Pharmacology and physiology of the biliary radiographic contrast materials

(1) Solubilization of Telepaque in the intestine is a limiting factor in the rate of intestinal absorption. Bilopaque and Oragrafin are more water-soluble and appear to be better absorbed than Telepaque. (2) Bile salts in the intestinal lumen increase the solubility of Telepaque. Therefore, a fatty meal administered with the Telepaque is desirable to evacuate bile salts from the gallbladder into the intestine. This is not required for the more water-soluble agents, Biopaque and Oragrafin. (3) The degree of protein binding of the contrast agents can be related to the degree of toxicity. Cholografin is the most highly bound and is the most toxic. (4) Hepatic receptor proteins may specifically bind the biliary contrast agents. This may be the reason that the renal contrast materials are poorly escreted in bile compared to the biliary contrast agents. (5) Telepaque is conjugated in the liver with glucuronide making the compound more soluble in bile. This prevents precipitation of Telepaque in the gallbladder and avoids reabsorpiton from the intestine. (6) The biliary excretion of Telepaque is facilitated by bile salts. Therefore, the administration of a fatty meal with Telepaque not only increases the rate of intestinal absorption of Telepaque but also the rate of biliary excretion. (7) The rate of biliary excretion of both the oral and the intravenous contrast agents appears to be limited by a hepatic transport maximum. Above a certain dose, increased amounts of the contrast agents do not result in more rapid excretion of the agents into bile. Rapid infusion of intravenous contrast agents results in high plasma concentration and greater urinary excretion, without increasing the biliary excretion. It does not appear to be indicated in clinical practice. (8) The biliary concentration of the contrast agents used for intravenous cholangiography is determined by their rate of biliary excretion, the choleretic effect of the contrast agent, and factors that determine the rate of basal bile flow. Fixed coupling of water with the biliary excretion of these contrast agents imposes an inherent limitation on the concentration of the contrast agent in bile. It appears that the biliary concentration of the intravenous contrast materials can be increased by having the patient fast prior to intravenous cholangiography. This decreases the enterohepatic circulation of bile salts and the rate of bile-salt-dependent bile flow. (9) Failure of the gallbladder to visualize after administration of Telepaque when there is adequate biliary excretion may be due to cystic duct obstruction, failure of the inflamed gallbladder mucosa to reabosrb water, or reabsorption or the contrast agent by the diseased gallbladder mucosa. (10) Maximum concentration of Telepaque occurs at 14-19 hr after ingestion. It is at this time that radiographs of the gallbladder should be made. With Bilopaque, peak concentration occurs at 10 hr so radiographs can be made earlier when Bilopaque is used.