A G-protein coupled acetylcholine receptor signaling pathway that contributes to a decrease in frequency or rate of heart contraction. Binding of acetylcholine to a G-protein coupled (muscarinic) receptor on the surface of the signal-receiving cell results in the alpha subunit of a coupled G-protein binding to GTP. This results in the separation of the beta-gamma complex from the alpha subunit. Both the alpha subunit, and the beta-gamma complex can continue to signal to bring about membrane hyperpolarization and a reduction in heart rate.