Low Doses Of Gentamicin Research Articles

Identifying innate immune receptors causing production of inflammatory cytokines by infection of Francisella tularensis.

Abstract Francisella tularensis is a pathogenic species and a causative agent of tularemia. This bacterium is incredibly infectious and often lethal in that F. tularensis induces the overproduction of pro-inflammatory cytokines. This study aims to identify the innate immune receptors responsible for the production of inflammatory cytokines in response to F. tularensis infection. The study uses an immortalized C57BL/6 Bone Marrow Macrophage (BMMac) cell line as the control and knockout BMMac deficient in various innate immune receptors. The cells are passaged and seeded before infecting them with F. tularensis for 2 hours. The cells were treated with a high dose of gentamicin for 2 hours, followed by a low dose of gentamicin for an incubation period of 24 hours. The supernatant from these cells is then collected and analysis processed by ELISA, an enzyme-linked immunosorbent assay, to measure the production of inflammatory cytokines. The results were compared with the positive control of wild-type BMMac and the negative control of uninfected BMMac cells. Analysis of these mutants reveals the involvement of innate immune receptors in triggering the pro-inflammatory cytokine storm in response to F. tularensis infection that leads to the host’s death. Supported by NIH (SC1 GM122699-01A1)

Identifying innate immune receptors causing production of inflammatory cytokines by infection of Francisella tularensis

Abstract Francisella tularensis is a pathogenic species and a causative agent of tularemia. This bacterium is incredibly infectious and often lethal in that F. tularensis causes the overproduction of pro-inflammatory cytokines. The objective of this study is to identify the innate immune receptors responsible for the production of inflammatory cytokines in response to F. tularensis infection. The study uses an immortalized C57BL/6 Bone Marrow Macrophage (BMMac) cell line as the control and knockout BMMac that are deficient in various innate immune receptors. The cells are passaged and seeded before infecting them with F. tularensis for 2 hours. The cells are treated with a high dose of gentamicin for 2 hours followed by a low dose of gentamicin for an incubation period of 24 hours. The supernatant from these cells are then collected and analysis processed by ELISA, an enzyme-linked immunosorbent assay, to measure production of inflammatory cytokines. These data were compared with the positive control of wild type BMMac and the negative control of uninfected BMMac cells. Analysis of these mutants reveal the involvement of innate immune receptors in triggering the pro-inflammatory cytokine storm in response to F. tularensis infection that leads to death of the host.

Hemopexin, a potential biomarker for the diagnosis of chronic predisposition to acute kidney injury

In the last years, the new concept of predisposition to acquire acute kidney injury (AKI) is emerging. This concept was observed in our group when experimental animals exposed to an absolutely subnephrotoxic acute treatment with certain drugs (e.g. gentamicin and cisplatin) developed AKI when they were treated with a second insult with another drug, while control animals exposed to the same second drug experimented no toxicity. On these grounds, we decide to study if chronic exposure to nephrotoxicants might induce this predisposition to AKI and investigate how to detect this condition by the search of predisposition biomarkers. To this end, rats (Sprague-Dawley) were treated with a subtoxic dosage of the experimental nephrotoxin uranyl nitrate (UN) in the drinking water for 22 weeks, or plain water (as control). After 21 weeks both groups were treated with subtoxic regime of getamicin during 7 days. Renal function was monitored by means of serum creatinine, serum urea, proteinuria, N-acetyl-beta-D-glucosaminidase and lactate dehydrogenase excretion measurement. After and before gentamicin treatment a subset of rats were sacrificed and their kidneys used for histology. With the purpose of identifying biomarker of predisposition, proteomic studies were performed before gentamicin administration at week 21. Chronic administration of UN in drinking water during 21 weeks did not modify renal function or renal tissue integrity. However, when rats exposed to UN during 21 weeks where challenged with low doses of gentamicin, they developed an overt renal failure as shown by an increase in creatinine, urea and by histological alterations. These alterations were not observed in the control group. Using a proteomic analysis, hemopexin was detected which was validated by Western blot. Urinary excretion of hemopexin was statically higher in the exposed group than in the control group. Our results suggest that chronic exposure to UN, at doses that apparently does not produce damage, predisposes to AKI when animals were subjected to a second insult at subtoxic doses. Hemopexin protein might be potentially used as marker of chronic predisposition to ARF. This new diagnostic tool might help to reduce AKI incidence and severity, and also the associated sanitary and socioeconomic costs.

