Viscoelastic properties of the contracting detrusor. II. Experimental approach.
Mechanical properties of detrusor muscle were studied with small-amplitude oscillatory volume perturbations in isometrically contracting bladders of anesthetized dogs. Contractions were studied at oscillatory frequencies (f) of 2 and 4 Hz and at bladder volumes (Vbl) ranging from 30 to 110 ml. The magnitude of bladder hydrodynamic stiffness (magnitude of G) increased linearly with mean detrusor pressure (Pdet) while the phase angle remained relatively constant during contraction. The slope (mG) of magnitude of G-Pdet relations had a positive dependence on f and a negative dependence on Vbl. Analysis of oscillatory data, described in the companion paper, was performed using incremental lumped-parameter models consisting of a spring with incremental constant (S = dF/dL), a viscous element with incremental viscosity (b = dF/du), and a mass (m). Only the model where elastic and viscous elements were placed in series with each other and in parallel with mass was compatible with the experimental data. Both S and b increased linearly with effective force (F), defined as Pdet times the cross-sectional area of the intravesical cavity. Slopes of the S-F and b-F relationships (ms and mb) were independent of Vbl and varied only slightly with f. The importance of this finding stems from recognizing that ms and mb correspond to the exponential coefficients of nonlinear series elastic and internal viscosity elements. These parameters, when normalized by resting muscle length, represent fundamental muscle properties independent of muscle cross-sectional area, stretch, or level of activation and compare well with parameters derived from other muscle systems using techniques such as quick releases and isotonic contractions.