ACTIVE AND PASSIVE COMPLIANCE OF THE FETAL BOVINE BLADDER

Purpose: Others have shown that the fetal bovine bladder is relatively noncompliant. Previous studies on compliance of fetal bovine bladders have demonstrated that the youngest fetal bladders had lowest and the oldest fetal bladders (near full-term) had greatest compliance. Our s tudy was designed to determine the level of participation of active te nsion in the compliance of fetal bladders during gestation. Materials and Methods: Fetal bovine bladders were obtained immediately after mat ernal harvest and crown-to-rump length was measured to determine gesta tional age. The fetus was inspected for genitourinary anomalies and th e bladder was immediately placed in chilled M199 media, Strips (1 x 0. 5 cm.) were excised from the anterior sagittal plane of the bladder an d subjected to length-tension analysis in oxygenated Tyrode's buffer a t 37C. Tension was measured using a force transducer and length was in creased using a micropositioner. Compliance refers to the length-tensi on studies performed in normal Tyrode's solution and consists of a com bination of active (smooth muscle tone) and passive properties. Passiv e compliance refers to length-tension studies performed after inactiva tion of bladder smooth muscle tone. Compliance with muscle tone intact was determined by incrementally stretching the strips to twice restin g length in physiological buffer and then permitting them to return to resting length. Passive compliance with muscle tone ablated was deter mined in the same fashion after overnight incubation in calcium-free T yrode's buffer in the presence of 5 mM. egtazic acid and 10 mM, sodium azide, An exponential function was fit to the normalized length-tensi on curves, where the exponential coefficient (EC) is numerically inver sely proportional to compliance. Results: Passive compliance was great est in the youngest bladders (EC = 0.5 in the first trimester) and gra dually decreased with increasing fetal age (EC = 1.2 in the third trim ester). Active compliance demonstrated the opposite pattern, since the younger bladders were more stiff (EC = 2.1 in the first and 1.6 in th e third trimesters). Conclusions: These studies demonstrate that passi ve compliance is greatest in the youngest bladders and progressively d ecreases with gestation. However, active smooth muscle tone is greates t in the youngest bladders and decreases with gestation. Thus, high ac tive smooth muscle tone in the youngest fetal bladders results in rela tively poor compliance of the early stage fetal bladder.