Abstract

Goldfish kidneys are able to repair gentamicin-induced injury by regeneration along the injured nephron and by developing distinctly new nephrons. The ability to develop new nephrons following nephro-toxicant injury is unique to fish. The new nephrons develop from clusters of basophilic cells found in the interstitial hemopoietic tissue. The origin of these clusters is currently unknown. Are they embryonic remnants which can provide only one round of new nephron development, or do they originate from some type of precursor cells as the need for new nephrons arises? To address this question we exposed goldfish to three sequential doses of gentamicin, six weeks apart. Data indicate that the ability of goldfish to produce nephrons is intact after multiple gentamicin exposures.

Introduction

Gentamicin nephrotoxicity in mammalian kidneys is characterized by necrosis of proximal tubule cells and sloughing of these cells off the basement membrane. This is followed over the next days by a regenerative response along the existing nephron. Surviving cells flatten, migrate along the basement membrane and divide to repopulate the nephron (1,2,3). The goldfish kidney, in addition to this regenerative response, is able to produce entirely new nephrons following gentamicin-induced toxicity. These nephrons develop from clusters of basophilic cells in the interstitial tissue (4). To determine if these clusters are embryonic remnants and can only produce one round of new nephrons we examined the response of the fish kidney to multiple exposures.

Materials and Methods

Goldfish were given up to three intraperitoneal injections of 50 mg/kg Gentamicin. Fish were then sacrificed at varying intervals throughout the course of the 19 week period (6,8,9,10,15,17,18,19 weeks). Kidneys were removed immediately following sacrifice and prepared for histology. Using light microscopy, sections were examined and the number of basophilic clusters, basophilic tubules, and young tubules quantified.

Results

The immediate effects of 50 mg/kg gentamicin on goldfish kidney is profound as demonstrated by Reimschuessel (4). Within approximately six weeks after the first injection, the kidney appeared well repaired and comparable with controls. On day 3 after the second injection, active necrosis was widespread. Two weeks later, basophilic clusters became active and at three weeks, regeneration along the nephron and developing nephrons were observed. By the fourth and fifth weeks after the second injection, the kidney appeared almost normal with some developing nephrons present and a somewhat reduced number of tubules.

The third injection, again induced widespread necrosis evident three days later. Two weeks following the third injection, fish exhibited slightly varying degrees of damage and repair. New nephrons at varying stages of development were present. Some of the "older" new nephrons, developing as a result of the second gentamicin injection, were resistant to the third injection. This is similar to what is seen in rat fetal kidney (5-8). At three weeks, developing nephrons continued to be produced. At the conclusion of the 19-week study, the kidneys of fish receiving three consecutive exposures demonstrated some evidence of toxic insult (reduced numbers of tubules and presence of developing nephrons). This was not unexpected since only four weeks had passed since the last injection of gentamicin. Controls, single-injected fish (19 weeks post-injection), and twice-injected fish (13 weeks post-injection), all had normal kidney profiles.

Conclusions

1.  Gentamicin-induced kidney damage in fish elicits a two-tier repair process which involves repopulation of damaged regions as well as development of new nephrons.

2.  Both levels of repair to injury persist after multiple rounds of gentamicin-induced injury.

3.  New nephrons in early stages of development appear resistant to toxic insult. The mechanism by which these cells are conferred resistance is unknown. It may be due to the absence of metabolic "machinery" required to process the gentamicin or could be due to inability of the cells to transport gentamicin into the cytoplasm.

References

1.  Spangler WL, Adelman RD, Conzelman Jr. GM, Ishizaki G: Gentamicin nephrotoxicity in the Dog: Sequential light and electron microscopy. Vet Pathol 17:206­217, 1980.

2.  Riviere, JE, Carver MP, Coppoc GL, Carlton WW, Lantz GC, Shy-Modjeska J: Pharmacokinetics and comparative nephrotoxicity if fixed-dose versus fixed-interval reduction of gentamicin dosage in subtotal nephrectomized dogs. Toxicol and Applied Pharmacol 75:496-509, 1984.

3.  Houghton DC, Harnett M, Campbell-boswell M, Porter G, Bennet W: A light and electron microscopic analysis of gentamicin nephrotoxicity in rats. Am. J. Pathology 82:589-612, 1976.

4.  Reimschuessel R, Williams D, Lipsky MM: Gentamicin toxicity induces development of new nephrons in goldfish. Presented at the 22nd annual International Assoc. for Aquatic Animal Medicine Conference, Orlando, Fl. May 1991.

5.  Beauchamp D, Gourde P, Theriault G, Bergeron MG: Age-dependent gentamicin experimental nephrotoxicity. Journal of Pharmacology & Experimental Therapeutics 260:444-449, 1992.

6.  Gilbert T. Nabarra B. Merlet-Benichou C: Light and electron-microscopic analysis of the kidney in newborn rats exposed to gentamicin in utero. Am J of Path 130:33-43, 1988.

7.  Provoost AP, Adejuyigbe O., Wolff ED: Nephrotoxicity of aminoglycosides in young and adult rats. Pediatric Res 19:1191-1196, 1985.

8.  8. Kacew S. Hewitt WR, Hook, JB: Gentamicin-induced renal metabolic alteration in newborn rat kidney: lack of potentiation by vancomycin. Toxicol and Appl Pharm 99:61-71, 1989.