Influence of muscle length on the force-velocity relation of K+-contractures in smooth muscle from rabbit urinary bladder

Force-velocity relations of K+-contractures of longitudinal smooth muscle from rabbit urinary bladder were studied by isotonic quick release at 37 degrees C. In order to minimize the influence of parallel elasticity the study was limited to the rising part of the length-tension curve. The force-velocity data fitted well with Hill's equation. The in situ length of the strip at a bladder volume of 10 ml is called L10. This length is 50% of that at which maximum active tension is developed. At L10 Vmax was 0.29 muscle lengths per second and it was estimated to be 0.36 lengths/s at optimum length. Constant b in Hill's equation had a value of 0.052 L10/s and it was unaffected by length changes over the interval 0.69 L10-1.44L10. At L10 a/Po was 0.17. In the interval given above, a/Po decreased with increasing length in proportion to the increase in Po, indicating that a was also length independent. According to Hill's equation [V = b(Po - P)/(P + a)], V should increase in proportion to (Po - P) when the muscle length is increased if a and b are constants. Such a linear relation was found at shorter lengths but at lengths close to or at the length for maximum active tension, V increased more than (Po - P). Two possible explanations were considered; firstly that b/(P + a) increased, and secondly that the load on the contractile element could be less that P due to an influence of the considerable tension in the parallel elastic element at these lengths. The series elastic recoil of the active muscle amounted to 3-4% of the muscle length when released to zero tension.