The Microdialysis Shunt Probe: The Influence of Bile Salts on Recovery of Caffeine and Phenolphthalein Glucuronide and Use of In Vitro Recovery Data for Calibration In Vivo

microdialysis probe enables the study of the concentration of analytes in the bile in awake animals where the bile flow is intact (1-3). A diagram of the shunt microdialysis probe is shown in F1. The probe’s shunt, which carries the bile flow, is of appropriate dimensions for implantation into the bile duct of an adult rat. Suspended inside the shunt is a linear microdialysis probe that continuously samples low molecular weight compounds from the bile. The probe can be used in awake, freely moving animals for an extended period of time (2). It is important to show that compounds of interest in the bile can be studied quantitatively using microdialysis. Compounds actively secreted in bile are usually polar molecules with molecular weights exceeding 300-400 g/mol. The actual molecular weight threshold for biliary excretion is species dependent, being 325 g/mol in the rat compared to 500 g/mol in man (4). Biotransformation of molecules in the liver to form adducts with glucuronic acid, glutathione, glycine or sulfate increases both the molecular weight and the polarity of the parent compound, making it more likely to find these conjugates in bile. Compounds secreted in bile are released to the gastro-intestinal lumen where they can undergo elimination, further metabolism or reabsorption into portal circulation. The process of reabsorption into the bile is referred to as enterohepatic circulation (EHC) and is important for compounds such as the bile acids, vitamins A, D3 and B12, folic acid, phenolphthalein, imipramine, indomethacin, antibiotics and estrogens (5). The goals of this study were (1) to demonstrate that bile salts can affect the recovery of analytes through the microdialysis probe and (2) to show that, when bile salts are taken into account, calibration of the probe in vitro can be used to determine the in vivo concentration in the bile. In a previous study, it was shown that an imbalance of bile salts across the microdialysis membrane resulted in a net flow of water to the side of the membrane with the highest concentration of bile salts (6). The use of a 2% solution of bile salts in Ringer’s (BSR) as the perfusate was sufficient to avoid loss in sample volume during in vivo dialysis experiments against bile. In this study it is shown that the bile salts can have strong interactions with some analytes, thus affecting their ability to transport across the membrane. Two contrasting analytes, caffeine and phenolphthalein glucuronide, were chosen for comparison (F2). Caffeine (FW 194.2) is a small, water soluble compound The Microdialysis Shunt Probe: The Influence of Bile Salts on Recovery of Caffeine and Phenolphthalein Glucuronide and Use of In Vitro Recovery Data for Calibration In Vivo