A study of the dynamics of imipramine accumulation in the isolated perfused rabbit lung

An isolated perfused rabbit lung preparation (IPL) was used to study the uptake, accumulation, and efflux of the tricyclic amine imipramine (IMIP). The rate of IMIP uptake into the IPL was resolved into two exponential components (rates 1 and 2 of uptake). The initial velocities for these uptake components were linearly related to the concentration of IMIP in the perfusate (Cp). This linear relationship indicates that IMIP accumulated in the IPL by diffusion and/or binding. The steady-state accumulation of IMIP was obtained by integration of the exponential expression relating the rate of IMIP uptake to time. The amount of IMIP accumulated at steady state by rate 1 was linearly related to Cp, whereas the amount of IMIP accumulated at steady state by rate 2 was saturable with respect to Cp. These steady-state data are in agreement with the steady-state accumulation data previously reported from experiments with the recirculating blood-perfused rabbit lung. In the absence of IMIP in the perfusate. IMIP that had previously accumulated in the IPL effluxed from the lung at three rates (t1/2 = 18 sec, 58 sec, and 8.25 min), which indicates that accumulated IMIP was in at least three pools in the lung. In addition, a noneffluxable pool was detected which was not the result of irreversible binding to tissue.ated in pools 1 and 2 by rate 1 of uptake. The IMIP in pool 3 and in the noneffluxable pool was accumulated in the IPL by rate 2 of uptake. Efflux pool 3 and the IMIP accumulated at a steady state by rate 2 of uptake were resolved by a Hofstee plot into a biphasic curve indicating two types of binding sites. The noneffluxable pool of IMIP in the IPL was saturable with increasing Cp and represented approximately 30% of the IMIP accumulated by rate 2 into pool 3. Rate 2 of uptake, pool 3, and particularly the noneffluxable pool of IMIP in the IPL are possibly responsible for the accumulation and persistence of Imip in the lung as seen in whole body distribution studies. The physicochemical properties of IMIP and other compounds known to be accumulated in lung tissue are discussed in relation to the possible involvement of lung phospholipids and the compartmentalization of IMIP in the concentric lamellar organelles of the lung.