Influence of Muscle Length on the Force-Velocity Relation of K+-contractures in Smooth Muscle from Rabbit Urinary Bladder

Abstract

Force-velocity relations of K+-contractures of longitudinal smooth muscle from rabbit urinary bladder were studied by isotonic quick release at 37.degree. C. In order to minimize the influence of parallel elasticity, the study was limited to the rising part of the length-tension curve. Force-velocity data fitted well with Hill''s equation. In situ length of the strip at a bladder volume of 10 ml was called L10. This length was 50% of that at which maximum active tension was developed. At L10 Vmax was 0.29 muscle lengths /s 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 L10-1.44 L10. At L10 a/Po was 0.17. In the interval given above, a/Po decreased with increasing length in proportion to increase in Po, indicating that a was length independent. According to Hill''s equation [V = b(Po -P)/(P + a)], V should increase in proportion to (Po-P). Two possible explanations were considered; that b/(P + a) increased, and that load on the contractile element may be less than P due to an influence of 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 0 tension.