Acetylcholine activation of K+ channels in cell-free membrane of atrial cells

The activation mechanisms of K+ channels by muscarinic acetylcholine (m-ACh) receptors were examined in isolated atrial cells by use of patch-recording technique. In "cell-attached" patch recordings, ACh, present in the pipette, activated an inwardly rectifying K+ channel. In "inside-out" patches, activation of the K+ channel by ACh diminished with time following excision of the patch, but it resumed when GTP was present in the solution bathing the intracellular side of the membrane. The A protomer of pertussis toxin, together with NAD, inhibited the channel activation in the presence of GTP. Since pertussis toxin specifically ADP-ribosylates GTP-binding proteins Ni and No, which can interact with m-ACh receptors, and inhibits their functions, it was concluded that m-ACh receptors communicate with the K+ channel via GTP-binding proteins, probably Ni and/or No, in atrial cell membrane.