Figure 2.

Functional effects of voltage-dependent inhibition. (A) current records from an adrenal chromaffin cell in the absence (black traces) and presence (red traces) of extracellular ATP. Chromaffin cells express P2Y receptors that are activated by ATP co-released with catecholamines. Hallmark features of voltage-dependent inhibition are shown and include reduced peak amplitude with prominent slowing of the activation kinetics (left pair of current traces). A conditioning prepulse to +100 mV applied just before the test pulse to +10 mV reversed most of the inhibition and normalized the activation kinetics of I_Ca. but had no effect under control conditions (black trace) (right pair of current traces) (B). Voltage-dependent relief of inhibition (facilitation) can also occur during stimuli that mimic brief trains of actions potentials. The example is taken from an adrenal chromaffin cell stimulated with a 45 Hz train of ten action potential-like stimuli (from −60 mV to +35 mV) in the presence of ATP to produce inhibition (red) or in control conditions (black traces). Currents were normalized to the first pulse of the train and mean data is shown on left. In control conditions there was a small decrease in current amplitude (probably due to inactivation), but in the presence of ATP there was a robust increase in current, reflecting partial reversal of the G protein mediated inhibition.