Spontaneous and Partial Repair of Ribbon Synapse in Cochlear Inner Hair Cells After Ototoxic Withdrawal.

Ototoxicity is one of the major causes of sensorineural deafness. However, it remains unclear whether sensorineural deafness is reversible after ototoxic withdrawal. Here, we report that the ribbon synapses between the inner hair cells (IHCs) and spiral ganglion nerve (SGN) fibers can be restored after ototoxic trauma. This corresponds with hearing restoration after ototoxic withdrawal. In this study, adult mice were injected daily with a low dose of gentamicin for 14 consecutive days. Immunostaining for RIBEYE/CtBP2 was used to estimate the number and size of synaptic ribbons in the cochlea. Hearing thresholds were assessed using auditory brainstem responses. Auditory temporal processing between IHCs and SGNs was evaluated by compound action potentials. We found automatic hearing restoration after ototoxicity withdrawal, which corresponded to the number and size recovery of synaptic ribbons, although both hearing and synaptic recovery were not complete. Thus, our study indicates that sensorineural deafness in mice can be reversible after ototoxic withdrawal due to an intrinsic repair of ribbon synapse in the cochlea.

Cochlear Inner Hair Cell Ribbon Synapse is the Primary Target of Ototoxic Aminoglycoside Stimuli

The ribbon synapses of inner hair cells (IHCs) play an important role in sound encoding and neurotransmitter release. However, it remains unclear whether IHC ribbon synapse plasticity can be interrupted by ototoxic aminoglycoside stimuli. Here, we report that quantitative changes in the number of IHC ribbon synapses and hearing loss occur in response to gentamicin treatment in mice. Using 3D reconstruction, we were able to calculate the number of IHC ribbon synapses after ototoxic gentamicin exposure. Mice were injected intraperitoneally with a low dose of gentamicin (100 mg/kg) once a day for 14 days. Double immunostaining was used to identify IHC ribbon synapses; histopathology and scanning electron microscopy were used to observe the morphology of cochlear hair cells and spiral ganglion neurons (SGNs), the hearing threshold shifts were recorded by auditory brainstem response examinations. Our study shows that the maximal number of IHC ribbon synapses appeared at the 7th day after treatment, followed by a significant reduction after the 7th day regardless of ongoing treatment. Correspondingly, the maximal elevation of hearing threshold was observed at the 7th day after treatment. Meanwhile, additional cochlear components included OHCs, IHCs, and SGNs were unaffected, suggesting that IHC ribbon synapses are more susceptible to ototoxic aminoglycoside stimulation. Our study indicated that quantitative changes in the number of IHC ribbon synapses is critical response to lower dose of ototoxic stimulation, and may contribute to moderate hearing loss. Additionally, our data indcated that ribbon synaptic plasticity may require the quantitative changes to play self-protective role adapted to ototoxic aminoglycoside stimuli.

Assessing low-dose gentamicin-induced kidney injury in rats by analysis of urine

Introduction Gentamicin is an important aminoglycoside antibiotic used in clinics to combat infections from especially gram negative bacteria. A frequent side-effect of aminoglycoside antibiotics is a kidney injury consisting of necrosis of proximal tubular cells. It is important both in clinics and in research directed to eliminate or ameliorate this side-effect that a method is available for detection of injury at an early stage. Methods We have therefore compared the sensitivity of four methods currently used in animal models to assess kidney injury induced by aminoglycoside antibiotics by applying them to the analysis of urine from male rats treated with low doses of gentamicin. Results Excretion of phospholipids was significantly increased in contrast to excretion of N-acetyl- β-glucosaminidase (NAG), neutrophil gelatinase-associated lipocalin (NGAL) and protein. Assessment of phospholipiduria thus is the more sensitive noninvasive way to monitor initial renal injury induced by aminoglycoside antibiotics. A protocol is given for the serial analysis of urinary phospholipids allowing a considerable number of determinations to be carried out in the course of 2–3 days. Discussion It is well known that all four considered methods do detect kidney damage induced in rats by high gentamicin doses far above doses used in clinics. The present investigation shows that only the analysis of the urinary phospholipids will detect damage induced by low doses of gentamicin. The method is relevant for animal model studies but will require considerable and innovative development for use in clinics.

