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Calcium currents in hair cells isolated from semicircular canals of the frog
M Martini1, M L Rossi, G Rubbini
1Istituto Nazionale per la Fisica della Materia, Dipartimento di Biologia dell'Università-Sezione di Fisiologia Generale, 44100 Ferrara, Italy.
Abstract:
L-type and R-type Ca(2+) currents were detected in frog semicircular canal hair cells. The former was noninactivating and nifedipine-sensitive (5 microM); the latter, partially inactivated, was resistant to omega-conotoxin GVIA (5 microM), omega-conotoxin MVIIC (5 microM), and omega-agatoxin IVA (0.4 microM), but was sensitive to mibefradil (10 microM). Both currents were sensitive to Ni(2+) and Cd(2+) (>10 microM). In some cells the L-type current amplitude increased almost twofold upon repetitive stimulation, whereas the R-type current remained unaffected. Eventually, run-down occurred for both currents, but was prevented by the protease inhibitor calpastatin. The R-type current peak component ran down first, without changing its plateau, suggesting that two channel types generate the R-type current. This peak component appeared at -40 mV, reached a maximal value at -30 mV, and became undetectable for voltages > or =0 mV, suggestive of a novel transient current: its inactivation was indeed reversibly removed when Ba(2+) was the charge carrier. The L-type current and the R-type current plateau were appreciable at -60 mV and peaked at -20 mV: the former current did not reverse for voltages up to +60 mV, the latter reversed between +30 and +60 mV due to an outward Cs(+) current flowing through the same Ca(2+) channel. The physiological role of these currents on hair cell function is discussed.
Insights
Frog semicircular canal hair cells possess distinct L-type and R-type calcium (Ca2+) currents. These currents exhibit differential sensitivity to blockers and voltage, suggesting unique roles in hair cell function.
Area of Science:
- Neuroscience
- Cell Physiology
- Ion Channel Research
Background:
- Semicircular canal hair cells are crucial for balance.
- Calcium currents play a vital role in sensory cell function.
- Characterizing specific calcium channel subtypes is essential for understanding cellular mechanisms.
Purpose of the Study:
- To identify and characterize L-type and R-type calcium currents in frog semicircular canal hair cells.
- To investigate the pharmacological and voltage-dependent properties of these currents.
- To explore the physiological implications of these calcium currents in hair cell function.
Main Methods:
- Electrophysiological recordings (patch-clamp) to detect Ca(2+) currents.
- Application of specific calcium channel blockers (nifedipine, mibefradil, conotoxins, agatoxin) and ions (Ni(2+), Cd(2+)).
- Voltage-clamp protocols to analyze current kinetics, voltage dependence, and reversal potentials.
Main Results:
- Two distinct Ca(2+) currents, L-type and R-type, were identified.
- L-type currents were non-inactivating and sensitive to nifedipine. R-type currents showed partial inactivation and sensitivity to mibefradil.
- The R-type current displayed characteristics of a novel transient current, with evidence suggesting two channel types contributing to its components.
- Both currents were sensitive to Ni(2+) and Cd(2+), and run-down was prevented by calpastatin.
Conclusions:
- Frog semicircular canal hair cells express distinct L-type and R-type calcium currents with unique electrophysiological and pharmacological profiles.
- The R-type current may involve multiple channel subtypes, contributing to its complex kinetics.
- These findings provide insights into the molecular basis of calcium signaling and its role in vestibular hair cell function.