Confocal Imaging of Peripheral Regions of Intact Rat Lungs Following Intratracheal Administration of 6-Carboxyfluorescein, FITC-insulin, and FITC-Dextran

Abstract

Purpose. This study compared the pulmonary disposition of 3 structurally diverse probe molecules following their intratracheal (i.t.) administration to anesthetized rats.

Methods. Following administration of 6-carboxyfluorescein (CF), FITC-insulin (FI) and FITC-dextran (FD), lungs were removed, inflated, perfused with a marker dye, and peripheral elements examined with confocal microscopy (CLSFM).

Results. At 5 min most of each probe remained within airspaces; the remainder distributed to interstitium, capillaries, Type II cells, and macrophages. At 60 min disposition differed significantly among probes. The smallest (CF, 376 Da) had almost completely exited airspaces and was found primarily in extracellular interstitial spaces, often behind Type II cells. Disposition was consistent with both entry into peripheral lymphatics and association with peripheral fibers. FI and FD (6 and 10 kDa, respectively) were retained substantially longer within airspaces. In contrast to CF, FI appeared to localize along septal and peripheral fibers, but its disposition was inconsistent with the involvement of peripheral lymphatics.

Conclusions. While all probes were ≤10 kDa, there was considerable disparity among both their rates of absorption and subsequent disposition within peri-alveolar elements. CLSFM appears to be a useful ancillary tool for studying the pulmonary absorption of drugs and macromolecules.