Molecular Pharmacology, Vol 13, 99-112, Copyright © 1977 by the American Society for Pharmacology and Experimental Therapeutics

Mechanism of Chloroquine Transport in the Isolated Retina

¹ Departments of Biological Chemistry and Pharmacology and Upjohn Center for Clinical Pharmacology, The University of Michigan Medical Center, Ann Arbor, Michigan 48109

The feasibility of using the isolated rat retina to study transport processes was ascertained. The incubated tissue was characterized by a constant extracellular space, high steady-state levels of energy reserves, and high ratio of cellular K⁺ to Na⁺. The mediated transport system for chloroquine was experimentally resolved from total drug uptake and was distinguished from the nonsaturable uptake component as well as from tissue binding. The saturable uptake process had an apparent K_m and V_max of 2.4 mM and 8 nmoles/mg, dry weight, per minute, and resulted in retinal accumulation of chloroquine relative to its concentration in the medium. The uptake was independent of monovalent cations and displayed a Q₁₀ of 1.62. Metabolic inhibitors and ouabain strongly enhanced the rate of chloroquine uptake. Exposure of the retina to these substances abolished the characteristics of the saturable transport system and resulted in drug uptake by a markedly stimulated, nonmediated process, apparently because of increased permeability of the plasma membrane. The saturable uptake increased at high pH, the data indicating an uncharged form of the drug during transport. Both uptake and exodus of chloroquine displayed the phenomenon of countertransport. Cellular disruption altered the sensitivity of uptake to temperature and pH, and abolished drug accumulation as well as countertransport. Quinacrine, amphetamine, and cocaine, but not biogenic amines, competitively inhibited the uptake of chloroquine and transaccelerated its transport. The results provide evidence for the existence in the retina of a mediated transport system for drugs structurally related to basic amines.