Hair cell Ca2+ channels were analysed by means of the whole-cell patch-clamp technique in mechanically isolated cells. Two Ca2+ currents were detected, most likely carried through two distinct channel types of the basolateral membrane. At -20 mV and in 4 mM extemal Ca2+, one current had a mean time constant (tau) of activation of about 0.5 ms and an average amplitude of about 130 pA; the other one had an activation tau smaller than or similar to the previous one, a mean peak amplitude of -42 pA and it inactivated with a tau of about 7 ms. The inactivating channel ran down much faster than the non-inactivating one; the protease inhibitor calpastatin prevented run-down of both channels. The inactivating current appeared at - 40 mV, peaked at -30 mV and became undetectable for depolarizations equal to or larger than 0 mV; the plateau current was appreciable at -60 mV and peaked at -20 mV. The plateau current reversed between +30 and +60 mV because of an outward Cs+ current flowing through the same Ca2+ channel. In some cells, the size of the non-inactivating current increased almost two-fold upon repetitive stimulation. The increase of current size was most likely due to the progressive opening of previously silent channels through a run-up mechanism that did not involve a cGMP- or a cAMP-dependent phosphorylation. Recovery from inactivation required about 100 ms at -120 mV and 300 ms at -70 mV. Several results indicated that the inactivation process was Ca2+ dependent: thus, the rather long recovery lag from inactivation most likely was the time necessary for the cell extrusion mechanisms to restore a physiological Ca2+ concentration. The inactivating Ca2+ channel may be functionally important in producing fast (synchronous) transmitter release in response to strong and short stimuli; the non-inactivating one may provide the spontaneous activity, and its modulation may be important for the response to weak and prolonged stimuli.
VOLTAGE-ACTIVATED CA2+ CHANNELS OF HAIR CELLS ISOLATED FROM SEMICIRCULAR CANALS OF THE FROG.
MARTINI, Marta;ROSSI, Marialisa;RUBBINI, Gemma;RISPOLI, Giorgio
1999
Abstract
Hair cell Ca2+ channels were analysed by means of the whole-cell patch-clamp technique in mechanically isolated cells. Two Ca2+ currents were detected, most likely carried through two distinct channel types of the basolateral membrane. At -20 mV and in 4 mM extemal Ca2+, one current had a mean time constant (tau) of activation of about 0.5 ms and an average amplitude of about 130 pA; the other one had an activation tau smaller than or similar to the previous one, a mean peak amplitude of -42 pA and it inactivated with a tau of about 7 ms. The inactivating channel ran down much faster than the non-inactivating one; the protease inhibitor calpastatin prevented run-down of both channels. The inactivating current appeared at - 40 mV, peaked at -30 mV and became undetectable for depolarizations equal to or larger than 0 mV; the plateau current was appreciable at -60 mV and peaked at -20 mV. The plateau current reversed between +30 and +60 mV because of an outward Cs+ current flowing through the same Ca2+ channel. In some cells, the size of the non-inactivating current increased almost two-fold upon repetitive stimulation. The increase of current size was most likely due to the progressive opening of previously silent channels through a run-up mechanism that did not involve a cGMP- or a cAMP-dependent phosphorylation. Recovery from inactivation required about 100 ms at -120 mV and 300 ms at -70 mV. Several results indicated that the inactivation process was Ca2+ dependent: thus, the rather long recovery lag from inactivation most likely was the time necessary for the cell extrusion mechanisms to restore a physiological Ca2+ concentration. The inactivating Ca2+ channel may be functionally important in producing fast (synchronous) transmitter release in response to strong and short stimuli; the non-inactivating one may provide the spontaneous activity, and its modulation may be important for the response to weak and prolonged stimuli.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
