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Binding of the neurotransmitter acetylcholine to its GPCR on heart muscle cells

activation of the G protein: Gi

activated beta-gamma complex directly opens a K+ channel in heart cell plasma membrane

K+ leaves the cardiac muscle cell (dec. the cell's excitability); K+ moves from cytosol to the extracellular space

Inactivation of the alpha subunit by hydrolysis of its bound GTP

G protein returns to its inactive state

allows K+ channel to close

causes confirmational change of the receptor

Displacement of GDP by GTP on the G alpha subunit

activated alpha subunit activates the beta gamma complex

Result: slows down heart by releasing K+ from the cell

Acetylcholine released to slow down heart