Ca2+ current of frog vestibular hair cells is modulated by intracellular ATP but not by long-lasting depolarisation

Some aspects of Ca2+ 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 Ca2+ macrocurrent. With 1 mM ATP in the pipette solution, about 60% of the recorded cells displayed a Ca2+ 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 similar to 280 and similar to 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 similar to 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 ATP gamma S, ADP, AMP or GTP. Long-lasting depolarisations (> 5 s) produced a progressive, reversible decay in the inward current despite the presence of intracellular ATP. Ca2+ channel blockade by Cd2+ unmasked a slowly activating outward Cs+ current flowing through a non-Ca2+ 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 Ca2+ macrocurrent.