Modulation of the muscarinic K+ channel by P-2-purinoceptors in guinea-pig atrial myocytes

1. Activation of muscarinic K+ (K-ACh) channels by P-2-purinergic agonists, such as ATP, decreases monotonically in the continued presence of agonist. We investigated the mechanisms underlying this process of decline in guinea-pig atrial myocytes using the patch-clamp technique. 2. External ATP reversibly depressed the acetylcholine (ACh, 5.5-11 mu M)-induced K-ACh current in a concentration-dependent manner with a half-maximal inhibitory concentration (IC50) of 5.4 mu M. 3. External ATP irreversibly reduced guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S)-induced K-ACh current both in control and pertussis toxin (PTX)-pretreated cells, suggesting (i) that the ATP-induced inhibition of K-ACh current occurred at some step(s) downstream from the activation of the PTX-sensitive G protein, G(K), and (ii) that a PTX-insensitive G protein was involved in the signal transduction pathway. 4. The potency order of ATP analogues in reducing K-ACh current was ATP greater than or equal to 2-methylthio-ATP greater than or equal to alpha,beta-methylene-ATP, indicating involvement of a P-2Y-type purinoceptor. 5. In the cell-attached patch recording, ATP (100 mu M) applied to the bath solution reduced the activity of the K-ACh channels activated by ACh in the pipette, in two out of eight experiments, suggesting the possible involvement of cytosolic second messengers in the inhibition of K-ACh channels. 6. The ATP-induced reduction of K-ACh current was not affected by a protein kinase C inhibitor, 1-(5-isoquinolinesulphonyl)-2-methylpiperazine dihydrochloride (H-7), suggesting that this response was not mediated by the activation of protein kinase C. 7. These results demonstrate that, in addition to the membrane-delimited activation through G(K), external ATP causes an inhibition of the K-ACh channel probably by activating a PTX-insensitive G protein and cytosolic second messenger(s), which may underlie the monotonic decrease of the ATP-activated K-ACh current.