Elimination kinetics of furosemide in perilymph and serum of the chinchilla. Neuropharmacologic correlates

A Tribute to the Pioneers of Fetal Pharmacology

Clinical pharmacology is a branch of the field of pharmacology that evolved following the recognition that the nature, duration, and intensity of drug action depend on both the intrinsic properties of the drug and an interaction with the host to whom the drug is given. Advances in drug development have placed highly specific and extremely potent therapeutic agents in the marketplace. While these advances have progressed rapidly in adult medicine, pediatric clinical pharmacology has not kept pace and until very recently has lagged behind the research and attention paid to the proper use of therapeutic and diagnostic drugs in adults. Recognition that advances in the science of developmental pharmacology and pediatric clinical pharmacology were essential in the development of new drugs to treat children came in the 1950s and 1960s mostly through the work of 2 pioneering scientists in fetal and perinatal clinical pharmacology, Drs Sumner Yaffe and Bernard Mirkin. Here we pay a tribute to these most influential pioneers in the United States who were instrumental in paving the path for advancing the field of fetal and perinatal pharmacology concepts and their incorporation into pediatric drug development programs.

Manipulation of the Endocochlear Potential Reveals Two Distinct Types of Cochlear Nonlinearity

The mammalian hearing organ, the cochlea, contains an active amplifier to boost the vibrational response to low level sounds. Hallmarks of this active process are sharp location-dependent frequency tuning and compressive nonlinearity over a wide stimulus range. The amplifier relies on outer hair cell (OHC)-generated forces driven in part by the endocochlear potential, the ∼+80 mV potential maintained in scala media, generated by the stria vascularis. We transiently eliminated the endocochlear potential in vivo by an intravenous injection of furosemide and measured the vibrations of different layers in the cochlea's organ of Corti using optical coherence tomography. Distortion product otoacoustic emissions were also monitored. After furosemide injection, the vibrations of the basilar membrane lost the best frequency (BF) peak and showed broad tuning similar to a passive cochlea. The intra-organ of Corti vibrations measured in the region of the OHCs lost the BF peak and showed low-pass responses but retained nonlinearity. This strongly suggests that OHC electromotility was operating and being driven by nonlinear OHC current. Thus, although electromotility is presumably necessary to produce a healthy BF peak, the mere presence of electromotility is not sufficient. The BF peak recovered nearly fully within 2 h, along with the recovery of odd-order distortion product otoacoustic emissions. The recovery pattern suggests that physical shifts in operating condition are a critical step in the recovery process.

Adaptation of Cochlear Amplification to Low Endocochlear Potential

Endocochlear potential (EP) is essential for cochlear amplification by providing the voltage source needed to drive outer hair cell (OHC) transducer current, which leads to OHC electromechanical force. An early study using furosemide to reversibly reduce EP showed that distortion product otoacoustic emissions (DPOAEs) recovered before EP. This indicated that cochlear amplification may be able to adjust to a new, lower EP. To investigate the mechanism of this adjustment, the extracellular OHC voltage, which we term local cochlear microphonic (LCM), was measured simultaneously with DPOAE and EP while using intraperitoneal (IP) and intravenous injection of furosemide to reversibly reduce EP. With IP injection, the DPOAEs recovered fully, whereas the EP was reduced, but LCM showed a similar time course as EP. The DPOAEs failed to accurately report the variation of cochlear amplification. With intravenous injection, for which both reduction and recovery of EP are known to occur relatively quickly compared to IP, the cochlear amplification observed in LCM could attain nearly full or even full recovery with reduced EP. This showed the cochlea has an ability to adjust to diminished operating condition. Furthermore, the cochlear amplifier and EP recovered with different time courses: cochlear amplification just started to recover after the EP was nearly fully recovered and stabilized. Using a Boltzmann model and the second harmonic of the LCM to estimate the mechanoelectric transducer channel operating point, we found that the recovery of cochlear amplification occurred with recentering of the operating point.

