ATP modulation of L-type calcium channel currents in guinea pig outer hair cells

Ca²⁺ channel currents and their modulation by adenosine 5′-triphosphate (ATP) in acutely isolated guinea pig outer hair cells (OHCs) were investigated using the whole-cell patch-clamp technique. The current-voltage (I-V) relation of OHCs indicated that the Ca²⁺ channel opened near -30 mV, and the current reached a maximum at + 10 and 0 mV in 20 mM Ca²⁺ and Ba²⁺ external solutions, respectively. BayK 8644 (BayK, 2 μM) caused a 3.5-fold increase in peak Ca²⁺ currents and shifted the I-V curves toward more negative potentials. These results suggest that the majority of Ca²⁺ channels in OHCs have L-type characteristics. The effects of ATP on Ca²⁺ channels of OHCs were heterogenous. ATP (100 μM) decreased Ca²⁺ channel currents by 31.7 ± 5.6% at 0 mV and shifted Ca²⁺ tail activation curves toward more depolarized potentials in some cells (N = 6). By contrast, in others, ATP enhanced the currents by 43.5 ± 12.5% at + 10 mV (N = 6). In the presence of BayK, however, ATP-induced inhibition or enhancement of Ca²⁺ channel currents was attenuated. In addition, 100 μM ATP produced little effect on Ca²⁺ channel currents in another subpopulation of cells (N = 12). This heterogenous neuromodulation of Ca²⁺ channel currents by ATP may reflect a functional diversity among OHCs.