GTP-binding proteins couple cardiac muscarinic receptors to a K channel

Abstract

Binding of acetylcholine (ACh) to cardiac muscarinic ACh receptors (mAChR) activates a potassium channel that slows pacemaker activity^1–3. Although the time course of this activation suggests a multi-step process with intrinsic delays of 30–100 ms^4–6 no second-messenger system has been demonstrated to link the mAChR to the channel. Changes in cyclic nucleotide levels (cyclic AMP and cyclic GMP) do not affect this K channel or its response to muscarinic agonists^7,8. Indeed, electrophysiological experiments argue against the involvement of any second messenger that diffuses through the cytoplasm⁹. We report here that coupling of the mAChR in embryonic chick atrial cells to this inward rectifying K channel requires intracellular GTP. Furthermore, pretreatment of cells with IAP (islet-activating protein from the bacterium Bordetella pertussis) eliminates the ACh-induced inward rectification. As IAP specifically ADP-ribosylates two GTP-binding proteins, N_i and N_o, that can interact with mAChRs¹⁰, we conclude that a guanyl nucleotide-binding protein couples ACh binding to channel activation. This represents the first demonstration that a GTP-binding protein can regulate the function of an ionic channel without acting through cyclic nucleotide second messengers.