Vesicular Storage of Adenosine Triphosphate in the Guinea-pig Cochlear Lateral Wall and Concentrations of ATP in the Endolymph during Sound Exposure and Hypoxia

Abstract

Previous studies have revealed putative vesicular stores of adenosine triphosphate (ATP) in the marginal cells of the cochlear stria vascularis which may serve as a source of ATP for purinergic signalling. This study aimed to provide further evidence of ATP storage in the cochlea and to see whether ATP levels in the endolymph are affected by noise and hypoxia. Tissues from the lateral wall and organ of Corti of the guinea-pig cochlea were fractionated to obtain vesicular (VF) and mitochondrial (MF) fractions. Free and total ATP were then measured by the luciferase-luciferin reaction from which membrane-bound vesicular ATP was calculated. In the lateral wall, the VF contained 2.02+/-0.04 nmol ATP/mg protein (n = 5), significantly greater (p < 0.001; paired Student's t-test) than the concentration of ATP in the MF (0.36+/-0.05). In the organ of Corti, the VF contained 0.69+/-0.08 nmol ATP/mg protein (n = 4), significantly smaller than the amount in the VF of the lateral wall tissues (p < 0.001; non-paired Student's t-test). Small amounts of fumarase. an enzyme of the mitochondrial matrix, in the VF, excluded the possibility of mitochondrial ATP contamination. To investigate the effect of hypoxia and noise on the ATP concentrations in the endolymph, fluid samples were collected from the first (basal) cochlear turn of anaesthetized guinea-pigs. As a result of hypoxia (15 min, 13% F1O2), ATP concentrations (nM, mean +/- SEM) increased from 6.2+/-2.3 to 9.3+/-4.5 (n = 4), but the difference was not statistically significant. As a result of noise (15 min, 10 kHz, 110 dB SPL. broad band), the ATP levels increased significantly from 7.4+/-1.2 to 16.0+/-1.8 (p = 0.01; Student's t-test: n = 4). This study has demonstrated the presence of a vesicular store of ATP in the stria vascularis of the cochlea and described an increase in the ATP levels in the endolymph during noise exposure. The findings suggest that ATP is actively secreted from the vesicular store under conditions of metabolic stress. The presence of ATP under basal conditions supports a role for ATP in the sound transduction process during normal function.

Sound-Induced Intracellular Ca2+ Dynamics in the Adult Hearing Cochlea

“…Previous studies have suggested that this increase in [Ca 2+ ] i is due in part to ATP release from connexin-based hemichannels [2–6]. Changes in [Ca 2+ ] i were measured in response to 103 dB SPL 1000 Hz sound in the presence of 40 U/l apyrase, to eliminate exogenous ATP stimulation, 100 μM CBX, to inhibit ATP release into the apical compartment through connexin hemichannels, 1 μM thapsigargin, an inhibitor of SERCA, which clears intracellular Ca 2+ through uptake into endoplasmic reticular stores, and 1 mM gentamicin, a blocker of mechanoelectrical transduction channels (Fig 3C).…”

“…In adult guinea pigs, noise exposure is associated with an acute increase in endolymphatic ATP levels [2], raising the possibility that ATP release into scala media is an early event in noise-induced ototoxicity. This finding was bolstered by a series of studies in neonatal organotypic mouse cochlear cultures implicating intracellular Ca 2+ signaling (ICS) waves in this process.…”

Na+/K+-ATPase α1 Identified as an Abundant Protein in the Blood-Labyrinth Barrier That Plays an Essential Role in the Barrier Integrity

“…Noise trauma results in depletion of intracellular ATP as well as in changes in vascular permeability [28]. The link between ATP depletion and permeability changes is the decreased Na + /K + -ATPase activity that ATP depletion engenders [29].…”

“…The mechanism by which noise exposure decreases Na + /K + -ATPase activity is thought to be through depletion of regional ATP or through direct affects on the structure of the enzyme [54]. Impaired energy metabolism, accompanied by rapid depletion of intracellular ATP, has been reported in noise-exposed animals [28]. As ATP is the prime substrate of the Na+/K+ pump, reduced ATP levels may have the effect of depressing Na + /K + -ATPase activity, leading to PKCη activation and occludin hyperphosphorylation.…”

Noise-induced hearing loss

“…Chemical analysis showed that ATP was secreted and consumed by anemones after hair bundle disruption, and that RP displayed ATPase activity, although the precise function of ATP in the repair process in not clear [32]. Currently, the possible role of ATP in vertebrate hair cell repair after sub-lethal trauma is also unknown, although ATP is present in the endolymph after noise trauma [33] and has known neurohumoral and apoptotic signaling functions [34]. …”