Distensibility of the mouse rectum: application of impedance planimetry for studying age-related changes

Rectal distensibility is an important parameter for investigation of anorectal function. The aim of the present paper was to study pressure-cross-sectional area (CSA) relations and distensibility of the rectum in mice aged 15-78 weeks using impedance planimetry. A four-electrode probe system located inside a balloon was designed to determine pressure-CSA relations in the mouse rectum. CSA was estimated from measurements of the electrical impedance of saline inside a balloon. The method was validated in studies in vitro showing high accuracy and good reproducibility. The method was applied in measurements of inter- and intra-individual variation and age-related variation of rectal pressure-CSA relations and distensibility. Repeated distension every third day in a cohort of 10 mice did not change the CSA during a 4-week period and there was no long-term effect of repeated measurements. There was an age-related increase in rectal CSA in the distended state until the mice were 26 weeks of age (P = 0.004). The unloaded size of the rectum did not change with age. The distensibility of the rectum did not vary significantly with age, though there was a tendency to a higher distensibility during growth followed by decline with age (P = 0.08). In conclusion, this study presents a simple, minimally invasive in vivo model for repeated evaluation of rectal distensibility in mice. The mouse model will provide a tool for analysing relative changes in rectal biomechanical properties in different future disease models.