Natural history of aminoglycoside nephrotoxicity in the dog

The natural history of aminoglycoside nephrotoxicity is not well described. This study investigated in the dog renal functional and electrolyte abnormalities during and for 20 days following a 10-day course of low-dose gentamicin (7 mg/kg/day), high-dose gentamicin (30 mg/kg/day), and netilmicin (30 mg/kg/day). Renal histology was examined at the end of the study. Renal functional abnormalities occurred only in animals receiving high-dose gentamicin. A fall in maximal urinary osmolality (1579 ± 347 mOsm/kg/H₂O to 450 ± 118, p < 0.05) was followed by renal glycosuria and a fall in GFR (66.9 ± 11.9 ml/min to 21.3 ± 8.6, p < 0.05). These three functional indices had recovered by day 30 in the survivors. Plasma potassium fell in animals receiving high-dose gentamicin (3.8 ± 0.02 mEq/L to 3.3 ± 0.4, p < 0.05) and reached the lowest values (2.7 and 2.9 mEq/L) just prior to death in two animals dying in uremia. Netilmicin also caused a significant fall in plasma potassium (4.3 ± 0.1 mEq/L to 3.9 ± 0.1, p < 0.05). Hypocalcemia (10.0 ± 1.3 mg/dl to 7.8 ± 1.4, p < 0.05) but not hypomagnesemia developed following high-dose gentamicin. Peak serum aminoglycoside levels after high-dose gentamicin and netilmicin were comparable, but trough levels rose only in high-dose gentamicin animals and paralleled the fall in GFR. Light microscopy of the kidney 3 weeks after high-dose gentamicin demonstrated no proximal tubular necrosis but extensive focal tubulointerstitial nephritis, especially in the juxtamedullary cortex. Similar but less extensive derangements were noted in animals receiving low-dose gentamicin, despite the absence of functional abnormalities. Minor histological abnormalities were noted in animals receiving netilmicin. To summarize: 1) major renal functional and electrolyte abnormalities developed only following high-dose gentamicin and included impaired urinary concentration, glycosuria, reduced GFR, hypokalemia, and hypocalcemia (except for a fall in plasma potassium, similar doses of netilmicin were not nephrotoxic); (2) tubulointerstitial nephritis, particularly in the juxtamedullary cortex, occurred with low-dose gentamicin as well as high-dose gentamicin and may be a factor in delayed or incomplete recovery from gentamicin nephrotoxicity; (3) in this model, netilmicin at comparable doses was substantially less nephrotoxic than gentamicin; (4) renal potassium wasting may be a heretofore unrecognized consequence of aminoglycoside administration.