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Gentamicin is a nephrotoxic antibiotic of the aminoglycoside group, which accumulates within the renal cortex. The present study is an attempt to localize precisely the sites of gentamicin accumulation along isolated tubular segments. We performed autoradiography of 3 H-gentamicin (3H-G) uptake on isolated tubules from kidneys of 6 rabbits previously treated by a single dose of this drug (125 muCi/kg of body wt; 140 microgram/kg of body wt). Isolated tubules were obtained by microdissection following collagenase incubation, 4 hours after 3H-G administration. Autoradiography of single isolated tubular segments was performed according to a dry-film technique. Results were as follows. Almost no gentamicin incorporation (less than 2 silver grains per 150 micrometer2) takes place along the distal parts of the nephron, from the beginning of the loop of Henle to the end of the medullary collecting duct. No differences were visible along these parts of the nephron, whatever their localization, cortical or medullary, In the proximal tubule (PT), we observed a gradual regular increase in 3H-G accumulation, from the glomerulus to the end of the pars recta. The silver grain density progressively increased along this structure from the very early PT (5 per 150 micrometer2) to the last millimeter of the pars recta (40 per 150 micrometers). No clear difference between superficial and juxtamedullary nephrons was detected. The possible mechanisms that could account for this observed variation in 3H-G cellular uptake along the PT are discussed.

1. In mice and guinea-pigs, the number of glomeruli was counted in kidneys during normal growth and in hypertrophy induced by unilateral nephrectomy. 2. In mice, the number of glomeruli increased sharply during the first 2 weeks in life, and more slowly afterwards. Unilateral nephrectomy, when performed during this period of natural increase, induced the formation of supplementary nephrons in the contralateral kidney. 3. In guinea-pigs, the number of glomeruli was almost complete at birth. No evidence of a supplementary increase in the number of nephrons was found in hypertrophied kidneys following unilateral nephrectomy. 4. These results, together wit previous data obtained in the rat, suggest that the ability to induce new nephrons after unilateral nephrectomy in different species would depend more on the state of kidney maturity at birth than on differences in the renal mechanisms which lead to hypertrophy.

Previous biochemical studies indicated that the spirolactone-mineralocorticoid receptor complexes are unable to translocate into the nucleus. The present study was designed to reinvestigate the intracellular distribution of spirolactone-binding sites, using autoradiography. For this purpose, rabbit kidney pyramids were incubated at 30 C with tritiated SC9420 or aldosterone. Thereafter, aldosterone-sensitive cortical collecting tubules were microdissected and processed for dry film autoradiography. The concentration was 2 nM for both steroids. Non-specific labeling was determined by incubations with tritiated steroids plus a 100-fold excess of unlabeled steroids. Results show the presence of specific nuclear labeling for both [3H] aldosterone and [3H]SC9420. Specific cytoplasmic labeling was very low for both [3H]aldosterone and [3H]SC9420. The nuclear labeling by [3H]SC9420 was equally and almost completely displaced by a 100-fold excess of unlabeled aldosterone or SC9420 (91% and 87%, respectively). We conclude that spironolactone-receptor complexes migrate into the nucleus. The difference between these results and those of previous studies with biochemical techniques, which failed to detect specific nuclear binding of spirolactone, may be due to methodological reasons.

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