Uptake of imipramine in rat liver lysosomes in vitro and its inhibition bybasic drugs

We investigated the uptake of imipramine (IMP) in highly purified lysosomes from rat liver and its inhibition by a variety of basic drugs in vitro. Th e uptake of [H-3]IMP into lysosomes peaked in less than 20 s, showing littl e temperature dependency or countertransport phenomena. It was accelerated by increase of extralysosomal pH, stimulated by Mg2+-ATP in KCl buffer, and suppressed by acidic ionophores. However, the uptake of [H-3]IMP in lysoso mes was approximately 140-fold higher than the value expected from the pH-p artition theory. IMP and other weak lipophilic bases like chlorpromazine an d propranolol raised the intralysosomal pH, and their potency was stronger than that of NH4Cl, a typical pH-perturbing weak base. A variety of basic d rugs inhibited the uptakes of [H-3]IMP and [C-14]methylamine into lysosomes , their 50% inhibitory concentrations (IC50) being almost the same for [H-3 ]IMP and [C-14]methylamine uptake (r = 0.842). A high correlation (r = 0.94 6) was observed between the IC50 values (for the inhibition of [H-3]IMP upt ake) and the lipophilicity (P-oct values). These results suggest that the a ccumulation of lipophilic basic drugs is driven primarily by the transmembr ane pH difference (pH-partition theory) but with the involvement of some ad ditional mechanism(s) related to drug lipophilicity, possibly binding (part ition or adsorption) to lipophilic substance(s) and/or aggregation within l ysosomes. Based on this idea, we have established a model that described an d successfully simulated the weak base-induced pH increase, the accumulatio n of a lipophilic weak base (IMP), and the inhibition of accumulation of IM P by lipophilic basic drugs.