This study investigates whether the phasic contractions of the opossum sphincter of Oddi (SO) delay bile flow by acting as a resistor or facilitate bile flow by acting as a pump. The common bile duct (CBD) and an adjacent segment of duodenum from eight opossums were studied in a propulsion evaluation system in vitro. This system required the production of hydrostatic work by the SO to transfer fluid from the CBD to the duodenum when the pressure in the duodenum was equal or greater than the pressure in the CBD. Fluid movement from the CBD to duodenum and duodenum to CBD was studied at pressure gradients up to 50 cmH2O before and after sodium nitroprusside (10(-7) M) inhibition of smooth muscle contractile activity. All preparations propelled fluid from the CBD to the duodenum against a pressure gradient ranging from 10 to 50 cmH2O. The SO emptied the CBD in a monoexponential fashion, with a time constant of 1.52 +/- 0.7 min, until CBD pressure was reduced to 8.5 +/- 3.2 cmH2O, when propulsion ceased. Superimposed on the CBD pressure waveform were pressure pulses of 1-2 cmH2O in amplitude that resulted from the contractions of the SO. CBD pressure was higher at the start than at the end of a periodic pressure pulse, whereas CBD pressure was stable between pulses. The frequency of the pressure pulses was greatest at the maximal CBD pressure (9.4/min) and decreased significantly when the basal pressure was reached (1.5/min, P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)