Mechanisms underlying facilitation by dopamine of ATP-activated currents in rat pheochromocytoma cells

Mechanisms underlying facilitation by dopamine of extracellular adenosine 5′-triphosphate (ATP)-activated current were investigated in rat pheochromocytoma PC12 cells using the whole-cell voltage-clamp techniques. Dopamine (10 and 100 μM) augmented the peak amplitude of an inward current elicited by ATP (3-100 μM). The activation time course of the ATP-evoked current was accelerated by dopamine; the presence of 10 μM dopamine shifted the dependence of activation rate constants on the concentration of ATP toward a lower concentration range two fold. Dopamine also accelerated the inactivation and the deactivation, which was determined from the current decay upon washout of ATP. Intracellular mediators responsible for the dopamine-induced facilitation was estimated by loading various compounds in patch pipettes. Facilitation was not observed when K-252a (1 μM), a protein kinase inhibitor, was included in the intracellular solution. In addition, facilitation was also attenuated by intracellular adenosine 5′-O-(thiotriphosphate)tetralithium salt (ATP γS (1 mM) or α-β-methylene ATP (1 mM). Inclusion of adenosine 3′, 5′-cyclic monophosphate sodium salt (cAMP, 100 μM), guanosine 3′,5′-cyclic monophosphate sodium salt (cGMP, 100 μM), 12-O-tetradecanoylphorbol-13-acetate (TPA, 1 μM) or phorbol-12,13-dibutylate (1 μM) in the intracellular solution did not affect the facilitation. Guanonsine 5′-O-(thiotriphosphate)tetralithium salt (GTP γS, 500 μM) or guanosine 5′-O(2-thiodiphosphate)-trilithium salt (GDP βS, 500 μM) did not modify the facilitation either. The results suggest that dopamine augments the ATP-activated inward current by facilitating association of ATP to its binding site, and that the augmentation may be mediated through some protein kinase which is different from cyclic-nucleotide-dependent protein kinases or protein kinase C.