Effect of intracellular dialysis of ATP on the hyperpolarization-activated cation current in rat dorsal root ganglion neurons

The mechanism of the effect of intracellular ATP on the hyperpolarization-activated non-selective cation current (I-h) in rat dorsal root ganglion neurons was investigated using a whole cell voltage-clamp technique. Under voltage-clamp conditions, I-h was activated by hyperpolarizing pulses raised to a voltage of between - 70 and - 130 mV. The activation curve of I-h in rat dorsal root ganglion (DRG) neurons shifted by about 15 mV in the positive direction with an intracellular solution containing 1 mM cAMP. When ATP (2 mM) was applied intracellularly, the half-maximal activation voltage (V-half) of I-h shifted from - 97.4 +/- 1.9 to - 86.8 +/- 1.6 mV, resulting in an increase in the current amplitude of I-h by the pulse to between - 80 and - 90 mV. In the presence of an adenylate cyclase inhibitor, SQ-22536 ( 100 muM), the intracellular dialysis of ATP also produced a shift in the voltage-dependence of I-h in rat DRG neurons, indicating that the effect of ATP was not caused by cAMP converted by adenylate cyclase. Intracellular dialysis of a nonhydrolysable ATP analog, AMP-PNP or ATP-gamma-S, also produced a positive shift in the voltage-dependence of I-h activation, suggesting that the effect of ATP results from its direct action on the channel protein. These results indicate that cytosolic ATP directly regulates the voltage dependence of I-h activation as an intracellular modulating factor.