Study of NO-synthase activity and detection of NO-dependent antimicrobial effects in primary culture of frog urinary bladder epithelial cells

We have shown previously that endogenous nitric oxide (NO) in frog urinary bladders modulates the influence of arginine-vasotocin on the increase in the osmotic water permeability of the bladder epithelium. The goal of the present work was to obtain a primary culture of epithelial cells from a frog urinary bladder in order to study in vitro the cellular activity of NO-synthase (NOS) and its regulation. The best conditions for cultivating the cells turned out to be with the use of a modified L-15 medium containing 10% fetal bovine serum and gentamicin (40 μg/ml) at room temperature. Under these conditions, at least 50% of the cells preserved their viability for 8 days. The NOS activity was estimated from the accumulation of nitrites (NO2−) in the cultivation medium; the amount of NO2− in the presence of nitro-L-arginine methyl ester (L-NAME), an inhibitor of all types of NOS, was considered unspecific and was subtracted from each value. The NO2− accumulation was linear in time for three days of cultivation and was inhibited by 1400W, an inducible NOS (iNOS) inhibitor, or by 7-nitroindazole, an inhibitor of constitutive NOSs, at concentrations of 5–50 and 10–200 μM, respectively. One-day incubation of the cells in the medium with low doses of gentamicin (1 or 2 μg/ml) led to a marked increase in the amount of bacteria in the cultivation m5 mM edium identified as E. coliand Acinetobacter sp. An addition of 5 mM L-NAME to the cultivation medium increased the amount of the bacteria by 1.5-and 2.5-fold in the presence of 2 and 1 μg/ml gentamicin/ml, respectively. Thus, the epithelial cells of frog urinary bladders have the NO-dependent antibacterial effect, which seems to be the result of enhanced iNOS expression. The obtained data indicate that the primary culture of the epithelial cells from frog urinary bladders is a prospective model for studying the role of NO and the regulation of NOS activity, which is of particular interest for studying protective NO effects in epithelial tissues.

Subclinical kidney injury incites endotoxin hyperresponsiveness

gentamicin, an aminoglycoside discovered in 1963 from micromonospora, is one of the most commonly used antibiotics against Gram-negative infections. Aminoglycosides achieve bacterial killing by binding to bacterial ribosomes, causing blockade of protein synthesis through failure of initiation and

Intratympanic gentamicin therapy for intractable Ménière's disease

Conclusions. Intratympanic gentamicin treatment using a low dose of gentamicin (∼21–24 mg per injection) applied at intervals of a minimum of 27 days, has been shown to be a successful treatment of vertigo. Objectives. The objective of this analysis was to evaluate the efficacy and side effects of intratympanic injections of gentamicin as treatment of intractable unilateral Ménière's disease. Patients and methods. This was a retrospective study in which 57 patients treated with intratympanic gentamicin for Ménière's disease were analysed. Patients received between 1 and 10 intratympanic injections of gentamicin in an outpatient setting. Results. Six months after treatment, overall complete or substantial vertigo control was reported by 80.7% of our patients (VCC class A, 61.4%; class B, 19.3%). Unilateral caloric weakness increased from 50.1% to 79.8% after treatment, and complete caloric areflexia (30°C and 44°C) was induced in 38.6%. In this study hearing worsened (>10dB = SNHL) in only 15.8% of our patients (range 10–29 dB) and loss of word recognition (WR) scores worsened (>15%) in 31% of the patients. Only 1 injection was necessary in 49.1% of the patients, the remaining 50.9% needed 2, 3, 4, 6 or 10 injections, always with a time interval of a minimum of 27 days.

MicroWick-delivered transtympanic gentamicin: Hearing effects

Problem: Transtympanic gentamicin administration has become a popular modality for the treatment of Ménière’s disease. This modality and other inner ear medical therapies are gaining increased clinical and scientific attention. We have previously described the kinetics and effects of gentamicin uptake into the inner ear after delivering the medicine into the middle ear using a variety of different techniques and sustained release modalities. In this study we focus on a newer sustained release device, the Silverstein MicroWick (Micromedical, Inc, Minneapolis, MN). Methods: We studied the effects of a single administration of gentamicin onto the microwick after implanting the wick into the round window membrane of Chinchilla laniger. One mg of gentamicin (10 mg/mL) was applied to a microwick after securing the wick in the round window niche of Chinchilla laniger. Hearing was measured before placing the wick and at the preset time points after. Results: All of the animals had normal preoperative hearing and there was no hearing loss demonstrated in the 4-hour time group. However, 40% of the animals in the 8-, 24-, and 48-hour time points exhibited a hearing loss. Conclusion: Despite the very low dose of gentamicin, we demonstrated more hearing loss at earlier time points than in previous experiments. The fact that 40% of animals demonstrated hearing loss at each time point suggests to us that the damage from gentamicin, delivered in this manner (a single low dose on a microwick), occurs early and those animals remain impaired for at least 48 hours. Meanwhile, animals whose hearing is intact for the first 8 hours likely do not suffer damage after that point. Significance: The information gained from this study and the follow-on studies will increase our scientific understanding about the effects of gentamicin on the inner ear and allow clinicians to more effectively treat patients with inner ear disorders. Support: None reported.

