ATP MODULATION OF L-TYPE CALCIUM-CHANNEL CURRENTS IN GUINEA-PIG OUTERHAIR-CELLS

Ca2+ channel currents and their modulation by adenosine 5'-triphosphat e (ATP) in acutely isolated guinea pig outer hair cells (OHCs) were in vestigated using the whole-cell patch-clamp technique. The current-vol tage (I-V) relation of OHCs indicated that the Ca2+ channel opened nea r -30 mV, and the current reached a maximum at +10 and 0 mV in 20 mM C a2+ and Ba2+ external solutions, respectively. BayK 8644 (BayK, 2 mu M ) caused a 3.5-fold increase in peak Ca2+ currents and shifted the I-V curves toward more negative potentials. These results suggest that th e majority of Ca2+ channels in OHCs have L-type characteristics.The ef fects of ATP on Ca2+ channels of OHCs were heterogenous. ATP (100 mu M ) decreased Ca2+ channel currents by 31.7 +/- 5.6% at 0 mV and shifted Ca2+ tail activation curves toward more depolarized potentials in som e 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, AT P-induced inhibition or enhancement of Ca2+ channel currents was atten uated. In addition, 100 mu M ATP produced little effect on Ca2+ channe l currents in another subpopulation of cells (N = 12). This heterogeno us neuromodulation of Ca2+ channel currents by ATP may reflect a funct ional diversity among OHCs.