SPHINCTER OF ODDI REGULATES FLOW BY ACTING AS A VARIABLE RESISTOR TO FLOW

Two models of transsphincteric flow and a model evaluating pumping activity were established in the anesthetized Australian brush-tailed possum to determine whether the sphincter of Oddi (SO) acts as a resistor or as a pump. A simple model of transsphincteric flow (inflow only) demonstrated that at physiological common bile duct (CBD) pressure, 9.5 +/- 0.3 cmH2O (n = 7), transsphincteric flow occurred between SO pressure waves (n = 10). A second more complex transsphincteric flow model was established that permitted simultaneous measurements of inflow, outflow, CBD pressure, SO basal pressure, SO contraction frequency, and amplitude. At physiological CBD pressure, inflow always equaled outflow (157.0 +/- 11.2 and 156.4 +/- 11.4 mul/min, respectively; n = 7). The SO displayed regular contractions superimposed on a basal pressure of 1.1 +/- 0.4 mmHg. Contraction amplitude was 12.6 +/- 3.0 mmHg and the frequency was 3.6 +/- 0.4 contractions/min (n = 7). Pressure waves recorded in the CBD corresponded to the SO contractions and reflected SO activity. Transsphincteric flow occurred between SO contractions and was obstructed by these contractions. Stimulation of SO activity (basal pressure and contraction frequency) with intra-arterial injections of motilin (200 ng/kg) or erythromycin (200 mug/kg) abolished transsphincteric flow. Reduction in SO contraction frequency to 72.7 +/- 7.2% (P < 0.01, paired t test) after administration of Cisapride (2 mg/kg iv) increased transsphincteric flow to 147.6 +/- 12.3% (n = 7, P < 0.05, paired t test). In six possums, possible SO pumping action was evaluated. A manometer was connected to the CBD, and a second manometer was connected to the duodenum surrounding the papilla. When the fluid level in each manometer was set to 10 cm above the level of the SO, no pumping was observed within 20 min (n = 6). When a hydrostatic gradient of +10 cmH2O was established (relative to the CBD), fluid flow from the CBD to the duodenum was observed but no pumping was detected in 20 min (n = 6). To test for duodenobiliary reflux, a -10-cmH2O pressure gradient (relative to CBD) was established. No fluid flowed from the duodenum into the CBD (n = 6). In the Australian brush-tailed possum the SO regulates flow from the CBD into the duodenum by acting as a variable resistor. Furthermore, this activity prevents any reflux of duodenal contents into the CBD.