Mechanisms of pelvic organ crosstalk: 1. Peripheral modulation of bladder inhibition by colorectal distention in rats

PURPOSE: Bladder activity can be inhibited by afferent input from the colorectum (inhibitory rectovesical reflex). We evaluated the functional response of the rat bladder to nonnoxious and noxious colorectal distention, and investigated the mechanical and pharmacological peripheral modulation of this response.

MATERIALS AND METHODS: In 70 female Sprague-Dawley® rats we evaluated the effect of nonnoxious (20 mm Hg) and noxious (40 and 60 mm Hg) colorectal distention on the micturition volume threshold and on bladder activity in a filled bladder. We also studied the effect of rectal balloon size (1.5 vs 3.5 cm long), and rectal administration of 2% lidocaine jelly or 1 mM allyl isothiocyanate solution on the inhibitory rectovesical reflex.

RESULTS: Colorectal distention at 60 mm Hg increased the micturition volume threshold (mean ± SE 0.640 ± 0.056 vs 0.448 ± 0.035 ml in controls, p <0.001). Bladder contraction frequency was significantly decreased by 40 and 60 mm Hg colorectal distention vs controls (mean 0.62 ± 0.06 and 0.33 ± 0.05 per minute, respectively, vs 0.77 ± 0.03, each p <0.001). These effects were reversible and pressure dependent (p <0.001), and more pronounced using a large rectal balloon (mean 40 vs 60 mm Hg colorectal distention 0.35 ± 0.12 vs 0.07 ± 0.04 per minute, p = 0.004). We noted no significant graded inhibition of bladder contraction amplitude or duration. The inhibitory rectovesical reflex was reversibly abolished by intrarectal lidocaine administration. Intrarectal allyl isothiocyanate administration significantly increased the effect of noxious colorectal distention on bladder contraction frequency.

CONCLUSIONS: Only noxious levels of colorectal distention initiated the inhibitory rectovesical reflex. The effect increased with rectal balloon size and with intrarectal administration of allyl isothiocyanate. It was reversibly abolished by lidocaine. Results suggest that spinal interneurons are the mechanism behind the inhibitory rectovesical reflex.