Furosemide alters nonlinear capacitance in isolated outer hair cells

The outer hair cell (OHC) from the organ of Corti plays a crucial role in hearing through its unique voltage-dependent mechanical responses. Furosemide, one of the loop diuretics, disrupts normal cochlear function. Here we report on direct effects of furosemide on OHC motility-related, voltage-dependent capacitance using the whole-cell patch-clamp technique. Extracellularly applied furosemide reversibly shifted the voltage at peak capacitance (V(pkC(m))) to positive levels. The shift, whose maximum approached 90 mV, evidenced a Hill coefficient of 1.5 and K(1/2) of 10 mM. Changes in the magnitude of nonlinear capacitance were not fully reversible. While it is clear that the overwhelming effect of furosemide on hearing results via its effects on the endolymphatic potential, the present results indicate that furosemide directly alters OHC motility and may, in part, contribute to sensory dysfunction.

Cochlear effects of indacrinone are not altered by penicillin

Indacrinone is a loop diuretic structurally related to ethacrynic acid. Indacrinone is a racemic mixture. Previous studies have shown that the (-) enantiomer caused reduction of endocochlear potential (EP) and elevation of compound action potential (CAP) threshold (Rybak and Whitworth, 1987a). It has been demonstrated that organic acids such as penicillin, probenecid and sodium salicylate prevent the reduction of EP normally observed after furosemide administration (Rybak et al., 1992a). The present study was designed to determine whether penicillin pretreatment could prevent changes in EP and CAP threshold in (-)-indacrinone treated chinchillas. Adult chinchillas were anesthetized with ketamine and pentobarbital. A microelectrode was advanced into the scala media using the round window approach, and CAP responses to clicks were measured. One group was treated with (-)-indacrinone 100 mg/kg via the jugular vein. A second group of animals received penicillin 50 mg/kg i.v. thirty minutes before (-)-indacrinone. The mean EP change in the indacrinone-treated animals was 38.38 +/- 19.32 millivolts (mv). The reduction of EP in the group receiving penicillin was 24.43 +/- 20.74 mv (P > 0.09). The mean CAP threshold changes in animals receiving indacrinone was 20 +/- 14.14 dB whereas those pretreated with penicillin showed a threshold shift of 21.43 +/- 20.35 dB (P > 0.05). These findings are consistent with previous studies which showed that the effect of ethacrynic acid on the EP and CAP was not changed by the pretreatment with penicillin (Rybak et al., 1990).

Pharmacokinetics of furosemide in endolymph

To investigate the pharmacokinetics of organic anions in the endolymph of the guinea pig, 100 mg/kg furosemide, an organic anion, was intravenously given to measure the concentration in the cochlear endolymph by high-performance liquid chromatography with fluorescence detection. In the endolymph, the concentration of the furosemide increased slowly for 1 hr to 1.6 micrograms/ml and gradually declined thereafter. Pretreatment with 200 mg/kg probenecid, an anion transport inhibitor, had no effect on the furosemide elimination in the endolymph except on the concentration at 2 hr. This was contrary to the drastic change observed in the perilymph of the scala tympani by the same pretreatment. Analogous to the effect in the endolymph, probenecid showed no change in the concentration of the serum, while a pronounced gradient of furosemide concentration existed between them. The present results suggest that the furosemide passively transfers from blood to the endolymph at a relatively low penetrability.

Comparative acute ototoxicity of loop diuretic compounds

A microelectrode was used to measure endocochlear potentials (EP) in adult chinchillas and to study the effects of a series of loop diuretics. EP was measured before, during and for several hours after the intravenous injection of the following loop diuretics: furosemide, piretanide, bumetanide, ethacrynic acid, indacrinone stereoisomers and ozolinone. The first four loop diuretics caused a substantial dose-related reduction of EP. The (-) isomer of indacrinone was found to cause a dose-related reduction of EP to a moderate degree. The (+) isomer of indacrinone and ozolinone caused very little change of EP, even in very high doses. Findings are consistent with data on the mechanism of action of these agents in the kidney.

