ATP modulation of calcium channels in chromaffin cells

1. The effects of externally applied micromolar concentrations of adenosine 5'-triphosphate (ATP) on Ca2+ currents (ICa) were studied in whole-cell clamped adrenaline-secreting chromaffin cells. 2. Ca2+ currents in chromaffin cells activated at about -40 mV, reached a maximum at 0 mV and had an apparent reversal potential at +50 to +60 mV, indicating the existence of only high voltage-activated Ca2+ channels. 3. ATP blocked Ca2+ current rapidly, reversibly and in a concentration-dependent manner (10(-9)-10(-4) M). 4. ATP did not completely block Ca2+ current even at the highest concentrations used (100 microM). The remaining component of Ca2+ current was characterized by slower activation and inactivation kinetics. 5. ATP blocked ICa even in the presence of nisoldipine and/or omega-conotoxin GVIA, suggesting that its modulatory role is not specific for L- and/or N-type Ca2+ channels. 6. Other adenine nucleotides also blocked the Ca2+ current partially. The order of potencies was ATP > or = ADP > AMP >> adenosine, indicating that the ATP effects are most probably mediated by a P2-type purinergic receptor. 7. Dialysis of the cells with an intracellular solution containing 1 mM guanosine 5'-O-thiodiphosphate (GDP-beta-S) or pre-incubation of the cells with pertussis toxin (PTX) blocked the inhibitory effects of ATP. 8. Intracellular application of the non-hydrolysable GTP analogue guanosine 5'-O-(3'-thiotriphosphate) (GTP-gamma-S; 50 microM) also decreased ICa in a manner similar to that seen for ATP and significantly reduced the ATP inhibitory effect. 9. Conditioning pulses to potentials positive to +80 mV partly reversed the inhibitory effects of ATP on the Ca2+ current. The prepulse-induced enhancement of ICa depended on [GTP]i-related G protein activity such that concentrations larger than 200 microM GTP, or GTP-gamma-S (50 microM) were required for significant prepulse potentiation of the Ca2+ current, while dialysis with GDP-beta-S prevented it. 10. We conclude that the ATP, co-released with catecholamines in the intact adrenal gland, may inhibit the secretory process by down-regulating the Ca2+ channel via a P2-type purinergic receptor coupled to a PTX-sensitive G protein.