Transport of gentamicin in rat proximal tubule

The intrarenal distribution of tritiated gentamicin (GM) was determined in rat by combined immunofluorescence and section freeze-dry autoradiography, techniques that permit subcellular localization before and after diffusional redistribution. Tissue from these kidneys was also examined by electron microscopy. After parenteral administration of 4 to 100 mg/kg, GM accumulates in S1 and S2 but not S3 segments of proximal tubules. Within 10 minutes, autoradiography demonstrates 3H-GM in the lumina of proximal and distal tubules; a subapical distribution consistent with pinocytotic uptake is prominent in many proximal cells. After 1 hour, 3H-GM is diffusely distributed within the cytoplasm in section freeze-dry autoradiographs with minimal evidence of intracellular sequestration. At this time, 3H-GM is presumably within endocytotic vacuoles, and electron microscopy reveals only rare vacuoles containing single myeloid bodies. Subsequently, section freeze-dry autoradiography shows sequestration of the aminoglycoside, but this intracellular localization is lost during tissue processing for fluorescent microscopy up to 6 hours after injection. At 6 hours large cytoplasmic vacuoles containing multiple well-organized myelin figures first appear in S1 and S2 segments. By 48 hours, 3H-GM is firmly bound in these vacuoles and is maintained in situ in both section freeze-dry autoradiographs and immunofluorescent preparations in association with increased numbers of vacuoles containing multiple myeloid bodies by electron microscopy. These studies thus demonstrate diffusible 3H-Gm within the cell which is available to initiate nephrotoxicity 1 to 6 hours after administration.