Furosemide alters organ of corti mechanics: evidence for feedback of outer hair cells upon the basilar membrane

A widely held hypothesis of mammalian cochlear function is that the mechanical responses to sound of the basilar membrane depend on transduction by the outer hair cells. We have tested this hypothesis by studying the effect upon basilar membrane vibrations (measured by means of either the Mössbauer technique or Doppler-shift laser velocimetry) of systemic injection of furosemide, a loop diuretic that decreases transduction currents in hair cells. Furosemide reversibly altered the responses to tones and clicks of the chinchilla basilar membrane, causing response-magnitude reductions that were largest (up to 61 dB, averaging 25-30 dB) at low stimulus intensities at the characteristic frequency (CF) and small or nonexistent at high intensities and at frequencies far removed from CF. Furosemide also induced response-phase lags that were largest at low stimulus intensities (averaging 77 degrees) and were confined to frequencies close to CF. These results constitute the most definitive demonstration to date that mechanical responses of the basilar membrane are dependent on the normal function of the organ of Corti and strongly implicate the outer hair cells as being responsible for the high sensitivity and frequency selectivity of basilar membrane responses. A corollary of these findings is that sensorineural hearing deficits in humans due to outer hair cell loss reflect pathologically diminished vibrations of the basilar membrane.

Furosemide has no effect on endocochlear potential and tuning properties of primary afferent fibres in the pigeon inner ear

In the pigeon no influence of high doses (135 mg/kg) of furosemide on endocochlear potential and sound evoked activity in single auditory nerve fibres was found. This finding contrasts strongly to results in mammals.

Distribution and metabolism of leukotriene C4 after cisternal injection in guinea pigs

Following cisternal injection of [3H8]LTC4 into guinea pigs, leukotriene metabolites were identified in the brain, cerebellum, perilymph, blood, liver and kidneys. LTC4 was metabolized into LTD4 and LTE4 in the cerebrospinal fluid and LTE4 was transported into the blood for general circulation and uptake into the liver and kidneys. The excretion of LTE4 from CNS to blood seemed to be the rate-limiting step in the elimination of leukotrienes from the body. Leukotrienes were also transported into the perilymph. The conversion of LTC4 into LTD4 and LTE4 was lower in perilymph as compared to the cerebrospinal fluid, suggesting a rate limiting function of the cochlear aqueduct that can be defined as a cerebrospinal fluid-labyrinth barrier.

Ototoxicity of indacrinone is stereospecific

Indacrinone (MK-196) is a loop diuretic which consists of a racemic mixture. The purpose of this study was to evaluate the individual enantiomers in the chinchilla model to determine whether these compounds affect auditory function and whether a difference in ototoxic potency exists. Very little change of endocochlear potential (EP) or compound action potential (CAP) was noted in animals receiving the (+)-enantiomer. On the other hand, chinchillas injected with the (-)-enantiomer were found to have a dose related reduction in both CAP and EP. These findings suggest the possibility that the diuretic receptor in the kidney and the receptor mediating ototoxicity in the cochlea, may have similar steric requirements for interacting with loop diuretics.

Auditory brainstem response in young burn-wound patients treated with ototoxic drugs

Burn-wound patients often require potentially ototoxic doses of aminoglycoside drugs in the treatment of gram-negative sepsis. Cochlear hearing impairment may be an unfortunate consequence of this medical therapy. We evaluated auditory sensitivity with the auditory brainstem response (ABR) in a group of 32 children with acute, severe thermal burns ranging in age from 18 months to 17 years. The mean percent of total body surface area burns was 64%. None of the subjects had a known history of hearing deficits or aminoglycoside therapy, and all yielded a normal baseline ABR upon hospital admission. Eight of the subjects (22%) showed either an abnormal ABR, or no response, at 40 dB prior to hospital discharge. The medical treatment for this group of subjects (gentamicin, amikacin, vancomycin, amphotericin B) was compared to that of a second subgroup of 7 subjects without auditory deficit but with a statistically comparable percentage of burns. The mean dosage of vancomycin was higher for the auditory impairment group than for the unimpaired group. Prediction of ototoxicity in the acute burned patient is extremely difficult as there are numerous factors that may influence the risk of cochlear damage. We conclude, however, that the ABR can be applied in early detection of auditory deficit. Follow-up audiometric assessment is advisable since auditory deficits in this population may be delayed or progressive after discontinuance of drug therapy.

