Ototoxic effects of ethacrynic acid

NKCC1: Newly Found as a Human Disease-Causing Ion Transporter

Among the electroneutral Na⁺-dependent chloride transporters, NKCC1 had until now evaded identification as a protein causing human diseases. The closely related SLC12A transporters, NKCC2 and NCC have been identified some 25 years ago as responsible for Bartter and Gitelman syndromes: two renal-dependent salt wasting disorders. Absence of disease was most surprising since the NKCC1 knockout mouse was shown in 1999 to be viable, albeit with a wide range of deleterious phenotypes. Here we summarize the work of the past 5 years that introduced us to clinical cases involving NKCC1. The most striking cases are of 3 children with inherited mutations, who have complete absence of NKCC1 expression. These cases establish that lack of NKCC1 causes deafness; CFTR-like secretory defects with mucus accumulation in lung and intestine; severe xerostomia, hypotonia, dysmorphic facial features, and severe neurodevelopmental disorder. Another intriguing case is of a patient with a dominant deleterious SLC12A2 allele. This de novo mutation introduced a premature stop codon leading to a truncated protein. This mutant transporter seems to exert dominant-negative effect on wild-type transporter only in epithelial cells. The patient who suffers from lung, bladder, intestine, pancreas, and multiple endocrine abnormalities has, however, normal hearing and cognition. Finally, new reports substantiate the haploinsufficiency prediction of the SLC12A2 gene. Cases with single allele mutations in SLC12A2 have been linked to hearing loss and neurodevelopmental disorders.

Na+ -K+ -2Cl- Cotransporter (NKCC) Physiological Function in Nonpolarized Cells and Transporting Epithelia

Two genes encode the Na+ -K+ -2Cl- cotransporters, NKCC1 and NKCC2, that mediate the tightly coupled movement of 1Na+ , 1K+ , and 2Cl- across the plasma membrane of cells. Na+ -K+ -2Cl- cotransport is driven by the chemical gradient of the three ionic species across the membrane, two of them maintained by the action of the Na+ /K+ pump. In many cells, NKCC1 accumulates Cl- above its electrochemical potential equilibrium, thereby facilitating Cl- channel-mediated membrane depolarization. In smooth muscle cells, this depolarization facilitates the opening of voltage-sensitive Ca2+ channels, leading to Ca2+ influx, and cell contraction. In immature neurons, the depolarization due to a GABA-mediated Cl- conductance produces an excitatory rather than inhibitory response. In many cell types that have lost water, NKCC is activated to help the cells recover their volume. This is specially the case if the cells have also lost Cl- . In combination with the Na+ /K+ pump, the NKCC's move ions across various specialized epithelia. NKCC1 is involved in Cl- -driven fluid secretion in many exocrine glands, such as sweat, lacrimal, salivary, stomach, pancreas, and intestine. NKCC1 is also involved in K+ -driven fluid secretion in inner ear, and possibly in Na+ -driven fluid secretion in choroid plexus. In the thick ascending limb of Henle, NKCC2 activity in combination with the Na+ /K+ pump participates in reabsorbing 30% of the glomerular-filtered Na+ . Overall, many critical physiological functions are maintained by the activity of the two Na+ -K+ -2Cl- cotransporters. In this overview article, we focus on the functional roles of the cotransporters in nonpolarized cells and in epithelia. © 2018 American Physiological Society. Compr Physiol 8:871-901, 2018.

Molecular and clinical implications of loop diuretic ototoxicity

Recent advances in molecular biology have been applied to inner ear research. Loop diuretic ototoxicity has been suggested, but not proven, to share a common mechanism with diuretic effects on renal tubules. The discovery of the molecular nature of the Na-K-2Cl cotransporter in the cochlea provided a better understanding of loop diuretic ototoxicity. In this review, we describe clinical reports of loop diuretic ototoxicity and other information obtained by physiological, biochemical and morphological investigations related to the mechanism sensitive to loop diuretics. Based on recent evidence for the molecular nature of the Na-K-2Cl cotransporter expressed in the mammalian cochlea, the underlying mechanisms of ototoxicity induced by loop diuretics are described.

Dose-response relationships for furosemide ototoxicity in rat

Furosemide is an ototoxic loop diuretic which is highly bound to serum albumin. Previous studies have shown that rats deficient in albumin are more susceptible to furosemide ototoxicity than are rats with normal serum albumin concentrations. The present study was designed to compare the dose-response relationships for furosemide ototoxicity in rats with normal serum albumin concentration to rats without albumin in their serum. Young adult rats 50-80 days of age from each group were anesthetized with Rompun, and the endocochlear potential (EP) and compound action potential (CAP) thresholds were measured before and after furosemide injection. Afer a stable EP and CAP threshold were measured, each animal was injected with a single dose of furosemide through a cannula in the jugular vein. Rats with normal serum albumin had very little change in the EP or CAP threshold until the dose of furosemide was 40 mg/kg or greater. The dose-response curves for EP reduction and CAP threshold elevation then rose steeply to reach a maximum at 50 mg/kg. Albumin-deficient rats were much more sensitive to the effects of furosemide. The dose-response curves for both EP and CAP were shifted to the left. The doses resulting in half-maximal effects in the albumin-deficient rats were about half that found in the normal rats. These findings support the hypothesis that the access of furosemide to its site of ototoxic action in the cochlea depends on the quantity of unbound furosemide in the serum.

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.

The effects of ethacrynic acid upon the potassium concentration in guinea pig cochlear fluids

After i.v. injection of 50 mg/kg ethacrynic acid (EA), potassium concentration in the endolymph (Ke+) measured with K+-specific microelectrodes decreases by 10 mM at the most and endocochlear potential falls to negative values. Potassium concentration in the perilymph (Kp+) generally does not change, but sometimes a transient decrease in Kp+ level of about 0.5 mM was observed, presumably due to the electrogenic effect of the time-related decrease of the endocochlear potential. When anoxia is induced approximately 120 min after EA administration Ke+ slowly decreases. The decrease in Ke+ 50 min after the arrest of ventilation is smaller when compared with the Ke+ anoxic decrease without preceding EA administration. The endocochlear potential, which falls to negative values during anoxia after EA administration, does not return to the zero level as in the case when only anoxia is applied. Similarly, during anoxia, which follows EA administration, the perilymphatic Ke+ concentration increases more slowly than in the case when only anoxia is introduced. It is assumed from the results that EA abolishes activity of the positive electrogenic K+ pump and reduces the passive permeability of the walls of the cochlear duct to the potassium ions.

Ethacrynic acid, furosemide, and vestibular caloric responses

Ethacrynic acid and furosemide, saluretic diuretics of high potency, have been shown to be toxic to the auditory system. In the present study, it was found that the depressant effect of ethacrynic acid extends into the vestibular system. Cats, given a 20 mg/kg intravenous bolus of ethacrynic acid, exhibited a 50% greater reduction of the vestibular nystagmus response to a caloric irrigation than control animals receiving normal saline. This was true for all three major parameters of the recorded nystagmus response (degrees of slow phase displacement, frequency of beats and slow phase velocity). This effect was very rapid in onset (present at 30 seconds postinjection), and quite transient (approaching normal by 20 minutes postinjection). Furosemide did not produce the vestibular depression. Nystagmus responses observed after injection of furosemide (at 20 mg/kg) were not significantly different from those of control animals receiving normal saline. Possible mechanisms of action and therapeutic implications of the ethacrynic acid-induced vestibular depression are discussed.