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Department of Physiology, Saga Medical School, Japan.

1. Whole-cell voltage clamp and cell-attached patch-clamp techniques were applied to single atrial myocytes enzymatically dissociated from adult guinea-pig hearts. 2. In whole-cell clamp conditions, external applications, of ATP activated the muscarinic K+ (KACh) current, identified by its inward rectification, its reversal potential near the calculated K+ equilibrium potential (EK) and its relaxation properties during step changes of whole-cell membrane potential. Theophylline, an antagonist for Pi-purinoceptors, did not affect the action of ATP on the KACh current, indicating that the response was evoked through P2-purinoceptors. 3. The concentration-response relationship for ATP was well described by a Hill equation with a half-maximal concentration of 1.84 microM and a Hill coefficient of 0.94. ATP (100 microM) produced a maximal increase of the KACh current to 10.92 microA microF-1, which corresponds to 44.9 and 80.9% of the maximal increases evoked by ACh (10 microM) and adenosine (100 microM), respectively. 4. The activation of KACh current gradually declined to a steady level despite the continuous presence of ATP (desensitization). Recovery from the desensitization was relatively rapid with a half-time of approximately 1.5 min. 5. The activation of KACh current by ATP was completely abolished by pre-incubating myocytes with pertussis toxin (PTX, 5 micrograms ml-1), indicating that P2-purinoceptors are coupled to PTX-sensitive G proteins to activate the KACh channel. 6. In the cell-attached patch recording, ATP (5 microM) applied to the pipette solution enhanced the activity of a channel with single-channel conductance of 52.7 +/- 0.9 pS (mean +/- S.E.M., n = 10), reversal potential near EK and mean open time of 1.1 +/- 0.1 ms. These conductance and kinetic properties are identical to those of the KACh channel in the heart. In contrast, ATP applied to the bath solution did not significantly affect the basal activity of KACh channel openings. These observations suggest that the mechanism coupling the P2-purinoceptor to the activation of the KACh channel involves membrane-delimited component(s) rather than soluble second messenger(s). 7. These results strongly suggest a direct coupling of the P2-purinoceptor to the KACh channel through PTX-sensitive G proteins, analogous to the coupling mechanism of the muscarinic ACh receptor and Pi-purinoceptor to this channel.

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