Vestibular destruction by slow infusion of gentamicin into semicircular canals.

Intratympanic or round window application of gentamicin is often used to alleviate disabling vertigo arising from unilateral Meniere's disease; however, treatment is often accompanied by hearing loss because the drug initially enters the cochlea before diffusing to the vestibular system. In order to enhance vestibular damage and reduce the risk of hearing loss, gentamicin was infused directly into the vestibular system. An osmotic pump containing 50, 100, 200 or 400 microg/ml of gentamicin was infused into the superior semicircular canal of the chinchilla for 7 days. Afterwards, vestibular damage was evaluated by measuring the decline in hair cell density in the utricle, saccule and superior semicircular canals. Auditory damage was assessed with distortion product otoacoustic emissions (DPOAE) and outer hair cell (OHC) and inner hair cell (IHC) loss. Infusion with the two lowest gentamicin concentrations resulted in significant hair cell loss and reduced duration of the nystagmus response, but had little or no effect on OHC or DPOAE. Higher doses of gentamicin damaged cochlear hair cells and reduced the DPOAE. In conclusion, slow infusion of a low dose of gentamicin into the semicircular canals mainly damages the vestibular hair cells and inactivates the nystagmus response without damaging cochlear hair cells or DPOAE.

Microdose gentamicin delivered via the round window microcatheter: A therapeutic option in Menière's disease

Transtympanic gentamicin therapy is becoming an increasingly popular treatment for Menière's disease. Despite its popularity, transtympanic therapy is not without controversy. Vertigo cure rates are high, but hearing loss rates are variable. The best total dose of medicine, the best route of administration, and the best end-point of therapy have not been established. Meanwhile, the mode of action of gentamicin in controlling Menière's disease is still not completely understood. In the last several years, many groups have started to use a variety of sustained release devices to more accurately control the amount of gentamicin administered to the ear. This report discusses the use of one such sustained release device, and the effects of its use with very low doses of gentamicin, on the symptoms of Menière's disease.

Development of a long-term ascending urinary tract infection mouse model for antibiotic treatment studies.

A model of ascending unobstructed urinary tract infection (UTI) in mice was developed to study the significance of the antibiotic concentration in urine, serum, and kidney tissue for efficacy of treatment of UTI in general and pyelonephritis in particular. Outbred Ssc-CF1 female mice were used throughout the study, and Escherichia coli was used as the pathogen. The virulence of 11 uropathogenic E. coli isolates and 1 nonpathogenic laboratory E. coli strain was examined. Strain C175-94 achieved the highest counts in the kidneys, and this strain was subsequently used as the infecting organism. The model gave reproducible bladder infections, i.e., bacteria were recovered from 22 of 23 control mice after 3 days, and histological examination of kidney tissue showed that of 14 infected kidneys, 7 (50%) showed major histological changes, whereas 3 of 36 uninfected kidneys showed major histological changes (P = 0.018). Once the model was established, the efficacies of different doses of cefuroxime and gentamicin, corresponding to active concentrations in urine only or in urine, serum, and kidney tissue simultaneously, were examined. All cefuroxime doses resulted in significantly lower counts in urine than control treatments, but the dose which produced concentrations of cefuroxime only in urine and not in serum or kidney tissue had no effect on kidney infection. Even low doses of gentamicin (0.05 mg/mouse) resulted in concentrations in renal tissue for prolonged times due to accumulation. All gentamicin doses had a significant effect (compared to the effect of the control treatment) on bacterial counts in urine and kidneys. The antibiotic effect on bacterial counts in bladders was negligible for unknown reasons. Use of the mouse UTI model is feasible for study of the effect of an antibiotic in the urinary system, although the missing antibacterial effect in the bladder needs further evaluation.

Low-dose intratympanic gentamicin treatment of Menière's disease.

