CHARACTERIZATION OF CIMETIDINE TRANSPORT IN LLCPK(1) CELLS

In this study, cimetidine uptake and its regulation by LLCPK1 monolaye rs were investigated. Uptake was temperature dependent with kinetic an d specificity characteristics typical of a carrier-mediated mechanism. With cimetidine uptake in the presence of an excess concentration of the potent inhibitor quinidine as a measure of nonspecific transport, the estimated kinetic parameters for cimetidine uptake at 37-degrees-C under steady-state conditions are K(m) = 32.3 +/- 6.4 muM and V(max) = 20.2 +/- 2.1 pmol/mg per minute. Amiloride, quinidine, and quinine i nhibited cimetidine uptake, whereas N1-methylnicotinamide, tetraethyla mmonium, and guanidine did not. The uptake of cimetidine was increased in the presence of a cell --> lumen H+ gradient, consistent with the behavior of a cimetidine-H+ antiport system. Furthermore, the activity of both the Na+-H+ exchanger and H+-ATPase acted to dissipate the cel l --> lumen H+ gradient, thereby decreasing net cimetidine transport. These results suggest that there is a cimetidine-H+ exchange system in LLCPK1 cells and that the net secretion of organic base in vivo may b e regulated by luminal acidification mechanisms,