Contraction pattern of opossum gallbladder during fasting and after feeding

Our major aims in this study were to 1) determine in unanesthetized opossums the gallbladder volume, pressure, and tone during fasting as well as after feeding, 2) model the kinetics of changes in gallbladder volume, and 3) correlate gallbladder contractility with concurrent spike burst activity in the upper gastrointestinal tract and sphincter of Oddi (SO). In 10 animals, we implanted electrodes on the gastric antrum, duodenum, SO, and jejunum. An indwelling catheter monitored gallbladder pressure and volume. In two animals, a force transducer sutured to the gallblader wall monitored gallbladder tone. In each animal, fasting migrating myoelectric complexes (MMCs) were recorded in the stomach and small bowel. Cyclic changes in SO spike bursts occurred in synchrony with the MMCs. During fasting, gallbladder pressure, tone, and volume showed cyclic variations. During the second half of the duodenal MMC cycle, the gallbladder exhibited phasic as well as tonic contractions associated with net gallbladder emptying, whereas during the first half of the cycle the gallbladder relaxed and refilled. Minimal gallbladder volume during the MMC cycle averaged 4.8 ± 0.7 (SE) ml compared with a maximal volume of 7.0 ± 0.9 ml. Decreases in gallbladder volume occurred as an exponential function, whereas increases in gallbladder were linear. After feeding, the gallbladder underwent a sustained tonic contraction, without superimposed phasic contractions. Decreases in gallbladder volume occurred exponentially to a residual volume of 2.0 ± 0.5 ml. Net postprandial gallbladder emptying was maximal by 40 min; then the volume remained unchanged for 2-3 h. We conclude that 1) during fasting the opossum gallbladder undergoes periodic partial net emptying and net refilling synchronous with gastrointestinal MMC cycles and empties almost completely after feeding; 2) decreases in gallbladder volume occur as an exponential function, whereas its increase in volume is linear; and 3) changes in intraluminal gallbladder pressure generally parallel gallbladder contraction and emptying.