Length dependence of the contractility of pig detrusor smooth muscle fibres.

Evidence on the length dependence of the contraction velocity of smooth muscle fibres is contradictory. Nevertheless, a thorough understanding of this dependence is essential for a correct urodynamic diagnosis of voiding problems. We studied muscle fibres of pig urinary bladders (n = 23). Force-velocity relations were measured at different muscle lengths with a stop test technique. This method involves measuring force generation of electrically stimulated muscle fibres during controlled shortening from a pre-shortening length at a pre-set velocity to a fixed stop-length. We normalized the length dependence of the measured properties to slack length, optimum length (the length at maximum isometric force generation), and passive force. Isometric force was found to be length dependent with an optimum length of 290+/-68% of the slack length (n = 11, P < 0.05). The maximum shortening velocity was 0.37+/-0.14 s(-1) related to the slack length and 0.13+/-0.05 s(-1) related to the optimum length and was not length dependent (n = 16, P < 0.05). Slack length is preferable to normalize the length dependence of smooth muscle.