Acetylcholine decrease hea rate by which of the following mechanisms?

In SA Node (SAN) and AV Node (AVN); the phase 4 of action potential is not straight as seen in action potential of non automatic myocardial fibres; hence the SAN and AVN depolarizes automatically. Sympathetic system ( beta receptors), increases the hea rate by increasing the slope of phase 4 of action potential curve of SAN and AVN. Parasympathetic stimulation (ACh) decreases the slope of phase 4 (known as diastolic depolarization) and hence causes bradycardia by action of ACh on M2 receptors (Gi GPCR) of mainly SA node. M2 receptors acts the following 3 mechanisms 1. opening of ACh sensitive potassium channels 2. inhibition of funny currents 3. inhibition of calcium channels.