Studies of the hepatocellular uptake of the hepatobiliary scintiscanning agent 99mTc-DISIDA.

Bilirubin (BR) and organic anionic dyes such as sulfobromophthalein (BSP), indocyanine green (ICG) and rose bengal (RB) enter the hepatocyte by a specific non-sodium-dependent membrane transport system. Two analogous but distinct Na+-dependent transport systems effect the uptake of conjugated bile acids such as taurocholate (TC) and free fatty acids such as oleate, respectively. The mechanism of uptake of the acetanilidoiminodiacetic acid (HIDA) class of biliary scintiscanning agents is unknown. Accordingly, rat hepatocytes were isolated by collagenase perfusion of the liver and differential centrifugation, and incubated with 99mTc-diisopropyl-HIDA (99mTc-DISIDA) alone, or in the presence of various concentrations of BSP, ICG, RB, oleate or TC, with and without bovine serum albumin (BSA). Initial uptake velocity (Vo) was determined from the initial slope of the cumulative radioactivity/time curve. In albumin free media, uptake of 99mTc-DISIDA was temperature- and pH-dependent, with maximal uptake at 37 degrees C. There was virtually no uptake of inorganic 99mTc. In incubations containing 15-1500 microM 99mTc-DISIDA, Vo was saturable, with estimated Vmax = 65 nmoles/min/10(6) hepatocytes, and Km = 1200 microM. In keeping with its weak albumin binding, 99mTc-DISIDA Vo was only minimally influenced by equimolar concentrations of BSA. 99mTc-DISIDA Vo was inhibited by BR, BSP, ICG and RB, as well as by a rabbit antibody to the rat liver plasma membrane BSP/BR binding protein. Surprisingly, Vo was also inhibited by TC and oleate, but not influenced either by ouabain or by substitution of Li+ for Na+ in the medium.(ABSTRACT TRUNCATED AT 250 WORDS)