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The site of localization of [3H]gentamicin within mouse kidney is shown to be the proximal renal tubule by coincidence of the radioactivity, as visualized by autoradiography, and the mucopolysaccharide-rich microvilli characteristic of proximal convoluted tubules, as visualized by histochemical staining.The extensive accumulation and avid retention of aminoglycoside antibiotics by the kidneys of both experimental animals (4, 9, 13) and humans (1, 3) may be related to the nephrotoxicity associated with these drugs. If these processes are related, then an understanding of the precise anatomic localization of aminoglycoside accumulation and retention may be fundamental to the prevention and treatment ofaminoglycosideinduced nephrotoxicity. We have investigated the intrarenal localization of the aminoglycoside gentamicin in the mouse kidney.Three ,uCi/mg with gentamicin sulfate from Sigma) in 0.2% ethanol. Three control male mice were injected with 5 ml of 0.2% ethanol per kg, and both kidneys from each of these mice were prepared and examined in the same manner as for the treated mice. Twenty-four hours later, both kidneys from each mouse were removed under pentobarbital anesthesia, and sections of the medial parts of the kidneys (both axes were sampled in different kidneys) were mounted on a copper microtome chuck and frozen in liquid nitrogen slush. Four-micrometer sections of these tissues were either thaw-mounted or drymounted onto emulsion-coated (Kodak NTB-3) slides and stored for approximately 1 month. This autoradiographic procedure has been developed to prevent or minimize the diffusion of small molecules from tissue sites (11). After development of the emulsion, the tissues were stained either with pyronine-y or by the periodic acid-Schiff technique, which visualizes mucopolysaccharide-rich microvilli characteristic of cells in the proximal convoluted tubules.A representative photograph of the resultant autoradiograph and histochemical staining is shown in Fig. 1. All kidney preparations from treated animals showed similar findings. Dense clusters of autoradiographic grains (ARG) were restricted to cortical regions of the kidney, where they were associated with most, but not all, tubule profiles. The ARG were strikingly absent over glomeruli and over structures in the medulla including the thin limbs, collecting ducts, and blood vessels. A very slight enrichment of ARG was observed over the straight parts of the tubules below the cortical layers containing the glomeruli. In periodic acid-stained sections, the ARG were found exclusively over tubules that stained positively for mucopolysaccharides near the lumen. Furthermore, the convoluted tubules with the dense cluster of ARG had larger cells when compared with the tubules with no ARG. Control slides prepared for positive and negative chemography showed no spurious generation of or fading of latent images. The same localization has also been visualized in rat kidney preparations (data not shown).From these data it is concluded that [3H]gentamicin is very ...

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