudo-steady-state passive tension at longer lengths compared with previous measurements at those lengths. The objective of the present study was to determine the mechanisms involved in APS generation. Increasing the number of KCl-induced contractions or the duration of a contraction increased the amount of APS generated. Furthermore, a fraction of APS was restored in calcium-free solution and was sensitive to the general serine and threonine protein kinase inhibitor staurosporine. Most importantly, rhythmic contraction (RC) generated APS, and because RC occurs spontaneously in human bladder, a physiological role for RC was potentially identified.

Studies have shown that the length-tension (L-T) relationships in airway and vascular smooth muscles are dynamic and can adapt to length changes over a period of time. Our prior studies have shown that the passive L-T relationship in rabbit detrusor smooth muscle (DSM) is also dynamic and that DSM exhibits adjustable passive stiffness (APS) characterized by a passive L-T curve that can shift along the length axis as a function of strain history and activation history. The present study demonstrates that the active L-T curve for DSM is also dynamic and that the peak active tension produced at a particular muscle length is a function of both strain and activation history. More specifically, this study reveals that the active L-T relationship, or curve, does not have a unique peak tension value with a single ascending and descending limb, but instead reveals that multiple ascending and descending limbs can be exhibited in the same DSM strip. This study also demonstrates that for DSM strips not stretched far enough to reveal a descending limb, the peak active tension produced by a maximal KCl-induced contraction at a short, passively slack muscle length of 3 mm was reduced by 58.6 +/- 4.1% (n = 15) following stretches to and contractions at threefold the original muscle length, 9 mm. Moreover, five subsequent contractions at the short muscle length displayed increasingly greater tension; active tension produced by the sixth contraction was 91.5 +/- 9.1% of that produced by the prestretch contraction at that length. Together, these findings indicate for the first time that DSM exhibits length adaptation, similar to vascular and airway smooth muscles. In addition, our findings demonstrate that preconditioning, APS and adaptation of the active L-T curve can each impact the maximum total tension observed at a particular DSM length.

No abstract

Until the 1990s, the active and passive length‐tension (L‐T) curves for smooth muscle were believed to be static, with single passive tension (Tp) and maximum active tension (Ta) values for each length. However, recent studies have shown that the L‐Ta curves in airway and vascular smooth muscles can adapt to length changes. Our prior studies have shown that the L‐Tp curve in detrusor smooth muscle (DSM) is not static and can shift along the length axis as a function of strain and activation history, a trait we labeled adjustable passive stiffness (APS). In the present study, calcium‐free Tp and peak total tension (Tt) during a maximal KCl‐induced contraction were measured in DSM strips, and Ta was found by subtracting Tp from Tt. Strips were contracted at their passive slack length (Ls, ~3 mm) and then passively stretched to and contracted 5 times at 3‐fold Ls. These contractions produced increasingly greater Ta that approached a steady value. Next, strips were contracted once at 4‐fold Ls, returned to 3‐fold Ls and contracted 5 times. Ta at 3‐fold Ls was reduced after the contraction at 4‐fold Ls, and the 4 subsequent contractions at 3‐fold Ls produced increasingly greater Ta that returned to the initial steady Ta. These results provide evidence that the L‐Ta curve in DSM can adapt to changes in length. Support was provided by the Edwin Beer Research Program in Urology and Urology‐Related Fields of The New York Academy of Medicine.