To control attacks of vertigo while preserving both hearing and labyrinthine function, low doses of gentamicin were instilled intratympanically in nine patients with intractable unilateral Meniere's disease. Each patient received six instillations of antibiotic of 4 mg each (total dose, 24 mg). Patients were then followed for 2-4 years. Long-term results of treatment are reported according to the American Academy of Otolaryngology-Head and Neck Surgery 1985 criteria. Of the nine cases, three experienced complete control of vertiginous attacks, while six received substantial control. Post-treatment hearing acuity was unaffected, although disability following treatment became worse in one patient, a 66-year-old man. Caloric responses after therapy were absent or severely reduced in three ears, moderately reduced in two ears and unchanged in four ears. In three patients, labyrinthine function was found damaged 4-8 days after administration of the last dose of drug. Overall, intratympanic instillations of low doses of gentamicin in patients with intractable Meniere's disease were found to control vertiginous attacks with less damage to the inner ear function than that reported in the literature.

Increased levels of protein- and lipid-bound sialic acids in the renal cortex of rats injected with low doses of gentamicin

Administration of the aminoglycoside antibiotic, gentamicin, even at therapeutic doses, causes renal lysosomal phospholipidosis. We now report that protein- and lipid-bound sialic acid levels are increased significantly in a time-dependent fashion in the renal cortex of rats injected with gentamicin (10 mg kg body wt. per day) for 4–10 days and a significant relationship could be observed between these two parameters. This elevation was not due to tissue regeneration, since it was not observed in cisplatin-treated animals.

Antimicrobial therapy of experimental endocarditis caused by nutritionally variant viridans group streptococci.

Rabbits with nutritionally variant viridans group streptococcal experimental endocarditis were treated three times daily for 3 days with procaine penicillin (1.2 X 10(6) U) alone or together with low-dose streptomycin (2 mg/kg), high-dose streptomycin (8 mg/kg), low-dose gentamicin (0.32 mg/kg), or high-dose gentamicin (1.05 mg/kg). The mean 0.5-h serum concentrations of streptomycin were 5.3 and 22.5 micrograms/ml in the low- and high-dose group, respectively, and the concentrations of gentamicin were 0.7 and 2.5 micrograms in the low- and high-dose groups, respectively. The combination of procaine penicillin with each dose of aminoglycoside was significantly more effective (P less than 0.001) than was procaine penicillin alone. In combination with procaine penicillin, the higher dose of streptomycin was significantly more effective (P less than 0.02) than the lower dose of streptomycin. The higher dose of streptomycin was not significantly more effective than either dose of gentamicin. The results of treatment with the high or low dose of gentamicin were virtually identical.

Choice of drug and dosage regimen: Two important risk factors for aminoglycoside nephrotoxicity

Since the clinical use of aminoglycosides may be limited by the development of nephrotoxicity, it is important to be aware of those risk factors associated with a greater incidence of renal damage. Some of these factors are related to the drug and its administration and others are related to the patient's clinical condition. In the human kidney, the toxicity mechanism is very likely the same for all aminoglycosides, although the risk of nephrotoxicity increases for a given aminoglycoside as cortical concentrations increase. Kinetic studies in the rat demonstrated a nonlinear increase in renal cortical uptake of gentamicin and netilmicin, a linear relationship for tobramycin uptake, and a mixed kinetic pattern for amikacin, that is, saturation kinetics at low serum concentrations and a linear pattern at high serum levels. At comparable steady-state low serum levels, amikacin and tobramycin showed lower cortical concentrations than gentamicin or netilmicin, demonstrating a lower affinity for the uptake of amikacin and tobramycin in the rat. Since drug uptake kinetics determine the extent of cortical concentrations achieved, dosing strategies may affect cortical accumulation of aminoglycosides. Our kinetic data show that continuous infusions of low doses of gentamicin and amikacin resulted in higher cortical levels, but the differences between regimens were more remarkable for gentamicin than for amikacin. For tobramycin, however, cortical concentrations were similar regardless of the dosing strategy used. In addition, our data show that dosage regimens also determine cortical accumulation in humans. A second major determinant of nephrotoxicity is intrinsic toxicity. At therapeutic doses, gentamicin, tobramycin, netilmicin, and amikacin induce an early lysosomal phospholipidosis in the human kidney cortex comparable to that observed in animals treated with low doses of these drugs. However, animal and human studies have shown that amikacin induces significantly less lysosomal overloading than the other aminoglycosides with no loss of phospholipase A1 activity. Based on the examination of cortical drug levels and the detection of early biochemical and morphologic alterations induced by aminoglycosides, the data suggest that amikacin has less pronounced nephrotoxic effects than gentamicin, netilmicin, or tobramycin, when used in strictly comparable clinical conditions.

