Comparison of the non-adrenergic action of phentolamine with that of vanadate on cochlear function

Low endolymph calcium concentrations in deafwaddler2J mice suggest that PMCA2 contributes to endolymph calcium maintenance

In vertebrates, transduction of sound into an electrochemical signal is carried out by hair cells that rely on calcium to perform specialized functions. The apical surfaces of hair cells are surrounded by endolymphatic fluid containing calcium at concentrations that must be maintained by active transport. The mechanism of this transport is unknown, but an ATP-dependent pump is believed to participate. Mutation of the Atp2b2 gene that encodes plasma membrane calcium ATPase type 2 (PMCA2) produces the deaf, ataxic mouse: deafwaddler2J (dfw2J). We hypothesized that PMCA2 might transport calcium into the endolymph and that dfw2J mice would have low endolymph calcium concentrations, possibly contributing to their deafness and ataxia. First, using immunocytochemistry, we demonstrated that PMCA2 is present in control mice inner and outer hair cell stereocilia where it could pump calcium into the endolymph and that PMCA2 is absent in dfw2J stereocilia. Second, using an aspirating microelectrode and calcium-sensitive fluorescent dye, we found that dfw2J mice endolymph calcium concentrations are significantly lower than those of control mice. These findings suggest that PMCA2, located in hair cell stereocilia, contributes significantly to endolymph calcium maintenance.

Calcium transport mechanism in the endolymph of the chinchilla

The Ca2+ transport mechanism between endolymph and perilymph was evaluated by the effects of vanadate and amiloride on the endocochlear potential (EP) and the Ca2+ concentration in endolymph using Ca2+-selective microelectrodes. Under normal conditions, the EP was 81.8 +/- 0.9 mV, and the Ca2+ concentrations in endolymph and perilymph were 16.6 +/- 1.3 microM and 1.85 +/- 0.11 mM (N = 12), respectively. Therefore, the uphill electrochemical potential gradient for Ca2+ from perilymph to endolymph, 20.2 +/- 2.0 mV, indicates the existence of an active uptake of Ca2+ into endolymph. Vanadate, the inhibitor of Ca2+-ATPase, topically applied to the round window membrane caused biphasic changes of the EP and the endolymph Ca2+ concentration; the former in a transient increase followed by a consistent decrease and the latter in a slow decrease followed by a slow increase. Amiloride induced a slight EP depression and a concomitantly slight elevation of the Ca2+ concentration in endolymph. The electrochemical potential gradient for Ca2+ between endolymph and perilymph vanished with the use of vanadate but was not affected by amiloride. These results suggest that Ca2+-ATPase, sensitive to vanadate, maintained the bulk of active Ca2+ transport in the cochlea and that the participation of Na+-Ca2+ exchange is negligible.

Ionic changes in cochlear endolymph of the guinea pig induced by acoustic injury

The effects of acoustic overstimulation on the endocochlear potential (EP) and on concentrations of ions (K+, Na+, Cl-, H+, HCO3-, and Ca2+) in endolymph were investigated using ion-selective microelectrodes. A slight but significant elevation of the EP and alkalinization of the endolymph were induced by acoustic overstimulation, whereas there was little change in the K+, Na+, Cl-, and HCO3- concentrations. The changes in H+ and HCO3- concentrations implied a depression of PCO2, suggesting an increase in blood flow to the cochlea. On the other hand, the Ca2+ concentration increased abruptly to 48 times the pre-exposure value. In contrast, no significant change in the Ca2+ concentration was observed in cochleae with damaged hair cells. We discuss the mechanism of the tone-induced Ca2+ elevation in endolymph and its effect on hearing acuity.

