ATP modulation of calcium channels in chromaffin cells.

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Abstract
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.