The selectivity of the Ca channels was studied in isolated semicircular canal hair cells recorded under whole-cell or perforated-patch conditions. As found prevìously, about 60 percent of the cells exhibited a steady Ca current generated by L- and R-type (termed R2) components, while the remaining ones exhibited an additional R-type current (termed RI) which inactivated in a Ca-dependent manner. Upon reducing external Ca (less than 10 nM), all cells exhibited a steady current, carried by Na, whose maximal amplitude was 5-6 times larger than the corresponding one in normal Ca solution. The I-V curves shifted to the left, the peak Na current being attained at 50 mV. The current activation in Ca was fitted by a single exponential (t1 = 0.7 +/- 0.1 ms) whereas the deactivation was bi-exponential (t2 = 1.2 +/- 0.2, t3 = 9.1 +/- 1.1 ms; n = 22). In 10 nM Ca, both t1 and t2 accelerated by a factor 1.7, whereas t3 was unaffected. The total current recorded in Ca was insensitive to calciseptine (2 microM) and calcicludine (20-100 nM), whereas nifedipine (10 microM) blocked between 60 and 78 percent of the maximal current, indicating that the L-type channels do not carry the same fraction of the total current in different cells. In cells exhibiting slow inactivation time constant (larger than about 10 ms) of the R1 component, nifedipine accelerated inactivation by a factor 1.6, indicating that this dihydropyridine was able to partially block the R1 channel in the open state. When Na was the charge carrier, nifedipine was able to partially block also the open R2 channel, the block being more effective at negative potentials, whereas no block was observed in normal Ca solution. In conclusion, ìn low Ca solution, all three channel types lose their Ca selectivity and they were all affected by nìfedìpine, although to a different extent.
The Permeation of Sodium Through the Voltage-Gated Calcium Channels of Frog Labyrinth Hair Cell
MARTINI, Marta;RISPOLI, Giorgio;FARINELLI, Federica;GONZALEZ, Edgardo Maximiliano;ROSSI, Marialisa
2003
Abstract
The selectivity of the Ca channels was studied in isolated semicircular canal hair cells recorded under whole-cell or perforated-patch conditions. As found prevìously, about 60 percent of the cells exhibited a steady Ca current generated by L- and R-type (termed R2) components, while the remaining ones exhibited an additional R-type current (termed RI) which inactivated in a Ca-dependent manner. Upon reducing external Ca (less than 10 nM), all cells exhibited a steady current, carried by Na, whose maximal amplitude was 5-6 times larger than the corresponding one in normal Ca solution. The I-V curves shifted to the left, the peak Na current being attained at 50 mV. The current activation in Ca was fitted by a single exponential (t1 = 0.7 +/- 0.1 ms) whereas the deactivation was bi-exponential (t2 = 1.2 +/- 0.2, t3 = 9.1 +/- 1.1 ms; n = 22). In 10 nM Ca, both t1 and t2 accelerated by a factor 1.7, whereas t3 was unaffected. The total current recorded in Ca was insensitive to calciseptine (2 microM) and calcicludine (20-100 nM), whereas nifedipine (10 microM) blocked between 60 and 78 percent of the maximal current, indicating that the L-type channels do not carry the same fraction of the total current in different cells. In cells exhibiting slow inactivation time constant (larger than about 10 ms) of the R1 component, nifedipine accelerated inactivation by a factor 1.6, indicating that this dihydropyridine was able to partially block the R1 channel in the open state. When Na was the charge carrier, nifedipine was able to partially block also the open R2 channel, the block being more effective at negative potentials, whereas no block was observed in normal Ca solution. In conclusion, ìn low Ca solution, all three channel types lose their Ca selectivity and they were all affected by nìfedìpine, although to a different extent.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