Effects of vanadate on EP and its distribution in guinea pig cochlea

Effects of sigma-sodium vanadate on endocochlear d.c. potential (EP) and cochlear microphonic potential (CM) were examined and its distribution was observed by an X-ray microanalyzer in the guinea pig cochlea. The perilymphatic space was perfused with 1, 5, and 10 mM solutions of sodium vanadate, using a Harvard Microperfusion pump. By perfusing the scala vestibuli, EP and CM showed a rapid decrease. On the other hand, by perfusing the scala tympani, EP showed an overshoot at first, then a gradual decrease, while CM showed only a gradual decrease. The rates of decrease of EP and CM were dependent upon perfusion time and vanadate concentration. After the electrophysiological examinations, the specimen of the cochlea was observed by X-ray microanalyzer. An accumulation of vanadium was confirmed, especially in the stria vascularis and the hair cells. From the results obtained, the possibility of the responsibility of vanadate for a hypothetical sudden deafness originating in the stria vascularis was discussed.

Effect of iodoacetic acid upon cochlear potentials

Lotz et al. reported that perilymphatic application of 5 X 10(-3) M iodoacetic acid (IAA) in the guinea pig does not influence the first-order cochlear microphonics (CM1) under aerobic conditions. However, in ischemia the rate of decline of the second-order microphonics (CM II, also called postmortem CM) was significantly increased by IAA. The authors concluded that glycolysis plays no role in maintaining the CMI, but that it is responsible for supporting the CMII. In carefully controlled experiments we found that in the respiring guinea pig, perilymphatic application of 5 X 10(-3) M IAA produced a rapid and pronounced effect upon both the endolymphatic potential and the CM. In particular, the CM dropped to less than 0.5% of its initial level within 40 min, due to IAA, whereas it took 120 min to drop to the same level in total ischemia (without IAA). We therefore reject the above-mentioned proposition that IAA is ineffective upon cochlear potentials under aerobic conditions; moreover, we find that even under aerobic conditions, the CM drops well below the usual CM II level substantially faster than under anaerobic conditions (without IAA). Other important findings, including an anoxia-sensitive negative component of the endolymphatic potential due to severe intoxication with IAA, and the effects of pretreatment of the organ of Corti with low concentrations of IAA upon the CM II are discussed.

The Ca2+ activity of cochlear endolymph of the guinea pig and the effect of inhibitors

The Ca2+ concentrations in cochlear perilymph and endolymph of the guinea pig were measured with double-barreled Ca2+-selective microelectrodes and showed 1.76 +/- 0.74 X 10(-3) M and 2.20 +/- 0.19 X 10(-5) M, respectively. The electrochemical potential gradient for Ca2+ between perilymph and endolymph was 23.2 mV and the existence of an active transport mechanism from the former to the latter was suggested. Vanadate given perilymphatically decreased the Ca2+ concentration in endolymph with a slight elevation of the endocochlear potential and was suspected of blocking the active transport. The Ca2+ concentration in endolymph was abruptly increased by anoxia or the intravenous administration of 60 mg/kg furosemide and was slightly increased by the intravenous administration of 30 mg/kg furosemide or 100 mg/kg acetazolamide. The endolymphatic pH measured with pH-microelectrodes under various conditions indicates that the mechanism of increase in the Ca2+ concentration is attributed not to the liberation of Ca2+ from the surrounding tissues caused by a fall in pH but to the increased influx of Ca2+ from perilymph due to the depression of the endocochlear potential.

Antagonistic effects of perilymphatic calcium and magnesium on the activity of single cochlear afferent neurons

