SINGLE POTASSIUM CHANNEL CURRENTS ACTIVATED BY EXTRACELLULAR ATP IN DEVELOPING CHICK SKELETAL-MUSCLE - A ROLE FOR 2ND MESSENGERS

1. In developing chick skeletal muscle, extracellular ATP activates an early excitatory current and a delayed potassium current. Previous wo rk had shown that the potassium current elicited by ATP is sensitive t o temperature and activates with a delay of nearly 1 s, suggesting tha t a second messenger is involved. The existence of a second messenger was confirmed by the observation that single potassium channels were a ctivated in cell-attached patches, when ATP was applied outside of the patch pipette. 2. Two classes of ATP-activated potassium-channel curr ents were observed in cell-attached patches: one had a slope conductan ce of 23 pS, whereas the other had a slope conductance of 51 pS. 3. Ph armacological manipulations suggested that activation of the whole-cel l potassium current by ATP did not require cyclic adenosine monophosph ate (cAMP), cyclic guanosine monophosphate (cGMP), inositol 1,4,5-trip hosphate (IP3), nitric oxide, or a rise in internal free calcium. Addi tional pharmacological experiments suggested that activation of the wh ole-cell potassium current might not require activation of a G protein and probably did not involve intracellular protein phosphorylation. 4 . The ability of arachidonic acid and its metabolites to activate pota ssium channels in chick skeletal muscle was also tested. Arachidonic a cid, several prostaglandins and several leukotrienes activated whole-c ell potassium currents. However, results with several inhibitors sugge sted that arachidonic acid and its metabolites are not necessary for a ctivation of the whole-cell potassium current by ATP. 5. In excised ou tside-out membrane patches, ATP activated a single class of potassium channels. The slope conductance of these channels indicated that they are likely to be identical to the smaller of the two classes of second messenger activated potassium channels observed in cell-attached patc hes. 6. The observation that the larger class of potassium channels ob served in cell-attached patches was absent in excised patches suggests that activation of these channels by ATP requires a cytosolic factor that is easily dialyzed away. In contrast, the observation that the sm aller class of potassium channels could still be activated by ATP in e xcised patches suggests that the two classes of potassium channels are activated by different mechanisms. These results also indicate that a ll the molecules involved in coupling ATP receptor activation to openi ng of the smaller class of potassium channels remain closely associate d with an excised patch. One possible explanation is that there might be an intramembranous second messenger.