Mechanics and Ca2+-sensitivity of human detrusor muscle bundles studied in vitro

Mechanical properties of isolated smooth muscle strips from human urinary bladder were investigated in vitro. Bladder tissue was obtained from tumour-free wall regions of bladders from male patients undergoing cystectomy for bladder carcinoma. In intact muscle strips, activated with high-K⁺ solution, half-maximal force occurred at about 0.9 mm extracellular Ca²⁺]. The length-active force relation was determined and the muscle strips were fixed for light and electron microscopy at optimal length for active force (I_o). The maximal active force per unit smooth muscle cross-sectional area was 208±49 mN/mm², n= 6. Chemically skinned preparations were obtained by treatment with triton X-100. These preparations had a steep Ca²⁺]-force relation in the micromolar range which was influenced by calmodulin. The skinned preparations could be maximally activated by irreversible thiophosphorylation of the regulatory light chains. The force-velocity relation was determined in the maximally activated skinned muscle at 22 °C at 0.5 1_o. When the muscle was shortened by 10%, force was reduced by 35% whereas the maximal shortening velocity was little affected.