The dependence of the spontaneous and sound driven activity of single cochlear nerve fibers on the calcium and magnesium content of the perilymph was studied by perfusion of the perilymphatic space. It was possible to study these effects under steady-state conditions by continuously perfusing scala tympani at low rates while simultaneously recording from units in the chinchilla auditory nerve. Preparations were stable for many hours. As previously reported [Robertson and Johnstone (1979) Pflügers Arch. 380, 7-12], perfusion with solutions containing elevated concentrations of magnesium reduces both the spontaneous and driven activity. When calcium was eliminated from the perfusate, activity was completely abolished for stimuli with sound pressure levels below 100 dB. During partial blocks, a relatively frequency-independent threshold elevation was seen for frequencies well below the characteristic frequency (CF) of the unit, with greater elevations closer to CF. When the threshold elevation at CF was 30-40 dB, the width of the 'tip' portion of the tuning curve was reduced, resembling that of naturally-occurring units with low spontaneous rates of discharge. These effects are similar to that of raising the criterion for response during threshold measurement and are probably related to a frequency-dependent nonlinearity exhibited by the motion of the basilar membrane. The dynamic range for the growth of average rate with level was increased and saturation was shifted to higher stimulus levels during elevated magnesium perfusion. Raising the calcium content of the perfusate increased both spontaneous and driven rates, even in the saturated portion of the rate-intensity plot. Under these conditions, the response of the unit may more directly correspond to the intracellular potential of the presynaptic hair cell. It is argued that the primary site of divalent cation interaction is in the control of transmitter release. Inner hair cells of the mammalian cochlea apparently do not release transmitter in the absence of a calcium influx. The size of the pool of 'readily-available' transmitter appears to be influenced by divalent cations. Even though this synapse is probably specialized for the transmission of auditory signals, the mechanism of synaptic transmission is probably not fundamentally different from that of other well-characterized synapses.

Hormonal regulation of adenylate cyclase in the stria vascularis of the mouse

The adenylate cyclase complex is a ubiquitous 'second-messenger' system mediating the actions of hormones and neurotransmitters. Its presence but not its physiological control and function had previously been established in the cochlea. In this study, the hormonal stimulation of adenylate cyclase activity of the stria vascularis of the CBA mouse was characterized. In the presence of the regulatory nucleotide, GTP, the enzyme was stimulated by isoproterenol and epinephrine with a half-maximal effect at about 10 microM and the stimulation was blocked by propranolol. This profile is consistent with the presence of adrenergic beta 2-receptors on the strial enzyme complex. Hormones and neuromodulators preferring other receptor subtypes were ineffective; the non-specific stimulator, forskolin, activated the enzyme. The finding that potential hormonal effectors of water and ion transport including vasopressin were inactive may be significant with regard to the physiological role of strial adenylate cyclase.

Characterization of potassium permeability of cochlear duct by perilymphatic perfusion of barium

The relative transepithelial "permeabilities" of the cochlear duct to K, Na, and Cl were investigated so as to identify the K-selective tissues and to determine the cellular origin of this selectivity. Single-ion substitutions were made for K, Na, and Cl with the impermeant species N-methyl-D-glucamine (NMDG) for K and Na and gluconate or sulfate for Cl in perilymph. Transepithelial potential changes were relatively slow and small for Na and Cl substitutions. However, either K for Na or K for NMDG substitutions demonstrated a pronounced K selectivity (rapid changes of electrical potential) of only the sensory-cell tissue (organ of Corti). The response to the K for Na substitution was most clearly seen after electrogenic K transport was inhibited by ischemia while the sensory cells were metabolically sustained via perilymphatic perfusion. Under this condition, perfusion of a medium containing 154 mM K gluconate reduced the negative potential (typically -25 to -40 mV) to within a few millivolts of zero. In a control medium, perilymphatic barium (0.5-5 mM) produced qualitatively similar effects, suggesting that this K selectivity is localized primarily at the basolateral membrane of the sensory cells rather than at the junctional complexes.

Support of cochlear metabolic and ion transport processes solely by perilymphatic perfusion

Morphologic considerations would seem to suggest that the cochlear duct could not be maintained in a fully functional state in the absence of a blood supply. We found, however, that perilymphatic perfusion could be used as a substitute for the normal vascular circulation. The criteria used to determine cochlear function included (1) normal endocochlear potential, (2) normal net secretory flux of rubidium (as a tracer for K), and (3) normal levels of ATP in both the organ of Corti and the stria vascularis. All criteria were satisfied by our perfusion regimen.

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.