Length adaptation of the active length‐tension curve in rabbit detrusor smooth muscle

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.