Control of intracellular calcium by ATP in isolated outer hair cells of the guinea‐pig cochlea.

Abstract

SUMMARY1. Intracellular calcium levels were monitored in isolated outer hair cells of the guinea-pig cochlea using the calcium-sensitive dye Fura-2.2. The calcium in the cells was studied during application of ATP externally applied from a pipette. ATP induced a rise of intracellular calcium which could be separated into two components: a rapid rise, peaking in 20 s, localized around the apical end of the cell, and a slower rise, peaking in 50-150 s but spread throughout the cell. The effects were observed with 5, 25 and 100 jtM-ATP concentrations.3. In the absence of external Ca2", ATP was still able to trigger a rise in Ca2", but with a longer delay. Under these conditions, the cells did not show the initial rapid Ca2+ rise. The result suggests that ATP can mobilize intracellular stores.4. A rise in intracellular Ca21 was also observed when 5 mM-caffeine was applied to the bath.5. Simultaneous measurements were made of whole-cell currents and intracellular calcium. ATP activated an inward current at resting potentials of -60 mV. Internal Ca21 levels increased during the inward current. In current-clamped cells Ca2+ levels also increased during the associated depolarization produced by ATP.6. Adenosine (150/,M) did not produce any measurable inward current. Acetylcholine (ACh, 100 ,tM-1 mM) produced only a small rise in Ca2+. However, applied simultaneously with ATP, ACh suppressed the rise in intracellular Ca2+ produced by ATP, with the kinetics of a competitive antagonist.7. Intracellular Ca2+ increased with step depolarizations of the cell above -20 mV during whole-cell clamp. Large rises in Ca2+ were also observed on depolarizing the cell with isotonic KC1.8. Calcium levels in supporting cells of the organ of Corti were sensitive to ATP. In these cells, rises in intracellular Ca2+ did not require the presence of extracellular c2+.

Fluorescence imaging of extracellular purinergic receptor sites and putative ecto-ATPase sites on isolated cochlear hair cells

“…It has been demonstrated previously that the application of ATP to isolated cochlear hair cells elicits an inwardly rectifying current by gating a nonselective cation channel (Ashmore and Ohmori, 1990;Nakagawa et al, 1990;, 1993bLin et al, 1993. This study has shown that TNP-ATP blocks this current, thus establishing, at least in cochlear hair cells, that this analog is a more effective antagonist of ATP binding to purinergic receptor sites than the competitive P, purinoceptor antagonist suramin (Dunn and Blakeley, 1988;Nakazawa et al, 1990, 199 1;Henning et al, 1992;Silinsky et al, 1992).…”

“…Studies have shown that extracellularly applied ATP can elicit changes in intracellular Ca2+ concentrations and transmembrane ion conductance in isolated hair cells (Ashmore and Ohmori, 1990;Nakagawa et al, 1990;Shigemoto and Ohmori, 1990;Dulon et al, 1991;Housley et al, , 1993aAshmore et al, 1993). These effects are consistent with a P, purinoceptor classification.…”

ATP-mediated potassium recycling in the cochlear supporting cells

“…In the cochlea, it has been reported that ATP can evoke inward currents and raise the intracellular Ca ++ concentration in the outer and inner hair cells, thereby modifying sound transduction and neurotransmission [10][11][12][13]. ATP can activate purinergic (P2) receptors to produce inward cationic currents [14,15].…”

Role of nitric oxide on purinergic signalling in the cochlea

“…ATP has been described as a neurotransmitter or a neuromodulator in auditory neurotransmission. ATP has been reported to induce an increase of intracellular Ca 2+ concentration ([Ca 2+ ] i ) in outer hair cells (OHCs) [4,5], inner hair cells (IHCs) [6][7][8], spiral ganglion neurons (SGNs) [9,10], and supporting cells [11][12][13]. These previous studies suggested that ATP may affect auditory neurotransmission by modulating [Ca 2+ ] i in the cochlear cells.…”