The effect of caged calcium release on the adaptation of the transduction current in chick hair cells.

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Abstract
1. Intracellular Ca2+ concentration ([Ca2+]i) was raised by photolysis of a caged calcium compound, nitr‐5, and its effects on the mechano‐electrical transduction (MET) current were studied by a whole‐cell patch electrode voltage clamp technique in dissociated hair cells of a chick. Nitr‐5 was loaded into the hair cell by incubation with the membrane‐permeable form of the compound (nitr‐5 AM). 2. Photolysis of nitr‐5 by ultraviolet (UV) light irradiation induced outward currents at ‐50 mV when recorded with a KCl‐based intracellular medium without Ca2+ chelating compounds. The average amplitude of the photo‐activated outward current was 115 +/‐ 82 pA (mean +/‐ S.D., n = 5). 3. The MET current generated at ‐50 mV showed a decay after step displacement of the hair bundle. This adaptation was accelerated after UV exposure of the cell. The adaptation was further accelerated by hyperpolarization of the membrane and was eliminated in 20‐100 microM Ca2+ extracellular media. 4. The displacement‐response relationship was shifted towards the positive direction after the UV irradiation. 5. The recovery of the transducer current after step displacement of the hair bundle was accelerated after UV irradiation, for both the inward‐going MET current recorded at ‐50 mV and the outward‐going MET current at +54 mV. However, the adaptation was not observed at positive membrane potentials even after the photolysis of nitr‐5. 6. The extent of MET current decay was reduced or disappeared in 20‐100 microM Ca2+ extracellular media and the offset time course was prolonged at the membrane potential of ‐50 mV. The current decay was not observed even after the photo‐release of intracellular Ca2+ in 50‐100 microM Ca2+ extracellular media. 7. These results (paragraphs 3‐6) suggest that the MET current adaptation is accelerated by the increase of [Ca2+]i, and that Ca2+ ions entering through MET channels are essential in the development of adaptation. 8. The adaptation of the MET current was reversibly reduced in a dihydrostreptomycin (DHSM, 20‐50 microM) medium. The time course of the adaptation changes lagged the changes in the MET current amplitude. 9. The adaptation developed or disappeared with a delay of 10‐20 s after the introduction of either the normal‐Ca2+ (2.5 mM) or the low‐Ca2+ (50‐100 microM) extracellular medium, respectively. These delays in the development and the subsidence of adaptation suggest a presence of a Ca2+ buffer site intracellularly between the adaptative site and the MET channel.