Effect of gentamicin on isolated glomeruli and proximal tubules of the rabbit

Alterations in both glomerular filtration rate and tubular transport occur in clinical gentamicin nephrotoxicity. We have studied the function of isolated tubules and glomeruli from rabbits treated with gentamicin. Gentamicin was administered subcutaneously to sexually immature (1400 to 1800 gm) or sexually mature (3800 to 4600 gm) New Zealand White rabbits in a dose of 15 mg/kg twice a day. Immature rabbits were treated for 28 to 31 days and developed only minimal renal insufficiency. About one-half of the mature rabbits developed azotemia. The mature rabbits that did not become azotemic were sacrificed after 28 to 30 days, and those that became azotemic were killed when their serum creatinine reached 2.5 mg/dl or higher (10 to 24 days). Animals were anesthetized and kidneys were removed for histologic examination and isolation of tubules and glomeruli. The ratio of p-aminohippuric acid (PAH) concentration in isolated tubule cells to that in medium after incubation in 3H-PAH (1 microM) at 37 degrees C for 30 minutes (T/M PAH) was used as an indicator of transport capacity of tubules. T/M PAH ratios averaged 196 +/- 18 and 111 +/- 21 for control immature and mature rabbits, respectively, and 135 +/- 22, 80 +/- 16, and 9 +/- 2 for gentamicin-treated immature and mature nonazotemic and mature azotemic rabbits, respectively. Glomeruli were isolated and filtration induced in vitro by a transcapillary oncotic gradient. Ultrafiltration coefficient, Kf, of glomeruli of immature and mature control rabbits averaged 3.78 +/- 0.29 and 5.84 +/- 0.51 nl/minute X mm Hg. Kf from gentamicin-treated immature rabbits averaged 2.82 +/- 0.20 and from mature azotemic rabbits 3.14 +/- 0.44 nl/minute X mm Hg. Kf of both mature and immature rabbits were decreased compared with controls (p less than 0.01). When all animals were considered, relative glomerular filtration rate, estimated from 1/serum creatinine, was positively correlated with the T/M PAH and Kf. When only experimental animals were studied, 1/serum creatinine and T/M PAH were also correlated. Decreased glomerular filtration rate and dysfunction of proximal tubules were also correlated with abnormal tubule histology. We suggest that injury to glomeruli and tubules may represent independent manifestations of gentamicin toxicity. Dysfunction may be present even when there are only mild histologic changes and glomerular filtration rate is near normal. Kf does not appear to limit glomerular filtration rate after treatment with gentamicin; rather, some direct or indirect effect of tubular injury may determine the decrement in glomerular filtration rate.