Gentamicin incorporation along the nephron: autoradiographic study on isolated tubules

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.