Acetylcholine released by the parasympathetic nerves has which one of the following functions in the heart pacemaker cells?
1
It binds to GPCR and activates G protein to slow the heart rate
2
It stimulates GABA-activated ion-channel coupled receptor to increase the heart rate
3
It binds to GPCR and inhibits G protein to slow the heart rate
4
It inhibits GABA-activated ion-channel coupled receptor to increase the heart rate