Carrier-mediated transport of the organic cation procaine amide ethobromide by isolated rat liver parenchymal cells.

Using hepatocytes isolated by collagenase perfusion, we studied the kinetic characteristics of the uptake process for procaine amide ethobromide (PAEB). Determination of initial uptake velocities (Vo) at substrate concentrations from 30 to 400 micrometer demonstrated a saturable process with a Km of 54 +/- 10 micrometer and a Vmax of 0.13 +/- 0.01 nmol/min/mg of protein. Pretreatment of cells with metabolic inhibitors and reduction of the incubation temperature significantly reduced the Vo of 100 micrometer PAEB. Replacement of sodium ions with lithium had no effect, while replacement with choline decreased Vo by 75%. The intracellular concentration of PAEB was 18 times the medium concentration after 90 min, but 33% of that was in the acetylated form. Uptake of N4-acetyl PAEB occurred at a much lower rate and reached a cell/medium ratio of only 6 after 90 min. Only one of seven quaternary amines tested inhibited PAEB uptake at an inhibitor/substrate ratio (I/S) of 7.5, while four out of five tertiary amines significantly decreased Vo at an I/S of 0.75 and all five decreased it at a ratio of 7.5. Some organic acids and steroidal compounds also significantly decreased PAEB Vo at an I/S of 0.75, while others from each group had no effect at an I/S of 7.5. Because uptake is saturable, requires metabolic energy, and occurs against an electrochemical gradient, it is suggested that the hepatic accumulation of PAEB occurs via an active, carrier-medicated transport process.