Changes in endolymph chloride concentration following furosemide injection

Endocochlear potential (EP) and chloride concentration in endolymph were monitored with microelectrodes in the basal turn of the cochlea of the chinchilla. After intravenous injection of furosemide (25-100 mg/kg), the EP dropped precipitously and rapidly reached its minimum value, however, the chloride activity in endolymph decreased more gradually. Possible mechanisms for this phenomenon include a reduced electrostatic attraction of chloride ions to the scala media due to a decreased EP and a reduction of passive influx of chloride into endolymph, resulting from a reduction of active inward potassium transport by furosemide.

Furosemide ototoxicity: clinical and experimental aspects

Furosemide is an ototoxic diuretic. Furosemide injection is followed by a rapid, but reversible decrease of the endocochlear potential and eighth nerve action potential with a more gradual decrease of the endolymph potassium concentration. In contrast to the reversible effects of furosemide alone on the cochlea, the combination of kanamycin with furosemide resulted in irreversible changes in cochlear function which were associated with elevated levels of kanamycin in the blood and perilymph of the experimental animals. There was a striking similarity between the blood level measured by high pressure liquid chromatography at the time of recovery of auditory function in experimental animals and the ototoxic blood levels proposed by others in clinical literature. These findings help to provide a pharmacologic basis for the clinical observation of furosemide-induced hearing loss.

Effect of glycerol on furosemide-caused EP decrease in guinea pig neonates

Furosemide, a loop diuretic, reduces the endocochlear potential in guinea pigs. In our previous experiment using adult guinea pigs, glycerol was thought to open the blood-cochlea barrier and to help furosemide to reach its site of action in the stria vascularis. The present study was designed to determine whether or not the same potentiation by glycerol occurs in neonate guinea pigs. In one group, furosemide (10-30 mg/kg) was administered, while in another group, 50 v/v% glycerol (1 ml/kg) was administered 15 minutes prior to the injection of furosemide. Furosemide reduced the EP significantly more in neonates than in adults. This effect was not potentiated in the glycerol-pretreated group. From these findings, it seemed conceivable that the neonate blood-cochlea barrier still remains immature in some functions.

The effect of bumetanide on the stria vascularis: a stereological analysis of cell volume density

The purpose of this research was to determine the effect of bumetanide on the volume density (Vv) of the cells, capillaries and intercellular spaces of the stria vascularis (SV). 29 chinchillas were divided into seven groups. There were 3 experimental groups, three control groups and one normal, untreated, group of animals. After either a 20 mg/kg intravenous injection of bumetanide or an injection of a control solution, the animals were killed at 10 min, 1 h and 24 h. One complete radial section of the SV was analyzed in each animal. This section was located at 70% of the length of the basilar membrane as measured from the cochlear apex. Marginal cell volume decreased by 24% and 15% at 10 min and 1 h, respectively, after bumetanide administration. Intermediate cell volume increased by 31% and 27% at 10 min and 1 h, respectively, after bumetanide administration. Intercellular space volume increased by 14% and 21% at 10 min and 1 h, respectively, after bumetanide administration. No significant alteration in the Vv was observed in the strial capillaries or basal cells. A hypothetical model of the ion transporting properties of the SV is presented.

Structure side-effect sorting of drugs. VI. Ototoxicities

From a literature survey, over 130 (about 7.8%) drugs and chemicals have been associated with ototoxicities. The major classes are basic aminoglycoside and other antibiotics, anti-inflammatory drugs, antimalarials, beta-blockers, antineoplastic agents, heavy metals, diuretics, some topical agents and various miscellaneous drugs. Possible mechanisms of action are presented and discussed. These include inhibition of protein synthesis, the glycolytic cycle, the TCA cycle, energy utilization, energy generation and the respiratory system within the mitochondria membrane of the hair cell, and also alteration of the permeability of the endolymphatic membrane or alteration of the excretion system for the basic aminoglycosides in the lateral wall of the membranous cochlea. The relative rank order of ototoxicity and reactivity toward mucopolysaccharides of five aminoglycosides is found to be related to the number of basic groups in each molecule.

