Ca2+ current of frog vestibular hair cells is modulated by intracellular ATP but not by long-lasting depolarisation.
Some aspects of Ca(2+) channel modulation in hair cells isolated from semicircular canals of the frog (Rana esculenta) have been investigated using the whole-cell technique and intra and extracellular solutions designed to modify the basic properties of the Ca(2+) macrocurrent. With 1 mM ATP in the pipette solution, about 60% of the recorded cells displayed a Ca(2+) current constituted by a mix of an L and a drug-resistant (R2) component; the remaining 40% exhibited an additional drug-resistant fraction (R1), which inactivated in a Ca-dependent manner. If the pipette ATP was raised to 10 mM, cells exhibiting the R1 current fraction displayed an increase of both the R1 and L components by approximately 280 and approximately 70%, respectively, while cells initially lacking R1 showed a similar increase in the L component with R1 becoming apparent and raising up to a mean amplitude of approximately 44 pA. In both cell types the R2 current fraction was negligibly affect by ATP. The current run-up was unaffected by cyclic nucleotides, and was not triggered by 10 mM ATPgammaS, ADP, AMP or GTP. Long-lasting depolarisations (>5 s) produced a progressive, reversible decay in the inward current despite the presence of intracellular ATP. Ca(2+) channel blockade by Cd(2+) unmasked a slowly activating outward Cs(+) current flowing through a non-Ca(2+) channel type, which became progressively unblocked by prolonged depolarisation even though Cs(+) and TEA(+) were present on both sides of the channel. The outward current waveform could be erroneously ascribed to a Ca- and/or voltage dependence of the Ca(2+) macrocurrent.
Martini
Marta
M
Dipartimento di Biologia ed Evoluzione, Sezione di Fisiologia e Biofisica, e Centro di Neuroscienze, Università di Ferrara, Via L. Borsari 46, Ferrara, Italy.
Farinelli
Federica
F
Rossi
Maria Lisa
ML
Rispoli
Giorgio
G
D000255:Adenosine Triphosphate / Q000494:pharmacology / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D018069:Hair Cells, Vestibular / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D011893:Rana esculenta / Q000502:physiology*; D012665:Semicircular Canals / Q000166:cytology / Q000502:physiology*
D015220:Calcium Channels; D000255:Adenosine Triphosphate; D002118:Calcium