Channel mechanisms in SA nodal cells 2:GIRK channels (G-protein coupled Inward-Rectifying K(GIRK)channels/ M2 muscarinic receptors) Outside cell eurotransmitter GPCR CR SGPCR GIRK Inside cell GIRK channels are ACH-gated K channels found in cells of the SA node Parasympathetic neurons release acetylcholine,which acts on M2 muscarinic receptors, and GIRK channels are gated open.They allow an outward current of potassium, which counteracts If and slows the heart