Effect of glycerol on the EP decrease caused by furosemide

A change in endocochlear potential (EP) following furosemide injection was observed in regard to prior glycerol administration in two groups of guinea pigs. In one group, various doses of furosemide (20, 30, 40, 50 mg/kg) were injected, while in the other, glycerol (50 v/v %, 1 ml/kg) was injected prior to the furosemide (20, 30, 40 mg/kg) injection. In the glycerol-furosemide group, the decrease in EP was 40% greater than in the furosemide group. Therefore, glycerol was thought to potentiate the EP lowering action of furosemide. Such an effect of glycerol was assumed to be resulted by facilitating the access of furosemide to the site of action in the stria vascularis.

Effect of furosemide upon endolymph potassium concentration

Chinchillas were anesthetized with ketamine (40 mg/kg i.m.) and endocochlear potential (EP) and potassium concentration in endolymph (Ke+) were determined in control animals and in animals injected with various doses of furosemide (25, 50 or 100 mg/kg i.v.) by means of microelectrodes inserted into scala media. Control EP and Ke+ in the chinchilla were 81.3 +/- 3.8 mV and 158.5 +/- 3.2 mequiv./l, respectively. Following injection of furosemide, a dose-related fall in EP and Ke+ was observed. However, the EP declined much more rapidly than the Ke+, and recovered more quickly than the latter. The recovery of Ke+ tended to lag behind the EP recovery. The debate over whether potassium transport into endolymph and endocochlear potential generation are related or independent events is discussed in the light of recent literature and the present study.

Fluorescein kinetics in perilymph and blood: a fluorophotometric study

A modified slit lamp fluorophotometer was used to determine fluorescein concentration changes in the perilymph, cerebrospinal fluid and blood of chinchillas after intravenous injection of 0.2 ml of fluorescein sodium. This new technique provides a means of determining quantitative changes of fluorescein concentration in the perilymph without the need to withdraw fluid samples through the round window membrane of cochlear wall. Fluorescein was observed to enter the perilymph between 1 and 2 minutes after injection, and it reached its peak concentration in the mean time of 23 minutes. The mean peak concentration was 4.20 X 10-6 g/ml. Both increasing and decreasing fluorescein concentration changes in the perilymph followed an exponential time course. Although the observations of cerebrospinal fluid fluorescence were thought to represent a composite of the fluorescence of the cerebrospinal fluid itself and the underlying brain stem blood vessels, the peak fluorescence did not exceed that observed in the perilymph. These observations support the view that most of the perilymph is produced in the cochlea by ultrafiltration from the cochlear blood vessels. The slit lamp fluorophotometer appears to be a satisfactory means of recording fluorescein concentration changes in the perilymph without disturbing the cochlear physiology by penetrating the labyrinth to obtain fluid samples.

Pure-tone and speech intelligibility disturbances in patients with ototoxic disorders

The prevalence and severity of hearing impairment caused by ototoxic drugs are surprisingly high. This emphasizes the importance of the questions arising from the treatment. In 33.7% of the cases, the hearing impairment caused by ototoxic antibiotics was of severe degree and in 25.4%, it was extremely severe. Parenteral, topical or oral administration of aminioglycoside antibiotics is dangerous. Because of the very poor speech intelligibility, most probably not only the spiral organ but the vestibulocochlear nerve and the higher auditory pathways are also affected by these antibiotics. In some cases, the severe distortion in sound perception cannot be compensated even by a hearing aid of the best quality, and lip-reading which was advised occasionally was without any result. To prevent these toxic effects, these drugs should be administered very parsimoniously and then under very strict conditions and close control.

