Molecular Pharmacology, Vol 10, 837-848, Copyright © 1974 by the American Society for Pharmacology and Experimental Therapeutics

Characterization of the Transport System for Benzomorphans in Leukocytes

¹ Department of Biological Chemistry and Upjohn Center for Clinical Pharmacology, The University of Michigan Medical School, Ann Arbor, Michigan 48104

The biological transport of pentazocine in leukocytes displayed characteristics of an active, carrier-mediated process. Within 5 min the uptake reached cellular equilibrium levels several fold higher than those in the medium. The transport process followed simple saturation kinetics, with an apparent K_m of 40 µM and V_max of 100 nmoles/g of cells 5 sec. The uptake was sodium-independent. At decreased temperatures and in the presence of metabolic inhibitors, the transport of pentazocine was subject to noncompetitive inhibition. For temperatures between 0° and 37° the V_max for uptake had a Q₁₀ of 1.88. Poisons of glycolysis were more effective in blocking drug uptake than were inhibitors of aerobic energy production, and oxygen deprivation did not affect the transport process. The rate of transport of pentazocine into leukocytes decreased linearly with the fall in cellular ATP content. Uptake of pentazocine in cells depleted of ATP amounted to one-fifth of the uptake in control cells and was equal to that in cells previously treated by heating or freezing. Efflux of the drug from leukocytes was rapid and sensitive to temperature. Exodux appeared to be the result of two first-order processes with greatly different rate constants. Only the higher of these rate constants was affected by temperature, and displayed a Q₁₀ of 1.86. Both uptake and exodus exhibited the phenomenon of countertransport, showing transacceleration. Uptake of pentazocine and inhibition of this process at various pH values indicated that the drug was transported when it bore no net charge. Benzomorphan analogues and amines of wide structural variety, excluding quaternary amines or those with acidic character, competitively inhibited the uptake of pentazocine. Additional structural requirements in the uptake process have been elucidated. On the basis of its characteristics, the transport system for benzomorphans in leukocytes represents a novel process for the cellular uptake of amines.