title: Different properties of the atrial G protein-gated K+ channels activated by extracellular ATP and adenosine.
abstract: Extracellular ATP (ATPo) and adenosine activate G protein-gated inwardly rectifying K+ currents in atrial cells. Earlier studies have suggested that the two agonists may use separate pathways to activate the K+ current. Therefore, we examined whether the K+ channels activated by the two agonists have different properties under identical ionic conditions. In cell-attached patches, K+ channels activated by 100 microM ATP in the pipette had a single-channel conductance and mean open time of 32.0 +/- 0.2 pS and 0.5 +/- 0.1 ms, respectively, compared with 31.3 +/- 0.3 pS and 0.9 +/- 0.1 ms for the K+ channels activated by adenosine (140 mM KCl). With ATPo as the agonist, the K+ channel activity in cell-attached patches was approximately threefold lower than that in inside-out patches with 100 microM GTP in the bath. Applying ATP to the cytoplasmic side of the membrane (ATPi) produced a biphasic concentration-dependent effect on channel activity: an increase at low [mean affinity constant (K0.5) = 190 microM] and a decrease at high (K0.5 = 1.3 mM) concentrations. In contrast, with adenosine as the agonist, K+ channel activity in cell-attached patches was approximately fourfold greater than that in inside-out patches with 100 microM GTP in the bath. In inside-out patches, ATPi only augmented the K+ channel activity (K0.5 = 32 microM). These results show that although both ATPo and adenosine activate kinetically similar K+ channels in atrial cells, the channels are regulated differently by intracellular nucleotides.
mesh_terms: D000241:Adenosine / Q000494:pharmacology / Q000502:physiology*; D000255:Adenosine Triphosphate / Q000494:pharmacology / Q000502:physiology*; D000818:Animals; D003600:Cytosol / Q000378:metabolism; D005110:Extracellular Space / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000502:physiology*; D006325:Heart Atria; D007425:Intracellular Membranes / Q000378:metabolism; D015640:Ion Channel Gating; D009206:Myocardium / Q000378:metabolism*; D009711:Nucleotides / Q000494:pharmacology / Q000502:physiology; D009994:Osmolar Concentration; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D051381:Rats
chemical_list: D009711:Nucleotides; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D000255:Adenosine Triphosphate; D019204:GTP-Binding Proteins; D000241:Adenosine