Comparative nephrotoxicity of hydroxygentamicin and other aminoglycosides in rats

The nephrotoxicity of hydroxygentamicin and amikacin was examined in young adult Fischer 344 rats. Serum creatinine (SCr) and urea nitrogen (BUN) levels were not significantly affected following sc injection of 80 or 160 mg/kg/day of hydroxygentamicin for 15 days. However, 250 mg/kg of amikacin produced significant increases in both parameters and in kidney/body weight ratios. The ratios were also significantly increased after 80 or 160 mg/kg of hydroxygentamicin, but kidneys of rats receiving amikacin were considerably heavier than those of rats treated with hydroxygentamicin. The antibacterial potency of 250 mg/kg of amikacin is comparable to that of 100mg/kg of hydroxygentamicin. Additional studies, directly comparing hydroxygentamicin, a mutational biosynthetic, with gentamicin or netilmicin, all at 40, 80, and 160 mg base/kg, and incorporating renal function parameters as well as SCr, BUN, organ weight, tissue concentration, and kidney histopathology, revealed a characteristic pattern typical of aminoglycoside nephrotoxicity in mature adult male rats. In most parameters, values in rats given hydroxygentamicin or netilmicin were normal and comparable to those in controls, but kidney/body weight ratios were significantly increased at high doses. However, kidneys of rats medicated with gentamicin at comparable doses were considerably heavier than those of hydroxygentamicin-treated rats. Significant nephrotoxicity also was seen in rats given low doses of gentamicin or netilmicin. Eosinophilic granulation and vacuolization of renal proximal tubular epithelium, interstitial inflammation, and tubular dilation were observed microscopically with all three drugs in the following descending order of severity: gentamicin > netilmicin > hydroxygentamicin. The effects on proximal tubular epithelial cells following treatment with amikacin, netilmicin, or hydroxygentamicin correlated reasonably well with renal drug concentrations, but drug concentrations of gentamicin, which produced the most extensive kidney injury, were lower than those of the other three aminoglycosides. Elevated SCr or BUN were indicative of the presence of nephrosis, but early stages of tubular epithelial degeneration were not predicted by increases in BUN or SCr. Although minimal or mild nephrosis was seldom predicted by polyuria, proteinuria, or changes in osmolality, effects observed in renal function parameters usually correlated well with renal histopathology. However, a decrease in osmolality correlated best with enlarged kidneys and changes in renal morphology.

Tubular regeneration in rat kidney cortex during treatment with gentamicin at a low dose.

Tissue injury induced in rat kidney cortex by gentamicin at a low dose (10 mg/kg) has been studied by measuring subsequent regeneration after treatment for 4, 7, and 14 days. Cell proliferation was demonstrated by the incorporation of [3H]-thymidine into kidney cortex DNA. In treated animals, the specific radioactivity of DNA increased up to 4.6 times the mean value found in the controls. Autoradiography showed labelling of nuclei in proximal tubular cells. Electron microscopy showed that a subpopulation of tubular cells, devoid of myeloid bodies, which are characteristic of gentamicin intoxication, is also poorer in peroxisomes, and therefore may belong to less differentiated, regenerating cells. It is concluded that a low dose of gentamicin induces measurable tissue regeneration.

Increased renal DNA synthesis in vivo after administration of low doses of gentamicin to rats.

Kidney cortex DNA synthesis was studied in female rats treated with a low dose of gentamicin (10 mg/kg) up to 14 days. Synthesis was measured by incorporation of [3H]thymidine into DNA 1 h after intraperitoneal injection of the labeled precursor (200 muCi per animal). Gentamicin given in one injection per day resulted in a greater incorporation of [3H]thymidine into DNA after both 7 and 14 days of treatment as compared with control animals. When the daily dose was divided into three equal injections given at 8-h intervals, a statistically significant increase in thymidine incorporation was observed as early as 4 days after starting gentamicin administration. Excellent agreement was found between DNA specific radioactivity and kidney cortex nuclear labeling, as measured by histoautoradiography. The greatest amount of [3H]thymidine incorporation occurred within proximal tubular cells and interstitial cells. We conclude that a finite duration of gentamicin treatment at low dosage induces an increased DNA synthesis in vivo in rat kidney cortex. We suggest that this reaction results from cellular proliferation and could reflect a regenerative process after focal necrosis induced by gentamicin at low doses. The demonstrated early increase in DNA synthesis could be a useful tool to measure kidney cortex alterations caused by various aminoglycosides at low, therapeutic doses.