Labyrinthine barriers and cochlear homeostasis

The blood-labyrinth barrier is a concept used to explain the stability of composition of the labyrinthine fluids in spite of systemic alterations in blood composition. This blood-labyrinth barrier concept was tested by injecting various test substances into the systemic circulation of experimental animals and recovering these compounds in perilymph. The concentration of each test substance in perilymph lagged behind that of serum, and the transport of a series of test substances was found to be inversely related to the molecular weight and diameter. Among the osmotic agents injected, glycerol and urea penetrated into perilymph to a considerable degree, however, mannitol did not enter perilymph in any significant amount. This may explain the clinical differences noted with these agents in testing for Meniere's disease. Furosemide, an ototoxic diuretic, was found to penetrate into perilymph after intravenous injection into chinchillas. The concentration of furosemide, measured by high pressure liquid chromatography, was fairly constant at the time of full recovery of endocochlear potential after doses of 50-200 mg/kg. The principle of correlating drug concentration in serum and in inner ear fluids with pathophysiologic effect may provide a prediction of threshold concentration of ototoxic effect by measurement of serum concentration of the drug. More extensive studies are necessary to clarify the role of the blood-labyrinth barrier in the regulatory mechanisms which maintain the homeostasis in the inner ear and the pathology which may follow when this homeostasis is disrupted.

Nature of blood-labyrinth barrier in experimental conditions

The blood-labyrinth barrier is concept that has evolved based on marked difference in chemical composition between perilymph and blood. Studies reported here have been designed to manipulate physiologic, metabolic, and pharmacologic conditions in experimental animals in order to determine the characteristics of this regulatory mechanisms. Tracer studies of uptake of sodium, calcium, and albumin from blood into perilymph showed that these substances penetrate into inner ear fluids quite slowly. Injections of ototoxic substances (kanamycin, furosemide) show limited transport of these agents into perilymph. Administration of an osmotic agent (urea) resulted in a parallel but delayed elevation of perilymph concentration. The possible role of a alteration of blood-labyrinth barrier in inner ear disorders has been discussed.

Disposition and pharmacodynamics of diuretics and antihypertensive agents in renal disease

The pharmacodynamic actions and disposition of diuretic and antihypertensive agents may be significantly modified in subjects with renal disease. Most studies on this question have dealt with alterations in the elimination kinetics of these drugs and, while they generate descriptive data, minimal insight about changes in dose-response relationships or mechanisms of drug action are provided by such investigations. Several basic principles which may serve as useful guidelines in determining how renal failure will influence the response to drugs have been considered. They include the following: degree of renal malfunction, intrinsic toxicity of the drug, alternative pathways for drug metabolism and elimination, elimination pharmacokinetics and dose-response characteristics. Several classes of diuretic agents (thiazides, furosemide) and antihypertensive drugs (hydralazine, methylopda, propranolol, prazosin, and clonidine) have been used as models to define how basic knowledge of renal and non-renal pathways for elimination of drugs and their pharmacodynamic actions may assist in establishing rational therapeutic regimens for these agents in patients with renal failure.

Cochlear effects of locally applied inhibitors

The round window has been used as a route for introducing ototoxic substances into the inner ear in order to bypass barriers between the systemic circulation and the inner ear. We have used this method to administer locally sodium potassium-ATPase inhibitors and chloride transport inhibitors to the inner ear of the chinchilla. Drugs dissolved in saline solution were applied on the round window membrane. Endocochlear potential (EP) was recorded from the basal turn using the round window approach. The EP was not altered following application of saline solution as a control. Following application of ouabain (1 mM), the EP steadily declined. After vanadate (27 mM), the EP initially increased, and subsequently declined. Sanguiarine did not alter the EP. The loop diuretics furosemide and piretanide caused a marked decline in the EP after local application. However, the stilbene derivative DIDS did not alter the EP after topical application. These findings raise questions about whether the loop diuretics have any effect on chloride transport in the cochlea and make appear unlikely that active chloride transport contributes to the normal EP.