Relation between cell length and force production in urinary bladder smooth muscle.

Guinea-pig and rabbit urinary bladders were fixed in glutaraldehyde at different volumes. Strips were dissected out, embedded and cut for phase contrast and electron microscopy. Muscle wall thickness decreased with increased bladder volume as did the radial number of muscle cells. Cell length, measured by a morphometric method increased linearly with bladder radius, indicating that no slippage between the muscle cells occurred. Number of cells per mm2 cross sectional area increased linearly with bladder radius. Volume-active force relations were obtained by pelvic nerve stimulations of guinea pig bladders filled to different volumes. Maximum pressure was obtained at 0.15 ml bladder volume, and maximum wall tension at a volume of 2.5 ml which corresponds to a cell length of 400 micrometers and a cell packing density of about 107 000 x mm-2. Estimation of the length-active tension curve for the average muscle cell in the guinea-pig bladder indicated a maximum active force of 5.5 microN/cell. Maximum active force per cm2 muscle bundle was calculated to be about 59 N. No compensation for extracellular space and nonmuscular tissue within the muscle bundle was made.