The Ca2+ current was analyzed by means of the whole cell patch-clamp technique in mechanicady isolated hair cells. Pharmacological dissection of the current revealed a nifedlpine-sensitive component (L current) which did not inactivate and a resistent current (unaffected by w-ctx GVIA or w-ctx MVIIC or w-agatx IVA), which showed an incomplete Ca2+-dependent inactivation (R current). The peak component of the R current ran down first, without changing its plateau suggesting the contribution of two channel types R1 and R2, respectively). Eventually, run down occurred for all currents. but was prevented by the protease inhibitor calpastatin. The RI component appeared at -40 mV. reached a maximal value at -30 mV and became undetectable for volteges >= 0 mV. Its inactivation was removed when Ba2+ was the charge carrier. The L and R2 currents were appreciable at -60 mV and peaked at -20 mV: the former current dìd not reverse far voltages up to +60 mV; the latter reversed between +30 and +60 mV, contaminated however by an outward Cs+ current flowing through the same Ca2+ channel. The average amplitude of the plateau current was 130 pA (70% carried by the L channel and the remaining by the R2 channel) while that of the R1 component was 42 pA. The mean activation time constants of the L and R2 currents were 0.33 and 0.68 ms, respectively, and their deactivation time constants yielded 0.24 and 11.3 ms. The mean activation time constant of the R1 component was 0.18 ms and its inactivation time constant was 6.7 ms. In some cells, the L current size increased almost two-fold upon repetitive stimulation (run-up). The recovery from inactivation was time- and voltage-dependent suggesting the involvement of a Ca2+ exchanger working to restore the physiological [Ca2+]i. The inactivating R1 channel may be functionally important in producing fast transmitter release ín response to strong and short stimuli; the non-inactivating R2 and L channels may sustain the resting receptor activity and the afferent response to weak and prolonged stimuli.
THREE Ca2+ CHANNELS ARE DETECTED IN THE HAIR CELLS FROM FROG SEMICIRCULAR CANALS
MARTINI, Marta;RISPOLI, Giorgio;RUBBINI, Gemma;ROSSI, Marialisa
2000
Abstract
The Ca2+ current was analyzed by means of the whole cell patch-clamp technique in mechanicady isolated hair cells. Pharmacological dissection of the current revealed a nifedlpine-sensitive component (L current) which did not inactivate and a resistent current (unaffected by w-ctx GVIA or w-ctx MVIIC or w-agatx IVA), which showed an incomplete Ca2+-dependent inactivation (R current). The peak component of the R current ran down first, without changing its plateau suggesting the contribution of two channel types R1 and R2, respectively). Eventually, run down occurred for all currents. but was prevented by the protease inhibitor calpastatin. The RI component appeared at -40 mV. reached a maximal value at -30 mV and became undetectable for volteges >= 0 mV. Its inactivation was removed when Ba2+ was the charge carrier. The L and R2 currents were appreciable at -60 mV and peaked at -20 mV: the former current dìd not reverse far voltages up to +60 mV; the latter reversed between +30 and +60 mV, contaminated however by an outward Cs+ current flowing through the same Ca2+ channel. The average amplitude of the plateau current was 130 pA (70% carried by the L channel and the remaining by the R2 channel) while that of the R1 component was 42 pA. The mean activation time constants of the L and R2 currents were 0.33 and 0.68 ms, respectively, and their deactivation time constants yielded 0.24 and 11.3 ms. The mean activation time constant of the R1 component was 0.18 ms and its inactivation time constant was 6.7 ms. In some cells, the L current size increased almost two-fold upon repetitive stimulation (run-up). The recovery from inactivation was time- and voltage-dependent suggesting the involvement of a Ca2+ exchanger working to restore the physiological [Ca2+]i. The inactivating R1 channel may be functionally important in producing fast transmitter release ín response to strong and short stimuli; the non-inactivating R2 and L channels may sustain the resting receptor activity and the afferent response to weak and prolonged stimuli.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
